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ADXL05 Datasheet(PDF) 16 Page  Analog Devices 

ADXL05 Datasheet(HTML) 16 Page  Analog Devices 
16 / 20 page ADXL05 REV. B –16– control of system and mounting resonances are critical to proper measurement. Refer to the application note AN379, available from Analog Devices. CALIBRATING THE ADXL05 If a calibrated shaker is not available, both the 0 g level and scale factor of the ADXL05 may be easily set to fair accuracy by using a selfcalibration technique based on the 1 g (average) accelera tion of the earth’s gravity. Figure 30 shows how gravity and package orientation affect the ADXL05’s output. Note that the output polarity is that which appears at VPR; the output at VOUT will have the opposite sign. With its axis of sensitivity in the vertical plane, the ADXL05 should register a 1 g acceleration, either positive or negative, depending on orientation. With the axis of sensitivity in the horizontal plane, no acceleration (the 0 g bias level) should be indicated. 0 g (a) 0 g (b) –1 g (c) +1 g (d) INDICATED POLARITY IS THAT OCCURRING AT V PR . Figure 30. Using the Earth’s Gravity to SelfCalibrate the ADXL05 To selfcalibrate the ADXL05, place the accelerometer on its side with its axis of sensitivity oriented as shown in “a.” The 0 g offset potentiometer, Rt, is then roughly adjusted for midscale: +2.5 V at the buffer output (see Figure 25). Next, the package axis should be oriented as in “c” (pointing down) and the output reading noted. The package axis should then be rotated 180 ° to position “d” and the scale factor poten tiometer, R1a, adjusted so that the output voltage indicates a change of 2 gs in acceleration. For example, if the circuit scale factor at the buffer output is 400 mV per g, then the scale factor trim should be adjusted so that an output change of 800 mV is indicated. Adjusting the circuit’s scale factor will have some effect on its 0 g level so this should be readjusted, as before, but this time checked in both positions “a” and “b.” If there is a difference in the 0 g reading, a compromise setting should be selected so that the reading in each direction is equidistant from +2.5 V. Scale factor and 0 g offset adjustments should be repeated until both are correct. REDUCING POWER CONSUMPTION The use of a simple power cycling circuit provides a dramatic reduction in the ADXL05’s average current consumption. In low bandwidth applications such as shipping recorders, this simple, low cost circuit can provide substantial power reduction. If a microprocessor is available, only the circuit of Figure 31 is needed. The microprocessor supplies a TTL clock pulse to gate buffer transistor Q1 which inverts the output pulse from the µP to a 5 ° tilt error over the entire temperature range. Straight forward calibration schemes discussed in this data sheet may be used to reduce or compensate for temperature drift to improve the absolute accuracy of the measurement. Using the ADXL05 in Inertial Measurement Applications Inertial measurement refers to the practice of measuring accel eration for the purpose of determining the velocity of an object and its change in position, or distance traveled. This technique has previously required expensive inertial guidance systems of the type used in commercial aircraft and military systems. The availability of a low cost precision dc accelerometer such as the ADXL05 enables the use of inertial measurement for more cost sensitive industrial and commercial applications. Inertial measurement makes use of the fact that the integral of acceleration is velocity and the integral of velocity is distance. By making careful measurements of acceleration and math ematically integrating the signals, one can determine both veloc ity and the distance traveled. The technique is useful for applications where a traditional speed and distance measure ment is impractical, or where a noncontact, relative position measurement must be made. A practical inertial measurement system uses multiple acceler ometers to measure acceleration in three axes, and gyroscopes to measure rotation in three axes, the requirement for a 6 degree of freedom system. For simpler systems where one or more of the axes can be constrained, it is possible to build a system with fewer accelerometers and gyros. The measurement system must take the acceleration sensor and calibrate out all static errors including any initial inaccuracy or temperature drift. A mathematical model is used to describe the performance of the sensor in order to calibrate it. If these errors are not removed, then the process of double integration will quickly cause any small error to dominate the result. Most prac tical systems use microprocessors for error correction and a tem perature sensor for temperature drift compensation. Another approach is to maintain all of the sensors at a controlled tem perature. The microprocessors have the additional advantage of providing a low cost method of performing the single and double integration of the acceleration signal. The stability and repeatability of the accelerometer is the most important specification in an inertial system. The ADXL05 is “well behaved” that is, its response and temperature characteris tics are easy to model and correct, and once modeled they are very repeatable. For example, temperature performance can be adequately modeled using first order, (straight line) approxima tions for most applications, and other errors such as onaxis and pendulous rectification are minimal. This greatly simplifies the math required to correct the sensor. Vibration and Shock Measurement Applications The ADXL05 can measure shocks and vibrations from dc to 4 kHz. Typical signal processing for vibration signals includes fast Fourier transforms, and single and double integration for velocity and displacement. It is possible to build a single inte grator stage using the ADXL05’s output buffer amplifier in order to provide a velocity output. The sensitivity of the accelerometer will typically vary only ±0.5% over the full industrial temperature range, making it one of the most stable vibration measurement devices available. In vibration measurement applications, mechanical mounting and OBSOLETE 
