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Connect Sensors with Care
Jon Titus, Contributing Editor -- Design News, June 15, 2009
Many accelerometers rely on a piezoelectric element, such as a quartz crystal, that develops a small charge on its surfaces in direct proportion to an applied force. Attaching one side of the crystal to a small known mass (m) and the other side to a device under test (DUT) lets us use the basic physics equation f = m*a to relate acceleration (a) to force (f), and thus directly to the charge on the crystal.
Piezoelectric sensors typically require circuits that amplify small signals and provide a low-impedance output for data-acquisition instruments. These circuits and external connections can add noise to acceleration signals and distort measurements. That noise can come from ambient electrical conditions, ground loops and triboelectric effects. To reduce the latter, secure output wires to the sensor and the DUT to prevent charge from accumulating due to friction, or cable whip, between wires and surfaces.
If you use shielded cables to help remove ambient noise, ground them at the instrument end only. Grounding at both ends can produce a ground loop, as shown in the diagram, top right. If a ground loop exists because of a common ground between your instruments and the DUT, look for an electrically isolated mounting stud for your accelerometers. Never use a shield as a current-return path!
The capability of an accelerometer to provide useful information also depends on its mechanical connection with a DUT. Mounting often involves an adhesive, such as an epoxy, wax or cyanoacrylate, or a mechanical component such as a bolt or mounting stud. The references, right, provide detailed mounting information and helpful diagrams. A rigid mechanical mount offers the best frequency response, while silicone-rubber glue, a slightly flexible mounting pad or double-sided tape can act like a mechanical low-pass filter and reduce the sensor's high-frequency response.
Mechanical mounting usually involves drilling into the DUT and using a threaded stud or cap screw. A thread-lock liquid such as Loctite 271 will guard against loosening. You must ensure a smooth mounting surface and, during attachment, do not exceed the manufacturer's torque spec. For temporary attachments, consider magnetic mounts.
Manufacturers provide accelerometers in many configurations that use shear, flexure and compression to cause a mass to distort a piezoelectric material. Each sensor type has its own characteristics that best suit specific environments and measurement types. In the next Tips column, I'll examine accelerometers that use a micro electromechanical sensor (MEMS).
A ground loop caused by a slight difference in ground voltages (a) can add noise to a sensor
