Abstract:
A quadrature error signal cancellation circuit and technique can detect an undesired quadrature signal component in the output of a MEMS gyroscope and null the quadrature signal component. In one embodiment, the cancellation circuit is configured in a feedback loop with the MEMS gyroscope and includes components to detect and condition the quadrature signal, digitize the conditioned quadrature signal, and generate a quadrature error cancellation signal which is provided back to the MEMS gyroscope.

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 61/978,264 filed on Apr. 11, 2014, entitled METHOD AND APPARATUS FOR DETECTING AND CORRECTING QUADRATURE ERROR SIGNALS FROM A MEMS GYROSCOPE, the subject matter of which is incorporated herein by this reference for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The disclosure relates to Microelectromechanical Systems (MEMS) devices, and, more specifically, to a system and technique for detecting and correcting error signal components in the output signal generated by MEMS devices. 
     BACKGROUND OF THE INVENTION 
     An imperfect MEMS gyroscope generates an undesired quadrature signal that is out of phase to the desired “rate” signal that indicates rotation about an axis. Such quadrature signal introduces an error component into the rate signal, leading to less than optimal output results from the MEMS gyroscope. In some instances, quadrature signal characteristics can overwhelm the rate signal generated by the MEMS gyroscope. 
     Accordingly, a need exists for a system and technique to eliminate the quadrature signal component in the output of a MEMS gyroscope. 
     A further need exists for a system and technique detect and cancel quadrature signal components generated by a MEMS gyroscope. 
     SUMMARY OF THE INVENTION 
     Disclosed herein is a quadrature error signal cancellation circuit and technique that can detect a quadrature error signal component in the output of a MEMS gyroscope and cancel or null the quadrature error signal component. More specifically, the disclosed system and method allows for continuous quadrature error signal cancellation via a feedback loop configuration with the MEMS gyroscope. In one embodiment, a plurality of circuit elements or modules create a signal path that can detect and condition the quadrature error signal, digitize the conditioned signal, convert the digitized signal to a digital domain signal, and, using a digital PID controller, generate a quadrature error cancellation signal capable of nulling the quadrature signal. Using the disclosed system and method, a MEMs gyroscope&#39;s quadrature error can be detected and nulled, offering a substantial advantage over gyroscopes that don&#39;t contain quadrature nulling capability for those that are calibrated only during manufacturing. 
     According to one aspect of the disclosure, a method for cancelling a quadrature error signal in an output signal of a microelectromechanical (MEMS) device comprises: A) isolating the quadrature error signal in the output signal of the MEMS device; B) processing the isolated quadrature error signal; C) generating a quadrature error cancellation signal; and D) providing the quadrature error cancelation signal to the MEMS device. 
     According to another aspect of the disclosure, a system for cancelling the quadrature error signal associated with microelectromechanical (MEMS) apparatus comprises: A) an isolation module receptive to an output signal from MEMS device isolating a quadrature error signal in the output signal of the MEMS device; B) an analog-to-digital converter for converting the isolated quadrature error signal into a sampled quadrature error signal; C) a proportional signal processing module configured for generating a proportional component quadrature error signal by scaling the sampled quadrature error signal with a proportional scaling signal; D) an integral signal processing module configured for generating a integral component quadrature error signal by scaling the sampled quadrature error signal with an integral scaling signal and summing the scaled sampled quadrature error signal with a value of a previously generated integral component quadrature error signal, E) a derivative signal processing module configured for generating a derivative component quadrature error signal by forming a difference value between the sampled quadrature error signal and a value of a previously generated sampled quadrature error signal, and scaling difference value with a derivative scaling signal; F) a first summing element for combining any of the proportional component quadrature error signal, integral component quadrature error signal, derivative component quadrature error signal into a quadrature error cancellation signal; and G) a converter for converting a digital signal representing the quadrature error cancellation signal into an analog signal. In one embodiment, the system further comprises a conditioning module comprising a filter and a voltage source for adding a direct current offset voltage to the isolated quadrature error signal. In another embodiment, the system is combined with a MEMS device configured for receiving the quadrature error cancellation signal and adjusting input parameters of the MEMS device to minimize the amount of quadrature error signal in the output signal of the MEMS device. 
    
    
     
       DESCRIPTION THE DRAWINGS 
       Embodiments of the disclosed subject matter are described in detail below with reference to the following drawings in which: 
         FIG. 1  illustrates conceptually an exemplary circuit and signal path for quadrature error detection and cancellation in accordance with the disclosure; and 
         FIG. 2  illustrates conceptually a module for generating a quadrature error cancellation signal in accordance with the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a system  5  comprises a quadrature error cancellation circuit  20  and MEMS gyroscope  10  intercoupled into a feedback loop configuration. Quadrature cancellation circuit  20  disclosed herein may be used with any number of commercially available MEMS gyroscopes  10  including those disclosed in as disclosed in United States Patent Application Publication 2012/0227,487, and U.S. Pat. No. 7,023,065. The quadrature error cancellation circuit  20  connects to the MEMS gyroscope  10  to form a closed feedback loop, as illustrated in  FIG. 1 , thereby enabling continuous quadrature error signal cancellation. The outputs of the MEMS gyroscopes  10  are coupled to the quadrature error cancellation circuit  20 , which, in turn, is coupled to the inputs of MEMS gyroscope  10 , in accordance with the disclosure contained herein. 
     In an illustrative embodiment, described in greater detail herein, the quadrature error cancellation circuit  20  comprises and isolation module  22 , conditioning module  24 , converter module  28 , analyzer module  30 , and a second converter module  34  coupled in a signal path which provides a feedback loop to MEMS gyroscope  10 , as described in greater detail herein. 
     Quadrature Error Cancellation Circuit 
     The output signal from MEMS gyroscopes  10  contains both rate and quadrature signal components summed together, such out signal designated as Vrf in  FIG. 1 . The Vrf signal is coupled to circuit  20  through an isolation component  22  which functions to isolate the quadrature error signal component of the gyroscope output signal from the rate signal component of the MEMS gyroscope output signal. Such isolation is performed by multiplying the received Vrf signal with a signal of the same frequency as the rate signal but phase shifted an amount representing the amount of phase relationship of the quadrature error signal to the rate signal. In the illustrative embodiment, the quadrature error signal is orthogonal, or 90°, out of phase to the rate signal that indicates rotation about an axis. 
     In an illustrative embodiment, the above functionality is achieved with an isolation component  22  comprising a demodulator  21  and a phase shifter  23 . Demodulator  21  may be implemented with any number of existing components, including, but not limited to, an analog multiplier model AD633JRZ-R7, commercially available from Analog Devices, Inc., Norwood, Mass. 02062-9106. Demodulator  21  is adapted to receive the Vrf signal and the output of a phase shifter  23 , as illustrated. Phase shifter  23  may be implemented with any number of know designs. In the illustrative embodiment, a local oscillator signal, Vlo signal in  FIG. 1 , that may be generated to detect the rate signal, is provided to phase shifter  23  where it is phase shifted by a predetermined amount representing the out of phase relationship between the quadrature error signal and the rate signal, e.s. 90° in the illustrative embodiment. It will be obvious to those reasonably skilled in the arts that the amount of phase shift may vary depending on the out of phase relationship of the quadrature error signal to the rate signal inherent in the design of the gyroscope  10 . 
     The phase shifted Vlo signal is then supplied to demodulator  21  where it is multiplied with the received Vrf signal by the demodulator  21 , as shown in  FIG. 1 . Such multiplication operation demodulates the only quadrature error signal while rejecting the rate signal. Depending upon the implementation, demodulator  21 , may also produce an undesired high frequency signal, at approximately twice the Vlo frequency, in the isolated quadrature error signal output from demodulator  21 . 
     The isolated quadrature error signal output from demodulator  21  is provided in the illustrative embodiment to a conditioning module  24 . Conditioning module  24  comprises a filter  25 , amplifier  26  and voltage offset component  27 , as illustrated in  FIG. 1 . In the illustrative embodiment, filter  25  may be implemented with a low-pass filter having a filter frequency cut off set to reject any undesirable high-frequency components introduced into the isolated quadrature error signal received from demodulator  21 . Filter  25  may be implemented with any number of existing designs including an R-C filter circuit configuration. Depending on the implementation of the modulator  21 , the presence or absence of undesirable frequency components in the isolated quadrature error signal may vary. Accordingly, the necessity for filter  25  and the nature of the filter may likewise vary or, optionally, be eliminated altogether. 
     The filtered quadrature error signal is a Direct Current (DC) signal and is provided to amplifier  26 , which then amplifies the filtered quadrature error signal to provide sufficient gain for the quadrature error cancellation feedback loop of system  5 , illustrated in  FIG. 1 . In one embodiment, amplifier  26  may be implemented to have a programmable gain over a predetermined range, for example from −12 dB up to +25 dB, with a plurality of selectable intermediate gain values therebetween. Alternatively, other amplification amount may be required for compatibility with the parameters of other components within the feedback loop of system  5 . 
     Next, the amplified quadrature error signal output by amplifier  26  is received by offset voltage generator  27  which generates and adds an offset voltage to change the feedback loop set point. In an illustrative embodiment, offset voltage generator  27  may be implemented with a digital to analog converter (DAC) that generates a DC offset voltage by pulling current through a resistor. In one embodiment, the amount of offset voltage may be at the discretion of the system designer. 
     The offset isolated quadrature error signal is supplied to a conversion module  28  which comprises an analog-to-digital convertor (ADC)  29  and any associated support circuitry. In the illustrative embodiment of circuit  20 , analog-to-digital conversion of the isolated and conditioned quadrature error signal facilitates easier back-end processing by analysis module  30 . In an illustrative embodiment, analog-to-digital convertor  29  may be implemented with a design which generates an 8-bit successive approximation (SAR) of the analog signal being sampled. 
     Following digitization by ADC  29 , the quadrature error signal is provided to the analysis module  30  which comprises, in a illustrative embodiment, a PID controller  32  which analyzes the digitized quadrature error signal in the digital domain and generates a quadrature error cancellation signal. The PID controller  32  uses the digitized samples of the quadrature error sign to calculate the proportional (P), integral (I), and derivative (D) components of the quadrature error cancellation signal. To that extent, PID controller  32  contains enough onboard memory or has access to memory capable of storing at least one prior digital sample representing a previously sampled quadrature error signal. The P, I, and D signals are scaled by individual weighting factors to affect the feedback loop accuracy, settling time, and stability, or possibly any other parameters as needed based on the exact implementation of the feedback loop within system  5 . For additional signal conditioning, a digital offset and/or amplifier block, similar to those described herein, can be added at the input or output of the PID controller  32 . 
     In an illustrative embodiment, PID controller  32  may be implemented as illustrated in the block level schematic diagram of  FIG. 2 . Referring to  FIG. 2 , PID controller  32  comprises a proportional process module  36 , integral process module  42 , derivative process module  48 , summing elements  38  and  40 , and clipping element  39  in the configurations illustrated in  FIG. 2 . 
     The tc_adc signal in  FIG. 2  is the digital output of the analog-to-digital converter, i.e. the digitized error signal that indicates the amount of quadrature error. This signal is provided to a multiplier functional element in each of the proportional, integral and derivative modules. The tc_kp, tc_ki, tc_kd signals are multiplier or scaling for signal the proportional, integral, and derivative modules, respectfully within the PID controller  32 , as illustrated. 
     Latches  47  and  50  in the signal path of integrator module  42  and derivative module  48 , respectively, store the previous sample(s) of the quadrature error signal following scaling by their respective multiplier element for use in performing the derivative or integral functions. Latches  47  and  50  are clocked by the tc_latch_clk signal. 
     Signals adc_kp_out, adc_ki_out, and adc_kdout, the outputs of modules  36 ,  42  and  48 , respectively, are summed together to form the pid_out 1  signal which can then be clipped to an absolute maximum value that is less than a full allowable value, in the event the MEMS device behaves non-linearly for large quadrature/error signal levels. Clipping elements  39  and  46  (the boxes designated “OF” for overflow) monitor input levels beyond a specified amount and limit their respective outputs thereof to a predetermined maximum specified level. 
     In the illustrative equations below X[n] represents the present value of a signal X, and X[n−1] represents the previous value of signal X, the previous sample X[n−1] being generated with a delay element, e.g. a flip flop or D latch, or other memory device that is updatable via a PID clock signal. 
     Proportional process module  36  comprises multiplier element  37  and performs a scaling function, generating an output signal adc_kp_out[n] representing the input signal tc_adc[n] proportionally scaled by signal tc_kp as defined in Equation 1 below:
 
 adc _ kp _out[ n]=tc _ kp*tc _ adc[n]   (Equation 1)
 
     Integral process module  42  comprises multiplier element  44 , summing element  45 , clipping element  46  and delay element  47  in the signal path configuration, as illustrated in  FIG. 2 . Integral process module  42  performs an integrating function and a generates an output signal adc_ki_out[n] representing the product of the input signal tc_adc[n] and scaling signal tc_ki further summed with a prior stored output signal of the integral process as defined in Equation 2 below:
 
 adc _ ki _out[ n]=tc _ ki*tc _ adc[n]+adc _ ki _out[ n− 1]  (Equation 2)
 
     Derivative process module  48  comprises multiplier element  49 , delay element  50  and summing element  52  in the signal path configuration, as illustrated in  FIG. 2 . Derivative process module  48  performs a derivation function and generates an output signal adc_kd_out[n] representing the difference between the current and previous input signal vales tc_adc[n] and tc_adc[n−1] further scaled by signal tc_kd, as defined in Equation 3 below:
 
 adc _ kd _out[ n]=tc _ kd *( tc _ adc[n]|tc _ adc[n− 1])  (Equation 3)
 
     The output values of process module  36 , integral process module  42 , and derivative process module  48  are combined by summing elements  38 , as defined in Equation 4 below:
 
 pid _out1[ n]=adc _ kp _out[ n]+adc _ ki _out[ n]+adc _ kd _out[ n]   (Equation 4)
 
     The pid_out 1 [ n ] signal may then be clipped, as necessary, by a clipping element  39 , prior to being summed with the tc_init signal. The tc_init signal is the initial set point for the entire quadrature error cancellation feedback loop. The system  5  is calibrated with tc_init in nominal, ambient conditions for zero quadrature. When environmental conditions change e.g. supply, temperature, etc. the output of PID controller  32 , pid_out 2 , becomes non-zero so the tc_out signal changes to the sum of tc_init and pid_out 2 . The tc_out signal represents the output of PID controller  32  and is then supplied to conditioning module  34  and DAC  33 . 
     The digital PID controller  32  described herein offers several important advantages over prior art analog servo loop nulling systems. The PID has programmable clock rate and coefficients that 1) affect the energy level and frequency of undesired tones due to the clock, which appear at the gyroscope rate output; 2) allow for optimization of the systems&#39; transient response to quadrature errors; and 3) allow the quadrature cancellation system to be optimized for variations in gyroscopes, i.e. different quality factors, bandwidths, temperature behavior, etc. 
     Prior art analog servo loops generate in-band analog noise. An advantage to the implemented digital PID controller  32  is that no noise is added to the system  5  by the quadrature error processing circuit  20 , if noise from any of the circuit elements falls below the resolution rate of ADC  29 . The only “noise” that may be added is the undesired previously mentioned tone(s) that can then be minimized via programming of the control loop update rate and PID coefficients, tc_kp, tc_ki, tc_kd. 
     The digital output of PID controller  32 , which represents the quadrature error cancellation signal, is then applied to a conversion module  34  which comprises a digital-to-analog convertor (DAC)  33 , the output of which is provided back to the MEMS gyroscope  10  to calibrate the MEMS device quadrature signal. DAC  33  converts the low voltage digital output signal of PID controller  32  to high voltage control signals needed to tune the MEMS device for zero quadrature error. 
     For additional signal conditioning, a digital offset and/or amplifier element, similar to those described herein, can be added at the output of the PID controller  32  as part of conversion module  34 . In one embodiment, a high voltage source component capable of generating a high dynamic range may implemented as either as part of, or in addition to, DAC  33  and may comprise one or more cascode cells  150  interconnected between a pull up resistance module, which provides an output voltage to an output terminal Vout, and a low voltage domain current mirror (or current sink), an example of such apparatus being disclosed in commonly owned co-pending U.S. patent application Ser. No. 13/798,361, filed Mar. 13, 2103, by Ronald J. Lipka et al., and entitled APPARATUS FOR GENERATING HIGH DYNAMIC RANGE, HIGH VOLTAGE SOURCE USING LOW VOLTAGE TRANSISTORS, the subject matter of which is incorporated herein by this reference for all purposes. 
     The entire feedback loop of system  5  reduces the quadrature signal to zero. Since the quadrature cancellation error signal is digitized, a clock signal can be used to control the update rate and the quadrature cancellation loop can be enabled on an as needed basis if the application uses channel multiplexing or only needs infrequent updates. 
     It will be obvious to those recently skilled in the art that modifications to the apparatus and process disclosed here in may occur, including substitution of various component values or nodes of connection, without parting from the true spirit and scope of the disclosure. For example, the quadrature cancellation loop may comprise component integrated in all or partially on a semiconductor device, such as a configurable ASICS semiconductor device, or may be formed with discrete components or any combination thereof to realize the system disclosed herein.