Sensor device

Abstract

This sensor device includes a sensor element (11), a switch (12) connected to the sensor element (11), an amplifier (14) connected to the switch (12), a load resistance (13) connected between an input of the amplifier (14) and a ground terminal, and a control unit (7) for causing the switch (11) to perform a switching operation. The control unit (7) sets an off period during which the switch (11) is turned off, on the basis of an output voltage of the amplifier (14).

Description

Sensor Device

[0001] The present invention relates to a sensor device.

[0002] When a piezoelectric film is distorted (including expansion and contraction) by deformation, polarization proportional to the strain occurs on the surface of the piezoelectric film, generating an electric charge. Piezoelectric sensors are used to detect various pressures by utilizing this piezoelectric effect. For example, when a piezoelectric sensor is fixed to a detection site, the piezoelectric sensor deforms due to pressure (e.g., human muscle movement and pulse) or vibration, generating an electric charge in each sensor element included in the piezoelectric sensor. This electric charge is converted into a voltage, and pressure is detected based on this voltage. Because the detected pressure is easy to analyze, visualize, and record, it is expected to be effectively used in various fields such as medicine, sports, and gesture user interfaces (UI).

[0003] Taking advantage of the flexibility of piezoelectric film, it is possible to attach a piezoelectric sensor to the human body or to wear it in a way that conforms to a curved surface. Depending on the degree of deformation of the area where it is worn, the pressure and the amount of deformation of the film vary, and the output voltage of the piezoelectric sensor can change over a wide range. Therefore, ingenuity is required in the piezoelectric sensor material, detection circuit, and detection method so that it can detect a wide range of pressures, from very small to large.

[0004] Furthermore, if the piezoelectric sensor circuit is designed to accommodate large pressures, the output voltage from minute pressures will be at the same level as noise (such as power supply noise), making it difficult to detect pressure. Conversely, if the piezoelectric sensor circuit is designed to accommodate minute pressures, the output voltage will always exceed the voltage range that can be handled, making it impossible to determine the overall picture of pressure changes.

[0005] JP 2005-185631 A International Publication No. 2022 / 130684 JP 2020-156659 A

[0006] The present disclosure provides a sensor device capable of improving the dynamic range.

[0007] According to a first aspect of the present invention, there is provided a sensor device comprising a sensor element, a switch connected to the sensor element, an amplifier connected to the switch, a load resistor connected between an input of the amplifier and a ground terminal, and a control unit that causes the switch to perform a switching operation, wherein the control unit sets an off period for turning off the switch based on the output voltage of the amplifier.

[0008] According to a second aspect of the present invention, there is provided a sensor device according to the first aspect, wherein the control unit relatively shortens the off period when the output voltage of the amplifier is equal to or greater than a threshold value, and relatively lengthens the off period when the output voltage of the amplifier is less than the threshold value.

[0009] According to a third aspect of the present invention, there is provided the sensor device according to any one of the above aspects, wherein the control unit sets the off period based on a switching signal from an external device.

[0010] According to a fourth aspect of the present invention, there is provided a sensor device according to any one of the above aspects, further comprising an A / D converter connected to the output of the amplifier.

[0011] According to a fifth aspect of the present invention, there is provided a sensor device according to any of the above aspects, wherein the control unit controls the operation of the A / D converter so that the switching period of the switch and the sampling period of the A / D converter are the same.

[0012] According to a sixth aspect of the present invention, there is provided the sensor device according to any one of the above aspects, wherein the sensor element is a piezoelectric element.

[0013] According to a seventh aspect of the present invention, there is provided a sensor device comprising a plurality of sensor elements, a multiplexer connected to the plurality of sensor elements, an amplifier connected to the multiplexer, a load resistor connected between an input of the amplifier and a ground terminal, and a control unit that controls operation of the multiplexer, wherein the multiplexer operates to select one of the plurality of sensor elements, and the control unit determines a charge accumulation period based on an output voltage of the amplifier and operates the multiplexer for each of the charge accumulation periods.

[0014] According to an eighth aspect of the present invention, there is provided a sensor device according to the seventh aspect, wherein the control unit relatively shortens the charge accumulation period when the output voltage of the amplifier is equal to or greater than a threshold value, and relatively lengthens the charge accumulation period when the output voltage of the amplifier is less than the threshold value.

[0015] According to a ninth aspect of the present invention, there is provided the sensor device according to any one of the above aspects, wherein the control unit operates the multiplexer to select the plurality of sensor elements in sequence.

[0016] According to a tenth aspect of the present invention, there is provided the sensor device according to any one of the above aspects, wherein the control unit sets the charge accumulation period based on an external switching signal.

[0017] According to an eleventh aspect of the present invention, there is provided a sensor device according to any one of the above aspects, further comprising an A / D converter connected to the output of the amplifier.

[0018] According to a twelfth aspect of the present invention, there is provided a sensor device according to any of the above aspects, wherein the control unit controls the operation of the A / D converter so that the switching period of the multiplexer and the sampling period of the A / D converter are the same.

[0019] According to a thirteenth aspect of the present invention, there is provided the sensor device according to any one of the above aspects, wherein each of the plurality of sensor elements is a piezoelectric element.

[0020] According to the present disclosure, it is possible to provide a sensor device capable of improving the dynamic range.

[0021] FIG. 1 is a block diagram of a sensor device according to a first embodiment of the present invention. FIG. 2 is a block diagram of a sensor unit, a switch unit, an amplifier unit, and an A / D converter unit shown in FIG. 1. FIG. 3 is a circuit diagram showing an example of an amplifier. FIG. 4 is a flowchart illustrating the operation of the sensor device. FIG. 5 is a block diagram of a sensor element and its peripheral circuits according to a comparative example. FIG. 6 is a diagram illustrating an example of a detection signal in the comparative example. FIG. 7 is a diagram illustrating an example of a detection signal in the embodiment. FIG. 8 is a diagram illustrating a measurement method according to a first example. FIG. 9 is a diagram illustrating measurement conditions for the first example. FIG. 10 is a diagram illustrating measurement results for the first example. FIG. 11 is a diagram illustrating measurement results for the first example. FIG. 12 is a diagram illustrating an example of a detection signal for the second example. FIG. 13 is a diagram illustrating an example of a detection signal for the second example. FIG. 14 is a flowchart illustrating the operation of a sensor device according to a second embodiment of the present invention. FIG. 15 is a block diagram of the main components of a sensor device according to a third embodiment of the present invention. FIG. 16 is a flowchart illustrating the operation of the sensor device. FIG. 17 is a block diagram of the main components of a sensor device according to a fourth embodiment of the present invention.

[0022] Hereinafter, embodiments will be described with reference to the drawings. However, the drawings are schematic or conceptual, and the dimensions and proportions of each drawing are not necessarily the same as those of the actual drawing. Furthermore, even when the same parts are shown in different drawings, the dimensional relationships and proportions may be different. In particular, the following embodiments are illustrative of devices and methods for embodying the technical concept of the present invention, and the shape, structure, arrangement, etc. of the components do not specify the technical concept of the present invention. In the following description, elements having the same function and configuration are designated by the same reference numerals, and redundant description will be omitted.

[0023] In the following embodiment, a piezoelectric sensor (also called a pressure sensor) will be described as an example of the sensor device 1.

[0024] [1] First Embodiment [1-1] Configuration of Sensor Device 1 Fig. 1 is a block diagram of a sensor device 1 according to a first embodiment of the present invention. The sensor device 1 includes a sensor unit 2, a signal processing unit 6, a control unit 7, a storage unit 8, an external interface 9, and a voltage generation circuit 10.

[0025] The sensor unit 2 detects the applied pressure. The sensor unit 2 is capable of detecting pressure in each of a plurality of partial regions obtained by dividing the detection region (the region in which the sensor unit 2 is disposed). The sensor unit 2 outputs a plurality of detection signals corresponding to the plurality of partial regions, respectively. The detection signals output by the sensor unit 2 consist of electric charges.

[0026] The signal processing unit 6 receives a plurality of detection signals from the sensor unit 2 and performs signal processing on the plurality of detection signals. The signal processing unit 6 includes a switch unit 3, an amplifier unit 4, and an A / D converter unit 5.

[0027] The switch unit 3 is connected to the sensor unit 2. The switch unit 3 switches contacts (also referred to as current paths) on and off under the control of the control unit 7. The switch unit 3 switches the contacts for each of a plurality of detection signals. The switch unit 3 is configured, for example, by an analog switch.

[0028] The amplifier 4 is connected to the switch unit 3. The amplifier 4 receives a plurality of detection signals from the switch unit 3. The amplifier 4 amplifies the electrical signals input thereto.

[0029] The A / D converter unit 5 is connected to the amplifier unit 4. The A / D converter unit 5 converts the analog signal output from the amplifier unit 4 into a digital signal. The detection signal (digital signal) output from the A / D converter unit 5 is sent to the control unit 7.

[0030] The control unit 7 comprehensively controls the operation of the sensor device 1. The control unit 7 is configured to include one or more processors such as a CPU (Central Processing Unit) or an MPU (Micro Processing Unit). The control unit 7 executes programs stored in the storage unit 8 to realize various functions.

[0031] The control unit 7 receives a plurality of detection signals from the A / D converter unit 5. Based on the plurality of detection signals, the control unit 7 controls the operation of the switch unit 3. Based on the plurality of detection signals, the control unit 7 also causes each of a plurality of switches 12 (described later) to perform a switching operation.

[0032] The memory unit 8 stores various data necessary for the operation of the sensor device 1. The memory unit 8 is also used as a work area for the control unit 7. The memory unit 8 includes a volatile memory and a non-volatile memory. The volatile memory includes a random access memory (RAM) and a register, etc. The non-volatile memory includes a read only memory (ROM), a hard disk drive (HDD), a solid state drive (SSD), etc.

[0033] The external interface 9 is connected to an external device via a wired or wireless connection and has a function of communicating with the external device, and executes interface processing based on a predetermined standard between the external interface 9 and the external device.

[0034] The voltage generating circuit 10 generates a plurality of voltages required for the operation of the sensor device 1. The voltage generating circuit 10 supplies voltages to the corresponding circuits.

[0035] Next, a detailed description will be given of the configuration of the sensor unit 2 and the signal processing unit 6. Fig. 2 is a block diagram of the sensor unit 2, switch unit 3, amplifier unit 4, and A / D converter unit 5 shown in Fig. 1.

[0036] The sensor unit 2 includes a plurality of sensor elements 11. Each of the plurality of sensor elements 11 is formed, for example, by a piezoelectric element. A piezoelectric element is a sensor element that utilizes the piezoelectric effect and is capable of outputting an electrical signal in response to pressure applied to a piezoelectric body. The plurality of sensor elements 11 are formed, for example, on the same substrate. The piezoelectric element is formed to include a lower electrode, an upper electrode, and a piezoelectric layer sandwiched between them.

[0037] The switch unit 3 includes a plurality of switches 12. The plurality of switches 12 are connected to the plurality of sensor elements 11, respectively. The switches 12 are configured as single-pole single-throw (SPST) switches that can switch between one pole and one contact. The on / off states of the switches 12 are controlled based on a control signal supplied from the control unit 7.

[0038] The amplifier unit 4 includes a plurality of load resistors 13 and a plurality of amplifiers 14. The amplifiers 14 amplify and output an input signal. The amplifiers 14 are configured, for example, as operational amplifiers. The inputs of the plurality of amplifiers 14 are connected to the plurality of switches 12, respectively. The plurality of load resistors 13 are connected to the inputs of the plurality of amplifiers 14, respectively. The other end of the load resistor 13 is connected to the ground terminal GND, and a ground voltage (0 V) is supplied to the other end of the load resistor 13. The load resistor 13 has the function of converting the charge output from the sensor element 11 into a voltage.

[0039] 3 is a circuit diagram showing an example of the amplifier 14. The amplifier 14 is configured, for example, as a non-inverting amplifier. FIG. 3 shows an example of the configuration of a voltage follower.

[0040] The input terminal IN is connected to the switch 12, and the output terminal OUT is connected to an A / D converter (described later). A positive power supply V+ (e.g., 5 V) is supplied to the positive power supply terminal of the amplifier 14. A negative power supply V− (e.g., −5 V) is supplied to the negative power supply terminal of the amplifier 14.

[0041] The non-inverting input terminal (+) of the amplifier 14 is connected to the input terminal IN. One end of the load resistor 13 is connected to the non-inverting input terminal of the amplifier 14, and the other end is connected to the ground terminal GND. 0 V is supplied to the ground terminal GND.

[0042] The inverting input terminal (-) of the amplifier 14 is connected to the output of the amplifier 14. The output of the amplifier 14 is connected to the output terminal OUT. The amplifier 14 amplifies the input voltage and outputs it. In the example of FIG. 3, the amplification factor of the amplifier 14 is 1.

[0043] The A / D converter unit 5 includes a plurality of A / D converters 15 (ADC). Each of the A / D converters 15 is connected to a plurality of amplifiers 14. The A / D converters 15 convert analog signals into digital signals. The A / D converters 15 sample the input voltage at predetermined sampling intervals and output the sampled voltage as a digital signal.

[0044] [1-2] Operation The operation of the sensor device 1 configured as described above will now be described. Figure 4 is a flowchart illustrating the operation of the sensor device 1. Below, the operation of one sensor element 11 will be described, but the same operation as described below is performed for each of the multiple sensor elements 11.

[0045] As an initial state, for example, the control unit 7 turns on the switch 12. Alternatively, the control unit 7 causes the switch 12 to perform a switching operation at an initially set switching period.

[0046] The sensor element 11 detects the pressure applied thereto and outputs an electric charge as the detection result. The electric charge output from the sensor element 11 is supplied to the load resistor 13. The load resistor 13 converts the electric charge output from the sensor element 11 into a voltage.

[0047] The amplifier 14 amplifies and outputs the voltage of the load resistor 13. The A / D converter 15 converts the output voltage of the amplifier 14 into a digital signal. The output signal of the A / D converter 15 is called a detection signal.

[0048] The control unit 7 receives the detection signal from the A / D converter 15 (step S100). Then, the control unit 7 determines the voltage level of the detection signal (step S101). Note that the voltage level of the output signal of the amplifier 14 and the voltage level of the output signal of the A / D converter 15 have the same meaning, except for the difference between an analog signal and a digital signal.

[0049] Next, the control unit 7 determines the off period of the switch 12 based on the voltage level of the detection signal (step S102). Specifically, the control unit 7 compares the voltage level of the detection signal with a predetermined threshold, and sets the off period to be relatively short if the voltage level of the detection signal is equal to or greater than the predetermined threshold, and sets the off period to be relatively long if the voltage level of the detection signal is less than the predetermined threshold.

[0050] The off-period of the switch is related to the sampling period of the ADC, and it is undesirable to set it unnecessarily long beyond the sampling period. Therefore, the maximum value of the off-period is limited by the sampling period of the ADC. Furthermore, if the off-period is too long, the output voltage will be high and may exceed the detection range (referred to as the ADC range). Furthermore, only the high values ​​of the ADC range will be used, resulting in reduced resolution. Ideally, the average output voltage should be approximately 50% of the ADC range. In this embodiment, the upper limit of the off-period of the switch is preferably a period during which the average output voltage (referred to as the average output voltage) is 90% of the ADC range. The lower limit of the off-period of the switch is preferably a period during which the average output voltage is 10% of the ADC range, or between two and three times the average noise level. This makes it possible to achieve both high resolution and stability of the output signal while appropriately utilizing the effective range of the ADC.

[0051] Next, the control unit 7 causes the switch 12 to perform a switching operation based on the off period set in step S102 (step S103). Specifically, the control unit 7 supplies a switching signal to the switch 12 to cause the switch 12 to perform a switching operation. In the switching operation, the switch 12 repeatedly turns on and off, and the off period is set to the off period set in step S102. The off period of the switch 12 also includes 0 (ms). An off period of the switch 12 of 0 (ms) means that the switch 12 is always on.

[0052] The specific operation of the sensor device 1 will be described below.

[0053] 5 is a block diagram of a sensor element 11 and its peripheral circuitry according to a comparative example. The comparative example does not include a switch 12 for switching the connection state between the sensor element 11 and the amplifier 14. In other words, the sensor element 11 is always connected to the load resistor 13.

[0054] Fig. 6 is a diagram illustrating an example of a detection signal in the comparative example. The detection signal in Fig. 6 corresponds to the output of the amplifier 14. The horizontal axis of Fig. 6 represents time, and the vertical axis represents voltage (arbitrary units).

[0055] The detection signal changes depending on the pressure applied to the sensor element 11. The dots added to the waveform in Fig. 6 represent the sampling period (also called ADC sampling) of the A / D converter 15. In the comparative example, when the voltage level of the detection signal is low, it becomes difficult to detect the pressure.

[0056] Fig. 7 is a diagram illustrating an example of a detection signal in this embodiment. Fig. 7 shows the waveform of the detection signal and a switching signal from the control unit 7. The solid line waveform in the detection signal in Fig. 7 corresponds to the output of the amplifier 14. The dashed line waveform in the detection signal in Fig. 7 corresponds to a virtual voltage waveform that explains the dynamic range. The dashed line waveform in the detection signal in Fig. 7 corresponds to the voltage waveform of the comparative example described above.

[0057] The control unit 7 supplies a switching signal to the switch 12. The switch 12 performs a switching operation based on the switching signal. When the switching signal is at a high level (H), the switch 12 turns on, and when the switching signal is at a low level (L), the switch 12 turns off. The switching signal is made up of a plurality of pulse signals having the same pulse width.

[0058] The period when the switch 12 is off (referred to as an off period) corresponds to the period when the sensor element 11 accumulates electric charge (referred to as a charge accumulation period). The interval between two consecutive pulses of the switching signal is the off period (i.e., the charge accumulation period). The control unit 7 sets the off period based on the voltage level of the detection signal.

[0059] When the switch 12 is turned on, the sensor element 11 suddenly releases charge. The charge in the sensor element 11 is converted into a voltage by the load resistor 13. The amplifier 14 amplifies the voltage of the load resistor 13 and outputs it. If the pressure is changing, the voltage level of the detection signal increases when the switch 12 is turned on. After the accumulated charge is released while the switch 12 is on, the detection signal becomes a voltage corresponding to the current pressure change. When the switch 12 is turned off, the detection signal becomes 0 V.

[0060] The dots added to the waveform in Fig. 7 represent the sampling period (ADC sampling) of the A / D converter 15. The control unit 7 controls the A / D converter 15 so that the switching period of the switch 12 and the sampling period of the A / D converter 15 are the same. Based on the control signal from the control unit 7, the A / D converter 15 samples the voltage, converts it into a digital signal, and outputs this digital signal. This allows the A / D converter 15 to output a voltage value corresponding to the spike of the detection signal.

[0061] 7, in this embodiment, the voltage value of the detection signal of the amplifier 14 can be made larger than in the comparative example, and therefore the dynamic range of the sensor device 1 can be improved.

[0062] Increasing the OFF period of switch 12 increases the charge accumulation period of sensor element 11, resulting in a relatively large voltage value for the detection signal from amplifier 14. On the other hand, shortening the OFF period of switch 12 shortens the charge accumulation period of sensor element 11, resulting in a relatively small voltage value for the detection signal from amplifier 14. Control unit 7 sets a threshold value so as to obtain the desired voltage value, and also sets the OFF period of switch 12.

[0063] The control unit 7 sets the OFF period of the switch 12 relatively short when a large pressure is applied to the sensor unit 2, and sets the OFF period of the switch 12 relatively long when a small pressure is applied to the sensor unit 2. When the pressure applied to the sensor unit 2 is sufficiently large, the control unit 7 sets the OFF period of the switch 12 to 0 (ms).

[0064] The control unit 7 has information about a plurality of off periods, and may successively change the off periods so that the off periods become longer in order until a desired voltage value (threshold value) is obtained. The information about the plurality of off periods is stored in the storage unit 8.

[0065] If the leakage current of the switch 12 (leakage current when the switch is off) is large, the charge of the sensor element 11 will leak. Therefore, it is desirable that the leakage current of the switch 12 is as small as possible. It is desirable that the leakage current of the switch 12 be 100 pA or less.

[0066] [1-3] Examples Next, a first example will be described. Figure 8 is a diagram for explaining a measurement method according to the first example. The sensor unit 2 is placed on a base 20.

[0067] A digital force gauge 22 is fixed to a test stand 21 above the sensor unit 2. The digital force gauge 22 is a device for measuring load. The digital force gauge 22 includes a rod-shaped metal part 22A and a rubber part 22B attached to the tip of the metal part 22A. Pressure is applied to the sensor unit 2 by the rubber part 22B of the digital force gauge 22, and the pressure at that time is measured by the digital force gauge 22.

[0068] 9 is a diagram illustrating the measurement conditions of Example 1. In Example 1, measurements were performed under two measurement conditions. In Condition 1, the sensor unit 2 was placed on a metal plate. In Condition 1, the maximum pressure was 2.5 (N), the movement distance of the digital force gauge 22 was 10.8 (mm), the movement speed of the digital force gauge 22 was 350 (mm / min), and the force change δF was 1.4 (N / s).

[0069] In condition 2, the sensor unit 2 was placed on an elastomer. Elastomer is a type of elastic body. In condition 2, the maximum pressure was 0.2 (N), the movement distance of the digital force gauge 22 was 9.2 (mm), the movement speed of the digital force gauge 22 was 350 (mm / min), and the force change δF was 0.1 (N / s).

[0070] 10 and 11 are diagrams illustrating the measurement results of the first embodiment. The off periods are 0 ms, 4 ms, 8 ms, 12 ms, and 16 ms. An operation in which the off period is 0 ms means that the switch 12 is always on and the sensor element 11 does not accumulate charge (referred to as "no charge accumulation"). An operation in which the off period is longer than 0 ms means that the sensor element 11 accumulates charge (referred to as "charge accumulation"). In the first embodiment, when pressure is applied to the sensor unit 2, the voltage value is negative, and when pressure is removed from the sensor unit 2, the voltage value is positive.

[0071] It can be seen that when the off period is longer than 0 ms, the voltage amplitude is two to four times larger than when the off period is 0 ms. By storing charge in the sensor element 11, it becomes possible to detect minute pressures that would be undetectable without charge storage. The control unit 7 sets an optimal threshold value and sets the off period of the switch 12 so as to obtain the desired voltage.

[0072] Next, a second embodiment will be described. FIGS. 12 and 13 are diagrams illustrating an example of a detection signal according to the second embodiment. FIGS. 12 and 13 show waveforms of the detection signal and pressure. In FIGS. 12 and 13, the left vertical axis represents voltage (V), the right vertical axis represents force (N), and the horizontal axis represents time. FIG. 12 shows a voltage waveform without charge accumulation, and FIG. 13 shows a voltage waveform with charge accumulation. The charge accumulation period in FIG. 13 is 16 ms. Since the pressure is very small in FIGS. 12 and 13, the results after averaging 200 points are shown.

[0073] When the pressure changes from 0 (N) to 2.5 (N), the voltage drops, and when it changes from 2.5 (N) to 0 (N), the voltage rises. The voltage generated without charge accumulation is minute and is equivalent to the noise level, making it difficult to detect pressure. In contrast, when charge accumulation occurs, the voltage is about three times higher than the noise, making it possible to detect pressure.

[0074] While the sensor element 11 is storing electric charges, positive and negative charges may be generated due to pressure changes and may cancel each other out. Therefore, it is desirable to set the switching period of the switch 12 to 50% or less of the period during which pressure changes between positive and negative voltages occur.

[0075] [1-4] Effects of the First Embodiment According to the first embodiment, when pressure is applied to the sensor element 11, turning off the switch 12 allows charge to be accumulated in the sensor element 11. Then, turning on the switch 12 allows the charge accumulated in the sensor element 11 to be released to the load resistor 13. This makes it possible to detect a small pressure applied to the sensor element 11 as a large voltage. This makes it possible to increase the voltage amplitude and dynamic range of the detection signal. Consequently, it is possible to realize a sensor device 1 that can improve the dynamic range.

[0076] Furthermore, the voltage of the detection signal can be adjusted by adjusting the OFF period of the switch 12. This makes it possible to output a desired voltage value as the detection signal.

[0077] Furthermore, when a large pressure is applied to the sensor element 11, the OFF period can be set to 0 (ms), which makes it possible to detect a large pressure.

[0078] Furthermore, the control unit 7 can adjust the off period of the switch 12 (i.e., the charge accumulation period of the sensor element 11) by comparing the voltage level of the detection signal with a threshold value. This eliminates the need to switch circuit components to adjust the amplification factor of the amplifier 14, and eliminates the need to provide multiple load resistors. This allows for simplification of the circuit and miniaturization of the circuit board. It also eliminates the need for component selection and component replacement.

[0079] [2] Second Embodiment In the second embodiment, the OFF period of the switch 12 is changed based on a signal from an external device.

[0080] 14 is a flowchart illustrating the operation of the sensor device 1 according to the second embodiment of the present invention. The control unit 7 has information about a plurality of off periods. The information about the plurality of off periods is stored in the storage unit 8.

[0081] The control unit 7 monitors whether a switching signal has been received from an external device (step S200). The switching signal includes information specifying an OFF period. The control unit 7 receives the switching signal via the external interface 9.

[0082] Next, the control unit 7 determines the off period of the switch 12 based on the switching signal (step S201). Next, the control unit 7 causes the switch 12 to perform a switching operation based on the off period determined in step S201 (step S202). Specifically, the control unit 7 supplies a switching signal to the switch 12 to cause the switch 12 to perform a switching operation. In the switching operation, the switch 12 repeatedly turns on and off, and the off period is set to the off period determined in step S201. The off period of the switch 12 can be set to 0 ms. For example, in the example of FIG. 11, the off period can be selectively set to 0 ms, 4 ms, 8 ms, 12 ms, or 16 ms.

[0083] According to the second embodiment, the switch 12 can be switched on and off based on an external signal. Other effects are the same as those of the first embodiment.

[0084] [3] Third Embodiment In the third embodiment, a plurality of sensor elements 11 are selectively connected to one amplifier 14 .

[0085] 15 is a block diagram of the main parts of a sensor device 1 according to a third embodiment of the present invention. The sensor device 1 includes a sensor unit 2, a signal processing unit 6, a control unit 7, a storage unit 8, an external interface 9, and a voltage generating circuit 10. Although not shown in the figure, the block configurations of the control unit 7, the storage unit 8, the external interface 9, and the voltage generating circuit 10 are the same as those in the first embodiment.

[0086] The sensor unit 2 includes a plurality of sensor elements 11. The signal processing unit 6 includes a multiplexer (MUX) 30, a load resistor 13, an amplifier 14, and an A / D converter 15.

[0087] The multiplexer 30 is a circuit that selects and outputs one of a plurality of inputs. A plurality of sensor elements 11 are connected to each of the plurality of inputs of the multiplexer 30. The multiplexer 30 selects one of the plurality of sensor elements 11 based on a control signal from the control unit 7, and outputs the signal of the selected sensor element 11.

[0088] The output of the multiplexer 30 is connected to one end of the load resistor 13 and the input of the amplifier 14. The output of the amplifier 14 is connected to the A / D converter 15. The output of the A / D converter 15 is supplied to the control unit 7.

[0089] 16 is a flowchart illustrating the operation of the sensor device 1. The control unit 7 receives a detection signal from the A / D converter 15 (step S300), and then determines the voltage level of the detection signal (step S301).

[0090] Next, the control unit 7 determines the charge accumulation period based on the voltage level of the detection signal (step S302). Specifically, the control unit 7 compares the voltage level of the detection signal with a predetermined threshold, and sets the charge accumulation period relatively short if the voltage level of the detection signal is equal to or greater than the predetermined threshold, and sets the charge accumulation period relatively long if the voltage level of the detection signal is less than the predetermined threshold. The control unit 7 has information on multiple charge accumulation periods and can set the charge accumulation period so that the voltage level of the detection signal becomes a target level.

[0091] Next, the control unit 7 sends a selection signal to the multiplexer 30 so that each of the plurality of sensor elements 11 is in the charge accumulation period determined in step S302 (step S303). The multiplexer 30 connects one sensor element 11 to the amplifier 14 based on the selection signal.

[0092] The control unit 7 controls the A / D converter 15 so that the switching period of the multiplexer 30 is the same as the sampling period of the A / D converter 15. Based on the control signal from the control unit 7, the A / D converter 15 samples the voltage, converts it into a digital signal, and outputs this digital signal.

[0093] In the third embodiment, unselected sensor elements 11 among the plurality of sensor elements 11 can accumulate charge. For example, the plurality of sensor elements 11 are selected in turn for each predetermined charge accumulation period, thereby enabling each sensor element 11 to perform a switching operation. This allows each of the plurality of sensor elements 11 to accumulate charge so as to output a detection signal at a more optimal voltage level.

[0094] The second embodiment may be applied to the third embodiment. That is, the control unit 7 may receive a switching signal from the outside and determine the charge accumulation period based on this switching signal.

[0095] [4] Fourth Embodiment The fourth embodiment is a modification of the third embodiment, in which the sensor device 1 includes a plurality of multiplexers 30.

[0096] 17 is a block diagram of the main parts of a sensor device 1 according to a fourth embodiment of the present invention. The block diagram of the entire sensor device 1 is the same as that of FIG.

[0097] The switch section 3 includes a plurality of multiplexers 30. A plurality of sensor elements 11 are connected to the inputs of one multiplexer 30.

[0098] The amplifier unit 4 includes a plurality of load resistors 13 and a plurality of amplifiers 14. The load resistors 13 and amplifiers 14 are connected to the output of the multiplexer 30.

[0099] The A / D converter section 5 includes a plurality of A / D converters 15. The A / D converters 15 are connected to the outputs of the amplifiers .

[0100] The operation of each multiplexer 30 is the same as in the third embodiment. According to the fourth embodiment, it is possible to realize a sensor device 1 having a larger number of sensor elements 11. Other effects are the same as in the third embodiment.

[0101] Although the above-described embodiments have been described with reference to examples in which the sensor device 1 is a piezoelectric sensor, the present invention can also be applied to other types of sensors. For example, the above-described embodiments can also be applied to a pyroelectric sensor or the like.

[0102] The present disclosure is not limited to the above-described embodiments, and various modifications can be made in the implementation stage without departing from the spirit of the present disclosure. Furthermore, the embodiments may be implemented in appropriate combinations, in which case the combined effects can be obtained. Furthermore, the above-described embodiments include various inventions, and various inventions can be extracted by combining selected elements from the disclosed elements. For example, if the problem can be solved and the desired effect can be obtained even if some elements are deleted from all elements shown in the embodiments, the configuration from which these elements are deleted can be extracted as an invention.

[0103] 1...sensor device, 2...sensor section, 3...switch section, 4...amplification section, 5...A / D converter section, 6...signal processing section, 7...control section, 8...memory section, 9...external interface, 10...voltage generation circuit, 11...sensor element, 12...switch, 13...load resistor, 14...amplifier, 15...A / D converter, 20...base, 21...test stand, 22...digital force gauge, 22A...metal part, 22B...rubber part, 30...multiplexer

Claims

1. A sensor device comprising: a sensor element; a switch connected to the sensor element; an amplifier connected to the switch; a load resistor connected between the input of the amplifier and a ground terminal; and a control unit that causes the switch to perform a switching operation, wherein the control unit sets an off period for turning off the switch based on the output voltage of the amplifier.

2. The sensor device according to claim 1, wherein the control unit relatively shortens the off period when the output voltage of the amplifier is equal to or greater than a threshold value, and relatively lengthens the off period when the output voltage of the amplifier is less than the threshold value.

3. The sensor device according to claim 1, wherein the control unit sets the off period based on an external switching signal.

4. The sensor device of claim 1, further comprising an A / D converter connected to the output of said amplifier.

5. The sensor device according to claim 4, wherein the control unit controls the operation of the A / D converter so that the switching period of the switch and the sampling period of the A / D converter are the same.

6. A sensor device according to any one of claims 1 to 5, wherein the sensor element is a piezoelectric element.

7. A sensor device comprising: a plurality of sensor elements; a multiplexer connected to the plurality of sensor elements; an amplifier connected to the multiplexer; a load resistor connected between an input of the amplifier and a ground terminal; and a control unit that controls the operation of the multiplexer, wherein the multiplexer operates to select one of the plurality of sensor elements, and the control unit determines a charge accumulation period based on the output voltage of the amplifier and operates the multiplexer for each charge accumulation period.

8. The sensor device according to claim 7, wherein the control unit relatively shortens the charge accumulation period when the output voltage of the amplifier is equal to or greater than a threshold value, and relatively lengthens the charge accumulation period when the output voltage of the amplifier is less than the threshold value.

9. The sensor device according to claim 7, wherein the control unit operates the multiplexer to select the plurality of sensor elements in sequence.

10. The sensor device according to claim 7, wherein the control unit sets the charge accumulation period based on an external switching signal.

11. The sensor device of claim 7, further comprising an A / D converter connected to the output of said amplifier.

12. The sensor device according to claim 11, wherein the control unit controls the operation of the A / D converter so that the switching period of the multiplexer and the sampling period of the A / D converter are the same.

13. A sensor device according to any one of claims 7 to 12, wherein each of the plurality of sensor elements is a piezoelectric element.

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