WO2022245328A1

WO2022245328A1 - Sensor compensation

Info

Publication number: WO2022245328A1
Application number: PCT/US2021/032708
Authority: WO
Inventors: Miguel Angel ALEGRE LOPEZ
Original assignee: Hewlett-Packard Development Company, L. P.
Priority date: 2021-05-17
Filing date: 2021-05-17
Publication date: 2022-11-24

Abstract

A sensor apparatus comprises a resistive-bridge sensor to produce an output voltage comprising a desired component and a first offset. The sensor apparatus comprises an indirect current feedback amplifier comprising first and second input terminals to receive the output voltage, an output terminal to output an amplified output voltage comprising an amplified desired component and a second offset related to an operating range of the output voltage of the resistive-bridge sensor, the amplified desired component related to a voltage difference between the first and second input terminals and the second offset, and a negative feedback terminal to receive a compensation voltage to compensate for the first offset. The sensor apparatus comprises a processor comprising a digital-to-analog converter to deliver the compensation voltage to the negative feedback terminal. The processor sets the compensation voltage as proportional to the first offset, and offset by the second offset.

Description

SENSOR COMPENSATION

BACKGROUND

[0001] Resolution of measurement systems that rely on signals from resistive sensing elements is typically very low. Amplifying circuitry may be employed in order to improve the resolution. The maximum usable gain from the amplifying circuitry is limited by signal offsets resulting from systematic errors and component tolerances.

BRIEF DESCRIPTION OF THE DRAWINGS [0002] Figure 1 is an illustration of a sensor apparatus;

[0003] Figure 2 is an illustration of a compensation circuit;

[0004] Figure 3 is an illustration of a control loop according to an example; and

[0005] Figure 4 is a flow chart of an example method.

DETAILED DESCRIPTION

[0006] An example sensor apparatus 10, is shown in Figure 1 , comprises a resistive-bridge sensor 12 operable to produce an output voltage. The output voltage may vary according to a sensed condition, for example a physical magnitude to be measured. The output voltage comprises a desired output component, and a first offset voltage. The desired output component may relate to the sensed condition. The first offset voltage may correspond to a systematic error from the resistive-bridge sensor 12,

[0007] Depending on the bridge topology, the resistive-bridge sensor 12 may comprise a plurality of passive resistors and at least one sensing element. The resistive-bridge sensor 12 may comprise a quarter-bridge configuration, a half-bridge configuration, or a full-bridge resistive-bridge sensor 12. [0008] The resistive-bridge sensor 12 may comprise a strain gauge as a sensing element, to measure deformation In mechanical components. Measured deformation may be as a result of static or dynamic loading, and may represent a weight or other force appiied to the component. The resistive-bridge sensor 12 may comprise a sensing element having a variable resistance due to a physical effect such as strain, or a semiconductor effect such as a photoelectric effect. [0009] The resistive-bridge sensor 12 may comprise an ink ievel measurement sensor to measure an ink ievei based on the output voltage of the resistive-bridge sensor 12, As such, the sensor apparatus 10 may be suitable for use in printing devices.

[0010] Under ideal conditions, resistance of the sensing element depends linearly on the sensed condition. In this case, the output voltage V₀ of the resistive- bridge sensor 12 may be expressed by the following equation:

Wherein m corresponds to a magnitude of the sensed condition, k is a value dependent on the selected bridge topology, and V_cc is a voltage provided by an external voltage source to excite the resistive-bridge sensor 12.

[0011] Systematic errors from the resistive-bridge sensor 12, for example errors produced by tolerance of components such as passive resistors and sensing elements, contribute to a first offset voltage. Taking the first offset voltage into consideration, the output voltage V₀ ot the resistive-bridge sensor 12 may be expressed as:

[0012] The sensor apparatus 10 comprises a processor 18, for example a microcontroller, comprising a digital-to-anaiog converter 32. The processor 18 may comprise an ana!og-to- digital converter 30, as shown in the example of figure 2. The analog-to-digital converter 30 may be embedded within the processor 18.

[0013] In order to maximise a resolution of the sensor apparatus 10, consideration is given to a dynamic range of the analog-to-digitai converter 30. The dynamic range of the anaiog-to- digitai converter 30 represents a range of signal amplitudes which the anaiog-to-digital converter 30 is able to resolve, and is related to a number of bits that are used to digitise the analog signal originating from the resistive-bridge sensor 12. As the value of a voltage produced by the resistive-bridge sensor 12 is smaller than the largest input resolvable by the anaiog-to-digital converter 30, amplification of the voltage output is used to scale the output voltage range to correspond to the usable input range of the analog-to-digitai converter 30.

[0014] The sensor apparatus 10 comprises an amplifier 14 arranged in an indirect current feedback configuration. Such configuration may enable compensation of the first offset voltage produced by the resistive-bridge sensor 12 even if the value of the offset is such that it saturates the amplifier 14. [0015] As shown in Figure 2, a compensation circuit 20 according to an example comprises the amplifier 14 and the processor 18,

[0016] The amplifier 14 comprises first and second input terminals 22 arranged to receive the output voltage produced by a sensor, for example the resistive-bridge sensor 12, and produce an amplified output voltage based on a voltage difference between the first and second input terminals 22, The amplified output voltage comprises an amplified desired component related to the voltage difference, and a second otfset related to an operating range of the output voltage from the resistive-bridge sensor 12. The second otfset may be generated by an offset generation circuit 34.

[0017] Since the sensed condition m may be represented by a positive or a negative value, and the analog-to-digitai converter 30 employs a unipolar supply voltage, the second offset voltage may correspond to a desired offset value added during the amplification stage to maximise the input dynamic range of the analog-to-digital converter 30. Thus, an input voltage of the analog- to-digitai converter 30, after being amplified by a factor A and adding a second offset

may be expressed as:

Wherein m corresponds to a magnitude of the sensed condition, k is a value dependent on the selected bridge topology, and Vcc is a voltage provided by an external voltage source to excite the resistive-bridge sensor 12. Through the addition of the second offset voltage, it may be possible to ensure that the input voltage matches the dynamic range of fhe anaiog-to-dlgital converter 30 for a range of voltages produced by the resistive-bridge sensor when the magnitude of the sensed condition ranges from the maximum, to

, the minimum. In the

case of a unipolar supply and m which may be signed either positive or negative, the second offset voltage may represent a midpoint of the expected range. For a bipolar supply, or m which is of only a single sign, the second offset voltage may be varied accordingly so as to likewise maximise the effective operating range of the output voltage from the resistive-bridge sensor, when amplified as described herein,

[0018] The amplified output voltage is output by an output terminal 26 of the amplifier 14. The output terminal 26 may be operatively coupled to the input of the analog-to-digital converter 30. Increasing the gain of the amplifier 14 may match the amplified output voltage to the dynamic range of the analog-to-digital converter 30, thereby improving resolution of the sensor apparatus 10. However, an upper limit to gain of the amplifier 14 can be defined in order to prevent saturation of the amplifier 14, A saturated amplifier may produce a distorted output signal, leading to an incorrect measurement being produced by the sensor apparatus. For instance, using a three-stage instrumentation amplifier may allow for removal of the first offset only if a first stage is not saturated. That is, using the three-stage instrumentation amplifier, the maximum gain of the amplifier may have an upper limit that depends on the first offset.

[0019] A sensor compensation method is employed to mitigate the effect of the first offset voltage produced by the resistive-bridge sensor 12, increase the effective dynamic range of the analog-to-digita! converter 30, and thereby increase the maximum achievable resolution of the sensor apparatus 10. These benefits may be achieved while avoiding the risk of saturating the amplifier 14. By providing the amplifier 14 in the indirect current feedback configuration, limitations in terms of complexity, feasibility and cost may be addressed.

[0020] The amplifier 14 comprises a negative feedback terminal 28 arranged to receive a compensation voltage to compensate for the first offset, in an indirect current feedback configuration. The dig itai-to-anaiog converter 32 may be operatively coupled to the amplifier 14. An output terminal of the digital-to-anaiog converter 32 is connected to the negative feedback terminal 28 of the amplifier 14 to deliver a voltage to the negative feedback terminal 28.

[0021] As shown in Figure 2, a compensation circuit according to an exampie may comprise a first resistor 40, a second resistor 42 and a third resistor 44, The ratio of the first resistor 40 and the second resistor 42 may set the gain of the ampiifier 14, In order to eliminate the first offset voltage, a current may be provided through the third resistor 44 into the negative feedback terminal 28.

[0022] The processor 18 is configured to set the compensation voltage generated by the offset generation circuit 34 to a value proportional to the first offset voitage, and offset by the second offset voltage. This may dynamically compensate for the first offset voltage produced by the resistive-bridge sensor 12, thereby increasing the effective resolution that can be achieved for the sensor apparatus 10. in exampie embodiments, the offset generation circuit 34 is configured as a distinct unit in operative communication with the processor 18, or alternatively functionality of the offset generation circuit 34 is provided by the processor 18 itself, such as directly by the processor 18 itself, under the control of suitable program instructions.

[0023] The processor 18 may be arranged to set the compensation voitage during a calibration operation performed at a calibration point corresponding to a known sensed condition. The calibration point may correspond to a no-load sensed condition. The sensor apparatus 10 may comprise non-volatile memory to store a value of the compensation voitage.

[0024] In order to maximise the input dynamic range of the analog-to-digital converter 30 when the resistive-bridge sensor 12 is unloaded - that is, when the sensed condition is null - the input voitage of the analog-to-digitai converter 30 is set as equal to the second offset voitage. This condition may be achieved by setting the compensation voitage to a vaiue expressed by a following equation:

Wherein Ri corresponds to the first resistor 40, i¾ corresponds to the second resistor 42, and R_A corresponds to the third resistor 44.

[0025] The processor 18 may be switchabie between the calibration operation and a measurement operation during which a physical vaiue related to the sensed condition is measured.

[0026] The processor 18 may be configured to perform the calibration operation prior to performing the measurement operation. While the processor 18 performs the calibration operation, the resistive-bridge sensor 12 may be unloaded, and the output of the sensor apparatus 10 may be continuously monitored. Under such conditions, the sensor apparatus 10 may operate as a closed-loop control system, for instance a proportionai-integral-derivative, "PID", control loop. Figure 3 shows a schematic PID control loop. The input voltage of the analog-to-digital converter 30 may be set to a given reference value, for example calculated under the no-load condition, i.e. where m=0:

Wherein Ri corresponds to the first resistor 40, Rs corresponds to the second resistor 42, and R_A corresponds to the third resistor 44. [0027] A steady-state value for the compensation voltage may be calculated during execution of the PiD control loop by the processor 18. The processor 18 may set the compensation voltage to the calculated steady-state value. The calculated steady-state value may be employed during the measurement operation of the sensor apparatus 10.

[0028] The processor 18 may communicate with machine-readable storage. The machine- readable storage may be any electronic, magnetic, optical· or other physical storage device that stores executable instructions. Thus, machine-readable storage may be, for example, Random Access Memory (RAM), an Eiectricaliy-Erasable Programmabie Read-Only Memory (EEPROM), a storage drive, an optical disc, and the like. As described in detail below, machine- readable storage medium may be encoded with executable instructions for setting the compensation voltage. [0029] The machine-readable storage medium comprises instructions to receive an output voltage of a resistive-bridge sensor, the output voltage comprising a desired component and a first offset. The machine-readable storage medium comprises instructions to receive an amplified output voltage of an amplifier, the amp!ified output voltage comprising an amplified desired component and a second offset related to an operating range of the output voltage, the amplified component related to the output voltage. The machine-readable storage medium comprises instructions to set a digital-to-ana!og converter to output a compensation voltage proportional to the first offset voltage, and offset by the second offset voltage as a feedback signal to compensate for the first offset.

[0030] Figure 4 shows a flowchart of an exampie method 100. The method may be executable by the sensor apparatus 10 shown In figure 1 .

[0031] The method comprises, In block 102, producing, by a resistive-bridge sensor, an output voltage comprising a desired component and a first offset. In an example, the desired component may relate to a sensed condition, and the first offset may relate to a systematic error from the resistive-bridge sensor.

[0032] The method comprises, in block 104, receiving, by an indirect current feedback amplifier, the output voltage produced by the resistive-bridge sensor. Employing the indirect current feedback amplifier may eliminate the effect of the offset produced by the resistive-bridge sensor, thereby improving the resolution of the sensor apparatus.

[0033] The method comprises, in block 106, outputting, by the amplifier, an amplified output voltage comprising an amplified desired component and a second offset. The amplified desired component relates to the output voltage produced by the resistive-bridge sensor. The second offset is selected in the example embodiments to best match the operating range of the output voltage from the resistive bridge sensor, as amplified and output by the amplifier, to the input range of the analog-to-digita! converter. The second offset may correspond to a desired offset value, added at the amplification stage to improve the resolution of the sensor apparatus by ensuring that the amplified output voltage matches a dynamic range of an analog-to-digital converter regardless of a magnitude of the sensed condition.

[0034] The method comprises, in block 108, setting, a compensation voltage to be output by a digitai-to-analog converter. The compensation voltage is proportional to the first offset, and offset by the second offset. The compensation voltage may compensate for the first offset.

[0035] The setting the compensation voltage may be performed during a calibration operation performed at a calibration point corresponding to a known sensed condition. The calibration process, and accordingly the setting of the compensation voltage may be performed by a processor. The method may comprise switching between the calibration operation and a measurement operation during which a physical value related to the sensed condition is measured. The calibration operation may be performed before the measurement operation. The compensation voltage may be used to set a desired amplified output voltage at unloaded conditions in order to maximise the dynamic range of the sensor apparatus.

[0036] The method may comprise storing a value of the compensation voltage in non-volatile memory. This may enable the sensor apparatus to retrieve the previously calculated value for use during the measurement operation.

[0037] The method comprises, in block 110, receiving, by the amplifier, the compensation voltage as a feedback voltage to compensate for the first offset. For example, the compensation voltage can be used to set the desired amplified output voltage, conveniently at unloaded conditions, or any other known set point condition, in order to maximize the dynamic range of the measurement system as described.

[0038] The first offset may therefore be dynamically corrected, allowing for maximisation of the resolution of the resistive-bridge sensor.

[0039] Examples described herein may provide a cost-effective means of improving effective resolution of a sensor apparatus by eliminating undesired offset components originating from systematic, errors in resistive-bridge sensors without employing high-cost components such as programmable resistors. Examples described herein may improve performance of a measurement system by providing a repeatable mechanism for compensating undesired offsets. Performing the offset compensation at various points in a lifespan of the measurement system may allow for accurate offset compensation even if the amount of offset produced by the resistive-bridge sensor changes, for example due to aging of components.

[0040] Examples described herein may maximise a resolution of a sensor apparatus by employing an amplifier in an indirect current feedback configuration as such configuration does not set an upper limit on the amount of gain produced by the amplifier. Examples described herein may eliminate undesired offsets produced by resistive-bridge sensors to a high degree of precision stemming from a high resolution of digital-to-anaiog converters.

Claims

1 . A sensor apparatus comprising: a resistive-bridge sensor to produce an output voltage comprising a desired component and a first offset; an indirect current feedback amplifier comprising, first and second input terminals to receive the output voltage, an output terminal to provide an amplified output voltage comprising a second offset and an amplified desired component, the amplified desired component related to a voltage difference between the first and second input terminals, the second offset related to an operating range of the output voltage from the resistive-bridge sensor, and a negative feedback terminal to receive a compensation vottage to compensate for the first offset; and a processor comprising a dig ita!-to-analog converter to deliver the compensation voltage to the negative feedback terminal, wherein the processor sets the compensation voltage as proportional to the first offset, and offset by the second offset.

2. The sensor apparatus of claim 1 , wherein the processor is arranged to set the compensation voltage during a calibration operation performed at a calibration point corresponding to a known sensed condition.

3. The sensor apparatus of claim 2, wherein the calibration point corresponds to a no-load sensed condition.

4, The sensor apparatus of claim 3, wherein the processor is switchab!e between the calibration operation and a measurement operation during which a physical value related to the sensed condition is measured.

5. The sensor apparatus of eiaim 4, wherein the processor is configured to perform the calibration operation prior to performing the measurement operation.

6. The sensor apparatus of any one of claims 1 , wherein !he resistive-bridge sensor comprises a strain gauge as a sensing element.

7. The sensor apparatus of any one of claims 1 , wherein the resistive-bridge sensor comprises an ink level measurement sensor to measure an ink level based on its output voltage. S. The sensor apparatus of claim 1 , wherein the resistive-bridge sensor is one from a group of resistive-bridge sensors comprising: a quarter-bridge configuration, a half-bridge configuration, and a full-bridge configuration.

9, A method, comprising: producing, by a resistive-bridge sensor, an output voltage comprising a desired component and a first offset; receiving, by an indirect current feedback amplifier, the output voitage; outputting, by the amplifier, an amplified output voitage comprising an amplified desired component and a second offset, the amplified desired component related to the output voitage, the second offset related to an operating range of the output voltage; setting, by a processor, a compensation voltage to be output by a digita!-to-analog converter, the compensation voltage proportional to the first offset, and offset by the second offset; and receiving, by the amplifier, the compensation voitage as feedback voitage to compensate for the first offset.

10. The method of claim 9, wherein the setting the compensation voltage is performed during a calibration operation performed at a caiibration point corresponding to a known sensed condition. 11. The method of claim 10, comprising switching between the calibration operation and a measurement operation during which a physical vaiue related to the sensed condition is measured.

12. The method of claim 10, wherein the caiibration operation is performed before the measurement operation.

13. The method of claim 10, comprising storing a vaiue of the compensation voltage in nonvolatile memory. 14. A non-transitory machine-readable storage medium encoded with instructions executable by a processor, the machine-readable storage medium comprising: instructions to receive an output voltage of a resistive-bridge sensor, the output voltage comprising a desired component and a first offset; instructions to receive an amplified output voltage of an amplifier, the amplified output voitage comprising an amplified desired component and a second offset, the amplified desired component related to the output voltage, the second offset reiated to an operating range of the output voitage; instructions to set a digital-to-ana!og converter to output a compensation voltage proportional to the first offset voltage, and offset by the second offset voltage as a feedback signal to compensate for the first offset.

15. A compensation circuit comprising: an indirect current teedback amplifier comprising, first and second input terminals to receive a voltage output by a sensor, an output terminal to output an amplified output voltage comprising an amplified desired component and a second offset, the amplified desired component reiated to a voltage difference between the first and second input terminals, the second offset reiated to an operating range of the output voltage from the sensor, and a negative feedback terminal to receive a compensation voltage to compensate tor the first offset; and a processor comprising a digifa!-to-anaiog converter to deliver the compensation voltage to the negative feedback terminal, wherein the processor sets the compensation voltage as proportionai to the first offset, and offset by the second offset.