Yaw rate sensors

The latest development are micromechanical sensors. Their development began with the large-scale introduction of silicon micromachined pressure sensors to the automotive industry in the nineties, which entailed a massive price reduction. Then acceleration sensors for airbag firing, yaw rate sensors and more were introduced. Many devices are still being discovered. The next step is product evolution, with introduction times between a few years and over a decade, as shown in Tab. 2.2. Once customers in the industry have accepted a product, investment in large-scale production can go ahead. It helps to find more applications for the product The time scale for the product evolution process varies from about five... 

M. Lutz, W. Golderer, J. Gerstenmeier, J. Marek, B. Maihofer, S. Mahler, H. Munzel, U. Bischof, A precision yaw rate sensor in silicon surface micromachining, Proceedings of Transducers 97... 

Yaw-rate sensors recognize roll-over accidents and activate safety equipment, such as head airbags, to protect the passengers. 

Navigation systems record the position of a vehicle using satellite-based location systems (GPS) and match the determined position with the navigation system s digital street map. Yaw-rate sensors, in combination with wheel-speed sensors, permit interpolation in situations where no satellite reception is possible, such as when driving through tunnels or if the GPS signals cannot be clearly interpreted due to multiple reception as a result of reflections from houses in urban situations, for example. 

Fig. 7.2.3 Block schematic of a capacitive yaw-rate sensor system 1, sensor element 2, drive control unit 3, capacitance to voltage converter 4, synchronus demodulator 5, amplifier 6, low pass filter 7, signal conditioner for the reference phase 8, control unit...

The amplifier (5) boosts the signal, eliminates temperature coefficients, and adjusts the offset. The low pass filter (6) limits the bandwidth. The drive circuit is a closed loop system to achieve a stable drive oscillation. It consists of a structure (not drawn) that detects the oscillation movement of the drive of the yaw-rate sensor element, a control unit (2), and an actuator (not drawn). The start circuit (9) initiates the drive oscillation at power on. The block (8) generates all necessary adjustment signals and includes the logic circuit for the trim and an EPROM (erasable programmable read-only memory) for the storage of the trim data. 

Fig. 7.2.8 Sil icon micromachined yaw-rate sensor with electromagnetic drive 1, coupling spring 2, magnet 3, oscillating direction 4, oscillating mass 5, acceleration sensor 6, direction of Coriolis acceleration 7, spring to...

Fig. 7.2.10 Sensor module 1, yaw-rate sensor element 2, evaluation circuit 3, acceleration sensor mounted on a ceramic substrate. Source Robert Bosch GmbH...

Fig. 7.2.11 Exploded view of the complete yaw-rate sensor system. 1, plastic housing 2, damper 3, sensor module 4, PCB with microcontroller and CAN interface. Source Robert Bosch GmbH...

Fig. 7.2.13 Single chip yaw-rate sensor with a range of 150 °/s and a resolution of 0.5 °/s. The sensor element works under ambient air pressure conditions with a non-resonant evaluation and is packaged in a 7 mmx7 mm ceramic housing. Source Analog Devices Inc...

Fig. 7.2.14 Principle of the micromachined rotational yaw-rate sensor 1, comb structure for drive and drive detection 2, rotating mass 3, sensitive direction Fc, Coriolis force ...

In reality, the nodes move as well, but with a distance factor to the turning of the cylinder. This distance factor is called the Bryan factor. The moving of the nodes can be detected and is a measure for the yaw rate. This principle, when produced by precision mechanics, gives an excellent resolution (about 0.01 °/s) and is used in the aircraft industry. The following yaw-rate sensor for automotive application works on this basis (Fig. 7.2.17). 

They operate as drive, amplitude control of the drive, detectors for the oscillation nodes, and as actuators to keep the nodes in place. The principle enables a complete self-test by simulating a yaw rate via the piezoactuators of the closed loop. A yaw-rate sensor based on the principle of a vibrating cylinder is used in VDC systems (Fig. 7.2.18). This principle has been used in a similar way in silicon micromachining and is used in a yaw-rate sensor for VDC [9]. 

The application of yaw-rate sensors in vehicles requires particular attention. The dynamic requirements for the sensors are high Coriolis accelerations in the range of milli-g must be detected correctly, and at the same time the accelerations occurring in the range of several g must not interfere with the sensor function (such as when driving over potholes). In the case of the roll-over sensor this even applies for accelerations > 30 g, such as in a crash. 

Measurement errors occur if the linear range of the Coriolis detectors is exceeded. It is therefore recommended that yaw-rate sensors, especially when permanent availability is required as in the case of vehicle dynamics control, are placed as centrally as possible inside the vehicle to reduce the influence of disturbing accelerations. 

Yaw-rate sensors are spring/mass systems that can be stimulated by external influences in an undesired way on one of the operating frequencies, that is on the drive frequency, the detection resonance, and (depending on the sensor system) also on the difference and summation frequencies. 

The remedy to this is to place the resonance frequencies of the bodywork and the sensor as far apart as possible, and to make sensors with high working frequencies. The mounting of mechanical dampers to prevent transfer of the bodywork resonance frequencies to the sensor is common practice. Mechanical dampers within the sensor itself are also used. Typical working frequencies for yaw-rate sensors are 2-40 kHz, bodywork and printed circuit board resonance frequencies are typically below 5 kHz, or for very stiff light metal bodywork, up to 20 kHz. 

The sensitivity of the sensors should also be taken into consideration in the car assembly. The shock resistance level for typical sensors allows shock acceleration in the range of 5000 g, even for the typical fragile structures of yaw-rate sensors. Undamped impacts, from assembly tooling for example, however, reach accelerations of over 10 000 g and thus can destroy the sensors or cause preliminary damage. A more robust layout of the sensor is basically in conflict with the desired high sensitivity. 

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