Introduction to accelerometers

How do piezoelectric accelerometers — vibration sensors — work? The force caused by vibration or a change in motion (acceleration) causes the sensor's mass to “squeeze” contained piezoelectric material, in turn producing an electrical charge proportional to the force exerted upon it. Because the charge is proportional to the force, and the mass is a constant, the charge is also proportional to the acceleration.

Piezoelectric accelerometers come in two models: The first type is a “high impedance” charge output accelerometer, in which the piezoelectric crystal produces an electrical charge that is connected directly to measurement instruments. The charge output requires special accommodations and instrumentation most commonly found in research facilities. This type of accelerometer is also used in high temperature applications (greater than 120° C) where low impedance models cannot be used.

The second type of accelerometer is a “low impedance” output accelerometer, which has a charge accelerometer as its front end, but has a tiny built-in microcircuit and FET transistor that converts the charge into a low impedance voltage that can easily interface with standard instrumentation. This type of accelerometer is commonly used in industry.

Accelerometer specifications

Specifying an accelerometer for a particular application requires familiarity with some basic definitions. Here we review several terms:

Dynamic range: The +/- maximum amplitude that the accelerometer can measure before distorting or clipping the output signal, typically specified in gs.

Frequency response: Determined by mass, piezoelectric properties of the crystal, and resonance frequency of the case; it is the frequency range where the output of the accelerometer is within a specified deviation, typically +/- 5%.

g: 1g is the acceleration due to Earth's gravity, which is 32.2 ft/sec², 386 in/sec², or 9.8 m/sec².

Grounding: There are two types of signal grounding in accelerometers. Case grounded accelerometers have the low side of the signal connected to their case. As the case is part of the signal path and may be attached to a conductive material, care must be taken to avoid noise from the ground plain. Ground isolated accelerometers have the electrical components isolated from the case and are much less susceptible to ground-induced noise.

High frequency limit: The frequency where the output exceeds the stated output deviation, governed by the accelerometer's mechanical resonance.

Low frequency cut-off: The frequency where the output starts to fall below stated accuracy. The output does not “cut off,” but sensitivity decreases rapidly with lower frequencies.

Noise: Electronic noise, generated by the amplifying circuit, can be specified either broadband (specified over the a frequency spectrum) or spectral (designated at specific frequencies); noise levels are specified in gs. Noise typically decreases as frequency increases.

Resonance frequency: The frequency at which the sensor resonates or rings; frequency measurements should be well below the accelerometer's resonance frequency.

Sensitivity: The output voltage produced by a certain force, measured in gs. Accelerometers typically fall into two categories, producing either 10 mV/g or 100 mV/g. The frequency of the ac output voltage will match the frequency of the vibrations, and the output level will be proportional to the amplitude of the vibrations. Low-output accelerometers are used to measure high vibration levels while high-output accelerometers are used to measure low-level vibrations.

Temperature sensitivity: The voltage output per degree of measured temperature. Sensors are temperature-compensated to keep the change in output to within the specified limits for a change in temperature.

Temperature range: Limited by the electronic microcircuit that converts the charge to a low-impedance output. Typically, the range is -50° to 120° C.

Accelerometer types

Here we review the four main types of accelerometers.

Premium grade

These accelerometers are made from premium crystals and use low noise circuitry to produce a premium, low noise accelerometer. Their 316L stainless steel case is hermetically sealed against the environment so they can survive harsh industrial environments. They also have FM and CSA intrinsically safe options available.

Industrial grade

Industrial grade accelerometers are the workhorses of industry. They are used on everything from machine tools to paint shakers.

High vibration

Accelerometers used to monitor high vibration levels have a lower output (10 mV/g) and lower mass than industrial accelerometers.

Triaxial

Triaxial accelerometers measure vibration in the X, Y, and Z axes. They have three crystals positioned so that each one reacts to vibration in a different axis. The output has three signals, each representing the vibration for one of the three axes.

Mounting advice

An acceleration sensor must be mounted directly to a machine's surface to correctly measure vibrations. This can be accomplished with several types of mounts:

Magnet mounts are generally temporary mountings and are used to mount accelerometers to ferromagnetic materials commonly found in machine tools, structures, and motors. They allow the sensor to be easily relocated from site to site for multiple location readings. Two-pole magnetic mounts are used to mount an accelerometer to a curved ferromagnetic surface.

Adhesives and threaded studs are considered permanent mountings. Adhesives such as epoxy or cyanoacrylate are proven to provide satisfactory bonding for most applications. Note: Keep the film as thin as possible to avoid any unwanted vibration dampening due to the film's flexibility. To remove an adhesive-mounted accelerometer, use a wrench on the case's wrench flats and twist to break the adhesive bond. Caution: Do not use a hammer, as striking the accelerometer will damage it.

Mounting studs are preferred for mounting. They require the structure to be drilled and tapped, but provide reliable mountings. Be sure to follow the specified torque settings to avoid damaging the sensor or stripping the threads.