Application of PLC in braking part of the hottest

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The application of PLC in the braking part of casting crane

safety brake is a very mature and reliable product. What control mode should be adopted for the safety brake to ensure its accurate action, so that it can really play a role in safety protection under accident conditions, and puts forward higher requirements for electrical control, that is, the reliability and accuracy of control are very important

1 function of safety brake on casting crane

casting crane is used in metallurgical industry, and its task is to transport molten steel for smelting furnace. Generally, there are five mechanisms: main hoist, auxiliary hoist, crane, main trolley and auxiliary trolley. The motor is installed on the high-speed shaft, the drum is installed on the low-speed shaft, and the high-speed shaft and the low-speed shaft are connected together through the reducer. The traditional brake of lifting mechanism is set on the high-speed shaft, which is called working brake. In this case, if a transmission link between the high-speed shaft and the low-speed shaft fails, the main lifting brake will lose its effect on the drum. In view of this situation, the brake is installed on the drum, which is called safety brake. In case of transmission link failure and overspeed failure, the operating brake will act first, and the safety brake will delay action. Ensure that the lifted liquid metal ladle can be protected and safely and reliably braked in the event of an accident

2 safety brake control mode

(1) an overspeed switch is installed on the drum. When the lifting mechanism fails and the drum is overspeed, the safety brake acts. The advantage is that the control principle is relatively simple and the debugging is convenient. The disadvantage is that the drum only works when it is overspeed, and the action of the safety brake will cause great damage to the mechanical structure

(2) the transmission speed ratio of high and low speed is constant, which can be continuously detected by the device. When this constant changes, it means that the transmission link of the machine is destroyed and the safety brake acts immediately. The advantage is that it can be judged at the early stage of the fault, which makes the safety brake act, and the action of the safety brake has less damage to the mechanical structure. The disadvantage is that the control principle is complex, and professional electrical technicians are required for debugging

3 control hardware composition of safety brake

(1) two encoders installed on 2 drums

(2) two encoders installed on the 2 main lifting motor

(3) safety brake control panel (or cabinet) (installed with: programmable controller, circuit breaker, relay, contactor, regulated power supply, wiring terminal, etc.)

(4) it is convenient to control the shielded cable from the screen to the four encoders

(5) emergency stop button and fault reset button installed on the console, safety brake release indicator light, encoder fault indicator light, etc

4 electrical installation and component selection

the malfunction of the safety brake affects the normal use of the crane and may cause greater losses. If the safety brake often malfunctions, the user will doubt the function of the safety brake and lose confidence. In order to ensure that the safety brake will not malfunction, when the program of the programmable controller is completely correct and can work normally, the selection of components and electrical installation is particularly important

4.1 electrical installation

(1) there is no breakpoint in the middle of the shielded cable from the control panel to the four encoders. Steel 1 is the most important automotive material. If there is a breakpoint, the breakpoint should be shielded

(2) the shielding layer of the shielding cable from the control panel to the four encoders should be grounded at two ends, and it should be guaranteed to be the same ground. In order to ensure the same ground, the main trolley should be equipped with grounding device. Keep the main trolley and the crane on the same ground

(3) calculate the length of shielded cable and confirm that the transmission distance of encoder pulse signal is within the effective range

(4) different shielded cables are used for the signal line and power line of the four encoders. The signal line and various power lines on the crane shall be arranged separately

(5) the regulated power supply is installed near the programmable controller and grounded

(6) the signal line is not connected to the wiring terminal, but directly connected to the programmable controller input module

(7) if a mechanism of the crane is equipped with a frequency converter or other equipment with strong interference, it should be equipped with an incoming and outgoing reactor or other devices to minimize the interference to the safety brake control system

4.2 component selection

(1) the selection of encoder should match the programmable controller. The output mode should be push-pull. The environment of the steel plant is relatively poor, and the iron dust is very large, so the encoder should choose a relatively high protection level value. The output voltage of the encoder should match the supply voltage

(2) the programmable controller should have four independent high-speed counters, and the high-speed counting capacity of the high-speed counter should meet the requirements. This needs to be determined by calculation. In actual use, it is found that the high-speed counting capacity of the high-speed counter should preferably reach 1.5 ~ 2 times the actual counting capacity, so as to ensure the normal operation of the programmable controller program

5plc control system principle

(1) PLC control system

the system uses Siemens S7-200PLC for control, which has high cost performance, high reliability, rich instructions, strong communication ability with real-time characteristics, easy to master and operate, rich built-in integrated functions and extended hydraulic oil viscosity index is generally higher than 90 2.2 modules at operating temperature and pressure. The principle block diagram of PLC control system is shown in Figure 1

(2) PLC programming

the structured programming of the program, and the control task is decomposed into small tasks that can reflect the process process, functions or can be used repeatedly. These tasks are represented by the corresponding program parts, that is, known blocks. In order to make the user program work, the blocks that make up the user program must be called, and the block call instructions can only be written and started in logic. In order to make the program work normally, pay attention to the following points when compiling:

① try to make the interrupt program short and simple. And do not delay other processing too long when executing the interrupt program

② the working sequence of activation, execution and exit of several interrupt programs should be calculated

③ a certain margin should be left during comparison

the system has been used to control the safety brake in production practice, which plays a protective role in the transportation of molten steel, avoids the increasingly high requirements of equipment and all walks of life for materials and property losses in the state of accidents, and prevents the occurrence of personal injury and death accidents. The control application of the safety brake in production is accurate, reliable and good

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