Accuracy accelerometers

The accuracy of MWD directional measurements is generally much better than the single- or multishot-type measurements since the sensors are more advanced and the measurements more numerous. The azimuth measurement is made with the three components of the earth magnetic field vector and only with the horizontal component in the case of the single shot or multishot. The accelerometer measurements of the inclination are also more accurate whatever the value of the inclination. The average error in the horizontal position varies from... 

A benefit of the use of accelerometers is that they do not require a calibration program to ensure accuracy. However, they are susceptible to thermal damage. If sufficient heat radiates into the piezoelectric crystal, it can be damaged or destroyed. However, thermal damage is rare since data acquisition time is relatively short (i.e. less than thirty seconds) using temporary mounting techniques. 

The best method of ensuring that the point of measurement, its orientation, and the compressive load are exactly the same each time is to permanently or hard mount the transducers, which is illustrated in Figure 43.26. This guarantees accuracy and repeatability of acquired data. However, it also increases the initial cost of the program. The average cost of installing a general-purpose accelerometer is about 300 per measurement point or 3000 for a typical machine-train. 

Surveyors always try to verify their results with independent, redundant measurements. They use the redundant measurements for finding a statistically significant result, for raising the accuracy and for increasing the signal to noise ratio. As an independent sensor in our problem we need a second sensor, which can react on high-frequency movements which have a short relaxation time. The sensor can have a one dimensional sensitivity, because we want to measure only movements in the direction of the brake power. Therefore we used an accelerometer, which we mounted on the abutment with its sensitive axis in the direction of the brake power (Fig. 7). 

To evaluate the accuracy of the equipment under test, 8 bench trials applying accelerations with different amplitudes and directions were performed. Acceleration values obtained by the tested equipment were converted in RMS values to allow the comparison with readings performed by the calibrated accelerometer. Obtained data from devices and comparison are presented in the Table 3. 

The major advantages of MEMS sensors for measurement of stmcture seismic response are their small size, low power consumption, and low price in contrast with their high accuracy in measurement. For example, a typical price of MEMS accelerometers is less than 10 USD. In spite of this low cost, a MEMS accelerometer with 3 axes, 2 g full scale, 5 mm x 5 mm x 2 mm size, 1.1 mA current consumption at the input voltage of 2.7 V, and noise density of 175 fig/v/Hz is available off the shelf. 

The advantage of this application over unsuccessful attempts for MEMS accelerometer-based structural health monitoring is the required accuracy in the acceleration measurement. The measurement accuracy of the commercially available MEMS accelerometers is good enough for constructing restoring force characteristics. 

---