Abstract: This paper introduces a hoist speed control optimization system implemented using a PLC and a fully digital speed control device—a hoist speed control system based on torque following switching. The advantages of this method over two commonly used hoist speed control systems in practice are analyzed from the perspectives of speed and reliability.
Keywords: hoist; fully digital system; optimization
1. Introduction
The hoist speed control system is the core of the material feeding system and plays a crucial role in industrial production. Its performance and reliability directly impact production efficiency and economic benefits. The performance of the hoist speed control system primarily refers to the acceleration and deceleration time during operation; the shorter the acceleration and deceleration time, i.e., the faster the acceleration and deceleration rate, the better the system performance. However, reliability must be the prerequisite. The reliability of the hoist speed control system mainly depends on the smoothness of speed and torque changes during acceleration and deceleration. Otherwise, it will cause significant damage to the hoisting wire rope, reduce its service life, and may even lead to the wire rope breaking and serious accidents. When designing a hoist speed control system, we must ensure reliability first before considering how to improve the system's performance.
2. Operation process of the hoisting system
This example uses the hoist speed control system of an ironmaking plant. Its task is to continuously transport iron ore and coke separately from the charging pit area to the blast furnace for unloading. Because the charging car is a potential energy load, when it stops, the car accelerates downwards while simultaneously dragging the motor in reverse, so a brake must be used. First, the opening torque is established, the brake is released, and the car runs at a small acceleration for time t1. Then, a second acceleration is performed to reach the rated maximum speed, and then it runs at that speed at a constant speed until t2, when it enters the first deceleration zone and the car slows down. At t3, it enters the second deceleration zone, and the car speed continues to decrease, running at a lower speed. At t4, the brake is released, and the car speed drops to zero. The operating curve of the hoist speed control system of the ironmaking plant is shown in Figure 1.
Figure 1. Operation curve of the hoisting system
In the diagram, n represents the speed of the hoist motor, and t represents the running time.
3. Analysis and Optimization of the Hoist Speed Control System
Currently, there are two common types of hoist speed control systems in practice: one is a direct dual closed-loop control of speed and torque; the other is a dual closed-loop control system of speed and torque with torque-speed switching, which we will refer to here as a conventional switching system. The following is an analysis of these two systems.
3.1 Hoist Speed Control System with Dual Closed-Loop Control of Speed and Torque
In the dual-loop speed and torque control system, the speed setpoint, after being processed by the speed regulator (i.e., the speed loop PI regulator), outputs as the setpoint for the torque (current) regulator. The speed setpoint increases from zero after being processed by the ramp function generator. Therefore, the torque (current) regulator setpoint also starts from zero. This results in the output torque being less than the load torque for a period of time initially, causing the material trolley to experience a descent and subsequent ascent. The operating curve of the hoist speed control system with dual-loop speed and torque control is shown in Figure 2.
Figure 2 shows the operating curves of the hoisting system with dual closed-loop torque control.
Where T represents torque, n represents the speed of the hoist motor, and n(T) indicates that torque and speed are expressed in the same coordinate system. TL is the load torque, curve 1 is the output torque curve, and curve 2 is the actual speed curve. It is easy to see from the graph that when the system's output torque is less than the load torque, the actual speed decreases. When the system's output torque equals the actual load torque of the material cart, the speed decrease reaches its maximum, after which the material cart begins to accelerate in the opposite direction, with the speed changing from negative to positive. This results in a longer acceleration time from zero speed to the rated maximum speed, reducing system performance; furthermore, oscillations occur during speed regulation, reducing system reliability.
3.2 Hoist Speed Control System with Conventional Switching Control
In a conventional switching control hoist speed regulation system, the opening torque must be established in torque mode before the brake is released. This opening torque should be slightly larger than the load torque. After the opening torque is established, the brake is released and the system switches to a dual closed-loop control mode of speed and torque (current). When switching directly to the dual closed-loop mode, since the speed loop setpoint processed by the ramp generator still starts from zero, the output of the speed regulator also starts from zero. In the dual closed-loop system, the output of the speed regulator is the input of the current regulator, meaning the current regulator's output also starts from zero. The torque will inevitably undergo a process of first decreasing and then increasing, which also causes speed fluctuations. The operating curve of the conventional switching control hoist speed regulation system is shown in Figure 3.
Figure 3. Operating curves of the hoist speed control system with conventional switching control.
Where T is the torque, n is the speed of the hoist motor, and n(T) represents the torque and speed in the same coordinate system. TL is the load torque, T0 is the opening torque, t1 is the opening time, and at t2, T0=TL. Curve 1 is the output torque curve, and curve 2 is the actual speed curve. As shown in the figure, when the hoist is opened at time t1, the torque drops rapidly from T0 and then rises again. When the output torque drops from T0 to TL, the trolley is lifted, and the speed gradually increases; when the output torque drops below TL, the speed begins to decelerate in the forward direction, and may even reverse. When the output torque increases back to TL, the speed of the trolley reaches a minimum, and then the speed gradually increases. Clearly, during the transition process, the speed undergoes forward acceleration, forward deceleration, possible reverse acceleration and deceleration, and then another forward acceleration, resulting in a long transition time and reduced system performance. Furthermore, the speed regulation process may experience one or even two oscillations, affecting the reliability of the system.
3.3 Hoist Speed Regulation Optimization System—Dynamic Torque Following Switching System
From the analysis of the two systems above, we can see that to ensure system reliability, the torque and speed transition process should be smooth and free of oscillations. Using a dynamic torque follower switching system effectively solves the oscillation problem during the transition process, thus significantly optimizing the hoist speed control system.
The dynamic torque following system, like the conventional switching system, operates in torque mode before brake release to establish the opening torque T0. The difference is that after tension is established, the system accelerates the hoisting under the opening torque with a smaller acceleration. When the actual hoisting speed reaches 2% of the rated speed, dynamic torque following switching occurs, switching to a dual closed-loop control mode. During switching, the output of the torque regulator is set to T0 as the initial value to achieve a smooth, oscillation-free operation. The operating curve of the hoisting speed control system with dynamic torque following switching control is shown in Figure 4.
Figure 4. Operating curve of the hoist speed control system with dynamic torque following switching control.
Where T represents torque, n represents the speed of the hoisting motor, and n(T) indicates that torque and speed are expressed in the same coordinate system. TL is the load torque, T0 is the opening torque, t1 is the opening moment, t2 is the dynamic torque following switching moment, curve 1 is the output torque curve, and curve 2 is the actual speed curve. As can be seen from the motion curves, during opening, the switching is smooth, and both torque and speed increase smoothly. Performance and reliability have been improved.
4. Hardware and software design for optimizing the hoist speed control system
This paper uses a Siemens 300 series PLC and a Siemens fully digital speed controller to optimize the hoist speed control system. The PLC controls the opening and closing of the gate, the start and stop of the fully digital speed controller, provides the main setpoint to the fully digital speed controller, and sends dynamic torque following switching commands.
When the start command arrives, the fully digital speed controller operates in torque mode, with the main setpoint serving as the torque setpoint, valued at T0. After the brakes are released, the speed begins to increase at a relatively small acceleration under the influence of T0. Once the preset value of 2% of the rated speed is reached, the PLC sends a switching command. Under this command, the fully digital speed controller switches into the speed loop; it initializes the speed loop PI controller, which in turn initializes the input value of the torque loop PI controller. This initial value should be equal to T0, ensuring that the initial torque is T0 during switching. A deviation of +3% of the rated speed is added to the speed loop feedback comparison circuit. Because this value is greater than 2% of the rated speed of the actual operating speed, it ensures a smooth increase in the speed loop output, thus achieving dynamic torque-following switching.
5. Conclusion
The dynamic torque-following switching control hoist speed regulation system is an intelligent system that effectively achieves functions that ordinary control systems cannot. It significantly improves both the performance and reliability of hoist speed regulation systems. In practical applications, it can greatly enhance enterprise production efficiency and economic benefits while reducing safety accidents.
References
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About the author: (Chai Lin, male, born in 1979, research interests include novel electric drives and intelligent control)
