MEMS accelerometer self-test using variable excitation voltage and fixed timing

Abstract

The present disclosure relates to MEMS accelerometer self-test using a variable excitation voltage and fixed timing. A microelectromechanical system (MEMS) accelerometer sensor has a moving mass and a sense capacitor. To self-test the sensor, a test signal having a variable controlled excitation voltage and a fixed pulse width is applied to the sense capacitor. The leading and trailing edges of the test signal are aligned to coincide with a reset phase of a sense circuit coupled to the sense capacitor. The variable controlled excitation voltage of the test signal is configured to produce an electrostatic force that produces a desired physical displacement of the moving mass. During a read phase of the sense circuit, a change in capacitance of the sense capacitor due to an actual physical displacement of the moving mass is sensed for comparison to the desired physical displacement.

Description

Technical Field

[0001] This invention generally relates to a microelectromechanical system (MEMS) sensor of the accelerometer type, and more particularly to a technique for performing self-tests on MEMS accelerometers. Background Technology

[0002] Capacitive microelectromechanical systems (MEMS) accelerometer sensors typically consist of two stator nodes and one rotor node. An application-specific integrated circuit (ASIC) is electrically connected to all three nodes of the MEMS sensor. Acceleration in response to an applied force causes a moving (test) mass (electrically coupled to one rotor node) to displace relative to a pair of stationary electrodes (electrically coupled to the two stator nodes respectively), resulting in a change in capacitance. The ASIC applies a square-wave drive voltage to the rotor node and monitors the capacitance changes at the two stator nodes to determine the acceleration caused by the applied force.

[0003] MEMS accelerometer sensors are well-known for their use in safety-critical applications, such as passenger safety systems in motor vehicles. For example, MEMS accelerometer sensors can operate to assist electronic stability control functions, mechanical fault detection, and collision detection (for triggering airbag deployment and / or seatbelt pretensioning). Therefore, MEMS accelerometer sensors must function correctly. Self-testing of MEMS accelerometer sensors is necessary to ensure proper sensor operation.

[0004] There is a need in the field for self-testing techniques for use in conjunction with MEMS accelerometer sensors. Summary of the Invention

[0005] In one embodiment, a method for self-testing an accelerometer system is provided, the accelerometer system including a plurality of microelectromechanical systems (MEMS) accelerometer sensors, a charge sensing circuit, and a multiplexer configured to selectively couple sensing outputs of the MEMS accelerometer sensors to the charge sensing circuit during cyclically repeating sensing cycles, wherein each sensing cycle includes a reset phase and a readout phase. The method includes: during a first sensing cycle, coupling the sensing output of a first MEMS accelerometer sensor among the plurality of MEMS accelerometer sensors to the charge sensing circuit via the multiplexer; during the first sensing cycle, applying a self-test voltage to a second MEMS accelerometer sensor among the plurality of MEMS accelerometer sensors, the self-test voltage having a leading edge consistent with the reset phase of the first sensing cycle, and wherein the self-test voltage has a variable controlled excitation voltage; during a second sensing cycle occurring after the first sensing cycle, coupling the sensing output of the second MEMS accelerometer sensor to the charge sensing circuit via the multiplexer; and wherein the self-test voltage also has a trailing edge consistent with the reset phase of the second sensing cycle.

[0006] In one embodiment, a method for self-testing an accelerometer system is provided, the accelerometer system including a first microelectromechanical system (MEMS) accelerometer sensor, a second MEMS accelerometer sensor, and a third MEMS accelerometer sensor, a charge sensing circuit, and a multiplexer configured to selectively couple the sensing outputs of the first MEMS accelerometer sensor, the second MEMS accelerometer sensor, and the third MEMS accelerometer sensor to the charge sensing circuit in a cyclic repetitive sensing cycle, wherein each sensing cycle includes a reset phase and a readout phase. The method includes: during a first sensing cycle, coupling the sensing output of a first MEMS accelerometer sensor to a charge sensing circuit via a multiplexer; during a second sensing cycle, coupling the sensing output of a second MEMS accelerometer sensor to the charge sensing circuit via a multiplexer; during a third sensing cycle, coupling the sensing output of a third MEMS accelerometer sensor to the charge sensing circuit via a multiplexer; and during the second and third sensing cycles, applying a first self-test voltage to the first MEMS accelerometer sensor, wherein the first self-test voltage has a first variable controlled excitation voltage, the first variable controlled excitation voltage having a leading edge consistent with the reset phase of the second sensing cycle and a leading edge consistent with the reset phase of the first sensing cycle. During the third sensing cycle and the first sensing cycle, a second self-test voltage is applied to a second MEMS accelerometer sensor, wherein the second self-test voltage has a second variable controlled excitation voltage, the second variable controlled excitation voltage having a leading edge consistent with the reset phase of the third sensing cycle, and a trailing edge consistent with the reset phase of the second sensing cycle; and during the first sensing cycle and the second sensing cycle, a third self-test voltage is applied to a third MEMS accelerometer sensor, wherein the third self-test voltage has a third variable controlled excitation voltage, the third variable controlled excitation voltage having a leading edge consistent with the reset phase of the first sensing cycle, and a trailing edge consistent with the reset phase of the third sensing cycle. Attached Figure Description

[0007] To better understand the embodiments, reference will now be made to the accompanying drawings by way of example only, in which:

[0008] Figure 1 This is a block diagram of a single-axis microelectromechanical system (MEMS) accelerometer sensing system;

[0009] Figure 2 This is a block diagram of a three-axis MEMS accelerometer sensing system;

[0010] Figure 3A and Figure 3B The diagrams are shown below. Figure 1 and 2The timing diagram of the system operation;

[0011] Figure 4A and Figure 4B The self-test timing diagrams of a single-axis sensor using a variable duty cycle self-test signal and a variable voltage level self-test signal are shown respectively.

[0012] Figure 5A The diagram illustrates the use of Figure 4A Self-test timing diagram of a triaxial sensor with variable duty cycle self-test signal;

[0013] Figure 5B The diagram illustrates the use of Figure 5B Self-test timing diagram of a triaxial sensor with variable voltage level self-test signal;

[0014] Figure 6 This is a circuit diagram of a multiplexer that supports normal sensing mode and self-test mode;

[0015] Figure 7 Details of the self-test voltage generator circuit are shown; and

[0016] Figure 8 An alternative embodiment of a MEMS accelerometer sensor is illustrated. Detailed Implementation

[0017] Now refer to Figure 1 , Figure 1A block diagram of a single-axis microelectromechanical system (MEMS) accelerometer sensing system 10 is shown. System 10 includes a MEMS accelerometer sensor 12 and an application-specific integrated circuit (ASIC) 14 electrically connected to the sensor 12. It should be noted that the MEMS accelerometer sensor 12 is typically formed on a single integrated circuit die, and the ASIC 14 is typically formed on a separate integrated circuit die (however, in some embodiments, it may be integrated with the sensor 12 on a common die). The MEMS accelerometer sensor 12 includes a rotor node 20 and two stator nodes 22 and 24. The rotor node 20 is driven using a rotor drive signal Rd generated by a driver circuit 30 of the ASIC 14. At stator nodes 22 and 24, capacitive sensing signals Sc1 and Sc2 are output for processing by the ASIC. The rotor drive signal Rd is typically a square wave signal with a specific drive frequency and drive voltage Vrot. ASIC 14 includes a capacitance-to-voltage (C2V) converter circuit 32 coupled to receive capacitive sensing signals Sc1, Sc2 from stator nodes 22 and 24, and the C2V converter circuit 32 operates to convert the sensed capacitance change (i.e., difference) of the MEMS accelerometer sensor 12 into an analog voltage. Analog-to-digital converter (ADC) circuit 34 is used to convert the analog voltage into a digital signal indicating the sensed acceleration in a single-axis direction for processing by a digital signal processor (DSP) 36 within ASIC 14. In embodiments, ADC circuit 34 may include an accumulating incremental ADC (sigma-delta ADC) or a successive approximation (SAR) ADC (successive approximation ACD) or other suitable ADCs known to those skilled in the art.

[0018] Now refer to Figure 2 , Figure 2A block diagram of a triaxial microelectromechanical system (MEMS) accelerometer sensing system 10' is shown. System 10' includes three MEMS accelerometer sensors 12x, 12y, and 12z (one sensor for each of the orthogonal sensing directions X, Y, and Z) and an application-specific integrated circuit (ASIC) 14. ASIC 14 includes a multiplexer (MUX) 13, which is selectively electrically connected to each sensor 12x, 12y, and 12z in response to a selection signal Sel generated by control circuitry 33 of ASIC 14. It should be noted that the three MEMS accelerometer sensors 12x, 12y, and 12z are typically formed on a single integrated circuit die, but in some embodiments, they may be provided as separate circuits, and ASIC 14 is typically located on a separate integrated circuit die (but in some embodiments, it may be integrated with the sensors 12x, 12y, and 12z on a common die). Each MEMS accelerometer sensor 12x, 12y, 12z includes a rotor node 20 and two stator nodes 22 and 24 (nodes 22x, 24x, or 22y, 24y, or 22z, 24z, respectively). The rotor node 20 is driven by a rotor drive signal Rd generated by the driver circuit 30 of the ASIC 14. At the two stator nodes 22 and 24, capacitive sensing signals Sc1 and Sc2 (signals Sc1x, Sc2x, or Sc1y, Sc2y, or Sc1z, Sc2z, respectively) are output for selection by the MUX 13 and processed by the circuitry within the ASIC. The rotor drive signal Rd is typically a square wave signal with a specific drive frequency and drive voltage Vrot. ASIC 14 includes a capacitance-to-voltage (C2V) converter circuit 32, coupled by MUX 13, to receive selected capacitive sensing signals Sc1o, Sc2o from stator nodes 22 and 24 of selected sensors 12x, 12y, 12z in a single pass. The C2V converter circuit 32 operates to convert the sensed capacitance changes (i.e., differences) of the selected MEMS accelerometer sensors 12x, 12y, 12z into analog voltages. An analog-to-digital converter (ADC) circuit 34 operates to convert the analog voltages into digital signals indicating the sensed acceleration for processing by a digital signal processor (DSP) 36 within ASIC 14. In embodiments, the ADC circuit 34 may include an incremental ADC or a successive approximation (SAR) ADC, or other suitable ADCs known to those skilled in the art.

[0019] The general structural configuration of MEMS accelerometer sensors 12, 12x, 12y, or 12z is well known to those skilled in the art. Generally, MEMS accelerometer sensors 12, 12, 12y, and 12z include a stator and a moving (test) mass. The stator and moving mass can be made of semiconductor material connected by elastic (spring) suspension elements. The stator and moving mass typically each include multiple fixed arms and moving arms, which are interdigitated in a comb-finger configuration to form a pair of capacitors having a common electrical terminal coupled to rotor node 20 and separate electrical terminals coupled to two stator nodes 22 and 24, respectively. The capacitance of each capacitor in the pair depends on the relative position of the arms, which in turn depends on the position of the moving mass relative to the stator. In response to an applied force, the moving mass moves relative to the stator, causing a corresponding change in capacitance. This change in capacitance is sensed by an ASIC via capacitive sensing signals Sc1 and Sc2 and converted into a digital value indicating acceleration. MEMS accelerometer sensors 12, 12x, 12y, or 12z can be electrically represented by a first sensing capacitor Cs1 and a second sensing capacitor Cs2, such as Figure 1 and 2 As shown, the first sensing capacitor Cs1 has a first terminal electrically connected to the rotor node 20 and a second terminal electrically connected to the stator node 22, and is configured to generate a capacitive sensing signal Sc1 in response to the rotor drive signal Rd. The second sensing capacitor Cs2 has a first terminal electrically connected to the rotor node 20 and a second terminal electrically connected to the stator node 24, and is configured to generate a capacitive sensing signal Sc2 in response to the rotor drive signal Rd.

[0020] The C2V converter circuit 32 and ADC circuit 34 of ASIC 14 are controlled to operate cyclically according to a sensing cycle, which includes a reset phase and a readout phase. During the sensing cycle, the rotor node 20 of the MEMS accelerometer sensor 12 is driven using a square wave rotor drive signal Rd. In the reset phase of the sensing cycle, the sensing circuitry of the C2V converter circuit 32 is reset. For example, in the context of a C2V converter circuit 32 with a switched capacitor integrated circuit, the integrated capacitor is reset. During the reset phase, the two single-ended inputs of the differential C2V converter circuit 32 may be shorted together by a switching circuit, and the two single-ended outputs of the differential C2V converter circuit 32 may be shorted together by a switching circuit. In the readout phase of the sensing cycle, the sensing circuitry of the C2V converter circuit 32 is coupled to stator nodes 22 and 24 (in... Figure 2In this implementation, the capacitance change at the first sensing capacitor Cs1 and the second sensing capacitor Cs2 is sensed via MUX 13 and from sensing signals Sc1 and Sc2. This capacitance change indicates the acceleration of the moving mass block within the MEMS accelerometer sensor 12. The analog voltage output from the C2V converter circuit 32 is converted into a digital signal by the ADC circuit 34, where the digital signal indicates the sensed acceleration.

[0021] Figure 3A The diagram illustrates the timing of the operation of the sensing system 10, and Figure 3B The diagram illustrates the timing of the operation of the sensing system 10'. The square wave rotor drive signal Rd switches between a reference voltage (e.g., ground) and a drive voltage Vrot. The drive signal Rd has a drive period T. D In each drive cycle T D At the end, a reset phase is executed concurrently with the trailing edge of the square wave rotor drive signal Rd. The drive cycle T between consecutive reset phases... D Within this process, each readout phase occurs during which capacitive sensing signals Sc1 and Sc2 from sensor 12 are sensed. In the triaxial implementation, control signal Sel controls MUX 13 to cyclically change the selection of one of sensors 12x, 12y, and 12z (consistent with the reset phase), and between successive reset phases, sensing of capacitive sensing signals Sc1 and Sc2 from the selected sensor 12 occurs.

[0022] As previously mentioned, it is important to ensure the proper functioning of the MEMS accelerometer sensor 12, and for this purpose, the ASIC implements a self-test operating mode. The diffusion method for generating a self-test signal for the MEMS accelerometer sensor 12 is to introduce a voltage difference between stator nodes 22 and 24. This voltage difference creates an unbalanced force between the stators, causing displacement of the moving (test) mass. This displacement can then be read using the ASIC. The force applied to displace the moving mass is a function of the square of the difference between the voltage applied to stator node Vst and the voltage applied to rotor Vrot. If the same stator voltage is applied to stator nodes 22 and 24, the applied forces cancel each other out. However, if different stator voltages are applied to stator nodes 22 and 24, a net force exists that causes displacement of the moving mass.

[0023] Assuming the simplified timing scheme involves Figure 1The single-axis sensor shown allows for the analytic expression of the applied self-test force. The self-test voltage applied to one of the two stator nodes 22 and 24 is a DC signal. It should also be noted that mechanical components will filter out any high-frequency force contributions. Therefore, the relevant information is the average value of the self-test force given by the following equation:

[0024]

[0025] Where T st This is the time period during which the self-test was executed. This integral can be easily solved graphically as follows:

[0026]

[0027] In the aforementioned equation, c is a proportionality constant, and V ST It is the self-test voltage applied to one of the two stator nodes 22 and 24, with a self-test period T. st The duty cycle is D, and V S It is the reference voltage (e.g., common-mode voltage V) applied to the other stator node in the two stator nodes 22 and 24. CM The aforementioned equation shows that the change in duty cycle D or the self-test excitation amplitude V ST The change can be used to modify the magnitude of the self-test force applied to the sensing mass block.

[0028] Figure 4A The self-test timing diagram is shown, in which the self-test excitation amplitude V of the self-test signal is shown. ST It is fixed, and the duty cycle D can be changed to control the applied self-test force. The use of parallel vertical lines on the trailing edge of the self-test signal Vst1 is intended to indicate the trailing edge during the self-test period T. st The position within is variablely controlled using a variable duty cycle. Figure 4B The self-test timing diagram is shown, in which the self-test excitation amplitude V of the self-test signal is... ST It can be changed, and the duty cycle D is fixed to control the applied self-test force. The use of a parallel horizontal line for the pulse of the self-test signal Vst1 is intended to indicate the amplitude during the self-test period T. st The internal control is achieved through the use of a variable excitation voltage level. In each example, a first self-test signal Vst1 (with a variable duty cycle or a variable voltage level) is applied to the first stator node 22, and a second self-test signal Vst2 (at a fixed voltage Vs) is applied to the second stator node 24. It should be understood that the self-test signals may alternatively be applied to the opposite stator nodes.

[0029] Refer again Figure 3BRegarding the triaxial sensing system, it is important to note that when the MUX 13 selectively connects one of the MEMS accelerometer sensors 12x, 12y, or 12z to the C2V converter 32 to perform a readout, the other two sensors are disconnected from sensing. This disconnection of the other two sensors provides an opportunity to incorporate the self-test process using the same timing as used for normal sensor operation. When the stator nodes 22 and 24 of one of the MEMS accelerometer sensors 12 (e.g., sensor 12x) are coupled by the MUX 13 to the C2V converter 32 to sense the displacement of the moving mass block through the sensed capacitance on capacitors Cs1x and Cs2x, the ASIC 14 can transmit the self-test signal (based on...) Figure 5A shown Figure 4A Variable duty cycle technology or Figure 5B shown Figure 4B A variable voltage level technique is applied to the stator nodes 22 and 24 of the other two MEMS accelerometer sensors 12 (e.g., sensors 12y, 12z and capacitors Cs1y, Cs2y, Cs1z, Cs2z) to apply a self-test force F. ST This also causes the corresponding moving mass block to shift. In each case, the leading edge of the self-test signal pulse aligns with the reset phase. For... Figure 4A and Figure 5A The variable duty cycle self-test technique typically uses a pulse whose trailing edge typically occurs during the sensing phase for different sensing axes (i.e., between reset phases). Conversely, for... Figure 4B and Figure 5B The variable voltage level self-test technology aligns the trailing edge of the self-test signal pulse with the subsequent reset phase. Figure 4B and Figure 5B In the test, the self-test signals Vst1 and Vst2 are equal to the self-test time period T. st During this period, the voltages VSc1 and VSc2 at the corresponding stator nodes.

[0030] When using variable duty cycle self-test (WBT) techniques, the trailing edge transition of the WBT signal pulse during the sensing phase can introduce unwanted behavior in the sensing axis that is not excited by the WBT signal, and can interfere with the sensing operation of the sensing axis due to the injection of spurious charge at the input of the C2V converter 32. Therefore, using variable voltage level WBT techniques has the advantage of aligning the leading and trailing edges of the WBT signal pulse with the reset phase. During the reset phase, when the C2V converter 32 is inactive and being reset, the transition of the WBT signal prevents spurious charge injection and does not interfere with the sensing operation of the sensing axis.

[0031] Now refer to Figure 6 , Figure 6 The circuit diagram of a multiplexer (MUX) 100 is shown. The MUX 100 can be used as... Figure 2 The MUX 100 includes a multiplexer 13 in its triaxial sensor to support self-test operation. The MUX 100 has a first pair of inputs coupled to stator nodes 22x and 24x of the x-axis MEMS sensor 12x using input signal lines 102 and 104, a second pair of inputs coupled to stator nodes 22y and 24y of the y-axis MEMS sensor 12y using input signal lines 106 and 108, and a third pair of inputs coupled to stator nodes 22z and 24z of the z-axis MEMS sensor 12z using input signal lines 110 and 112. The MUX 100 also includes a pair of outputs coupled to a C2V converter of the ASIC using output signal lines 114 and 116.

[0032] The first switch S1 selectively connects signal line 102 to the first self-test voltage V generated by the self-test voltage generator circuit. STX Furthermore, the second switch S2 selectively connects signal line 102 to the common-mode voltage V of C2V converter 32. CM The third switch S3 selectively connects signal line 104 to the first self-test voltage V. STX Furthermore, the fourth switch S4 selectively connects signal line 104 to the common-mode voltage V. CM First self-test voltage V STX and common-mode voltage V CM The signal is selectively applied via MUX 100 to apply a self-test signal to the stator node of sensor 12x during self-test operation. A fifth switch S5 selectively connects input signal line 102 to output signal line 114, and a sixth switch S6 selectively connects input signal line 104 to output signal line 116.

[0033] The seventh switch S7 selectively connects signal line 106 to the second self-test voltage V generated by the self-test voltage generator circuit. STY Furthermore, the eighth switch S8 selectively connects signal line 106 to the common-mode voltage V. CM The ninth switch S9 selectively connects signal line 108 to the second self-test voltage V. STY Furthermore, the tenth switch S10 selectively connects signal line 108 to the common-mode voltage V. CM Second self-test voltage V STY and common-mode voltage V CMThe signal is selectively applied via MUX100 to apply a self-test signal to the stator node of sensor 12y during self-test operation. Eleventh switch S11 selectively connects input signal line 106 to output signal line 114, and twelfth switch S12 selectively connects input signal line 108 to output signal line 116.

[0034] The thirteenth switch S13 selectively connects signal line 110 to the third self-test voltage V generated by the self-test voltage generator circuit. STZ Furthermore, the fourteenth switch S14 selectively connects signal line 110 to the common-mode voltage V. CM The fifteenth switch S15 selectively connects signal line 112 to the third self-test voltage V. STZ Furthermore, the sixteenth switch S16 selectively connects signal line 112 to the common-mode voltage V. CM The third self-test voltage V STZ and common-mode voltage V CM The signal is selectively applied via MUX 100 to apply a self-test signal to the stator node of sensor 12z during self-test operation. Seventeenth switch S17 selectively connects input signal line 110 to output signal line 114, and eighteenth switch S18 selectively connects input signal line 112 to output signal line 116.

[0035] The actuation of switches S1 to S18 is controlled using a selection signal Sel (configured as a multi-bit digital signal) generated by the ASIC control circuitry. The MUX 100 supports normal sensing operation (in which the sensing voltage VSc at the stator node is sensed) and self-test operation (in which the self-test voltage V...). STX V STY and V STZ It is selectively applied to the rotor nodes, and the voltage Vst is sensed.

[0036] The normal operation of the sensor will now be described (see also: Figure 3B ).

[0037] When the x-axis MEMS sensor 12x (see attached label 140) is sensed, Figure 3BWhen the ASIC control circuit generates a selection signal Sel, it actuates (i.e., closes) switches S5 and S6 (connecting stator nodes 22x and 24x of sensor 12x to the C2V converter circuit). Switches S2 and S4 are deacted (and switches S1 and S3 are never actuated during normal operation). The selection signal Sel further deacts (i.e., opens) switches S11, S12, S17, and S18 (disconnecting the y-axis MEMS sensor 12y and the z-axis MEMS sensor 12z), and switches S8, S10, S14, and S16 are actuated (i.e. closed) to bias the stator nodes of sensors 12y and 12z to the common-mode voltage V. CM .

[0038] When the y-axis MEMS sensor 12y (see attached label 142) is sensed, Figure 3B When the ASIC control circuit generates a selection signal Sel, it actuates (i.e., closes) switches S11 and S12 (connecting the stator nodes 22y and 24y of sensor 12y to the C2V converter circuit). Switches S8 and S10 are deactivated (and switches S7 and S9 are never actuated during normal operation). The selection signal Sel further deactivates (i.e., opens) switches S5, S6, S17, and S18 (disconnecting the x-axis MEMS sensor 12x and the z-axis MEMS sensor 12z), and switches S2, S4, S14, and S16 are actuated (i.e., closed) to bias the stator nodes of sensors 12x and 12z to the common-mode voltage V. CM .

[0039] When the z-axis MEMS sensor 12z (see attached label 144) is sensed, Figure 3B When the ASIC control circuit generates a selection signal Sel, it actuates (i.e., closes) switches S17 and S18 (connecting the stator nodes 22z and 24z of sensor 12z to the C2V converter circuit). Switches S14 and S16 are deactivated (and switches S13 and S15 are never actuated during normal operation). The selection signal Sel further deactivates (i.e., opens) switches S5, S6, S11, and S12 (disconnecting the x-axis MEMS sensor 12x and the y-axis MEMS sensor 12y), and switches S2, S4, S8, and S10 are actuated (i.e., closed) to bias the stator nodes of sensors 12x and 12y to the common-mode voltage V. CM .

[0040] The sensor self-test operation using variable duty cycle, fixed amplitude technology will now be described (see also: [link to documentation]). Figure 5A ).

[0041] Assuming the x-axis MEMS sensor 12x test and the x-axis self-test cycle T stXExecute accordingly. During the x-axis self-test cycle T. stX Within, the drive signal Rd has two drive cycles T D These two drive cycles correspond to the sensing operations performed on the y-axis MEMS sensor 12y and the z-axis MEMS sensor 12z. This is immediately followed by the x-axis self-test cycle T. stX Previous drive cycle T D The end of the drive cycle (i.e., the period during which sensor 12x is sensed) coincides with the trailing edge of the rotor drive signal Rd, and corresponds to the x-axis self-test period T. stX In the first of the two drive cycles, the ASIC's control circuit generates a selection signal Sel to deactivate (i.e., disconnect) switches S5 and S6, disconnecting the x-axis MEMS sensor 12x from the ASIC's C2V converter, and actuates (i.e., closes) switches S1 and S4 to apply a first self-test voltage V to the stator node 22x via signal line 102. STX And a common-mode voltage V is applied to the stator node 24x of the x-axis MEMS sensor 12x via signal line 104. CM It is important to note here that switches S2 and S3 remain open from the immediate preceding drive cycle, during which sensing by sensor 12x is being performed. It is also important to note that, alternatively, switches S2 and S3 are closed, and switches S1 and S4 remain open to apply the first self-test voltage V to stator node 24x via signal line 104. STX And a common-mode voltage V is applied to the stator node 22x of the x-axis MEMS sensor 12x via signal line 102. CM 12x self-test of x-axis MEMS sensor from common-mode voltage V CM Up to the first self-test voltage V STX The leading edge transition occurs during the reset phase. The first self-test voltage V... STX The voltage level is fixed, but the duration for which this voltage is applied to the stator node is variablely controlled by the ASIC control circuit via the selection signal Sel, in order to set the self-test force F. ST The quantity, self-testing force F ST Displacement of the mass block is applied to move (test) the x-axis MEMS sensor 12x. During the x-axis self-test cycle T... stX In the second of the two drive cycles, the deactivation of switches S2, S3, S5, and S6 continues, but the ASIC's control circuit generates a selection signal Sel to deactivate (i.e., open) switch S1 and actuate (i.e., close) switch S2 to terminate the first self-test voltage V. STX The application of the common-mode voltage V through line 102 according to the variable duty cycle. CMApply to stator node 22x. Note: Instead, combine the opposite application of open switch S3 and closed switch S4. x-axis MEMS sensor 12x self-test from the first self-test voltage V STX to common-mode voltage V CM The trailing edge transition is controlled by the desired duty cycle and is likely to occur during the x-axis self-test cycle T. stX At some point in the middle or before / after of the second drive cycle out of two drive cycles. During the x-axis self-test cycle T stX After completion, the ASIC control circuit generates a selection signal Sel to close switches S5 and S6 during the drive cycle in which sensor 12x is sensed. During this phase, the stator voltages at nodes 22x and 24x are biased by the C2V converter 32 to the common-mode voltage V. CM Level. Then, due to the previously applied self-test force F ST The movement (test) of the x-axis MEMS sensor 12x caused by this movement can be sensed by the capacitances of sensing capacitors Cs1x and Cs2x. The magnitude of the sensed displacement is then compared with the applied self-test force F. ST The expected displacement is compared to confirm the correct operation of the x-axis MEMS sensor 12x. The driving of the aforementioned switches causes the stator voltages VSc1x and VSc2x to exhibit... Figure 5A The time behavior described.

[0042] It should be noted that due to the system's interleaved sensing operation, during the x-axis self-test cycle T... stX During the first of the two drive cycles, the ASIC's control circuit generates a selection signal Sel to actuate (i.e., close) switches S11 and S12 to connect the stator nodes 22y and 24y of the y-axis MEMS sensor 12y to the ASIC's C2V converter for sensing capacitances Cs1y and Cs2y. Furthermore, during the x-axis self-test cycle T... stX During the second of the two drive cycles, the ASIC's control circuit generates a selection signal Sel to actuate (i.e., close) switches S17 and S18 to connect the stator nodes 22z and 24z of the z-axis MEMS sensor 12z to the ASIC's C2V converter for sensing capacitances Cs1z and Cs2z. Therefore, the first self-test voltage V... STX The sensing operation is performed concurrently with the sensing operation on the other sensing axes by one of the stator nodes 22x (VSc1x) and 24x (VSc2x) of the x-axis MEMS sensor 12x.

[0043] Similar operations are performed for testing the y-axis MEMS sensor 12y (via stator nodes 22y and 24y) and the z-axis MEMS sensor 12z (via stator nodes 22z and 24z). The corresponding self-test cycle T... stY and T stZ It's merely a temporal shift (offset and partial overlap). Specifically, in the drive cycle T associated with z-axis sensing and x-axis sensing... D During this period, a second self-test voltage V is applied to the variable duty cycle, fixed amplitude self-test. STY This causes the y-axis MEMS sensor 12y to sense (test mass block) displacement. Similarly, during the drive cycle T associated with the x-axis and y-axis sensing... D During this period, a third self-test voltage V is applied to the variable duty cycle, fixed amplitude self-test. STZ This causes the z-axis MEMS sensor 12z to sense (inspection mass block) displacement.

[0044] Given that a self-test voltage V is applied during the self-test signal STX V STY V STZ The variable duty cycle, when one of these other sensing axes is connected by the MUX 100 to the ASIC's C2V converter circuit (i.e., not during the reset phase), may cause a common-mode voltage V to occur. CM The trailing edge transition of the level can cause unwanted charge injection, interfering with sensing measurements. Using a variable amplitude and a fixed duty cycle for the self-test signal can resolve this unwanted charge injection issue.

[0045] The self-test operation of sensors using variable amplitude, fixed duty cycle technology will now be described (see also...). Figure 5B ).

[0046] Assuming the x-axis MEMS sensor 12x test and the x-axis self-test cycle T stX Execute accordingly. During the x-axis self-test cycle T. stX Within, the drive signal Rd has two drive cycles T D These two drive cycles correspond to the sensing operations performed on the y-axis MEMS sensor 12y and the z-axis MEMS sensor 12z. This is related to the x-axis self-test cycle T. stX Previous drive cycle T D The end (i.e., the drive cycle in which sensor 12x is sensed) coincides with the trailing edge of the rotor drive signal Rd, and corresponds to the x-axis self-test period T. stXIn the first of the two drive cycles, the ASIC's control circuit generates a selection signal Sel to deactivate (i.e., disconnect) switches S5 and S6, disconnecting the x-axis MEMS sensor 12x from the ASIC's C2V converter, and actuates (i.e., closes) switches S1 and S4 to apply a first self-test voltage V to the stator node 22x via signal line 102. STX And a common-mode voltage V is applied to the stator node 24x of the x-axis MEMS sensor 12x via signal line 104. CM It is important to note here that switches S2 and S3 remain open during the immediate preceding drive cycle, during which sensing by sensor 12x is being performed. It is also important to note that, alternatively, switches S2 and S3 are closed, and switches S1 and S4 remain open to apply the first self-test voltage V to stator node 24x via signal line 104. STX And a common-mode voltage V is applied to the stator node 22x of the x-axis MEMS sensor 12x via signal line 102. CM 12x self-test of x-axis MEMS sensor from common-mode voltage V CM Up to the first self-test voltage V STX The leading edge transition occurs during the reset phase. The duration for which the voltage is applied to the stator node is fixed, but the first self-test voltage V applied to the stator node... STX The voltage level is variablely controlled by the ASIC control circuit via the selection signal Sel, so as to set the self-test force F applied to cause the x-axis MEMS sensor 12x to move (test) the displacement of the mass block. ST The quantity. During the x-axis self-test cycle T. stX In the second of the two drive cycles, the deactivation of switches S2, S3, S5, and S6 continues. This is related to the x-axis self-test cycle T. stX The second drive cycle of the two drive cycles ends simultaneously, and based on the fixed duty cycle of the self-test signal, the ASIC control circuit generates a selection signal Sel to deactivate (i.e., open) switch S1 and actuate (i.e., close) switch S2 to terminate the first self-test voltage V. STX The application of the common-mode voltage V through line 102. CM Apply to stator node 22x. Note: Instead, combine the opposite application, open switch S3 and close switch S4. x-axis MEMS sensor 12x self-test from the first self-test voltage V STX to common-mode voltage V CM The trailing edge transition occurs during the subsequent reset phase, therefore the fixed duty cycle is specifically equal to two drive cycles. During the x-axis self-test cycle T... stXAfter completion, the ASIC control circuit generates a selection signal Sel to close switches S5 and S6 during the drive cycle in which sensor 12x is sensed. During this phase, the stator voltages at nodes 22x and 24x are biased to the common-mode voltage V by the C2V converter 32. CM Level. Then, due to the previously applied self-test force F ST The displacement of the mass block caused by the movement of the x-axis MEMS sensor 12x (test) can be sensed by the capacitance of sensing capacitors Cs1x and Cs2x. The magnitude of the sensed displacement is then compared with the applied self-test force F. ST The expected displacement is compared to confirm the correct operation of the x-axis MEMS sensor 12x. The driving of the aforementioned switches causes the stator voltages VSc1x and VSc2x to exhibit... Figure 5B The time behavior described.

[0047] It should be noted that due to the system's interleaved sensing operation, during the x-axis self-test cycle T... stX During the first of the two drive cycles, the ASIC's control circuit generates a selection signal Sel to actuate (i.e., close) switches S11 and S12 to connect the stator nodes 22y and 24y of the y-axis MEMS sensor 12y to the ASIC's C2V converter for sensing capacitances Cs1y and Cs2y. Furthermore, during the x-axis self-test cycle T... stX During the second of the two drive cycles, the ASIC's control circuit generates a selection signal Sel to actuate (i.e., close) switches S17 and S18 to connect the stator nodes 22z and 24z of the z-axis MEMS sensor 12z to the ASIC's C2V converter for sensing capacitances Cs1z and Cs2z. Therefore, the first self-test voltage V... STX The sensing operation is performed concurrently with the sensing operation on the other sensing axes by one of the stator nodes 22x (VSc1x) and 24x (VSc2x) of the x-axis MEMS sensor 12x.

[0048] Similar operations are performed for testing the y-axis MEMS sensor 12y (via stator nodes 22y and 24y) and the z-axis MEMS sensor 12z (via stator nodes 22z and 24z). The corresponding self-test cycle T... stY and T stZ It's merely a temporal shift (offset and partial overlap). Specifically, in the drive cycle T associated with z-axis sensing and x-axis sensing... D During this period, a second self-test voltage V is applied to the variable amplitude, fixed duty cycle self-test. STY This results in the displacement of the y-axis MEMS sensor 12y (inspection mass block). Similarly, during the drive cycle T associated with the x-axis and y-axis sensing...D During this period, a third self-test voltage V is applied to the variable amplitude, fixed duty cycle self-test. STZ This causes the z-axis MEMS sensor 12z to sense the displacement of the mass block.

[0049] Now refer to Figure 7 , Figure 7 Details of a self-test voltage generator circuit used to generate a self-test signal incorporating a variable amplitude, fixed duty cycle, are shown. A digital-to-analog converter (DAC) circuit is provided to generate the self-test voltage V. STX V STY V STZ Each self-test voltage in the circuit. Each DAC circuit receives a digital control signal from the control circuitry of the ASIC. The bits of the digital control signal specify a particular voltage level generated by the DAC circuitry to set the variable amplitude self-test voltage V. STX V STY V STZ The voltage level. For example, the power supply voltage of the DAC circuit can be provided by a charge pump (CP) circuit (note that the use of a CP circuit is not mandatory at all, and other options for generating voltage exist and are well known to those skilled in the art). In a preferred embodiment, a voltage regulator (VReg) can be used.

[0050] Figure 8 An alternative embodiment of MEMS accelerometer sensor 12' is illustrated. Sensor 12' differs from sensor 12 in that sensing capacitors Cs1 and Cs2 are not used to apply displacement force to the moving (test) mass. Instead, sensor 12' includes two additional self-test capacitors Cst1 and Cst2 for use during self-test operation. Capacitors Cs1, Cs2, Cst1, and Cst2 have a common electrical terminal and individual electrical terminals. The common electrical terminal is coupled to rotor node 20, and the individual electrical terminals are coupled to two sensing sub-nodes 22 and 24 and two self-test stator nodes 23 and 25, respectively. Sensing sub-nodes 22 and 24 are coupled to the ASIC via MUX circuitry 13. Self-test stator nodes 23 and 25 are coupled to receive self-test signals Vst1 and Vst2 from self-test circuitry 40. As described herein, self-test signals Vst1 and Vst2 may include fixed-amplitude, variable-duty-cycle self-test signals or variable-amplitude, fixed-duty-cycle self-test signals.

[0051] While the invention has been illustrated and described in detail in the accompanying drawings and foregoing description, such illustrations and descriptions are to be considered illustrative or exemplary, and not restrictive; the invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments may be understood and implemented by those skilled in the art in practicing the claimed invention, upon studying the drawings, the disclosure, and the appended claims.

Claims

1. A method for self-testing an accelerometer system, the accelerometer system including a plurality of microelectromechanical system (MEMS) accelerometer sensors, a charge sensing circuit, and a multiplexer configured to selectively couple the sensing outputs of the MEMS accelerometer sensors to the charge sensing circuit in a cyclic repetitive sensing cycle, wherein each sensing cycle includes a reset phase and a readout phase, the method comprising: During the first sensing cycle, the sensing output of the sensing capacitor of the first microelectromechanical system accelerometer sensor connected to the plurality of microelectromechanical system accelerometer sensors is coupled to the charge sensing circuit via the multiplexer; During the first sensing cycle, a self-test voltage is applied to the self-test input of the self-test capacitor of the second microelectromechanical system accelerometer sensor connected to the plurality of microelectromechanical system accelerometer sensors, the self-test voltage having a leading edge that coincides with the reset phase of the first sensing cycle, and wherein the self-test voltage has a variable controlled excitation voltage. During the second sensing cycle, the sensing output of the sensing capacitor connected to the second microelectromechanical system accelerometer sensor is coupled to the charge sensing circuit via the multiplexer, the second sensing cycle occurring after the first sensing cycle; The self-test capacitor and the sensing capacitor of the second microelectromechanical system accelerometer sensor are different capacitors. The self-test voltage also has a trailing edge that coincides with the reset phase of the second sensing cycle.

2. The method according to claim 1, further comprising: The value of the variable controlled excitation voltage applied to the self-test input of the self-test capacitor connected to the second microelectromechanical system accelerometer sensor is set to achieve the desired displacement of the sensing mass block of the second microelectromechanical system accelerometer sensor.

3. The method according to claim 2, further comprising: During the second sensing cycle, the charge sensing circuit senses the actual displacement of the sensing mass block of the second microelectromechanical system accelerometer sensor through the sensing capacitor of the second microelectromechanical system accelerometer sensor; as well as The actual displacement is compared with the expected displacement.

4. The method according to claim 1, further comprising: During the first sensing period, the displacement of the sensing mass block of the first microelectromechanical system accelerometer sensor is sensed by the charge sensing circuit.

5. The method according to claim 1, further comprising: A force signal is applied to the rotor of each of the plurality of microelectromechanical system accelerometer sensors, wherein the force signal is a square wave signal having a drive period equal to the sensing period, and wherein the edge of the force signal coincides with the reset phase.

6. The method of claim 5, wherein the edge of the force signal is a trailing edge.

7. A method for self-testing an accelerometer system, the accelerometer system including a first microelectromechanical system (MEMS) accelerometer sensor, a second MEMS accelerometer sensor, and a third MEMS accelerometer sensor, a charge sensing circuit, and a multiplexer, the multiplexer being configured to selectively couple the sensing outputs of the first MEMS accelerometer sensor, the second MEMS accelerometer sensor, and the third MEMS accelerometer sensor to the charge sensing circuit during cyclic repetitive sensing cycles, wherein each sensing cycle includes a reset phase and a readout phase, the method comprising: During the first sensing cycle, the sensing output of the sensing capacitor connected to the first microelectromechanical system accelerometer sensor is coupled to the charge sensing circuit via the multiplexer; During the second sensing cycle, the sensing output of the sensing capacitor connected to the second microelectromechanical system accelerometer sensor is coupled to the charge sensing circuit via the multiplexer; During the third sensing cycle, the sensing output of the sensing capacitor connected to the third microelectromechanical system accelerometer sensor is coupled to the charge sensing circuit via the multiplexer. During the second sensing cycle and the third sensing cycle, a first self-test voltage is applied to the self-test input of the self-test capacitor connected to the first microelectromechanical system accelerometer sensor, wherein the first self-test voltage has a first variable controlled excitation voltage, the first variable controlled excitation voltage having a leading edge consistent with the reset phase of the second sensing cycle and a trailing edge consistent with the reset phase of the first sensing cycle, wherein the sensing capacitor of the first microelectromechanical system accelerometer sensor and the self-test capacitor of the first microelectromechanical system accelerometer sensor are different capacitors; During the third sensing cycle and the first sensing cycle, a second self-test voltage is applied to the self-test input of the self-test capacitor connected to the second microelectromechanical system accelerometer sensor, wherein the second self-test voltage has a second variable controlled excitation voltage, the second variable controlled excitation voltage having a leading edge consistent with the reset phase of the third sensing cycle and a trailing edge consistent with the reset phase of the second sensing cycle, wherein the sensing capacitor of the second microelectromechanical system accelerometer sensor and the self-test capacitor of the second microelectromechanical system accelerometer sensor are different capacitors; as well as During the first sensing cycle and the second sensing cycle, a third self-test voltage is applied to the self-test input of the self-test capacitor connected to the third microelectromechanical system accelerometer sensor, wherein the third self-test voltage has a third variable controlled excitation voltage, the third variable controlled excitation voltage having a leading edge consistent with the reset phase of the first sensing cycle and a trailing edge consistent with the reset phase of the third sensing cycle, wherein the sensing capacitor of the third microelectromechanical system accelerometer sensor and the self-test capacitor of the third microelectromechanical system accelerometer sensor are different capacitors.

8. The method according to claim 7, further comprising: During the second and third sensing cycles, the value of the first variable controlled excitation voltage is set to achieve the desired displacement of the sensing mass block of the first microelectromechanical system accelerometer sensor through the self-test capacitor of the first microelectromechanical system accelerometer sensor; During the first sensing period, the charge sensing circuit senses the actual displacement of the sensing mass block of the first microelectromechanical system accelerometer sensor through the self-test capacitor of the first microelectromechanical system accelerometer sensor; as well as The actual displacement is compared with the expected displacement.

9. The method according to claim 7, further comprising: During the third sensing cycle and the first sensing cycle, the value of the second variable controlled excitation voltage is set to achieve the desired displacement of the sensing mass block of the second microelectromechanical system accelerometer sensor through the self-test capacitor of the second microelectromechanical system accelerometer sensor; During the second sensing cycle, the charge sensing circuit senses the actual displacement of the sensing mass block of the second microelectromechanical system accelerometer sensor through the self-test capacitor of the second microelectromechanical system accelerometer sensor; as well as The actual displacement is compared with the expected displacement.

10. The method of claim 7, further comprising: During the first sensing period and the second sensing period, the value of the third variable controlled excitation voltage is set to realize the desired displacement of the sensing mass block of the third microelectromechanical system accelerometer sensor through the self-test capacitor of the third microelectromechanical system accelerometer sensor; During the third sensing cycle, the charge sensing circuit senses the actual displacement of the sensing mass block of the third microelectromechanical system accelerometer sensor through the self-test capacitor of the third microelectromechanical system accelerometer sensor; as well as The actual displacement is compared with the expected displacement.

11. The method of claim 7, further comprising: A force signal is applied to the rotor of each of the first, second, and third microelectromechanical system (MEMS) accelerometer sensors, wherein the force signal is a square wave signal having a drive period equal to the sensing period, and wherein the edge of the force signal coincides with the reset phase.

12. The method of claim 11, wherein the edge of the force signal is a trailing edge.

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