Gyroscopes run at a specific resonant frequency. When this frequency and the frequency of vibrations in the environment are close, mechanical disturbances can distort the measurements. France-based CEA-Leti and Italy-based Politecnico di Milano said they have addressed this distortion issue by developing a gyroscope that operates in the high frequency range, at about 50kHz, beyond the parasitic vibration frequencies occurring in harsh environments such as automobile, aerospace and industrial applications.
“This is the first time that a high-frequency gyroscope has reached such a high level of performance, which means it is possible to detect bias stability, sensor resolution, and therefore rotation speeds of less than one degree per hour,” Philippe Robert, MEMS business development manager and senior expert at CEA-Leti, told EE Times. “This is due to the use of nanogauge detection.”
Over the last ten years, there has been a strong demand for low-cost, miniaturized inertial sensors such as gyroscopes and accelerometers. To miniaturize sensors without lowering sensitivity and resolution performances, CEA-Leti has developed the M&NEMS concept of piezoresistive detection with silicon nanogauges. Based on 25 patents, the M&NEMS combines on the same device NEMS and MEMS technologies (micro- and nano-electromechanical systems). It consists of a thin layer for the detecting elements, i.e., the piezoresistive nanogauge, and a thick layer to define the internal mass and the deformable springs.
The nanogauge allows a large constraint concentration, and “we have progressively realized that a three-axis detection could be implemented in this new architecture of sensors,” said Robert. For inertial sensors, it is indeed valuable to detect changes in vibration on the x, y and z axis. Starting with accelerometers, Leti then used the piezoresistive nanogauge detection for pressure sensors, microphones, and more recently for gyroscopes.
Six years ago, Leti started working with Politecnico di Milano (Polimi) as part of a European-funded project. Dubbed Nirvana, the project aimed at developing advanced 9-axis inertial micro-sensors (3D gyroscopes, 3D accelerometers, and 3D magnetometers) based on silicon nanowires for consumer and automotive applications, as well as a 3-axis gyroscope for medical applications. While Polimi was in charge of the MEMS gyroscope design, Leti was responsible for the accelerometer and magnetometer development and fabrication.
The project demonstrated promising results with the gyroscope in terms of bias, noise, and linearity specifications. Once finalized, CEA-Leti and Polimi decided to pursue their research using their own funds. “We were not able to finalize an industrial contract, but we believed in it wholeheartedly and continued to collaborate,” said Robert. “Little by little, we realized that one of the interests of this nanogauge detection for gyroscopes is to be able to maintain high levels of performance even when increasing the resonant frequency of these sensors.”
Capacitive vs. piezoresistive
The M&NEMS gyroscope uses piezoresistive nanogauges not only for rotational speed detection, but also for drive detection.
State-of-the-art, low-power MEMS gyroscopes are based on capacitive sensing. “The problem with capacitive detection is that it detects displacement and, when you want to work at high resonant frequencies, the structures become stiffer,” said Robert. Nonetheless, “the stiffer the structure, the more it deforms.”
With piezoresistive detection, he continued, “we don’t measure displacements, but constraints. We have shown experimentally that we can increase the resonant frequency without losing performance. We even managed to improve it.” This is a significant advantage because almost all gyroscopes are sensitive to vibration and when the vibration reaches the resonant frequency of the sensor, the sensor loses the information, said Robert.
Vibration sensitive environments
To maintain acceptable sensitivity, conventional gyroscopes must operate at a relatively low frequency (15-20kHz). In a plane or a car, however, the vibrations of the environment are often beyond 20kHz. Based on discussions with carmarkers, CEA-Leti and Polimi then worked on a gyroscope whose resonance frequency goes beyond this spectrum to “become environment insensitive”. Operating at frequencies in the order of 50kHz, the co-developed gyroscope has characteristics that exceed the state of the art in terms of bias, noise, linearity, Robert claimed.
For automotive applications, gyroscopes must be able to detect a variation of one degree per hour, i.e., a speed of rotation about ten times slower than that of the earth, in environments that are subjected to strong, consistent vibrations. Cars, most particularly autonomous cars, can’t lose navigation, even when they can no longer receive a GPS signal (e.g., in tunnels or urban canyons). “We need a backup system to take over, therefore a reliable navigation system with high sensitivity and very good resolution,” said Robert. “We have proven that we could do it.”
The gyroscope with modes around 50 kHz and a 1.45mm² footprint has indeed been tested against a 20kHz twin device, and the sensor demonstrated a scale factor of 1.3 mdps/√Hz ARW and 0.5° stability, validating theoretical estimates. “We reconciled small size, low cost, performance and co-integration with other sensors,” said Robert. “Our technology is aimed at all applications, including consumer, but where we are really going to stand out is in applications where there are severe vibration environments, such as industry, automotive, aerospace, and defense.”
The technology is advanced, and “our objective is to find industrial partners to go further,” said Robert. “Several MEMS foundries in Europe and in Asia have shown interest, but a transfer is expensive and it will only happen when we have a large industrial partner. We currently address other types of sensors such as accelerometers and pressure sensors for which we benefit from the support of industrial partners.”
The technology is ready to be transferred. Nonetheless, Robert admitted that there is still work to be done before arriving at a product. “We are at a relatively low TRL [technology readiness level], which means we have proofs of concept, but we need to go further on packaging and electronic integration aspects.”
Depending on the application and the specific need of the industrial partner, Robert said it can take one, two, three years to put into production and have a final product.
In parallel, Polimi and CEA-Leti are continuing their research work. They said they have just reached a new performance milestone, but do not wish to discuss it until they have validated it experimentally. “The idea thereafter, in terms of component development, is to continue to improve performance. In terms of bias, today we are at 0.5° per hour, and the objective is to go below 0.1° per hour,” Robert concluded.
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