The safety brake is a very mature and reliable product. What kind of control method is used for the safety brake to ensure its accurate and error-free action, so that it does play a safety protection role in an accident state, and puts forward higher requirements for electrical control, That is, the reliability and accuracy of the control is very important.
1.The role of safety brakes on casting cranes
Foundry cranes are used in the metallurgical industry and their task is to transport molten steel to smelting furnaces. There are generally five major institutions: the main hoist, the sub hoist, the big car, the main car, and the sub car. The motor is installed on the high-speed shaft, and the reel is installed on the low-speed shaft. The high-speed shaft and the low-speed shaft are connected together through a reducer. The traditional brake of the lifting mechanism is located on the high-speed shaft, and is called the 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, a brake is installed on the drum, which is called a safety brake. In the case of transmission link faults and overspeed faults, the running brake is activated first, and the safety brake is delayed. 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 method
(1) The reel is equipped with an overspeed switch. When the lifting mechanism fails and the reel is overspeed, the safety brake is activated. The advantage is that the control principle is relatively simple and the debugging is convenient. The disadvantage is that the reel only works when the speed is over, and the safety brake action damages the mechanical structure.
(2) The transmission ratio of high and low speeds is constant, and the constant is continuously detected by the device. When this constant changes, it means that the mechanical transmission link is broken and the safety brake is immediately activated. The advantage is that it can be judged at the early stage of the occurrence of the failure, so that the safety brake is activated, and the safety brake action will damage the mechanical structure less. The disadvantage is that the control principle is more complicated, and professional electrical technicians are needed for debugging.
3.Control hardware composition of safety brake
(1) 2 encoders mounted on 2 reels.
(2) 2 encoders mounted on 2 main hoisting motors.
(3) Safety brake control screen (or cabinet) (installed with: programmable controller, circuit breaker, relay, contactor, regulated power supply, terminal block, etc.).
(4) Shielded cable from control panel to 4 encoders.
(5) Emergency stop button and fault reset button, safety brake release indicator, encoder fault indicator, etc. installed on the console.
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 fails to operate, the user will doubt and lose confidence in the role of the safety brake. In order to ensure that the safety brake does not malfunction, the selection of components and electrical installation are particularly important when the programmable controller program is completely error-free and can work normally.
4.1 Electrical installation
(1) There is no breakpoint in the middle of the shielded cable from the control screen to the 4 encoders. If there is a breakpoint, shield the breakpoint.
(2) The shielding layer of the shielded cable from the control screen to the four encoders needs to be grounded at both ends and must be the same ground. To ensure the same ground, the main car should be equipped with a grounding device. Keep the main car and the big car in the same place.
(3) Calculate the length of the shielded cable and confirm that the transmission distance of the encoder pulse signal is within the valid range.
(4) Use different shielded cables for the signal and power lines of the 4 encoders. The signal lines and various power lines on the crane should 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 terminal, and is directly connected to the programmable controller input module.
(7) If a mechanism of the crane is equipped with a frequency converter or other relatively noisy equipment, it should be equipped with a line reactor or other device to minimize interference with the safety brake control system.
4.2 . Component selection
(1) The selection of the encoder must match the programmable controller. For the output method, select the push-pull output method. The environment of the steelmaking plant is relatively poor, the iron dust is very large, and the encoder should choose a relatively high protection level value. The output voltage of the encoder must match the value of the supply voltage.
(2) The programmable controller must have 4 independent high-speed counters, and the high-speed counting capacity of the high-speed counters can 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 is best to reach 1.5 to 2 times the actual required counting capacity, so as to ensure that the programmable controller program works normally.
5.PLC control system principle
(1) PLC control system
This system uses Siemens S7-200PLC for control. It has extremely high cost performance, high reliability, rich instructions, strong real-time communication capabilities, easy to master, easy to operate, rich built-in integrated functions and expansion modules.
(2) PLC programming
Structured programming of the program, the control tasks are decomposed into small tasks that can reflect the process technology, functions, or can be used repeatedly. These tasks are represented by the corresponding program parts, which are known blocks. To make the user program work, the user program The block must be called, and the block call instruction can only be written and started in logic. In order to make the program work properly, pay attention to the following points when compiling:
① Make interrupt procedures as short and simple as possible. And do not delay too long for other processing when executing the interrupt routine;
② The sequence of activation, execution, and exit of several interrupt programs should be calculated;
③ There must be a certain margin when comparing.
The system’s control of the safety brake has been used in production practice, and has played a protective role in the transportation of molten steel. It has avoided equipment and property damage in the event of an accident, prevented the occurrence of personal injury and death. The control application is accurate, reliable and good.