What is a Vibration sensor ?

The sensors used to measure, display and analyze frequency, displacement, velocity or acceleration are called vibration sensors. There are various vibration sensor types but the most commonly used vibration sensor is accelerometer that produces electrical signal proportional to acceleration of vibrating component. The other types are Piezoelectric, pin and spring, capacitive, optical, strain guage, magnetic etc.

Vibration sensors respond to repetitive mechanical motion. Most of the sensors are available with their contacts "normally open" type. The contacts close when the sensor vibrates in its designed frequency range. Frequency range and sensitivity are manually adjustable in some of the sensors.

Mechanical sensors or switches which respond to displacement are best suited for low frequency vibrations. Piezoelectric sensors are sensitive to acceleration and hence are best suited for high frequency vibrations.

Applications

Following are the applications of different types of vibration sensors:
• It is used as automatic shutdown switches due to excessive vibration of machinery.
• It is used to shut down domestic appliances such as washing machines (when it is out of balance during spin cycle).
• It is used in toys and games.
• It is used as shock sensor while device is being transported.

Working principle of Vibration Sensor Types

Vibration sensors are devices designed to detect and measure vibrations or oscillations in a physical system. They are widely used in various applications viz. industrial monitoring, automotive systems, structural health monitoring and more. There are several types of vibration sensors each with its own working principles.

Accelerometer

It measures acceleration which can be used to infer vibrations. It works based on principle of Newton's second law of motion (F= m *a). When a mass within accelerometer experiences acceleration, a force proportional to that acceleration is applied to mass. This force is converted into electrical signal that can be measured.
Examples include piezoelectric, capacitive and MEMS accelerometers.

Pin and Spring

• It is simplest type of vibration sensor. It consists of small and thin pin in the center of a coil spring. The spring is anchored at its base and the other end is free to vibrate. If the vibration reaches sufficient amplitude, the spring touches the pin and completes the circuit between the two leads of the device. Figure-1 depicts one such sensor manufactured by chinese supplier Elecrow.
• Advantages: Low cost, ability to respond two out of three axes, no power supply requirement, ability to switch DC or AC.
• Disadvantages: It requires latching components such as flipflop or 555 timer as internal contact is extremely brief.

Piezoelectric vibration Sensor

This sensor works based on the piezoelectric effect, which is the ability of certain materials to generate an electric charge in response to applied mechanical stress. The basic structure of this sensor consists of a piezoelectric material (e.g. ceramic or polymer) sandwiched between two electrodes. When mechanical vibrations are applied to the sensor, the piezoelectric material experiences stress. This generates a proportional electric charge. This charge accumulates on the electrodes, creating a measurable voltage across the sensor.

Let us understand Piezoelectric vibration sensor working principle.
• It is usually made of piezoelectric polymer film laminated to a polyester substrate. The film is designed to be anchored at one end and the other end is free to vibrate. The figure-2 depicts one such sensor developed by "Measurement Specialties".
• It is available in unweighted version and weighted version (as shown in the figure). The addition of weight changes the resonant frequency of the vibration sensor.
• It generates about 7 V DC between the two leads with deflection of mere 2 mm. Larger deflection produces higher voltages.
• It is possible to connect CMOS component with the sensor and Op-Amp may be used for signal conditioning purpose.
• Surface mount version of piezoelectric vibration (Murata PKGS series) sensor is available with analog output for connection through an op-amp.
• Advantages: High sensitivity to dynamic vibrations across a wide frequency range, Compact size and light weight, rugged construction, fast response time etc.
• Disadvantages: The sensor's own mass can influence measurement in some applications, limited sensitivity to low frequency vibrations, accuracy is affected due to non-linearity and temperature variations etc.

Capacitive Sensor

It uses changes in capacitance to detect vibrations. It consists of two parallel plates with one attached to vibrating object. As the distance between these plates changes due to vibration, the capacitance between them changes. This variation in capacitance is converted into electrical signal to be analyzed or measured.

Optical Sensor

It uses light to detect vibrations. It works by measuring changes in intensity or frequency of light reflected from vibrating surface. For example, laser doppler vibrometers measure frequency shift of light reflected from moving surface to determine its vibration frequency and amplitude.

Strain Gauges

They are sensors that measure deformations of an object under stress, which can be correlated with vibrations. They work by using resistive element that changes its electrical resistance when subjected to strain. As the object vibrates, the strain gauge experiences strain which causes its resistance to change. This resistance change is converted into electrical signal to be measured.

Magnetic type

It is used to detect excessive vibration in machines or other devices which contain heavy rotating mechanical parts. These sensors may be larger in size and handle currents of 1 Ampere or significantly higher.

There are many other vibration sensor types such as vibrometer which measures vibration using laser beam and so on. Accelerometer, Velocity sensor, proximity sensor and small are also used as vibration sensors. Refer advantages and disadvantages of vibration sensor for more information on merits and demerits of the different types of vibration sensors.

Advantages and Disadvantages of other Sensor Types

Different Types of sensors and Transducers

RF and Wireless Terminologies

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