What Is a Contact-Type Displacement Sensor?
A contact-type displacement sensor is a sensor that measures the distance between the tip of a sensor and an object.
It can measure the distance within the range of physical sliding. A similar example is a dial gauge that is often seen in lathe shops. This is a measuring instrument that is shaped like a rotating meter with a pointer, whereby the dial gauge visually reads the meter’s needle.
In contrast, contact-type displacement sensors can be captured as electrical information and are components that can be used in measuring instruments and FA control sensors. Other sensors with similar properties include measuring elements that can measure length using lasers or ultrasonic waves. The major difference between length measurement sensors and contact-type displacement sensors is that the value obtained by the former is an absolute value, while the result obtained by the later is a relative distance.
Uses of Contact-Type Displacement Sensors
Contact-type displacement sensors are used to incorporate information as a factor in determining surface distortion, roughness, and processing quality. They are suitable for applications where the presence or absence or shape of a non-measured object is to be measured with high precision on the order of micrometers.
1. Application for Detecting Presence/Absence
When the object to be measured comes into contact with contact-type displacement sensors, the distance between the contact points changes from the distance in the released state. Therefore, by capturing this change, it is possible to determine whether or not the object is in a predetermined position.
2. Application for Detecting the Amount of Change
For example, when measuring the roughness of a surface that has been flattened by cutting, the object comes into contact with the contact-type displacement sensors, while the distance between the contact points changes from the distance in the released state to a constant value, which is replaced by a zero point. If the surface is uneven or distorted, the contact portion of the contact-type displacement sensor will oscillate, and the value of the oscillation will be captured as the amount of change and measured as the surface roughness.
As described above, the sensor can measure the amount of change from an arbitrary reference point to the surface roughness with high accuracy in the μm range.
3. Application as Differential Detection
For example, there are cases where an object has a relatively non-uniform thickness and a long shape, and where scratches, steps, etc. are to be detected. In such a case, the thickness of the object is measured while moving it in the longitudinal direction, but if the thickness of the object is non-uniform, it is not possible to use a method where a threshold value is set by the length measurement to discriminate.
Therefore, a method to measure steps by differential detection is adopted. Contact-type displacement sensors are suited for this type of processing because the values are obtained as relative values.
Principle of Contact-Type Displacement Sensors
1. LVDT Method
In the LVDT method, an actuating transformer is built into the sensor, and physical changes in the contact area of the contact-type displacement sensors are linked to the core of the actuating transformer, causing the inductance of the transformer to fluctuate. The change in the contact area is extracted as an electrical signal.
2. Scale Method
The scale method is also known as the pulse counting method. Scale systems can be further classified into two types, namely: magnetic pulse counting systems that use magnetism and optical pulse counting systems Make use of light.
The advantage of the magnetic pulse counting method is that it does not depend on the environment, such as dust or dirt, at the work site. Also, it is possible to work without worrying about the temperature of the work site.
Other Information on Contact-Type Displacement Sensors
Advantages and Disadvantages of Contact-Type Displacement Sensors
1. LVDT Method
- Advantage: Measured values do not fluctuate.
- Disadvantage: Magnetic field may not be stable depending on the position of the iron core in the coil.
2. Scale Method
- Advantage: Not easily affected by the work site or environment.
- Disadvantage: If the contactor moves suddenly, it may not respond correctly. (Measured values may fluctuate.)