Successful application of PLC in the transformation of diamond six-sided top press

Requirements for the control device of the six-sided press

The six-sided top press is a key equipment for synthetic diamond synthesis. It has the characteristics of multiple specifications and high degree of automation. In the past, it was controlled by relay-contactor mode. Its logical relationship is cumbersome and a large number of relays are used (more than 40), so the swelling rate is high. The failure of the relay action often causes the pressing block to be smashed or even damages the top hammer, which increases the consumption of raw and auxiliary materials and affects the normal operation of the equipment. In addition, the six-sided top press also has certain requirements for the positioning accuracy and synchronization performance of the six pressure cylinders. The past relay-contactor control method has the problems of slow response speed, slow action, sticky armature, poor contact, etc., which makes the positioning and synchronization performance of the six cylinders worse and increases the chance of damage to the hard and gold top hammers. Therefore, the six-sided top press has the following requirements for the control device:

1. High reliability

2. Six-cylinder positioning and synchronization control should be good

In view of the above two basic requirements, combined with the process characteristics of the six-sided top press, we use PLC to control the press to work according to the following procedures (as shown in the right figure):

PLC Model Selection

The focus of PLC model selection is nothing more than the following aspects: 1. Determine the control scale, that is, the number of I/O points; 2. Price; 3. Whether the after-sales service is guaranteed. After sufficient research and considering the convenience of future maintenance, we finally decided to use the SR-21PLC produced by the Sino-foreign joint venture Wuxi Huaguang Electronics Co., Ltd. This is a small PLC with relatively high performance and price. The maximum number of I/O points is 168 points, the maximum capacity is 1.7K~3.7K instruction words, modular structure, flexible configuration, and a variety of I/O modules and special function modules. This PLC has rich instructions, data processing functions, and can be connected to the host computer to form an industrial local network. The peripheral equipment that matches it can basically meet user requirements, with a printer interface, EPROM writer, and can be connected to a tape recorder.

Control device configuration

Based on the configuration principles established by the press process characteristics, the basic requirements for the control device and the cost of the entire device, we decided to use a PLC with 80 I/O points. In order to facilitate future operation, a programmer and printer interface unit were also configured.

Circuit Design

We connect the six definition numbers 122~127 to the proximity switch input signal, which are used as monitoring for whether the right, front and upper three-cylinder pistons are offside when moving forward in the empty stroke and whether the six cylinders (including the left, right and lower three-cylinder pistons) are running at the same time (i.e. synchronous action) when filling. The rest of the I/O is connected to buttons, travel switches, external time relays, contactors, 220V AC solenoid valves, indicator lights and other electrical components. In the actual installation process, in order to prevent electromagnetic interference, all input lines are strictly separated from strong current wires; the proximity switch input signal line uses twisted pair wires; an isolation transformer is installed on the PLC power supply side; and RC absorbers are connected to both ends of all solenoid valves and contactor coils. Considering the cost and the PLC's interference suppressor from the power supply.

Software design and application of data processing functions

1. Software design:

For the sake of convenience and space saving, we only list the automatic engineering flow chart here.

The segmented working procedure is basically the same as the automatic working procedure, except that the pressure will not be automatically released immediately after the pressure maintenance is completed. The operator needs to press the supercharger pressure relief (i.e. segmented pressure relief) button to release the pressure, and then the procedure ends. The adjustment procedure is mainly used to manually adjust the position of the six-cylinder piston.

2. Application of data processing functions

Due to space limitations, we will only illustrate here the use of SR-21 data processing instructions to prevent multiple buttons from being operated simultaneously during six-cylinder synchronous monitoring and adjustment, and will explain them one by one below.

(1) Synchronous monitoring program module

The left figure is a block diagram of the synchronous monitoring program module. The delay in the block diagram is set according to the speed of the six-cylinder piston movement response in the specific equipment. A short time requires that the six-cylinder pistons basically start moving at the same time when filling, and a long time allows the six-cylinder pistons to not start moving at the same time at the moment of filling. Due to the adjustment of the hydraulic system, the length of the high-pressure oil circuit, the friction resistance of the piston, the size of the flow rate and other factors that may affect the speed of each cylinder piston response, there will always be a phenomenon of slow movement of the piston movement of a few cylinders. When this phenomenon is more serious, it may cause the six top hammers to be not on the center line when the six cylinders are over-pressurized, resulting in a fault. The purpose of synchronous monitoring is to send out an asynchronous alarm signal when the sluggish phenomenon is more serious, and stop the six-cylinder piston movement at the same time, so that the operator can make corresponding treatment in time.

(2) Synchronous monitoring ladder diagram

The ladder diagram is shown in the figure, which is a specific application of the above flowchart. It is worth mentioning that in the ladder diagram, 772, 773, and 774 are used as the result of compare. When the six-cylinder pistons are synchronized, the comparison result is equal to the constant 63 (BCD number), and the flag relay 773 is established. Otherwise, 772 and 774 are separated, and an out-of-sync alarm is triggered.

(3) Adjust the anti-misoperation program flowchart

See the following diagram for this part of the block diagram. It should be noted that this is only the procedure for manual adjustment of the pistons of the upper, front, right and lower cylinders. Some other adjustment actions belong to different groups and have similar principles. This anti-misoperation procedure can effectively prevent the operator from pressing a certain button and preventing other groups of buttons from being pressed by mistake and causing equipment failure.

(4) Ladder diagram to prevent misoperation

Conclusion

Practice has proved that the application of PLC in the transformation of six-sided top press is fruitful. The data processing function adopted enables the designed monitoring and anti-misoperation procedures to meet the expected requirements. PLC can be used in harsh working environments without having to consider the impact of voltage fluctuations, electromagnetic interference, ambient temperature and humidity on it too much. The entire control device can operate stably and reliably.