MEASUREMENT USING ELECTRICAL TRANSDUCERS

Abstract: A method and apparatus for measuring the value of a physical parameter is based on the detection of a change in the electrical properties of measurement transducer. For example, the transducer may be associated with the feeler arms of a well logging tool. The method comprises transmitting a signal to the transducer and receiving a return signal, the height or other property of which determines the value to be measured. The method may employ a reference transducer to which identical pulses are sent to counteract system errors and an auto-zero step is also provided.

Description: "MEASUREMENT USING ELECTRICAL TRANSDUCERS" DESCRIPTION This invention relates to a method of measurement using electrical transducers, In. a particular application the invention is applied to transducers in the form of electrically- conductive coils located in a remote situation, for example, in downhole logging instruments used in oil wells, Coils, used in various ways, form suitable transducers for downhole logging instruments because they are more capable of surviving in the high temperature environment than active transducers. Examples of the use of coils for such a purpose are described in U,S, Patent Specification 2908085 and U.K, Specification 1513965; the latter describes a method of using coils as "eddy current" proximity detectors. The present Applicant has described a measuring instrument in his co-pending PCT Patent Application GB 84/00426 which employs transducer coils and to which the present invention can be applied. The contents of the Applicant's aforementioned co-pending application are incorporated by reference into the present specification as illustrating in detail one example of an instrument employing coil transducers to which the invention can be applied- However the present invention is not limited in its application to coil-type transducers. When used with coil-type transducers it may be applied to a single inductor, to transformers of various types, including the linear differential transformer described in the Applicant's co-pending application, or any other coil arrangement where it is desired to measure a change in the electrical properties of the coil. In a conventional system using coil transducers for measurement, the value of a physical parameter being measured modifies the mutual or self-inductance of the coil transducer, ' The value of the inductance is determined by including the coil in an alternating current circuit. Typically the primary winding of a linear variable differential transformer is excited by a sinusoidally varying voltage of constant amplitude and frequency. The nature of the output waveform from the secondary winding of the transformer depends upon the position of the core of the transformer which in turn is arranged to move in accordance with the physical parameter being measured. The waveform produced in the secondary winding will be in the form of a sine wave having amplitude and phase values depending upon the core position. In a typical case this signal is processed by being rectified and transmitted through a low pass filter, thereby to obtain a d,c voltage, the value of which varies with the core position. In more sophisticated versions of the method phase sensitive detectors are used to give a substantially linear relationship between the core displacement and the output signal from the detector. These prior methods give rise to relatively slow response times due to the need to obtain samples of the output waveform over a number of cycles for an accurate. result. This sampling and averaging is achieved in the example mentioned earlier by the low pass filter, A further disadvantage is that the a.c. supply to the low impedance coil system consumes a relatively large amount of power and this is particularly undesirable in a downhole logging instrument, A main requirements of a downhole logging instrument, although not exclusive to that instrument, is for a coil system that provides a rapid data acquisition cycle since it is required to measure the dimensions of a pipe at a number of points around its circumference while the tool maintains a reasonable speed along the axis of the pipe. One approach to at least some of these problems is described in European Patent Application No, 0082772, This comprises the application of a transient voltage to a transducer coil and the measurement of the time taken"for current in the coil to fall to a predetermined level. It is claimed that this approach allows data to be obtained from the coil more rapidly than in the prior proposals, but there is an attendant loss in the obtainable resolution. If in that system, the sample rate is S and the resolution is R then the time interval must be measured with an accuracy of ^ - Thus to measure to an accuracy of one part in one thousand in one hundred microseconds, the resolution needs to be 0,1 microseconds, requiring lOMhz clock rates in the counting circuitry. In a practical embodiment of this approach the clock rate would have to be doubled since the coil current has also to be established taking a similar duration to the discharge time- The method disclosed in this document is also subject to further limitations because the multiplexor switch resistance as well as the coil and interconnection resistances, since they are in series with the quantity to be measured, have a first order effect on the final measured value. The present invention seeks to provide an improved method of measurement using an electrical transducer- According to the present invention ' there is provided a method of measuring the value of a physical parameter detected by a change in the electrical properties of a measurement transducer comprising the transmission of an electrical pulse to the transducer, and determination of a property of the resultant signal received from the transducer, which property is a function of said change in the electrical properties of the transducer, The method may further include the step of converting the determined property into a constant voltage signal, The determined property may for example be the height of a pulse returned from the transducer. In a preferred embodiment of the invention said transducer is connected in series with a reference transducer such that both simultaneously receive said electrical pulse, and the method includes determining said property of the resultant signal for both the measurement transducer and the reference transducer, and obtaining the ratio of the determined values to constitute a measurement of the physical parameter. The transducer may comprise a linear transformer with a movable core, the core being moved in response to the physical parameter being measured, thereby to vary the mutual inductance of the transformer, said electrical pulse being fed to one winding of the transformer, and said received signal being the resultant pulse induced in the other winding of the transformer, In accordance with a further aspect of the invention there is provided a method for automatically treating signals obtained from a transducer in data acquisition equipment, the equipment being based on analogue circuitry but controlled by a digital processor, and the output signal from the equipment being obtained via an analogue to digital converter, the analogue circuitry being supplied with an offset voltage controlled by the processor via a digital to analogue converter, the method comprising setting the input signal from the transducer to zero, automatically registering the output from the analogue to digital converter, changing the offset value to nullify the output from the analogue to digital converter. This method for treating signals may be used in the method of measurement as aforesaid, and therein providing an auto-zero technique which is applied to both signal and reference channels. The method as aforesaid is particularly suitable for well logging tools where it may be applied to the transducers associated with the feeler arms in a caliper-type well logging tool. In such a system the coils are located in the caliper tool and communicate via electrical connections to the signal processing circuitry at the top of the well- Embodiments of the invention will now be described, by way of example only with reference to the accompanying drawing which is a block circuit diagram of a measurement circuit operating in accordance with the present invention- Referring to the figure, the method is illustrated with reference to five measurement transducers each comprising a respective linear variable displacement transformer 0 n - T4,. Such transducers are well known and comprise a transformer wound about a straight axis whose core may be displaced in accordance with some measured parameter- This displacement modifies the mutual inductance of the transformer- A further transformer T R „ is included which acts as a reference- The core of this transformer is maintained in a fixed position- One terminal of the primary winding of the reference transformer is connected to circuit ground and its other terminal is connected via a common line to a first terminal of the primary winding 3 of each of the transformers T0 Λ - T4., The remaining terminal of the primary winding of each transformer T Q - T, is connected to a respective switching contact in a primary multiplexor unit 6- Thus each primary winding of the transformers T Q - T. is in series with the primary winding of the reference transformer T R - ' The primary multiplexor unit 6 enables each of the primary windings of the transformers T Q - T. to be selectively connected to a primary pulse voltage source 7 via a switch 8- In a typical system where transducers including the transformers T Q - T. are 0 cyclically excited their primary windings are connected successively by the multiplexor 6 to the pulse source 7. One terminal of the secondary winding of each of the 5 transformers ~ - T. and one terminal of the secondary winding of the reference transformer are connected by a common line to circuit ground- The remaining terminal of each of the secondary windings of the transformers T Q - T, is connected to a respective terminal of a secondary multiplexor 8 which is operative to switch any one of these connections to the non-inverting input terminal of an instrumentation amplifier 9- The remaining terminal of the secondary winding of the reference transformer !_.„„ is connected to the non-inverting input terminal of a second instrumentation amplifier 10- The inverting input terminals of the instrumentation amplifiers 9 and 10 are connected via a common line to circuit ground. The output terminal of the instrumentation amplifier 9 is connected to a peak detector 11 which produces a constant voltage output of value dependent on the detected peak value of its input signal- The output from the peak detector is connected to the inverting input terminal of a summing amplifier 12 whose non-inverting input terminal is connected to circuit ground- Reference signal ' processing circuitry comprises the reference instrumentation amplifier 10 feeding a peak detector 13 and a summing amplifier 14, corresponding to the peak detector 11 and summing amplifier 12 in the signal processing circuitry- The signal from the output of the summing amplifier 12 is fed to the signal input of an analogue to digital converter 15 via a buffer amplifier 16, The output from the reference summing amplifier 14 is fed to the reference input of the analogue to digital converter 15 also via a respective buffer amplifier 17. The conduction path between the buffer amplifier 16 and the output of the summing amplifier 12 is routed via a multiplexor 18 having connections to enable alternative signals to be switched to the signal input of the analogue to digital converter 15, as is described below. One of these alternative signals is the output from the reference summing amplifier 14 which has an additional connection to the multiplexor 18. Similarly the reference summing amplifier 14 is connected to its buffer amplifier 17 via a further multiplexor 19 enabling other connections to be made to the reference input of the analogue to digital . converter 15 as is also described below. The digital output of the analogue to digital converter 15 is fed to a microprocessor (not shown) ; it is stored, processed or displayed by standard techniques. A digital to analogue converter 20, controlled by the microprocessor, provides an offset voltage for the signal processing circuitry- . The output from the DAC 20 is connected to the inverting input terminal of the summing amplifier 12. The output from the DAC 20 is also connected to provide a further input to the multiplexor 18 enabling the output from the DAC 20 to be connected directly to the analogue to digital converter 15 via the buffer 16. A further digital to analogue converter 21 provides an offset voltage for the reference signal processing circuitry and its output is connected to the inverting input terminal of the summing amplifier 14- The output of DAC 21 is also connected to provide a further input to the multiplexor 19 and hence may be connected to the reference input of the analogue to digital converter 15. Both of the digital to analogue converters 20 and 21 have an 8-bit resolution and are capable of producing output signals on either side of the circuit ground potential; ground potential generally corresponding to one half of the maximum digital input- The remaining input to the multiplexor 19 is a voltage reference signal as is further mentioned below. The circuitry functions by sampling each of the transformers T Q - , in succession and derives a ratio of the voltages across the secondary of the transformer with the voltage across the secondary of the transformer T REp for each of thee transformers Tr. - T. in turn. The measurement process is initiated by applying a narrow pulse to a primary winding of one of the transformers determined by the primary multiplexor 6. Since each of the primary windings of the transformers T n 0 - T4. is in series with the primary winding of the transformer T E this pulse is also applied to the primary winding of the reference transformer. The secondary multiplexor 18 is synchronised with the multiplexor 6 such that the secondary winding of the transformer to which the pulse has been applied is connected by the secondary multiplexor 8 to the instrumentation amplifier 9, This amplifier provides a high input impedance and any changes in the secondary circuit resistance has a negligible effect on the pulse signal. The resultant pulse output of the amplifier 9 is fed to the peak detector 11 which produces a constant voltage output of value dependent on the height of the signal from the amplifier 9. The output from the peak detector 7 is combined with the offset voltage developed by the > - 14 - digital analogue converter 20 and fed to the summing amplifier 1, The voltage levels and resitor values are such that the least significant bit of the digital to analogue converter 20 corresponds to the least significant bit of the output signal of the analogue to digital converter 15, The summing amplifier has the effect of dividing the output from the digital to analogue 0 converter effectively by 16 and since the analogue to digital converter 15 typically has a resolution of 12 bits then the 8 bit resolution of the DAC 20 is achieved when both the analogue to digital converter 15 and the DAC 20 are connected to the same reference 5 voltage (typically 10,24 volts). Prior to each coil sampling cycle the DAC 20 is loaded with a voltage which is the inverse of the offset voltage for the signal processing channel. The 0 microprocessor maintains a list of channel offset values in a modifiable memory and loads these into the DAC 20 as necessary. These values are updated continually by means of an auto-zero cycle as will be described. 5 \ - 15 - The signal derived from the secondary winding of the reference transformer T-,..-- is fed through the si-gnal processing circuit i.e. the amplifier 10 and the peak detector 13 and is combined with an inverse offset voltage from the DAC 21 also loaded by the microprocessor. The combined signal is then fed to the summing amplifier 14. During measurement operation the output from the summing amplifier 12 is fed to the signal input of the A-D signal 15 and the output from the summing amplier 14 is fed to the reference input of the converter 15. The A-D converter 15 is of the ratioraetric type so that the digital output from the converter 15 represents the ratio of the signal input to the reference input, . The method thus uses a fixed transducer as a reference for each measurement so that even instantaneous fluctuation in the circuit resistances or driving voltages do not effect the accuracy and consistency of the final output from the analogue to digital converter 15, This final signal is dependent solely on the ratio of the voltages across the primary windings of the measurement transducer and the reference transducer and depends only on the V - 16 - properties of the prima y coils- It therefore represents an accurate and consistant measurement of the change in inductance of the measurement coil and hence an accurate measurement of the parameter causing this change- The auto zero cycle referred to above is carried out separately for the reference and signal channels- It is conducted by sampling each channel with no primary 0 drive pulse present- The digital to analogue converter 20 has the previously loaded offset value and the processor then detects any residual output from the analogue to digital converter 15 and updates the offset value loaded into the DACS to cancel this 5 residue- The similar procedure is used to correct the offset voltage applied to the DAC 21 in the reference signal processing circuitry by connecting the multiplexor 18 to the output of the summing amplifier 14, This output is thereby connected to the signal 0 input of the A-D converter 15 and the reference offset voltage corrected- The multiplexor 19 is connected to the reference voltage in both auto-zero cycles- The system allows other diagnostic techniques to be ■ carried out- The output from either of the offset DACS 20 or 21 can be routed directly to the analogue to digital converter 15 by appropriate switching of the multiplexors 18 and 19, This allows a closed loop diagnostic technique to be carried out on the system to self-test by loading values to the DACS 20 and 21 and reading the .resultant signal back from the analogue to digital converter. The interfaces between the microprocessor and the various components in the signal processing circuitry are not shown in the diagram but can be accomplished by standard techniques well known to those skilled in the art. The sequence of operations performed by the mirocprocessor to obtain the measurement data and to execute the auto * zero cycles are as follows: Initialise Routine (to be performed on power-up or • system reset) 1, Set signal digital offset to the equivalent of 0 volts (this is usually half scale, or 128 for an 8 bit DAC) for each channel, 2, Set reference digital offset to the equivalent of 0 Volts 3, Load reference digital offset into reference DAC, 4, Set signal and reference A-D input multiplexors to respective signal and reference channels. Auto-zero Routine for signal Channels 1, Select required channel on Secondary multiplexor 8 and Primary multiplexor 6. 2- Select voltage reference (V REF) on Reference multiplexor 19 3. Load channel digital offset into signal DAC 20. 4. Discharge peak detector to negative supply voltage. 5- Set peak detector active (detect positive peaks) - 6. Read A-D converter 15. 7- Subtract digital result in step 6 from channel digital offset value. 8. Repeat above 8 steps for each signal channel. 9. Select reference channel on reference multiplexor 19 Auto-zero routine for reference channel 1. Select voltage reference on reference multiplexor 19. Select reference channel on signal multiplexor 18. 3. Discharge peak detector. 4. Set peak detector. 5. Read A-D converter 15. 6- Subtract digital result in step 5 from xef. digital offset value- V - 20 - 7. Load digital offset into reference DAC 21- 8- Select signal and ref- channels on respective multiplexors 18 and 19, > Measurement Routine 1- Select required channel on Secondary multiplexor 8 and Primary multiplexor 6- 0 2, Load channel digital offset into signal DAC 20, Discharge peak detectors. 5 4, Set peak detectors- Send pulse to primary coils- Read A-D for channel results. 0 7- Repeat above 6 steps for each channel. Each routine leaves the system in a "reset" state with the multiplexors 18 and 19 switched to their 5 respective channels and the reference DAC 21 with the current reference digital offset value. The primary and secondary multiplexors 6 and 8 and the signal DAC 20 are not set to any particular value and must be explicitly set by each routine. In a typical arrangement repeated measurement cycles would be followed by a reference and signal auto zero cycle once every second- The method described enables the coil system to be sampled to give a very rapid acquisition time and a resolution and accuracy limited only by the capability of commercially available analogue to digital converters- Typically this allows an acquisition cycle time of less than 50 microsecond ' s, with 12 bit resolution and 10 bit accuracy. There are no fundamental limitations in the method and if a particular application required improvement in the values it can be achieved by the use of suitable precision components. The accurate values may be achieved over a broad temperature range without the need to maintain constant voltage or current sources or the need to use low temperature drift components. The effects of multiplexor switches and interconnection resistances have a negligible effect on the final result. \ - 22 - Use of a microprocessor does not involve large amounts of computational programming and in particular the basic digital result is obtained without multiplication or division operations- The speed and accuracy of the method are obtained at the cost of loss in linearity compared with prior methods but in many applications this is not a disadvantage as an overall system linearity correction can be applied through an EPROM type look-up table. This in itself has its own 0 advantage as it completely frees the coil system design from linearity restrictions, that is to say it is no longer required to maintain a linear relationship between core movement and signal output, and as a result an extremely compact coil transducer 5 can be constructed having removed this restriction. When the transducers are the linear variable- displacement type, for example in a system as described in the Applicant's co-pending application no. 8333190, further design restrictions are 0 removed since there is no need for a linear mechanical linkage i.e. a linear relation between one end of the linkage and the other, between the point of measurement and the transducer core. The embodiment of the invention described above is the preferred one, but it is possible to modify this and remain in accordance with the present invention. In particular the system may be constructed without, a microprocessor by using a high grade, low drift, signal processing circuit. In such an arrangement all gain errors in the transducers, pulse processing and analogue to digital elements of the circuit that are common to the reference and signal circuits are eliminated. However the use of a microprocessor allows zero drift errors also to be corrected and provides for diagnostic procedures. Other modifications would be apparent to those skilled in the art for example in place of a peak detector to measure the pulse height a track and hold circuit may be used to sample the pulse height at a fixed time interval after the commencement of the pulse. Alternatively an integrator may measure the voltage-time product. As a further alternative a root mean square integrator circuit may measure the energy in the pulse. Within the scope of the invention it is also envisaged that the measurement process may be carried out without a reference transformer, although such a method has a disadvantage that changes in pulse voltage or in the primary circuit resistance will cause a variation in the primary pulse shape and hence inconsistent results. Similarly gain changes in the analogue to digital converter or secondary multiplexor, and voltage reference drift in the analogue to digital converter will also cause errors- Although the embodiment of the invention described in detail above is applied to the sampling of transformer coils, the method may be used with advantage for sampling other types of transducers, for example a d-c- voltage normally applied to a strain gauge- The narrow sampling pulse that is used in the method produces smaller heating effects within the device than does a constant excitation voltage- Thus a signal multiplexor and associated electronics may be connected and used for both coil type transducers and d.c. transducers.

Claims: CLAIMS 1- A method of measuring the value of a physical parameter detected by a change in the electrical properties of a measurement transducer comprising the transmission of an electrical pulse to the transducer, and determination of a property of the resultant signal received from the transducer, which property is a function of said change in the electrical properties of the transducer. 2- A method as claimed in Claim 1 further' including the step of converting the determined property into a constant voltage signal. 3. A method as claimed in Claim 1 or Claim 2 wherein the determined property is the height of a pulse returned from the transducer. 4. A method as claimed in any one of the preceding claims wherein said transducer is connected in series with a reference transducer such that both simultaneously receive said electrical pulse, and the method includes determining said property of the resultant signal for both the measurement transducer ) - 26 - and the reference transducer, and obtaining the ratio of the determined values to constitute a measurement of the physical parameter. 5. A method as claimed in any one of the preceding claims wherein the transducer comprises a linear transformer with a movable core, the core being moved in response to the physical parameter being measured, thereby to vary the mutual inductance of the 0 transformer, said electrical pulse being fed to one winding of the transformer, and said received signal being the resultant pulse induced in the other winding of the transformer. 5 6, A method as claimed in any one of the preceding claims including the step of automatically treating signals obtained from a transducer in data acquisition equipment, the equipment being based on analogue circuitry but controlled by a digital 0 processor, and the output signal from the equipment being obtained via an analogue to digital converter, the analogue circuitry being supplied with an offset voltage controlled by the processor via a digital to analogue converter, the method comprising setting the 5 input signal from the transducer to zero, automatically registering the output from the analogue to digital converter, changing the offset value to nullify the output from the analogue to digital converter. 7. A method for automatically treating signals obtained from a transducer in data acquisition equipment, the equipment being based on analogue circuitry but controlled by a digital processor, and the output signal from the equipment being obtained via an analogue to digital converter, the analogue circuitry being supplied with an offset voltage controlled by the processor via a digital to analogue converter, the method comprising setting the input signal from the transducer to zero, automatically registering the output from the analogue to digital converter, changing the offset value to nullify the output from the analogue to digital converter. 8. A method as claimed in Claim 6 dependent on Claim 4 wherein the step of automatically treating signals is used to provide an auto-zero technique which is applied to both signal and reference channels- -9. Apparatus for performing the method as claimed in Claim 1 wherein the transducer is located in a well logging tool and is associated with the feeler arms thereof and the signal processing circuitry is located remote from the transducer at a position at the top of the well-

Inventors: ATHERTON ERIC JOHN, ATHERTON, ERIC, JOHN
Applicants: HANSON RES, HANSON RESEARCH LIMITED
Application year: 1985
Main classification: G01D5
IPC codes: E21B47, G01B7, G01D1, G01D3, G01V3