Dr Jean Guerard (ONERA)
This paper reports the prototyping and performance evaluation of a vibrating structure connected to the Digital Programmable Controller (DPC) developed by Thales-Belgium (TAS-B) in a gyroscope application. ONERA has been developping vibrating MEMS inertial sensors for various applications. The VIA cell (Vibrating Beam Accelerometer family) is already in use in the french civil and defence industry. The VIG cell (Coriolis Vibrating Gyroscope family) has been proposed for space applications, in the frame of low cost assistance gyroscope associated with star trackers on satellite platforms : detumbling, slowing down satellite rotation to allow star tracker acquisition or recovery. During previous activities several aspects of the VIG have been investigated, in the objective of a future qualification. The gyroscope quartz cells endured radiations successfully up to geostationary dose, and could also withstand launcher vibrations. Shocks were also acceptable with intercalated passive absorber. Concerning electronics, the design and realization of an ASIC was initiated, but prohibitive cost prevented from further development. Nevertheless it was assessed that the electronic architecture of the ONERA gyroscope was compatible with the DPC developed by TAS-B. Electronic architecture of the gyroscope has been mapped on the DPC cores and peripherals, and requirements set in terms of A/D D/A converters, voltages, CPU usage, and communication with host. Once the chip was released recently, the opportunity to build a demonstrator was found with the french space agency. A DPC Reference Kit (DRK) is used in this project. It consists in a true qualified DPC, mounted on an evaluation board with power supplies, I/O connectors and programmer access through JTAG. The DRK comes together with software tools (compiler, debugger, configuration manager), offering the developper a complete toochain from C source code to oscilloscope view of signals. In the present work, the core functions of the gyroscope application have been developed and characterized. First a Direct Digital Synthesizer (DDS) has been implemented with a particularly high resolution of 0.001 Hz, in order to accurately drive at resonance the high quality factor vibrating cell. A pair of ADC are operated synchronously with the synthesizer to acquire the amplitude and phase of Drive and Sense signals coming out of the vibrating cell. ADC are digitally demodulated in real time (~100 kHz) using the hardware Multiplier/Accumulator of the DPC, and deliver in-phase and quadrature components to the second DPC core. Further processing is performed, such as an embedded PLL for the resonator, and a decimation filter to scale down the raw data stream to the sampling rate configured by the user, which takes place in a standard desktop computer running the demonstration OBC software. The gyroscope data frames are composed and transmitted by the third core on a serial port. All these functions perfectly match the intentional asymmetric core design of the DPC, and all three cores are in use in the application. The program memory is tiny for each core (4K, 8K, 16K), but keeping an eye on assembler generated by the compiler allows the programmer to write clean yet efficient code. The performance of several functions of the DPC have been evaluated in real conditions, such as ADC resolution. the word length of a single acquisition is 13 bits; when measuring a constant voltage, the ADC resolution is 0.1 lsb after averaging, which is equivalent to 16 bits at 10 ms, limited by 1/f noise. But when measuring a modulated signal on a carrier, the 1/f noise disappears and the resolution is 0.0005 lsb at 100 s (corner frequency is about 0.1 Hz), which is equivalent to 23 bits. Therefore we conclude on the use of the DPC for metrology applications.
Dr Jean Guerard (ONERA)