Force sensing system

Abstract

This disclosure relates to a compensation circuit for compensating for thermal effects in an output signal from a force sensor. The compensation circuit includes a monitoring circuit configured to monitor one or more electrical parameters of a resistive force sensor and a processing circuit. The processing circuit is configured to determine the absolute resistance value of the force sensor based on the one or more monitored electrical parameters, and to adjust one or more operating parameters of the force sensor system based at least in part on the determined absolute resistance value.

Description

Technical Field

[0001] This disclosure relates to the field of force sensing systems. Background Technology

[0002] Electronic devices such as mobile phones and tablet computers typically include one or more mechanical switches or buttons (i.e., user input transducers) for receiving user input, such as adjusting the volume of the audio output from the device. Such mechanical switches and buttons have several disadvantages, including: susceptibility to damage from water, dust, and other debris; limited lifespan due to mechanical wear; and relatively large size and / or cost compared to some other types of user input transducers.

[0003] Force sensors are increasingly being used as replacements for traditional mechanical switches and buttons, serving as user input transducers to detect user inputs such as touch and button presses. Compared to mechanical switches, buttons, and other types of user input transducers or devices, force sensors are generally less susceptible to the adverse effects of aging because they typically contain no moving parts, or fewer moving parts than mechanical switches or buttons.

[0004] Furthermore, force sensors are typically implemented in such a way that there is no gap (i.e., discontinuity) through which water, dust, or other debris can enter the sensor or the device containing the sensor, making them particularly suitable for applications where preventing the ingress of water, dust, and other debris is critical. For example, a resistive force sensor can be implemented by printing a pattern of resistive ink onto a suitable substrate or carrier.

[0005] Furthermore, force sensors typically occupy less physical space than mechanical switches, buttons, etc. with equivalent functions. Therefore, using force sensors can increase the amount of space available for other components of the device or reduce the overall size of the device. Both of these are major advantages in the design and development of modern small form factor devices such as mobile phones, for which integrating multiple different functions within a limited space has always been a challenge.

[0006] In addition, the use of force sensors can enhance feature content by allowing the recognition of the shape and force of button presses and mapping them to specific functions, and can, for example, allow the entire edge of the phone to be implemented as a continuous “button” strip, thereby improving device functionality and user experience.

[0007] Therefore, force sensors represent a viable and commercially attractive alternative to traditional mechanical switches and buttons for user input transducers. However, using force sensors as input devices presents other challenges. Embodiments of this disclosure aim to address at least some of these challenges in part. Summary of the Invention

[0008] According to a first aspect, the present invention provides a compensation circuit for compensating for thermal effects on a resistive force sensor in a force sensor system, the compensation circuit comprising:

[0009] A monitoring circuit, configured to monitor one or more electrical parameters of the resistive force sensor; and

[0010] Processing circuitry, wherein the processing circuitry is configured to:

[0011] The absolute resistance value of the force sensor is determined based on one or more monitored electrical parameters; and

[0012] One or more operating parameters of the force sensor system are adjusted, at least in part, based on the determined absolute resistance value.

[0013] One or more monitored electrical parameters may include the current passing through a force sensor.

[0014] Alternatively, one or more monitored electrical parameters may include the voltage across the force sensor.

[0015] The processing circuit can be configured to determine the compensation factor based on the offset between the determined absolute resistance value of the force sensor and the initial absolute resistance value.

[0016] The processing circuitry can be further configured to apply a compensation factor to adjust one or more operating parameters of the force sensor system in order to generate a compensated force sensor output signal.

[0017] The processing circuitry can be further configured to process the compensated force sensor output signal to determine whether the compensated force sensor output signal corresponds to a valid user input.

[0018] The processing circuitry can be configured to compare the compensated force sensor output signal with a threshold to determine whether the compensated force sensor output signal corresponds to a valid user input.

[0019] The processing circuitry can be configured to compare the compensated force sensor output signal with a known signature of a valid user input to determine whether the compensated force sensor output signal corresponds to a valid user input.

[0020] One or more operating parameters may include one or more of the following:

[0021] Simulated gain;

[0022] Digital gain;

[0023] Time constant;

[0024] User input signal validity threshold;

[0025] Filter coefficients;

[0026] Cutoff frequency;

[0027] The operating point of the force sensor;

[0028] The bias voltage of the force sensor; and

[0029] Bias current of the force sensor.

[0030] The processing circuit can be further configured as follows:

[0031] Monitor the output voltage of the force sensor; and

[0032] The force sensor system's one or more operating parameters are adjusted based on the determined absolute resistance value and the monitored output voltage.

[0033] The processing circuit can be configured to determine the thermal gradient compensation factor based on the offset between the determined absolute resistance value of the force sensor and the initial absolute resistance value.

[0034] The thermal gradient compensation factor may include a scaling factor to be applied to a given absolute resistance value to generate a compensated absolute resistance value.

[0035] The processing circuitry can be configured to determine a measure of signal validity based on the compensated absolute resistance value and the monitored voltage.

[0036] The processing circuitry can be configured to compare a signal validity metric with a signal validity threshold.

[0037] The processing circuit can be configured to process the force sensor output signal or the compensated force sensor output signal when the signal validity metric exceeds the signal validity threshold.

[0038] The processing circuitry can be configured to compare the force sensor output signal or the compensated force sensor output signal with a threshold to determine whether the compensated force sensor output signal corresponds to a valid user input.

[0039] The processing circuitry can be configured to compare the force sensor output signal or the compensated force sensor output signal with a known signature of a valid user input to determine whether the compensated force sensor output signal corresponds to a valid user input.

[0040] The processing circuitry can be operated to adjust one or more operating parameters of the force sensor system in response to predetermined conditions, based at least in part on a determined absolute resistance value.

[0041] The pre-determined conditions may be based on one or more of the following:

[0042] The force sensor's determined absolute resistance value is compared with a predetermined threshold.

[0043] A comparison of the relative change in the absolute resistance of the force sensor over time with the change in the target;

[0044] The comparison between the determined absolute resistance of the force sensor and the output voltage of the force sensor; or

[0045] Comparison of the absolute resistance of multiple different force sensors.

[0046] According to a second aspect, the present invention provides a compensation circuit for compensating for thermal effects on a resistive force sensor in a force sensor system, the compensation circuit comprising:

[0047] A monitoring circuit, configured to monitor the absolute resistance of the force sensor; and

[0048] An amplifier circuit configured to amplify the output signal from the resistive force sensor, wherein the gain of the amplifier circuit varies based on the absolute resistance of the force sensor.

[0049] According to a third aspect, the present invention provides a compensation circuit for compensating for thermal effects on a resistive force sensor in a force sensor system, the compensation circuit comprising:

[0050] A monitoring circuit, configured to monitor the absolute resistance of the force sensor; and

[0051] A processing circuit configured to process the output signal from the resistive force sensor, wherein the processing circuit is configured to monitor the voltage of the output signal and adjust one or more operating parameters of the force sensor system based on a determined absolute resistance value and the monitored output voltage.

[0052] According to a fourth aspect, the present invention provides an integrated circuit comprising a compensation circuit according to any one of the first to third aspects.

[0053] According to a fifth aspect, the present invention provides a force sensor circuit comprising a resistive force sensor and a compensation circuit according to any one of the first to third aspects.

[0054] According to a sixth aspect, the present invention provides an apparatus including a force sensor circuit according to a fifth aspect.

[0055] The device may include a mobile phone, tablet computer, laptop computer, portable media player, gaming device, game controller, in-vehicle entertainment system, or battery-powered device.

[0056] According to a seventh aspect, the present invention provides a resistive force sensor comprising a plurality of resistors arranged in a bridge circuit, wherein the resistors cause an equal and opposite change in resistance values ​​on opposite sides of the bridge circuit when a force is applied to the sensor.

[0057] According to an eighth aspect, the present invention provides a resistive force sensor according to claim 26, wherein the plurality of resistors includes four resistors, and wherein the resistors are arranged on a sensor substrate such that when a force is applied to the sensor, two resistors are in a tensioned state and two resistors are in a compressed state.

[0058] Each resistor in the resistor can be shaped such that when a force is applied to the force sensor, the change in resistance of the two resistors in the compressed state is equal to and opposite to the change in resistance of the two resistors in the tensioned state.

[0059] Alternatively, the value of each resistor in the resistor can be set such that when a force is applied to the force sensor, the change in resistance value of the two resistors in the compressed state is equal to and opposite to the change in resistance value of the two resistors in the tensioned state.

[0060] Alternatively, the resistivity or resistance value of each resistor in the resistors is configured such that when a force is applied to the force sensor, the change in resistance value of the two resistors in the compressed state is equal to and opposite to the change in resistance value of the two resistors in the tensioned state.

[0061] According to a ninth aspect, the present invention provides a resistive force sensor comprising a plurality of resistors arranged such that when the resistivity of one or more of the resistors changes due to an applied force, the absolute resistance of the resistive force sensor remains unchanged. Attached Figure Description

[0062] Embodiments of the invention will now be described strictly by way of example only with reference to the accompanying drawings, in which:

[0063] Figure 1 This is a schematic representation of a differential resistive force sensor;

[0064] Figure 2a and Figure 2b The arrangement of the force sensors is shown;

[0065] Figure 2c A force sensor mounted on the wall of the main unit is shown;

[0066] Figure 2d The force pair is shown Figure 2c The effect of the installed force sensor;

[0067] Figure 3 A force sensor is schematically shown mounted on the wall of the host unit using adhesive, with a heat source close to the wall of the host unit;

[0068] Figure 4 The effect of temperature on the absolute resistance of a resistive force sensor and the resulting change in the force sensor's sensitivity are shown.

[0069] Figure 5 This is a schematic block diagram of a force sensor system, which includes a resistive force sensor and a compensation circuit for compensating for thermal effects on the force sensor.

[0070] Figure 6 This is a flowchart illustrating the operation performed by a processing circuit to process the signal output by the force sensor, thereby compensating for changes in the force sensor sensitivity due to temperature variations.

[0071] Figure 7 The differences between the absolute resistance values ​​of the force sensor and those with a valid user input are shown, as well as the differences between the differential output voltages of the force sensor and those with a valid user input.

[0072] Figure 8 This is a flowchart illustrating the operation performed by a processing circuit to process the signal output by the force sensor to compensate for the effect of the temperature gradient in the force sensor;

[0073] Figure 9 The effects of some operations performed by the processing circuitry are illustrated graphically.

[0074] Figure 10 This is a schematic diagram of a force sensor system, which includes a resistive force sensor and an alternative compensation circuit for compensating for thermal effects on the force sensor; and

[0075] Figure 11 This is a schematic diagram of a force sensor system, which includes a resistive force sensor and other alternative compensation circuitry for compensating for thermal effects on the force sensor. Detailed Implementation

[0076] First refer to Figure 1The force sensor is generally shown as 100. In the example shown, the force sensor 100 is a resistive force sensor that includes a first resistor 102, a second resistor 104, a third resistor 106, and a fourth resistor 108 (shown in dashed outline at 120) arranged in a Wheatstone bridge configuration. Thus, the first resistor 102 and the second resistor 104 are connected in series between a first power supply rail or terminal 110 that receives a bias voltage Vbias from a voltage source such as a battery (typically via a regulator such as a low dropout regulator (LDO)) and a second power supply rail or terminal 112 coupled to a reference voltage such as ground (Gnd), thereby forming a first resistive voltage divider that produces a first output voltage Vp at node 114 between the series-connected first resistor 102 and second resistor 104. Similarly, the third resistor 106 and the fourth resistor 108 are connected in series between the first power supply rail or terminal 110 and the second power supply rail or terminal 112 to form a second resistive voltage divider (connected in parallel with the first voltage divider). The second resistive voltage divider generates a second output voltage Vn at node 116 between the series-connected third resistor 106 and fourth resistor 108.

[0077] Resistors 102, 104, 106, and 108 can be selected such that the ratio of the value R1 of the first resistor 102 to the value R2 of the second resistor 104 is equal to the ratio of the value R3 of the third resistor 106 to the value R4 of the fourth resistor 108, i.e., R1:R2 = R3:R4. Therefore, in the use of force sensor 100, when no force is applied to force sensor 100, the value of the first output voltage Vp is equal to the value of the second output voltage Vn, making the differential output voltage Vout of sensor 100 (i.e., Vp-Vn) equal to zero. When force is applied to force sensor 100, the resistance values ​​R1-R4 of one or more resistors 102-108 change, causing the magnitude of the first output voltage Vp to differ from the magnitude of the second output voltage Vp, and thus the differential output voltage Vout of force sensor 100 takes a non-zero value, which is a function of the amount of applied force. In this way, the force sensor 100 can output a differential sensor output voltage signal Vout, which indicates the magnitude of the force applied to the force sensor 100.

[0078] Force sensor 100 can be configured such that when a force is applied, two resistors (e.g., first resistor 102 and fourth resistor 108) are in a compressed state, and therefore their resistance values ​​decrease compared to when no force is applied, while the other two resistors (e.g., second resistor 104 and third resistor 106) are in a tensioned state, such that their resistance values ​​increase compared to when no force is applied. In this way, when a force is applied to force sensor 100, the differential output voltage Vout can be maximized, which can help increase the likelihood of correctly detecting user input on force sensor 100.

[0079] The differential output voltage Vout is processed by downstream analog or digital processing circuitry, for example, to detect user input (e.g., a press or touch on force sensor 100) based on the differential output voltage Vout.

[0080] Force sensors of the type described above (where some resistors may be in a compressed state when a force is applied, while others may be in a tensioned state) are typically configured as multilayer sensors in which the resistors are stacked along the axis of the force, and the material thickness between the resistors is known. Therefore, when a sensor containing stacked resistors bends due to a force, the resistors on the inner radius of the bend are in a compressed state, while those on the outer radius of the bend are in a tensioned state.

[0081] For the above types of force sensors and Figure 1 The force sensor shown, force sensor 100, has an absolute resistance value that can be defined based on the voltage across the force sensor and the current flowing through it. For example, in... Figure 1 In the example force sensor 100, the absolute resistance value can be defined as:

[0082] Rabsolute = Vbias / Isense

[0083] Where Vbias is the bias voltage applied to the force sensor 100 and Isense is the current through the force sensor 100.

[0084] Figure 2a The front surface of the resistive force sensor 200 is schematically shown, and the first resistor 102 and the fourth resistor 108 of the Wheatstone bridge 120 positioned on the front side of the sensor substrate 210 are shown. Figure 2b The rear side of the resistive force sensor 200 is schematically shown, illustrating the second resistor 104 and the third resistor 106 of the Wheatstone bridge positioned on the rear side of the sensor substrate 210.

[0085] The above types in Figure 2a and 2bThe force sensor shown is typically mounted on the inner surface of an outward-facing wall of a host device such as a mobile phone, tablet computer, or laptop computer, so that the force applied to the wall is transmitted to the force sensor 200. Figure 2c This arrangement is shown in the figure. Figure 2c A force sensor 200 is shown mounted on the wall 220 of the host device. Figure 2d The effect of the force 230 applied to the wall 220 is shown (in an exaggerated manner), showing that the first resistor 102 and the fourth resistor 108 of the force sensor 200 are in a compressed state due to the force 230, while the second resistor 104 and the third resistor 106 of the force sensor 200 are in a tensile state due to the force 230.

[0086] The force sensor 200 can be attached to the wall 220 of the main unit using adhesive 240, such as... Figure 3 As shown. When the force sensor 200 is attached to the wall 220 in this manner using adhesive 240, when the host device is exposed to a heat source (which... Figure 3 When the heat source 300 is in the middle, the sensitivity of the user input transducer containing the force sensor 200 to the applied force may decrease.

[0087] The heat source 300 can be external to the host device, such as a heat sink, the user's body (e.g., if the host device is stored in the user's pocket), or if the user's fingers are warmer than the host device (e.g., if the body or chassis of the host device is cold), or even the user's fingers on the host device.

[0088] Alternatively, the heat source 300 may be located inside the main unit. For example, the processing circuitry and / or battery of the main unit may become hot during use, especially during extended periods of use.

[0089] When exposed to heat source 300 (whether external or internal to the main unit), adhesive 240 may soften. Due to this softening, the amount of force that must be applied to the wall 220 to which force sensor 200 is attached in order to generate a given output voltage Vout may be greater than the amount of force that must be applied to force sensor 200 to generate that output voltage in the absence of heat source 300. Therefore, in the presence of heat source 300, the adhesive 240 may soften. Figure 2c The sensitivity of the user input transducer in a force sensor arrangement of the type shown may be reduced. This is undesirable because the resulting non-uniform response of the user input transducer to user input can lead to a poor user experience.

[0090] In addition, temperature changes in the force sensor 200 will cause changes in the resistance values ​​of the constituent resistors 102-108, which may lead to changes in the sensitivity of the force sensor 200.

[0091] The latter effect is Figure 4 As shown in the figure, the topmost curve 410 shows that an increase in the temperature of the force sensor (trace 412) results in a corresponding increase in the absolute resistance value of the force sensor (trace 414).

[0092] Figure 4 The bottom graph shows the effect of increasing the temperature of the force sensor on the force required to produce a given force sensor output voltage. As can be seen from trace 422, without any compensation, the force required to produce a given force sensor output voltage increases with increasing temperature of the force sensor 200. Trace 424 shows the compensation factor that needs to be applied to maintain a uniform response (trace 426) as the temperature of the force sensor 200 increases.

[0093] Additionally, the presence of heat source 300 (external or internal to the host device) may cause a temperature gradient within force sensor 200 (in... Figure 3 (Indicated by arrow 310), this may cause the output voltage Vout to be incorrectly interpreted as a change in user input (touch, press, etc.) on the user input transducer containing the force sensor 200.

[0094] For example, such as Figure 2c As shown, the first resistor 102 and the fourth resistor 108 of the force sensor 200 are positioned near the wall 220 of the host device, and the second resistor 104 and the third resistor 106 of the force sensor 200 are physically separated from the first resistor 102 and the fourth resistor 108 by the thickness of the force sensor substrate 210. Therefore, when a heat source 300 is present in the host device, the temperature of the first resistor 102 and the fourth resistor 108 will differ from the temperature of the second resistor 104 and the third resistor 106 for a certain period of time, which continues until thermal equilibrium is reached within the force sensor 200 (i.e., until all resistors 102-108 are at the same temperature). Therefore, during this period, a temperature gradient will exist within the force sensor 200, such that the heating effect of the heat source 300 has a greater impact on the resistance values ​​of the first resistor 102 and the fourth resistor 108 than on the resistance values ​​of the second resistor 104 and the third resistor 106.

[0095] As will be understood, if the heat source is inside the host device (e.g., if the internal circuitry or internal battery of the host device acts as the heat source), causing the second resistor 104 and the third resistor 106 to be exposed to the elevated temperature before the first resistor 102 and the fourth resistor 108, then a temperature gradient in the opposite direction may occur.

[0096] Since the output voltage Vout of the force sensor 200 varies depending on the resistance values ​​of the individual resistors 102-108 that constitute the force sensor, any thermal effects that affect the resistors 102-108 differently may cause the output voltage Vout to be misinterpreted as an indication of changes in user input (such as touch or press) on the user input transducer containing the force sensor 200.

[0097] Figure 5 This is a schematic block diagram showing a circuit including a compensation circuit for processing the force sensor output signal to compensate for thermal effects in the output voltage Vout of a resistive force sensor of the type described above.

[0098] exist Figure 5 The circuit, generally shown at 500, includes a force sensor 200 of the type described above, which receives a bias voltage Vbias from a first voltage rail 502. The input of a differential amplifier circuit 510 is coupled to the differential output of the force sensor 200, and the differential amplifier circuit 510 operates to amplify the differential voltage Vout output by the force sensor 200 and output an amplified output signal Vamp.

[0099] Circuit 500 also includes a compensation circuit 520, which includes a current sensor circuit 530, an analog-to-digital converter (ADC) circuit 540, and a signal processing circuit 550.

[0100] A current sensor circuit 530 is connected in series between the force sensor 200 and a second voltage rail 504, which provides a reference voltage (e.g., ground or a 0-volt reference voltage) to the circuit 500. The current sensor circuit 530 may include, for example, a current-sensing resistor with a known resistance value and an associated voltage detection circuit configured to measure the voltage drop across the current-sensing resistor. It should be understood that, although... Figure 5 The example shown performs low-side current sensing, but other examples may perform high-side current sensing, as will be understood and known to those skilled in the art.

[0101] The current sensor circuit 530 is configured to output a current sensing signal indicating the current Isense passing through the force sensor 200 to the ADC circuit 540. The ADC circuit 540 then converts the current sensing signal into a digital signal indicating the current Isense, and outputs the digital signal to the signal processing circuit 550 to compensate for changes in sensor sensitivity caused by thermal effects on the force sensor 200 and / or errors in the differential voltage Vout output by the force sensor 200, as will be described in detail below.

[0102] It will be understood that if the current sensor circuit 530 includes its own ADC circuit and is therefore configured to output a digital signal indicating the current Isense, then the ADC circuit 540 can be omitted. Alternatively, if the signal processing circuit 550 is an analog signal processing circuit, the ADC circuit 540 can also be omitted.

[0103] Signal processing circuit 550 (as indicated above) (which may be an analog signal processing circuit, a digital signal processing circuit, or a combination of both) is configured to monitor the output of amplifier circuit 510 to determine whether force sensor 200 has received valid user input such as pressing or touching.

[0104] For this purpose, the signal processing circuit 550 can be configured to sample the signal output by the amplifier circuit 510 at a predetermined sampling frequency and determine whether the force sensor 200 has received valid user input based on the value of the sample. For example, the signal processing circuit 550 can be configured to compare each sample value with a predetermined threshold or with one or more values ​​of one or more predetermined signatures of valid user input in order to determine whether the sample value corresponds to a sample value of valid user input.

[0105] If the signal processing circuit 550 determines that the force sensor 200 has received valid user input, the signal processing circuit can output a signal indicating valid user input to a downstream processing circuit, such as a host device, which can take appropriate action in response to the detection of valid user input.

[0106] As described above, thermal effects can adversely affect the sensitivity of the user input transducer, including the force sensor 200. The signal processing circuit 550 is configured to compensate for such thermal effects in order to compensate for changes in the sensitivity of the user input transducer and / or reduce the risk of erroneous detection of user input on the transducer.

[0107] By determining the current absolute resistance value of the force sensor 200 and comparing it with a known or predetermined initial absolute resistance value of the force sensor (e.g., a value determined during the initial calibration of circuit 500), signal processing circuit 550 can determine a compensation factor to be applied before or during processing the sensor output signal to compensate for sensitivity variations in the user-input transducer.

[0108] In addition, the signal processing circuit 550 can use the determined current absolute resistance value in conjunction with the output voltage Vout of the force sensor 200 to determine whether the change in the output voltage Vout is caused by a valid user input or by a temperature gradient in the force sensor 200, and can accordingly change the mechanism used to detect valid user input, thereby reducing the risk of false detection of user input.

[0109] Figure 6 The flowchart illustrates the operation performed by signal processing circuitry 550 to compensate for changes in the sensitivity of the user input transducer that may be caused by thermal effects on force sensor 200.

[0110] Signal processing circuit 550 is configured to determine the current absolute resistance value of force sensor 200 based on a known bias voltage Vbias and the current Isense flowing through force sensor 200 (as determined by current sensor circuit 530 at a specific sampling time or within a specific sampling period). Therefore, signal processing circuit 550 determines (operation 610) the current absolute resistance value Rcurrent of force sensor 200 according to the following calculation:

[0111] Rcurrent = Vbias / Isense.

[0112] At operation 620, signal processing circuit 550 retrieves the initial absolute sensor resistance Rinitial. The initial absolute sensor resistance Rinitial may be, for example, a calibration value of the absolute resistance of sensor 200 determined by signal processing circuit 550, wherein force sensor 200 is at a known temperature and is stored in a memory, register, etc. associated with signal processing circuit 550.

[0113] At operation 630, signal processing circuit 550 determines the absolute sensor resistance offset value Roffset by subtracting the determined current absolute sensor resistance Rcurrent from the initial absolute sensor resistance Rinitial, i.e.:

[0114] Roffset=Rcurrent-Rinitial.

[0115] At operation 640, signal processing circuit 550 determines a compensation factor based on a defined offset value Roffset. The compensation factor is applied by signal processing circuit 550 (operation 650) to compensate for thermal effects on the output Vout of force sensor 200.

[0116] The compensation factor can be an analog gain compensation factor applied to amplifier circuit 510 to adjust the gain of amplifier circuit 510 in order to "normalize" the signal output by amplifier circuit 510, thereby at least partially compensating for the thermal effects on the output Vout of force sensor 200.

[0117] Alternatively, the compensation factor may change one or more other parameters, such as digital gain, time constant, one or more user input signal validity thresholds, one or more filter coefficients, cutoff frequency, etc., which can be used to process the sensor output signal Vout into output signal Vout and / or samples of output signal Vout by signal processing circuit 550 and / or amplifier circuit 510.

[0118] In addition, the compensation factor may be used, either additionally or alternatively, to adjust the operating point of the force sensor 200 (e.g., bias voltage or bias current) to at least partially compensate for thermal effects on the output Vout of the force sensor.

[0119] At operation 650, signal processing circuit 550 determines whether the compensated or normalized output signal (e.g., the sample value to which amplifier circuit 510 or signal processing circuit 550 applies compensation, as described above) corresponds to a valid user input. For example, signal processing circuit 550 may compare the compensated sample value with a known signature value of a valid user input to determine whether the compensated sample value corresponds to a valid user input. Alternatively, signal processing circuit 550 may compare the compensated sample value with a threshold to determine whether the compensated sample value corresponds to a valid user input.

[0120] If the signal processing circuit 550 determines that the compensated sample value corresponds to a valid user input, then at operation 670, an output signal indicating that a valid user input has been detected is output by the signal processing circuit 550 to the downstream processing circuit.

[0121] Otherwise, the signal processing circuit 550 does not output an output signal, or alternatively, at operation 680, an output signal indicating that no valid user input has been detected can be output by the signal processing circuit 550 to the downstream processing circuit.

[0122] Therefore, the signal processing circuit 550 is able to compensate for sensitivity changes that may be caused by thermal effects on the force sensor 200.

[0123] The signal processing circuit 550 can also compensate for errors that may be caused by thermal gradients in the force sensor 200, as will now be referred to. Figures 7 to 9 Described.

[0124] Figure 7 (In the topmost graph 710) the difference between the change in the absolute resistance of the force sensor 200 due to the temperature gradient in the force sensor 200 and the change in the absolute resistance of the force sensor 200 due to effective user input is shown. Figure 7(In the bottom graph 720) the difference between the change in the output voltage Vout of the force sensor 200 due to the temperature gradient in the force sensor 200 and the change in the output voltage Vout of the force sensor 200 due to the effective user input is also shown.

[0125] As can be seen from section 712 of graph 710, the effect of the temperature gradient in force sensor 200 is that the absolute resistance value of force sensor 200 increases relatively significantly. This is because the resistance values ​​of all component resistors 102-108 of force sensor 200 change in the same way due to the temperature change of the force sensor. Therefore, in the presence of a heat source (e.g., heat source 300), the resistance values ​​of all resistors 102-108 will increase (but to varying degrees, due to the thermal gradient in force sensor 200), resulting in a relatively large increase in the absolute resistance value of force sensor 200.

[0126] Conversely, as can be seen from portion 714 of graph 710, when the force sensor 200 is subjected to valid user input such as pressing or touching, the change in the absolute resistance value of the force sensor 200 is less than the change caused by the temperature gradient in the force sensor 200. This is because the effect of the increase in the resistance values ​​of the resistors 104 and 106, which are under tension due to the force applied by the user, on the absolute resistance value of the force sensor 200 is at least partially offset by the decrease in the resistance values ​​of the resistors 102 and 108, which are under compression due to the force.

[0127] Another effect of the temperature gradient in force sensor 200 is that the increase in the differential output voltage Vout of force sensor 200 is relatively small, as can be seen from part 722 of graph 720. This is also because the resistance values ​​of all the component resistors 102-108 of the force sensor change in the same way due to the temperature change of the force sensor. Therefore, in the presence of a heat source (e.g., heat source 300), the resistance values ​​of all resistors 102-108 will increase (but to varying degrees, due to the thermal gradient in force sensor 200), which results in a relatively small difference between the voltage Vp generated at node 114 and the voltage Vn generated at node 116, and therefore a relatively small change in the differential output voltage Vout of force sensor 200.

[0128] Conversely, as shown in section 724 of graph 720, when force sensor 200 is subjected to valid user input such as pressing or touching, the change in differential output voltage Vout of force sensor 200 is much greater than the change caused by the temperature gradient in force sensor 200. This is because the resistance values ​​of resistors 104 and 106, which are in a tensioned state due to the force applied by the user, increase, and the resistance values ​​of resistors 102 and 108, which are in a compressed state due to the force, decrease, causing the output voltages Vp and Vn to shift in different directions (e.g., Vp increases and Vn decreases, or vice versa). Therefore, when force sensor 200 is subjected to valid user input, the differential output voltage Vout of force sensor 200 undergoes a relatively large change.

[0129] By monitoring the absolute resistance value and the differential output voltage Vout of the force sensor 200, the signal processing circuit 550 can distinguish between the effects of the thermal gradient in the force sensor 200 and the effects of the valid user input on the force sensor 200, and thus can reduce the possibility of erroneous user input detection, as will now be referred to. Figure 8 and Figure 9 As described.

[0130] Figure 8 This is a flowchart illustrating the operation performed by signal processing circuitry 550 to compensate for the effect of thermal gradients in force sensor 200, thereby reducing the risk of erroneous user input detection. Figure 9 The effects of some of the operations performed by the signal processing circuit 550 are illustrated graphically.

[0131] The operation that causes the determination of the absolute sensor resistance offset value Roffset is similar to that in 810-830. Figure 6 The corresponding operations are 610-630, and therefore will not be described in detail here.

[0132] At operation 840, the signal processing circuit determines a thermal gradient compensation factor to be applied to the force sensor 200 based on the determined current absolute resistance value Rcurrent. For example, the thermal gradient compensation factor could be a scaling factor to be applied to Rcurrent.

[0133] At operation 850, signal processing circuitry 550 applies a thermal gradient compensation factor to the determined current absolute resistance value Rcurrent to generate a compensated absolute resistance value Rcomp. The compensated absolute resistance value Rcomp (or the signal indicating it) can be processed in a manner familiar to those skilled in the art. For example, one or more gains and / or one or more time constants and / or one or more thresholds can be applied.

[0134] At operation 860, signal processing circuit 550 determines a signal validity metric based on the compensated absolute sensor resistance value Rcomp and the sensor output voltage Vout. For example, the signal validity metric ValidSignal can be determined by simply subtracting the sensor output voltage Vout value from the corresponding compensated absolute sensor resistance value Rcomp, i.e.:

[0135] ValidSignal = RComp - Vout.

[0136] Therefore, the signal validity metric takes into account both the absolute resistance value of the force sensor 200 and its differential output voltage Vout, and can thus be used to distinguish between valid user input and effects caused by thermal gradients in the force sensor 200. As those skilled in the art will understand, the signal validity metric can be calculated in any other manner that takes into account both the absolute resistance value of the force sensor 200 and its differential output voltage Vout. Furthermore, the compensated absolute sensor resistance value Rcomp (or the signal it represents) can be subjected to independent filtering to remove noise or to better determine the resistance signature before calculating the signal validity metric.

[0137] At operation 865, the signal validity metric is compared with a predetermined signal validity threshold to determine whether the absolute resistance value of the force sensor and the output voltage Vout of the force sensor correspond to a valid user input or the effect of a thermal gradient in the force sensor 200.

[0138] If the signal validity metric exceeds the signal validity threshold, such as Figure 9 As shown in section 912 of graph 910, the signal processing circuit 550 determines that the detected change is not caused by the thermal gradient, and therefore the signal processing circuit passes the sensor differential output voltage Vout data (i.e., the sampled value of the sensor differential output voltage Vout) (at operation 870) for further processing to determine whether a valid user input has been received.

[0139] If so, the processing moves to operation 880, which is similar to operation 650 described above, where signal processing circuit 550 determines whether the sample value (which has been compensated for by amplifier circuit 510 or signal processing circuit 550 as described above) corresponds to a valid user input. If so, at operation 890, an output signal indicating that a valid user input has been detected is output by signal processing circuit 550 to downstream processing circuits (such as...). Figure 9 (As shown in part 922 of graph 920). Otherwise, the signal processing circuit 550 does not output an output signal (e.g., Figure 9(As shown in part 924 of the graph 920), or alternatively, at operation 895, an output signal indicating that no valid user input has been detected can be output by the signal processing circuit 550 to the downstream processing circuit.

[0140] If, at operation 865, the signal processing circuit 550 determines that the signal validity metric does not exceed the signal validity threshold, then... Figure 9 As shown in section 914 of graph 900, the signal processing circuit 550 determines that the detected change is caused by the thermal gradient, and therefore the processing moves to operation 895 because no valid user input is detected.

[0141] Figure 10 This is a schematic block diagram showing a circuit including an alternative compensation circuit for processing the force sensor output signal to compensate for thermal effects in the output voltage Vout of a resistive force sensor of the type described above.

[0142] exist Figure 10 The circuit shown generally as 1000 includes a force sensor 200 of the type described above, which receives a known bias current Ibias. The input of a differential amplifier circuit 510 is coupled to the differential output of the force sensor 200, and the differential amplifier circuit 510 operates to amplify the differential voltage Vout output by the force sensor 200 and output an amplified output signal.

[0143] Circuit 1000 also includes a compensation circuit 1020, which includes a voltage monitoring circuit 1030, an analog-to-digital converter (ADC) circuit 1040, and a signal processing circuit 1050.

[0144] Voltage monitoring circuit 1030 (performs high-side voltage detection in the illustrated example, but can also be configured to perform low-side voltage detection) is configured to output a voltage sensing signal to ADC circuit 1040, the voltage sensing signal indicating the voltage VSeminor across force sensor 200. ADC circuit 1040 then converts the voltage sensing signal into a digital signal indicating the voltage VSeminor and outputs this digital signal to signal processing circuit 1050.

[0145] As will be understood by those skilled in the art, if the voltage sensor circuit 1030 includes its own ADC circuit and is therefore configured to output a digital signal indicating the voltage across the force sensor 200, the ADC circuit 1040 may be omitted. Similarly, if the signal processing circuit 1050 is an analog signal processing circuit, the ADC circuit 1040 may also be omitted.

[0146] Signal processing circuitry 1050 (as indicated above) (which may be analog signal processing circuitry or alternatively digital signal processing circuitry) is configured to monitor the output of amplifier circuitry 510 to determine whether force sensor 200 has received valid user input such as a press or touch. Signal processing circuitry 1050 is typically referenced above. Figures 6 to 9 The operation described differs in that the bias current Ibias is known and the voltage VSeminor across the force sensor is measured. Therefore, in operations 610 and 810, the absolute resistance of the force sensor 200 is calculated by the signal processing circuit 1050 according to the following calculation:

[0147] Rabsolute = VSensor / Ibias.

[0148] All other operations performed by the signal processing circuitry to compensate for changes in sensitivity caused by thermal effects on the force sensor 200 and / or to reduce the likelihood of false alarms in user input detection caused by thermal gradients in the force sensor 200 are as described above. Figures 6 to 9 As described.

[0149] Figure 11 This is a schematic block diagram of a circuit that includes alternative compensation circuitry for processing the force sensor output signal to compensate for thermal effects in the output voltage Vout of a resistive force sensor of the type described above.

[0150] exist Figure 11 The circuit shown generally at 1100 includes a force sensor 200 of the type described above, which receives a bias voltage Vbias. The input of a differential amplifier circuit 510 is coupled to the differential output of the force sensor 200, and the differential amplifier circuit 510 operates to amplify the differential voltage Vout output by the force sensor 200 and output an amplified output signal.

[0151] Circuit 1100 also includes a compensation circuit 1120, which includes a voltage monitoring circuit 1030, an analog-to-digital converter (ADC) circuit 1040, and the aforementioned reference circuit. Figure 10 The described type of signal processing circuit 1150, and the above reference Figure 5 The types of current sensor circuits 530 and ADC circuits 540 described are as follows.

[0152] The voltage monitoring circuit 1030 is configured to output a voltage sensing signal indicating the voltage VSeminor across the force sensor 200 to the ADC circuit 1040. The ADC circuit 1040 then converts the voltage sensing signal into a digital signal indicating the voltage VSeminor and outputs the digital signal to the signal processing circuit 1150.

[0153] If the voltage sensor circuit 1030 includes its own ADC circuit and is therefore configured to output a digital signal indicating the voltage across the force sensor 200, the ADC circuit 1040 can be omitted. Alternatively, if the signal processing circuit 1150 is an analog signal processing circuit, the ADC circuit 1040 can also be omitted.

[0154] The current sensor circuit 530 is configured to output a current sensing signal indicating the current Isense passing through the force sensor 200 to the ADC circuit 540. The ADC circuit 540 then converts the current sensing signal into a digital signal indicating the current Isense and outputs the digital signal to the signal processing circuit 1150.

[0155] If the current sensor circuit 530 includes its own ADC circuit and is therefore configured to output a digital signal indicating the current Isense, then the ADC circuit 540 can be omitted. Alternatively, if the signal processing circuit 1150 is an analog signal processing circuit, then the ADC circuit 540 can also be omitted.

[0156] Figure 11 The examples shown include a high-side voltage monitoring circuit and a low-side current monitoring circuit, but those skilled in the art will understand that other examples may include a high-side current monitoring circuit and a low-side voltage monitoring circuit, or a high-side or low-side voltage and current monitoring circuit.

[0157] Signal processing circuit 1150 (as indicated above) (which may be an analog signal processing circuit or alternatively a digital signal processing circuit) is configured to monitor the output of amplifier circuit 510 to determine whether force sensor 200 has received valid user input such as a press or touch. Signal processing circuit 1150 is generally operable in the manner described above, except that it measures both the current Isense through force sensor 200 and the voltage VSeminor across force sensor 200. Therefore, in operations 610 and 810, the absolute resistance of force sensor 200 is calculated by signal processing circuit 1150 according to the following calculation:

[0158] Rabsolute=-VSensor / Isense.

[0159] All other operations performed by the signal processing circuitry to compensate for changes in sensitivity caused by thermal effects on the force sensor 200 and / or to reduce the likelihood of false alarms in user input detection caused by thermal gradients in the force sensor 200 are as described above. Figures 6 to 9 As described.

[0160] The above discussion describes dynamic compensation for thermal effects on force sensor 200. However, it should be understood that the above techniques can also be performed during the initial calibration of a system containing force sensor 200 and circuits 500, 1000, 1100, and such systems can be recalibrated occasionally or periodically thereafter using the above techniques.

[0161] The above-mentioned compensation technique can be triggered by multiple conditions, such as: if the measured or determined value of the absolute resistance of the force sensor 200 exceeds a predetermined threshold; if the relative change of the absolute resistance of the force sensor 200 over time differs from the target change; based on the comparison of the absolute resistance of the force sensor 200 and the differential output voltage Vout of the force sensor 200; the comparison of the absolute resistances of multiple different force sensors; or any combination of two or more of the above factors.

[0162] To further improve the performance of a system that includes the force sensor and compensation circuit described above, an improved force sensor can be employed.

[0163] As referenced above Figure 1 The resistive force sensor includes a first resistor 102, a second resistor 104, a third resistor 106, and a fourth resistor 108 arranged in a Wheatstone bridge configuration. In the improved force sensor according to this disclosure, the Wheatstone bridge can be designed such that bending stress caused when a force is applied to the force sensor (e.g., by a user pressing a pressure sensor or the wall of the host device on which the force sensor is mounted) results in a change in the resistance value of the resistors 102 and 108 in a compressed state, said change being equal to and opposite to the change in the resistance value of the resistors 104 and 106 in a tensioned state.

[0164] In this arrangement, the absolute resistance of the force sensor does not change when a force is applied, and therefore any change in the absolute resistance of the force sensor will only be due to temperature changes in the force sensor. Thus, this arrangement improves the detection of temperature changes that may affect the force sensor. This arrangement also improves the detection of temperature gradients in the force sensor, since any change in the output voltage and the change in the absolute resistance of the force sensor must be due to a temperature gradient.

[0165] Various properties of resistors 102-104 can be adjusted to achieve this effect. For example, the shape of each resistor in resistors 102-104 can be selected such that the bending stress caused by the application of force to the force sensor results in a change in the resistance value of resistors 102 and 108 in a compressed state, the change being equal to and opposite to the change in the resistance value of resistors 104 and 106 in a tensioned state. Alternatively, the magnitude of each resistor in resistors 102-108 can be selected to achieve this effect. Furthermore, the resistivity or resistance value of each resistor in resistors 102-108 can be selected to achieve this effect. Any single property of resistors 102-108 or any combination of these properties can be selected or adjusted to achieve the desired effect.

[0166] The above reference Figures 5 to 11 The described compensation circuit can be configured as a standalone module or circuit, which can be coupled to the force sensor circuit. Alternatively, the compensation circuit or module can be housed within a package that includes the force sensor circuit. For example, the compensation circuit and the force sensor can be mounted on a common substrate (e.g., a printed circuit board) to form a combined force sensor / compensation circuit or module. As another alternative, the compensation circuit or module and / or the force sensor circuit can be housed within a package that includes force sensing signal acquisition and / or processing circuitry.

[0167] Compensation circuitry (whether configured as a standalone module or packaged with a force sensor) can be incorporated into devices that use one or more force sensors as user input transducers (e.g., portable devices such as mobile phones, tablets or laptops, portable media players, in-vehicle entertainment systems, gaming devices, or controllers). Such devices are typically battery powered.

[0168] As will be understood from the foregoing discussion, this disclosure provides an effective mechanism for compensating for thermal effects in the output of a force sensor, thereby enabling accurate detection of the desired sensing signal in the force sensor output.

[0169] It should be noted that the above embodiments are illustrative and not limiting of the invention, and those skilled in the art will be able to devise many alternative embodiments without departing from the scope of the appended claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in the claims, "an" or "a" does not exclude a plurality / type, and a single feature or other unit may perform the function of several units recited in the claims. Any reference numerals or labels in the claims should not be construed as limiting their scope.

Claims

1. A compensation circuit for compensating for thermal effects on a resistive force sensor in a force sensor system, the compensation circuit comprising: A monitoring circuit configured to monitor the voltage across the resistive force sensor and the current through the resistive force sensor; as well as Processing circuitry, wherein the processing circuitry is configured to: The absolute resistance value of the force sensor is determined based on the monitored voltage across the resistive force sensor and the monitored current flowing through the resistive force sensor; and One or more operating parameters of the force sensor system are adjusted, at least in part, based on the determined absolute resistance value.

2. The compensation circuit of claim 1, wherein the processing circuit is configured to determine a compensation factor based on the offset between the determined absolute resistance value of the force sensor and the initial absolute resistance value.

3. The compensation circuit of claim 2, wherein the processing circuit is further configured to apply the compensation factor to adjust the one or more operating parameters of the force sensor system in order to generate a compensated force sensor output signal.

4. The compensation circuit of claim 3, wherein the processing circuit is further configured to process the compensated force sensor output signal to determine whether the compensated force sensor output signal corresponds to a valid user input.

5. The compensation circuit of claim 4, wherein the processing circuit is configured to compare the compensated force sensor output signal with a threshold to determine whether the compensated force sensor output signal corresponds to a valid user input.

6. The compensation circuit of claim 4, wherein the processing circuit is configured to compare the compensated force sensor output signal with a known signature of a valid user input to determine whether the compensated force sensor output signal corresponds to a valid user input.

7. The compensation circuit according to claim 1, wherein the one or more operating parameters include one or more of the following: Simulated gain; Digital gain; Time constant; User input signal validity threshold; Filter coefficients; Cutoff frequency; The operating point of the force sensor; The bias voltage of the force sensor; and The bias current of the force sensor.

8. The compensation circuit according to claim 1, wherein the processing circuit is further configured to: Monitor the output voltage of the force sensor; and The force sensor system's one or more operating parameters are adjusted based on the determined absolute resistance value and the monitored output voltage.

9. The compensation circuit of claim 8, wherein the processing circuit is configured to determine a thermal gradient compensation factor based on the offset between the determined absolute resistance value of the force sensor and the initial absolute resistance value.

10. The compensation circuit of claim 9, wherein the thermal gradient compensation factor includes a scaling factor to be applied to the determined absolute resistance value to generate the compensated absolute resistance value.

11. The compensation circuit of claim 9, wherein the processing circuit is configured to determine a signal validity metric based on the absolute resistance value of the compensation and the monitored voltage.

12. The compensation circuit of claim 11, wherein the processing circuit is configured to compare the signal validity metric with a signal validity threshold.

13. The compensation circuit of claim 12, wherein the processing circuit is configured to process the force sensor output signal or the compensated force sensor output signal when the signal validity metric exceeds the signal validity threshold.

14. The compensation circuit of claim 13, wherein the processing circuit is configured to compare the force sensor output signal or the compensated force sensor output signal with a threshold to determine whether the compensated force sensor output signal corresponds to a valid user input.

15. The compensation circuit of claim 13, wherein the processing circuit is configured to compare the force sensor output signal or the compensated force sensor output signal with a known signature of a valid user input to determine whether the compensated force sensor output signal corresponds to a valid user input.

16. The compensation circuit of claim 1, wherein the processing circuit is operable to adjust one or more operating parameters of the force sensor system in response to predetermined conditions, at least in part based on a determined absolute resistance value.

17. The compensation circuit of claim 16, wherein the predetermined condition is based on one or more of the following: The force sensor's determined absolute resistance value is compared with a predetermined threshold. A comparison of the relative change in the absolute resistance of the force sensor over time with the change in the target; The comparison between the determined absolute resistance of the force sensor and the output voltage of the force sensor; or Comparison of the absolute resistance of multiple different force sensors.

18. A compensation circuit for compensating for thermal effects on a resistive force sensor in a force sensor system, the compensation circuit comprising: A monitoring circuit configured to monitor the absolute resistance of the force sensor, wherein the absolute resistance of the force sensor is determined based on the voltage across the force sensor and the current through the force sensor. as well as An amplifier circuit configured to amplify the output signal from the resistive force sensor, wherein the gain of the amplifier circuit varies based on the absolute resistance of the force sensor.

19. A compensation circuit for compensating for thermal effects on a resistive force sensor in a force sensor system, the compensation circuit comprising: A monitoring circuit configured to monitor the absolute resistance of the force sensor, wherein the absolute resistance of the force sensor is determined based on the voltage across the force sensor and the current through the force sensor. as well as A processing circuit configured to process the output signal from the resistive force sensor, wherein the processing circuit is configured to monitor the voltage of the output signal and adjust one or more operating parameters of the force sensor system based on a determined absolute resistance value and the monitored output voltage.

20. An integrated circuit comprising the compensation circuit according to claim 1.

21. A force sensor circuit comprising the resistive force sensor and compensation circuit according to claim 1.

22. An apparatus comprising the force sensor circuit according to claim 21.

23. The apparatus of claim 22, wherein the apparatus comprises a mobile phone, tablet computer, laptop computer, portable media player, gaming device, game controller, in-vehicle entertainment system, or battery-powered device.

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