Vibration sensors or accelerometers form critical components in condition monitoring systems for industrial gearboxes and machineries. This approach is commonly implemented in wind turbines, industrial pumps, compressors, HVAC systems, machine spindles, conveyor belts, sorting tables, and machine tools. Vibration in industrial machinery can either be a normal part of machine operations and a sign of problem. The question remains “How does the maintenance professional distinguish the difference between the acceptable vibration level and the kind of vibration that requires immediate attention?” Consistently monitoring vibration will allow them to distinguish anomalies that may indicate potential issues that lead to equipment breakdown and failures.
There are a wide range of accelerometers currently available in the market such as piezoelectric accelerometers, IEPE accelerometers, piezoresistive accelerometers and MEMS variable capacitance accelerometers. They also come with different ranges of sizes, shapes, power supply options and mounting configurations which make it challenging to select the appropriate type for individual requirements. In addition, not all types of accelerometers are suitable for condition monitoring applications. It is essential to understand key criteria in selecting the correct type of accelerometers or vibration sensors as it is important to get signals that accurately represent the machinery state.
There are several conditions that need to be considered when choosing the right type of accelerometers for industrial condition monitoring.
Measurement Requirements
Typical condition monitoring application measuring vibration on machinery to monitor the back-and-forth movements or oscillation of machines and components such as motors, bearings, shafts, gears and other elements that make up the mechanical system.
Engineers must consider the type of vibration experienced by the machine. When the motion is mostly linear, a single-axis vibration sensor will be enough. It is often used to measure mechanical vibration levels. Conversely, a 3-axis vibration sensor can offer more mounting flexibility, as the orthogonal orientation enables pickup on one or more axes regardless of the vibration direction. Tri-axial vibration sensors provide more information concerning machinery health and type of vibration than conventional single-axis units. Triaxial sensors are also easier to mount than three individual sensors.
Frequency Response
Physical structures and dynamic systems respond differently to different excitation frequencies. The general-purpose vibration sensors provide a frequency response starting from around 1 Hz. This is suitable for the majority of industrial applications. However, when it comes to measuring vibrations in a large structure, such as a bridge, or wind turbine, a dedicated low-frequency sensor that can measure down to DC (0Hz) is required.
Ideally, the frequency response of the vibration sensor should be 40 to 50 times the shaft RPM (Revolutions Per Minute) for bearing monitoring. For fans and gearboxes, the minimum upper limit of the sensor frequency should be 4 to 5 times the blade passing frequency. The lower frequency limit is less critical as a frequency less than 2Hz is rarely required.
Vibration Sensors with lower frequency ranges tend to have lower electronic noise floors. Lower noise floors help in increasing the sensor’s dynamic range and may be more important to the application than the high frequency measurements. However, it is important to select a sensor with a usable frequency range that includes all frequencies of vibration required in measurements.
Sensitivity Range
Sensitivity denotes the conversion between vibration and voltage at a reference frequency. The sensitivity range of a sensor is selected based on the range of vibration amplitude levels to which the sensor will be exposed during measurements. As a general rule of thumb, if a machine produces high amplitude vibrations (greater than 10 g rms) at the measurement point, a low sensitivity (10 mV/g) sensor is preferable. On the other hand, if the vibration is less than 10 g rms, a 100 mV/g sensor should generally be used.The sensitivity of industrial accelerometers typically range between 10 and 100 mV/g. Higher sensitivity accelerometers are used for special applications, such as low frequency/low amplitude measurements.
One thing to keep in mind while selecting a sensitivity range is that the peak g level should never exceed the acceleration range of the sensor. This would result in amplifier overload and signal distortion that can generate erroneous data.
Operating Environment
The environment in which vibration measurements are performed can have a significant impact on their accuracy. Temperature transients (hot air or oil splash) can result in metal case expansion. This can produce erroneous output during low frequency measurements (<5Hz). Therefore, for such conditions, temperature compensation will be needed to offset these effects. Additionally, cable connectors and jackets are also used to withstand high humidity or wet environments.
Vibration sensors certified as Intrinsically Safe can be used for measurement in areas subject to hazardous concentrations of flammable gas, vapor, mist, or combustible dust in suspension. That certification ensures that the sensor does not contain parts that can act as ignition source in any operating conditions. In Australia, IECEx certification is required for sensors to be used for monitoring in explosive environments.
Mounting Configurations
Another factor to be considered is how the sensors are mounted. Depending on the situation, it affects the resonant frequency of a vibration sensor which may affect the measurement. Hence, it’s important to avoid using a mounting method that shifts the resonance into the frequency range of a vibration environment.
Deciding the suitable mounting configuration for an accelerometer highly depends on the type of accelerometers, the required accuracy of frequency response and the type of surface to be mounted. Stud mounted accelerometers generally allow the widest range of usable frequency ranges for general applications. This is generally the case for IEPE and piezoelectric accelerometers. Adhesive mounting or magnetic mounting techniques adds additional mass to the accelerometer which lowers the resonance frequency of the sensing system. Therefore, using these mounting configurations may limit the usage and accuracy of the frequency response of the accelerometers. Soft materials, such as those in rubber mounting pads, also affect the frequency response by generating a filtering effect which will dampen the transmissibility of high frequency data.
Hence, choosing the right monitoring solution can help operators get the best performance out of their equipment throughout their life. We offer a wide selection of displacement sensors and accelerometers that enable vibration measurements for condition monitoring with great accuracy.
