Image credit: Meggit Sensing Systems / Wilcoxon Research | Weschler | PCB Piezotronics
Vibration transmitters are signal conditioning packages that provide calibrated output from vibration sensors or accelerometers.
Vibration transmitters convert input data from one or more sensors into a standard signal. The sensor may be discrete, such as an accelerometer, or integral to the transmitter. Like vibration sensors, transmitters form an important part of a vibration measurement system. Such systems are used to predict and diagnose "machine health" by detecting abnormal vibrations during operation. Through early detection of such abnormalities, plant staff can schedule a suitable repair instead of dealing with the loss of time and efficiency caused by an unexpected catastrophic machine failure.
For example, vibration transmitters may be used to sense shaft misalignment by measuring vibration. Shaft misalignment occurs when rotating machinery shafts become misaligned due to movement that results from the operation of the machine at less than optimal conditions. When shafts become misaligned, forces on the machine produce stress on various components, causing abnormal vibration.
Vibration transmitters commonly use current loop output, an analog signal which is compatible with most meters and data analyzers. Current loop transmitters typically operate over the 4-20 mA or 10-50 mA range. A 4-20 mA loop establishes a 4 mA output as "off" (in this case "no vibration") and 20 mA as "maximum load." When no vibration is detected, the transmitter transmits a constant 4 mA current. When a vibration sensor or accelerometer detects vibration, it relays the vibration reading to the transmitter, which then alters its output to a current value proportional to the vibration reading it received.
The image below illustrates an example of a fairly broad proportional scale. As discussed above, no vibration results in a 4 mA output, while the maximum load of 5 inches per second (IPS) would trigger a 20 mA maximum output. This scale divides the output into increments of 2 mA and displays the corresponding vibration needed to trigger this output. This particular transmitter has been calibrated to sound an alarm if vibration levels reach 3.75 IPS (or 16 mA output) and shut down the machine if it reaches 4.38 IPS (or > 18 mA output).
Image credit: SI-TES
While current loop output is the most common type, vibration transmitters may also use voltage, frequency, or digital output signals.
Vibration transmitters may be manufactured, used, and tested according to broader standards related to vibration measurement. Examples include:
ANSI/ASA S2.27 (Measurement and evaluation of vibration of ship propulsion machinery)
BS 7385 (Measurement of vibrations and evaluation of their effects on buildings)
BS ISO 10815 (Measurement of vibration generated in railway tunnels by the passage of trains)
IHS Globalspec - Vibration Sensors