Putting cars in to motion: Page 4 of 5

April 03, 2014 //By Jari Nieminen, Murata
Putting cars in to motion
MEMS sensors are increasingly providing a crucial element in closed-loop automotive control systems.
as the single-axis accelerometer recently introduced by Murata (SCA720-D01), which joins its growing range of MEMS sensors targeting suspension and engine vibration applications in the automotive market. With a simple 3-wire connection and analogue output, the sensor is small and robust enough to be mounted directly on the suspension system, providing fast and accurate feedback to the electronically controlled suspension system’s ECU.

Figure 2 shows a block diagram of the SCA720-D01.

Manufactured using bulk micro-machining and housed with a signal conditioning ASIC, the SCA720-D01 achieves a nominal sensitivity of 0.9375 V/g. From this, acceleration can be calculated as a function of the output voltage and sensitivity, as shown in Equation 1. Note that the Vout(0g) is the nominal output voltage at 0g position, with a 5V supply. In order to maintain accuracy, the device also employs a ratiometric output feature, which ensures that the output voltage is less susceptible to supply voltage variations; a ratiometric output indicates that if the supply voltage fluctuates then the output voltage will fluctuate proportionally. Furthermore, when the same reference voltage is used for both the sensor and the A/D converter in the signal chain, any error caused by the reference voltage variation is automatically compensated for.

The SCA720 also includes a built-in self-test mode, which simulates acceleration (or deceleration) using an electrostatic force. This force simulates acceleration high enough to deflect the proof mass to the extreme positive position, causing the output voltage to reach its maximum value.

The operating environment including EMI/EMC for this type of sensor can be classed as harsh and as the amount of electronics used in cars increases so too due the (global) list of requirements to which they must adhere. The requirements of reliability are also much more stringent in the automotive field. It is now commonplace to test components to in excess of 2000 temperature cycles, where the requirement used to be less than 1000 cycles between

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