Patent Description:
This invention pertains generally to technology for measuring the position of the core in a differential variable reluctance transducer. More particularly, the invented technology can be deployed in a caliper to accurately and stably measure the caliper position. For example, the technology may be used in a multi-arm caliper designed to measure the diameter of a wellbore or to measure the corrosion, scaling, or pitting of tubulars (e.g., casing in a wellbore).

Caliper tools are often used in the oil-and-gas industry to measure characteristics of the wellbore environment. For example, a multi-arm caliper logging tool may be positioned in a wellbore (e.g., via wireline) to measure the diameter of the wellbore at various depths in the wellbore. The diameter measurement may be taken at various axes to provide a diameter profile. When positioned in a tubular, such as casing in a wellbore, the caliper tool provides information about the condition of the inner wall of the tubular. An overview of caliper tools is provided in Applicant's <CIT>, the entirety of which patent is incorporated herein by reference.

One way to determine the radial position of a caliper arm (or "finger") in a caliper logging tool (the distance of a point on the arm from the longitudinal axis defined by the caliper tool) is to mechanically link the position of the arm to the position of a core relative to a transformer or inductor (e.g., in a differential variable reluctance transducer). The electrical signal provided by the transformer/inductor transducer is a function of the position of the core and thus is a function of the position of the arm. An overview of such an approach is provided in <CIT>.

The circumferential resolution of a caliper tool may be increased by increasing the number of caliper arms. For example, a <NUM>-arm caliper tool has a greater circumferential resolution than a <NUM>-arm caliper tool which has a greater circumferential resolution than a <NUM>-arm caliper tool. This increased circumferential resolution comes at a cost. Namely, more caliper arms means more data competing for limited processing resources. This manifests in a logging tool as slower logging speeds to allow time to capture data at the various depths within the borehole without degrading depth resolution. Stated another way, to maintain vertical (depth) resolution while increasing circumferential resolution without increasing processing capacity (and, thereby, the size and power requirements of the tool), the logging process will require more time. Accordingly, there is a need for a caliper-position sensor that acquires high-resolution caliper-sensor information quickly so as to reduce the need to increase processing capacity in order to increase circumferential resolution.

US Patent Publication <CIT>) and <CIT>) are useful for understanding the invention.

The present invention is directed to technology to satisfy the need for high-resolution (circumferential, radial, and depth), high-speed caliper measurements with small size and power-consumption constraints.

These and other features, aspects, and advantages of the present invention will be better understood with reference to the following description, appended claims, and accompanying drawings where:.

In the summary above, and in the description below, reference is made to particular features of the invention in the context of exemplary embodiments of the invention. The features are described in the context of the exemplary embodiments to facilitate understanding. But the invention is not limited to the exemplary embodiments. And the features are not limited to the embodiments by which they are described. The invention provides a number of inventive features which can be combined in many ways, and the invention can be embodied in a wide variety of contexts. Unless expressly set forth as an essential feature of the invention, a feature of a particular embodiment should not be read into the claims unless expressly recited in a claim.

Except as explicitly defined otherwise, the words and phrases used herein, including terms used in the claims, carry the same meaning they carry to one of ordinary skill in the art as ordinarily used in the art.

Because one of ordinary skill in the art may best understand the structure of the invention by the function of various structural features of the invention, certain structural features may be explained or claimed with reference to the function of a feature. Unless used in the context of describing or claiming a particular inventive function (e.g., a process), reference to the function of a structural feature refers to the capability of the structural feature, not to an instance of use of the invention.

Except as otherwise stated herein or as is otherwise clear from context, the inventive methods comprising or consisting of more than one step may be carried out without concern for the order of the steps.

The terms "comprising," "comprises," "including," "includes," "having," "haves," and their grammatical equivalents are used herein to mean that other components or steps are optionally present. For example, an article comprising A, B, and C includes an article having only A, B, and C as well as articles having A, B, C, and other components. And a method comprising the steps A, B, and C includes methods having only the steps A, B, and C as well as methods having the steps A, B, C, and other steps.

Terms of degree, such as "substantially," "about," and "roughly" are used herein to denote features that satisfy their technological purpose equivalently to a feature that is "exact. " For example, a component A is "substantially" perpendicular to a second component B if A and B are at an angle such as to equivalently satisfy the technological purpose of A being perpendicular to B.

Except as otherwise stated herein, or as is otherwise clear from context, the term "or" is used herein in its inclusive sense. For example, "A or B" means "A or B, or both A and B.

An exemplary caliper-arm-position sensor incorporating an embodiment of the invention is depicted in <FIG>. The sensor includes a differential variable reluctance transducer ("DVRT") <NUM> comprising two coils 106A, 106B and a core 106C. The windings 106A, 106B are wound about a bobbin and are connected electrically in series to form a compound coil. The coils 106A, 106B consist of the same number of turns and split the winding area of the bobbin in half. In the middle of the bobbin, a center tap lead 106D ("C3") is attached where the coils 106A, 106B are connected together. The core 106C is configured to move relative to the coil 106A/106B and thus change the impedance (and reluctance) of the coils 106A, 106B. The core 106C is mechanically attached to a caliper arm (not shown) such that each unique caliper-arm position corresponds to a unique position of the core 106C relative to the windings 106A, 106B. By measuring a voltage at the center tap, the position of the core 106C relative to the coils 106A, 106B can be inferred (and thus the position of the caliper arm may be inferred).

Each end of the coil is driven with a differential constant-current signal 105A, 105B. The frequency of the differential drive signal 105A, 105B is selected to be exactly the zero phase frequency of the windings of the coils 106A, 106B. For temperature stability of the measurement, the frequency of the drive signal 105A, 105B is set slightly lower than the self-resonant frequency of the DVRT <NUM>. For a drive-signal frequency significantly above the self-resonant frequency of the DVRT <NUM>, the DVRT operates as a low-Q band-pass filter in the capacitive region, resulting in loss of information regarding the position of the core 106C relative to the coils 106A, 106B. For a drive-signal frequency significantly below the self-resonant frequency of the DVRT <NUM>, the DC-coupled instrumentation circuit will be loaded by the inductive reactance of the DVRT <NUM>, resulting in poor electrical efficiency.

The differential drive signal 105A, 105B is produced by a current-pump <NUM> and a processor <NUM> (e.g., a digital-signal processor, "DSP"). The processor <NUM> drives the pump <NUM> with a high-drive signal 103A ("DRV_H") and a low-drive signal 103B ("DRV_L"). As depicted in <FIG>, The DRV_H 103A and DRV_L 103B signals are complementary: DRV_H 103A is high when DRV_L 103B is low and DRV_H 103A is low when DRV_L 103B is high. Also as depicted in <FIG>, the current pump <NUM> converts the DRV_H 103A and DRV_L 103B square-wave voltage signals to phase-complementary roughly sinusoidal voltage signals 105A, 105B: the differential drive signal comprises a first sinusoidal signal 105A (solid line) applied to one end of the compound coil 106A/106B (the C1 end) and a second, <NUM>-degree phase shifted, sinusoidal signal 105B (dashed line) applied to the other end of the compound coil 106A/106B (the C2 end). In one exemplary embodiment, the DRV_H signal 103A is a repeated pattern of high for <NUM> microsecond and low for <NUM> microsecond for a <NUM> signal (the DRV_L signal 103B would thus be the inverse pattern, low for <NUM> microsecond and high for <NUM> microsecond).

The signal at the center tap 106D of the DVRT is a function of the position of the core 106C relative to the coils 106A, 106B. The center tap 106D is connected to an analog-to-digital converter ("ADC") <NUM> through a buffer <NUM>. This signal at the center tap 106D is sampled once per drive-signal cycle at a specific moment relative to the zero crossing of the DRV_H/DRV_L drive signal. The ADC <NUM> is triggered by the processor <NUM> with a "trigger" signal 103C. At this point, the ADC <NUM> samples the instantaneous signal from the center tap 106D of the DVRT <NUM> and provides the information to the processor via a bus <NUM>. The DC value of this signal will track the physical position of the core 106C, resulting in a very accurate, high resolution, non-contact position measurement. This measurement spans two quadrants with an output that reads zero when the core 106C is positioned at one extreme, full scale when the core 106C is positioned at the other extreme, and mid-scale when the core 106C is positioned in the middle of the coil of the DVRT <NUM>. The processor <NUM> provides for adjustment of the timing of the trigger point of the ADC <NUM> in order to phase null the system which is made up of the analog and digital circuitry. A delay is required to compensate for the phase shift of the DVRT signal as it passes through the amplifiers of the analog circuitry. For example, the processor <NUM> may provide an adjustable index register which controls the trigger timing for the ADC <NUM>. In one embodiment, the processor may adjust the timing via application of <NUM> steps of <NUM> degrees phase, from zero to three-hundred-sixty degrees relative to the starting of the sine wave couplet 105A/105B. Thus, for example, in an embodiment having an analog-portion delay of <NUM> degrees, the adjustment index would be two steps of <NUM> degrees. By adding eight steps to the index (<NUM>-degrees of delay), the measurement will have a negative slope. The delay, and thus the appropriate index, is a function of the analog circuitry to process the signal at the center tap 106D of the DVRT <NUM>. For set circuity, the index may be set as constant in the processor <NUM>. While <FIG> depicts separate circuit blocks, the blocks do not necessarily correspond to distinct devices. For example, ADC <NUM> and processor <NUM> may be integrated into a single device (e.g., a DSP with an integrated analog-to-digital converter).

The timing of an exemplary trigger for sampling by the ADC <NUM> is depicted in <FIG>. Here, the ADC-trigger signal 103C is shown relative to the DRV_H 103A and DRV_L 103B signals. This shows a phase-shift delay <NUM> of the trigger 103C relative to the zero-crossing points <NUM>.

By driving the DVRT <NUM> at a high frequency (e.g., <NUM>), it is possible to improve the throughput of the measurement (relative to lower frequency operation) while maintaining an appropriate signal-to-noise ratio and at the same time improve resolution of the measurement. For example, in one embodiment, the ADC <NUM> is measured <NUM> times in a row at a rate of <NUM> samples per second (providing <NUM> synchronous measurements of the DVRT <NUM> signal at the center tap 106D). The first <NUM> samples are discarded to eliminate crosstalk/distortion; thus, the DC value of the signal at the center tap 106D is sampled <NUM> times. The <NUM> samples are grouped into three groups of <NUM>. The samples in each group are summed and then divided by four to yield three numbers with four times the resolution of the samples (a gain of <NUM> bits of resolution). The three yielded numbers are then summed and the sum divided by <NUM> to yield an additional resolution gain of <NUM>%. This decimation provides low pass filtering which improves the noise performance of the measurement. Other read rates and decimation schemes/factors may be used, as appropriate for the application (e.g., different numbers of transducers or different communications bandwidth may benefit from different sampling and decimation schemes). The high-speed measurement of a single sensor enables reading more sensors in a set time frame. Thus, for example, the high-frequency DVRT drive allows more caliper arms on a caliper logging tool without reducing the logging rate. The circumferential resolution of the measurement is increased without any increase in logging time.

An exemplary embodiment of current pump <NUM> is depicted in <FIG>. The embodiment comprises two MUX switches <NUM>, <NUM> that work in combination with an operational amplifier <NUM> to receive the DRV_H 103A and DRV_L 103B signals (as switch-position-selection signals) and therefrom generate a roughly sinusoidal signal at the output of the operational amplifier <NUM>. A feedback capacitor <NUM> on the operational amplifier <NUM> bandwidth limits the drive signal to prevent ringing in the DVRT <NUM> and further provides a simple anti-alias filter for the ADC <NUM> to limit the spectrum of the signal at the center tap 106D that is sampled by the ADC <NUM>.

The roughly sinusoidal signal at the output of the operational amplifier <NUM> drives a Howland current pump <NUM> comprising three operational amplifiers <NUM>, <NUM>, <NUM>. One amplifier <NUM> is the controller amplifier for the pump. A second amplifier <NUM> is configured as a voltage follower which preserves the high output impedance of the pump while sourcing a low impedance signal for the pump current feedback and drive signal for the phase-splitter amplifier <NUM>. The phase-splitter amplifier <NUM> is controlled by control amplifier <NUM> through a feedback signal across a resistor <NUM> to ensure true differential constant current operation. The Howland current pump <NUM> drives the differential drive signal 105A, 105B. This current drive is used to excite the DVRT <NUM> as it compensates for temperature changes in the resistance of the coils 106A, 106B. The current drive also compensates for changes in the on resistance of the MUX switches used to selectively connect the sensor circuitry to one of multiple transducers (as explained below).

An exemplary embodiment of the buffer <NUM> is depicted in <FIG>. This embodiment scales the signal at center tap 106D in addition to buffering the signal for sampling by the ADC <NUM>.

An exemplary embodiment of the coil assembly of the DVRT <NUM> is depicted in the various views of <FIG>. The exemplary coil assembly includes two coils 106A, 106B each comprising approximately <NUM> turns about a bobbin <NUM>. The first coil 106A is wound about the bobbin <NUM> at a first portion of the bobbin 602A. The second coil 106B is wound about the bobbin <NUM> at a second portion of the bobbin 602B. The first and second portions 602A, 602B of the bobbin have the same dimensions. The dimensions shown in <FIG> are millimeters.

An exemplary caliper-arm-position sensor incorporating an embodiment of the invention is depicted in <FIG>. This is similar to the embodiment depicted in <FIG>, with the difference being that the <FIG> embodiment is configured with multiple transducers 706A, 706B, 706N supported by a single processing/drive circuit. As described above with respect to the DVRT <NUM> in <FIG>, each DVRT in <FIG> includes three connection points, one at each end of the compound coil (C1, C2) and one at the center tap (C3). The DVRTs 706A, 706B, 706N are connected to the processing/drive circuit through three MUX switches <NUM>, <NUM>, <NUM>, each switch corresponding to a DVRT connection point (C1, C2, C3) and each position in a switch corresponding to one of the DVRTs 706A, 706B, 706N. The position of each of the MUX switches <NUM>, <NUM>, <NUM> is controlled by the processor <NUM> through a bus 103D, thus the MUX switches <NUM>, <NUM>, <NUM> are used to select which of the multiple transducers 706A, 706B, 706N are accessed at a given time. While three transducers 706A, 706B, 706N are depicted in <FIG>, more or fewer transducers may be incorporated into the embodiment. For example, for multi-arm caliper tools, there could be one transducer for each arm. Thus, a <NUM>-arm tool would have <NUM> transducers and a <NUM>-arm tool would have <NUM> transducers. Some portion or all of these transducers may be supported by a single processor, ADC, and current pump. For example, an embodiment of the circuit of <FIG> may support up to <NUM> transducers and multiple such circuits could be deployed in a multi-arm caliper tool with more than <NUM> transducers (e.g., <NUM> processing/drive circuits for a <NUM>-arm tool).

Another exemplary caliper-arm-position sensor incorporating an embodiment of the invention is depicted in <FIG>. This is similar to the embodiment depicted in <FIG>, with the difference being that each of the DVRTs 706A, 706B, 706N is current driven at the compound coil's center tap (C3) rather than at the coil's ends (C1, C2). In this embodiment, a processor <NUM> controls a current source <NUM> through a control line(s) or bus 803A in order to provide current to the selected DVRT 706A, 706B, 706N. For example, the DVRT coil may be driven at the center tap with one of the output-voltage signals 105A, 105B depicted in <FIG>. The processor <NUM> controls a MUX switch <NUM> through a bus 103D to selectively provide the drive current to a DVRT. A driver <NUM> may optionally be disposed between the current source <NUM> and the MUX switch <NUM>. In this embodiment, the signals at the coil ends (C1, C2) 805A, 805B of the MUX-selected DVRT are sampled by an ADC <NUM> controlled by the processor <NUM> through a control line(s) or bus 803C. The processor may analyze the difference between the two end signals 805A, 805B through, for example, software or programmable logic implementing a differencing algorithm. The resulting differential signal is indicative of the core position in the DVRT.

Another exemplary caliper-arm-position sensor incorporating an embodiment of the invention is depicted in <FIG>. This is similar to the embodiment depicted in <FIG>, with the difference being that the signals at the coil ends (C1, C2) 805A, 805B are first processed by a differential receiver <NUM> and the resulting differential signal is digitized by the ADC <NUM>. The resulting differential signal is indicative of the core position in the DVRT.

Claim 1:
A caliper-arm-position sensor comprising:
(a) a compound coil comprising:
(i) a first coil (106A) having a first end connection and a second end connection, and
(ii) a second coil (106B) having a first end connection and a second end connection,
(iii) wherein the second end connection of the first coil is electrically connected to the first end connection of the second coil;
(b) a core (106C) positioned adjacent to the coil;
(c) a caliper arm mechanically connected to the core;
(d) a voltage driver (<NUM>, <NUM>; <NUM>, <NUM>) having an alternating-voltage output signal electrically connected to at least one of the group consisting of the first end connection of the first coil, the second end connection of the first coil, and the second end connection of the second coil;
(e) an analog-to-digital converter (<NUM>; <NUM>) electrically connected to at least one of the group consisting of the first end connection of the first coil, the second end connection of the first coil, and the second end connection of the second coil; and
(f) a timer (<NUM>; <NUM>);
characterised in that:
the timer (<NUM>; <NUM>) is provided with an output electrically connected to the analog-to-digital converter, wherein the output of the timer is configured to trigger sampling by the analog-to-digital converter at a predetermined period of time relative to a phase point of the alternating-voltage input signal.