Application of the hottest measuring light curtain

Research on the application of measuring light curtain in fruit grading equipment

Abstract This paper introduces the application of measuring light curtain in fruit grading equipment, puts forward a new fruit grading method, and analyzes the operating principle, main technical functions and design points of equipment workers not to store the instrument in the sun for a long time

keyword measurement light curtain; Fruits; classification; Design

1 overview of fruit grading technology

the main function of grading fresh fruit is to make the specifications of products of the same grade consistent, so as to realize independent or collective packaging, so that packaged products can enter the supermarket with a unified high-quality appearance, attract consumers' attention, and enhance consumers' desire to buy

fruit grading forms include volume grading and weight grading. Since the purpose of grading is mainly to unify product appearance specifications for packaging and improve sales grade, volume grading is the most widely used. The traditional volume classification equipment includes drum classifier, three roller classifier and belt classifier. Their principles are similar. They all use hole frames or gaps of several levels to screen. When the shape of the material is slightly smaller than the pores, it can pass through or fall out by its own gravity, so as to realize classification

pore type screening classification is a simple and practical form, which not only has high production efficiency, but also can set multiple levels. However, there are some defects in this classification form, which are mainly manifested in: (1) the whole process of mechanical overturning and screening of materials will inevitably cause bruises, abrasions and crushing of the skin of fresh fruits; (2) The classification accuracy is not high, and the grade difference cannot be set too small, generally not less than 5mm. (3) It is prone to the chaos of "cascade" of size; (4) The adaptability is poor, and it is only suitable for nearly round and hard skinned fruits, such as oranges, while it should be used with caution for fruits with weak skin, such as apples, pears, peaches, persimmons, mangoes, etc

in view of this, after in-depth research, the author conceives an innovative volume classification form by using light curtain measurement technology, designs a new type of fruit classification and sorting equipment, and realizes high-speed continuous single discrimination and classification on the machine. This classification technology is especially suitable for the classification of fresh fruits with uneven shape. Compared with the traditional aperture classification method, it is a qualitative leap, because it realizes the non-contact classification form, and fundamentally eliminates the defect that the damaged fruits in the hole frame are difficult to clamp the steel wire rope test sample due to the loosening force and breaking strength, and it is accurate and high-speed

2 light curtain measurement principle

photoelectric sensor is an electronic device that senses the change of light intensity it receives, including optical system, amplifier and switching value output device. All photoelectric sensors use modulated light to eliminate the possible influence of surrounding light sources. When working, the photoelectric sensor emits light. When the detected object passes by, according to the different detection modes, the object or absorbs light or reflects light to the light receiver of the photoelectric sensor, resulting in the change of the light intensity received by the light receiver. Its change value triggers the switch signal output to realize the detection function

the light curtain for measurement is a special photoelectric sensor, which, like the ordinary opposed photoelectric sensor, includes two parts, the transmitter and the light receiver, which are separated from each other and placed relatively, but its overall size is large and it is a long tube. The detection light produced by the measuring light curtain emitter is not only a beam like an ordinary sensor, but a light array is generated at a fixed distance along the length direction to form a "light curtain". In a scanning way, it cooperates with the controller and its software to realize the function of monitoring and measuring the shape and size of the object

the light curtain measurement system needs to be equipped with a controller to work normally. The controller is programmed with software, which can provide a variety of scanning modes and detection and analysis modes. In addition, it can communicate with PLC or computer through serial interface to transmit control or scanning results. In addition, it can also access trigger signals to control the scanning process

2.1 light screen scanning mode

there are two commonly used light screen scanning modes, namely, linear scanning mode and cross scanning mode. The former is the most commonly used and simplest scanning mode. The transmitter continuously generates a fixed spacing light array, which is arranged in parallel from beginning to end, and received by the light receiver one by one

in the cross scan mode, oblique rays are staggered between the straight-line scanning rays. The first oblique scanning light beam is established between the second channel of the emitter and the first channel of the light receiver; The second oblique scanning beam is established between the third channel of the emitter and the second channel of the light receiver. And so on, until the last oblique scanning beam is established between the last channel of the transmitter and the channel before the end of the light receiver. The last channel of the transmitter is activated again after emitting the inclined light, emitting the straight-line light to the last channel of the light receiver, so as to complete the whole scanning process. The optical resolution can be improved by using the cross scanning mode. The accuracy of the middle 1/3 of the detection area is the highest, and the minimum size of the detected object can be reduced to 2/3 of that of the linear scanning

2.2 light curtain detection and analysis mode

light curtain detection and analysis mode has a variety of settings, and the most commonly used are "head and tail light blocking mode" and "head and tail light transmission mode"

head and tail light blocking mode when the object enters the light curtain area, block the light, identify the number of the blocked first beam of light through the controller, and then calculate the total number of blocked light from bottom to top until the last blocked light, accumulate the value, so as to obtain the measured direction size of the object

head and tail light transmission mode when the object enters the light curtain area, the controller recognizes the transmitted light, calculates from the first transmitted light, accumulates the values in turn until the last transmitted light, calculates the total number of transmitted light, and then obtains the measured direction size of the object

3 design of grading equipment

from the above analysis, the measurement light curtain can provide an ideal and reliable detection mode, and can realize non-contact automatic measurement of the overall dimension of the tested object. Therefore, applying this technology to fruit grading equipment will produce an ideal effect that mechanical grading is difficult to compare

the fruit grading equipment designed by the author using light curtain measurement technology is mainly composed of three parts, including the light curtain measurement and grading device of the core part, the front fruit sorting and arranging device and the rear automatic sorting mechanism

3.1 light curtain measuring and grading device

the external dimensions of common fruits are not large, small ones such as plum, litchi, longan, etc., and large ones such as apple, pear, orange, etc., and their outer diameter is generally between 20 and 100mm. Obviously, the smaller the shape of the fruit, the smaller the grade difference requirements. Taking the above-mentioned small fruit as an example, the similar grade difference is generally only 2 ~ 3mm. Therefore, the measurement light curtain suitable for fruit grading requires small size and high precision

the high-precision measurement light curtain used by this equipment has a length of 163mm (6.4 ") for both the transmitting and receiving light sensing sections, and can generate a total of 64 scanning rays with a light spacing of 2.5mm (0.1"). When using linear scanning, the minimum size of the detected object is 2.5mm, while when using cross scanning, the size of the detected object can be as small as 1.7mm, which can realize the discrimination with a grade difference of 1.7 ~ 2.5mm. Therefore, it is applicable to most fruit grading

before the tested fruits enter the light curtain area, they have been arranged in a neat single queue, which is driven by the conveyor belt in turn

the conveyor belt can adopt flat belt, which is made of polyethylene PE, polyurethane PU, silica gel Si, etc. that meet the requirements of food hygiene. Unscrew the speed valve handle in a counterclockwise direction to supply oil, so that the piston can lift away from the cylinder bottom ideally, and its bottom is supported by a smooth support plate to ensure that the flat belt is straight and does not sag. Because the surface of the flat belt is flat and elastic, it can be in flexible contact with the transported fruit, which can play a role of cushioning and protection

the transmitter and light receiver of the measuring light curtain are correspondingly arranged on both sides of the conveyor belt, and the No. 1 scanning light is level with the surface of the conveyor belt. Because the fruits are all entities with certain shapes, the "head and tail light blocking mode" is the most suitable detection and analysis mode, and the fruits entering the detection range of the light curtain are all in the same plane, so the first blocking light is No. 1 light, and the controller only needs to collect the last blocking light number to analyze the detection data and calculate the size of the measured direction of the fruit section

the fruit can be distinguished and measured in the process of flat belt conveying, which has neither additional mechanical effect nor movement such as drop collision, which can fundamentally prevent the occurrence of fruit injury in the grading process. The railings on the conveyor belt can adjust the spacing inside and outside to adapt to the size of different fruits and play the role of limiting and guiding. It must be noted that a narrow gap should be left in the handrail at the light curtain to facilitate the scanning light

since the transmitter and light receiver devices of the measuring light curtain are on both sides of the conveyor belt, the light has a certain distance from each other, which should be within the detection distance allowed by the sensor, so the width of the conveyor belt is designed to be limited. The application environment of the measuring light curtain will affect the reliability of its long-term work. When the sensor is in the state of maximum detection distance, the intensity of the received light will be weakened due to the pollution of the optical lens by air dust and other reasons, resulting in unstable work. Therefore, the best detection distance should be determined according to the excess gain curve of the selected product. The so-called excess gain refers to the measurement that the light intensity received by the photoelectric sensor exceeds the light energy required to start its amplifier. An excess gain of 1 represents the minimum energy required to start the amplifier; When the excess gain is 10, it means that the optical energy received by the light receiver is 10 times of the energy required to start the amplifier. Each type of photoelectric sensor has a corresponding excess gain curve. By selecting the correction coefficient in combination with environmental factors, the maximum reliable detection distance under different conditions can be estimated

for some fruits with stems, when entering the light curtain measurement area, the upward stems will also trigger photoelectric signals, resulting in inaccurate measurement values. Therefore, this kind of fruit must be sorted out before entering the light curtain measurement area, so that each individual is in the same state conducive to measurement. Taking Apple detection as an example, two parallel conveyor belts are configured in the measurement area, leaving a certain gap between each other. The finished apple is supported between the two flat belts, and the handle is downward to clip into the belt gap. Therefore, the interference of the handle is avoided when measuring through the light curtain, and the measured values are the dimensions of the same part in the same direction, making the detection more accurate and reliable

3.2 arrangement device

the arrangement device is in front of the light curtain measurement and classification device. Its function is to arrange the disordered fruits into a neat single line, so that the fruits enter the light curtain measurement area in turn at a certain distance, which is very important to ensure accurate measurement. For different fruits, the design of the device is different, and the targeted design must be made according to the morphological characteristics of the fruit. In general, some mutually coordinated mechanical conveying devices can be used to guide the movement of fruits to achieve the purpose, such as conveyor belts arranged horizontally or obliquely, carding baffles, centrifugal turntables, guide chutes, etc., which will not be described here one by one

3.3 sorting mechanism

the sorting mechanism is installed immediately after the measurement light curtain, and acts after receiving the measurement signal. When the tested fruit passes through the measuring light curtain, the photoelectric signal is triggered. After the controller calculates the value and compares it with the set grading size, the command is issued to drive the sorting mechanism to act and send the fruit to the appropriate grade line