Understanding Vibration Sensors: Types and Applications

Vibration sensors are devices used to measure, display, and analyze frequency, displacement, velocity, or acceleration. While various types exist, the most common is the accelerometer, which produces an electrical signal proportional to the acceleration of a vibrating component.

Other types include:

• Piezoelectric
• Pin and Spring
• Capacitive
• Optical
• Strain Gauge
• Magnetic

Vibration sensors respond to repetitive mechanical motion. Most are available with “normally open” contacts, closing when the sensor vibrates within its designed frequency range. Some sensors allow manual adjustment of frequency range and sensitivity. Mechanical sensors (switches) responding to displacement are best for low-frequency vibrations, while piezoelectric sensors, sensitive to acceleration, are ideal for high-frequency vibrations.

Applications

Vibration sensors have diverse applications:

• Automatic shutdown switches for machinery experiencing excessive vibration.
• Shutdown mechanisms for domestic appliances like washing machines (detecting out-of-balance spin cycles).
• Toys and games.
• Shock sensors during device transportation.

Working Principle of Vibration Sensor Types

Vibration sensors detect and measure vibrations or oscillations in a physical system, widely used in industrial monitoring, automotive systems, structural health monitoring, and more.

Accelerometer

Measures acceleration to infer vibrations, based on Newton’s second law of motion (F=m*a). When a mass within the accelerometer experiences acceleration, a proportional force is applied. This force is converted into an electrical signal for measurement. Examples include piezoelectric, capacitive, and MEMS accelerometers.

Pin and Spring

• The simplest type, consisting of a thin pin in the center of a coil spring. The spring is anchored at its base, with the other end free to vibrate.
• If the vibration reaches sufficient amplitude, the spring touches the pin, completing the circuit between the device’s two leads.
• Advantages: Low cost, ability to respond to two out of three axes, no power supply requirement, ability to switch DC or AC.
• Disadvantages: Requires latching components like a flip-flop or 555 timer as internal contact is extremely brief.

Piezoelectric Vibration Sensor

This sensor uses the piezoelectric effect, where certain materials generate an electric charge in response to applied mechanical stress.

The basic structure consists of a piezoelectric material (e.g., ceramic or polymer) sandwiched between two electrodes. When mechanical vibrations are applied, the material experiences stress, generating a proportional electric charge that accumulates on the electrodes, creating a measurable voltage.

• Usually made of piezoelectric polymer film laminated to a polyester substrate. The film is designed to be anchored at one end, with the other end free to vibrate.
• Available in unweighted and weighted versions (weight changes the sensor’s resonant frequency).
• Generates about 7V DC between the two leads with a mere 2 mm deflection. Larger deflections produce higher voltages.
• It is possible to connect CMOS component with the sensor, and an Op-Amp may be used for signal conditioning.
• Surface mount versions (e.g., Murata PKGS series) are available with analog output for connection through an op-amp.
• Advantages: High sensitivity to dynamic vibrations across a wide frequency range, compact size, lightweight, rugged construction, fast response time.
• Disadvantages: The sensor’s mass can influence measurement, limited sensitivity to low-frequency vibrations, accuracy affected by non-linearity and temperature variations.

Capacitive Sensor

Detects vibrations using changes in capacitance. It consists of two parallel plates, one attached to a vibrating object. As the distance between the plates changes due to vibration, the capacitance between them changes. This variation is converted into an electrical signal for analysis or measurement.

Optical Sensor

Uses light to detect vibrations, measuring changes in the intensity or frequency of light reflected from a vibrating surface. For example, laser Doppler vibrometers measure the frequency shift of light reflected from a moving surface to determine its vibration frequency and amplitude.

Strain Gauges

Measure deformations of an object under stress, which can be correlated with vibrations. They use a resistive element that changes its electrical resistance when subjected to strain. As the object vibrates, the strain gauge experiences strain, causing its resistance to change. This resistance change is converted into an electrical signal.

Magnetic Type

Used to detect excessive vibration in machines with heavy rotating mechanical parts. These sensors may be larger in size and handle currents of 1 Ampere or significantly higher.

There are many other types of vibration sensors such as vibrometers, which measure vibration using a laser beam. Accelerometers, velocity sensors, and proximity sensors are also used as vibration sensors.