Vibration sensor

Vibration can be defined as the mechanical oscillation about an equilibrium position of a machine or component or simply the back and forth motion of a machine or component.
Vibration in industrial equipment is sometimes part of the normal operation nut sometimes it can be a sign of trouble.
In machine monitoring we are dealing with two types of vibrations-Axial or thrust vibration and radial vibration.

Axial vibration is a longitudinal shafting vibration or parallel to the shaft of a motor. For example, a shaft misalignment could cause axial vibration.

Radial vibration occurs as a force applied outward from the shaft. Radial vibration would occur if there is a heavy spot in the motor as it rotates. If there is a deformed fan blade, as the fan spins the deformed fan blade would pull outwardly on the shaft of the motor causing radial vibration. Different types of sensors to monitor vibration which are accelerometer.

Accelerometers are devices that measure the vibration, or accelerometer of motion of a structure. They have a transducer that converts mechanical force caused by vibration or change in motion into an electrical current using the piezoelectric effect. There are two types of piezoelectric accelerometer high impedance and low impedance.
High impedance accelerometer produces an electrical charge which is connected directly to the measurement instruments. They require special accommodations and instrumentation so they are found in research facilities or high temperature applications.
Low impedance accelerometers have a charge accelerometer as its front end as well as a built-in microcircuit and transistor that convert that charge into a low impedance voltage. This type of accelerometer easily interface with standard instrumentation which makes it commonly used in the industry.

A strain gauge measures the strain on a machine component. A strain gauge is a sensor whose resistance varies with applied force; it converts force, pressure, tension, weight, etc. into a change in electrical resistance which can then be measured. When external forces are applied to a stationary object, stress and strain are the results. When there is a strain applied to any metallic wire length of that wire increases and the diameter decreases. This increase in length and decrease in diameter will change the resistance of the wire which will give us our measurement or strain on our machine component.

Eddy current sensors are non-contact devices that measure the position or change of position of a conductive component. These sensors operate with magnetic fields. The sensor has a probe which creates an alternating current at the tip of the probe. The alternating current creates small currents in the component we are monitoring called eddy-currents. The sensor monitors the interaction of these two magnetic fields. As the field interaction changes the sensor will produce a voltage proportional to the change in the interaction of the two fields.
When using the eddy current sensors it is important for the component to be at least three times larger than the sensor diameter for normal operation; otherwise, advanced calibration would be required.

When choosing the vibration sensor foe your application it is important to look at factors such as, range and accuracy, environment condition, and shape of the measuring surface.
Out of the three sensors that we have discussed the accelerometer is the most common because it has a good range of frequency, meaning it can sense slow and fast applications. Along with the frequency, accelerometers are priced affordably and are durable.
They do have to be mounted directly to the machine which is common for vibration sensors.
Eddy-current or capacitive sensors have medium accuracy and are not optimal for high resolution applications.
They are very durable making them a good option for dirty environments.
Just like accelerometers they have to be directly mounted to the machine being monitored.
Lastly strain gauges are both versatile and accurate while still suitable for hazardous environments.Unfortunately, they can be hard to install correctly and to get proper data your application will need amplifiers which can drive up the noise.

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