Getting MEMS Gyros Into Your Designs

3rd June 2013
Posted By : ES Admin
Getting MEMS Gyros Into Your Designs
MEMS gyros and accelerometers are now incorporated into a broad range of consumer electronic devices, becoming an essential component of automotive navigation and electronic stability control systems. But, how do they work and what does an engineer need to consider when contemplating designing-in a MEMS gyro to their end-product? By Ulf Meriheinä, Senior MEMS Application Specialist at Murata Electronics.
MEMS gyros and accelerometers manufactured by Murata use a capacitive detection approach where one or more capacitors change value with an applied mechanical signal. The accelerometer includes one or more (multi-axis) proof masses that sense gravity and acceleration. The force generated on the proof mass is then gauged with a silicon spring converting it to a deflection, to be sensed as a change in the distance between one or more pairs of capacitor plates.

This technology makes it straightforward to achieve various levels of damping by modifying the gas pressure of the cavity where the proof mass is located. Typical levels of damping are over-damped, critically damped or under-damped. Multi-axis elements are generated by shifting the location of the centre of gravity of the proof mass relative to the spring suspension, shifting the axis of rotation.

The springs are made of single crystal silicon, providing a very stable transfer function with a temperature dependence of gain close to that of the elastic constant of silicon, i.e. less than 1% over 100°C, which is easy to compensate for. The symmetrical structures result in internal self-compensating effects, giving a very stable zero point over temperature and time. The large signal from the sensing element makes the design of the interface electronics easy and enables optimisation for very low power, very high resolution or high speed (little filtering).

The angular rate sensor, or gyro, utilises the Coriolis effect. A proof mass is brought into angular motion, in this case vibrating angular motion. An external angular rate perpendicular to this motion will cause a force proportional to both the vibration motion and the angular rate and perpendicular to both of them. This Coriolis force is detected through synchronised capacitive sensing. Symmetrical structures result in stability and reduced cross sensitivity to linear or angular acceleration.

Figure 1: Main components of a high performance gyro

Market Specifics

Experience of the demands of different markets and applications has led Murata to design two different types of gyro product. One is for maximum performance and reliability while the second has been developed for minimal power consumption. A high performance low noise gyro is typically designed for use in automotive and instrument grade applications. The latter is aimed at battery operated consumer devices such as smart phones and personal GPS navigation units. The automotive gyro is a single axis component, but the consumer gyro is a 3-axis device, that consists of one element with one proof mass only, but with different detection electrodes for the three axes.

When selecting a MEMS gyro component, engineers should carefully review the manufacturer’s technical specifications. For automotive safety systems, and in particular for Electronic Stability Control applications the key attributes include reliability, both in terms of stability and durability, for use in a harsh environment. Typically the sensor would need to cover the extended temperature range of −40 to +105°C, and be capable of achieving over 3000 cycles across the same temperature range. Sensor functions look for the sensitivity of the output signal compared to input to be approximately 2% and an adequate measuring range in the order of 100° turning movement per second. Also, measurement of directional accuracy to be within 1° and zero point to approximately 1.5° per second. Lack of susceptibility to any electromagnetic inference is also important.

Figure 2: Example code flowchart for Murata’s CMA3000 ultra low power consumer grade accelerometer

In navigation and similar high accuracy instruments the considerations mentioned are important as well as that of noise. Noise caused by white noise and 1/f-noise and resolution are important; as is the avoidance of “random walk” caused by white noise, and stability that is a consequence of 1/f-noise and sensor drift. Stability has several influences that need to be kept to a minimum, temperature behaviour and sensitivity to linear acceleration being two of the most notable.

In consumer applications, and in particular in handheld 6DoF AHRS (6 degrees of freedom Attitude and Heading Reference Systems) such as those used in remote control and autonomous aerial vehicles, the emphasis is on finding an extremely low cost, compact and low power consumption sensor that has an adequate measuring range in the order of 1,000 to 2,000° per second. Insensitivity to vibration, having low noise characteristics and offering good short-term stability are also important.

Digital gyro sensor devices are typically connected over an I2C or SPI printed circuit board level bus. For example, the SPI interface on the SCA100T MEMS gyro from Murata has a 4-wire synchronous serial interface. Data communication is enabled with a low active Slave Select or Chip Select wire (CSB). Data is transmitted with a 3-wire interface consisting of wires for serial data input (MOSI), serial data output (MISO) and serial clock (SCK). The SPI interface is designed to support any microcontroller that uses SPI bus. Communication can be carried out by software or hardware based SPI.

Figure 3: An example application combining a gyro, accelerometer, and magnetometer for a 6 DoF AHRS application

Figure 3 illustrates an example application that combines a gyro, accelerometer and magnetometer for a 6 DoF AHRS application for a smart phone. All sensors communicate across the same PCB level bus, in this case I2C. The application processor is running Android OS and the sensor drivers are implemented in the kernel. The sensor fusion function, that combines all the sensor signals together, is implemented on the hardware abstraction layer (HAL). HAL provides the Sensor Manager with the results of the combined sensor events.

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