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A few notes about the application of accelerometers will help in understanding how the selection of a sensor is made in a particular case.
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Steady-State Acceleration
In steady-state accelerations, we are interested in a measure of acceleration that may vary in time but that is nonperiodic. Thus, the stop-go motion of an automobile is an example of a steady-state acceleration. For these steady-state accelerations, we select a sensor having (1) adequate range to cover expected acceleration magnitudes and (2) a natural frequency sufficiently high that its period is shorter than the characteristic time span over which the measured acceleration changes. By using electronic integrators, the basic accelerometer can provide both velocity (first integration) and position (second integration) information.EXAMPLE 5.15
An accelerometer outputs 14 mV per g. Design a signal-conditioning system that provides a velocity signal scaled at 0.25 volt for every m/s, and determine the gain of the system and the feedback resistance ratio.
Solution
First we note that 14 mV/g becomes
FIGURE 5.29 An integrator can be used to obtain velocity information from an accelerometer.
Now we need an integrator to get the velocity and amplifier to provide the proper scale. Such a circuit is shown in Figure 5.29. We chose T = RC = 1 so that the integrator output is scaled at
Thus, we can make R2 = 175 kW and R1 = 1 kW so that The application of accelerometers for vibration first requires that the applied frequency is less than the natural frequency of the accelerometer. Second, one must be sure the stated range of acceleration measured will never exceed that of the specification for the device. This assurance must come from a consideration of Equation (5.29) under circumstances of maximum frequency and vibration displacement.
Shock
The primary elements of importance in shock measurements are that the device have a natural frequency that is greater than 1 kHz and a range typically greater than
500 g. The primary accelerometer that can satisfy these requirements is the piezoelectric type (Section 5.4.3).
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Types of Motion
Accelerometer Principles
Types of Accelerometers
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