Pendulum-type accelerometer sensor with conditional capacitance detection

Abstract

The accelerometer sensor comprises a casing (2), a pendulum (3) fixed relative to the casing, a mobile electrode (5.3) carried by the pendulum and connected to a detection circuit (7), a first electrode (5.1) and a second electrode (5.2) rigidly mounted relative to the casing, the first electrode (5.1) and the second electrode (5.2) forming with the mobile electrode two capacitors whose capacitance varies depending on the distance between the mobile electrode and the first fixed electrode and between the mobile electrode and the second fixed electrode, and a control unit (8) designed to perform detection operations to measure the variable capacitance of the capacitors and control operations of the mobile electrode depending on the measured capacitance by applying a logic signal (bs) to control a switch (9) for selectively connecting the fixed electrode to an excitation circuit (6) that delivers a control signal (u) to the fixed electrode to maintain the pendulum in a predetermined position. The control unit is configured to apply a first detection signal to one of the fixed electrodes selected according to the logic level of the control signal and a second detection signal to the other fixed electrode during each calibration period, and the control signal (u) is applied to the electrode to which the second detection signal is applied.

Description

[Technical Field]

[0001] The present invention relates to a closed-loop pendulum accelerometer sensor with electrostatic control and detection used for detecting physical quantities, and a method for controlling such a sensor. This sensor is, for example, a MEMS (Micro-Electro-Mechanical Systems) technology sensor. [Background technology]

[0002] A pendulum-type electrostatic accelerometer comprises a casing and a vibrating mass connected to the casing by one or more hinges, the hinges positioned to form a pendulum in which the vibrating mass is movable relative to the casing in either a translational or rotational direction. The movement of this vibrating mass under acceleration is typically detected by three electrodes.

[0003] The first fixed electrode and the second fixed electrode are integrated with the casing and connected to the drive circuit.

[0004] The movable third electrode is supported by a pendulum and connected to the detection circuit.

[0005] Each fixed electrode, together with the movable electrode, forms a capacitance whose value depends on the distance between them. In the absence of manufacturing defects and when the sensor is not accelerated along its sensing axis, the pendulum maintains a neutral position and the two capacitances are equal. On the other hand, when the pendulum is accelerated along its sensing axis, the pendulum moves, causing the capacitance formed by one of the movable and fixed electrodes to decrease and the capacitance formed by the movable electrode and the other fixed electrode to increase.

[0006] Furthermore, this capacitance fluctuation also depends on the deformation of the casing and the pendulum.

[0007] In closed-loop operation, the pendulum's position is servo-controlled to a neutral position or a target position midway between the fixed electrodes by applying an electrostatic force to the pendulum that should counteract the acceleration applied along the sensing axis. This electrostatic force is obtained as a result of applying a voltage to these electrodes to maintain a capacitance difference of zero.

[0008] The sensor is equipped with a drive circuit for each fixed electrode, and this drive circuit is configured to supply power to the electrode in order to generate the electrostatic force.

[0009] The quadratic index of electrostatic force with respect to applied voltage complicates the design of the control circuit that performs servo control of the pendulum and estimation of its acceleration.

[0010] To circumvent this difficulty, it is known that a calibrated voltage pulse can be used to control the pendulum in an all-or-nothing manner.

[0011] These pulses are applied to one of these electrodes depending on whether the pendulum should be pulled or pushed to return it to the target position. In this case, the pulse density for pushing or pulling the pendulum, i.e., the number of pulses per time interval, becomes an affine function of the acceleration to be measured.

[0012] Therefore, zero acceleration is compensated for by the equality of the average number of pulses in both directions.

[0013] However, the symmetry of the pulses applied to these two electrodes can be imperfect due to the difference between the duration of the pulse applied to the first fixed electrode and the duration of the pulse applied to the second fixed electrode.

[0014] In this case, the pulse density is modified by servo control to maintain the pendulum at the target position, thereby biasing the acceleration estimation.

[0015] In order to improve the performance of this type of sensor, in Document WO2014 / 128027, it is proposed to use a common drive circuit to suppress the problems of manufacturing asymmetry and aging changes of the elements of the drive circuit.

[0016] Also, in Document WO2017 / 85142, a fine-tuning phase for transmitting a moderate control pulse that enables the realization of optimal performance for a smaller measurement range, and an expansion operation control phase for transmitting a high-amplitude control pulse to bias the sensor at full scale while expanding the measurement range and possibly with low performance in some cases are proposed to be implemented.

[0017] Although the aforementioned sensors are advantageous in many aspects, their power consumption is relatively high.

Prior Art Documents

Patent Documents

[0018]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0019] In view of the above, an object of the present invention is to provide a pendulum-type electrostatic accelerometer sensor that maintains performance improvement while consuming low power.

[0020] Furthermore, another object of the present invention is to propose such a sensor having a simple mounting structure.

Means for Solving the Problems

[0021] Accordingly, the present invention proposes an accelerometer sensor comprising: a casing; a pendulum fixed to the casing; a movable electrode supported by the pendulum and connected to a detection circuit; a first fixed electrode and a second fixed electrode integrated with the casing, which together with the movable electrode form two capacitors whose capacitance is variable depending on the distance between the movable electrode and the first fixed electrode and between the movable electrode and the second fixed electrode; and a control unit configured to perform a detection operation for measuring the variable capacitance of the capacitors and a control operation of the movable electrode according to the measured capacitance, by applying a logic signal to control a switch for selective connection of the fixed electrode to a drive circuit that delivers a control signal to the fixed electrode to maintain the pendulum in a predetermined position.

[0022] The control unit is configured to apply a first detection signal to one of the fixed electrodes selected according to the logic level of the control signal during each calibration period, and to apply a second detection signal to the other fixed electrode. The control signal is applied to the electrode to which the second detection signal is applied.

[0023] Therefore, by applying two detection and control signals, the number of times the switch is switched on is reduced, resulting in decreased sensor power consumption and improved performance.

[0024] According to another characteristic, the first detection signal and the second detection signal are signals in the form of slots.

[0025] In addition, the switch comprises a first input terminal for a reference potential supplied by the drive circuit and a second input terminal for zero potential, thereby selectively connecting the fixed electrode to the drive circuit or zero potential.

[0026] In one embodiment, the drive circuit includes a digital-to-analog converter that is connected to a switch and controlled by a control unit.

[0027] The detection circuit may include an amplifier stage, which has an input connected to a movable electrode and an output connected to an analog-to-digital converter, and the analog-to-digital converter has an output connected to a control unit.

[0028] For example, the control unit includes a first pendulum position estimator, which is connected at its input to the output of a detection circuit and at its output to the negative input of a comparator, the comparator having an output connected to the input of a corrector, the corrector having an output connected to a sequencer and a second estimator, the second estimator having a first output connected to the positive input of the comparator and a second output that supplies an estimated value of acceleration.

[0029] In one embodiment, the control unit is configured to apply a second detection signal and control signal using a common detection and control pulse.

[0030] Furthermore, an object of the present invention is also a method for controlling an accelerometer sensor as defined above. This method is - During each calibration period, the variable capacitance of the capacitor is detected by applying a first detection signal to one of the fixed electrodes selected according to the logic level of the control signal, and a second detection signal to the other fixed electrode. - A step of controlling a movable electrode according to capacitance measured by applying a logic signal to control a switch for selective connection of a fixed electrode to a drive circuit that delivers a control signal to apply a control signal to the electrode to which a second detection signal is applied. Includes.

[0031] According to this method, advantageously, the second detection signal and control signal are applied by a common detection and control pulse.

[0032] Other objects, features, and advantages of the present invention will become apparent by reading the following description, which is presented solely as a non-limiting example and with reference to the accompanying drawings. [Brief explanation of the drawing]

[0033] [Figure 1] This is a schematic diagram of a sensor according to one embodiment of the present invention. [Figure 2] This is a timing graph showing the application of the first detection signal, the second detection signal, and the control signal to a fixed electrode according to the logic level of the control signal. [Figure 3] This is a timing graph showing the application of the first detection signal, the second detection signal, and the control signal to a fixed electrode according to the logic level of the control signal. [Figure 4] This timing graph shows another embodiment of the present invention in which the detection signal and control signal applied to the controlled electrode are applied by common detection and control pulses. [Modes for carrying out the invention]

[0034] Figure 1 shows an accelerometer sensor according to the present invention, which is generally indicated by reference numeral 1.

[0035] Here, the accelerometer sensor is a micro-electromechanical system, also known as MEMS, fabricated by etching a plate of crystalline or semi-crystalline material such as silicon.

[0036] The sensor comprises a casing 2. A solid 3 is articulated to the casing 2 by a hinge 4. The hinge 4 is positioned so that the solid 3 pivots to form a movable pendulum relative to the casing 2.

[0037] Sensor 1 comprises a first fixed electrode 5.1 and a second fixed electrode 5.2, which are integrated with the casing and connected to the drive circuit indicated by reference numeral 6. A third electrode 5.3, supported by solid 3, is connected to the detection circuit 7. The control unit 8 is connected to the drive circuit 6 and the detection circuit 7.

[0038] The drive circuit 6 includes an output connected to a switch 9. The switch 9 has two positions, one connected to the first electrode 5.1 and the other connected to the second electrode 5.2, so as to selectively connect the first electrode 5.1 and the second electrode 5.2 to the drive circuit 6.

[0039] More specifically, the switch 9 comprises a switch I1 configured to connect a first electrode 5.1 to either the output of the drive circuit 6 or ground, and a switch I2 configured to connect a second electrode 5.2 to either the output of the drive circuit 6 or ground.

[0040] Switch 9 is controlled by control unit 8.

[0041] The control unit 8 includes a first estimator 10. This first estimator 10 is connected to the detection circuit 7 at its input and to the negative input of the comparator 11 at its output. The comparator 11 has an output connected to the input of the compensator 12, and the output of the compensator 12 is connected to the sequencer 13.

[0042] The control unit 8 further comprises a second estimator 14. This second estimator 14 has an input connected to the output of the compensator 12, an output connected to a further input of the comparator 11, and an output that supplies an estimate of the acceleration γe.

[0043] Furthermore, the drive circuit 6 includes a digital-to-analog converter 15 that is connected to the switch 9 and controlled by the control unit 8.

[0044] The detection circuit 7 includes a main amplifier stage 16, which includes a charge amplifier 17 comprising a loop capacitor 18 with capacitance Cref and a switch I3.

[0045] This amplifier stage has an input connected to the movable electrode 5.3 and an output connected to the input of the analog-to-digital converter 19, the analog-to-digital converter 19 having an output connected to the first estimator 10 of the control unit.

[0046] This sensor operates as follows:

[0047] The control unit 8 manages the operation of the sensor and, in particular, the history of various operations calibrated at frequency FS. The sequencer 13 sequentially and cyclically controls the digital-to-analog converter 15 by control u, the analog switches I1 and I2 by control s, the analog-to-digital converter 19 by control c, and the analog switch I3 by control r, thereby sequencing the operations within the calibration period Ts.

[0048] According to the logic state of control s, one of electrodes 5.1 and 5.2 is connected to the output V of the digital-to-analog converter 15, while the other electrode 5.2 and 5.1 are simultaneously connected to ground. Therefore, the electrode connected to the output of this converter is positioned at the reference potential supplied by the drive circuit 6.

[0049] The accelerometer sensor is controlled by a control unit to perform, in each calibration period TS, a phase in which it detects the variable capacitances Ch and Cb formed between the first fixed electrode 5.1 and the movable electrode 5.3, and between the second fixed electrode 5.2 and the movable electrode 5.3, respectively, and a control phase in which an excitation signal is applied to one of these fixed electrodes to return the pendulum to its target position by the electrostatic force applied to the capacitor plate of the fixed electrode selected by control s. At the end of the detection phase, the compensator 12 determines the sign of the logical control signal bs to determine whether the control signal u should be applied to the fixed electrode 5.1 or the fixed electrode 5.2.

[0050] When bs = +1, a voltage is applied to electrode 5.2, and then this electrode 5.2 attracts the pendulum.

[0051] When bs = -1, a voltage is applied to electrode 5.1, and then this electrode 5.1 attracts the pendulum.

[0052] Capacitance detection is performed conditionally according to the sign of the control signal bs, and the chronological order of these detections is determined by the sign of bs.

[0053] The capacitance C of the first fixed electrode, called the "high" fixed electrode. h and the capacitance C of the second fixed electrode, called the "low" fixed electrode. b The linearized representation of is given by the following relation.

[0054]

number

[0055] The relative positions of the pendulums are given by the following relationship:

[0056]

number

[0057] Here, C0 represents the initial capacitance, C1 represents the active capacitance, z is the position of the pendulum, e is the width of the air gap, i.e., the distance between electrode 5.1 and electrode 5.3 or between electrode 5.2 and electrode 5.3, which are equal when at rest, and V ref Q is a reference voltage supplied by the analog-to-digital converter 19 and applied to these electrodes. b and Q h This is the charge transferred to the detection circuit 7, corresponding to the charge fluctuation at the terminals of the variable capacitor, which is governed by the voltage rise edge in the range of 0 to Vref.

[0058] Therefore, during each calibration period, two capacitance readings are performed in the middle of the detection phase to estimate the pendulum's position and power the compensator.

[0059] During the calibration period, the pendulum's position hardly changes between these two measured values.

[0060] The detection of variable capacitance is conditional, and the order of capacitance detection is conditional on the sign of the logic control signal bs received from the compensator 12. Therefore, a pseudo-random substitution of the order of these detections is performed. This substitution is pseudo-random due to the characteristics of the control signal bs in a sigma-delta type loop, which is a characteristic of white noise filtered by the high-pass transfer function determined by the compensator 12.

[0061] The first detection is performed on the uncontrolled electrode, the second detection is performed on the electrode to be controlled, and the control logic signal is available from the start of the real-time period, as the calculation of the control logic signal begins immediately after the detection values ​​from the previous calibration period become available.

[0062] Referring to Figure 2, for example, when bs = +1, the first detection D1 is performed on the high electrode 5.1 by applying a readout pulse, and then the second detection D2 is performed on the low electrode by applying a second measurement pulse.

[0063] Next, a control pulse is supplied to the low electrode Vb by controlling switch 9.

[0064] Referring to Figure 3, when bs = -1, the pulse detection order is reversed.

[0065] This conditional detection makes it possible to limit the number of times switch 9 is switched, and as a result reduce power consumption. This control is applied directly to the electrode being controlled. In fact, it is no longer necessary to operate switch 9, which remains in the same state between the detection phase D2 and the control phase.

[0066] Furthermore, it should be noted that the detection order is frequently replaced. Consequently, the electron-derived bias generated by the control pulses is converted into noise due to the pseudo-random characteristics of these controls.

[0067] In fact, any detection bias generates noise that has the same spectral appearance as the pulsed control signal bs. Conditional detection significantly reduces these biases by multiplying them by the average value of the control signal bs, leaving only noise that increases the tolerance.

[0068] Therefore, conditional detection becomes pseudo-random in terms of both the applied parasitic force and the position measurement itself.

[0069] Furthermore, in prior art, the detection pulses were periodic, and the spectra of these detection pulses consisted of lines. However, this substitution has a spectral diffusion effect, which makes it possible to limit the excitation of high-frequency parasitic modes and, in particular, to adjust this excitation through control. This excitation becomes persistent and slow-velocity variable, or in other words, its period is not very constant.

[0070] Furthermore, referring to Figure 4, in another embodiment, the second capacitance detection pulse and control pulse form a common detection and control pulse.

[0071] In other words, the rising edge of the control signal is shifted so that it occurs simultaneously with the falling edge of the second detection signal.

[0072] This implementation makes it possible to detect and control capacitance using the same pulse signal.

[0073] The electrostatic force applied to a pendulum whose direction is determined by the control signal bs is composed of the difference between the force applied by the detection and control pulses and the force applied by the first detection signal applied to the other electrodes.

[0074] This detection is performed by the carrier of the detection and control signals, and the control is performed in the baseband.

[0075] Furthermore, this implementation theoretically makes it possible to reduce power consumption by one-third by decreasing the switching of switch 9.

[0076] Similarly, a one-third reduction in bias error caused by the waveform, which depends on the number of pulses during the calibration period, is achieved.

[0077] Finally, the scale factor error caused by the waveform is completely eliminated.

[0078] In fact, the acceleration equivalent to the applied force is described as follows:

[0079]

number

[0080] Here, C1 is the active capacitance, e is the width of the air gap, m is the mass of the pendulum, and σh 2 and σb 2 This is the mean square of the voltages applied to the high and low electrodes during each calibration period.

[0081] In an implementation using three detection and control pulses, the applied force was described as follows:

[0082]

number

[0083] Here,

[0084]

number

[0085] corresponds to the bias,

[0086] [Number]

[0087] corresponds to the scale factor.

[0088] With the implementation of the first detection pulse and the second detection and control pulse, the applied force is described as follows.

[0089] [Number]

[0090] Assuming that the time constant is short compared to the period of these pulses, any waveform disturbance σerr 2 will have the same effect superimposed on the theoretical root mean square for either the detection pulse or the control pulse.

[0091] ε t By paying attention to the asymmetric part of this disturbance, it becomes as follows.

[0092] [Number]

[0093] In the 3-pulse implementation, it becomes as follows.

[0094] [Number]

[0095] Here, 3·ε t ·σ err 2 corresponds to the bias, and (σ c,th 2 +σ err 2 ) corresponds to the scale factor.

[0096] In a 2-pulse implementation, the following applies:

[0097]

number

[0098] Here, 2·ε t ·σ err 2 This corresponds to the bias, (σ c,th 2 +σ d,th 2 ) corresponds to the scale factor.

[0099] Therefore, it can be seen that errors caused by the control pulse are compensated for by errors caused by the detection pulse.

[0100] Scale factor errors that may become significant at specific operating temperatures and during sensor aging are excluded and should not be considered during the pre-calibration phase. [Explanation of Symbols]

[0101] 1. Accelerometer sensor, sensor 2 Casing 3 solid 4 hinges 5.1 First fixed electrode, first electrode, high electrode 5.2 Second fixed electrode, second electrode 5.3 Third electrode, movable electrode 6. Drive Circuit 7. Detection circuit 8 Control Unit 9 switches 10 The First Estimator 11 Comparator 12 Corrector 13 Sequencer 14. The Second Estimator 15 Digital-to-Analog Converter 16 Main amplifier stage 17 Charge Amplifier 18 Loop Capacitors 19. Analog-to-digital converter bs logic control signals, pulse control signals, control signals c control Ch, C h Variable capacitance, capacitance Cb, C b Variable capacitance, capacitance Cref Capacitance D1 First detection D2 Second detection FS frequency I1 switch, analog switch I2 switches, analog switches I3 switch, analog switch r control s control Ts, TS calibration period u control, control signals V output γe acceleration

Claims

1. An accelerometer sensor comprising: a casing (2); a pendulum (3) fixed to the casing; a movable electrode (5.3) supported by the pendulum and connected to a detection circuit (7); a first fixed electrode (5.1) and a second fixed electrode (5.2) integrated with the casing, which together with the movable electrode form two capacitors with variable capacitance depending on the distance between the electrodes; and a control unit (8) configured to perform a detection operation for measuring the variable capacitance of the capacitors, and, after the detection operation, a control operation of the movable electrode according to the measured capacitance by applying a logic signal (bs) to control a switch (9) for selective connection of the fixed electrode to a drive circuit (6) that sends a control signal (u) to the fixed electrode to maintain the pendulum in a predetermined position, wherein the accelerometer sensor comprises: a casing (2); a pendulum (3) fixed to the casing; a movable electrode (5.3) supported by the pendulum and connected to a detection circuit (7); a first fixed electrode (5.1) and a second fixed electrode (5.2) integrated with the casing, which together with the movable electrode form two capacitors with variable capacitance depending on the distance between the electrodes; and a control unit (8) configured to perform a detection operation for measuring the variable capacitance of the capacitors; and, after the detection operation, a control signal (bs) is applied to control a switch (9) for selective connection of the fixed electrode to a drive circuit (6) that sends a control signal (u) to the fixed electrode to maintain the pendulum in a predetermined position. The control unit is configured to apply a first detection signal to one of the fixed electrodes selected according to the logic level of the control signal during each calibration period, and to apply a second detection signal to the other fixed electrode, wherein the control signal (u) is applied to the electrode to which the second detection signal is applied. The control unit (8) is configured to apply the second detection signal and the control signal using a common detection and control pulse, and is an accelerometer sensor.

2. The accelerometer sensor according to claim 1, wherein the first detection signal and the second detection signal are signals in the form of a square wave.

3. The accelerometer sensor according to claim 1, wherein the switch (9) comprises a first input terminal for a reference potential supplied by the drive circuit (6) and a second input terminal for zero potential, thereby selectively connecting the electrode to the drive circuit or the zero potential.

4. The accelerometer sensor according to claim 1, wherein the drive circuit (6) includes a digital-to-analog converter connected to the switch (9) and controlled by the control unit (8).

5. The accelerometer sensor according to claim 1, wherein the detection circuit (7) includes an amplifier stage, the amplifier stage having an input connected to the movable electrode (5.3) and an output connected to an analog-to-digital converter, and the analog-to-digital converter having an output connected to the control unit.

6. The accelerometer sensor according to claim 1, wherein the control unit (8) includes a first pendulum position estimator (10), the first pendulum position estimator (10) is connected at an input to the output of the detection circuit and at an output to the negative input of a comparator (11), the comparator (11) has an output connected to the input of a corrector (12), the corrector (12) has an output connected to a sequencer (13) and a second estimator (14), and the second estimator (14) has a first output connected to the positive input of the comparator and a second output that supplies an estimated value of acceleration.

7. A method for controlling an accelerometer sensor according to claim 1, - During each calibration period, the variable capacitance of the capacitor is detected by applying a first detection signal to one of the fixed electrodes (5.1, 5.2) selected according to the logic level of the control signal, and a second detection signal to the other fixed electrode. - After the step of detecting the variable capacitance of the capacitor, the step of controlling the movable electrode (5.3) according to the measured capacitance by applying a logic signal to control a switch (9) for selective connection of the fixed electrode to a drive circuit (6) that delivers the control signal, so that a control signal (u) is applied to the electrode to which the second detection signal is applied. It is characterized by including A method wherein the second detection signal and the control signal are applied by a common detection and control pulse.

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