|
As a rule mechanical vibrations are measured using contacting sensors, preferably electro-dynamic or piezo-electric sensors. These sensors are similar in construction to seismographs and acquire the absolute motions of a measurement point in space.
Should it be necessary to acquire the relative motion between a measurement point and a vibrating reference point, then relative sensors are to be used. Typical measurement tasks are:
· Measurement of the shaft vibrations of rotors
· Measurement and monitoring of radial clearance in machines, e.g. between rotor journal and the sleeve bearing surface
· Measurement of the axial shaft displacement
It is common in the examples mentioned that the measuring point (rotor, shaft journal) rotates and thus a contacting vibration measurement is, as a rule, usually not possible.
A solution for this task is the employment of non-contacting vibration sensors.
This vibration displacement sensor operates according to the eddy-current principle. In contrast to an inductive sensor, it can be employed for vibration measurement at all electrically-conducting materials. The sensor is oil- and vacuum-tight and extremely resistant to corrosion and its temperature operating range is between -30°C and +180 °C. It has a diameter of approximately 8 mm and can be delivered with a wide range of body threads. The vibration measurement is executed over an air-gap of 1 to 2 mm where the variations in this gap range are not influenced by oil, water or vacuum.
Each sensor is provided with an oscillator which can be supplied as an external device or can be built into the casing of the sensor.
At the output of the oscillator a signal is produced which contains two components of measurement information:
-
a DC component which is proportional to the static air-gap between the sensor tip and the measurement surface, and
-
an AC component which is a measure of the dynamic vibration displacement.
The measurement, display and monitoring of these two values can be carried out using portable measuring instruments or VIBROCONTROL permanently-installed machine protection systems. A large variety of modules for the measurement of
- relative shaft vibrations
- axial displacement and
- relative expansion
are available. |
