What Are Contact Sensors?
Contact sensors are measuring instruments that measure the distance of the object to be measured by directly contacting a dedicated detector.
Since the distance is measured by bringing the detector into contact with the object to be measured, the advantage of contact sensors is that they are more accurate than non-contact sensors. However, it has the disadvantage of causing a small damage to the object to be measured because it is necessary to place the detector on the object.
Uses of Contact Sensors
Contact sensors are widely used in industrial applications.
The following are some examples of applications of contact sensors:
- Displacement measurement of thickness of products and test pieces
- Product inspection and position confirmation of conveying and processing equipment
- Liquid level control of water storage tanks
- Providing feedback signals for opening and closing valves and dampers
Contact sensors are typically used for contact displacement sensors. Contact sensors are mainly used to measure the thickness of products and test pieces. In addition to displacement, they may also be used to measure liquid level.
Most non-contact sensors measure distance by projecting a laser or other object and receiving the reflected light. Contact sensors have the advantage of being able to measure large inclined surfaces, which are impossible to measure with reflected light. Taking advantage of this, it is possible to measure the surface profile of a three-dimensional object.
It is also used to measure the displacement of the coarse stage on which the object to be measured is mounted and its movement is controlled, and to measure the runout when the stage is moved in one direction. It can also be used for feedback control from measurement and understanding of errors to adjustment.
Principle of Contact Sensors
Contact sensors, the most common type of contact displacement sensors, generally come with a dedicated probe. The probe has a spindle structure that mechanically extends and retracts up and down. When the probe is not in contact with anything, it is at its maximum extension by spring force.
When the probe comes into contact with the object to be measured, the probe contracts and a constant pressure is applied toward the object by the spring force. The displacement of the probe shaft at that time is detected and converted into length information. Therefore, the range that can be measured is within the range of the probe’s expansion and contraction.
The probe is often wound with a coil, and the shaft part. The shaft part, which serves as the iron core, expands and contracts. However, the expansion and contraction of the iron core depends on the position of the shaft, the impedance in the coil changes and the output changes. Since the impedance is fixed according to the position of the shaft, absolute position can be sensed.
Types of Contact Sensors
Various types of contact sensors are available. The following are examples of contact sensors.
1. Differential Transformer (LVDT) Type Displacement Sensor
This sensor converts the amount of vertical displacement generated by moving a contactor pressed against the object to be measured into an electrical signal to read the shape of the object to be measured. There is an iron core above the contactor, and the vertical movement of the contactor causes the coil impedance in the vicinity to change, resulting in an electrical signal output.
The structural feature of this sensor is that there is little jump in measurement values. On the other hand, since it uses the magnetic field of the coil, the magnetic field characteristics may not be stable depending on the position of the iron core in the coil.
2. Scale Type Displacement Sensor
Scale sensors digitally measure the amount of displacement of contact sensors. There are two types: magnetic type and optical counting type.
Magnetic Type
The magnetic type measures the displacement by detecting the vertical movement of a scale with alternating S and N poles using a magnetic sensing element. On the other hand, the optical counting type measures the displacement by projecting light onto a scale with many slits and counting the light passing through the slits with a light-receiving element.
Optical Counting Type
The optical counting type is a digital measurement method that is free from noise and can measure with high accuracy. However, rapid movement of the contactor may cause the magnetic sensing element or light-receiving element to react incorrectly, in which case the measured value will jump.
3. Limit Switches and Microswitches
Limit switches and microswitches are contact sensors that output the position of an object as a contact signal. A dog called an actuator is attached to the tip, which is driven by contact with an object to open and close the internal contacts. Simple and robust in structure, these parts are widely used in industry.
4. Float-Type Level Sensor
This sensor has a float attached, and the float moves up and down in accordance with the vertical movement of the liquid level to output the level. Because of its simple structure and low cost, it is used in many situations to control the liquid level in storage tanks. However, since the float must be floated in the tank, it is not suitable for tanks that are agitated.