Micro Inertial Measurement System

A micro inertial measurement system comprising of a system of accelerometers and gyroscopes is being developed in the National University of Singapore. The aim of the inertial measurement system is to track the six degrees of freedom of the object of interest. In this way, the position and orientation of the object can be known. Such a system finds application in many areas such as the military, space exploration, industrial robots, vehicles and even in toys! In this article, research done on the two principal components of the inertial micro system - microaccelerometer and microgyroscope - will be described. The detection technique for both is by capacitive means. Some of the key features of these components are as follows: (a) small in size - with a total sensing element area of approximately 1mm square with features at the micron scale; (b) low cost due to batch processing, and (c) the sensor is easily integrated with electronics as it is made of silicon which is the same substrate as that of microelectronics.

In recent years, there is a rapidly increasing demand in the market for microaccelerometers. High precision accelerometers are currently widely used in aircraft, ships, vehicles and the military. The number of other applications, such as active suspension, adaptive brakes, alarm system and robotics, are also increasing.

In our work, a low-g capacitive silicon microaccelerometer with resolution 15mg (1g=9.81m/s²) and sensitivity 36fF/g has been developed. Figure 1(a) shows the design while figure 1(b) shows the 3D model of the accelerometer. It has a full measurement range of ± 2g. The microaccelerometer uses symmetrical differential capacitor electrodes for an open loop differential capacitive detection. Combined with the high accuracy circuit, the accelerometer offers a high-level voltage output. The fabricated microaccelerometer has also been tested successfully, and the test results show that the accelerometer has an excellent thermal performance. The accelerometer has self-test function and has high reliability. It can survive a shock of 1000g with duration of 0.5ms. The experimental results agreed well with the design analysis.

For the gyroscope, the modal analysis was done using PATRANä . The sensing resonant frequency in the y-direction is 3.66 kHz while in the actuation direction it resonates at 4.42 kHz. The fish hook spring design provides an almost equal compliance in both sensing and actuation directions. The curve beam was added to reduce the stress effects on the beams.

Static capacitance tests have been done on the first batch of devices to characterise the actuation and detection combs cells. The capacitance distribution shows a close agreement with the design values. Experimental results also show that the electrostatic force - which is proportional to the square of the voltage - and the capacitance is within the linear range. Therefore, the electrostatic spring constant is a simple numerical value. Further tests reveal that the structure collapses at a voltage of 7.5 V. Such a test is necessary to determine the maximum operating voltage for the gyroscope. These works are carried out in collaboration with EG&G Heimann.1103. This project is carried out in collaboration with Dr Y.C. Liang and Mr V.J. Logeeswaran

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