What is an Accelerometer?

When you have machinery such as turbines or motors, it is best to monitor them to make sure everything is moving correctly or not moving at all. A movement might not even be desirable and could even be destructive to the machine or even yourself, which is why you would add a sensor to it. An accelerometer is a sensor that measures changes in gravitational acceleration. When attached to a machine it can be used measure the vibration, acceleration, tilt or shock detection.

What are high temperature accelerometers used for?

Some industries include aerospace for turbine engines, oil & gas use it as a measurement while drilling and wind farming for gearbox monitoring. There are plenty of other industries such as quarrying and railway that would also need technology like this as well.

Accelerometer components

The basic design consists of a piezoelectric sensing crystal which is attached to a seismic mass. An acceleration causes  the mass to produce a stress in the sensing crystal which produces an electric charge / output. The signal is then amplified and interpreted.

Types of high temperature piezoelectric elements

High temperature accelerometers require a high temperature piezoelectric element. There are two primary accelerometer types to choose from: single crystal, such as quartz, or ceramic/ polycrystalline, such as PZT or Ionix HPZ.

The advantage of using a ceramic element vs quartz is primarily the controllable polarization and high output allowing for high sensitivity accelerometers with very low noise floors. The limitations of piezoelectric crystals compared to quarts however are intrinsically linked to their limited high temperature properties: reduced long term stability, higher thermal expansion coefficient and their pyrocitric sensitivity.

The industry standard piezoceramic material, PZT, is generally limited to a maximum continuous operating temperature of 200°C. Single crystals ay be used for extreme high temperature accelerometers.

An interesting piezoelectric ceramic for high temperature accelerometers is the Ionix HPZ. HPZ has a maximum continuous operating temperature of 580°C. It has the highest sensitivity of high temperature piezoelectric crystals and is unique in having a shear coefficient comparable to PZT. It has a comparatively low thermal expansion coefficient and is much less susceptible to the pyroelectric effect than PZT. Ionix HPZ is an exciting material for accelerometer manufacturers to consider for low noise applications which exceed the operating limits of PZT.

Designs

There are three key mode designs used to create the piezoelectric element. These different modes are used in different applications.

Compression Mode – One of the earlier designs, the piezo ceramics are stacked on top of each other to form a compression disk that can move up and down. The stacks are often loaded by a screw to a pre-set torque to improve sensitivity. A high toughness ceramic such as HPZ can improve component reliability.

Shear Mode – this allows the piezo to sense vibrations primarily in one axis, and is left succestible to vibrations in the other two axes. This can be useful for a number of applicaitons

Overall

There is no single ideal solution for a piezoelectric accelerometer and compromises on properties will be made based on the application. For high temperature applications, the high temperature limitations of PZT often push accelerometer manufacturers to sacrifice sensitivity and use a more stable single crystal based element.

The Ionix HPZ material offers an opportunity for accelerometer manufacturers to bridge the gap between PZT and single crystals by offering a high sensitivity ceramic with the reduced thermal expansion of pyroelectric sensitivity required for some applications. Combined with the increased toughness, HPZ has the potential to expand the range of existing ceramic based accelerometers.

Ionix offers HPZ ceramic supply as well as support for integration, ready to go high temperature assemblies and sensors to take your sensors, actuators and transducers to the next level. Contact Ionix now to see how you could access more high value applications and markets with your exiting technology.