Accelerometers

An accelerometer, a sensor that represents acceleration as a voltage, comes in two axial types. The most common accelerometer measures acceleration along only a single axis. This type is often used to measure mechanical vibration levels. The second type is a tri-axial accelerometer. This accelerometer can create a 3D vector of acceleration in the form of orthogonal components. Use this type when you need to determine the type of vibration—lateral, transverse, rotational, and so on—that a component is undergoing or the direction of acceleration of the component.

Both types of accelerometers come with either both leads insulated, or isolated, from the case or with one lead grounded to the case. Some accelerometers rely on the piezoelectric effect to generate voltage. To measure acceleration with this type of sensor, the sensor must be connected to a charge-sensitive amplifier.

Other accelerometers have a charge-sensitive amplifier built inside them. This amplifier accepts a constant current source and varies its impedance with respect to a varying charge on the piezoelectric crystal. You can see this change in impedance as a change in voltage across the inputs of the accelerometer. Thus, the accelerometer uses only two wires per axis for both sensor excitation, or current, and signal output, or voltage. The instrumentation for this type of accelerometer consists of a constant current source and an instrumentation, or differential, amplifier. The current source provides the excitation for the built-in amplifier of the sensor, while the instrumentation amplifier measures the voltage potential across the leads of the sensor.

When choosing an accelerometer, pay attention to the most critical parameters. If the sensor must operate in extreme temperatures, you are limited to a sensor that relies on the piezoelectric effect to generate voltage. If the environment is very noisy, a sensor with a charge-sensitive amplifier built in might be the only usable choice.

To reduce errors when using an accelerometer, consider these factors:

• If the sensor is DC coupled, the DC offset of the accelerometer can drift with both temperature and age. This applies to both types of sensors because charge-sensitive amplifiers are prone to drift. AC coupling the output of the amplifier can minimize the drift in the system.
• Motors, transformers, and other industrial equipment can induce noise currents in the sensor cables. These currents can be an especially large source of noise with sensor systems that rely on the piezoelectric effect to generate voltage. Carefully routing sensor cables can minimize the noise in the cables.
• Accelerometers might have ground loops. Some accelerometers have their cases tied to a sense wire, while others are completely isolated from their cases. If you use a case-grounded sensor in a system with a grounded input amplifier, you set up a large ground loop, creating a source of noise.