Pendulous acceleration sensor with conditional capacitance detection

Abstract

This acceleration sensor comprises a casing (2), a pendulum (3) fixed to the casing, a movable 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) fixedly connected to the casing to form with the movable electrode two capacitors having a variable capacitance depending on the distance between the electrodes, and a control unit (8) designed to: perform a detection operation in order to measure the variable capacitance of the capacitors; and perform a control operation of the movable electrode in order to maintain the pendulum in a predetermined position, by applying a logic signal (bs) to control a switch (9) to selectively connect the fixed electrodes to a drive circuit (6), so as to transmit a control signal (u) to the fixed electrodes, as a function of the measured capacitance. The control unit is configured to apply, at each calibration period, a first detection signal at one of the fixed electrodes selected as a function of the logic level of the control signal, and a second detection signal at the other fixed electrode; the control signal (u) being applied to the electrode at which the second detection signal is applied.

Description

Technical Field

[0001] The present invention relates to a closed-loop pendulum acceleration sensor for detecting physical quantities and having electrostatic control and detection, and a method for controlling the sensor. The sensor is, for example, a micro-electromechanical system (MEMS) technology sensor. Background Art

[0002] An electrostatic pendulum accelerometer consists of a housing and a seismic mass connected to the housing by one or more hinges positioned so that the seismic mass forms a pendulum that is movable (either translationally or rotationally) relative to the housing. The motion of the seismic mass under acceleration is typically detected by three electrodes.

[0003] The first fixed electrode and the second fixed electrode are integrally formed with the housing and connected to the driving circuit.

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

[0005] Each fixed electrode forms a capacitance with the movable electrode, the value of which depends on the spacing between them. In the absence of manufacturing defects and when the sensor is not subjected to acceleration along its sensitive axis, the pendulum remains in its neutral position, where the two capacitances are equal. On the other hand, when the pendulum is subjected to acceleration along the sensor's sensitive axis, it moves, causing the capacitance formed between the movable electrode and one of the fixed electrodes to continuously decrease, while the capacitance formed between the movable electrode and the other fixed electrode to increase.

[0006] This change in capacitance also depends on the deformation of the housing and the deformation of the pendulum.

[0007] In closed-loop operation, the position of the pendulum is servo-controlled to a neutral or target position between these fixed electrodes by applying an electrostatic force to the pendulum that must compensate for the acceleration applied along the sensitive axis. This electrostatic force is generated by the voltage applied to the electrodes to maintain the capacitance difference at zero.

[0008] The sensor comprises a driving circuit for each fixed electrode, the driving circuits being arranged to supply power to the electrodes to generate the electrostatic force.

[0009] The quadratic nature of the electrostatic force, which is related to the applied voltage, complicates the design of the control circuitry that performs the servo control of the pendulum and the estimation of the acceleration.

[0010] To overcome this difficulty, it is known to use calibrated voltage pulses to control the pendulum completely or not at all.

[0011] These pulses are applied to either of these electrodes, depending on whether the problem is to pull or push the pendulum to return it to its target position. The density of pulses (i.e. the number of pulses per time interval) aimed at pushing or pulling the pendulum, respectively, is then an affine function of the acceleration to be measured.

[0012] Therefore, zero acceleration is compensated by an average equal number of impulses in both directions.

[0013] However, the pulses applied to the two electrodes may not be completely symmetrical due to a 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 the servo control to keep the pendulum in the target position, which biases the estimation of the acceleration.

[0015] In order to improve the performance of this type of sensor, the document WO 2014 / 128027 proposes using a common driving circuit, thereby limiting the problems of manufacturing asymmetry of the driving circuit and aging of electronic components.

[0016] Document WO 2017 / 85142 also proposes: performing a precision control phase to send moderate control pulses, thereby allowing optimal performance to be obtained within a reduced measurement range; and performing an extended operation control phase, in which high-amplitude control pulses are sent to extend the measurement range to bias the sensor at full scale, which may reduce performance.

[0017] The above-described sensors, while advantageous in many respects, have relatively high power consumption. Summary of the Invention

[0018] In view of the above circumstances, an object of the present invention is to provide an electrostatic pendulum acceleration sensor having reduced power consumption while maintaining improved performance.

[0019] In addition, another object of the present invention is to provide such a sensor with a simple implementation structure.

[0020] Therefore, the present invention proposes an acceleration sensor, which includes a shell, a pendulum fixed to the shell, a movable electrode carried by the pendulum and connected to a detection circuit, a first fixed electrode and a second fixed electrode, and a control unit; the first fixed electrode and the second fixed electrode are integrated with the shell to form two capacitors with variable capacitance with the movable electrode, and these variable capacitances depend on the distance between the first fixed electrode and the movable electrode, and the distance between the second fixed electrode and the movable electrode; the control unit is configured to: perform a detection operation to measure the variable capacitance of these capacitors; and according to the measured capacitance, perform a control operation on the movable electrode by applying a logic signal to control the switch to selectively connect the fixed electrode to the drive circuit, thereby transmitting a control signal to the fixed electrode to keep the pendulum in a predetermined position.

[0021] The control unit is configured to: apply a first detection signal at one of the fixed electrodes selected according to the logic level of the logic signal, and apply a second detection signal at the other fixed electrode in each calibration cycle; and a control signal is applied to the electrode to which the second detection signal is applied.

[0022] Therefore, by applying two detection signals and one control signal, the number of switching times of the switch is reduced, thereby achieving reduced sensor consumption and improved performance.

[0023] According to another feature, the first detection signal and the second detection signal are signals in the form of time slots. The first detection signal and the second detection signal are signals in the form of square waves.

[0024] According to yet another feature, the switch comprises a first input terminal at a reference potential provided by the driving circuit and a second input terminal at zero potential to selectively connect the fixed electrode to the driving circuit or zero potential.

[0025] In one embodiment, the driving circuit comprises a digital-to-analog converter connected to the switch and controlled by the control unit.

[0026] The detection circuit may comprise an amplifier stage having an input connected to the movable electrode and an output connected to an analog-to-digital converter having an output connected to the control unit.

[0027] For example, the control unit comprises a first estimator connected at an input to the output of the detection circuit and at an output to a negative input of a comparator having an output connected to an input of a corrector having an output connected to a sequencer and a second estimator having a first output connected to a positive input of the comparator and a second output providing an estimated acceleration.

[0028] In one embodiment, the control unit is configured to apply the second detection signal and the control signal by means of common detection and control pulses.

[0029] The present invention also relates to a method for controlling an acceleration sensor as defined above, comprising the following steps:

[0030] - detecting the variable capacitance of the capacitors by applying, during each calibration cycle, a first detection signal at one of the fixed electrodes selected according to the logic level of the logic signal and a second detection signal at the other fixed electrode;

[0031] - Based on the measured capacitance, the movable electrode is controlled by applying a logic signal to control a switch to selectively connect the fixed electrode to a drive circuit that transmits a control signal to apply a control signal to the electrode to which the second detection signal is applied.

[0032] According to the method, the second detection signal and the control signal are advantageously applied by means of a common detection and control pulse. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Other objects, features and advantages of the present invention will emerge on reading the following description, given purely by way of non-limiting example, and made with reference to the accompanying drawings, in which:

[0034] Figure 1 FIG. 1 is a schematic diagram of a sensor according to an embodiment of the present invention.

[0035] Figure 2 and Figure 3 FIG. 1 is a timing diagram showing application of the first detection signal and the second detection signal, and the control signal to the fixed electrode according to the logic levels of the logic signals.

[0036] Figure 4 FIG2 is a timing diagram illustrating another embodiment of the present invention, in which the detection signal and the control signal applied to the controlled electrode are applied via a common detection and control pulse. DETAILED DESCRIPTION

[0037] Figure 1 An acceleration sensor according to the invention is shown and designated by the general reference numeral 1 .

[0038] Here, the acceleration sensor is a microelectromechanical system, also called MEMS, which is realized by etching a plate of crystalline or semi-crystalline material such as silicon.

[0039] The sensor comprises a housing 2 to which a solid body 3 is hinged by a hinge 4 positioned in such a way that the solid body 3 forms a pendulum movable relative to the housing 2 according to a pivoting movement.

[0040] Sensor 1 includes a first fixed electrode 5.1, a second fixed electrode 5.2, and a third electrode 5.3. First and second fixed electrodes 5.1, 5.2 are integral with housing 2 and connected to a drive circuit, designated 6. Third electrode 5.3 is carried by solid body 3 and connected to a detection circuit 7. A control unit 8 is connected to drive circuit 6 and detection circuit 7.

[0041] The drive circuit 6 comprises an output connected to a switch 9 having two positions connected to the first fixed electrode 5 . 1 and the second fixed electrode 5 . 2 for selectively connecting the first fixed electrode 5 . 1 and the second fixed electrode 5 . 2 to the drive circuit 6 .

[0042] More specifically, the switch 9 includes a switch I1 configured to connect the first electrode 5.1 to either the output of the drive circuit 6 or to ground, and a switch I2 configured to connect the second electrode 5.2 to either the output of the drive circuit 6 or to ground.

[0043] The switch 9 is controlled by the control unit 8 .

[0044] The control unit 8 comprises a first estimator 10 connected at an input to the detection circuit 7 and at an output to the negative input of a comparator 11 having an output connected to an input of a corrector 12 , the output of which is connected to a sequencer 13 .

[0045] The control unit 8 further comprises a second estimator 14 having an input connected to the output of the corrector 12 , an output connected to the positive input of the comparator 11 , and an output providing the estimated acceleration γe.

[0046] Furthermore, the drive circuit 6 comprises a digital-to-analog converter 15 , which is connected to the switch 9 and controlled by the control unit 8 .

[0047] The detection circuit 7 comprises a main amplifier stage 16 comprising a charge amplifier 17 equipped with a switch I3 and a capacitor C ref The loop capacitor 18.

[0048] The amplifier stage has an input connected to the movable electrode 5 . 3 and an output connected to an input of an analog-to-digital converter 19 having an output connected to the first estimator 10 of the control unit.

[0049] The sensor operates as follows.

[0050] Control unit 8 manages the operation of the sensor, in particular, the timing of the various operations calibrated at frequency FS. Sequencer 13 sequences the operations within calibration period Ts by continuously and periodically controlling digital-to-analog converter 15 via control u, analog switches I1 and I2 via control s, analog-to-digital converter 19 via control c, and analog switch I3 via control r.

[0051] Depending on 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 or 5.1 is grounded. Thus, the electrode connected to the output of the converter is positioned at the reference potential provided by the driver circuit 6.

[0052] The acceleration sensor is controlled by the control unit to perform detection of the variable capacitance C in each calibration period Ts. h and C b stage and control stage; variable capacitor C h The variable capacitor C is formed between the first fixed electrode 5.1 and the movable electrode 5.3. b A logic signal bs is formed between the second fixed electrode 5.2 and the movable electrode 5.3. During the control phase, an excitation signal (i.e., control signal u) is applied to one of these fixed electrodes, causing the pendulum to return to its target position due to the electrostatic force applied to the plates of the capacitor (whose fixed electrode has been selected by control signal s). At the end of the detection phase, the corrector 10 determines the sign of the logic signal bs to determine whether the control signal u should be applied to the first fixed electrode 5.1 or the second fixed electrode 5.2.

[0053] If bs=+1, a voltage is applied to electrode 5 . 2 , which pulls the pendulum toward this electrode 5 . 2 .

[0054] If bs=−1, a voltage is applied to electrode 5 . 1 , which pulls the pendulum toward this electrode 5 . 1 .

[0055] The detection of the capacitances is performed conditionally according to the sign of the logic signal bs, the temporal order of these detections being determined by the sign of bs.

[0056] The capacitance C of the first fixed electrode, called the "high" fixed electrode, is given by the following relation: h The linear expression of the capacitance C of the second fixed electrode, called the "low" fixed electrode b The linear expression of:

[0057]

[0058]

[0059] And, the following relationship gives the relative position of the pendulum:

[0060]

[0061] Where C0 represents the initial capacitance, C1 represents the effective capacitance, z represents the position of the pendulum, e represents 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. When the electrodes are at rest, the distance between electrode 5.1 and electrode 5.3 is equal to the distance between electrode 5.2 and electrode 5.3. V ref is the reference voltage provided by the digital-to-analog converter 15 and applied to the electrodes, and Q b and Q h The charges transferred to the detection circuit 7 correspond to the charges received from 0 to V ref The amount of charge change at the ends of the variable capacitor affected by the changing voltage rise.

[0062] Therefore, for each calibration cycle, during the detection phase, a reading of the two capacitances is performed to estimate the position of the pendulum and to power the corrector.

[0063] During a calibration cycle, the position of the pendulum changes very little between measurements.

[0064] The detection of the variable capacitance is conditional, and the order of capacitance detection depends on the sign of the logic signal bs from the corrector 12. Therefore, the order of detection is pseudo-randomly permuted; this permutation is pseudo-random due to the characteristics of the control signal bs in a Σ-Δ (Sigma-Delta) loop (these characteristics are, in particular, those of white noise filtered by a high-pass transfer function determined by the corrector 12).

[0065] The first test is performed on the non-controlled electrode and the second test is performed on the electrode to be controlled; the logic signal is available from the beginning of the real-time cycle, since its calculation is started as soon as the test performed in the previous calibration cycle is valid.

[0066] refer to Figure 2 For example, when bs=+1, the first detection D1 is performed on the high fixed electrode 5.1 by applying a read first detection pulse (first detection signal), and then the second detection D2 is performed on the low fixed electrode by applying a second detection pulse (second detection signal).

[0067] Then, by controlling the switch 9, a control pulse (control signal u) is supplied to the lower fixed electrode Vb.

[0068] refer to Figure 3 , in the case of bs=-1, the pulse detection order is reversed.

[0069] This condition detection makes it possible to limit the number of switchings of the switch 9 and thus reduce consumption; the control is applied directly to the electrode to be controlled. In fact, it is no longer necessary to operate the switch 9 which remains in the same state between the detection phase D2 and the control phase CDE.

[0070] It should also be noted that the order of detection is changed at high frequency. Therefore, the electron source deviation caused by the control pulse is converted into noise due to the pseudo-random nature of the control pulse.

[0071] In practice, any detection deviation generates noise with a spectrum identical to that of the logic signal bs. Due to conditional detection, this deviation is significantly reduced by multiplying by the mean value of the control signal bs, leaving only the noise unchanged, thus improving the noise margin.

[0072] Therefore, both the parasitic forces applied to the condition detection and the position measurement itself become pseudo-random.

[0073] Furthermore, given that in the prior art the detection pulses are periodic and the spectrum of these detection pulses consists of lines, the arrangement has a spectrum spreading effect which makes it possible to limit the excitation of high-frequency parasitic modes and in particular to modulate this excitation by a control which becomes permanent and slowly varying (or in other words less monotonic in frequency).

[0074] In addition, reference Figure 4 According to another aspect, the second detection pulse and the control pulse form a common detection and control pulse.

[0075] In other words, the rising edge of the control signal is shifted to coincide with the falling edge of the second detection signal.

[0076] This embodiment allows capacitance detection and control to be performed by the same pulse signal.

[0077] 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 electrode constitutes the electrostatic force applied to the pendulum, the direction of which is determined by the logic signal bs.

[0078] Detection is performed by detecting the carrier of the control signal, while control is performed in baseband.

[0079] In theory, this embodiment still allows reducing the consumption by one third by reducing the switching of the switch 9 .

[0080] Similarly, a one-third reduction in the offset error due to the waveform, which depends on the number of pulses within the calibration period, is achieved.

[0081] Finally, the scale factor error caused by the waveform is completely eliminated.

[0082] In fact, the acceleration equivalent to the applied force is written as:

[0083]

[0084] Where: C1 is the effective capacitance, e is the width of the air gap, m is the mass of the pendulum, and is the mean square of the voltage applied to the upper and lower electrodes during each calibration cycle.

[0085] In an embodiment with three detection and control pulses, the applied force is written as:

[0086]

[0087] in, corresponds to the deviation, and Corresponds to the scale factor.

[0088] By implementing a first detection pulse, and a second detection and control pulse, the applied force is written as:

[0089]

[0090] Assuming the time constant is shorter than the pulse duration, any waveform error Whether it is on the detection pulse or the control pulse, it will have the same effect added to the theoretical mean square.

[0091] By noting the defect t For the asymmetric part, we get:

[0092]

[0093]

[0094]

[0095]

[0096] In the three pulse implementation, we obtain:

[0097]

[0098] in, corresponds to the deviation, and (σ c,th 2 +σ err 2 ) corresponds to the scale factor.

[0099] By implementing the two pulses, we obtain:

[0100]

[0101] in, corresponds to the deviation, and (σ c,th 2 -σ d,th 2 ) corresponds to the scale factor.

[0102] It can be seen that the error caused by the detection pulse compensates the error caused by the control pulse.

[0103] Scale factor errors are eliminated and no longer need to be considered during the initial calibration phase, which can be important at certain operating temperatures and during sensor aging.

Claims

1. An acceleration sensor comprising a housing (2), a pendulum fixed to the housing, a movable electrode (5.3) carried by the pendulum and connected to a detection circuit (7), a first fixed electrode (5.1) and a second fixed electrode (5.2), and a control unit (8); the first fixed electrode (5.1) and the second fixed electrode (5.2) are integrally formed with the housing to form two capacitors with variable capacitance with the movable electrode; the variable capacitance depends on the distance between the first fixed electrode and the movable electrode, and the distance between the second fixed electrode and the movable electrode; the control unit (8) is configured to: perform a detection operation to measure the variable capacitance of the two capacitors; and based on the measured capacitance, control a switch (9) by applying a logic signal (bs) to selectively connect the fixed electrode to a drive circuit (6), thereby transmitting a control signal (u) to the fixed electrode, thereby performing a control operation on the movable electrode to keep the pendulum at a predetermined position, characterized in that The control unit is configured to: apply a first detection signal at one of the fixed electrodes selected according to the logic level of the logic signal, and apply a second detection signal at the other fixed electrode in each calibration period; and the control signal (u) is applied to the electrode to which the second detection signal is applied.

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

3. The sensor according to claim 1 or 2, wherein: The switch (9) comprises a first input terminal at a reference potential provided by the drive circuit (6) and a second input terminal at a zero potential, so as to selectively connect the electrode to the drive circuit or the zero potential.

4. The sensor according to claim 1 or 2, wherein: The drive circuit (6) comprises a digital-to-analog converter, which is connected to the switch (9) and controlled by the control unit (8).

5. The sensor according to claim 1 or 2, wherein: The detection circuit (7) comprises an amplifier stage having an input connected to the movable electrode (5.3) and an output connected to an analog-to-digital converter having an output connected to the control unit.

6. The sensor according to claim 1 or 2, wherein: The control unit (8) comprises a first estimator (10) connected at an input to the output of the detection circuit and at an output to a negative input of a comparator (11), the comparator (11) having an output connected to an input of a corrector (12), the corrector (12) having an output connected to a sequencer (13) and to a second estimator (14), the second estimator (14) having a first output connected to a positive input of the comparator and a second output providing an estimated acceleration.

7. The sensor according to claim 1 or 2, wherein: The control unit (8) is configured to apply the second detection signal and the control signal via common detection and control pulses.

8. A method for controlling an acceleration sensor according to any one of claims 1 to 7, characterized in that: The method comprises the following steps: - detecting the variable capacitance of the two capacitors by applying a first detection signal at one of the fixed electrodes (5.1, 5.2) selected according to the logic level of the logic signal and a second detection signal at the other fixed electrode during each calibration cycle; - Based on the measured capacitance, the movable electrode (5.3) is controlled by applying a logic signal to control a switch (9) to selectively connect the fixed electrode to a drive circuit (6) that transmits a control signal, so as to apply the control signal (u) to the electrode to which the second detection signal is applied.

9. The method according to claim 8, wherein The second detection signal and the control signal are applied by a common detection and control pulse.

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