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Traditionally, accelerometers were used for heavy, high-end machinery such as windmills, industrial pumps, compressors and HVAC systems. In more recent years, demand has risen for better predictive maintenance on high-volume, smaller systems including machine spindles, conveyer belts, sorting tables and machine tools. This increase in demand for predictive machine maintenance has been driven by an increase in automation and digital industrial transformation. Machine downtime has negative consequences for customers and profitability.
For industrial condition monitoring and predictive maintenance applications, the following vibration specification parameters are considered critical to ensure long-term reliable, stable and accurate performance. All or some of the performance parameters discussed will help the customer make an intelligent decision on the technology chosen for the condition monitoring installation.
Wide Frequency Response
To detect all possible failure modes of machinery, the frequency response of the accelerometer should be 40 to 50 times the shaft RPM (revolutions per minute) for bearing monitoring.
Measurement Resolution and Dynamic Range
The measurement resolution of the vibration sensor is a function of the amplitude of the output signal to the
broadband noise of the onboard electronics. An accelerometer with superior signal output will allow the measurement of smaller vibration levels in the machinery. The ability to measure lower vibration amplitude enables the end user to predict a fault much earlier than a sensor with a lower dynamic range.
Long-Term Stability with Minimum Drift
Long-term drift is a shift in the sensitivity and/or zero output measurement (zero output drift applies to MEMS
sensors only). A shift in the sensitivity of the accelerometer could trigger a false alarm over time in the monitoring application.
Operating Temperature Range
Sensors must be able to survive temperature extremes of the application environment. The sensitivity variation versus temperature must be acceptable to the measurement requirement.
Packaging Options and Ease of Installation
For embedded condition monitoring installations of smaller machinery, the size and mounting options can become an important factor in choosing an accelerometer. Larger machinery typically uses an externally TO-5 stud mounted accelerometer but for machinery with smaller bearings and rotating shafts, it will be necessary to use an embedded or miniature accelerometer.
Sensor Output Options
Depending on installation and application, a choice of sensor output signal options may be necessary. Most current predictive maintenance installations require an analog signal from the sensor so the end user can decide on which parameters to monitor for particular machinery. Typically, the signal output is driven by the DAQ or PLC interface and an analog output (±2V or ±5V) is the most common choice. However, for installations requiring long cable lengths, loop powered 4-20mA sensors are also common. For future digital factories and Industry 4.0, the need for digital output signals will become more common as will smart sensors with on board microprocessors that can make immediate maintenance decision for the end user.
Main types of technologies for condition monitoring applications
Piezoelectric vibration sensors
Piezoelectric (PE) accelerometers incorporate piezoelectric crystals that are self-generating and provide a signal when stressed by external excitation such as vibrating machinery.
Most piezoelectric sensors are based on Lead Zirconate Titanate ceramics (PZT) which are poled to align the
dipoles and make the crystals piezo-electric. PZT crystals are ideal for condition monitoring applications since
they offer wide temperature range, broad dynamic range, and wide frequency bandwidth (usable to >20kHz).
The piezoelectric sensor is versatile, reliable and the most popular vibration sensor for machinery monitoring.
Variable capacitance vibration sensors
Variable Capacitance (VC) sensors derive the acceleration measurement from a change in capacitance of a seismic mass moving between two parallel capacitor plates. The change in capacitance is directly proportional to the applied acceleration. VC accelerometers require an IC to be closely coupled to the sensing element to convert the very small capacitance changes into a voltage output. This conversion process often results in poor signal to noise ratio and limited dynamic range.
Download the whitepaper
To learn more about how vibration sensors are used in predictive maintenance applications and how to choose the right technology, download the TE Connectivity (TE) whitepaper ‘Condition Monitoring’ here: https://www.te.com/usa-en/campaigns/transportation-solutions/predictive-maintenance-with-vibration-sensors-whitepaper.html
TE is a global industrial technology leader creating a safer, sustainable, productive and connected future. TE’s broad range of connectivity and sensor solutions have been proven in the harshest environments and enable advancements in transportation, industrial applications, medical technology, energy, data communications and the home.
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