Application of Laser Displacement Sensor in Precision Thickness Measurement

Laser triangulation diffuse reflection displacement sensor has obvious advantages in thickness measurement:

- Very small measurement spot, which is the spot light area. For example, the spot area of ​​ZLDS10X series of Zhenshangyou Company is about 1mm. Compared with area-type non-contact capacitance and eddy current sensors, it has almost no requirements on the area of ​​the measured object and is suitable for measuring the thickness of very small area size;

- Farther measuring range starting distance. It is much larger than the starting distance of non-contact capacitance and eddy current sensors. For example, the measuring range of ZLDS100 laser displacement sensor of Zhenshangyou Company is 100mm, and the measuring distance can reach 1000mm. In this way, the sensor can be far away from the measured object, avoiding damage and thermal radiation of the measured object;

- It has a large measuring range. For example, the measuring range of ZLDS101 laser displacement sensor of Zhenyou Company reaches 2000mm, which is difficult for other sensors to achieve.

- It has nothing to do with the material of the object being measured, that is, metal and non-metal objects, non-transparent surfaces with diffuse reflection conditions can be measured.

Like other sensors, the following conditions must be taken into consideration to achieve accurate thickness measurement, otherwise even the best sensors will not be able to measure accurately:

1. Use a single laser displacement sensor to measure thickness

The object to be measured is placed on the measuring platform, the distance from the sensor to the platform surface is measured, and then the distance from the sensor to the surface of the object to be measured is measured, and the thickness is measured after calculation. It is required that there is no air gap between the object to be measured and the measuring platform, and the object to be measured has no warping. These strict requirements can only be achieved offline.

2. Dual laser sensors for thickness measurement

A laser displacement sensor is installed above and below the object to be measured. The thickness of the object to be measured is D=C-(A+B). Among them, C is the distance between the two sensors, A is the distance between the upper sensor and the object to be measured, and B is the distance between the lower sensor and the object to be measured. In the figure: SMR refers to the starting distance of the sensor range, and MR refers to the sensor range.

The advantage of this method for online thickness measurement is that it can eliminate the influence of the vibration of the measured object on the measurement result. However, it also has requirements for the installation and performance of the sensor.

The conditions to ensure measurement accuracy are: the two sensors must emit coaxial light beams, and the two sensors must scan synchronously (see the attached figure in this article). Coaxiality is achieved by installation, while synchronization depends on the selection of laser sensors with synchronization ends. For example, the ZLDS10X series laser displacement sensors all have synchronization ends.

Synchronous working diagram:

Asynchrony will cause large errors: if the object being measured has a vibration frequency of 20HZ and an amplitude of 1mm, if the signal is asynchronous and delayed by 1ms, it will cause an error of 125µm.

Schematic diagram of installing two laser coaxial cables:

The installation makes the two lasers coaxial, which not only ensures the thickness of the object at the same position, but also reduces the error caused by the tilt of the object. For example, when the moving direction of the object is different from the axis, when the axis is different by 1mm, the tilt of the object by 2° can cause an error of 35µm.