Manual measurement is the most traditional way of measurements, but this kind of measurement always needs a lot of labour and time, which is inefficient and low accuracy. Therefore machines measurement can be used to instead of manual measurement, and mechanical inspection can be considered. The advantage of mechanical inspection system is that it has a low requirement of environment and low cost, but the disadvantage is that the accuracy is low and it will always cause some errors.Therefore mechanical inspection system is suitable for the small quantities productions with low requirements of accuracy. Through measuring the distance between the instrument and the bottom of the wooden pallets directly, to get the data and analyse it, thus pide substandard products. From this the labour resources can be saved greatly and the company expenses will be reduced.

3.2    Optical inspection
The idea of optical inspection system is similar to the principle of the optical alarm system. Alarms issue optical light, if there is an obstacle cut off the light, alarms will receive the signals and the sensor of alarms will detect the abnormality signals. Due to the light detector belongs to the equipment of weak signal detection, therefore while installing should be paid attention to some details, such as sensitivity, height and so on.
3.2.1    The optical alarm
Infrared alarm is one kind of  the optical alarm, it is pided into active and passive two kinds. Active infrared alarm, alarm is active emits infrared, infrared will bounce back when it encountered obstacles and be receiving by alarm probe. If a probe monitoring, the infrared is motionless, is constantly issued a red line and continue to rebound, the alarm will not alarm.When a moving object violated this invisible line, the probe will detected that there are abnormal, the alarm will beam.
Infrared alarm is sensitive to the temperature, the higher the temperature of objects infrared radiation that is stronger, when the induction to the environment in the presence of high background radiation intensity, will trigger an alarm. In thecase of abnormal temperature, also can produce false alarm condition.
The working principle of optical detectors (optical alarm) is to use optical light through the LED emitting diodes, then doing focus process by optical mirror so that the light can be transmitted longer distance and received by the sensor, an alert will be issued after the light is blocked. Optical light is a non-visible light which also can diffuse, and will be formed as a cone beam after projecting. Optical light can emit 1000 light beam constantly per second, so it is defined as pulsating optical beam.
This is a kind of detectors work by the principle of beam interruption, if detected object pass by detection devices through conveyor belt, the light cut down will cause alarm. It is always using pairs: one transmitter, one receiver. The transmitter emits one or more invisible optical light to form the warning line. While an object passed, the light will be blocked, and the signal of receiver will be changed. After analysing the data, the alarm will warn.
3.3    Vision technique
Machine vision systems use camera to instead of human eyes to make variety of measurement and judgment. This is an important branch of computer science, which combines optical, mechanical, electronics, computer hardware and software and other aspects of the technology. And also a number of areas related to computers, image processing, pattern recognition, artificial intelligence, signal processing, optical and electrical integration, etc… With the rapid development of image processing and pattern recognition techniques, the development of machine vision is also greatly promoted.
3.3.1    The introduction of vision technique
In the modern industrial automated production, include a variety of testing, production monitoring and parts identification applications, these works are normally with highly reproducible and intelligent accomplished can only be achieved by human eyes. Sometimes, however, such as precise and rapid measurement of small size, shape matching or colour recognition cannot be distinguished continuously and stably, other physical sensors are also difficult to complete. At this time, people start to considering about capture images with a CCD camera, sent data into computer or dedicated image processing module, though digital processing, according to the pixel distribution and brightness and colour information to distinguish the size, shape and colour. 木板钉子缺陷检查系统设计(4):http://www.751com.cn/zidonghua/lunwen_13067.html