Abstract:
An improved energy efficient intelligent rotary pulser for generating a mud pulse in a MWD (measurement while drilling) application. In the rotary pulser, a control circuit activates a brushless motor that rotates a windowed restrictor relative to a fixed housing to act as a shutter and window respectively. Opening of the windowed restrictor allows generally unrestricted mud flow. Closing of the windowed restrictor generally restricts mud flow. The windowed restrictor is powered both in opening and closing operations by the motor.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to a telemetry system, and in particular to a measurement while drilling (MWD) system. More particularly, the present invention relates to an actuator for a downhole mud pulser for sending information from downhole to surface. 
       BACKGROUND OF THE INVENTION 
       [0002]    The desirability and effectiveness of well logging systems where information is sensed in the well hole and transmitted to the surface through mud pulse telemetry has long been recognized. Mud pulse telemetry systems provide the driller at the surface with means for quickly determining various kinds of downhole information, most particularly information about the location, orientation and direction of the drill string at the bottom of the well in a directional drilling operation. During normal drilling operations, a continuous column of mud is circulating within the drill string from the surface of the well to the drilling bit at the bottom of the well and then back to the surface. Mud pulse telemetry repeatedly restricts the flow of mud to propagate signals through the mud upward to the surface, thereby providing a very fast communication link between the drill bit and the surface. Depending on the type of drilling fluid used, the velocity may vary between approximately 3000 and 5000 feet per second. 
         [0003]    A telemetry system may be lowered on a wireline located within the drill string, but is usually formed as an integral part of a special drill collar inserted into the drill string near the drilling bit. The basic operational concept of mud pulse telemetry is to intermittently restrict the flow of mud as it passes through a downhole telemetry valve, thereby creating a pressure pulse in the mud stream that travels to the surface of the well. The information sensed by instrumentation in the vicinity of the drilling bit is encoded into a digital formatted signal and is transmitted by instructions to pulse the mud by intermittently actuating the telemetry valve, which restricts the mud flow in the drill string, thereby transmitting pulses to the well surface where the pulses are detected and transformed into electrical signals which can be decoded and processed to reveal transmitted information. 
         [0004]    Representative examples of previous mud pulse telemetry systems may be found in U.S. Pat. Nos. 3,949,354; 3,958,217; 4,216,536; 4,401,134; and 4,515,225. 
         [0005]    Representative samples of mud pulse generators may be found in U.S. Pat. Nos. 4,386,422; 4,699,352; 5,103,430; and 5,787,052. 
         [0006]    A telemetry system capable of performing the desired function with minimal control energy is desirable, since the systems are typically powered by finite-storage batteries. One such example is found in U.S. Pat. No. 5,333,686, which describes a mud pulser having a main valve biased against a narrowed portion of the mud flowpath to restrict the flow of mud, with periodic actuation of the main valve to allow mud to temporarily flow freely within the flowpath. The main valve is actuated by a pilot valve that can be moved with minimal force. The pilot valve additionally provides for pressure equalization, thereby increasing the life of downhole batteries. 
         [0007]    Another example of an energy efficient mud pulser is described in U.S. Pat. No. 6,016,288, the mud pulser having a DC motor electrically powered to drive a planetary gear which in turn powers a threaded drive shaft, mounted in a bearing assembly to rotate a ball nut lead screw. The rotating threaded shaft lifts the lead screw, which is attached to the pilot valve. 
         [0008]    Solenoid-type pulser actuators have also been used to actuate the main pulser valve, however, there are many problems with such a system. The use of a spring to bias the solenoid requires the actuator (servo) valve to overcome the force of the spring (about 6 pounds) and of the mud prior to actuating the main valve. A typical solenoid driven actuator valve is capable of exerting only 11 pounds of pressure, leaving only 5 pounds of pressure to actuate the pulser assembly. Under drilling conditions requiring higher than normal mud flow, the limited pressures exerted by the solenoid may be unable to overcome both the pressure of the return spring and the increased pressure of the flowing mud, resulting in a failure to open the servo-valve, resulting in the main valve remaining in a position in which mud flow is not restricted, and therefore failing to communicate useful information to the surface. 
         [0009]    A further problem with the use of a solenoid to actuate the pulser assembly is the limited speed of response and recovery that is typical of solenoid systems. Following application of a current to a solenoid, there is a recovery period during which the magnetic field decays to a point at which it can be overcome by the force of the solenoid&#39;s own return spring to close the servo-valve. This delay results in a maximum data rate (pulse width) of approximately 0.8 seconds/pulse, limiting the application of the technology. 
         [0010]    Moreover, the linear alignment of the solenoid must be exactly tuned (i.e. the magnetic shaft must be precisely positioned within the coil) in order to keep the actuator&#39;s power characteristics within a reliable operating range. Therefore, inclusion of a solenoid within the tool adds complexity to the process of assembling and repairing the pulser actuator, and impairs the overall operability and reliability of the system. 
         [0011]    Existing tools are also prone to jamming due to accumulation of debris, reducing the range of motion of the pilot valve. Particularly when combined with conditions of high mud flow, the power of the solenoid is unable to clear the jam, and the tool is rendered non-functional. The tool must then be brought to the surface for service. 
         [0012]    Stepper motors have been used in mud pulsing systems, specifically, in negative pulse systems (see for example U.S. Pat. No. 5,115,415). The use of a stepper motor to directly control the main pulse valve, however, requires a large amount of electrical power, possibly requiring a turbine generator to supply adequate power to operate the system for any length of time downhole. 
         [0013]    Repair of previous pulsers has been an as yet unresolved difficulty. Typically, the entire tool has been contained within one housing, making access and replacement of small parts difficult and time-consuming. Furthermore, a bellows seal within the servo-poppet has typically been the only barrier between the mud flowing past the pilot valve&#39;s poppet and the pressurized oil contained within the servo-valve actuating tool, which is required to equalize the hydrostatic pressure of the downhole mud with the tool&#39;s internal spaces. Therefore, in order to dissemble the tool for repair, the bellows seal had to be removed, causing the integrity of the pressurized oil chamber to be lost at each repair. 
         [0014]    Furthermore, a key area of failure of MWD pulser drivers has been the failure of the bellows seal around the servo-valve activating shaft, which separates the drilling mud from the internal oil. In existing systems, the addition of a second seal is not feasible, particularly in servo-drivers in which the servo-valve is closed by a spring due to the limited force which may be exerted by the spring, which is in turn limited by the available force of the solenoid, and cannot overcome the friction or drag of an additional static/dynamic linear seal. 
         [0015]    It remains desirable within the art to provide a pulse generator that has an energy efficiency sufficient to operate reliably and to adapt to a variety of hostile downhole conditions, has reduced susceptibility to jamming by debris, and is simpler to repair than previous systems. 
       SUMMARY OF THE INVENTION 
       [0016]    It is an object of the present invention to obviate or mitigate at least one disadvantage of previous mud pulsers and pulse generators. 
         [0017]    In a first aspect, the present invention provides a downhole measurement-while-drilling rotary pulser having a windowed restrictor movable between an open position which permits mud flow through a fixed housing having a flow passage and a restricted position which restricts mud flow through the fixed housing having a flow passage, the windowed restrictor powered between the open position and the restricted position by a reversible electric motor. 
         [0018]    The windowed restrictor is preferably movable between the open position and the restricted position in a reciprocating rotary motion relative to the fixed housing. Preferably, the fixed housing is held stationary and the windowed restrictor is rotatable, driven in rotation by the motor. 
         [0019]    Preferably the rotary pulse includes a controller for controlling the electric motor. The controller may be adapted to sense the position of the windowed restrictor relative to the fixed housing and/or the absolute position of the windowed restrictor. 
         [0020]    Preferably the position of the windowed restrictor is sensed when the mud flow through the flow passage is restricted, and wherein the amount and direction of rotations of the motor from that position are counted and stored by the controller. Preferably, the sensed position of the windowed restrictor is calibrated as the fully closed position of the windowed restrictor in controlling the windowed restrictor&#39;s travel during operation to avoid excess collision or frictional wear between the windowed restrictor and the windowed restrictor stop. Preferably the controller senses the position of the windowed restrictor by sensing whether movement of the windowed restrictor is impeded, and wherein the controller counts the number of rotations of the motor until the windowed restrictor is impeded and compares the number of rotations to an expected number of rotations to determine the position of the windowed restrictor with respect to the fixed housing. Preferably, the expected number of rotations is preset to allow a predetermined rate of mud flow past the windowed restrictor when the windowed restrictor is moved away from the fixed housing by the preset expected number of rotations. 
         [0021]    Preferably, the controller includes a debris clearing command that is initiated when the number of rotations counted is not equal to the expected number of rotations. Preferably, the debris clearing command causes the motor to reciprocate the windowed restrictor to dislodge any debris present between the windowed restrictor and the fixed housing. 
         [0022]    In a further aspect, the present invention provides a method for causing the generation of a pressure pulse in drilling mud by a controlled windowed restrictor valve including the steps of powering a rotary pulser windowed restrictor valve in a first direction using a rotary motor such that drilling mud is permitted to flow past a fixed housing; and powering the windowed restrictor in a second direction using the rotary motor such that drilling mud flow past the fixed housing is restricted, wherein the pressure pulse is generated. 
         [0023]    Preferably, the power flow to the motor may be cut or interrupted to hold the windowed restrictor in a particular position within its range of motion to tailor the pressure and duration characteristics of a mud pulse. 
         [0024]    In a further aspect, the present invention provides a controller for use with a downhole measurement-while-drilling rotary pulser, the controller having a sensor, memory, control circuitry, and an operator interface. 
         [0025]    Preferably, the sensor is a mud flow sensor, pressure sensor, temperature sensor, rotation-step counter, position sensor, velocity sensor, current level sensor, battery voltage sensor, timer, or error monitor. Preferably, the memory is adapted to store time-stamped or counted sensed events together with an event-type indication. Preferably, the control circuitry is programmable to cause an action within the actuator responsive to a sensed event, a time, an elapsed time, a series of sensed events, or any combination thereof. Preferably, the operator interface provides information from memory to the operator. Preferably, the operator interface allows an operator to alter the programming of the control circuitry. 
         [0026]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein: 
           [0028]      FIGS. 1A and 1B  are longitudinal cross sectional views of the upper and lower portions of an embodiment of the mud pulser of the present invention, depicting a windowed restrictor in a restricted position; 
           [0029]      FIG. 2A and 2B  are a longitudinal cross sectional view of the upper and lower portions of an embodiment of the mud pulser of the present invention, depicting a windowed restrictor in an open position; 
           [0030]      FIG. 3  is a fixed housing of the present invention; and 
           [0031]      FIG. 4  is a windowed restrictor of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    The present invention relates to an apparatus and method for actuating a mud pulser telemetry system used during well-drilling or well logging operations. The present apparatus allows a pulser valve to be powered both in opening (e.g. to allow generally unrestricted flow) and in closing (e.g. to generally restrict flow,) and does not rely on a solenoid system. 
         [0033]    The powered opening and closing of a windowed restrictor results in various functional and economic advantages, including the ability to clear debris from the restricted portion of the mud flowpath, and faster data rates due to elimination of inherent operating delays in the solenoid systems of previous tools, with the end result of providing a pulser driver which consumes a minimal amount of power (e.g. DC or AC electricity) while providing more force with which to drive the windowed restrictor in each direction. Therefore, the pulser remains functional at a comprehensive range of downhole drilling conditions. 
         [0034]    Furthermore, in the embodiment shown in the Figures, the present device is designed to have several independent, interconnected housings, and employs a double seal between the oil compartment and the drilling mud, which simplifies assembly and repair of the tool. The assembly/disassembly is simplified to reduce repair turnaround time by using modular components. 
         [0035]    Additionally, the use of a brushless motor, electric load sensors, and control circuitry in a powered-both-directions valve system allows for self-calibration of the tool and self-diagnosis and error correction unavailable in other systems. 
         [0036]    Communication of information to the well surface is accomplished by encoded signals, which are translated to produce pressure changes in the downward flow of the pressurized drilling mud. It is recognized that although the drilling fluid is generally referred to as mud, other drilling fluids are also suitable for use with the present invention, as is well known in the art. 
         [0037]    With reference to the Figures, the rotary pulser  10  of the present invention generally includes a plurality of serially interconnected housings  20 ,  30 ,  40 ,  50 ,  60 ,  70 , and  80 , an electrical connector  90 , and a controller  100  for controlling the operation of the rotary pulser  10 . 
         [0038]    A preferred embodiment includes a motor  110 , such as a brushless motor, AC motor, DC motor, 3 phase motor etc., which may be monitored and controlled by the controller  100 , the rotary movement of the motor  110  being converted into rotary movement of a windowed restrictor  120  through a rotary gear reduction system  130 , thereby moving the windowed restrictor  120  between an open position (see  FIG. 2b ) and a restricted position (see  FIG. 1   b ). Preferably, the windowed restrictor  120  is movable, as in a rotor. The windowed restrictor  120  functionally cooperates with a fixed housing  140 . Preferably, the fixed housing  140  is static, as in a stator. The fixed housing  140  may include a window  150  forming a flow passage  160 . The windowed restrictor  120  may include a shutter  170 . The shutter  170  may be adapted to substantially block the flow passage  160  when the windowed restrictor  120  is in the restricted position, and substantially open the flow passage  160  when the windowed restrictor  120  is in the open position. 
       Mechanical System 
       [0039]    The rotary gear reduction system  130  is used to translate the torque from the motor  110  into rotary movement of the windowed restrictor  120 , which is preferably a series of gear reductions through gear and pinion or worm gear type gear reductions. The rotary gear reduction system  130  may have a gear reduction generally in the ranges of 10:1, 100:1, or 1000:1. The rotary gear reduction system  130  includes seals which serve to isolate the rotating mechanism from the operating fluids. 
         [0040]    In the embodiment pictured in  FIG. 1A and 1B , the motor  110 , is electrically powered through an electrical connection  90 , by a power source (not shown), such as batteries. When activated, the motor  110  rotates the rotary gear reduction system  130 , causing rotation of an output shaft  180 , which is operatively connected with the windowed restrictor  120 . The output shaft  180  may be supported by bearings  190  and  200 . 
         [0041]    The output shaft  180  is surrounded by lubricating fluid, which must be pressurized against the downhole hydrostatic pressure. As shown, a pressure compensator in the form of a membrane or bellows  210  allows reservoir fluid to substantially equalize the pressure via a port  220 . The pressure compensator may be a membrane, bellows, piston type or other type known in the industry. Seals  230 ,  235 ,  240 , and  245  maintain the integrity of the lubrication chamber during operation and during replacement of the membrane or bellows  210  during maintenance. 
         [0042]    In a preferred embodiment, the construction of the rotary pulser  10  allows a significant number of downhole clogs to be easily cleared, as described below (a clog being an event where debris in the mud may interfere with the windowed restrictor  120  and impede the capability of the windowed restrictor  120  from substantially blocking the window  150  and interfering with the flow passage  160 , therefore reducing the ability of the rotary pulser  10  to produce a distinct or sharp pressure pulse in the mud). The serially interconnected housing design allows simplified and reduced repair time of the tool when necessary. 
         [0043]    The windowed restrictor  120  and/or at least a portion of the fixed housing  140  are preferably composed of a wear resistant material or coated with a wear resistant material such as tungsten carbide or ceramic to increase the efficiency of the tool and to reduce maintenance of the tool, and is preferably replaceable. 
         [0044]    The windowed restrictor  120  preferably comprises a plurality of shutters  170  which correspond to a plurality of windows  150  within the fixed housing  140 . Most preferably, the windowed restrictor  120  includes a set of three shutters  170  spaced apart by 120° to correspond to three windows  150  spaced apart by 120° within the fixed housing  140 . Preferably, the windows  150  provide a relatively large flow are to allow relatively large debris to flow unimpeded through the windows  150  and to reduce velocity/abrasion related “wash” or wear of components. 
         [0045]    Preferably, at least a portion of an edge or edges of the shutter  170  (associated with the windowed restrictor  120 ) and/or at least a portion of an edge or edges of the window  150  (associated with the fixed housing  140 ) may be beveled, chamfered, or tapered, or otherwise channeled to adjust the flow characteristics and/or reduce wear. 
         [0046]    Preferably, the windowed restrictor  120  is located towards a bottom end of the rotary pulser  10 . 
         [0047]    Preferably, the mud flow is generally radially inwards (e.g. from outside to center) to match the natural flow of mud, eliminating the apparatus associated “center out” type pulsers that utilize additional flow channeling to take at least a portion of the mud flow naturally occurring outside the tool, channel the mud into a central portion of a tool, and then pass it through the pulser&#39;s valve in the central portion of the tool (i.e. center-out), and then release the mud back to the annulus around the tool. In the present invention, abrasion or “wash” is reduced due to the much improved flow path. Decreased turbulence may also provide sharper pulse-edge characteristics in the mud&#39;s flow. 
       Operation 
       [0048]    When restriction of mud flow through the rotary pulser  10  (i.e. to generate a positive pressure pulse), the motor  110  will be activated by the controller  100  in the direction to move the windowed restrictor  120  into the restricted position (See  FIG. 1   b ). The current-consuming portion of the circuit is then shut down until a further signal is received from the controller  100 . The lack of current to the motor  110  results in the motor  110  being immovable and therefore acting as a brake to prevent further movement of the windowed restrictor  120  until further activation of the motor  110 . 
         [0049]    Subsequently, when the controller  100  initiates reverse motion by the motor  110  to move the windowed restrictor  120  into the open position (See  FIG. 2   b ). The current-consuming portion of the circuit is then shut down until a further signal is received from the controller  100 . The motor  110  again acts as a brake until further power is applied (by shorting its coils together). 
         [0050]    Use of a rotary motor powering the windowed restrictor in both directions also allows the system to be more responsive than solenoid systems, resulting in a faster data rate with more accurate or precise pulse-edge timing. Experimental results indicate that data rates of 0.25 seconds/pulse are possible with this system, as compared to 0.8 to 1.5 seconds/pulse in solenoid systems. Faster or modulated pulses may be obtained. 
       Flow Detection &amp; Diagnostic Software 
       [0051]    The controller  100  may be programmed to put the rotary pulser  10  in a dormant or power conserving state until a triggering event is detected. For example, the rotary pulser  10  may remain in the dormant or power conserving state until it senses a no flow-to-flow condition without rotation. This combination versus a flow state change with rotation instructs the rotary pulser  10  to create binary weighted flow restrictions, as programmed by the controller  100 . 
         [0052]    The controller  100  may detect the position of the windowed restrictor  120  against relative to a windowed restrictor stop  250 . The windowed restrictor stop  250  may comprise a transverse slot  270  along a at least a portion of the perimeter of the windowed restrictor  120 , the slot  270  corresponding generally to the working angle of rotation of the windowed restrictor  120  as it is movable between a restricted position and an open position. A pin  280  may extend from the fixed housing  140  to engage the slot  270 . 
         [0053]    The windowed restrictor stop  250  allows the controller  100  to sense when the windowed restrictor  120  open position and the windowed restrictor  120  restricted position. In addition, the controller  100  may be programmed to recognize that a certain number of rotations of the motor  110  are needed to move the windowed restrictor  120  between the open position and the restricted position. The controller may also sense rotation of the motor  110  and count rotations and direction of rotation. 
         [0054]    Debris may enter the rotary pulser  10  with the mud, potentially causing jamming or other interference. The controller  100  may be programmed to detect and clear jams from the windowed restrictor  120  and/or the window  150  of the fixed housing  140  (e.g. any partial or complete obstruction of the flow passage  160 ). For example, debris may become lodged between the windowed restrictor  120  and the fixed housing  140 , preventing the full opening or restricting of the flow passage  160 . In such a situation, the controller  100  could detect an increase in current drawn by the motor  110  at an unexpected position of the windowed restrictor  120  (i.e. an increase in current would be expected when the windowed restrictor  120  is at the open position or the restricted position either end, as the windowed restrictor stop  250  is engaged, but would not be expected elsewhere in the working angle of rotation). This mid-travel increase in current draw may be recognized by the controller  100  as debris, and the controller  100  may then enter a clearing program to attempt to automatically clear the debris. In the clearing program, the windowed restrictor  120  may be reciprocated (e.g. slowly or quickly), or it may be repeatedly moved in an opening direction and moved in a closing direction, in order to “chew” on the debris until it is cut through. Due to the power of the motor  110  and the rotary gear reduction system  130 , the windowed restrictor  120  is able to shear right through most types of debris commonly encountered. 
         [0055]    The ability to detect and clear most jams within the tool allows a more robust design of the tool in other respects. For example, as the tool can easily clear particulate matter from the assembly, the tool can be provided with larger and fewer mud ports, and may include reduced amounts of screening. Screening is susceptible to clogging, and so reducing screening leads to longer mean time between operation failure of the device in-hole; and will reduce the velocity of any mud flow through the tool, reducing wear on the bladder and other parts. Further, the removal of several previously necessary components (such as the return spring, transformer, and solenoid and related electronics) contributes to a tool of smaller size (in both length and diameter) that is more versatile in a variety of situations. For example, embodiments with outside diameter less than 1⅜″ (approaching 1″) or length less than four feet have been achieved, although these dimensions are not by way of limitation, but by example only. 
         [0056]    The rotary pulser  10  has generally been described in creating a positive pressure pulse, that is, moving the windowed restrictor  120  into the restricted position to create an increase in pressure. The rotary pulser  10  may also be used to create a negative pressure pulse, for example by moving the windowed restrictor  120  from the restricted position (or a partially restricted position) to the open position, to create a decrease in pressure. The rotary pulser  10  may also be used to create combinations of positive and negative pressure pulses. 
       Landing Sleeve (Muleshoe) 
       [0057]    The rotary pulser  10  of the present invention may be received in a landing sleeve  260 . The landing sleeve  260  may be compatible with both a vertical tool (or other “event trigger” type monitoring and reporting tools) and real-time MWD tools which allows the rotary pulser  10  of the present invention to be retrieved from the landing sleeve  260  and replaced in the landing sleeve  260  with a real-time MWD tool without having to trip the pipe out of the hole. This feature allows drilling with an event trigger type tool, such as a vertical tool, at a cost savings over the equipment and operations cost of a real-time MWD tool. In the event that the drilling operations run into unexpected circumstances (e.g. a vertical tool detects a vertical deviation outside the parameters and reports that deviation to surface via the rotary pulser  10 ), the event type trigger type tool can be retrieved from the landing sleeve  260  and a real-time MWD tool seated in the landing sleeve  260  to fully assess the situation and provide telemetry to surface, again via the rotary pulser  10 , to allow correction, e.g. through directional drilling. 
         [0058]    In addition, the rotary pulser  10  of the present invention is retrievable from the landing sleeve  260  and reseatable in the landing sleeve  260 . 
         [0059]    Custom software also has the ability to track downhole conditions, and also uses a sensor to detect mud flow. When mud flow is detected, a signal is sent to the Directional Module Unit (not shown), to activate the overall system. The system also has the ability to time stamp events such as start or end of mud flow, incomplete cycles or system errors, low voltages, current, and the like, as well as accumulated run-time, number of pulses, number of errors, running totals of rotations or motor pulses. Wires or conductors may also be easily passed by the pulser section to service additional near-bit sensors or other devices. The software that detects the mud flow can be configured for different time delays to enable it to operate under a larger variety of downhole drilling conditions than its predecessors. The mud flow detection capability can also be used to calibrate or confirm the open position and/or the closed position of the windowed restrictor  120 . 
         [0060]    In addition, a user may monitor such data as well as any downhole sensors using a user interface attachable to the tool. Such sensors may include pressure or temperature sensors, rotation step-counters, travel or depth sensors, current levels, battery voltage, or timers. The user could monitor each component of the actuator to determine when the tool must be removed from downhole for repair. A user may, in turn, program an activity to cause an action or correction in response to a sensed event. 
         [0061]    The present invention has been described as being applicable to measurement while drilling (MWD) systems. As used herein, that includes, but is not limited to, any drilling or well servicing operations involving sending a signal from downhole to surface through the working fluid, and includes “triggering event” type monitoring tools (e.g. as a vertical tool monitoring declination and only reporting in the event of a triggering event, e.g. vertical angle outside of parameters) and includes “polling” type tools that can be polled to send back a reading (e.g. a vertical tool that monitors declination, but only reports to surface when sent a polling signal, such as no flow-to-flow condition without rotation, or at a polling time interval) and includes real-time MWD tools (e.g. that provide continuous or nearly continuous reporting of parameters to surface). 
         [0062]    The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.