Systems and methods for controlling a voltage multiplier ladder for optimal efficiency and minimal component stress

A system comprises a voltage multiplier ladder, a driver that provides an input voltage to the voltage multiplier ladder, and a controller that regulates the driver such that a voltage stress in the ladder is evenly distributed and do not exceed a maximum allowable stress and meanwhile the ladder is operating at an optimal efficiency.

BACKGROUND

This disclosure relates to controlling a frequency of an input voltage to a voltage multiplier ladder to achieve a more optimal operation of the voltage multiplier ladder.

Electronic radiation generators are used in many downhole tools used for well-logging. The electronic radiation generators may use high-voltage sources to cause charged particles to accelerate toward a target. When the charged particles strike the target, radiation such as neutrons or x-rays may be generated. The radiation may exit the downhole tool and penetrate into a geological formation adjacent a wellbore where the downhole tool is located. Measurements of the radiation that returns to the downhole tool may provide an indication of where hydrocarbon resources may be located, as well as other characteristics of the geology of the formation.

Owing to the constraints of a downhole tool, some high-voltage power supplies to the radiation generators may use a modified Cockcroft-Walton voltage multiplier ladder. In one example, the voltage multiplier ladder may be operated at a constant frequency that is expected to be optimal given the components of the voltage multiplier ladder. Yet the properties of the components of the voltage multiplier ladder may vary substantially as the downhole tool is subjected to the various high temperatures and high pressures that may arise in the well. This may cause the voltage multiplier ladder component characteristics to change, leading to a less-than-optimal operation of the voltage multiplier ladder.

SUMMARY

In some embodiments, a system is disclosed that comprises a voltage multiplier ladder having a plurality of multiplier stages N including an input stage, an output stage, and an intermediate point stage n between the input and output stage. At least one loading coil is disposed along the voltage multiple ladder. The system further comprises a driver that is configured to provide an input voltage having an input voltage frequency and an input voltage magnitude to the voltage multiplier ladder. The system further comprises a controller that is configured to regulate the driver such that a voltage stress in the ladder is evenly distributed and do not exceed a maximum allowable stress.

In some embodiments, a system is disclosed that comprises a voltage multiplier ladder comprising a plurality of voltage multiplication stages and at least one loading coil. The system further comprises a driver that is configured to provide an input voltage to the voltage multiplier ladder, and a controller that is configured to receive a measured value from an intermediate position along the voltage multiplier ladder and adjust a parameter of the input voltage in accordance with the received measurement.

DETAILED DESCRIPTION

A modified Cockcroft-Walton voltage multiplier ladder may be used to supply high voltage (e.g., 60 kv or higher for a typical neutron generator, 200 kV or higher for a typical X-ray generator) to an electronic radiation generator in a downhole tool. The radiation generator may use the high voltage from the voltage multiplier ladder to accelerate charged particles toward a target material. When the charged particles strike the target material, the interaction with the target material may produce radiation such as x-rays or neutrons. The radiation may be used by a downhole tool in a well to assess properties of a well, which may indicate the presence or absence of hydrocarbons at particular locations in the geological formation that surrounds the well. The downhole tool may also use the radiation to identify many other properties of the geological formation, such as porosity, mineralogy, density, and so forth.

The voltage multiplier ladder may be subject to changes in temperature and pressure as the downhole tool is moved through the well. These changes in temperature and pressure may impact the electrical characteristics of the voltage multiplier ladder. For instance, the capacitance, loading coil inductance, and parasitic capacitance of the voltage multiplier ladder may vary. To account for such changes in the electrical characteristics of the voltage multiplier ladder, a controller may determine an input voltage frequency that causes the voltage efficiency not to be held to a particular constant value, but rather to adapt to match a voltage efficiency at one of the internal stages of the voltage multiplier ladder. This adaptive control may allow for improved voltage and frequency efficiency that may produce a correspondingly improved voltage distribution over the voltage multiplier ladder, which may reduce the voltage stress on components of the voltage multiplier ladder. This may increase the operational reliability of the voltage multiplier ladder.

In addition, by measuring the voltage efficiency across an internal stage of the voltage multiplier ladder, a system health parameter that is indicative of system problems may be identified. This system health parameter may allow the voltage multiplier ladder to be indirectly monitored for temperature and voltage stress on components of the voltage multiplier ladder. In addition, the remaining operational time of failure could be predicted from this measurement and used to provide a predictive, proactive maintenance scheduling system. Additionally, the diagnostic health parameter may be used to adjust the operation of the high voltage ladder, e.g. lowering the per stage voltage, in order to prevent a total failure during a job by maintaining a lower but still adequate radiation output of the device.

With this in mind,FIG. 1illustrates a well-logging system10that may employ the systems and methods of this disclosure. The well-logging system10may be used to convey a downhole tool12that includes such scintillator detectors through a geological formation14via a wellbore16. The downhole tool12may be conveyed on a cable18via a logging winch system20. Although the logging winch system20is schematically shown inFIG. 1as a mobile logging winch system carried by a truck, the logging winch system20may be substantially fixed (e.g., a long-term installation that is substantially permanent or modular). Any suitable cable18for well logging may be used. The cable18may be spooled and unspooled on a drum22.

Although the downhole tool12is described as a wireline downhole tool, it should be appreciated that any suitable conveyance may be used. For example, the downhole tool12may instead be conveyed as a logging-while-drilling (LWD) tool as part of a bottom hole assembly (BHA) of a drill string, conveyed on a slickline or via coiled tubing, and so forth. For the purposes of this disclosure, the downhole tool12may be any suitable measurement tool that generates radiation using an electronic radiation generator powered by a voltage multiplier ladder controlled in the manner of this disclosure. The downhole tool12may provide radiation measurements (e.g., counts of detected gamma-rays or x-rays) to a data processing system24via any suitable telemetry (e.g., via electrical signals pulsed through the geological formation14or via mud pulse telemetry). The data processing system24may process the radiation measurements to identify certain properties of the wellbore16(e.g., porosity, permeability, relative proportions of water and hydrocarbons, and so forth) that may be otherwise indiscernible by a human operator.

By way of example, the data processing system24may include a processor, which may execute instructions stored in memory and/or storage. As such, the memory and/or the storage of the data processing system24may be any suitable article of manufacture that can store the instructions. The memory and/or the storage may be ROM memory, random-access memory (RAM), flash memory, an optical storage medium, or a hard disk drive, to name a few examples. A display, which may be any suitable electronic display, may provide a visualization, a well log, or other indication of properties of the wellbore16.

One example of the downhole tool12is shown inFIG. 2. The downhole tool12may include a radiation source26to emit radiation28into the geological formation14. The radiation source includes an electronic radiation generator, such as an electronic x-ray generator or an electronic neutron-generator. The radiation source26emits radiation28out of the downhole tool12. For example, the radiation28may enter the geological formation14, where it may scatter or collide with atoms of the geological formation14to generate other radiation that also may scatter. Some of the radiation28or radiation that results from interactions with the radiation28in the geological formation14may scatter and return to the downhole tool12, to be detected by a radiation detector30. In general, the radiation detector30may detect when ionizing radiation enters the downhole tool12and generate an electrical signal, such as a count rate of detected radiation or spectrum of detected radiation that may provide an indication of characteristics of the wellbore16or the geological formation14.

One example of the radiation generators shown inFIG. 3. In the illustrated example, the radiation generator is an X-ray generator that includes an X-ray tube100that is grounded at a target (i.e., anode) end102, although floating target configurations may also be used in some embodiments. The X-ray tube100further illustratively includes a cathode103on the opposite end of the tube from the target end102. The cathode103is coupled to a voltage multiplier ladder104that includes mid-stage loading coils105a,105b. In some embodiments, the voltage multiplier ladder104may take a form as described by U.S. Published Application No. 2015/0055747, “Energy Radiation Generator With Bi-Polar Voltage Ladder,” and U.S. Published Application No. 2015/0055748, “Energy Radiation Generator With Uni-Polar Voltage Ladder,” which are incorporated by reference in its entirety for all purposes. The voltage multiplier ladder104may be coupled to a transformer106(shown inFIG. 5). The X-ray tube100, voltage multiplier ladder104, and the transformer106are enclosed within one or more insulating sleeves108(e.g., PFA), which in turn is enclosed within a generator housing110. An insulating gas may be inserted in an inner space117within the generator housing110. The voltage multiplier ladder104further includes an input116to receive an AC voltage. The grounded target configuration shown schematically inFIG. 3provides a simplification in the mechanical design and assembly, which may also help in maintaining mechanical stability of the target, maintaining thermal management of the target, as well as the radiation exposure of the insulating material108.

The cathode103releases electrons in response to exposure to heat, although in some embodiments “cold” cathodes (e.g., Carbon nanotubes, etc.) may also be used. As shown inFIG. 4, the voltage multiplier ladder104applies a voltage to the cathode103, and the introduction of current heats the cathode103and causes it to release electrons. A grid204is optionally provided to move electrons released from the cathode103toward an electron-accelerating section206. The accelerating section206speeds electrons toward a target208. Upon collision with the target208, X-rays are generated which may be used in various applications, such as downhole well-logging measurements.

A basic unipolar voltage multiplier ladder configuration may be inadequate for achieving very high voltages if the required voltage is on the order of hundreds of KV) within the space confines dictated for downhole use. That is, given the space constraints of the downhole tool pad or sonde housing in which a voltage multiplier ladder is deployed, it may be difficult to achieve desired voltage levels with the basic unipolar configuration. More particularly, this is due to voltage efficiency, which may be defined as the ratio of the output voltage and the input voltage multiplied by the number of stages. For example, a 30- or 40-stage basic unipolar voltage multiplier ladder may have a voltage efficiency of about 40% to 60%. For an input voltage of 15 kV, which is roughly the maximum voltage rating for most components currently available commercially (e.g., capacitors and diodes) at sizes appropriate for downhole tools, the output voltage may be plotted against the number of stages. Cascading stages reduces the voltage efficiency. The output voltage converges to a given value, which is around 250 kV. Adding a relatively large number of stages may therefore not provide desired high operating voltages. The inability of such configurations to generate high voltages may further be attributed to the stray capacitance across the stages.

To generate higher voltages (e.g., of 400 kV or more) using a unipolar ladder, one or more loading coils may be positioned at appropriate intermediate locations or positions in the voltage multiplier ladder. In a particular embodiment illustrated inFIG. 5, the first and second coils105a,105bmay be positioned approximately two-fifths and four-fifths stage positions, respectively, down the length of the voltage multiplier ladder104. The number of loading coils and the position of the loading coils may vary depending on the application. The first coil105aand the second coil105bmay be substantially identical to one another, respectively positioned at about ⅖thand ⅘thalong the length of the voltage multiplier ladder104. In this configuration an optimal voltage efficiency Foptmay be derived using the C, Csand N by using the square root of the ratio of the Csand C multiplied by the N, such as:

Fopt=tanh(2⁢N⁢CSC)2⁢N⁢CSC,(1)
where C is the voltage multiplier ladder series capacitor, Csis the stray capacitance, and N is the number of voltage multiplication stages of the voltage multiplier ladder104. If the optimal voltage efficiency were treated as a constant value, the optimal frequency foptof the voltage multiplier ladder may be given by the equation:

fopt=12⁢π⁢1L⁢CCS·1tanh(25⁢N⁢CSC),(2)
where C is the voltage multiplier ladder series capacitor, Csis the parasitic capacitance between the AC and DC leg of the voltage multiplier ladder, and N is the number of voltage multiplication stages of the voltage multiplier ladder104.

It should be noted that in the example illustrated inFIG. 5, the capacitor of each stage is depicted as being the same for the entire length of the voltage multiplier ladder. However, this example is provided for the simplicity of illustration only. Variations to this design are possible. For example, different capacitors and/or diodes with different ratings can be used in different stages of the voltage multiplier ladder. A “tapered” configuration is also possible, such as having higher rated capacitors and/or diodes at the beginning of the voltage multiplier ladder and lower rated capacitors and/or diodes towards the end of the voltage multiplier ladder. The tapering can be gradual (from stage to stage), or stepped (from a group of stages to a next group of stages), or a combination thereof. All such variations can be modeled and/or mathematically calculated based on variations to the equations disclosed herein.

As noted above, however, the electrical characteristics of the voltage multiplier ladder104may vary as the downhole tool12moves through the wellbore16. To achieve optimal performance, the voltage multiplier ladder104shown inFIG. 5is regulated by a controller220that adjusts the operating frequency and a magnitude of an input voltage supplied by a high-voltage driver222into the voltage multiplier ladder104via the transformer106. The controller220may include, for example, an application-specific integrated circuit (ASIC); a programmable logic device, such as a field-programmable gate array (FPGA); a processor and memory storing instructions to carrying out a method of this disclosure; some combination of these; or the like.

Before discussing the operation of the controller220shown inFIG. 5, certain characteristics of the operation of the voltage multiplier ladder104will first be described. Turning toFIG. 6, for example, a plot240illustrates a relationship between voltage efficiency F (ordinate242) and input voltage frequency f (244). A resonant curve246includes two resonant peaks of a first peak value F1occurring at a frequency f1and a second peak value F2occurring at a frequency f2, due to the two loading coils105aand105b. To avoid overstressing the components of the voltage multiplier ladder, it is advisable to avoid driving the ladder at a resonating frequency. Stated in another way, the voltage multiplier ladder104may be driven at an optimal frequency foptand optimal voltage efficiency Fopt, out of resonance of the voltage multiplier ladder. For example, in the illustrated embodiment inFIG. 6, the optimal frequency foptis chosen at a frequency above the second resonant peak occurring at the frequency f2. In this embodiment, the optimal voltage efficiency is about 90% and it occurs at the optimal frequency fopt. Other input frequency and voltage efficiency can also be chosen depending on the specific condition an operator would consider as optimal.

FIG. 7is a plot260of voltage V across the ladder capacitor C (ordinate262) over the stages (abscissa264) of the voltage multiplier ladder104when driven at the optimal frequency foptand the optimal voltage efficiency Fopt. V1represents the maximum voltage and V2represents the minimum voltage across a capacitor C in the ladder. When the voltage multiplier ladder104ofFIG. 5is driven at the optimal frequency and voltage efficiency, the voltage across each element of the voltage multiplier ladder at each stage is well defined and optimal as shown by a curve266. This optimal voltage distribution ensures that the voltage across each capacitor C is well defined, fluctuating between a minimum voltage V1and a maximum voltage V2. The maximum voltage V2is kept at or below the input voltage Vinapplied to the first stage of the voltage multiplier ladder104. This way, an operator can easily control the stress level at each stage to ensure the voltage does not exceed the voltage ratings of the voltage multiplier ladder capacitors C and loading coils105a,105b. This is particularly valuable for systems in restricted spaces, such as radiation generators used in downhole logging tools, which, owing to size limitations, are restricted in space and the voltage margin may be relatively small in relation to the input voltage Vin.

The example shown inFIGS. 6 and 7is based on an assumption of a constant optimal voltage efficiency can be relied upon during an entire operation. In reality, however, the assumed optimal voltage efficiency, as shown in the equations above, may actually change with variations in the voltage multiplier ladder capacitance C, the inductance L of the loading coils105a,105band the parasitic capacitance Cs. These components, in particular the voltage multiplier ladder capacitances and parasitic capacitances, may vary with temperature and applied voltage. As such, when a system such as a downhole tool12is in the wellbore16, the optimal voltage efficiency may, in fact, be constantly changing. The amount of variation in the components depends on the components and insulating materials that are used, but these variations can be quite substantial. Capacitance variations of 30% over the operating temperature range and variations of up to 50% over the applied voltage may be seen. Variations in the parasitic capacitance, too, may affect the optimal voltage efficiency if potting material or other insulating materials are used that have a strong temperature dependency. Other variations, such as diode current leakage and parasitic resistance changes, may also have indirect impacts on the voltage efficiency.

Some of these changes are shown by a plot280ofFIG. 8, which shows simulation results of voltage (ordinate282) per stage (abscissa284) for an optimal frequency regulation of an N-stage voltage multiplier ladder104with varying capacitances. In particular, the plot280shows the effect of a parasitic capacitance C1at 100% (curve286), C2at 55% (curve288), and C3at 27% (curve290) when the voltage multiplier ladder104is driven at a different optimal frequency foptthat corresponds to a different optimal voltage efficiency Fopt. As the capacitance of the voltage multiplier ladder104decreases, the voltage sag between stages also decreases. At C1, the voltage sags to approximately 90% of the maximum voltage, at C2the voltage sags to approximately 82% and at C3the voltage sags down to 67%. The optimal frequency foptincreases throughout this capacitance decrease also increases by about 10%. The optimal voltage efficiency due to the capacitance drop ensures that Fopt1>Fopt2>Fopt3. Similar effect can be seen if other variables, such as ladder capacitance, diode leak current, etc. are changed.

Ignoring these variations in operation by driving the voltage multiplier ladder104based on a fixed optimal frequency foptwith a fixed optimal voltage efficiency Foptmay produce an uneven voltage distribution across the voltage multiplier ladder104, overstressing certain stage or stages of the ladder which may eventually lead to malfunctioning or breakdown of the ladder. An example is illustrated inFIG. 9, where a plot300is depicted to plot voltage V (ordinate302) over stage N (abscissa304) of a voltage multiplier ladder104. The plot300shows the effect of differences in parasitic capacitance—C1at 100% (curve306), C2at 55% (curve308), and C3at 27% (curve310)—when the voltage multiplier ladder104is driven to an optimal frequency foptthat assumes a constant optimal voltage efficiency Fopt1. In these three cases of ladder capacitance C1, C2and C3, the output voltage is equal but the voltage distribution across the capacitors C of the voltage multiplier ladder104is sub-optimal. The larger sagging stage voltage in C2and C3indicate a downward shift in the voltage efficiency Foptwhich would not be optimal if the optimal frequency foptwere assumed to be constant. Under such conditions, the resulting regulation would cause the voltage per stage at higher stages to increase above the input voltage, reaching, in this simulation example, 30% above the initial voltage at stage 4 N/5 for ladder capacitance C3. This suboptimal voltage distribution leads to overstress on the voltage multiplier ladder104capacitors, which will decrease the reliability of the voltage multiplier ladder104by appreciable amounts.

With reference once again toFIG. 5, the controller220may avoid such a suboptimal voltage distribution by regulating the optimal voltage efficiency Foptin a control loop. In particular, test circuitry224,226, and228may measure the voltage at certain stages of the voltage multiplier ladder104. For example, a test circuitry224may measure the voltage at the input stage of the voltage multiplier ladder104. One or more test circuitry226may measure the voltage at one or more stage n of the voltage multiplier ladder104. Further, a test circuitry228may measure the voltage at the output stage of the voltage multiplier ladder104. Although the test circuitry226is shown once in the example ofFIG. 5, other examples may include many instances of the test circuitry226at various stages n throughout the total N stages of the voltage multiplier ladder104between the input stage and the output stage of the voltage multiplier ladder104. The stage n that is selected for testing may be any suitable interim stage that is used to regulate the optimal voltage efficiency Foptin the control loop. In some embodiments, the test circuitry226is located at a stage n that is adjacent to a loading coil105aor105b. In some embodiments, the test circuitry226is electrically coupled to a loading coil105aor105b.

In an example illustrated inFIG. 5, the test circuitry224,226, and228may use a bleed resistor (R) and a current (I) sensor to obtain voltage (e.g., V=IR), though any other suitable circuitry may be used. The efficiency at any stage n or stages in the voltage multiplier ladder104may be measured with the respective bleed resistor Rnand an appropriate measurement circuit for current sensing to determine the bleed current In, along with the first stage voltage Vin, using the equation below:

For example, the controller220may receive three inputs: measurements that enable determination of a voltage input signal Vin, an output voltage efficiency FNrepresenting the voltage efficiency at the output stage of the voltage multiplier ladder104, and an intermediate stage voltage efficiency Fnrepresenting the voltage efficiency at a stage n of the voltage multiplier ladder104. Using measurements from the test circuitry224, the voltage input signal Vinmay be identified according to the following relationship:
VN=IIN×RIN(4),
where RINrepresents a resistance of the bleed resistor of the test circuitry224and IINrepresents a current measured at the test circuitry224.

Using measurements from the test circuitry224and228, the output voltage efficiency FNmay be identified according to the following relationship:
FN=IOUT×ROUT/(N×VIN)  (5),
where ROUTis the resistance of the bleed resistor of the test circuitry228and the IOUTis the current measured by the test circuitry228, N is the number of the last stage of the voltage multiplier ladder104, and VINis the input voltage measured at the test circuitry224according to Equation 4.

Using measurements from the test circuitry224and226, the intermediate stage voltage efficiency Fnmay be identified according to the following relationship:
Fn=In×Rn/(n×VIN)  (6),
where Rnis the resistance of the bleed resistor of the test circuitry226, Inis the current measured by the test circuitry226, n is the number of the stage of the voltage multiplier ladder104where the test circuitry226is located, and Vinis the input voltage measured at the test circuitry224according to Equation 4. It should be appreciated that the values from Equations 4, 5, and 6 may be calculated by the controller220or may be provided as inputs (e.g., via some prior calculation circuitry that provides the outputs of Equations 4, 5, and 6 as inputs into the controller220).

Moreover, it should be appreciated that in the example illustrated herein, the capacitor and diode of each stage are assumed to be the same for each stage of the voltage multiplier ladder. However, this example is provided for the simplicity of illustration only. Variations to this design are possible. For example, as discussed above, different capacitors and/or diodes with different ratings and properties can be used in different stages of the voltage multiplier ladder. A “tapered” configuration is also possible, such as having higher rated capacitors and/or diodes at the beginning of the voltage multiplier ladder and lower rated capacitors and/or diodes towards the end of the voltage multiplier ladder. The tapering can be gradual (from stage to stage), or stepped (from a group of stages to a next group of stages), or a combination thereof. All such variations can be modeled or mathematically calculated based on variations to the equations disclosed herein.

The control loop of the controller220thus can be used to regulate the driver222to ensure an optimal efficiency with minimal component stress across the voltage multiplier ladder104ladder. In one particular embodiment, the voltage efficiency at a given intermediate stage n is compared with the voltage efficiency FNat the last stage N such that Fnhas a pre-determined functional relationship with FN. If Fndeviate from FNsignificantly, an instructional signal can be generated by the controller220and delivered to the HV driver222so that a different voltage frequency (and/or a magnitude of input voltage) can be adopted to bring Fnback to the pre-determined functional relationship with FN. This allows for dynamic control over the voltage multiplier ladder104despite changes to the optimal due to variations in components, parasitic characteristics, and environmental conditions such as temperature. Besides the optimal frequency, the controller220may also regulate the desired output voltage, the input voltage to the driver222, and/or the parameters of transformer106, etc.

This exemplary embodiment is further illustrated inFIG. 10. Plot320depicts simulation results of voltage (ordinate322) over stage (abscissa324) where the efficiency is calculated at each stage n in the voltage multiplier ladder104from 1 to N when the voltage multiplier ladder104is operating at the optimal frequency fopt1(curve326), fopt2(curve328), or fopt3(curve330) for the corresponding capacitance C1, C2and C3of the voltage multiplier ladder104. When stage n is greater than N/5, the measured efficiency Fn≈FNfor stages n thereafter with a very low standard deviation in the data. In fact, as the frequency varies away from the optimal value, the measured efficiencies trend higher or lower. The regulation controller220thus may vary the drive frequency f to obtain Fnto be substantially the same as FN. The frequency at this point is foptand the efficiency is Fopt. The input drive voltage V may be increased or decreased until the desired output voltage is obtained.

This exemplary embodiment can be further illustrated by a flowchart350ofFIG. 11, the controller220may generate an input voltage at an input frequency f and a voltage magnitude V (block352). The voltage multiplier ladder104may be driven at the input voltage (block354). One or more of the test circuitry224,226, and228may obtain measurements that allow certain values of voltage efficiency such as Fnand FNto be identified as discussed above (block356).

If the values of voltage efficiency Fnand FNare substantially equal (decision block358), the controller220may adjust the input voltage frequency according to any suitable method (block360) and blocks352,354,356, and358may repeat on occasion, periodically, or continuously. For example, the controller220at block360may increase or decrease the input frequency and observe whether the voltages efficiencies Fnand FNget closer or farther apart from one another, and aim to minimize a difference between the voltage efficiencies Fnand FN.

When the values of voltage efficiency Fnand FNare substantially equal (decision block358), this may suggest that the optimal frequency foptand optimal voltage efficiency Foptgiven the current conditions of the voltage multiplier ladder104have been achieved. Accordingly, the controller220may select the magnitude of the input voltage V to achieve a desired output voltage Vout. If the output voltage Voutis the desired value (decision block362), the controller220may not change the magnitude of the input voltage V and blocks352,354,356,358, and362may repeat on occasion, periodically, or continuously, until a change occurs that causes the values of voltage efficiency Fnand FNnot to be substantially equal (decision block358) or the desired output voltage Vons not to be achieved (block362). For example, if the output voltage Voutis not at the desired value (decision block362), the controller220may adjust the magnitude of the input voltage V (block364) until the desired output voltage Voutis reached.

It should be appreciated that the above example is provided for illustration purpose only. Variations to the illustrated embodiments can be devised without departing from the inventions disclosed herein. For example, instead of providing the test circuitry228at the last stage N of the voltage multiplier ladder104, one may connect the test circuitry228at the second to the last stage, i.e. stage N−1, of the voltage multiplier ladder104, or stage N−2 of the voltage multiplier ladder104, and so on. In such an event, the operation in block358may be no longer whether Fnis substantially the same as FN, but rather whether Fnis substantially within a functional relationship with FN-1or FN-2, and so on. One example of such functional relationship is shown in plot320ofFIG. 10. Other functional relationship may also be used depending on the specific condition and designs of a particular ladder.

Similarly, one may appreciate that the measurement at the end of the voltage multiplier ladder104may be completely omitted if a functional relationship between the voltage frequency and the stage can be predetermined either experimentally or by simulation, such as the plot320inFIG. 10. In such an event, the measurement taken at an intermediate stage n can be compared to a pre-determined value. If there is a substantial deviation from the pre-determined value, the controller220may adjust one or more parameters of the HV driver, such as a voltage frequency, an amplitude of the input voltage, or a combination thereof. Therefore, the performance of the voltage multiplier ladder104can be dynamically regulated so that it may operate at an optimal efficiency with minimal possibility to overstress the components of the voltage multiplier ladder104.

Sometimes the voltage multiplier ladder104is folded to reduce length. The turning point of the voltage multiplier ladder104may be used as tap point for measurement. A few exemplary embodiments in this respect can be found in co-pending, co-assigned, and concurrently filed patent application titled “Collocation of Radiation Generator Components for Limited-Space Devices”, by Jani Reijonen, the entire content of which is incorporated herein by reference.

The measurements used to regulate the voltage multiplier ladder104(e.g., in accordance with the flowchart350ofFIG. 11) may also be used to ascertain a relative health status of the voltage multiplier ladder104. For instance, as shown by a flowchart370ofFIG. 12, the controller220may generate an input voltage at an input frequency f and a voltage magnitude V (block372). The voltage multiplier ladder104may be driven at the input voltage (block374). The test circuitry224,226, and228may obtain measurements that allow the values such as the voltage efficiency Fnto be substantially the same as discussed above (block376). If the voltage efficiency Fnis identified as less than some threshold value (decision block378), this may indicate a possible impending failure condition of the voltage multiplier ladder104. As such, the controller220may provide an indication message to the data processing system24at the surface or elsewhere within the downhole tool12(block380). The threshold may be any suitable value based on experimental or simulated failure conditions of the voltage multiplier ladder104. The indication message output based on the identification below-threshold voltage efficiency Fnmay indicate and, thus, may allow human operators to proactively identify, when the voltage multiplier ladder104may benefit from maintenance to prevent a more complete failure. These measurements may also allow for a determination of an expected lifetime of the voltage multiplier ladder104(e.g., by identifying a likely amount of remaining operational lifespan based on the voltage efficiency Fn).

In another example, shown by a flowchart390ofFIG. 13, the controller220may store a record of the occurrences where the controller220changes the input voltage frequency f during regulation of the voltage multiplier ladder104(e.g., as may occur during regulation based on the flowchart350ofFIG. 11). These changes in input voltage frequency f imply changes in the optimal frequency foptof the voltage multiplier ladder104. Likewise, changes in frequency foptof the voltage multiplier ladder104imply that the voltage multiplier ladder104is undergoing some kind of stress that is changing the electrical properties of its components. The controller220or the data processing system24at the surface may observe the rate of these changes (block392). If the rate of these changes exceeds some threshold (decision block394), this may indicate a possible impending failure condition of the voltage multiplier ladder104. As such, the controller220or the data processing system24may provide an indication message that identifies the state of the health of the voltage multiplier ladder104(block396) and/or adjust the operating parameters automatically or after operator verification to ensure that ladder can be operated at a lower stress so that a job may be finished without the complete failure of the ladder and rather with potentially less precise results. The threshold may be any suitable value based on experimental or simulated failure conditions of the voltage multiplier ladder104linked to the number of changes of optimal input frequency fopt. The indication message output may indicate and, thus, may allow human operators to proactively identify, when the voltage multiplier ladder104may benefit from maintenance to prevent a more complete failure. These measurements may also allow for a determination of an expected lifetime of the voltage multiplier ladder104(e.g., by identifying a likely amount of remaining operational lifespan based on the number of changes in the optimal input frequency fopt).

Thus, technical effects of the disclosure include, among other things, that the optimal frequency foptmay be maintained regardless of changes to the value of the optimal voltage efficiency Fopt. The optimal voltage efficiency Foptcan change due to the voltage multiplier ladder capacitance, loading coil inductance, or parasitic capacitance changing under voltage and temperature stress. Maintaining optimal voltage and frequency efficiency may increase the likelihood of an optimal voltage distribution ensuring that the voltage stresses on the voltage multiplier ladder capacitors are reduced, increasing operational reliability of the voltage multiplier ladder104.

Furthermore, measuring the voltage efficiency Fngives a system health parameter that is indicative of system problems and a method to indirectly monitor the temperature and voltage stress on components. This parameter may be used to estimate remaining operational time to failure and be used to enable a predictive, proactive maintenance scheduling system.

Although the above exemplified embodiments are described in the context of voltage efficiency F, one should readily appreciate that other variations are possible with the benefit of the current disclosure. For example, instead of or in addition to calculating a voltage efficiency F based on measurements from certain position(s) of the voltage multiplier ladder104, one may simply use a directly measured value at such position(s) without any calculation. Examples of such measured values may include, but are not limited to, a voltage, a current, a resistance, a frequency, or a combination thereof. Such measured value may be compared to a threshold. If the measured value differs from the threshold, or otherwise deviate from a pre-determined functional relationship from the threshold, the controller220would adjust one or more parameters of the HV driver, so that the voltage stress in the ladder is evenly distributed and do not exceed a maximum allowable stress, and at the same time the ladder is operating at an optimal efficiency.