Regulation of automatic control and regulation sys

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The adjustment of the automatic control and adjustment system of wetting liquid

offset printing is based on the principle that oil and water repel each other. The wetting device of the offset press applies wetting liquid to the printing plate to protect the non graphic part (blank part) of the printing plate from ink. Correctly controlling the amount of wetting liquid applied to the printing plate and maintaining a good water ink balance are important conditions for obtaining high-quality prints. Insufficient water supply will cause dirty version; Excessive water supply will not only deform the paper and affect registration, but also aggravate the emulsification of ink. Therefore, how to ensure the supply of sufficient and uniform water, and make the printing quality achieve the best effect and control the water supply to the minimum is a key problem in offset printing technology. At present, the offset printing workshop of printing plants in China generally adopts the method of visual inspection to judge whether the amount of wetting liquid on the printing plate is appropriate based on experience. Therefore, the quality of printed matter is very unstable. The scientific method is to use instruments to measure the amount of wetting liquid on the printing plate

1 types of wetting devices

there are many types of wetting devices, which can be divided into contact and non-contact according to the way of coating wetting liquid to the printing plate; According to the way of supplying wetting liquid, it can be divided into continuous water supply and intermittent periodic water supply; According to the way of obtaining wet liquid film from printing plate, it can be divided into mechanical type, electrostatic type, condensing type and pneumatic mechanical type

2 automatic control and regulation system of wetting liquid

this paper will introduce a ① automatic control and regulation system of wetting liquid with laser as light source, which first touches the DC (automatic damping control system) that creates large water droplets. It is a device for automatic measurement, monitoring and control. Its main structure includes: laser, servo amplifier, sensor, computer, controller and display screen. There are two sensors on the top of the system, which are composed of photoelectric converters equipped with silicone photodiodes. The working principle is shown in Figure 1

The scanning head of the

system is composed of a He Ne laser, which runs parallel to the roller axis. The laser is transmitted through the optical fiber, and the light is divided into two beams by the beam splitter. A beam of light is projected onto the plate cylinder at an angle of 75 ° through the reflector. After the non image part of the plate is irradiated by the laser, if the amount of wetting liquid is small, the diffusion of reflected light will be large; If the amount of wetting liquid is large, the diffusion of reflected light is small, which is close to specular reflection. According to this principle, the silicone photodiode in sensor 1 converts the laser intensity reflected on the measuring point into an electrical signal. Sensor 2 converts the intensity of another laser beam into an electrical signal. After being processed, the two electrical signals are sent to the computer, and the computer can know the amount of wetting liquid on the printing plate by comparing the recorded intensity of the two. Because of the high brightness and good monochromaticity of the laser, it is more accurate to use the laser as the light source to measure the wetting of the printing plate while avoiding the splashing of the punched sample to hurt people

the data acquisition and control circuit of the device adopts multi-channel synchronous acquisition and time-sharing input mode. The circuit schematic diagram is shown in Figure 2. According to the program design of each test, the laser performs cyclic scanning between 1 and 16 measurement points. Each measurement process includes from the operating side of the printing machine to the driving side and its return journey

the sensor adopts 2Cu series photodiodes. This type of sensor can be used in light detectors and automatic control instruments for photoelectric conversion, photoelectric coupling, laser reception, etc. The electrical signal collected by the sensor is amplified to its specified voltage, and the optical signal amplification circuit is shown in Figure 3

the electrical signal converted by the sensor and amplified by the amplifier often contains noise signals with multiple frequency components due to the electromagnetic interference of the test environment and the influence of the sensor and amplifier itself. In this case, it is necessary to adopt filtering measures to suppress unwanted stray signals and increase the signal-to-noise ratio of the system. Here, the active hydraulic stable low-pass filter is selected. The analog multiplexer uses a 16 channel 1-out-of-1 chipset composed of two ad7501 chips. Ad582 chip is selected as the sampling/holding device, and the capture time is less than 6 μ s. Disconnection time is 200 μ s? AD574 successive comparison type 12 bit a/d converter is selected as the a/d converter? The interface between a/d converter and computer adopts 8259A interrupt controller? When the a/d converter completes a conversion, the 8259A sends an interrupt application to the computer. The computer responds to the interrupt and records the conversion results. At the same time, it compares the two data on the same measurement point to obtain the wetting liquid volume of this measurement point? The computer adjusts the rotation speed of the bucket roll motor through the driver according to the set wetting liquid volume value? The driver here adopts tda1085c general motor speed control chip

tda1085c is a general speed control chip developed by American Motorola Company? The chip changes the motor voltage by controlling the conduction angle of the bidirectional thyristor, so as to adjust the motor speed? Can the digital signal of motor speed detected by the feedback speed measuring device be used as the signal of closed-loop control system? The speed regulation system composed of tda1085c has the advantages of high speed stability, simple structure, convenient parameter adjustment of external components, low cost and so on. It can be widely used in control systems that need speed regulation

with the automatic control and adjustment system of wetting liquid, the operator can input the measured value of wetting liquid at this time as the rated value into the computer when the printing machine reaches the required wetting liquid transmission. During the operation of the printing machine, the rated value and actual value are displayed on the display screen in a graphical manner. If the difference between the test results of the two experimental machines sometimes exceeds or is lower than the predetermined tolerance range, the computer will adjust the rotation speed of the bucket roll motor to make the actual value of the wetting liquid always within the tolerance range of the rated value. (by Zhang Yinhui, Zhang Haiyan)

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