THE PIEZOELECTRIC ACCELEROMETER
How do accelerometers and vibration sensors work ?
Accelerometers for the measurement of acceleration, shock or vibration come in many types using different principles of operation.
Inside a piezoelectric version, the sensing element is a crystal which has the property of emitting a charge when subjected to a compressive force.
In the accelerometer, this crystal is bonded to a mass such that when the accelerometer is subjected to a 'g' force, the mass compresses the crystal which emits a signal. This signal value can be related to the imposed 'g' force.
The sensing element is housed in a suitable sensor body to withstand the environmental conditions of the particular application. Body are usually made in stainless steel with welding of the various parts to prevent the ingress of dust, water, etc.
Electrical connection can be via a sealed cable or a plug/socket arrangement.
Many present accelerometers have internal electronic circuitry to give outputs which can be directed used by the associated acquistion or control systems.
Mechanical fixing of the sensor is important in order to achieve true transfer of the vibration or acceleration. Many fixing methods are used including beeswax, hard glues, threaded stud (male or female), magnetic mounts.
Accelerometers are used in many scientific and industrial applications such as predictive maintenance, aerospace, automotive, medical, process control, etc.
An introduction to vibration monitoring
Accurate data capture is vital as part of a predictive maintenance plan in modern manufacturing. Here we explain how modern vibration monitoring technology works and offers advice on how to successfully install and use accelerometers.
Automation is a powerful industrial tool for industry yet, for many companies in the manufacturing or processing sectors, gaining the economic advantage it brings has not always been possible. Today, the costs of designing, specifying, purchasing, maintaining and running automated systems is becoming increasingly more affordable but the technology is still costly enough to demand the protection of preventative maintenance. Automated machinery has to be constantly available to prevent downtime and so an efficient maintenance strategy is critical.
Condition monitoring is the key. For example, in some circumstances where vibration is carefully monitored it is possible to extend operating life beyond recommended maintenance intervals and make significant savings in manpower and running costs. At the other end of the scale, monitoring can prevent profit and reputation from being wiped out instantly when a rapid increase in vibration indicates an imminent catastrophic failure. In short, vibration monitoring is a key tool for plant and equipment maintenance today.
Vibration levels are measured using accelerometers, which, like most powerful engineering tools, provide excellent service as long as the engineer concerned takes a little time to appreciate how they work. Even the best accelerometer will not offer the precision or longevity of which it is capable if poorly installed or maintained. It is therefore in everyones interest to spend a little time considering what accelerometers are and how to effectively install them before specification and installation begins.
There are two main types of accelerometer: AC accelerometers, which are used with a data collector for monitoring the condition of higher value assets such as wind turbines, and 4-20mA accelerometers, which are used with a PLC to measure lower value assets such as fans and pumps. Both are capable of detecting imbalance, bearing condition and misalignment but AC accelerometers can also identify cavitation, looseness, gear defects and belt problems. The current array of sensors or accelerometers for vibration monitoring offered by market leaders such as Hansford Sensors can operate over a wide temperature range, measuring both high and low frequencies with low hysteresis characteristics and excellent levels of accuracy. These devices also offer robust and reliable service, thanks to stainless steel sensor housings that can prevent the ingress of moisture, dust, oils and other contaminants, enabling their use in a variety of applications.
Accelerometers typically contain a piezoelectric crystal element bonded to a mass. When the accelerometer is subject to an accelerative force, the mass compresses the crystal, causing it to produce an electrical signal that is proportional to the level of force applied. The signal is then amplified and conditioned using inbuilt electronics that create an output signal, which is suitable for use by higher level data acquisition or control systems. Output data from accelerometers mounted in key locations can either be read periodically using sophisticated hand-held data collectors, for immediate analysis or subsequent downloading to a PC, or routed via switch boxes to a centralised or higher level system for continuous monitoring.
To specify an accelerometer correctly, engineers need to work closely with a supplier that has appropriate industry experience and knowledge. This will ensure that all things have been considered, from the vibration level and frequency range to be measured, to weight or fitting restrictions and environmental conditions, such as ambient temperatures, the presence of moisture, chemicals or potentially explosive atmospheres.
When it comes to installation, the information you have already gathered will help you to understand the importance of achieving a firm foundation for your mounted accelerometers. For example, a device that offers such sensitivity to vibration may naturally be affected by its own movement if not properly installed. Engineers should mount the device directly onto the machine surface on a flat, smooth, unpainted surface that is larger than the base of the accelerometer. The installer should ensure that the surface is free from grease and oil, as close as possible to the source of vibration. By following these guidelines, you will have already supported your accelerometer, and thus your maintenance regime, by enabling the device to give the most accurate measurements of vibration levels.
For details of ACCELEROMETER Suppliers, who can provide detailed information, please click SUPPLIERS, then select ACCELERATION, VIBRATION or SHOCK sections...
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