Laser displacement sensors were sparingly used in the past as they lacked speed, accuracy and flexibility. However, advanced laser displacement sensors available now are more accurate (0.02 µm repeatability), faster (392 kHz sampling rate) and highly flexible, and can be employed in a wide range of applications in the solar cell sector. A few common applications of laser displacement sensors, specifically in solar panel manufacture, are non-contact thickness, flatness, perpendicularity, and warpage.
Key Properties of Laser Displacement Sensors
Speed of a laser displacement sensor is defined as a measure of the frequency of measurements taken by a sensor on a target (sampling rate). The more the number of measurements made by the sensor the more the measurement averaging and thus a more reliable stable reading is achieved.
Accuracy is the extent of errors encountered while making measurements within the measuring range of the laser displacement sensor. This error includes the error caused by the sensor (linearity), error due to temperature fluctuations, sensor mounting errors, and so on. More accurate measurements are possible by analyzing and limiting the sources of these errors. Accuracy is influenced by resolution, which is the smallest amount of change registered by the sensor, and repeatability, which is a measurement of differences observed during multiple measurements of a target under the same conditions. To summarise, increasing the resolution and repeatability is one way of enhancing the accuracy of laser displacement sensors.
Flexibility refers to the many ways in which a sensor head can be configured. Significant savings in cost can be achieved by using a single part for many dimensional measurements.
The key limiting factor for measurement speed is the rate of collection of information from the receiving CMOS sensor. The image below illustrates a conventional sensor diagram:
The number of pixels and the clock speed are the key factors affecting the readout time. The more the number of pixels to be read out, the longer the readout time, and the slower the measurement sampling rate. The image below depicts a more advanced sensor diagram:
Here the clock pulse is increased by 4 and the pixels are read out in parallel (2x as fast). Hence, the sampling rate is increased by a factor of 8. Therefore, in the same target area in the same time, the sensor can make eight measurements compared to the one measurement that was possible previously.
Resolution and Repeatability
There are several methods for enhancing the resolution and repeatability of a laser displacement sensor. One method is to double the number of available pixels on the CMOS receiver without increasing the sensor head’s total size. However, the effective beam spot size will need to decrease so as to maintain a maximum resolution. The small beam spot can be created by upgrading the optics in the sensor head.
Each displacement sensor would give an analog output, and the analog signal will be input into some other system for evaluation. Each interface can induce noise and thus error into the system. Enhanced systems permit connecting to several sensor heads at a time. Thus, several calculations can be done within the controller and the resultant output can be sent to a data collection device.
More and more companies are increasingly turning to non-contact laser displacement sensors to enhance quality, feed information back for critical processes, identify defects early, decrease scrap and increase throughput.
With the rapid development of factory automation and enhanced focus on inspection and R&D tools, KEYENCE, as the leading supplier of sensors, measurement devices and microscopes, is developing and manufacturing innovative and reliable products that meet customer requirements in every manufacturing industry.
This information has been sourced, reviewed and adapted from materials provided by KEYENCE.
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