Question

What are "compression mode" and "shear mode" accelerometers, and how do I know which is appropriate for my application?

Answer

Compression mode and shear mode refer to internal construction techniques of piezoelectric accelerometers. These two techniques comprise the majority of how piezoelectric accelerometers are made, although there are other techniques as well (for example, bimorph benders). It is impossible to tell whether an accelerometer is compression or shear mode by examining the outside of the sensor, so the manufacturer will typically state this on the product data sheet. The exception to this is ring-type accelerometers, featuring a mounting screw through the center of the sensor. These are almost always shear mode accelerometers.

Compression mode accelerometers are made by literally stacking the piezoelectric sensing elements one on top of the other, along with a seismic mass. A screw is then inserted through the center of this stack. The screw is torqued until the stack is compressed to a certain pre-designed load. This is one of the earliest techniques in accelerometer design and is still commonly found today, frequently in very high temperature rated accelerometers.

Shear mode accelerometers can be made in a number of ways. Nearly all involve the piezoelectric element (or elements) being mounted radially around a fixed center post. A seismic mass is then mounted around these elements. The entire structure is held together most commonly with an adhesive, but screws are also sometimes used, as well as special metal bands tightened around the structure. The piezoelectric element (or elements) are not in compression (in the sensitive direction), however, but have shear forces acting on them when the accelerometer is accelerated.

Both techniques have their positives and negatives. Compression mode accelerometers tend to have a higher mounted resonant frequency. Shear mode accelerometers usually show a better (lower) base strain and temperature transient sensitivity. But these are generalities. It is certainly possible to find a compression mode accelerometer with very low base strain sensitivity, just as it is possible to find a shear mode accelerometer with a very high mounted resonant frequency.

It should also be noted that there are some in the instrumentation community who claim that the compression mode technique is old technology and therefore no longer useful in today's marketplace. But compression mode is still very successful and considered one of the most robust and reliable construction techniques developed. Endevco has documented evidence of these accelerometers lasting many decades and having a sensitivity drift of a few percent or less over this timeframe.

From the accelerometer user's perspective, then, more emphasis should be placed on considering the appropriateness of the accelerometer's specifications to the application at hand, than on whether it is compression mode or shear mode. Focus on the accelerometer's specifications, not on how it is built internally.