What Is a Linear Displacement Transducer (LDT)?
A linear displacement transducer (LDT) is a device that uses a laser to measure the position and distance of an object.
A laser beam is irradiated onto the object’s surface, and the displacement of the object is detected and measured. It is a non-contact, highly accurate displacement measurement.
Another feature of this device is its high-speed detection capability. Since laser beam irradiation and light detection occur almost simultaneously, displacement can be measured instantaneously. This makes it ideal for analyzing high-speed movements and vibrations.
Applications of Linear Displacement Transducer (LDT)
Linear displacement transducers (LDT) are used in various applications due to their non-contact, high-precision measurement capability.
1. Material Deformation and Stress Analysis
By measuring minute displacements and deformations when a force is applied to an object, the elastic properties and rigidity of the material can be evaluated.
2. Vibration Analysis of Structures Such as Buildings and Machines
By measuring vibration frequencies, vibration modes, and resonance phenomena of objects, the stability and durability of structures can be evaluated. It also plays an important role in nanoscale research.
By measuring the vibration and displacement of materials and devices, their properties and operating principles can be analyzed.
Principle of Linear Displacement Transducer (LDT)
Linear displacement transducer (LDT) uses triangulation as its measuring principle. The main components consist of a light emitter and a light receiver. The light emitter consists of a light emitter and a light emitting lens, while the light receiver consists of a linear image sensor and a light receiving lens. These parts are used to survey the object and detect displacement with high accuracy.
First, light from the projection element is focused by the projection lens and projected onto the object. The object reflects the light, and the light reaches the linear image sensor through the light-receiving lens of the light-receiving part. This creates a spot of light on the linear image sensor, which detects the position of the object.
As the object moves, the light spot on the linear image sensor moves and the displacement of the object is detected by sensing the amount of movement. In addition to the linear image sensor, a position-sensitive device(PSD) may also be used.
Types of Linear Displacement Transducers (LDT)
There are various types of linear displacement transducers (LDT), the most common of which are dimensional, displacement, and geometrical measurements.
1. Dimensional Measurement
For dimension measurement in a stationary state, such as thickness measurement, step measurement, inner/outer diameter measurement, and so on. It measures the distance to the reference surface and the distance to the measurement surface, and outputs the difference as dimensions.
2. Displacement Measurement
Linear displacement transducer (LDT) is used for dynamic measurements such as amplitude measurement of a vibrating object or runout measurement of a rotating object. The distance to the measurement point is measured at a fixed sampling cycle, and the displacement is calculated from the maximum and minimum values and output as a displacement value.
3. Shape Measurement
Measures the flatness of a surface or the curvature of a surface. It measures the distance between multiple points and outputs the shape of the measurement range based on the coordinates and measured values of the measurement points. For this reason, many of them have a drive mechanism.
When the sensor head is attached to the drive mechanism, it is less rigid than a fixed sensor head and is therefore more susceptible to disturbances such as vibration. For high-precision measurement, the sensor head is generally mounted on a rigid fixed stand and the measurement object is moved.
Other Information on Linear Displacement Transducer (LDT)
Accuracy of Linear Displacement Transducer (LDT)
The accuracy of a laser displacement sensor depends on the sensor’s characteristics and the external environment. Accuracy is calculated by taking into account not only resolution, but also linearity, temperature characteristics, and other factors.
If the resolution is ±1 μm, linearity is ±0.2% F.S (measurement range 20 mm), and temperature characteristic is ±0.08% F.S/°C (measurement temperature range 1°C), linearity is 20 mm x 0.1% = 0.02 mm and temperature characteristic is 20 mm x 0.08 % = 0.016 mm. The sum of these values gives a measurement accuracy of ±0.037 mm. Therefore, measurements in microns are not possible.
Thus, even if the resolution is high, accurate measurement is not possible if the linearity or temperature characteristics are poor. For high-precision measurement, it is preferable to use a sensor with a small measurement range and good linearity and to measure in an environment with no temperature variation.
In addition, it is necessary to select a sensor that matches the condition of the object to be measured, since the measured value may change depending on the surface condition of the object.