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CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Patent Application 60/860,401, filed Nov. 21, 2006, the content of which is incorporated herein by reference for all purposes. 

   FIELD OF THE DISCLOSURE 
   The present disclosure relates generally to testing conducted in wells penetrating subterranean formations and, more particularly, to modular apparatus and methods of use. Still more particularly, the present disclosure relates to an apparatus and method to facilitate the placement of tool components close to the formation wall. 
   BACKGROUND 
   Drilling, completion, and production of reservoir wells involves monitoring of various subsurface formation parameters. For example, parameters of reservoir pressure and permeability of the reservoir rock formations are often measured to evaluate a subsurface formation. Fluid may be drawn from the formation and captured to measure and analyze various fluid properties of a fluid sample. Monitoring of such subsurface formation parameters can be used, for example, to determine the formation pressure changes along the well trajectory or to predict the production capacity and lifetime of a subsurface formation. 
   Traditional downhole measurement systems sometimes obtain these parameters through wireline logging via a formation tester tool. A formation tester tool may alternatively be coupled to a drill string in-line with a drill bit (e.g., as part of a bottom hole assembly) and even a directional drilling subassembly. The drill string often includes one or more stabilizer(s) to engage a formation wall during drilling to substantially reduce or eliminate vibration, wandering, and/or wobbling of the drill bit and the drill string during drilling operations. 
   A typical formation tester tool engages a formation wall to obtain measurements of the subsurface formation parameters. Therefore, measurement instruments or probes used to generate the subsurface formation parameters are sometimes configured to protrude from the drill string sufficiently to engage the formation wall. The amount of protrusion from the drill string is typically sufficient for the probes to meet or extend beyond the diameter of the stabilizer, which is typically configured to engage or about to engage the formation wall. 
   In some systems, each time a drill bit is selected or adjusted to drill a particular diameter well, the formation tester tool may also need to be replaced. One motivation for replacing the formation tester tool may be that the tester tool comprises an integral stabilizer no longer suitable for drilling a well of the selected diameter. A new formation tester tool is selected having an integral, larger diameter stabilizer to engage the wall of the larger diameter well. The formation tester tool may also need to be replaced so that its measurement instruments or probes extend further and engage the wall of the larger diameter well. In these systems, a drilling operation often requires a plurality of different formation tester tools to accommodate any of a number of well diameters. This requirement affects, for example, the cost of the service delivery. 
   SUMMARY 
   In accordance with one aspect of the disclosure, a system for testing an underground formation penetrated by a well is disclosed. The system includes a downhole tool, a plurality of stabilizing subs, a plurality of frames, and at least one measuring device. The toll is configured to be coupled to a work string and includes a body having an outer surface, a connection for coupling a stabilizing sub to the downhole tool, and at least one portion configured to receive a frame. The plurality of stabilizing subs are configured to be coupled to the downhole tool and include an outer surface defining an offset relative to the outer surface of the downhole tool. A first of the plurality of stabilizing subs has a first stabilizing sub offset. The plurality of frames are configured to be detachably mounted on the at least one portion of the downhole tool. Each frame has an offset relative to the outer surface of the downhole tool and an aperture for receiving a measuring device, wherein a first of the plurality of frames has a first frame offset determined by the first stabilizing sub offset. The at least one measuring device is configured to be secured in at least one of the plurality of frames. 
   In accordance with another aspect of the disclosure, a system for testing an underground formation penetrated by a well is disclosed. The system includes a downhole tool having an elongated tool body and at least one measuring device. In particular, the tool is configured to be coupled to a work string and the body has a bore that is disposed along a longitudinal axis thereof for circulating a fluid. A web is disposed across the bore such that at least one fluid passageway is provided around the web and such that the web at least partially frames a through hole disposed in the tool body. The measuring device is configured to be secured in the through hole. 
   In accordance with another aspect of the disclosure, a method for testing an underground formation penetrated by a well is disclosed. The method includes providing a downhole tool that is configured to be coupled to a work string and configured to convey a measuring device for testing the subterranean formation penetrated by the well. The method further includes, selecting a stabilizing sub configured to be coupled to the downhole tool and having an outer surface offset a first distance relative to an outer surface of said downhole tool; selecting a frame from a plurality of frames configured to be coupled to said downhole tool, wherein the frame is configured to protrude from the downhole tool outer surface by a second distance different from distances associated with others of the plurality of frames, and wherein the frame is selected based on the first distance associated with the stabilizing sub; coupling said selected stabilizer sub and said selected frame to the downhole tool; lowering the downhole tool in the underground formation; and testing the underground formation using the measurement device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an elevation view including a block diagram of a drilling rig and drill string that may incorporate the example apparatus described herein. 
       FIG. 2  depicts a block diagram that may be used to implement a logging while drilling tool of  FIG. 1 . 
       FIG. 3A  depicts a first side view and  FIG. 3B  depicts a second side view of an example tool collar that may be used to implement the example tool collar of  FIG. 1 . 
       FIG. 3C  depicts an exploded view of a stabilizer sleeve configured to be coupled to the tool collar of  FIGS. 3A and 3B . 
       FIG. 3D  depicts a cross-sectional view of the tool collar of  FIGS. 3A-3C . 
       FIG. 4  depicts the example tool collar of  FIGS. 3A-3C  having an example probe module implemented using a two-probe-per-pad configuration. 
       FIG. 5  depicts the example tool collar of  FIGS. 3A-3D  having another example probe module implemented using a five-probe-per-pad configuration. 
       FIG. 6  depicts an example tool collar having probe modules located at opposing ends of a stabilizer sleeve. 
       FIG. 7  illustrates the example tool collar of  FIGS. 3A-3D  having a removable probe module inserted therein. 
       FIG. 8  illustrates an exploded diagram in which the probe module of  FIG. 7  is removed from the tool collar. 
       FIG. 9  is a cross-sectional view A-A of the example tool collar of  FIG. 8 . 
       FIG. 10  is a partial cross-sectional view B-B of the example tool collar of  FIGS. 7 and 8  and depicts an example rotatable connector used to provide electrical and hydraulic connectors to the probe module of  FIGS. 7 and 8 . 
       FIG. 11  depicts an alternative example implementation in which a coaxial connector is used to provide electrical and hydraulic connectors. 
       FIG. 12  is another cross-sectional view C-C of the example tool collar of  FIGS. 7 and 8  in which the example probes of  FIGS. 7 and 8  are provided using an integrally formed probe module. 
       FIG. 13  illustrates the cross-sectional view C-C of the example tool collar of  FIGS. 7 and 8  in which each of the example probes of  FIGS. 7 and 8  is provided via a separate and respective probe module. 
       FIGS. 14 and 15  illustrate detailed diagrams of the example probe module  702  removably inserted in the example tool collar of  FIGS. 3A-3D . 
       FIG. 16  is a front view and  FIG. 17  is a cross-sectional side view of an alternative example probe having a shroud that can be used to implement the example probe module of  FIGS. 14 and 15 . 
       FIG. 18  depicts a state diagram representing an example method of operating the example probe module of  FIGS. 14 and 15 . 
       FIGS. 19 through 21  illustrate detailed diagrams of an example probe system that may be implemented within (e.g., integral with) a tool collar in a fixed or non-removable configuration or that may be used to implement a probe module removably insertable into a tool collar. 
       FIG. 22  depicts an alternative example implementation of the example probe system of  FIGS. 19-21  using a motor and lead screw configuration. 
       FIG. 23  depicts a state diagram of a drilling operation that represents an example method to operate the example probe system of  FIGS. 19-21 . 
       FIG. 24  depicts another example probe system implemented using a dual-probe configuration in which two probes are integrally formed so that they simultaneously extend and retract relative to a tool collar. 
       FIG. 25  depicts another example tool collar having a plurality of probes. 
       FIG. 26  depicts a probe assembly used to implement one of the probes of  FIG. 25 . 
   

   DETAILED DESCRIPTION 
   Certain examples are shown in the above-identified figures and described in detail below. In describing these examples, like or identical reference numbers are used to identify common or similar elements. The figures are not necessarily to scale and certain features and certain views of the figures may be shown exaggerated in scale or in schematic for clarity and/or conciseness. 
     FIG. 1  shows a drilling system and related environment. Land-based platform and derrick assembly  100  are positioned over a wellbore  102  penetrating a subsurface formation F. The wellbore  102  is formed by rotary drilling in a manner that is well known. However, those of ordinary skill in the art, given the benefit of this disclosure, will appreciate that the present invention also finds application in directional drilling applications as well as rotary drilling, and is not limited to land-based rigs. A drill string  104  is suspended within the wellbore  102  and includes a drill bit  106  at its lower end. The drill string  104  is rotated by a rotary table  108 , energized by means not shown, which engages a kelly  110  at the upper end of the drill string  104 . The drill string  104  is suspended from a hook  112 , attached to a traveling block (not shown), through the kelly  110  and a rotary swivel  114 , which permits rotation of the drill string  104  relative to the hook  112 . 
   A drilling fluid  116  is stored in a pit  118  formed at the well site. A pump  120  delivers the drilling fluid  116  to the interior of the drill string  104  via a port in the rotary swivel  114 , inducing the drilling fluid  116  to flow downwardly through the interior of the drill string  104  as indicated by directional arrow  122 . The drilling fluid  116  exits the drill string  104  via ports in the drill bit  106  to lubricate the drill bit  106  and then circulates upwardly through the region between an outer surface of the drill string  104  and the wall of the wellbore  102 , called the annulus  124 , as indicated by direction arrows  126 . The drilling fluid  116  is referred to herein as drilling mud when it enters the annulus  124  and flows through the annulus  124 . The drilling mud typically includes the drilling fluid  116  mixed with formation cuttings and other formation material. The drilling mud carries formation cuttings up to the surface as the drilling mud is routed to the pit  118  for recirculation and so that the formation cuttings and other formation material can settle in the pit  118 . 
   The drilling fluid  116  performs various functions to facilitate the drilling process, such as lubricating the drill bit  106  and transporting cuttings generated by the drill bit  106  during drilling. The cuttings and/or other solids mixed with the drilling fluid  116  create a “mudcake” that also performs various functions, such as coating the borehole wall. 
   The dense drilling fluid  116  conveyed by the pump  120  is used to maintain the drilling mud in the annulus  124  of the wellbore  102  at a pressure (i.e., an annulus pressure (“A P ”)) that is typically higher than the pressure of fluid in the surrounding formation F (i.e., a pore pressure (“P P ”)) to prevent formation fluid from passing from the surrounding formation F into the borehole. In other words, the annulus pressure (A P ) is maintained at a higher pressure than the pore pressure (P P ) so that the wellbore  102  is “overbalanced” (A P &gt;P P ) and does not cause a blowout. The annulus pressure (A P ) is also usually maintained below a given level to prevent the formation surrounding the wellbore  102  from cracking and to prevent the drilling fluid  116  from entering the surrounding formation F. Thus, downhole pressures are typically maintained within a given range. 
   The drill string  104  further includes a bottom hole assembly  128  near the drill bit  106  (e.g., within several drill collar lengths from the drill bit). The bottom hole assembly  128  includes capabilities for measuring, processing, and storing information, as well as communicating with surface equipment. The bottom hole assembly  128  includes, among other things, measuring and local communications apparatus  130  for determining and communicating measurement information associated with the formation F surrounding the wellbore  102 . The communications apparatus  130 , including a transmitting antenna  132  and a receiving antenna  134 , is described in detail in U.S. Pat. No. 5,339,037, commonly assigned to the assignee of the present application, the entire contents of which are incorporated herein by reference. 
   The bottom hole assembly  128  further includes a formation tester  136  that may comprise one or more drill collars such as drill collars  154  and  158 . Each of the collars  154  and  158  includes respective breakable connectors (e.g., the breakable connectors  301   a  and  301   b  of  FIG. 3A ) to breakably or detachably couple the collars  154  and  158  to one another and/or to other collars of the bottom hole assembly  128 . As used herein, detachable connectors are connectors that are capable of being attached to one another and detached or separated from one another. In other example implementations, the collars  154  and  158  may be a unitary piece (e.g., may be formed using one collar). Yet in other example implementations, such as described below in connection with  FIGS. 3A-3D , a tool collar having a plurality of threads on a portion of an outer diameter surface is configured to receive a stabilizer sleeve (e.g., a stabilizer sleeve  302  of  FIGS. 3A-3C ) having stabilizer blades and a plurality of threads on a portion of an inner diameter surface that enable mechanically coupling the stabilizer sleeve to the tool collar. 
   The formation tester  136  includes one or more measurement probe(s)  137   a - c  configured to perform measurement operations. The probe  137   a  may be located preferably, but not necessarily, on a raised portion  159  (e.g., a pad) of an outside diameter of the formation tester  136 . Alternatively, the probes  137   b  and  137   c  may be located in a stabilizer blade  156  of the formation tester  136 . Alternatively or additionally, probes may be anywhere on the formation tester  136 . 
   The bottom hole assembly  128  further includes a surface/local communications subassembly  138 . As known in the art, the surface/local communications subassembly  138  may comprise a downhole generator (not shown) commonly referred to as a “mud turbine” that is powered by the drilling fluid  116  flowing downwardly through the interior of the drill string  104  in a direction generally indicated by arrow  122 . The downhole generator can be used to provide power to various components in the bottom hole assembly  128  during circulation of the drilling fluid  116 , for immediate use or for recharging batteries located in the bottom hole assembly  128 . 
   The subassembly  138  further includes an antenna  140  used for local communication with the apparatus  130 , and also includes a known type of acoustic communication system (not shown) that communicates with a similar system (not shown) at the earth&#39;s surface via signals carried in the drilling fluid  116  or drilling mud. Thus, the surface communication system in the subassembly  138  includes an acoustic transmitter that generates an acoustic signal in the drilling fluid  116  or drilling mud that includes information of measured downhole parameters. 
   One suitable type of acoustic transmitter employs a device known as a “mud siren” (not shown). A mud siren may include a slotted stator and a slotted rotor that rotates and repeatedly interrupts the flow of the drilling fluid  116  or drilling mud to establish a desired acoustic wave signal in the drilling fluid  116 . The driving electronics in the subassembly  138  may include a suitable modulator, such as a phase shift keying (PSK) modulator, which conventionally produces driving signals for the mud siren. For example, the driving signals can be used to apply appropriate modulation to the mud siren. 
   The acoustic signals transmitted by the acoustic communication system are received at the surface by transducers  142 . The transducers  142  (e.g., piezoelectric transducers) convert the received acoustic signals to electronic signals. The outputs of the transducers  142  are coupled to an uphole receiving subsystem  144 , which demodulates the transmitted signals. An output of the receiving subsystem  144  is then coupled to a processor  146  and a recorder  148 . 
   An uphole transmitting system  150  is also provided, and is operative to control interruption of the operation of the pump  120  in a manner that is detectable by transducers  152  in the subassembly  138 . In this manner, the subassembly  138  and the uphole equipment can communicate via two-way communications as described in greater detail in U.S. Pat. No. 5,235,285, the entire contents of which are incorporated herein by reference. 
   In the illustrated example of  FIG. 1 , the bottom hole assembly  128  is further equipped with one or more stabilizer sections. The stabilizer sections comprise stabilizer blades or protuberances  156  and  157  that are used to address the tendency of the bottom hole assembly  128  to wobble and become decentralized as it rotates within the wellbore  102 , resulting in deviations in the direction of the wellbore  102  from the intended path (for example, a straight vertical line). Such deviation can cause excessive lateral forces on the drill string sections as well as the drill bit  106 , thereby producing accelerated wear. The stabilizer blades  156  and  157  are configured to overcome this action and centralize the drill bit  106  and, to some extent, the drill string  104 , within the wellbore  102 . The stabilizer blades  156  and  157  may be integral with the drill collar  154 , or they may be bolted on the drill  154 . In some example implementations, the thickness and/or shape of the stabilizer blades  156  and  157  may be selected based on the type of drilling operation to be performed and/or the desired handling or performance of the bottom hole assembly  128  during the drilling operation. 
   The order in which the local communications apparatus  130 , the formation tester  136 , and the surface/local communications subassembly  138 , are depicted on the bottom hole assembly  128  in  FIG. 1  is only one example implementation. In other example implementations, the components  130 ,  136 ,  138 , of the bottom hole assembly  128  may be rearranged or one or more components may be removed or added. In addition, the bottom hole assembly  128  may include fewer or more of any one or more of the components  130 ,  136 ,  138 , and/or any other components not shown. The example methods and apparatus described herein are also not restricted to drilling operations. Persons of ordinary skill in the art will appreciate that the example apparatus and methods described herein can also be advantageously used during, for example, well testing or servicing. Further, the example methods and apparatus, in general, can be implemented in connection with testing conducted in wells penetrating subterranean formations and in connection with applications associated with formation evaluation tools conveyed downhole by any known means. 
     FIG. 2  depicts a block diagram of a formation tester  200  that may be used to implement, for example, the formation tester  136  of  FIG. 1 . In the illustrated example of  FIG. 2 , lines shown connecting blocks in  FIG. 2  represent hydraulic or electrical connections, that may comprise one or more flow lines or one or more wires or conductive paths respectively. 
   To perform downhole measurements and tests, the formation tester  200  is provided with probes  202   a  and  202   b . In an example implementation, each of the probes  202   a - b  includes a respective sensor  204   a - b  and may include an analog-to-digital converter (ADC)  206   a - b . One or both of the probes  202   a  and  202   b  may be configured to be stationary within the formation tester  200 . The sensors  204   a - b  may be configured to measure formation parameters (e.g., resistivity, porosity, density, pressure, sonic velocity, natural radioactivity, or any other measurement). Alternatively or additionally, the probes  202   a  and  202   b  may be provided with actuators, such as coils or antennae, radioactive sources, piezo electrical actuators, etc. In some cases, the probes  202   a  and  202   b  may be configured to facilitate the performance of different types of measurements. For example, the measurement probe  202   a  may be configured to facilitate measuring a formation parameter while the measurement probe  202   b  may be configured to facilitate measuring another different formation parameter. In other cases, the probes  202   a - b  may be configured to perform the same type of measurement. 
   Example probe systems and/or example probe modules that may be used to implement measurement probe are described in greater detail below. For example, the probes  202   a  and  202   b  may be implemented using measurement/pad modules (e.g., the measurement/pad module of  FIGS. 3A-3D ). 
   In another example implementation, the probes  202   a  and  202   b  are preferably configured to protrude from the formation tester  200 , each of which may be substantially similar or identical to the measurement probes  137   a ,  137   b  and  137   c  of  FIG. 1 . Probes  202   a  and  202   b  are typically configured to recess in a cavity of the formation tester during drilling and to protrude from the formation tester  200  toward a borehole wall when a measurement is desired. Thus, the probes  202   a  and  202   b  facilitate the placement of tool components close to the borehole wall. 
   The probes  204   a  and  204   b  may be equipped with position sensors or displacement sensor (e.g., analog potentiometers, digital encoders, etc.) to determine and/or substantially continuously monitor the distances by which the probes  204   a  and  204   b  are extended from the formation tester  200 . Additionally or alternatively, the amount of hydraulic fluid used by a hydraulic system  230  to displace the probes  204   a  and  204   b  may be used for tracking or monitoring the extension distances of the probes  204   a  and  204   b . This hydraulic fluid amount may be estimated using, for example, motor revolution sensors on an optional motor  232 . Thus, the probes  202   a  and  202   b  may be used as a mechanical caliper to make a measurement of the borehole diameter. Alternatively or additionally, the probes  202   a  and  202   b  may be used for measuring rock elastic modulus and rock strength. 
   In another example implementation, the formation tester  200  may be configured to determine the formation pore pressure (“P P ”). The probes  202   a  and  202   b  are preferably configured to protrude from the formation tester  200  and seal a portion of the formation wall. As shown, each of the probes  202   a - b  includes a pressure sensor  204   a - b  and may include an analog-to-digital converter (ADC)  206   a - b . The sensors  204   a  and  204   b  may be quartz gages, but other known pressure gages may be used. The sensors  204   a  and  204   b  are in fluid communication with the sealed portion of the borehole wall through at least a fluid inlet in the probes  202   a - b  respectively. Usually, the hydraulic system  230  comprises a pump or a piston that is energized by the motor  232  for drawing formation fluid into the probe. 
   In some cases, each of the probes  202   a - b  includes a drawdown piston between the hydraulic system  230  and a respective probe inlet. The drawdown pistons may be equipped with position sensors or displacement sensors (e.g., analog potentiometers, digital encoders, etc.) to determine and/or substantially continuously monitor their position within the probes  204   a  and  204   b.    
   Example probe systems and/or example probe modules that may be used to implement a pressure probe are described in greater detail below. For example, the probes  202   a  and  202   b  may be implemented using probe modules (e.g., the probe module  702  of  FIGS. 14 and 15 ). 
   In yet another example implementation, at least one of the probes  202   a - b  may be used to sample formation fluid. This probe is preferably configured to protrude from the formation tester  200  and seal a portion of the borehole or formation wall. In this example, the hydraulic system  230  is used to draw formation fluid through the probes  202   a - b  into the formation tester  200 . The hydraulic system  230  may comprise a pump driven by, for example, the motor  232 , and one or more sample cavity(ies) to capture a sample of formation fluid and to carry the sample to the surface where further analysis of the retrieved fluid sample may be performed. The fluid sample is preferably taken as a representative sample of the area of the well from which the sample was drawn using known systems and methods. 
   Example probe systems and/or example probe modules that may be used to implement a sampling probe are described in greater detail below. For example, the sampling probe may be implemented using the probe module  602   a  of  FIG. 6 . 
   As described below, the probes  202   a - b  may be implemented using one or more removably insertable probe modules (e.g., the probe module  702  of  FIGS. 7 and 8 ). A removably insertable probe module may be modular and may be insertable into an opening (not shown) formed in the formation tester  200 . The removably insertable probe module may include mechanical, electrical, and/or hydraulic interfaces that are relatively easily connectable to corresponding interfaces on the formation tester  200 . In this manner, the bottom hole assembly  128  ( FIG. 1 ) need not be completely disassembled and reassembled to connect different modules each time different instrumentation (e.g., different probes or different sensors) is required to perform different measurements of a formation (e.g., the formation F of  FIG. 1 ). Instead, an interchangeable probe module can be removed from the formation tester  200  and replaced using another interchangeable probe module having different measurement capabilities, different dimensions (e.g., probe length), etc. 
   In alternative example implementations, the probes  202   a - b  and pads (e.g., the pad  159  of  FIG. 1 ) can be part of a pad/probe module that is removably insertable in or mountable to the formation tester  200 . 
   In yet other example implementations, measurement modules may not have sensors (e.g., the sensors  204   a - b ) mounted on an extendable probe, but may instead have sensors that are part of the measurement modules and the measurement modules may be removably insertable in or mountable to the formation tester  200 . In some cases, respective pads may be integrally formed the measurement modules, and each of the sensors  204   a - b  may be located substantially flush with respect to the outer surface of a respective pad. 
   To provide electronic components and hydraulic components to control the probes  202   a - b  and obtain test and measurement values, the formation tester  200  is provided with a chassis  208  that includes a tool bus  210  configured to transmit electrical power and communication signals. The chassis  208  also includes an electronics system  214  and a battery  216  electrically coupled to the tool bus  210 . The chassis  208  further includes the hydraulic system  230  and the optional motor  232 . 
   The tool bus  210  includes tool bus interfaces  212   a - b  to couple the tool bus  210  to tool buses of other collars to transfer electrical power and/or information signals between collars. For example, the tool bus  210  may be used to electrically connect the formation tester  200  to a surface/local communications subassembly such as, for example, the surface/local communications subassembly  138  in  FIG. 1 . Thus, the formation tester  200  may receive power generated by a turbine located in the surface/local communications subassembly  138 . Additionally, the formation tester  200  may and send and/or receive data from the surface via the subassembly  138  and the modem  226 . 
   To operate the probes  202   a - b , the chassis  208  is provided with the hydraulic system  230  coupled to the motor  232  via, for example, a gearbox (not shown). Motor  232  may be of any known kind such as, for example, a brushless direct-current (“DC”) motor, a stepper motor, etc. The hydraulic system  230  and the motor  232  may be used to extend and retract the probes  202   a - b  relative to the formation tester  200  toward and away from the wall of the wellbore (e.g., the wellbore  102  of  FIG. 1 ). 
   In the illustrated example, the hydraulic system  230  is fluidly coupled to an annulus pressure (A P ) port  234  to sense the pressure of drilling mud in the annulus  124  of the wellbore  102  ( FIG. 1 ). The hydraulic system  230  is also shown fluidly coupled to an internal pressure (I P ) port  236  to sense the pressure of drilling fluid (e.g., the drilling fluid  116  of  FIG. 1 ) that flows through a fluid passage  238  in the formation tester  200 . In some example implementations, the hydraulic system  230  may use the annulus and internal fluid pressures instead of or in addition to the motor  232  to extend and/or retract the probes  202   a  and  202   b , for example as described below in connection with  FIGS. 19-21 . 
   The battery  216  and/or the subassembly  138  provide electrical power to the motor  232  that, in turn, provides mechanical power to the hydraulic system  230 . Additionally or alternatively, the pressure differential between the annulus and internal fluid pressures provide hydraulic power to the hydraulic system  230 . In some cases, it may be advantageous to configure the formation tester  200  so that the hydraulic system  230  is capable of operating during circulation of the drilling fluid  116  and/or when circulation of the drilling fluid  116  has stopped. Thus, the formation tester  200  is preferably capable of making a measurement while a circulation pump is on and/or a measurement while a circulation pump is off. For example, the hydraulic system  230  may include an accumulator to store hydraulic energy during circulation of the drilling fluid  116  for later use, as described below in connection with  FIGS. 19-21 . An accumulator may also be used to store hydraulic energy over a long period of time to reduce the peak electrical consumption of the formation tester  200  as described below in connection with  FIG. 14 . 
   Although the hydraulic system  230  is shown as being implemented in the chassis  208 , in some example implementations, one or more portions of the hydraulic system  230  may be implemented in probe modules (e.g., the probe module  702  of  FIGS. 7 and 8 ). Example hydraulic systems that may be used to implement the hydraulic system  230  are described in detail below. 
   The electronics system  214  is provided with a controller  218  (e.g. a CPU and Random Access Memory) to implement test and measurement routines (e.g., to control the probes  202   a - b , etc.). To store machine accessible instructions that, when executed by the controller  218 , cause the controller  218  to implement test and measurement routines or any other routines, the electronics system  214  is provided with an electronic programmable read only memory (EPROM)  220 . In the illustrated example, the controller  218  is configured to receive digital data from various sensors in the formation tester  200 . The controller  218  is also configured to execute different instructions depending on the data received. The instructions executed by the controller  218  may be used to control some of the operations of the formation tester  200 . Thus, the formation tester  200  is preferably, but not necessarily, configured to sequence some of its operations (e.g. probe movement) according to sensor data acquired in situ. 
   In an example implementation, the electronics system  214  may be configured to adjust the force exerted on the formation surface by the probes  202   a  and  202   b  based on the data collected by the sensors  204   a  and  204   b . In addition, the electronics system  214  can be configured to maintain the setting force of the probes  202   a  and  202   b  against the formation surface while the formation tester  200  is moved up and down or rotated to obtain measurements at different locations of the formation surface. 
   Additionally or alternatively, the electronics system  214  may drive a motor controller (e.g., a stepper controller, a revolutions controller, etc.) and collect data from motor revolution sensors that enable tracking or monitoring the extension distances of the probes  204   a  and  204   b.    
   In some example implementations, the electronics system  214  may include controllers (e.g., pulse-width-modulation (“PWM”) controllers) for controlling hydraulic fluid flow to the probes  204   a  and  204   b  with substantially high precision. For example, a PWM controller may be used to control opening and closing of hydraulic fluid line valves (e.g., solenoid valves) to control the extension/retraction of the probes  204   a  and  204   b.    
   Examples of close loop sequencing that may be used to control the operations of formation tester  200  are described in detail below in connection with  FIG. 18 . 
   To store, analyze, process and/or compress test and measurement data, or any kind of data, acquired by formation tester  200  using, for example, the sensors  204   a - b , the electronics system  214  is provided with a flash memory  222 . To generate timestamp information corresponding to the acquired test and measurement information, the electronics system  214  is provided with a clock  224 . The timestamp information can be used during a playback phase to determine the time at which each measurement was acquired and, thus, the depth at which the formation tester  200  was located within a wellbore (e.g., the wellbore  102  ( FIG. 1 ) when the measurements were acquired. To communicate information when the formation tester  200  is still downhole, the electronics system  214  is provided with a modem  226  that is communicatively coupled to the tool bus  210  and the subassembly  138 . In the illustrated example, the formation tester  200  is also provided with a read-out port  240  to enable retrieving measurement information stored in the flash memory  222  when the testing tool is brought to surface. The read-out probe  240  may be an electrical contact interface or a wireless interface that may be used to communicatively couple a data collection device to the formation tester  200  to retrieve logged measurement information stored in the flash memory  222 . 
   Although the components of  FIG. 2  are shown and described above as being communicatively coupled and arranged in a particular configuration, persons of ordinary skill in the art will appreciate that the components of the formation tester  200  can be communicatively coupled and/or arranged different from what is shown in  FIG. 2  without departing from the scope of the present disclosure. Also, although the formation tester  200  is shown with two probes  202   a - b , any number of probes may be used in the formation tester  200 . 
     FIG. 3A  depicts a first side view and  FIG. 3B  depicts a second side view of an example formation tester  300  that may be used to implement the example formation tester  136  of  FIG. 1 . As shown in  FIG. 3A , the example formation tester  300  is provided with breakable connectors  301   a  and  301   b  to enable coupling the example formation tester  300  to a drill string (e.g., the drill string  104  of  FIG. 1 ) or work string. The breakable connectors  301   a  and  301   b  are shown, by way of example, as threaded sections. However, any other type of breakable connector may be used instead. 
   The example formation tester  300  is coupled to a stabilizer subassembly, in this case a stabilizer sleeve  302  (e.g., a screw-on stabilizer sleeve). The example stabilizer sleeve  302  includes stabilizer blades  303 , which may be substantially similar or identical to the example stabilizer blades  156  and  157  of  FIG. 1 . As shown in  FIG. 3C , the stabilizer sleeve  302  is configured to be removably attached to the formation tester  300  by sliding the stabilizer sleeve  302  onto a portion of the formation tester  300  in a direction generally indicated by arrows  304  so that the formation tester  300  and the stabilizer sleeve  302  are in substantial coaxial alignment. To enable removably attaching the stabilizer sleeve  302  to the formation tester  300 , the formation tester  300  includes an outer surface  305  (e.g., an outer diameter surface) and is provided with a plurality of threads  306  on a portion of the outer diameter surface  305  and the stabilizer sleeve  302  includes an inner surface (e.g., an inner diameter surface) is provided with a plurality of threads  307  on at least a portion thereof. The plurality of threads  306  of the formation tester  300  are configured to threadingly engage the plurality of threads  307  of the stabilizer sleeve  302  to enable mechanically coupling the stabilizer sleeve  302  to the formation tester  300 . In other example implementations, the stabilizer sleeve  302  may be configured to be coupled to the formation tester  300  via fastening interfaces or fastening elements other than threads. 
   In yet other example implementations, the stabilizer subassembly may comprise a collar with stabilizer blades coupled thereto or integral with the collar. This stabilizer subassembly may be substantially similar or identical to the collar  154  and the stabilizer blades  156  of  FIG. 1 . The stabilizer subassembly is configured to be coupled to a downhole tool similar or identical to the collar  158  of  FIG. 1 . In yet other example implementation, the stabilizer subassembly may comprise a reamer for enlarging the well. 
   The formation tester  300  is provided with example pads  308  and  310  having respective example measurement probes  312  and  314 . The pads  308  and  310  and the probes  312  and  314  are removably coupled to the formation tester  300  as shown in  FIGS. 7 and 8 . In this manner, the formation tester  300  can accept a plurality of different pads and/or probes. In the illustrated example, the pads  308  and  310  do not function as stabilizer blades (e.g., the stabilizer blades  303 ). 
   In an example implementation, the lengths of the probes  312  and  314  may then be selected from a plurality of different probe lengths based on the desired offset (e.g., distance d 1  of  FIG. 3B ) of the probes  312  and  314  from an outer surface  318  of the formation tester  300 . For example, the length of the probes  312  and  314  may be selected so that the distance d 1  is less than a distance d 2  from which an outer surface  320  of the stabilizer blade  303  is offset from an outer surface  322  of the stabilizer sleeve  302 . In other example implementations, the thickness of the measurement pads  308  and  310  may be selected so that the distance d 1  is substantially similar or equal to the distance d 2 . The thickness of the pads  308  and  310  may then be selected from a plurality of different pad thicknesses based on length of the selected probes  312  and  314 . 
   In addition, some pads may be implemented using pads that can be extended or retracted relative to an outer surface (e.g., the surface  318 ) of a tool collar using electrical, hydraulic, and/or mechanical devices. For example, the pads may be extended and retracted using powered devices (e.g., hydraulic or electrical actuators, motors, etc.). In this manner, the pads may contact the formations in cases for which such contact facilitates or is beneficial for performing a measurement. 
   In a typical drilling application, a stabilizer subassembly (e.g., the stabilizer sleeve  302 ) is often selected based on the size of a drill bit assembly (e.g., the drill bit  106  of  FIG. 1 ), which dictates the diameter of a wellbore (e.g., the wellbore  102  of  FIG. 1 ). For instance, in the illustrated example of  FIG. 1 , the drill collar  154  is selected so that the stabilizer blades  156  protrude a distance (e.g., the distance d 2  of  FIG. 3B ) sufficiently offset from an outer surface (e.g., the outer surface  318 ) of the drill collar  154  to ensure substantially continuous contact between the stabilizer blades  156  and a formation surface of the wellbore  102 . In this manner, the drill collar  154  can substantially reduce or prevent wobble in the bottom hole assembly  128 . 
   Formation measurements sometimes require measurement probes (e.g., the measurement probes  312  and  314 ) to extend toward and contact a formation surface of a wellbore (e.g., the wellbore  102  of  FIG. 1 ) or to extend relatively close to the formation surface without physically contacting the formation surface. In the illustrated example of  FIG. 3B , the pads  308  and  310  protrude a distance d 1  that may be substantially similar to or less than the distance d 2  associated with the stabilizer sleeve  302  to facilitate extending the probes  312  and  314  to a formation surface by minimizing the travel distance required by the probes  312  and  314  to reach the formation surface but still protecting the probes. That is, as shown in  FIG. 3B , in a non-measurement (retracted) position, the probes  312  and  314  can protrude from the formation tester  300  away from the outer surface  318  and be preferably, but not necessarily, positioned below outer pad surfaces  324  and  326  of the pads  308  and  310  so that the pads  308  and  310  protect the probes  312  and  314  during drilling. Then, during a measurement process, the probes  312  and  314  can be extended from within the pads  308  and  310  to a formation surface to, for example, draw formation material into the formation tester  300 . In the illustrated example, the amount of travel length required for the probes  312  and  314  to extend during a measurement process is reduced by the extra initial length of the selected probes  312  and  314  beyond the outer surface  318  of the formation tester  300 , and the protuberance of the selected probes  312  and  314  beyond respective ones of the outer surfaces  324  and  326  of the pads  308  and  310  when in a retracted position can be substantially reduced and/or eliminated by the extra thickness of the pads  308  and  310 . 
   In some example implementations, the example apparatus and methods described herein may be implemented using a measurement/pad module that does not include an extendable probe. Formation measurements sometimes require measurement sensors to be located close to the formation surface of the wellbore. In this case, the plurality of measurement/pad modules may have sensors (not shown), located preferably, but not necessarily, below respective ones of the outer surface  324  and  326  of the pads  308  and  310 , so that the pads  308  and  310  substantially protect the sensors during drilling. The pads  308  and  310  may also be configured to protrude a distance d 1  from an outer surface (e.g., the outer surface  318 ) of the drill collar  154 . When the stabilizer sleeve  302  is replaced with another stabilizer sleeve (or with a wear band or slick sleeve) having a different offset distance d 2  (or a different outermost circumference), the pads  308  and  310  can be changed as described below in connection with  FIGS. 7 and 8  so that the distance d 1  ( FIG. 3B ) is substantially similar to or less than the distance d 2  ( FIG. 3B ). 
   In the illustrated example of  FIG. 3D , a cross-sectional view of the formation tester  300  shows that the pads  308  and  310  are separate from a probe module  332  that includes the probes  312  and  314  so that the pads  308  and  310  and the probes  312  and  314  can be replaced using other pads and other probes without replacing the probe module  332 . However, in other example implementations, the pads  308  and  310  and the probes  312  and  314  can be part of a pad/probe module that is removably insertable in or mountable to the formation tester  300 . In this case, the pad/probe module together with the probes  312  and  314  can be replaced using other pad/probe modules. Alternatively, the pad  308  and the probe  312  can form a first pad/probe module and the pad  310  and the probe  314  can form a second pad/probe module. In the illustrated example of  FIG. 3D , the formation tester  300  includes recesses  338  formed therein to receive respective ones of the pads  308  and  310 . However, in some example implementations, recesses need not be provided to couple the pads  308  and  310  to a formation tester. 
   Also shown in  FIG. 3D , the formation tester includes a tool bus interfaces  334   a - b  substantially similar or identical to the tool bus interfaces  212   a - b  of  FIG. 2 . The tool bus (not shown) connects the tool bus interfaces  334   a - b  and runs through an upper mandrel chassis  340  and a lower mandrel chassis  341 . The upper mandrel chassis  340  and the lower mandrel chassis  341  are configured to hold a plurality of components  336  (e.g., some or all of the components  218 ,  220 ,  222 ,  224 , and  226  of the electronics system  214  of  FIG. 2 ), a battery (e.g., the battery  216  of  FIG. 2 ), components of a hydraulic system (e.g., the hydraulic system  230  of  FIG. 2 ), and/or a motor (e.g., the motor  232  of  FIG. 2 ). The upper mandrel chassis  340  and/or the lower mandrel chassis  341  typically include mechanical, electrical, and/or hydraulic interfaces that are relatively easily connectable to corresponding interfaces in the probe module  332 , as further described below, for example, in connection with  FIGS. 11 and 12 . 
   Probe modules (e.g., the probe module  332  of  FIG. 3D ) may also be interchanged with other probe modules having different sensor types or other different characteristics (e.g., shape, number of probe openings or inlets, etc.). For example, different probe modules may accommodate different probe sizes.  FIG. 4  depicts the example formation tester  300  of  FIGS. 3A-3D  having an example probe module  402  that is implemented using a two-probe-per-side probe module that includes two probes  404  and  406  recessed in a pad  408  and configured to, for example, measure formation fluid mobility. Each of the probes  404  and  406  may be provided to perform the same or different types of measurements and the probes  404  and  406  may be configured to operate independent of one another (e.g., extend and retract independent of one another and perform measurement operations independent of one another). 
     FIG. 5  depicts a pad  501  removed from the formation tester  300 , which, in the illustrated example, includes an example probe module  502  that is implemented using a multiple-probe-per-pad configuration. The probe module  502  may be configured to extend and retract its probes simultaneously. Inlets of the probes may be connected to a single flow line and a single pressure sensor to, for example, measure an average response of a formation over a distributed area. 
     FIG. 6  depicts an example configuration of the formation tester  300  having probe modules  602   a - b  and respective probe pads  604   a - b  located at opposing ends (e.g., above and below) of the stabilizer sleeve  302 . The example configuration of  FIG. 6  enables the same or different types of measurements to be performed simultaneously at different depths of a wellbore (e.g., the wellbore  102  of  FIG. 1 ). In addition, placing probe modules and pads on the formation tester  300  as shown in  FIG. 6  enables any number of different types of measurements to be performed simultaneously or at different times. In the illustrated example, the probe assembly  602   a  includes a guard probe and the probe assembly  602   b  includes a pressure probe similar to probe  1600  of  FIG. 17 . The guard probe of the probe assembly  602   a  has a first peripheral inlet configured to draw mud filtrate that may have infiltrated the formation along a wellbore (e.g., the wellbore  102  of  FIG. 1 ), and a second, central inlet so that formation fluid samples drawn by the central inlet of the probe assembly  602   a  are substantially clean (e.g., the formation fluid samples drawn by the central inlet are relatively cleaner than they would otherwise be without the use of the guard probe provided by the probe assembly  602   a ). 
   Although  FIGS. 4 ,  5 , and  6  show circular probes, the probes could have any other shape (e.g., an elliptical or elongated shape). Also, although  FIGS. 4 ,  5 , and  6  depict a drill string portion having one tool collar (e.g., the formation tester  300 ) in other example implementations, a drill string may have any number of tool collars. 
     FIG. 7  illustrates a partially assembled view of the example formation tester  300  of  FIGS. 3A-3D  having a probe module  702  removably inserted therein that includes the probe  312  of  FIGS. 3A ,  3 B, and  3 D and  FIG. 8  illustrates an exploded view in which the probe module  702  is removed from the formation tester  300 . In the illustrated example, the pad  308  of  FIGS. 3A ,  3 B, and  3 D is separate from the probe module  702  and is removed from the formation tester  300 . However, in other example implementations, the pad  308  is part of or integral with the probe module  702 . 
   As shown in  FIGS. 7 and 8 , the formation tester  300  is provided with an opening  704  (e.g., a slot, an aperture, etc.) into which the probe module  702  can be removably inserted. In addition, the formation tester  300  is provided with an area  705  on the outer surface  318  of the formation tester  300  substantially surrounding a perimeter formed by the opening  704 . The area  705  is configured to receive the pad  308 . Threaded apertures or holes  706  are formed on the outer surface  318  in the area  705  that can be used to fasten the pad  308  to the formation tester  300  using fastening elements  708  (e.g., screws  708 ) to, for example, hold the probe module  702  in the opening  704 . Although the probe module  702  is shown in  FIGS. 7 and 8  as being removable from the formation tester  300 , in some example implementations, the probe module  702  may be integral with the formation tester  300 . However, an operator may interchange the pad  308  with other pads as desired. 
     FIG. 9  is a cross-sectional view A-A and  FIG. 10  is a partial cross-sectional view B-B of the example formation tester  300  of  FIGS. 7 and 8 . The example formation tester  300  includes recesses  902  and  904  ( FIG. 9 ) to receive respective ones of the pads  308  and  310  ( FIG. 3B ) and the opening  704  to receive the probe module  702  ( FIGS. 7 and 8 ). In the illustrated example, the recess  904  is formed in the area  705 . In the illustrated example of  FIG. 9 , the opening  704  is shown as extending through the example formation tester  300 . However, in other example implementations, the opening  704  may extend from the outer surface  318  ( FIG. 3B ) of the formation tester  300  toward a central or longitudinal axis of the formation tester  300  only partially into the example formation tester  300 . 
   To enable drilling fluid (e.g., the drilling fluid  116  of  FIG. 1 ) to flow through a drill string (e.g., the drill string  104  of  FIG. 1 ), the example formation tester  300  is provided with drilling fluid passageways  906  and  908  ( FIGS. 9 and 10 ) formed on either side of and adjacent to the opening  704 . The fluid passageways  906  and  908  extend along a length of the formation tester  300  substantially parallel to a central or longitudinal axis of the formation tester  300  and are configured to hydraulically connect annular passageways within a drill string (e.g., the drill string  104  of  FIG. 1 ) through which drilling fluid (e.g., the drilling fluid  116  of  FIG. 1 ) flows toward a drill bit (e.g., the drill bit  106  of  FIG. 1 ). To receive electrical connectors  1002  and/or hydraulic connectors  1004  ( FIG. 10 ) from, for example, a chassis (e.g., the mandrel chassis  340  or  341  of  FIG. 3D ), the example formation tester  300  is provided with a passageway  914  ( FIGS. 9 and 10 ) extending along a length of the formation tester  300  substantially parallel to a central or longitudinal axis of the formation tester  300  and substantially parallel and adjacent to the fluid passageways  906  and  908 . In the illustrated example, the passageway  914  is coaxial with the central or longitudinal axis of the example formation tester  300 . 
   As shown in  FIG. 10 , the passageway  914  is configured to receive a chassis  1006  having a rotatable connector  1008  rotatably mounted thereon. The rotatable connector  1008  includes the electrical connectors  1002  and the hydraulic connectors  1004 . In the illustrated example, the passageway  914  includes a threaded portion  916  ( FIGS. 9 and 10 ), and the chassis  1006  includes a threaded portion  1010  configured to be threadingly coupled to the threaded portion  916  of the passageway  914 . To prevent the drilling fluid  116  from flowing into the opening  704 , the chassis  1006  is provided with o-rings  1012 . To align electrical and hydraulic connectors (not shown) of the probe module  702  with the electrical connectors  1002  and the hydraulic connectors  1004 , the rotatable connector  1008  is provided with a keyway  1014 . 
   To assemble the probe module  702  ( FIGS. 7 and 8 ) with the formation tester  300 , the chassis  1006  can first be threadingly coupled to the formation tester  300  causing the rotatable connector  1008  to extend into the opening  704 . The probe module  702  can then be inserted and slid into the opening  704 . The rotatable connector  1008  can be rotated to align the keyway  1014  with a key of the probe module  702  so that the electrical connectors  1002  and the hydraulic connectors  1004  align with electrical and hydraulic connectors of the probe module  702 . Note that although six electrical connectors are shown in  FIG. 10 , the rotatable connector  1008  may include any desired number of electrical connectors. Note also that although two hydraulic connectors are shown in  FIG. 10 , the rotatable connector  1008  may include any desired number of hydraulic connectors. Upon insertion of the probe module  702 , electric wires (not shown) in the chassis  1006  that are terminated at the electrical connectors  1002  are connected to electric wires (not shown) in the probe module  702 . The electrical connectors may include a pin socket assembly as well known in the art. Also, hydraulic or flow lines (not shown) in the chassis  1006  that are terminated at the hydraulic connectors  1002  are connected to hydraulic or flow lines (not shown) in the probe module  702 . The hydraulic connectors may comprise a hydraulic stabber well known in the art. Further details of the connectors can be found in  FIGS. 12 and 13 . The pad  308  ( FIGS. 3A ,  3 B,  3 D,  7 , and  8 ) can then be placed over the probe module  702  and fastened to the formation tester  300 . 
     FIG. 11  depicts an alternative example implementation of electrical and hydraulic connectors in which an example probe module  1101  is configured to electrically and fluidly engage a coaxial connector  1108  having electrical connectors  1102  and hydraulic connectors  1106 . In the illustrated example, the coaxial connector  1108  is coupled to a chassis  1110  substantially similar or identical to the mandrel chassis  340  or  341  of  FIG. 3D . In the illustrated example, the electrical connectors  1102  are provided on a surface of the coaxial connector  1108  and are configured to engage corresponding electrical connectors  1104  of the probe module  1101 . Wires  1112  electrically coupled to the electrical connectors  1102  are routed through a passage in the coaxial connector  1108  and are provided to transfer communication signals and/or electric power through the electrical connectors  1102  and  1104  and from, for example, an electronics system (e.g., the electronics system  214  of  FIG. 2 ) and/or a battery (e.g., the battery  216  of  FIG. 2 ) to components in the probe module  1101 . The hydraulic connectors  1106  are implemented using annular grooves (i.e., annular grooves  1106 ) provided about the coaxial connector  1108  between o-rings  1114  and are configured to fluidly engage similar annular grooves of the probe module  1101  and fluidly connect fluid passageways fluidly coupled to hydraulic components in the chassis  1110  to passageways  1116  formed in the probe module  1101  and fluidly coupled to components in the probe module  1101  including, for example, a compensator (e.g., a compensator  1436  of  FIG. 10 ), and/or an extending chamber (e.g., an extending chamber  1482   a  of  FIG. 10 ) used to move a probe. 
   As the coaxial connector  1108  is inserted into and engages the probe module  1101 , the electrical connectors  1102  engage their respective electrical connectors  1104  and the annular grooves  1106  engage respective grooves that fluidly couple fluid passageways in the chassis  1110  to the fluid passageways  1116 . In the illustrated example of  FIG. 11 , the coaxial connector  1108  configuration enables first inserting the probe module  1114  into the opening  704  and subsequently inserting and threadingly coupling the chassis  1110  (and, thus, the coaxial connector  1108 ) into the passageway  914  to electrically couple the electrical connectors  1102  and  1104  and to fluidly couple fluid passageway in the chassis  110  to the fluid passageways  1116 . 
     FIG. 12  is another cross-sectional view C-C of the example formation tester  300  of  FIGS. 7 and 8 . In the illustrated example, the probe module  702  is implemented using an integrally formed probe module that includes both of the example probes  312  and  314 . In this manner, inserting the probe module  702  into the opening  704  in a direction generally indicated by arrow  1201  provides the example formation tester  300  with both of the example probes  312  and  314  simultaneously. 
   In an alternative example implementation shown in  FIG. 13 , a first example probe module  1302  includes the example probe  312  and a second example probe module  1304  includes the example probe  314 . In the illustrated example of  FIG. 13 , the probe module  1302  may be removably inserted into the opening  704  in a direction generally indicated by arrow  1303  and the probe module  1304  may be removably inserted into the opening  704  in a direction generally indicated by arrow  1305 . In addition, each of the probe modules  1302  and  1304  may be interchangeable with each other. 
   As shown in  FIG. 12 , electrical and hydraulic interfaces  1202  and  1204  are provided on respective ends of the example probe module  702  to electrically and fluidly couple the example probe module  702  to other drill string segments (e.g., the upper chassis  340  and the lower chassis  341  of  FIG. 3D ). The electrical and hydraulic interfaces  1202  and  1204  include, for example, conductive pins (not shown) to engage the electrical socket  1002  ( FIG. 10 ) of the rotatable connector  1008  and fluid couplings (e.g., hydraulic fittings) to engage the hydraulic connectors  1004  ( FIG. 10 ) of the rotatable connector  1008 . 
   As shown in  FIG. 13 , to electrically and hydraulically connect the first probe module  1302  to the second probe module  1304 , each of the first and second probe modules  1302  and  1304  is provided with a respective electrical and hydraulic interface  1306  and  1308 . The electrical and hydraulic interfaces  1306  and  1308  are configured to electrically and fluidly couple to one another to enable electrical current flow and hydraulic fluid flow between the first and second probe modules  1302  and  1304 . 
     FIGS. 14 and 15  illustrate detailed cross-sectional (section C-C) diagrams of the example probe module  702  removably inserted in the example formation tester  300  of  FIGS. 3A-3D . As shown in  FIGS. 14 and 15 , the probe module  702  is held in place in part by the pads  308  and  310  that are fastened to the formation tester  300 . Also shown is an annular passageway  1401  that enables drilling fluid (e.g., the drilling fluid  116  of  FIG. 1 ) to flow through the formation tester  300 . The annular passageway  1401  is split to form passageways  906  and  908  of  FIG. 9  around an upper chassis  1403 , a lower chassis  1405 , and the probe module  702 . The upper chassis  1403  may be substantially similar or identical to the upper chassis  340  of  FIG. 3D  and may be configured to hold or contain, for example, hydraulic components (e.g., an actuator  1432  and an accumulator  1458 ). Although not shown in  FIGS. 14 and 15  for clarity, the upper chassis may be fluidly and/or electrically connected to the probe module  702  using, for example, the rotatable connector  1008  as discussed above in connection with  FIGS. 10 and 12  or the coaxial connector  1108  as discussed above in connection with  FIG. 11 . Of course, any other type of connector may be used. The lower chassis  1405  may be substantially similar or identical to the lower chassis  341  of  FIG. 3D  and may be configured to hold or contain, for example, an electronics module  1428  and a battery  1426 . Although not shown in  FIGS. 14 and 15  for clarity, the lower chassis  1405  may also be fluidly and/or electrically coupled to the probe module  702  in a similar way as the upper chassis is coupled to the probe module  702 . Although portions and components of the example probe module  702  are shown in a particular arrangement, in other example implementations the components of the example probe module  702  may be rearranged while maintaining connections and functional relationships therebetween to implement the same functionality as described below in connection with  FIGS. 14 and 15 . 
   To perform measurements associated with the formation F, the probe module  702  is provided with drawdown pistons  1402  and  1404  located within respective ones of the measurement probes  312  and  314 . The probes  312  and  314  are configured to extend and retract relative to respective probe openings  1406  and  1408  of the probe module  702  during a measurement process in directions generally indicated by arrows  1410  and  1412 . In addition, to draw formation material into the probes  312  and  314 , each of the drawdown pistons  1402  and  1404  is configured to move relative to its respective probe  312  and  314  in the directions generally indicated by the arrows  1410  and  1412 . To engage a formation surface of a wellbore (e.g., the wellbore  102  of  FIG. 1 ) and form a seal between the formation surface and the probes  312  and  314  to facilitate drawing the formation material into the probes  312  and  314 , each of the probes  312  and  314  is provided with a respective packer or seal  1414  and  1416  made of, for example, a substantially deformable elastomeric material. In an alternative example implementation, the probes  312  and  314  may be configured to perform measurements without engaging a formation surface. 
   In the illustrated example, the drawdown pistons  1402  and  1404  are preferably, but not necessarily, equipped with position sensors or displacement sensors (e.g., analog potentiometers, digital encoders, etc.) (not shown) to determine and/or substantially continuously monitor their position within the probes  312  and  314 . 
   In the illustrated example of  FIG. 14 , the probes  312  and  314  are shown in a retracted, home position at which the packers  1414  and  1416  are within the probe openings  1406  and  1408 . In the illustrated example of  FIG. 15 , the probes  312  and  314  are shown in an extended, measurement position in which the packers  1414  and  1416  are extended away from the openings  1406  and  1408 . Also in  FIG. 15 , the drawdown piston  1402  is shown in an extended, home position. However, to draw formation fluid from the formation surface through a formation fluid port  1418  into the probe  312 , the drawdown piston  1402  is configured to be retracted relative to the probe  312 . For example, the drawdown piston  1404  of the probe  314  is shown in a retracted position drawing formation fluid  1417  into the probe  314  via formation fluid port  1420 . 
   To perform measurements, the probe module  702  is provided with sensors  1422  and  1424  ( FIG. 14 ) located within respective ones of the drawdown pistons  1402  and  1404 . The sensors  1422  and  1424  may be implemented using, for example, pressure sensors, temperature sensors, etc. The sensors  1422  and  1424  may be the same or different sensor types. In the illustrated example, the sensors  1422  and  1424  are electrically and/or communicatively coupled to a battery  1426  ( FIG. 14 ) and an electronics system  1428  ( FIG. 14 ) via cables  1430  ( FIG. 14 ). In this manner, the cables  1430  may be used to provide electrical power to the sensors  1422  and  1424  from, for example, the battery  1426 . In addition, the cables  1430  may also be used to communicate control information between the electronics system  1428  and electrical components in the upper chassis  1403  of the formation tester  300  and/or in the probe module  702 , and communicate measurement information to the electronics system  1428 . A common serial bus protocol (e.g., RS-485) or a controller area network (“CAN”) bus protocol may be used in combination with the electronics system  1428  to communicate control information and/or measurement information. The electronics system  1428  may be substantially similar or identical to the electronics system  214  of  FIG. 2 . 
   The components of the example probe module  702  are configured to extend and retract the probes  312  and  314  and the drawdown pistons  1402  and  1404  using energy associated with an actuator  1432  that is preferably, but not necessarily, compensated to annulus pressure A P . Annulus pressure A P  refers to the pressure of drilling mud in the annulus  124 . To pressurize, for example, clean oil or hydraulic oil in the formation tester  300  to the annulus pressure A P , the probe module  702  is provided with a compensator  1434  having an annulus pressure chamber  1436  filled with the clean oil or hydraulic oil and separated from drilling mud by a piston or bellow  1440  having an o-ring  1442 . In the illustrated example of  FIGS. 14 and 15 , the pad  308  is shown as having an aperture  1439  formed therethrough to enable drilling mud to flow into the annulus fluid port  1438 . 
   To receive the probes  312  and  314  when the probes  312  and  314  are retracted, the probe module  702  is provided with back chambers  1508   a  and  1508   b . The probes  312  and  314  are provided with respective o-rings  1510   a  and  1510   b  to sealingly separate the back chambers  1508   a  and  1508   b  from the drawdown piston control chambers  1496   a  and  1496   b . The fluid line  1464  fluidly couples the back chambers  1508   a  and  1508   b  to the annulus pressure chamber  1436  of the compensator  1434 . 
   In the illustrated example, the actuator  1432  is implemented using a lead screw configuration. For example, a motor (not shown) that is substantially similar or identical to the motor  232  ( FIG. 2 ) is coupled to an actuator screw or ram  1444  preferably, but not necessarily, via a gearbox (not shown). A nut  1454  may be fixedly coupled to the chassis. In addition, an end of the screw  1444  may be coupled via a ball joint (not shown) to a flange  1448  that forms a piston-like structure having an o-ring  1450  that sealingly engages an actuation chamber  1452  to generate hydraulic pressure. The motor can be activated and deactivated using an electronic control circuit (e.g., the electronics system  1428 ) to move the actuator ram or screw  1444 . A back chamber  1455  formed by the screw  1444 , the nut  1454 , and the upper chassis  1403  is preferably, but not necessarily, filled with hydraulic oil and is fluidly coupled to the annulus pressure chamber  1436  of the compensator  1434  via an annulus pressure fluid line  1464 . Thus, the flange  1448  is pressure compensated at an annular pressure A P . The actuation chamber  1452  is fluidly coupled to the probe module  702  via a power fluid line  1488 . A solenoid valve  1466  is disposed between the actuation chamber  1452  and the annulus pressure fluid line  1464  to selectively discharge or vent the hydraulic pressure generated in the actuation chamber  1452 . Preferably, the solenoid valve  1466  is closed when energized, and is open when de-energized. In this manner, the pressure in the actuation chamber  1452  is equal to the pressure (e.g., a compensator pressure) of the annulus pressure chamber  1436  when the solenoid valve  1466  is de-energized. The motor may then be activated to rotate in a reverse direction to reset the actuator screw  1444  in its initial position. 
   The pressure in the actuation chamber  1452  may be sensed by a pressure sensor and transmitted to the electronics system  1428 . The electronics system  1428  can then use the value indicative of the pressure to determine and/or control the amount of force the packers  1414  and  1416  exert against the formation surface and to control the motion (e.g., extension and retraction) of the drawdown pistons  1402  and  1404 . 
   To relatively quickly pull down or retract the drawdown pistons  1402  and  1404  to generate a relatively high flow rate of the formation fluid  1417  into the probes  312  and  314 , the formation tester  300  is provided with an accumulator  1458  that can be charged by the actuator  1432 . The accumulator  1458  includes a piston  1460  and a coil spring  1462 . As the motor moves the actuator screw  1444  toward the accumulator  1458 , and the hydraulic fluid in the actuation chamber  1452  is prevented from discharging by expelling fluid into the power fluid line  1488 , the hydraulic fluid pushes against the piston  1460  causing the coil spring  1462  to compress and store energy. In this manner, the energy stored in the accumulator  1458  can subsequently be used to achieve a high flow rate in power fluid line  1488  to, for example, relatively quickly pull down or retract the drawdown pistons  1402  and  1404 . Specifically, a relatively quick extension of the coil spring  1462  causes a relatively quick dispersion of hydraulic fluid that might not be achievable when the motor alone is used. In some example implementations, the accumulator  1458  may be eliminated. 
   To store energy to retract the probes  312  and  314  into the probe openings  1406  and  1408  and/or maintain the probes  312  and  314  in a retracted position and/or to extend the drawdown pistons  1402  and  1404  with the probes  312  and  314 , the probe module  702  is provided with a retractor  1468 . The retractor  1468  includes a piston  1470  having an o-ring  1472  that sealingly separates a retractor storage chamber  1474  from a retractor spring chamber  1476 , which is fluidly coupled to the annulus pressure chamber  1436  of the compensator  1434  via the annular pressure flow line  1464 . The retractor spring chamber  1476  includes a coil spring  1478  inserted therein that provides a force against the piston  1470  in a direction generally indicated by arrow  1480 . 
   To extend and retract the probes  312  and  314  based on the actuator  1432 , the accumulator  1458 , and the retractor  1468 , the probe module  702  is provided with respective extending chambers  1482   a  and  1482   b  ( FIG. 15 ) and respective retracting chambers  1484   a  and  1484   b  ( FIGS. 14 and 15 ) for each of the probes  312  and  314 . The extending chambers  1482   a - b  are sealingly separated from the retracting chambers  1484   a - b  by respective o-rings  1486   a  and  1486   b . The extending chambers  1482   a - b  are fluidly coupled to the actuation chamber  1452  via a power fluid line  1488 . The retracting chambers  1484   a - b  and the retractor storage chamber  1474  are fluidly coupled via respective control fluid lines  1490   a  and  1490   b.    
   Solenoid valves  1492   a  and  1492   b  are provided along the control fluid lines  1490   a - b  to control the flow of hydraulic fluid between the retractor storage chamber  1474  and the retracting chambers  1484   a - b . In the illustrated example, the solenoid valves  1492   a  and  1492   b  may be configured to be normally open (when de-energized.). 
   To extend and retract the drawdown pistons  1402  and  1404  relative to the probes  312  and  314 , the probes  312  and  314  and the drawdown pistons  1402  and  1404  form respective drawdown piston actuating chambers  1494   a  and  1494   b  ( FIG. 15 ) and respective drawdown piston control chambers  1496   a  and  1496   b  ( FIG. 15 ). Each of the drawdown pistons  1402  and  1404  is provided with a respective o-ring  1498   a  and  1498   b  ( FIG. 15 ) to sealingly separate the drawdown piston actuating chambers  1494   a - b  from the drawdown piston control chambers  1496   a - b . In addition, to sealingly separate the drawdown piston control chambers  1496   a - b  from the retracting chambers  1484   a - b , the probes  312  and  314  are provided with o-rings  1502   a  and  1502   b.    
   Each of the drawdown piston control chambers  1496   a - b  is fluidly coupled to the retractor storage chamber  1474  via respective control fluid lines  1504   a  and  1504   b . The probe module  702  is provided with a solenoid control valve  1506   a  at the control fluid line  1504   a  and a solenoid control valve  1506   b  at the control fluid line  1504   b  to control fluid flow between the retractor storage chamber  1474  and the drawdown piston control chambers  1496   a - b . In the illustrated example, the solenoid valves  1506   a  and  1506   b  may be configured to be normally open (when de-energized). 
   To protect the probes  312  and  314  during a drilling operation, the retractor  1468  and the solenoid valves  1492   a - b ,  1506   a - b , and  1466  are configured to cause the probes  312  and  314  to remain in a retracted position and the drawdown pistons  1402  and  1404  to remain in an extended position when electrical power is removed from valves  1492   a - b ,  1506   a - b , and  1464  during, for example, normal operation or a power failure. In this manner, when power is removed from the valves  1492   a - b ,  1506   a - b , and  1464  during a drilling operation, the probes  312  and  314  do not inadvertently or unintentionally extend, which would otherwise cause the probes  312  and  314  to be damaged when subjected to the forces of a drill string (e.g., the drill string  102  of  FIG. 1 ) against a formation surface while drilling. In particular, energy stored in the coil spring  1478  can be used to retract the probes  312  and  314  and/or cause the probes  312  and  314  to remain in a retracted position. For example, in the event of a power failure, the solenoid valve  1466  opens, thereby, equalizing the pressure in the power fluid line  1464  to the annular pressure A P . The solenoid valves  1492   a - b  open allowing fluid to flow from the retractor storage chamber  1474  to the retracting chambers  1484   a - b  via the flow lines  1490   a - b . As the energy stored in the coil spring  1478  causes the coil spring  1478  to push against the piston  1470 , the piston  1470  causes fluid to flow from retractor storage chamber  1474  to the retracting chambers  1484   a - b , which causes the volumes of the retracting chambers  1484   a - b  to increase and/or prevents the volumes of the retracting chamber  1484   a - b  from decreasing. In turn, the probes  312  and  314  retract and/or remain in a retracted position for at least the amount of time during which power is removed from the solenoid valves  1492   a - b  or for at least the duration of a power failure. 
   The energy stored in the coil spring  1478  can also be used to extend the drawdown pistons  1402  and  1404  and/or ensure that the drawdown pistons  1402  and  1404  remain in an extended position. For example, in the event of a power failure, the solenoid valves  1506   a - b  open allowing fluid to flow from the retractor storage chamber  1474  to the drawdown piston control chambers  1496   a - b  via the flow lines  1504   a - b . As the energy stored in the coil spring  1478  causes the coil spring  1478  to push against the piston  1470 , the piston  1470  causes fluid to flow from retractor storage chamber  1474  to the drawdown piston control chambers  1496   a - b , which causes the volumes of the drawdown piston control chambers  1496   a - b  to increase and/or prevents the volumes of the drawdown piston control chambers  1496   a - b  from decreasing. In turn, the drawdown pistons  1402  and  1404  extend and/or remain in an extended position for at least the duration of the power failure. 
     FIG. 16  is a front view and  FIG. 17  is a cross-sectional side view of another example probe  1600  that can be used instead of the example probes  312  and  314  ( FIGS. 14 and 15 ) to implement the example probe module  702 . The example probe  1600  includes a seal or packer  1602  and a shroud  1604  surrounding packer  1602 . In the illustrated example, the shroud  1604  is configured to create a seal against the formation surface of the wellbore  102  ( FIGS. 1 ,  14 , and  15 ) when the probe  1600  is in an extended position. In this manner, the shroud  1604  can locally isolate the formation from the annulus  124  to substantially reduce or eliminate the infiltration of drilling mud in the formation. In another example implementation, the shroud  1604  can compact the formation around the probe to substantially reduce or eliminate erosion or disintegration of the formation. Although the shroud  1604  is shown as rectangular, the shroud  1604  may be implemented using any other shape. 
     FIG. 18  depicts a state diagram  1800  representing an example method of operating the example probe module  702  of  FIGS. 14 and 15 . The state diagram  1800  shows a plurality of states arranged in an example state transition sequence to show different ways of operating the probes  312  and  314  and pistons  1402  and  1404  of  FIGS. 14 and 15 . Although the state diagram  1800  shows a particular state transition sequence, the example probe module  702  may be operated using other state transition sequences. In addition, although the state diagram  1800  may show a previous state transitioning to a next state, the transition may not indicate the existence of a dependency between the previous and next states. In addition, other state transition sequences may be implemented by removing one or more states of  FIG. 18  or adding states or changing the order and sequence of the state transitions. 
   During a home position state  1802 , the example probes  312  and  314  are retracted within the probe module  702  so that the packers  1414  and  1416  are within their respective probe openings  1406  and  1408  as shown in  FIG. 14 . As shown in  FIG. 18 , the independent controllability of the probes  312  and  314  and the drawdown pistons  1402  and  1404  can be used to disable one of the probes  312  and  314  and its respective drawdown piston  1402  and  1404  to extend battery life by only operating one of the probes  312  and  314 . One of the probes  312  and  314  may also be disabled for any other reason such as, for example, to substantially reduce or eliminate the risk of damaging one or both of the probes  312  and  314  in substantially complex or risky operations. 
   The home position state  1802  may be the state when the drillstring  104  is used for drilling. The state transition sequence may be programmed in the electronics system  1428  or may be initiated from the surface using the two-way telemetry system described with respect to  FIG. 1  or a combination of programming and initiation from the surface. 
   In an example implementation, the two-probe extension state  1804  or the one-probe extension state  1816  may be triggered when the drilling operation pauses during, for example, a stand connection at the platform  100  ( FIG. 1 ). A surface operator using the uphole transmitting system  150  and controlling the interruption of the operation of the pump  120  in a manner that is detectable by the transducers  152  in the subassembly  138  may initiate any of the extension states  1804  or  1816 . Alternatively, downhole logic may detect a drilling pause by monitoring, for example, the drillstring rotation, the flow of drilling fluid  122 , and/or other drilling parameters to control the extension states  1804  and  1816 . In some example implementations, one or more probe(s) may be extended during drilling to obtain measurements at different locations of the formation surface. In other example implementations, the electronic system  1428  is configured to receive digital data from various sensors in the tool. In addition, the electronic system  1428  may be configured to execute different instructions depending on the data received. The instructions executed by the electronics system  1428  (e.g., by the controller  218 ) may be used to control some of the state transitions. Thus, the formation tester  300  is preferably, but not necessarily configured to perform some of its operations (e.g. probe movement) in, for example, a sequential manner based on sensor data acquired in situ. 
   During a two-probe extension state  1804 , both of the probes  312  and  314  are extended toward a formation surface of the wellbore  102 . To extend the probes  312  and  314 , the electronics system  1428  causes the closure of valves  1466  and causes the motor to actuate and extend the actuator screw or ram  1444  ( FIG. 15 ) to increase the hydraulic fluid pressure in the power fluid line  1488 . Preferably, but not necessarily, the electronics system  1428  drives a motor controller (e.g., a stepper controller, a revolutions controller, etc.). Additionally or alternatively, the number of motor revolutions may be measured and transmitted to the electronics system  1428 . The number of motor revolutions enables the computation of the fluid volume displaced by the motor, which in turn enables tracking or monitoring the extension distances of the probes  312  and  314 . A pressure sensor in communication with the electronics system  1428  may be used to monitor the pressure in the power fluid line  1488 . 
   To enable the probes  312  and  314  to extend using the pressure in the power fluid line  1488 , the electronics system  1428  opens the solenoid valves  1492   a - b  to allow hydraulic fluid to flow out of the retracting chambers  1484   a - b  and into the retractor storage chamber  1474 . As hydraulic fluid flows out of the retracting chambers  1484   a - b , the volume of the retracting chambers  1484   a - b  decreases and hydraulic fluid flows from the power fluid line  1488  into the extending chambers  1482   a - b  to increase the volume of the extending chambers  1482   a - b  and cause the probes  312  and  314  to extend as shown in  FIG. 15 . As the actuator screw or ram  1444  and the probes  312  and  314  extend, hydraulic fluid flows from the annulus pressure chamber  1436  of the compensator  1434  and from the retractor spring chamber  1476  to the back chambers  1508   a - b  and the actuator back chamber  1455  via the annulus pressure fluid line  1464  as the volumes of the chambers  1436  and  1476  decrease and the volumes of the chambers  1508   a - b  and  1455  increase. The complete extension of the probes  312  and  314  against the borehole wall may be detected by a pressure sensor (not shown) (e.g., a pressure sensor in the power fluid line  1488 ) and a displacement sensor (not shown) in the probes  312  and  314 . A relatively significant increase of pressure in the power flow line and/or a relatively significant decrease of the displacement speed of the probes  312  and  314  may indicate that the probes  312  and  314  are in engagement with or pressed against the formation surface of the borehole. When the probes  312  and  314  are extended, the electronics system  1428  closes the solenoid valves  1492   a - b  to maintain the probes  312  and  314  in the extended position. 
   In some example implementations, the electronics system  1428  may include pulse-width-modulation (“PWM”) controllers for controlling hydraulic fluid flow to the probes  312  and  314  with substantially high precision. For example, a PWM controller may be used to control the opening of solenoid valves  1492   a - b  to control the extension of the probes  312  and  314 . In this manner, the electronics system  1428  may be configured to independently control the extension speed of each of the probes  312  and  314  by selectively controlling the degree of opening of a respective one of the solenoid valves  1492   a - b.    
   In addition, the electronics system  1428  can be configured to maintain and/or control the setting force of the packers  1414  and  1416  against the formation surface to a predetermined level while, for example, the formation tester  300  is moved up and down or rotated to obtain measurements at different locations of the formation surface. The pressure level in the retracting chamber  1484   a  and/or the retracting chamber  1484   b  as well as the pressure level in the power fluid line  1488  may be communicated to the electronics system  1428 . A controller (e.g., the controller  218  of  FIG. 2 ) in the electronics system  1428  can then analyze these pressure levels and control the motor rotation and/or the degree of opening of the solenoid valve  1492   a  and/or the solenoid valve  1492   b  based on the analyzed pressure levels using, for example, close loop control techniques known in the art. In this manner, the setting force of the packer  1414  and/or the packer  1416  against the formation surface can be adjusted. The valve  1492   a  and/or the valve  1492   b  may then be closed to maintain the position of the probe  312  and/or the probe  314  in a substantially fixed position. 
   During a two-piston retraction state  1806 , the drawdown pistons  1402  and  1404  are retracted to draw the formation fluid  1417  into the probes  312  and  314 . In  FIG. 15 , the drawdown piston  1404  is shown retracted. To retract both of the drawdown pistons  1402  and  1404 , the electronics system  1428  causes the motor to actuate and extend the actuator screw or ram  1444  ( FIG. 15 ) to increase the hydraulic fluid pressure in the power fluid line  1488 . The electronics system  1428  opens the solenoid valves  1506   a - b  to allow hydraulic fluid to flow from the drawdown piston control chambers  1496   a - b  and into the retractor storage chamber  1474  via the control fluid lines  1504   a - b . As hydraulic fluid is expelled from the drawdown piston control chambers  1496   a - b , the volumes of the drawdown piston control chambers  1496   a - b  decrease and hydraulic fluid from the power fluid line  1488  and the extending chambers  1482   a - b  flows into the drawdown piston actuating chambers  1494   a - b . At the same time, the volumes of the drawdown piston actuating chambers  1494   a - b  increase causing the drawdown pistons  1402  and  1404  to pull or retract toward the drawdown piston control chambers  1496   a - b . When the drawdown pistons  1402  and  1404  are sufficiently retracted, the electronics system  1428  may close the solenoid valves  1506   a - b  to cause the drawdown pistons  1402  and  1404  to remain in the retracted position. The retraction of the drawdown pistons  1402  and  1404  may be stopped before a full stroke is achieved, and the retraction can be restarted later. 
   The electronics system  1428  may also be coupled to devices (not shown) used to measure the distances of extension and retraction of the drawdown pistons  1402  and  1404  relative to the probes  312  and  314 . The position (e.g., a position measured in motor revolutions) of any of the drawdown pistons  1402  and  1404  may be monitored with a displacement sensor (e.g., an analog potentiometer, a digital encoder, etc.) either directly coupled to or indirectly coupled to one or both of the drawdown pistons  1402  and  1404 . 
   In an example implementation, the electronics system  1428  can substantially continuously monitor the extension/retraction distances of the drawdown pistons  1402  and  1404  and use the measured distances to independently control the extension/retraction speeds of the drawdown pistons  1402  and  1404  and/or to determine the volume of the formation fluid  1417  in the probes  312  and  314 . In another example implementation, the electronics system  1428  can substantially continuously monitor the pressure level measured by the sensors  1422  and  1424  and adjust the amount of opening of the valves  1506   a - b  based on the measured pressure to, for example, achieve a predetermined pressure level in the formation fluid  1417 . 
   The control of the extension/retraction of the drawdown pistons  1402  and  1404  may be achieved by independently controlling the opening of the valves  1506   a - b  by, for example, partially energizing the valves using a PWM controller. The amount of opening of the valves  1506   a - b  may be adjusted using close loop control techniques known in the art. 
   If a high flow rate of the formation fluid  1417  into the probes  312  and  314  is desired, the motor can actuate the actuator screw or ram  1444  further to store hydraulic pressure in the accumulator  1458  ( FIG. 14 ) while the solenoid valves  1506   a - b  and  1466  are closed. In this manner, when the electronics system  1428  opens the solenoid valves  1506   a - b , the coil spring  1462  ( FIG. 14 ) of the accumulator  1458  expands quickly to relatively quickly expel hydraulic fluid from the actuation chamber  1452  and into the drawdown piston actuating chambers  1494   a - b , thereby causing the drawdown pistons  1402  and  1404  to relatively quickly retract or pull down and creating a high flow rate of the formation fluid  1417  into the probes  302  and  304 . 
   The pressure measured by sensors  1422  and/or  1424  can be continuously monitored by the electronics system  1428  during and following a piston retraction state when any of the pistons  1402  and  1404  remain in the retracted position (sometimes referred to as a build-up phase). These pressure data may be processed downhole to extract the formation pore pressure and other parameters of interest using known methods. The formation pore pressure is then preferably sent to the surface by telemetry to, for example, make a drilling decision, or the pore pressure can be used downhole to control a subsequent state. Alternatively, the pressure data may be compressed and sent by telemetry to the surface, and the formation pore pressure and/or any other parameters can be extracted at the surface. 
   In some example implementations, the analysis of the pressure measured by the sensor  1422  and/or the sensor  1424  may indicate that one or both of the probes  312  and  314  needs to be reset. The analysis of the pressure measured by the sensors  1422  and/or  1424  may be performed downhole by the electronics system  1428 . Alternatively or additionally, the data collected by the sensor  1422  and/or the sensor  1424  may be compressed and sent to a surface operator by telemetry for analysis. The data may be processed and/or displayed by the processor  146 . A command may be sent to the testing tool  300  to reset one or both of the probes  312  and  314 . During an example one-probe reset state  1808 , the solenoid valves  1492   b  and  1506   b  are opened while the solenoid valves  1492   a  and  1506   a  remain closed. The electronics system  1428  may cause the motor to retract the actuator screw or ram  1444  to draw hydraulic fluid out of the drawdown piston actuating chambers  1494   b  into the actuation chamber  1452  or may vent the pressure in the actuation chamber  1452  by opening the valve  1466 . When the valve  1506   b  is open, hydraulic fluid also flows from the retractor storage chamber  1474  into the drawdown piston control chambers  1496   b  via the valve  1506   b . The drawdown piston  1404  is extended away from the drawdown piston control chambers  1496   b  to expel the formation fluid  1417  and/or debris from the probes  314 . Retracting the actuator screw or ram  1444  and/or opening the valve  1466  also enables hydraulic fluid to flow out of the extending chambers  1482   b  and into the actuation chamber  1452 . When the valve  1492   a  is open, hydraulic fluid also flows from the retractor storage chamber  1474  into the retracting chamber  1484   b  via the valve  1492   b  to retract the probe  314  into the opening  1408 , thus reducing the volume of the back chamber  1508   b . When the drawdown piston  1404  is extended, the electronics system  1428  may close the solenoid valve  1506   b  to prevent hydraulic fluid from flowing out of the drawdown piston control chamber  1496   b  and to maintain the drawdown piston  1404  in an extended position. 
   The electronics system  1428  may then cause the motor to actuate and extend the actuator screw or ram  1444  ( FIG. 15 ) to increase the hydraulic fluid pressure in the power fluid line  1488 , which can cause the probe  314  to extend again toward a formation surface of the wellbore  102 . In addition, the setting force of the packers  1416  against the formation surface can be adjusted and the valve  1492   b  can be closed to maintain the probe  314  in a substantially fixed position. 
   In addition, the electronics system  1428  may be configured to control operation (e.g., extraction and retraction) of the drawdown pistons  1402  and  1404  in a sequential manner to enable one of the probes  312  and  314  to generate a pressure disturbance in the formation fluid  1417  that is subsequently measured by the other one of the probes  312  and  314 . For example, in a one-piston retraction state  1810 , one of the pistons  1402  and  1404  is retracted to draw the formation fluid  1417  into a respective one of the probes  312  and  314  while both of the probes  312  and  314  are in an extended position. In the illustrated example of  FIG. 15 , the drawdown piston  1404  is shown retracted. To retract the drawdown piston  1404 , the electronics system  1428  opens the solenoid valve  1506   b  while keeping the solenoid valve  1506   a  closed. In this manner, the drawdown piston  1404  retracts to draw the formation fluid  1417  as described above in connection with the two-piston retraction state  1806  while the other drawdown piston  1402  remains extended without drawing the formation fluid  1417  as shown in  FIG. 15 . When the drawdown piston  1404  is retracted, the electronics system  1428  closes the solenoid valve  1506   b  to maintain the drawdown piston  1404  retracted. 
   The pressure measured by the sensor  1422  and/or the sensor  1424  can be continuously monitored by the electronics system  1428  during and following a piston retraction state  1810 . These pressure data may be processed downhole to extract horizontal and/or vertical formation permeability and other parameters of interest. The formation permeability measurement values may then be sent to the surface by telemetry to, for example, make a drilling decision, or the formation permeability measurement values can be used downhole to control a subsequent state. Alternatively, the pressure data may be compressed and sent by telemetry to the surface, and the formation permeability and/or any other parameters can be extracted at the surface. 
   In a one-piston extension state  1812 , the drawdown piston  1404  is extended to expel the formation fluid  1417  from the probe  314 . The electronics system  1428  may cause the motor to retract the actuator screw or ram  1444  to draw hydraulic fluid into the actuation chamber  1452  or may vent the pressure in the actuation chamber  1452  by opening the valve  1466 . To extend the drawdown piston  1404 , the electronics system  1428  opens the solenoid valve  1506   b  to allow hydraulic fluid to flow into the drawdown piston control chamber  1496   b  causing the drawdown piston  1404  to extend. When the drawdown piston  1404  is extended, the electronics system  1428  may close the solenoid valve  1506   b  to maintain the drawdown piston  1404  in an extended condition. 
   In a two-probe reset state  1814 , both of the probes  312  and  314  are retracted into the example formation tester  300  to a home position as shown in  FIG. 14 . Also, both of the drawdown pistons  1402  and  1404  are extended into respective probes  312  and  314  to, for example, remove debris introduced in the fluid port  1418  and/or the fluid port  1420  during a piston retraction state. In the two-probe reset state  1814 , the electronics system  1428  opens the solenoid valve  1466  to vent the pressure in the actuation chamber  1452  and in the power fluid line  1488 . 
   To extend both of the drawdown pistons  1402  and  1404  away from the drawdown piston control chambers  1496   a - b  and to expel the formation fluid (and/or debris)  1417  from the probes  312  and  314 , the electronics system  1428  opens the solenoid valves  1506   a - b  to allow hydraulic fluid to flow from the retractor storage chamber  1474  into the drawdown piston control chambers  1496   a - b . As hydraulic fluid is drawn out of the drawdown piston actuating chambers  1494   a - b , the volumes of the drawdown piston actuating chambers  1494   a - b  decrease and the volumes of the drawdown piston control chambers  1496   a - b  increase causing the drawdown pistons  1402  and  1404  to extend. 
   To retract the probes  312  and  314 , the electronics system  1428  opens the solenoid valves  1492   a - b  to enable hydraulic fluid to flow into the retracting chambers  1484   a - b  from the retractor storage chamber  1474 . Specifically, as the coil spring  1478  ( FIG. 14 ) of the retractor  1468  ( FIG. 14 ) extends, the retractor  1468  displaces the hydraulic fluid into the retracting chambers  1484   a - b  via the control fluid lines  1490   a - b . Hydraulic fluid flows out of the extending chambers  1482   a - b  and into the actuation chamber  1452 . Hydraulic fluid also flows from the actuation chamber and the extending chambers  1482   a - b  into the annulus pressure chamber  1436  of the compensator  1434  via the annulus pressure fluid line  1464 . As hydraulic fluid flows out of the extending chambers  1482   a - b , the volumes of the extending chambers  1482   a - b  decrease and fluid flows from the retractor storage chamber  1474  into the retracting chambers  1484   a - b , thereby increasing the volumes of the retracting chambers  1484   a - b.    
   In the two-probe reset state  1814 , the electronics system  1428  also causes the motor to retract the actuator screw or ram  1444 . When the probes  312  and  314  are retracted, the electronics system  1428  may close the solenoid valves  1492   a - b  to maintain the probes  312  and  314  retracted at the home position state  1802 . When the drawdown pistons  1402  and  1404  are extended, the electronics system  1428  closes the solenoid valves  1506   a - b  preventing hydraulic fluid from flowing out of the drawdown piston control chambers  1496   a - b  and maintaining the drawdown pistons  1402  and  1404  in an extended condition. 
   In the illustrated example of  FIG. 18 , the example probe module  702  ( FIGS. 16 and 17 ) can transition from the home position state  1802  to a one-probe extension state  1816  in which one of the probes  312  and  314  is extended. To extend the probe  314 , the electronics system  1428  closes the solenoid valve  1466  and causes the motor  1454  ( FIG. 15 ) to actuate and extend the actuator screw or ram  1444  ( FIG. 15 ) to increase the hydraulic fluid pressure in the power fluid line  1488 . To enable the probe  314  to extend using the pressure in the power fluid line  1488 , the electronics system  1428  opens the solenoid valve  1492   b . However, the electronics system  1482  keeps the solenoid valve  1492   a  closed to prevent fluid from flowing out of the retracting chamber  1484   a . When the probe  314  is extended, the electronics system  1428  may close the solenoid valve  1492   b  to maintain the probe  314  in the extended position. 
   In a one-piston retraction state  1818 , the drawdown piston  1404  is retracted to draw the formation fluid  1417  into the probes  314 . To retract the drawdown piston  1404 , the electronics system  1428  maintains the solenoid valve  1466  closed, and the motor extends the actuator screw or ram  1444  to displace hydraulic fluid into the drawdown piston actuating chamber  1494   b . If a high flow rate of the formation fluid  1417  into the probe  314  is desired, the accumulator  1458  can be used as described above in connection with the two-piston retraction  1806  to store energy and relatively quickly release the energy to relatively quickly pull or retract the drawdown piston  1404 . The electronics system  1428  opens the solenoid valve  1506   b  to allow hydraulic fluid to flow from the drawdown piston control chamber  1496   b  and into the retractor storage chamber  1474  via the control fluid lines  1504   b . However, the electronics system  1428  keeps the solenoid valve  1506   a  closed to prevent hydraulic fluid from flowing out of the drawdown piston control chamber  1496   a , thereby causing the drawdown piston  1402  to remain extended. When the drawdown piston  1404  is sufficiently retracted as shown in  FIG. 15 , the electronics system  1428  may close the solenoid valve  1506   b  to maintain the drawdown piston  1404  in the retracted state. The retraction of the drawdown piston  1404  may be stopped before the full stroke is achieved, and restarted later. 
   The electronics system  1428  may be configured to acquire pressure data from the sensor  1424  to determine whether the packer  1416  is properly sealingly engaged to the formation surface of the wellbore  102  ( FIG. 1 ). The electronics system  1428  may also be configured to adjust the force exerted on the formation surface by the packer  1416  during the one-piston retraction state  1818  to overcome leaks between the packer and the formation surface when detected by the sensors  1424 . 
   The electronics system  1428  may also be configured to acquire pressure data from the sensor  1424  and to determine testing parameters based on the pressure data. For example, the pressure data collected during the one-piston retraction state  1818  may be analyzed and a desirable drawdown pressure and/or a desirable drawdown speed may be computed based on the analyzed pressure data. 
   In an example implementation, during the one-piston retraction state  1818 , the electronics system  1428  can substantially continuously monitor the retraction (or extension) distance of the drawdown piston  1404  and use the measured distance to adjust the retraction speed of the drawdown piston  1404  to a desired drawdown speed computed based on the data acquired in state  1818 . In another example implementation, the electronics system  1428  can substantially continuously monitor the pressure level measured by the sensor  1424  and adjust the level of opening of the valve  1506   b  based on the pressure level to, for example, achieve the desired drawdown pressure computed based on the data acquired in state  1818 . The control of the retraction of the drawdown piston  1404  may be achieved by controlling the opening of the valve  1506   b  by, for example, partially energizing the valves using a PWM controller. The amount of opening of the valve  1506   b  may be adjusted using close loop control techniques known in the art. 
   During a one-probe reset state  1822 , the probe  314  is retracted into the example formation tester  300  and the drawdown piston  1404  is extended into the probe  314 . The electronics system  1428  opens the solenoid valves  1492   b  and  1506   b . However, the electronics system  1428  keeps the solenoid valve  1492   a  and  1506   a  closed to prevent extension of the probe  312  and retraction of drawdown piston  1402 . As the coil spring  1478  ( FIG. 14 ) of the retractor  1468  ( FIG. 14 ) extends, the retractor  1468  displaces the hydraulic fluid to move the system back to a home position as shown in  FIG. 14 . In the one-probe reset state  1822 , the electronics system  1428  may also cause the motor  1454  to retract the actuator screw or ram  1444 . 
     FIGS. 19 through 21  illustrate detailed diagrams of an example probe system  1902  that may be implemented within (e.g., integral with) a tool collar (e.g., the formation tester  300  of  FIGS. 3A and 3B ) in a fixed or non-removable configuration. Alternatively, the example probe system  1902  may be used to implement a removably insertable probe module (e.g., the probe module  702  of  FIGS. 14 and 15 ). In the illustrated example, the components of the probe system  1902  are shown in a schematic representation for purposes of discussion to show the relationships between the various components. However, the components of the probe system  1902  may be rearranged while maintaining connections and functional relationships therebetween to implement the same functionality as described below in connection with the schematic illustrations of  FIGS. 19-21 . 
   To perform measurements associated with a formation (e.g., the formation F of  FIG. 1 ), the probe system  1902  is provided with an example probe  1904  and a drawdown piston  1906  located within the probe  1904 . The probe  1904  is configured to extend and retract relative to a probe opening  1908  of the probe system  1902  during a measurement process in directions generally indicated by arrows  1910  and  1912 . The drawdown piston  1906  is configured to move relative to the probe  1904  in the directions generally indicated by the arrows  1910  and  1912  to draw formation material into the probe  1904 . To engage a formation surface of a wellbore (e.g., the wellbore  102  of  FIG. 1 ) and form a seal between the formation surface and the probe  1904  to facilitate drawing the formation material into the probe  1904 , the probe  1904  is provided with a packer or seal  1914 . 
   In the illustrated example of  FIG. 19 , the probe  1904  is shown in a retracted, home position at which the packer  1914  is within the probe opening  1908 . In the illustrated example of  FIG. 21 , the probe  1904  is shown in an extended, measurement position in which the packer  1914  extends away from the opening  1908 . In addition, the drawdown piston  1906  is shown in a retracted position that draws formation material  1920  through a formation fluid port  1922  into the probe  1904 . 
   To perform measurements of the formation material  1920 , the probe system  1902  is provided with a sensor  1916  located within the drawdown piston  1906 . The sensor  1916  may be implemented using, for example, a pressure sensor, and/or a temperature sensor. In the illustrated example, the sensor  1916  is communicatively coupled to an electronic system (e.g., the electronics  218  of  FIG. 2 ) via wires or cable  1918  to communicate measurement information to the electronic system for storage. 
   The components of the probe system  1902  are configured to extend and retract the probe  1904  and the drawdown piston  1906  using energy associated with annulus pressure (A P ) and drill string internal pressure (I P ). Annulus pressure A P  refers to the pressure of formation material and other material (e.g., drilling mud) in the annulus (e.g., the annulus  124  of  FIG. 1 ). Drill string internal pressure I P  refers to the pressure of drilling fluid (e.g., the drilling fluid  116  of  FIG. 1 ) flowing through an internal passage (e.g., the passages  906  and  908  of  FIGS. 9 and 10 ) of the drill string  104 . 
   To sense the drill string internal pressure I P  the probe system  1902  is provided with an internal pressure chamber  1926  ( FIG. 19 ) that is filled with hydraulic fluid. A piston or bellow  1928  having an o-ring  1930  sealingly separates the internal pressure chamber  1926  from an internal fluid port  1932 . Drilling fluid (e.g., the drilling fluid  116  of  FIG. 1 ) flows through the internal fluid port  1932  and generates a force against the piston  1928 . To sense the annulus pressure A P , the probe system  1902  is provided with a compensator  1933  that includes an annulus pressure chamber  1934  ( FIG. 19 ) and an annulus fluid port  1936  sealingly separated by a piston or bellow  1938  having an o-ring  1940 . Drilling mud flows through the annulus fluid port  1936  and generates a force against the piston  1938 . 
   To store energy associated with the annulus pressure A P  and the internal pressure I P  to extend the measurement probe  1904 , the probe system  1902  is provided with an actuator  1941 . The actuator  1941  includes an actuator ram  1942  having a first flange  1944  (i.e., a first force element) that forms a piston-like structure having an o-ring  1946  that sealingly separates a balancing chamber  1948  from the internal pressure chamber  1926 . The actuator ram  1942  also includes a second flange  1950  (i.e., a second force element) that also forms a piston-like structure having an o-ring  1952  to sealingly separate an actuation chamber  1954  ( FIGS. 20 and 21 ) from an actuator reference chamber  1956  ( FIGS. 19 and 21 ). The balancing chamber  1948  and the actuation chamber  1954  are fluidly coupled to the annulus pressure chamber  1934  via a fluid passage or line  1960 . A solenoid check valve  1962  is disposed between the actuation chamber  1954  and the fluid line  1960  to control the flow of hydraulic fluid therebetween. Solenoid check valve  1962  is preferably normally open. When energized, solenoid check valve  1962  closes and prevents the discharge of hydraulic fluid from the actuation chamber  1954  into the annulus pressure chamber  1934 . When closed, solenoid check valve  1962  still allows hydraulic fluid to flow into the actuation chamber  1954 . 
   To store energy associated with the area of first flange  1944  and the area of second flange  1955 , the actuator ram  1942  is provided with a low pressure chamber  1964 . In the illustrated example, the low pressure chamber is filled with air, initially at atmospheric pressure. To sealingly capture the air within the air chamber  1964 , the probe system  1902  is provided with a piston rod  1966  inserted in the air chamber  1964 , and the actuator ram  1942  is provided with o-rings  1968  that sealingly engage the piston rod  1966 . 
   As shown in  FIG. 19 , the actuator  1941  includes the internal pressure chamber  1926 , the piston  1928 , the internal fluid port  1932 , the actuator ram  1942 , the balancing chamber  1948 , and the actuator reference chamber  1956 . In the illustrated example, the actuator  1941  is configured to work with the compensator  1933  to store energy based on differences between the annulus pressure A P , the internal pressure I P , and atmospheric pressure associated with the air stored in the air chamber  1964 . As described in greater detail below, the actuator  1941  uses the stored energy to extend the measurement probe  1904  and/or retract the drawdown piston  1906  to draw the formation fluid  1920  into the probe  1904 . 
   In an alternative example implementation shown in  FIG. 22 , an actuator  2202  is implemented using a lead screw configuration. The actuator  2202  is provided with an actuator ram  2204  having an outer diameter threaded portion  2206  (e.g., a first force element) at a first end and a first flange  2208  (e.g., a second force element) at a second end. The actuator  2202  of  FIG. 22  is provided with a nut  2210  with an inner diameter threaded portion  2212  that threadingly engages the outer diameter threaded portion  2206  of the actuator ram  2204 . Instead of storing energy associated with the annulus pressure A P  and the internal pressure I P  ( FIG. 19 ), the actuator  2202  uses a motor  2231  and an optional gear  2235  to rotate the nut  2210  and thus moving the actuator ram  2204 . The motor can be activated and deactivated using an electronic control circuit (e.g., the electronics  218  of  FIG. 2 ). The motor  2231  is preferably equipped with a rotary encoder  2233  for monitoring its position, and current sensors (not shown) for monitoring its torque. Measuring the motor position and currents allows, amongst other things, a precise control of the motor. The motor rotation may further be interpreted as a displaced volume and may be used for estimating the relative displacements of moving parts in a probe module. 
   Also shown in  FIG. 22  is a pressure sensor  2230 , measuring the differential pressure between the actuation chamber  1954  and the wellbore pressure. The signal generated by the sensor  2230  is preferably communicated to a downhole controller (such as controller  218 ). The controller  218  may utilize the signal from the sensor  2230 , for example, to adjust the speed of the motor  2231 . Thus, the controller  218  is capable of adjusting the extension rate of the probe  1904 , or of the drawdown piston  1906 . 
   In addition, the differential pressure between the actuation chamber  1954  and the wellbore pressure is related in part to the contact pressure of the probe packer  1914  against the wellbore wall. Thus, the controller  218  may be further capable of adjusting the contact pressure of the packer against the wellbore wall. In the embodiment of  FIG. 22 , the probe  1906  is instrumented with a displacement sensor  2234  for measuring the relative displacement of the probe in the retracting chamber. The displacement sensor may be one of a potentiometer or a linear encoder, or any other type of displacement sensor know in the art. The signal generated by the sensor  2234  may be used by a downhole controller (controller  218  for example) for adjusting the speed of the motor  2231 . In other embodiments, the signal generated by the sensor  2234  may be used by a downhole controller (controller  218 ) for adjusting valves, such as valves  1494   a - b  or  1506   a - b , which may be effectuated by utilizing a pulse width modulator controller. Thus, the controller  218  may adjust the position and/or speed of the probe  1904 . 
   In the embodiment of  FIG. 22 , the probe  1906  is also instrumented with displacement and pressure sensors in sensor block  2236 . The displacement measurement may be used for measuring the drawdown piston speed or position with respect to the probe. This measurement may also be used for controlling the tool operations, or for interpreting the pressure values recorded by the pressure sensor in sensor block  2236 . 
   Although the displacement sensors and the pressure chamber are shown in  FIG. 22  only, it should be understood that equivalent or similar sensor can be used in other embodiments of this disclosure. Also, although the pressure sensor is shown measuring the differential pressure between the actuation chamber  1954  and the wellbore pressure, other similar sensors may be used in other chambers for controlling the operation of the downhole tool. 
   Returning now to  FIG. 19 , to store energy for example to retract the measurement probe  1904  into the probe opening  1908 , the probe system  1902  is provided with a retractor  1976 . The retractor  1976  includes a piston  1978  having an o-ring  1980  that sealingly separates a retractor storage chamber  1982  ( FIG. 20 ) from a retractor spring chamber  1984  ( FIGS. 19 and 20 ). The retractor spring chamber  1984  includes a coil spring  1986  ( FIGS. 19 and 20 ) inserted therein that provides a force against the piston  1978  in a direction generally indicated by arrow  1988  ( FIG. 19 ). 
   To extend and retract the measurement probe  1904  based on the actuator  1941  and the retractor  1976 , the probe system  1902  is provided with an extending chamber  1990  ( FIG. 21 ) and a retracting chamber  1992  ( FIGS. 19 and 21 ). The extending and retracting chambers  1990  and  1992  are sealingly separated by an o-ring  1993  that sealingly engages the probe  1904 . The extending chamber  1990  is fluidly coupled to the actuation chamber  1954  ( FIGS. 20 and 21 ) via a power fluid line  1994 . The retracting chamber  1992  and the retractor storage chamber  1982  ( FIG. 20 ) are fluidly coupled via a control fluid line  1996 . A solenoid check valve  1998  is provided along the control fluid line  1996  to control the flow of hydraulic fluid between the retractor storage chamber  1982  and the retracting chamber  1992 . 
   To protect the probe  1904  during a drilling operation, the retractor  1976  and the solenoid check valve  1998  are configured to cause the probe  1904  to remain in a retracted position. In particular, energy stored in the coil spring  1986  can be used to retract the probe  1904  and/or cause the probe  1904  to remain in a retracted position. In this manner, inadvertent, accidental, or unintentional extensions of the probe  1904  are substantially reduced or prevented due to, for example, a power failure. Ensuring that the probe  1904  remains in a retracted position prevents damage to the probe  1904  during a drilling operation that may otherwise occur if the probe  1904  were extended while a drill string (e.g., the drill string  102  of  FIG. 1 ) moved during a drilling operation. For example, in the event of a power failure, the solenoid check valve  1962  closes allowing fluid to flow in one direction from the retractor storage chamber  1982  ( FIG. 20 ) to the retracting chamber  1992  via the flow line  1996 . As the energy stored in the coil spring  1986  causes the coil spring  1986  to push against the piston  1978 , the piston  1978  causes fluid to flow from retractor storage chamber  1982  to the retracting chamber  1992 , which causes the volume of the retracting chamber  1992  to increase and/or prevents the volume of the retracting chamber  1992  from decreasing. In turn, the probe  1904  retracts and/or remains in a retracted position for at least the duration of the power failure. 
   To extend and retract the drawdown piston  1906  relative to the probe  1904 , the probe  1904  and the drawdown piston  1906  form a drawdown piston actuating chamber  2002  ( FIG. 21 ) and a drawdown piston control chamber  2004  ( FIGS. 19 and 21 ). The drawdown piston  1906  is provided with an o-ring  2006  ( FIGS. 19 and 21 ) that sealingly engages an inner wall of the probe  1904  to sealingly separate the drawdown piston actuating and control chambers  2002  and  2004 . 
   To receive the probe  1904  when the probe  1904  is retracted, the probe system  1902  is provided with a back chamber  2008 . The probe  1904  is provided with an o-ring  2010  to sealingly separate the back chamber  2008  from the retracting chamber  1992  and the drawdown piston control chamber  2004 . The back chamber  2008  is fluidly coupled to the retractor spring chamber  1984  via an annulus pressure (A P ) fluid line  2012  ( FIGS. 20 and 21 ) and the retractor spring chamber  1984  is fluidly coupled to the annulus pressure chamber  1934  via another annulus pressure (A P ) fluid line  2014  ( FIGS. 20 and 21 ). 
     FIG. 23  depicts a state diagram of a drilling operation  2300  that represents an example method to operate the example probe system  1902  of  FIGS. 19-21 . In a drilling state  2302  of the drilling operation  2300 , while a drill bit (e.g., the drill bit  106 ) is drilling into a formation (e.g., the formation F of  FIG. 1 ), the example measurement probe  1904  is in a retracted or home position as shown in  FIG. 19 . That is, the probe  1904  and the packer  1914  are substantially completely retracted within the probe opening  1908  so that they are below an outer surface of a pad (e.g., the outer surface  324  of the pad  308  of  FIG. 3B ). Alternatively, if the example probe system  1902  is implemented so that the probe  1904  extends through a stabilizer blade (e.g., the stabilizer blade  303  of  FIGS. 3A and 3B ) instead of a pad, the probe  1904  and the packer  1914  are below a stabilizer blade surface (e.g., the outer surface  320  of the stabilizer blade  303  of  FIG. 3B ). 
   Also during the drilling state  2302 , drilling fluid (e.g., the drilling fluid  116  of  FIG. 1 ) flows through a drill string internal passage (e.g., the internal fluid passage  238  of  FIG. 2 ) creating a drill string internal pressure I P  and drilling mud flows through the annulus  124  ( FIG. 1 ) of the wellbore  102  ( FIG. 1 ) creating an annulus pressure A P . The internal fluid port  1932  receives the drilling fluid  116  and the annulus fluid port  1936  receives the drilling mud. During the drilling state  2302 , the drill string internal pressure I P  is higher than the annulus pressure A P . This difference in pressures causes the actuator ram  1942  ( FIG. 19 ) to shift toward the actuator reference chamber  1956  ( FIG. 19 ) and becomes set in an armed state shown in  FIG. 20 . In the armed state of  FIG. 20 , the actuator  1941  ( FIGS. 19 and 20 ) and the retractor  1976  ( FIGS. 19 and 20 ) store energy to subsequently extend the probe  1904  and retract the drawdown piston  1906 . In an alternative example implementation using the lead screw configuration of  FIG. 22 , instead of using the pressure difference between the drill string internal pressure I P  and the annulus pressure A P , the motor  2210  may be activated to move the actuator ram  2204 . 
   As the actuator ram  1942  shifts toward the actuator reference chamber  1956  ( FIGS. 19 and 21 ), hydraulic oil is expelled from the actuator reference chamber  1956  into the retractor storage chamber  1982  ( FIG. 20 ) and hydraulic oil is also expelled from the balancing chamber  1948  ( FIGS. 19 and 21 ) to the annulus reference chamber  1934  ( FIGS. 19 and 20 ) causing the volumes of the actuator reference chamber  1956  and the balancing chamber  1948  ( FIGS. 19 and 21 ) to be reduced. In addition, hydraulic oil flows into the actuation chamber  1954  ( FIGS. 20 and 21 ) through the solenoid check valve  1962  ( FIGS. 19-21 ) and the volume of the actuation chamber  1954  increases. The solenoid check valves  1962  and  1998  ( FIGS. 19-21 ) remain closed (i.e., solenoid check valves are not energized and allow flow in only one direction). For example, the solenoid check valve  1962  remains closed to prevent hydraulic fluid flow from the actuation chamber  1954  to the annulus pressure chamber  1934  and/or the balancing chamber  1948  via the fluid line  1960 . Keeping the solenoid check valve  1962  closed causes the actuator ram  1942  to remain armed as shown in  FIG. 20  regardless of changes in the drill string internal pressure I P  and/or the annulus pressure A P . Also, the solenoid check valve  1962  remains closed to prevent hydraulic fluid flow from the retracting chamber  1992  ( FIGS. 19-21 ) to the retractor storage chamber  1982  ( FIG. 20 ). Keeping the solenoid check valve  1962  closed prevents the probe  1904  from extending and, instead, causes the probe  1904  to remain in the retracted position shown in  FIGS. 19 and 20 . In the event of a power failure, the solenoid check valve  1962  closes allowing fluid to flow in one direction from the retractor storage chamber  1982  to the retracting chamber  1992  via the flow line  1996  to cause the volume of the retracting chamber  1992  to increase and, in turn, cause the probe  1904  to retract and to remain in the retracted position for at least the duration of the power failure. 
   In a drilling halt state  2304 , the drill bit  106  ( FIG. 1 ) stops turning and the drill string internal pressure I P  drops to become substantially equal to the annulus pressure A P . During the drilling halt state  2304 , the processor  146  ( FIG. 1 ) may communicate a downlink command to an electronics system (e.g., the electronics system  214  of  FIG. 2 ) to perform a measurement. The downlink command causes the probe system  1902  to enter a draw sample state  2306 . 
   In the draw sample state  2306  and in response to the downlink command, the solenoid check valve  1998  ( FIGS. 19-21 ) is opened (i.e., the solenoid check valve  1998  is energized) and the actuator ram  1942  moves toward the internal pressure chamber  1926  as shown in  FIG. 21  as hydraulic fluid is expelled from the actuation chamber  1954  ( FIGS. 20 and 21 ) into the extending chamber  1990  ( FIG. 21 ) causing the probe  1904  to extend through the probe opening  1908  as shown in  FIG. 21 . In addition, the solenoid valve  1998  is opened (i.e., energized) to allow hydraulic fluid to flow from the retracting chamber  1992  ( FIGS. 19 and 21 ) to the actuator reference chamber  1956  ( FIGS. 19 and 21 ). In addition, some of the energy stored in the coil spring  1986  is used to force hydraulic fluid into the actuator reference chamber  1956 . 
   As the probe  1904  extends and contacts a formation surface of the wellbore  102  ( FIG. 1 ), a tip  2016  of the probe  1904  extends through the packer  1914  and penetrates the mud cake on the formation surface. When the probe  1904  is set against the formation surface (e.g., when the probe  1904  can extend no further), hydraulic pressure in the extending chamber  1990  ( FIG. 21 ) increases and hydraulic fluid flows from the extending chamber  1990  into the drawdown piston actuating chamber  2002  ( FIG. 21 ) causing the drawdown piston  1906  to move toward the drawdown piston control chamber  2004  ( FIGS. 19 and 21 ). As the drawdown piston  1906  moves toward the drawdown piston control chamber  2004 , hydraulic fluid flows from the drawdown piston control chamber  2004  to the retracting chamber  1992  ( FIG. 21 ). In addition, the formation material  1920  ( FIG. 21 ) is drawn through the formation fluid port  1922  into a drawdown chamber  2018  ( FIG. 21 ) (i.e., a formation fluid chamber) of the probe  1940  and toward the sensor  1916 . When the drawdown piston  1906  is fully retracted, the pressure in the drawdown chamber  2018  becomes substantially equal to the pore pressure (P P ) (i.e., the pressure of the formation material  1920  in the formation F of  FIG. 1 ). To ensure that the probe  1904  extends and the drawdown piston  1906  retracts in the sequence described above, the resistance associated with extending the probe  1904  must be less than the resistance associated with retracting the drawdown piston  1906 . For example, o-ring sizes and material composition can be selected to create suitable resistances. 
   When the measurement performed by the sensor  1916  is complete (e.g., when the stabilization of pressure in the drawdown chamber  1918  is detected or when a time threshold is reached), the probe system  1902  enters into a retract probe state  2308  ( FIG. 19 ). In the retract probe state  2308 , the solenoid check valve  1998  is closed (i.e., de-energized) and the solenoid check valve  1962  is opened (i.e., energized). Hydraulic fluid flows from the actuating chamber  2002  ( FIG. 21 ) and the extending chamber  1990  ( FIG. 21 ) to the annulus pressure chamber  1934 . The energy remaining in the actuator  1941  ( FIGS. 19 and 20 ) assists in expelling the hydraulic fluid to the annulus pressure chamber  1934 . 
   Also, in the retract probe state  2308 , stored energy remaining in the retractor  1976  is used to return the probe  1904  to the retracted or home position shown in  FIG. 19  by pushing hydraulic fluid into the retracting chamber  1992  ( FIGS. 19 and 21 ) and the drawdown piston control chamber  2004  ( FIGS. 19 and 21 ). As the probe  1904  returns to the retracted position, the actuator ram  1942  returns to the starting position shown in  FIG. 19  and the solenoid check valve  1962  is closed (i.e., de-energized). 
     FIG. 24  depicts another example probe system  2400  implemented using a dual-probe configuration in which two probes  2402  and  2404  are integrally formed so that they extend and retract simultaneously relative to a tool collar  2406 . The example probe system  2400  also includes an actuator ram  2408  to extend and retract the probes  2402  and  2404  relative to the tool collar  2406 . A power fluid line  2410  extending through the actuator ram  2408  and the probes  2402  and  2404  provides hydraulic fluid for extending and retracting the probes  2402  and  2404 . To control the extension and retraction of the probes  2402  and  2404 , the probe system  2400  is provided with an actuator back chamber  2412  coupled to a probe control fluid line  2414  having a solenoid check valve  2416 . The solenoid check valve  2416  can be opened (e.g., energized) to enable hydraulic fluid to flow out of the actuator back chamber  2412  allowing the hydraulic fluid flowing through the power fluid line  2410  to extend the probes  2402  and  2404  as the volume of the actuator back chamber  2412  decreases. 
   Each probe  2402  and  2404  of the example probe system  2400  includes a respective drawdown piston  2418  and  2420  and sensor  2422  and  2424 . The drawdown pistons  2418  and  2420  extend and retract relative to the probes  2402  and  2404  to draw formation fluid into the probes  2402  and  2404 . Each of the drawdown pistons  2418  and  2420  retracts into a respective drawdown piston control chamber  2426  and  2428 . To control the retraction and extension of the drawdown pistons  2418  and  2420 , for each of drawdown piston  2420  and  2422 , the probe system  2400  is provided with a respective piston control fluid line  2430  and  2432 . Each of the piston control fluid lines  2430  and  2432  is provided with a solenoid check valve  2434  and  2436 . Opening (e.g., energizing) the solenoid check valves  2430  and  2432  causes hydraulic fluid to flow out of the drawdown piston control chambers  2426  and  2428  and through the piston control fluid lines  2430  and  2432 . The hydraulic fluid provided via the power fluid line  2410  then causes the pistons  2412  and  2414  to be drawn or retracted into the drawdown piston control chambers  2426  and  2428  to draw formation fluid into the probes  2402  and  2404 . 
   The probe system  2400  is also provided with annulus pressure (A P ) fluid lines  2438  that are fluidly coupled to a compensator (not shown) substantially similar or identical to the compensator  1933  of  FIG. 19 . The A P  fluid lines  2438  provide hydraulic fluid at an annulus pressure to urge the probes  2402  and  2404  to extend as described above in connection with  FIGS. 19-21  and  23 . 
   In an example implementation, the power fluid line  2410 , the control fluid lines  2414 ,  2430 , and  2432 , and the A P  line  2438  can be connected to power fluid lines, control fluid lines, and A P  fluid lines of the example probe system  1902  of  FIGS. 19-21  to control the probes  2402  and  2404  and the pistons  2418  and  2420  as described above in connection with the example probe system  1902 . 
     FIG. 25  depicts a portion of a tool collar  2500  having plurality of probes  2502   a - j  perform downhole measurements in connection with a drilling operation. Some or all of the probes  2502   a - j  may be configured to extend and retract relative to the tool collar  2500  to perform measurements. In the illustrated example, the probes  2502   a - j  are mounted in stabilizer blades  2504   a - b  ( 2504   b  not shown), which may be configured to spiral at least partially around the tool collar  2500 . In other example implementations, the stabilizer blades  2504   a - b  may instead be implemented using pads that provide substantially similar or identical functionality as described above in connection with the pads  308  and  310 . 
   In the illustrated example, the probes  2502   a - j  are mounted in respective ones of the stabilizer blades  2504   a - b  in groups of five. However, any other grouping quantities may be used. Implementing the stabilizer blades  2504   a - b  in spiral configurations about the tool collar  2500  causes each of the probes  2502   a - j  to be on a different horizontal and vertical plane. In this manner, each of the probes  2502   a - j  can perform a measurement (e.g., a pressure measurement) at a different elevation and radial location of a wellbore (e.g., the wellbore  102  of  FIG. 1 ). The configuration shown in  FIG. 25  enables substantially simultaneously collecting measurement information associated with different locations of the wellbore  102  spanning a surface of the wellbore  102  having a length substantially similar to the length of the stabilizer blades  2504   a - b . Mounting the probes  2502   a - j  along the length of the stabilizer blades  2504   a - b  facilitates obtaining measurements associated with a small or thin target area of the wellbore  102  by reducing the amount of positioning accuracy required to position any single probe adjacent to the target area of interest. In addition, the illustrated probe mounting configuration enables acquiring relatively a more accurate formation property (e.g. formation pressure) because more measurement points spreading over a larger surface area of the wellbore  102  can be acquired. 
   To perform measurements (e.g., pressure measurements), each of the probes  2502   a - j  is provided with a drawdown piston chamber (e.g., the drawdown piston chamber  2624  of  FIG. 26 ) described below in connection with  FIG. 26 . The measurement values can be stored in a memory (e.g., the FLASH memory  222  of  FIG. 2 ). The measurement values can be transmitted to the surface or can be downloaded when the tool collar  2500  is returned to the surface. In some example implementations, the measurement values can be analyzed by a controller (e.g., the controller  218  of  FIG. 2 ) while the tool collar  2500  is located in the wellbore  102 . 
   During a drilling operation, the probes  2502   a - j  are kept retracted below outer surfaces  2506   a - b  of the stabilizer blades  2504   a - b . The transmitter subsystem  150  ( FIG. 1 ) can then communicate a command from the surface to an electronics system (e.g., the electronics system  214  of  FIG. 2 ) associated with the tool collar  2500  to initiate a test sequence when, for example, drilling has been halted. In response to the command, the electronics system  214  can cause some or all of the probes  2502   a - j  to extend from the stabilizer blades  2504   a - b . For example, the tool collar  2500  is provided with one-way check valves  2508   a - b  that can be communicatively coupled to the electronics system  214 , and the electronics system  214  can open or close the one-way check valves  2508   a - b  to cause the probes  2502   a - j  to extend or retract. 
   To accumulate energy for extending the probes  2502   a - j , the tool collar  2500  is provided with a tool collar fluid passageway  2512  and a mud piston  2514  configured to move along a length of the fluid passageway  2512 . The mud piston  2514  includes a mud piston fluid passageway  2516  formed through and along a length of the mud piston  2514 . During a drilling operation, drilling fluid (e.g., the drilling fluid  116  of  FIG. 1 ) flows through the tool collar fluid passageway  2512  and the mud piston fluid passageway  2516  in a direction generally indicated by arrow  2518 . The size (e.g., the diameter) of the mud piston fluid passageway  2516  is smaller than the size (e.g., the diameter) of the tool collar fluid passageway  2512  and provides fluid flow resistance when the drilling fluid  116  flows through the tool collar fluid passageway  2512 . In turn, the fluid flow resistance provided by the mud piston fluid passageway  2516  causes the mud piston  2514  to move along the tool collar fluid passageway  2512  in the direction generally indicated by the arrow  2518 . 
   The tool collar  2500  is provided with a first spring chamber  2522  and a second spring chamber  2524  located along the tool collar fluid passageway  2512 . The first spring chamber  2522  includes a coil spring  2526  that engages a flange  2528  of the mud piston  2514 , and the second spring chamber  2524  includes an annular accumulator piston  2530  sealingly engaged to the mud piston  2514  and a coil spring  2532  that engages the annular accumulator piston  2530 . In the illustrated example, the coil spring  2532  has a spring force relatively greater (e.g., has a higher spring constant k) than the coil spring  2526 . 
   During a drilling operation, the mud piston  2514  is configured to generate energy based on the drilling fluid  116  that flows through the tool collar fluid passageway  2512 , and the coil spring  2532  is configured to store the energy generated by the mud piston  2514  for subsequent use to extend some or all of the probes  2502   a - j . In particular, the one-way check valves  2508   a - b  and valves  2534   a - b  and  2536   a - b  are closed during drilling so that hydraulic fluid from the first spring chamber  2522  can flow in only one direction to an accumulator chamber  2538  as the drilling fluid  116  flows through the tool collar fluid passageway  2512  causing the mud piston  2514  to move and compress the coil spring  2526 . The hydraulic fluid expelled from the first spring chamber  2522  increases a volume of the accumulator chamber  2538  causing the annular accumulator piston  2530  to compress the coil spring  2532  causing the coil spring  2532  to store energy. As the annular accumulator piston  2530  moves toward the coil spring  2532 , the annular accumulator piston  2530  expels drilling mud from the second spring chamber  2524  into the annulus  124  ( FIG. 1 ) of the wellbore  102  via mud fluid ports  2537 . The one-way check valves  2508   a - b  and the valves  2534   a - b  and  2536   a - b  prevent the hydraulic fluid from being expelled from the accumulator chamber  2538 , which, in turn, causes the coil spring  2532  to remain in a compressed state to store energy. 
   In response to receiving a measurement sequence command, the electronics system  214  causes one or more of the valves  2534   a - b  to open to allow the coil spring  2532  to extend using the stored energy and move the annular accumulator piston  2530  to expel the hydraulic fluid from the accumulator chamber  2538  to fluid passageways  2542   a - b . The fluid passageways  2542   a - b  are fluidly coupled to the probes  2502   a - j , and the hydraulic fluid flows to the probes  2502   a - j  via the fluid passageways  2542   a - b  to cause the probes  2502   a - j  to extend. To retract the probes  2502   a - j , the electronics system  214  opens the valves  2536   a - b  to enable hydraulic fluid to flow from the fluid passageways  2542   a - b  to the first spring chamber  2522 . 
     FIG. 26  depicts an example probe assembly  2600  having the probe  2502   a  of  FIG. 25 . To extend and retract the probe  2502   a , the example probe assembly  2600  is provided with a probe spring chamber  2602  having a coil spring  2604  therein. When the probe  2502   a  extends, a flange  2606  of the probe  2502   a  compresses the coil spring  2604 , which, in turn, stores energy. To form a seal between the probe  2502   a  and a formation surface of a wellbore, the probe  2502   a  is provided with a packer  2608  made of, for example, a substantially deformable elastomeric material configured to sealingly engage the formation surface when the probe  2502   a  is extended. To retract the probe  2502   a  when fluid is expelled from the fluid passageway  2452   a , the stored energy in the coil spring  2604  causes the spring  2604  to extend and push the flange  2606 , which, in turn, retracts the probe  2502   a.    
   The probe assembly  2600  includes a drawdown piston  2610  in the probe  2502   a  configured to draw formation fluid. In the illustrated example, the drawdown piston  2610  includes a pressure sensor  2612  configured to measure a pressure of formation fluid. To draw the formation fluid, the probe  2502   a  is provided with a drawdown piston spring chamber  2614  having a coil spring  2616 . The probe assembly  2600  also includes a check valve  2622  configured to control the flow of hydraulic fluid into and out of a drawdown piston chamber  2624 . When the check valve  2622  is closed (e.g., de-energized), hydraulic fluid flows from the fluid passageway  2542   a  into the drawdown piston chamber  2624  via a fluid passageway  2628  and a fluid passageway  2629  formed through the drawdown piston  2610  causing the volume of the drawdown piston chamber  2624  to increase as the drawdown piston  2610  moves toward the coil spring  2616  causing the spring  2616  to compress and store energy. As the drawdown piston  2610  retracts toward the spring  2616 , formation fluid is drawn into the pressure sensor  2612 . The probe  2502   a  includes a fluid passageway  2630  that enables fluid to flow into and out of the drawdown piston spring chamber  2614  to enable increasing and decreasing the volume of the drawdown piston spring chamber  2614  to extend and retract the drawdown piston  2610 . Optionally, the passageway  2630  is equipped with throttle valve  2650 , which may be an adjustable throttle valve. The throttle valve  2650  may be used for controlling the rate at which the drawdown piston  2610  retracts. Also, the probe  2502   a  may include a detent  2651  for preventing the drawdown piston to retract until the pressure in the drawdown piston chamber  2624  has reached a sufficient level. The pressure in the drawdown piston chamber  2624  depends, in part, on the level of the contact force between the packer  2608  and the formation. Thus, the detent  2651  may be used for controlling the level of contact force at which the drawdown is initiated. 
   To extend the drawdown piston  2610  and expel the formation fluid from the pressure sensor  2612 , the check valve  2622  is opened (e.g., energized) and the drawdown piston  2610  expels hydraulic fluid from the drawdown piston chamber  2624  to the fluid passageway  2452   a . The probe assembly  2600  includes a fluid passageway  2632  that enables fluid to flow into and out of the probe spring chamber  2602  to enable increasing and decreasing the volume of the probe spring chamber  2602  to extend and retract the probe  2502   a . The fluid passageway  2632  is fluidly coupled to a compensator chamber  2634  that holds the fluid that flows into and out of the probe spring chamber  2602  and the drawdown piston spring chamber  2614 . The compensator chamber  2634  is substantially similar or identical to the compensator  1933  of  FIG. 19  and can be used to sense an annulus pressure A P . 
   Although certain methods, apparatus, and articles of manufacture have been described herein, the scope of coverage of this patent is not limited thereto. To the contrary, this patent covers all methods, apparatus, and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.

Summary:
A system for testing an underground formation penetrated by a well includes a downhole tool that is configured to be coupled to a work string and that includes an outer surface, a connection for coupling a stabilizing sub to the downhole tool, and at least one portion configured to receive a frame. The system further includes a plurality of stabilizing subs that are configured to be coupled to the downhole tool, a plurality of frames configured to be detachably mounted on the at least one portion of the downhole tool, and at least one measuring device configured to be secured in at least one of the plurality of frames. The stabilizing subs each have an outer surface that defines an offset relative to the outer surface of the downhole tool, wherein a first of the plurality of stabilizing subs has a first stabilizing sub offset, and the plurality of frames each have an offset relative to the outer surface of the downhole tool and an aperture for receiving a measuring device, wherein a first of the plurality of frames has a first frame offset determined by the first stabilizing sub offset.