Displacement Sensor: Working Principle and Characteristics

Displacement Sensor

Displacement Sensor Introduction

The displacement sensor, also called a linear sensor, which converts the displacement into the electricity. There are many displacement measurement methods based on sensor. Small displacements are usually detected by strain gauge, inductive, differential transformer, eddy current, and Hall sensors. While large displacements are usually measured by inductive synchronizer, grating, tolerance, and magnetic grid sensing technology.
There are many types of displacement sensors, with different applications fields. Understanding different displacement sensors will help people select more suitable ones. Here introduces the working principles and characteristics of different displacement sensors according to their types. Reading the following content can provide you with a basic knowledge of selecting displacement sensors.
300mm Linear Displacement Sensor
Fig 1. 300mm Linear Displacement Sensor

Types of Displacement Sensors

Potentiometer Displacement Sensor

Potentiometer displacement sensor, which converts the displacement into a resistance or voltage output with a linear or arbitrary function. Both ordinary linear potentiometers and circular potentiometers can be used as linear displacement and angular displacement sensors, respectively. However, potentiometers designed for the purpose of measuring displacement require a definite relationship between the displacement and the resistance change. The movable brush of the potentiometer sensor is connected with the measured object. The displacement of the object causes the resistance of the moving end of the potentiometer to change. The amount of change in resistance reflects the amount of displacement, that is, the resistance increases or decreases indicates the direction of displacement.
75mm Linear Displacement TransducerFig 2. 75mm Linear Displacement Transducer
Usually the power supply voltage is passed on the potentiometer to convert the resistance change into a voltage output. The wire-wound potentiometer changes with the turn resistance as a ladder when the brush moves, and its output characteristics are also ladder-shaped. If this kind of displacement sensor is used as a displacement feedback element in a servo system, an excessive step voltage will cause the system to oscillate. Therefore, the resistance value of each turn should be reduced as much as possible in the production of the potentiometer. Another major disadvantage of the potentiometer sensor is that it is easy to wear. While its advantages are: simple structure, large output signal, convenient use and low price.

Magnetostrictive Displacement Sensor

It mainly uses the principle of magnetostriction. That is, producing a strain pulse signal through the intersection of two different magnetic fields to accurately measure the position.

Linear Displacement Sensor

In order to convert the linear mechanical displacement into an electrical signal, the variable resistance slide rail is usually fixed, and different resistance values are measured by the displacement on the slide rail. The sensor slide rail is connected to a steady-state DC voltage, allowing a small current of microamperes to flow, and the voltage between the slide and the starting end is proportional to the length of the slide.

Angle Displacement Sensor

Angle displacement sensor is applied to obstacle handling. For example, use angle sensor to control your wheels to find obstacles indirectly.
Its principle is very simple. If the motor angle sensor is configured to operate, but the gear does not rotate, it means that your machine has been blocked by obstacles. This technology is very simple to use and very effective. But the moving wheels cannot slip on the floor (or slip too many times), otherwise you will not be able to detect obstacles. An idling gear connected to the motor can avoid this problem. The wheel is not driven by the motor but driven by the movement of the device. During the rotation of the driving wheel, if the idler stops, it means you have encountered an obstacle.

Hall Displacement Sensor

Its measuring principle is to keep the excitation current of the Hall element (semiconductor magnetic sensor) constant, and to move it in a uniform gradient magnetic field. The displacement is proportional to the output Hall potential. The greater the magnetic field gradient, the higher the sensitivity; the more uniform the gradient change, the closer the relationship between Hall potential and displacement is linear.

Photoelectric Displacement Sensor

It measures the displacement or geometric size of the object according to how much the measured object blocks the luminous flux. It is characterized by non-contact measurement and continuous measurement. Photoelectric displacement sensors are often used for continuous measurement of wire diameter or as edge position sensors in strip edge position control systems.

Wire-wound Displacement Sensor

It is made by using constantan wire or nickel-chromium alloy wire as a resistor body and winding it on an insulating frame. Wire-wound potentiometers are characterized by small contact resistance, high accuracy, and low temperature coefficient. Its disadvantages are poor resolution, low resistance, and poor high-frequency characteristics. Mainly used as voltage divider, rheostat, zero adjustment and working point in the instrument, etc.

Conductive Plastic Displacement Sensor

Use a special process to cover the DAP (dipropylene phthalate) resistance slurry on the insulating body, heat and polymerize it to form a resistance film, or press the DAP resistor powder into the groove of the insulating substrate to form a solid body. The characteristics are: good smoothness, excellent resolution, good wear resistance, long life, low dynamic noise, high reliability, and chemical corrosion resistance. It is widely used in space installations, missiles, aircraft radar antennas servo systems, etc.

Metal Glass Uranium Displacement Sensor

The metallic glass uranium resistor paste is coated on the ceramic substrate by screen printing according to a certain pattern, and then it is sintered at high temperature. The characteristics are: wide resistance range, good heat resistance, strong overload capacity, moisture resistance, abrasion resistance, etc. The disadvantage has large contact resistance and current noise.

Sensor Characteristics

Metal Film Displacement Sensor

The resistor body of the metal film sensor can be composed of alloy film, metal oxide film, metal foil, etc. respectively. It is characterized by high resolution, high temperature resistance, low temperature coefficient, low dynamic noise and good smoothness.

Photoelectric Displacement Sensor

It eliminates mechanical contact, and has the advantages of long life and high reliability. While the shortcomings include digital signal output and cumbersome processing.

Magnetic Displacement Sensor

It eliminates the mechanical contact, and featured with long life and high reliability. While has the shortcomings of higher requirements for the working environment.
KPC Linear Displacement Sensor

Fig 3. KPC Linear Displacement Sensor

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