A 3-Axis Gyroscope with Continuous Self-Test
February 25, 2015, ISSCC, San Francisco—Ganesh Balachandran from Bosch presented the design of a 3-axis gyroscope for electronic stability control (ESC) with continuous self test. Active and passive control systems are the driver for more functionality and capability within a car and these functions create a need for more sensors.
In a car, a gyroscope used to detect yaw for stability control, pitch for the active suspension, and roll for rollover detection and seatbelt pre-tensioning. All of the functions require inputs from many sensors and electronic stability control is one of the most complex. The control system has to calculate the difference between intended and actual trajectories and selectively applies the brakes to prevent skidding.
The ESC needs low noise, robustness to sensor parasitic resonant modes, and continuous signal monitoring for safety. The system must have low offset drift and immunity to vibration, angular acceleration, and EMI. These requirements demand an ASIC, MEMS sensor, and specialized packaging.
The MEMS gyroscopes use a test mass and measurement electronics to detect displacement. An open-loop rate gyro has a simple readout and low power, while a closed-loop (force feedback) gyro is insensitive to the MEMS parameters and has good offset drift.
Both types need to perform quadrature cancellation and detect Coriolis forces. The issues related to quadrature error include phase errors in the signal path, which can be demodulated with the other out of phase signals. These signals are temperature and stress dependent and lead to offset drift and phase noise. The quadrature error is proportional to the x-axis displacement while the Coriolis forces are proportional to velocity, which is the first derivative of x, so the two signals are 90° out of phase.
A closed-loop design offers a solution to the quadrature and Coriolis issues. The electro-mechanical forces can be used to cancel the quadrature forces, leaving only the in-axis motion for detection. The MEMS converts the changes in capacitance to voltage, which is digitized and fed back to the electro-static correction plates. To keep the test mass at the zero displacement position.
Any ESC component must meet the highest level of safety and operation, as these functions are mission critical. One key parameter is the gyro must detect a 5 percent change in the output signal in 20 ms. In comparison to other ESC systems, this one does not require many sensors and has very relaxed computational requirements. The ESC subsystem uses continuous self test using out of band tones to minimize offset drift and noise and confirm proper operation. The test tones are injected into the force feedback loop and use some simplifications in the feedback algorithms to simplify the transfer functions.
Even thought he gyros are operating at fairly low frequencies, it is important to manage far-out and close-in phase noise. The test tones are at 25 kHz +/- 1 kHz, so the sensor needs very low phase noise near 25 kHz, and also needs low far-out phase noise to ensure that the noise doesn’t alias into the operating bands. The MEMS portions need to manage parasitic mechanical modes and have very little phase noise. The mechanical modes come from out of plane displacements and imbalanced spring constants in the cantilever sections.
A digital PLL and AGC help to control the mechanical parasitic signals and are also needed for setting the gain from digital to rate and self test. The low close-in phase noise requirement is addressed by a band-pass sigma-delta modulator in front of the PLL and AGC. All together, the back-end processing requires a 6.4 MHz VCO sigma-delta modulator, a 3-bit finite impulse response DAC and a 200 kHz RC filter.
The final device shows 0.0039 degree per second per root hertz with the test tones. Full scale response is 630 degrees per second in a bandwidth of 80 Hz. Offset over the automotive temperature range is less than +/- 0.1 degree per second.
