Journal of Electronic Testing

, Volume 28, Issue 5, pp 599–614

FPGA-based Novel Adaptive Scheme Using PN Sequences for Self-Calibration and Self-Testing of MEMS-based Inertial Sensors

Authors

    • Department of Electrical and Computer EngineeringUniversity of British Columbia
  • Ankit Kansal
    • Department of Electrical EngineeringIndian Institute of Technology
  • Mrigank Sharma
    • Department of Electrical and Computer EngineeringUniversity of British Columbia
  • Edmond Cretu
    • Department of Electrical and Computer EngineeringUniversity of British Columbia
Article

DOI: 10.1007/s10836-012-5336-x

Cite this article as:
Sarraf, E.H., Kansal, A., Sharma, M. et al. J Electron Test (2012) 28: 599. doi:10.1007/s10836-012-5336-x
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Abstract

We propose a novel adaptive technique based on pseudo-random (PN) sequences for self-calibration and self-testing of MEMS-based inertial sensors (accelerometers and gyroscopes). The method relies on using a parameterized behavioral model implemented on FPGA, whose parameters values are adaptively tuned, based on the response to test pseudo-random actuation of the physical structure. Dedicated comb drives actuate the movable mass with binary maximum length pseudo-random sequences of small amplitude, to keep the device within the linear operating regime. The frequency of the stimulus is chosen within the mechanical spectral operating range of the micro-device, such that the induced response leads to the identification of the mechanical transfer function, and to the tuning of the associated digital behavioral model. In case of a micro-gyroscope, experimental results demonstrate the adaptive tracking of the damping coefficient from 5.57 × 10−5Kg/s to 7.12 × 10−5Kg/s and of the stiffness coefficient from 132 N/m to 137.7 N/m. In the case of a MEMS accelerometer, the damping and stiffness coefficients are correctly tracked from 3.4 × 10−3Kg/s and 49.56 N/m to 4.57 × 10−3Kg/s and 51.48 N/m, respectively—the former values are designer-specified target values, while the latter are experimentally measured parameters for fabricated devices operating in a real environment. Hardware resources estimation confirms the small area the proposed algorithm occupies on the targeted FPGA device.

Keywords

PN sequencesImpulse-responseFPGAMEMSAccelerometerGyroscopeSelf-testingSelf-calibration

1 Introduction

In today’s MEMS’s world, highly sensitive MEMS-based microsystems are made possible by incorporating complex fabrication methodologies and high resolution readout electronics, combined with costly packaging assemblies. The challenge for low-cost and increased functionality marks a growing interest in built-in-self-test (BIST) and built-in-self-calibration (BISC) for MEMS-based inertial sensors, where functional aspects can be verified and calibrated at rest and/or in operation.

The advantages of BIST and BISC can be summarized as follows:
  1. i.

    From the manufacturer’s perspective, BIST is an advantageous solution for an overall reduced fabrication cost and increased reliability. Failure-sources in the micro-manufacturing process (such as over-etching, stiction and packaging-induced stress) may contribute to microsystems malfunctioning, or induce changes in the operating characteristics. Functional testing costs less and is significantly faster than calibration. Therefore, the typical approach for large volume manufacturing is to perform functional testing before packaging (to avoid the high cost of packaging for known-bad sensors), followed by a calibration procedure after the microsystem has been packaged.

     
  2. ii.

    From the user perspective, BIST is an essential component in crucial, usually safety applications, which require continuous monitoring, such as airbag deployment. BISC, on the other hand, is important for maintaining high accuracy in demanding applications, such as adaptive path prediction systems, in particular for recalibrating the mechanical performance when the equivalent stiffness and/or damping coefficients are affected by changes in the operating environment (temperature, humidity and pressure). This is also crucial in micro-g measurement applications such as oil explorations, earthquake detection, bridge monitoring and non-invasive surgery.

     

Testing provides only a yes/no answer to decide if the device satisfies basic functional threshold operating requirements. Moreover, according to Deb and Blanton [5], the testing process has limitations which sometimes can cause good micro-devices to be rejected (aspect known as the yield loss), or bad micro-devices to be accepted (aspect known as test escape). The testing scheme needs to provide the right compromise between reliability and overall cost. The BIST can help identify unstable defects [5], provide adequate mechanisms for improving device reliability and ensure its correct performance [3, 11]. Nevertheless, a simple functional test is not sufficient in many cases (e.g. for high sensitivity inertial sensors), where the specifications are so tight that a periodic re-calibration of the microsystem is necessary. Variations in the mechanical sensor characteristics can be compensated through the correlated tuning of the subsequent electronic stages. Difficulties arise when, once calibrated at the manufacturing site, the devices change their behavior at the user side, while in operation. This happens due to large variations in the operating conditions, such as temperature and humidity. This aspect generates the need for low-cost calibration schemes applicable at the user side, to guarantee the continuous reliability and accuracy of the measured data. Failure in reliability and accuracy could lead to catastrophic consequences in the case of the aforementioned applications.

Most of the micro-accelerometers and micro-gyroscopes available on the market today, such as Analog Devices© [2] and ST Microelectronics© [15], have a self-testing feature: an applied test voltage generates an electrostatic force (testing stimulus) that causes a predetermined movement of the movable structure. This force should activate the device to its full working range, i.e. maximum output, and failure to demonstrate a full output operating range within a prescribed tolerance level reflects that the device is faulty, as stated by Xiong et al. [16].

Pseudorandom (PN), or maximum length sequences, have been popular for testing and calibration of electronic systems [10], and have been recently applied as well to the test and calibration of MEMS-based microstructures. They offer several advantages, both theoretical and in terms of hardware implementation, as they are similar to white noise-like input stimuli [10], and avoid generation of complex and device-dependent test signals. PN sequences for MEMS testing have been employed for instance for on-chip measurement of both the thermal and mechanical dynamics of piezoresistive cantilever beams [6, 13]; one of the authors has further extended the methodology to accelerometers, and used it for instance to evaluate the static and dynamic response of ADXL103 [2] devices. Dumas et al. [7] used this methodology to ensure that the output test signal from the accelerometer was not degraded by noise—they were able to achieve 20 dB of noise rejection and 2.55 s test time.

The present paper1 reports the use of PN sequences as actuating voltages that induce small perturbation in the dynamics of the MEMS structures, in order to adaptively extract their time-varying damping and stiffness coefficients. The whole calibration algorithm is efficiently implemented on FPGA. The method is an extension along the testing concepts presented in [6]. Xiong et al. [16] mentioned that very soon MEMS devices will be fabricated on the same chip with digital analog, memory, and FPGA circuit technologies. An efficient, reliable and low cost system-level solution can be then obtained by integrating the sensor with BISC and BIST schemes targeted on cheap, reconfigurable and efficient technologies such as FPGAs.

Our contributions are:
  1. i.

    Implementation of a model-based adaptive identification method based on PN sequences for self-testing and self-calibration of MEMS-based accelerometers and gyroscopes. A reference behavioral model of the device (DUT-R), implemented on FPGA, is initialized with factory default values, and the difference between its response and the real device response to PN sequences excitation is used to adjust the stiffness and the damping coefficients. This self-calibration mechanism ensures maintaining high measurement accuracy over larger environment-induced changes in sensor characteristics, for the parameter values of the behavioral model can be used to adjust the calibration of the subsequent electronic subsystem.

     
  2. ii.

    Beside the reference model, the entire self-calibration module is implemented in reconfigurable hardware (FPGA), with a low occupancy area, suitable for limited computational resources.

     

Our longer-term goal is to apply the technique to a three degrees-of-freedom (DOFs) inertial measurement unit, for the independent self-calibration of the two accelerometers and the gyroscope MEMS devices.

This paper is organized as follows. Section 2 describes the functioning principle of the proposed scheme and its FPGA implementation. The experimental setup and the results for the micro-accelerometer and micro-gyroscope, along with the necessary hardware resources, are presented and thoroughly discussed in Section 3. Future work is outlined in Section 4, with some final conclusions elaborated in Section 5.

2 Proposed Adaptive Technique and Implementation

As time-varying operating environment conditions (temperature, pressure, humidity, etc.) modify the sensitivity of the MEMS sensors, maintaining a desired high-accuracy in the measurement process requires a periodic re-calibration of the system sensitivity. An adaptive, on-board, real-time self-calibration scheme is able to track and adjust the changes in sensor characteristics. We propose in the present work an adaptive error-correction scheme based on a digital behavioral reference model, and demonstrate its implementation on FPGA.

2.1 System Architecture and FPGA-based Hardware Blocks

The system level block diagram is illustrated in Fig. 1 [8]. It comprises the micro-device (fabricated in MEMSCAP© SOI-MUMPs technology), analog readout electronics (either as a separate CMOS module or with discrete components), an analog to digital converter (ADC) block to interface to FPGA, a digital to analog converter (DAC) for the configuration and actuation of the MEMS device, and the FPGA block (for the self-calibration controller). The FPGA implementation of the proposed technique using Xilinx© blocks is shown in Fig. 2. To minimize the physical circuit area, the PN sequence generator was targeted on FPGA as part of the controller block (see Fig. 2(a)) and its output is fed to both DUT-R model (see Fig. 2(e)) and to the actual device-under-test (DUT), via a simple one-bit digital-to-analog converter.
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Fig. 1

System-level architecture

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Fig. 2

FPGA implementation of controller blocks. a PN sequence generator, b damping controller, c stiffness controller, d correlation cells, e DUT-R

A 1,023−bit maximum length sequence (MLS) is implemented by using 10 interconnected shift registers. The configuration shown corresponds to a generator polynomial [10, 7, 0]. The input–output cross correlation function for a linear time invariant (LTI) system can be used to estimate its impulse response from the response to an MLS input [10]. Assuming h[n] as the impulse response of the LTI system, its output to an input signal x[n] is given by the convolution:
$$ y\left[ n \right]=x*h\left[ n \right]=\sum\limits_{k=0}^{\infty } {x\left[ {n-k} \right]\cdot h\left[ k \right]} $$
(1)
In the case of using a maximum length sequence as input x[n], we can recover the unknown impulse response from the cross correlation between the test input (x[n]) and measured output (y[n]) signals, \( {\varPhi_{xy }}\left[ n \right]=x\odot y\left[ n \right] \), for it will be proportional to the impulse response h[n]of the system:
$$ \begin{array}{*{20}c} {x_{MLS }}\odot y\left[ n \right]={x_{MLS }}\odot \left( {{x_{MLS }}*h} \right)\left[ n \right]=\left( {{x_{MLS }}\odot {x_{MLS }}} \right)*h\left[ n \right] \\ =\left( {\delta \left[ n \right]} \right)*h\left[ n \right]=h\left[ n \right] \\\end{array} $$
(2)
This is due to both the associativity between cross-correlation and convolution operators, and to the fact that the autocorrelation function Φxx[n] of a MLS (PN) is, except for a small DC level (dependent on the length of MLS), a good delta function approximation:
$$ {\varPhi_{{{x_{MLS }}{x_{MLS }}}}}\cong \delta \left[ n \right] $$
(3)
As can be seen from Eq. 2, if the input is an MLS sequence, the input–output cross correlation is an estimate of system impulse response. For the discrete time case, the cross correlation operator between two N-length periodic sequences is defined as:
$$ {\varPhi_{xy }}\left[ n \right]=\sum\limits_{k=0}^{N-1 } {x\left[ k \right]\cdot y\left[ {k+n} \right]} $$
(4)
In the corresponding FPGA implementation, for a binary input sequence the multipliers can be replaced with a multiplexer, as shown in Fig. 2. In the case of a periodic maximum length amplitude (MLA) sequence as input, for every distinct delay k we get the associated signature h[k], representing the value of the impulse response at the sample time t=kTs. With corresponding added delays in the hardware, the coefficients can be identified from the post-processing of the output signal. For an MLS of length N = 2 M−1, we have an N-length signature pattern (k = 0, 1,..,N−1). When used for identifying the parameter values of a reference model, only the first coefficients will carry most of the relevant information in practical cases.. The width of the significant portion is equal to Fs/BW, where Fs = 1/Ts is the sampling rate (the clock rate of MLS here) and BW is the 3 dB bandwidth of LTI system [10]. This relation reflects two important points:
  1. i.

    For a high value of FS or a low value of BW, the information is spread over a large number of redundant coefficients h[k] in the signature.

     
  2. ii.

    To standardize the proposed approach, we choose the MLS chip rate as a function of 3-dB frequency of the LTI system.

     

Using Nyquist criterion, the chip rate for MLS was chosen to be twice the 3-dB frequency. From the simulation results it was observed that is usually sufficient to use only the first 10−15 coefficients in the signature. In fact, it has been shown [10] that for an LTI system with p poles and q zeros, only {p+q+1} arbitrary impulse response terms are sufficient to characterize the system.

Inertial sensors (e.g. accelerometers) and resonators can be modeled as second order linear systems, or as two coupled second order systems in the case of vibratory gyroscopes. The linear constitutive equation of one degree of freedom resonant mode (valid for small displacement amplitudes) is given by [8]:
$$ \begin{array}{*{20}c} {m\frac{{d^2 x}}{{d{t^2}}}+\gamma \frac{dx }{dt }+k\cdot x=f(t)\Rightarrow X(s)=\frac{F(s) }{{s^2 m+s\gamma +k}}=\frac{F(s)/m }{{s^2 +s{\omega_0}/Q+\omega_0^2}}} \\ {\Rightarrow \frac{{{V_{out }}(s)}}{{{A_{ext }}(s)}}=\frac{C}{{s^2 +s{\omega_0}/Q+\omega_0^2}}} \\ \end{array} $$
(4)
Here x(t) is the displacement of the proof mass (with X(s) as its Laplace Transform pair), γ is the damping coefficient, k is the spring constant, ω0 is the natural resonant frequency of the device, Q is the quality factor, C is the readout gain constant and f(t) (or F(s) in its Laplace form) is the excitation force. Based on the values of the parameter γ the system can be under-damped, critically-damped or over-damped. From (5) we can see that under constant actuation (s = 0) the output depends on the effective stiffness constant of the inertial sensor and on the applied dc force (static regime), while the effect of damping and inertia become noticeable only in dynamic mode (the structure is operated at atmospheric pressure), at higher frequencies [14]. The linear model of the reference resonator (DUT-R) was implemented digitally in FPGA using a discretized state-space representation (Fig. 2e), with velocity and displacement as internal state variables. In order to adjust the values of the parameterized reference model knowing the measured impulse response of the device, it is important to know how do the variations in m, k and γ reflect in changes of the h[n] samples. While the proof mass will remain constant in operation, k and γ may vary in time, reflecting environment changes. The sensitivity analysis, illustrated in Fig. 3, shows that the reconstructed digital impulse response h[n]=hn depends indeed on both γ and k, with different h[n] coefficients having distinct sensitivities to their independent variation. We have analyzed the sensitivity of the impulse response terms h1, h2, h3… to induced variations in the damping and stiffness coefficients, \( \frac{{\partial {h_i}}}{{\partial \gamma }},\frac{{\partial {h_i}}}{{\partial k}} \), concluding that only the first samples hi are significant.
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Fig. 3

Impulse response sensitivity analysis to variations in k and γ

While both the equivalent elastic and damping coefficients can vary in time due to changes in operating conditions (e.g. operating temperature), the mass is taken as an invariant parameter for a given device. The self-calibration procedure has two distinct phases: the dc response is firstly used to determine the equivalent spring constant, while the dynamic response to the MLS excitation is used for recalibrating the value of the damping coefficient in the digital reference model.

The model-based recalibration technique uses a replica of the behavioral model (DUT-R) of the device. It is stored on FPGA and coupled to an adaptive algorithm that compares its response with the one of the device under test (DUT). The differences are used to recalibrate the DUT-R parameters (initially set to the values estimated after the fabrication) to current operating values.

The efficiency of the rapid prototyping methodology for FPGA implementation facilitated the conversion of the lumped element behavioral blocks (such as integrators) into hardware blocks. Two integrators, converting acceleration to velocity and velocity to position, have been implemented using Xilinx© equivalent blocks. The a priori values used for the damping and stiffness parameters of the gyroscope are 5.57 × 10−5Kg/s and 132 N/m, respectively, and 3.4 × 10−3Kg/s and 49.56 N/m for the accelerometer. They have been determined from experimental measurements on different sample devices using Polytec MSA-500 video-stroboscopy equipment and associated planar motion analysis software, as illustrated in Fig. 4.
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Fig. 4

Experimental characterization of the micro-devices, (a) Frequency response of the micro-accelerometer with varying ac voltages (area varying combs), (b) frequency response of the micro-gyroscope

The self-calibration block is the processing engine that adaptively computes and stores the values of the stiffness and damping coefficients. It contains the correlation cells for damping control, along with the damping and stiffness controllers. The output of the PN sequence generator acts as test input for both the DUT and DUT-R (FPGA model). The proposed technique allows the use of the same binary generator, with proper initialization, as stimulus towards extracting both stationary and dynamic response (as quantified by the error in most sensitive impulse response coefficients) of the micro-device.

Compared to the functional testing methods for mixed signal systems based on PN, the present work takes the concept further, and uses both the steady-state and dynamic responses of the device in a closed loop fashion to characterize and hence calibrate the micro-device in real time. The algorithm of operation is shown in Fig. 5 and can be succinctly described by the following steps:
  1. i.

    Starting with the equivalent spring constant, a dc force is applied to the DUT. This is achieved by initializing the state of the test generator with all ones, stored as initial values in a linear feedback shift register (LFSR). The approximate settling time of DUT serves as a guide for choosing the order of the test PN sequence.

     
  2. ii.

    For the first 10 clock cycles (10 being the order of PN sequence used), the difference in static response between the DUT and the DUT-R is computed and fed to the stiffness controller. The binary actuation test signal will remain “1” during this time interval.

     
  3. iii.

    Using the steady state error (difference between the experimentally measured response and the output of the reference model), the stiffness parameter k of the DUT-R is adjusted by the stiffness controller for a maximum matching between the responses of DUT and DUT-R.

     
  4. iv.

    The counter starts counting from the initial state, up to the total length of PN sequence.

     
  5. v.

    For the damping coefficient adjustment, the complete responses y[k], yR[k] of the DUT and DUT-R respectively, corresponding to the entire length of PN sequence x[k], are fed into the correlation cells. The algorithm computes through digital cross-correlation the significant coefficients of {hDUT[n], hDUTR[n]}, in order to perform the adaptive correction on γDUT-R.

     
  6. vi.

    The damping coefficient of the DUT-R, γDUTR, is being adjusted by the damping controller such that the error between the two signatures is minimized.

     
  7. vii.

    The iteration cycle for the stiffness and damping coefficients (κDUTR and γDUTR) of the reference model repeats until the resulting error falls below a desired threshold, imposed by the accuracy constraints. At this point, the self-calibration is achieved, and the reference model parameters reflect the values of the real device.

     
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Fig. 5

Algorithmic steps for the proposed technique

The stiffness controller (see Fig. 2(c)) (responsible for recalibrating the spring constant parameter) is active only during the first few clocks of every test sequence cycle, when the actuation signal is constant. Because for the physical DUT the test sequence maps to binary actuation voltages (one-bit DAC), while it can directly digitally interface with DUT-R model, it is necessary to use a scaling constant A for the actuation term on DUT-R. Its value is chosen to correspond to the equivalent displacement-to-voltage gain of the MEMS device response. A gain of A = 7,000 is used in our case. The final output of the controller is the sum of the a priori stiffness coefficient and the output of an accumulator block.

The correlation block, illustrated in Fig. 2(d), contains the cells for computing the cross-correlation expression∑x[n]⋅y[nk] between the binary PN input sequence x[n] and the measured (DUT) or digitally computed (DUT-R) output y[n]. The PN sequence is associated in the mechanical domain with two actuation force levels, {±F0} (left or right actuation with forces of constant magnitude). The binary nature of the sequence means that the convolution sum reduces to a sequence of multiplications of y[n] (or delayed versions) by {+1} or {−1} (a multiplexer selects y[n] or −y[n] based on the input x[n]), followed by the accumulation of the partial result. An adaptive scheme implements the error correction: the operating value of the damping coefficient to be stored in the behavioral model is iteratively adjusted, proportional with the accumulated difference (cumulated error) between the reconstructed impulse responses {hDUT[n], hDUTR[n]} of the DUT and DUT-R for a full PN cycle (every 1,023 clock cycles).

3 Experimental Results and FPGA Hardware Resources

The proposed adaptive technique for self-calibration and self-testing has been experimentally verified for tuning the stiffness and damping coefficients of fabricated accelerometers, and for the sensing mode of a MEMS gyroscope. This is a complete system and comprises MEMS-based components, FPGA board, readout electronics, DAQ; each individual component has been verified separately, in order to decrease the probability of erroneous signals.

3.1 Calibration of the Reference Model DUT-R

The proposed scheme relies on the accuracy of the reference model, responsible for providing the appropriate time-varying damping and stiffness coefficients. To address the range of the quantization error, a fixed/floating point study was performed on the DUT-R model, to identify a suitable word length representation in binary format. For this, considering a single DOF system, the dynamic range has been fixed according to the targeted sensitivity. It should be noted that the chosen dynamic range also affects the limitation of the BIST/BISC, as the reference model should be tuned or programmed to match the DUT. Since we target high sensitivity and low-g measurements, the dynamic range has been fixed to [−1 g, 1 g]. In this section the model parameters (mass, stiffness and damping coefficients) are discussed for a MEMS accelerometer. The same study was separately performed for the MEMS gyroscope.

The digital implementations are made using a rapid prototyping methodology, where Matlab/Simulink© models are mapped to Xilinx© FPGA configurations. The DUT-R is firstly evaluated using a Simulink floating point representation, and subsequently the regular Simulink blocks are converted, using Xilinx System Generator module, to fixed point Xilinx blocks. The amplitude of the signal is monitored at the input and output of each arithmetic block for word-length verification. The same input acceleration signals are applied to both, floating-point (Simulink regular blocks) and a fixed-point (Xilinx System Generator blocks, as shown in Fig. 2 (e)) models. In the first test case we apply a step-like input (Fig. 6(a) at 0.05 s). A second case considered a sine-like input, as in Fig. 6(b), with a frequency of 90 Hz. The acceleration magnitude is within the interval of [−1 g,+1 g] in both test cases.
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Fig. 6

Calibration of the DUT-R (accelerometer model) with Floating-point vs Fixed-point simulations

As the resolution of the reference model is of utmost importance, the chosen word-length for the fixed point FPGA computation varies from one arithmetic block to another, depending on the magnitude of the input signal. The word-length is represented by two numbers, one for the number of bits representing the real part (left), and the other (right) for the number of bits representing the fraction part of the signal. For low magnitude input signals the used word-length is (32, 30), with a large amount of bits is reserved for the fractional portion of the signal, while for large magnitudes a word-length of (24, 6) is used, where the large portion of bits is reserved for the real part of the signal. Additionally, the resolution of the DUT-R strongly depends on the resolution of the two digital integrators (one for velocity and the other for position). Figure 6 shows that the proposed hardware fixed-point implementations, highly concurrent, match well the generic floating point models.

3.2 Experimental Set Up

Figure 7 illustrates the experimental setup. A capacitive readout circuit interfaces the microsensor with the FPGA-based controller. It consists of an oscillator generating a carrier signal at 100 kHz, well outside the mechanical bandwidth, a transimpedance amplifier and a demodulator. The output voltage is proportional with the differential change in the capacitance and thus implicitly with the relative displacement of the proof mass, for small displacement values. An analog-to-digital converter (ADC) maps the voltage into a digital signal supplied as input to the FPGA module, where it is used for the tuning the digital reference model. The digital binary sequence generated in FPGA is converted into the two electrostatic actuation forces, {±F0}, applied on a dedicated pair of actuation comb capacitors. This is accomplished using a multifunctional DAQ from Measurement Computing© [9] (USB-1208 Series), with 1.2ksample/s to 1Msample/s sampling rate for DAC and ADC operations.
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Fig. 7

Experimental set up

The DUT is excited with maximal length sequences of small amplitude, with a frequency within the mechanical bandwidth of the movable structure. The input–output cross correlation gives an estimate of the impulse response (whose digitized values h[n] will be referred to as ‘coefficients’) of the micro-device. The analysis and simulation results have indicated that knowing the static response to a constant actuation voltage and the few first h[n] coefficients is sufficient to tune the reference model parameters to match the DUT, and to subsequently perform the calibration of the microsystem. Tests were carried out by imposing a dc voltage of 15 V and varying the ac signal amplitude for a fixed frequency of 8.5 kHz for the gyroscope, and a dc voltage of 20 V and varying the ac signal amplitude for a fixed frequency of 4.8 kHz for the accelerometer.

3.3 Self-calibration of Micro- accelerometer and Gyroscope

Both micro-devices have been designed and simulated using COVENTORWARE© software tools, fabricated in MEMSCAP© SOIMUMPs® technology (25 μm thickness for the structural layer), packaged in 40-DIP ceramic packages and experimentally characterized using a POLYTEC MSA-500 (Micro-System Analyzer) equipment. The planar motion analysis module of the Polytec MSA-500 was used to detect (0x) and (0y) motions down to 2 nm resolution, based on video-stroboscopy techniques.

Actuation of the proof mass (mimicking an external acceleration input) is achieved by applying ac voltages between the moving and the fixed fingers of the actuators and a bias dc voltage to the proof mass of the sensor, in order to linearize the effect of the electrostatic forces. The experimental tests on the MEMS-FPGA closed loop self-calibration were carried out by actuating the MEMS-based accelerometer with the PN sequences. The essential parameters, such as resonant frequency, damping coefficient and quality factor, have been estimated from the experimental measurements performed on the MEMS devices using Polytec MSA-500 micro-motion analyzer. The measured frequency responses for the accelerometer and for the sensing mode of the micro-gyroscope are shown in Fig. 4.

The value of the damping coefficient γ is obtained by half power method. That is, we use \( \gamma =\varDelta {f_0}/2{f_0}=1/2Q \) where ∆f0 is 3 dB-bandwidth in Hz and f0 is the resonant frequency. The stiffness coefficient is given by k=m(2πf0)2, where the mass m of the structure is a given data, estimated from the layout and material properties.

The resonant frequency and quality factor for each resonant mode are \( {\omega_0}=\sqrt{{{k \left/ {m} \right.}}} \) and \( Q=\sqrt{{{km \left/ {\gamma } \right.}}} \) respectively, where the mass m is assumed invariant to external variations, while k and γ can be time-varying parameters.

The principle of operation of the single axis one degree of freedom accelerometer can be derived from the resonator model in the previous subsection. The performance characteristics and the micro-structure geometry of the micro-accelerometer are illustrated in Table 1 and Fig. 8 respectively. The micro-structure exhibits different set of comb drives (longitudinal and lateral) for different actuating, sensing and feedback purposes. The basic structure of the micro-gyroscope is a proof mass suspended by crab-leg flexures with two planar degrees of freedom (DOFs). The micro-device features three sets of actuating and sensing comb drives.
Table 1

Performance characteristics of the micro-accelerometer

Parameter

Value

Proof mass [nKg]

50.43

Stiffness coefficient [N/m]

49.5627

Damping coefficient [Kg/s]

0.0034

Nominal capacitance [pF]

1.3

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Fig. 8

SEM shots of the micro-devices, (a) micro-accelerometer and (b) micro-gyroscope

Vibratory gyroscopes have two coupled modes, a driving and a sensing one. The driving mode is electrostatically excited, and in the absence of an external angular rate, the mechanical energy of its vibratory motion remains confined into the driven mode. A non-zero external angular rate will generate a non-inertial Coriolis force proportional to it, which couples the two resonant modes, and converts a part of the energy of the driving mode into an oscillation of the sensing mode. The external angular rate value is therefore determined by measuring the amplitude of the sensing mode vibration. The typical MEMS gyroscope structures use linear motions for the driven (0x) and the sensing (0y) coupled resonant modes [1].

In our case, the fabricated gyroscope has a driven mode resonant frequency of 8.5 KHz, with a forced amplitude of the (0x)-driven displacement of 5 μm. The quality factors for the drive and sense mode are Qx≈129and Qy≈44, respectively. Table 2 illustrates the physical characteristics of the fabricated gyroscope [8].
Table 2

Performance characteristics of the micro-gyroscope

Parameter

Actuation

Sensing

Stiffness coefficient [N/m]

122.28

132.23

Damping coefficient [Kg/s]

18.9 μ

57 μ

Proof mass [nKg]

48.67

48.67

Quality factor

128.8

43.63

Nominal capacitance [pF]

0.36

0.6

Assuming that we have matched modes (identical resonance frequencies for drive and sense modes), the Coriolis-induced displacement in the sense mode is given by \( y=2{Q_y}{\varOmega_z}x/{\omega_x} \), resulting in (0y) (sense) displacement amplitudes of 143.7 nm for 1 deg/s external angular rate. For a 0.05 rad/sec mismatch (see Fig. 9) between the driven and sensing modes, the sensitivity will drop by 50 % (displacement will drop from 4.623 μm to 1.791 μm). The tuning of the two resonant modes (driving, sensing) is crucial in high performance gyroscopes, where any mismatch could lead to a fast decrease in the sensitivity to external angular rates [14].
https://static-content.springer.com/image/art%3A10.1007%2Fs10836-012-5336-x/MediaObjects/10836_2012_5336_Fig9_HTML.gif
Fig. 9

Transfer function of the sensing mode displacement vs. primary actuation voltage, for a fixed reference angular rate. The sensitivity decreases from 4.6 μm to 2.832 μm for 100 Hz frequency mismatch between the two gyroscope resonant modes

The dynamical behavior of the gyroscope is modeled by a set of two coupled differential equations, for the driven and sensing mode, respectively:
$$ m\frac{{d^2 x}}{{d{t^2}}}+{\gamma_x}\frac{dx }{dt }+{k_x}x={F_{actuation }}+2m{\varOmega_Z}\frac{dy }{dt } $$
(5)
$$ m\frac{{d^2 y}}{{d{t^2}}}+{\gamma_y}\frac{dy }{dt }+{k_y}y=-2m{\varOmega_Z}\frac{dx }{dt } $$
(6)

Factuation is the electrostatic force that puts the gyroscope in motion in the 0x direction, directly proportional to the square of applied voltage. The Coriolis force \( -2m{\varOmega_Z}{dy \left/ {dt } \right.} \) induces the motion in the (0y) direction; a reciprocal Coriolis force, \( 2m{\varOmega_Z}{dy \left/ {dt } \right.} \), will be induced by the (0y) motion of the sense mode, but this is negligible relative to the electrostatic actuation. The two resonant modes may have different equivalent spring constants and damping coefficients.

We have considered in the present paper only the monitoring and adaptive recalibration of the sensing mode of the micro-gyroscope. PN sequences (converted into binary actuation voltages) are applied on the sensing mode (gap-varying) comb drives, in order to trigger a response. If one considers only their action, in the absence of an external angular rate, then (6) becomes:
$$ m\frac{{d^2 y}}{{d{t^2}}}+{\gamma_y}\frac{dy }{dt }+{K_y}y=\frac{1}{2}\frac{C_0 }{{{{{\left( {d-y} \right)}}^2}}}{V^2} $$
(7)

Here C0 is the initial capacitance, V is the actuation voltage, d is the initial distance between the gap-varying plates, and y is the displacement due to the applied electrostatic force. For convenience, the damping and stiffness coefficients are renamed from γy to γ and ky to k, respectively, since they are the only one to be determined in this case.

The experimental tests on the MEMS-FPGA closed loop adaptive reference model tuning were carried out by actuating the MEMS-based device with the generator sequence, as described earlier. It should be noted that the capacitive readout electronics introduces its own noise, adding a strong limitation in terms of displacement measurement resolution.

The a priori values used for the damping and stiffness parameters for the accelerometer (in the virtual model implemented on FPGA) are 3.4 × 10−3Kg/s and 49.56 N/m respectively. The same parameters for the gyroscope are 5.57 × 10−5Kg/s and 132 N/m, respectively, as estimated from measurements performed on different devices (from the same fabrication batch).

The temporal traces for the damping and stiffness coefficients tuning are shown in Fig. 10 for the accelerometer and in Fig. 11 for the gyroscope. The goal of the proposed methodology is to track, in an adaptive manner, changes in stiffness and damping coefficients due to external factors. The output of the PN sequence generator actuates both the DUT (MEMS accelerometer/gyroscope) and the DUT-R (FPGA-based model of accelerometer/gyroscope). As a result, the difference in the impulse responses of the DUT and DUT-R is calculated and used for adaptive tuning of the DUT-R parameters, as previously described. For the first phase mode (tuning of the stiffness coefficient), Figs. 10(a) and 11(a) show the decreasing error as the stiffness coefficients for the accelerometer and the gyroscope are recalibrated. As a result, the adjusted stiffness coefficient is the sum of the a priori stiffness coefficient (accelerometer/gyroscope) and the output of the accumulator block. This tuning process of the adaptive scheme led to the new values of stiffness coefficients, 51.48 N/m and 137.7 N/m for the accelerometer and gyroscope respectively. The dynamic response to the MLS excitation is used for recalibrating the value of the damping coefficient in the digital reference model. The adaptation of the damping coefficient, as illustrated in Figs. 10(b) and (b), requires more clock cycles than the stiffness coefficient tuning. This adaptive tuning process led to the new values of damping coefficients, 4.57 × 10−3Kg/s and 7.12 × 10−5Kg/s for the accelerometer and gyroscope respectively.
https://static-content.springer.com/image/art%3A10.1007%2Fs10836-012-5336-x/MediaObjects/10836_2012_5336_Fig10_HTML.gif
Fig. 10

Experimental tuning results for the stiffness (a) and damping (b) coefficients of the micro-accelerometer

https://static-content.springer.com/image/art%3A10.1007%2Fs10836-012-5336-x/MediaObjects/10836_2012_5336_Fig11_HTML.gif
Fig. 11

Experimental tuning results for the stiffness (a) and damping (b) coefficients of the micro-gyroscope

3.4 Hardware Resources and Trade-Off

The proposed adaptive scheme, including the PN sequence generator, is implemented on FPGA, leading to a reliable, cheap and flexible system [4, 12]. Replacing or cutting down the cumbersome, less flexible analog implementation with more flexible technologies such as FPGAs follows the route envisaged by Xiong et al [16] for the future of integrated circuits. The hardware resources estimate include numbers of slices, lookup tables (LUT), flip-flops (FF) and embedded multipliers. These estimates make it easy to determine how design choices affect hardware requirements. Tables 3 and 4 summarize the hardware resources required by the digital tuning algorithm. Mainly due to the large amount of high throughput multipliers (for increased accuracy), the high-resolution behavioral model used as reference for both the accelerometer and gyroscope cases occupies 75 % of the used FPGA implementation. The BISC for gyroscope occupies a slightly larger area than the one for accelerometer, due to the different requirements between the two inertial micro-devices, in terms of dynamic range and physical characteristics, such as proof mass, damping and stiffness coefficients.
Table 3

Hardware resources of the proposed scheme (for the MEMS accelerometer)

 

Slices

FF

LUT

MULT

Hardware Resources (for MEMS accelerometer)

 Block

  PN Sequence generator

12

11

1

0

  Stiffness Controller

248

58

484

0

  Damping Controller

207

99

365

0

  Correlation Cells

424

127

811

0

  DUT-R

2618

1495

4865

21

  Overall System

3547

1857

6461

21

 

BISC Percentage of occupation for accelerometers (%)

 Device

  XC2VP2

N/A

65.94

N/A

N/A

  XC2VP7

71.97

18.84

65.55

47.72

  XC2VP30

25.89

6.77

23.58

15.44

  XC2VP70

10.71

2.80

9.76

6.40

  XC2VP100

8.04

2.10

7.32

4.72

Table 4

Hardware resources of the proposed scheme (for the MEMS gyroscope)

 

Slices

FF

LUT

MULT

Hardware Resources (for MEMS gyroscope)

 Block

  PN Sequence generator

12

11

1

0

  Stiffness Controller

252

58

492

0

  Damping Controller

175

98

293

0

  Correlation Cells

1694

2536

2827

0

  DUT-R

2533

934

4646

35

  Overall System

4686

3646

8275

35

 

BISC Percentage of occupation for gyroscope (%)

 Device

  XC2VP2

N/A

N/A

N/A

N/A

  XC2VP7

95.08

36.9

83.95

79.54

  XC2VP30

34.21

13.31

30.2

25.73

  XC2VP70

14.16

5.5

12.5

10.67

  XC2VP100

10.62

4.13

9.38

7.88

The proposed scheme is massively parallel and is suitable for fixed-point FPGA implementation. With the exception of the DUT-R hardware implementation, an average of (16,12) word-length has been used for the stiffness and damping controllers, and for the correlation cells. The implementation shown in Fig. 2 has been verified through end-to-end VHDL simulations along with actual hardware implementation, to determine the device resources and speed. All blocks have the same throughput, with quite similar latencies. The circuit has a maximum speed of 14.217 MHz for the BIST/BISC (accelerometer) and 17.727 MHz for the BIST/BISC (gyroscope). The achieved clock frequency in both implementations (accelerometer/gyroscope) is acceptable. Pipelining techniques can be used to increase the speed, if necessary.

The hardware resources, illustrated in Tables 3 and 4, show the percentage of occupation of the proposed adaptive methodology for BISC, for several FPGAs from the Xilinx family. The one used for the experiments is a Virtex-II pro XC2VP100, which contains relatively small hardware resources compared to the state-of-the-art FPGA on the market today. Nevertheless, the proposed methodology has low hardware resources demands even in this case. Therefore, with the tremendous FPGA development following Moore’s law, area occupancy trade-off has become less critical than other trade-offs, such as speed and resolution. In our work, resolution was not compromised in the reference model, due to its importance in the proposed methodology. We can conclude, based on these values, that the methodology is suitable for the self-calibration of a more complex 3-DOF inertial measurement unit (IMU), using a cheap and small FPGA like XC2VP30. In this case the PN sequence generator can be a shared resource, and three reference models, for the 0x and 0y accelerometers and the 0z angular rate sensors, respectively, will be implemented to operate concurrently on the same FPGA chip.

4 Future Work

BIST guarantees the safety and functionality of operation of the device via continuous testing. Adaptation of stiffness and damping coefficients due to a change in temperature, pressure or humidity change was proven in this work through an adaptive, off-line, calibration methodology. An iterative process, as shown in Fig. 12, is performed regularly, in order to adapt the reference model parameters and the related calibration values – a maximum accuracy is maintained to the user side in such a manner.
https://static-content.springer.com/image/art%3A10.1007%2Fs10836-012-5336-x/MediaObjects/10836_2012_5336_Fig12_HTML.gif
Fig. 12

System level architecture with tuning block

A future challenge is to be able to perform online self-calibration and self-tuning, when the test actuation with PN sequences superimposes over the natural mechanical actuation given by an external acceleration or angular rate.

5 Conclusion

We have developed a novel, adaptive, self-testing and self-calibration scheme for MEMS inertial sensors and resonators. The algorithm relies on the use of a digital reference behavioral model implemented on FPGA, whose parameters (stiffness and damping coefficients) are tuned based on the accumulated error between the reconstructed impulse responses of the device under test and the reference model. The impulse responses are reconstructed using low amplitude PN sequences as actuation signals (so that the sensor remains within its linear operating regime), and subsequently computing the input–output cross-correlation. A low-cost capacitive readout circuit, together with ADC and DAC blocks, interface between the MEMS device under test and the digital module. The self-calibration procedure implemented on FPGA requires low hardware resources, and it is easy extensible to MEMS systems for multiple degrees of freedom.

Footnotes
1

Portion of this Manuscript has appeared as a conference abstract (Kansal et al 2011[8])

 

Acknowledgment

The authors would like to thank CMC Microsystems, MITACS, Auto21 Network of Centres of Excellence, and the Natural Sciences and Engineering Research Council of Canada (NSERC) for making this research possible.

Copyright information

© Springer Science+Business Media New York 2012