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The machine vision system is the main factor that affects the accuracy of component assembly. In the process of its work, the position of the PCB is first confirmed. When the PCB is transported to the placement position, the CCD installed on the head of the placement machine first recognizes the positioning marks set on the PCB to achieve the Confirmation of PCB position, after CCD confirms the positioning mark, it feeds back to the computer through the bus (BUS), calculates the patch dot position error (Ax, Ay), and feeds it back to the control system to realize the PCB identification process. After confirming the PCB position, it is followed by the confirmation of the components, including whether the shape of the component is consistent with the program, whether the center of the component is centered, and the coplanarity and deformation of the component pins. Among them, the process of component alignment is: after the placement head absorbs the components, the vision system images the components and converts them into digital image signals. The geometric center and geometric dimensions of the components are analyzed by the computer, and they are compared with the data in the control program. Compare, calculate the difference between the center of the nozzle and the center of the component at Ax.Ay and M, and feed it back to the control system for correction in time to ensure that the component pins coincide with the PCB pads.
Requirements for the application of SMT statistical technology
1)%, / positioning system
%, ＞The positioning system is the key mechanism of the placement machine and the main index to evaluate the accuracy of the placement machine. It includes, y transmission mechanism and *, y servo system. It has two functions: one is to support the placement head, that is, the placement head is installed on the% guide rail, and the guide rail moves in the y direction, so as to realize the whole process of patching in the xy direction. This kind of structure is used in general placement machines. It is more common; the other is to support the PCB bearing platform and realize the movement of the PCB in the %-y direction. This type of structure is commonly used in turret rotary head placement machines. In this type of high-speed machine, its placement The film head only makes a rotating movement, and relies on the horizontal movement of the feeder and the movement of the PCB board bearing plane to complete the placement process. There is also a type of placement machine. The placement head is installed on the * rail and only moves in the% direction, while the PCB bearing table only moves in the y direction. The two work together to complete the placement process.
There are two main types of drive mechanism, one is ball screw linear guide, and the other is synchronous toothed belt linear guide. With the reduction of SMC/SMD size and the continuous improvement of accuracy, the requirements for placement accuracy of the placement machine are getting higher and higher. In other words, the requirements for the x and y positioning systems are getting higher and higher. The x^y positioning system is guaranteed by the x^y servo system, that is, the AC servo motor drives the "inch transmission mechanism, and realizes precise positioning under the command of the displacement sensor and the control system. Among them, the accuracy of the displacement sensor plays a key role. The role of the current placement machine used in the displacement sensor has three types of circular grating encoder, magnetic scale and grating ruler. These three measurement methods can obtain high motion positioning accuracy, but only for single-axis motion position The deviation of the navigation is detected, but the orthogonal or rotation error caused by the deformation, bending and other factors of the navigation cannot be detected.
2) Z-axis positioning system
In the general placement machine, the base supporting the placement head is fixed on the guide rail, and the base itself does not move in the z direction. The z-axis control system here specifically refers to the positioning of the nozzle of the placement head during the movement, and its purpose is to meet the needs of different thickness PCBs and different height components. There are two common forms of z-axis control system: circular grating encoder-AC/DC motor servo system and cylindrical cam control system.
3) Rotational positioning of z-axis
The rotation control of the z-axis/suction nozzle of the early placement machine was realized by air cylinders and stoppers, which could only achieve 0° and 90°. Control, the current placement machine has directly installed the micro pulse motor inside the placement head to achieve. Directional high-precision control. The resolution of the micro pulse motor of Panasonic MSR type placement machine is 0.072°/pulse. It directly drives the suction nozzle device through a high-precision harmonic drive (reduction ratio of 30:1). Because the harmonic drive has the advantages of high concentricity between the input shaft and the output shaft, small gap, low vibration, etc., the actual resolution of the suction nozzle in 6 directions can be as high as 0.0024. /Pulse, improve the placement accuracy.
