The drive control system of a reversible cold rolling mill must handle frequent starts, stops, and changes in rolling direction across multiple passes. It also needs to coordinate motor speed, torque, and strip tension throughout the rolling process. These requirements become more demanding when rolling ultra-thin strip, where drive response and tension stability directly affect strip running conditions and product quality.
The main design considerations include load balancing between the main mill rolls, tension control for the uncoiler and coiler, main drive component selection, and dynamic response optimization.
Main mill load balancing control
The main mill uses separate upper and lower drive motors to operate the work rolls. The two rolls must maintain synchronized rotational speeds and share the rolling load properly. A speed difference between them can cause uneven load distribution, motor overload, strip slippage, and variations in product thickness.
Load balancing is achieved through several control functions:
Upper and lower roll synchronization: The electrical control system coordinates the rotational speeds of the upper and lower rolls to reduce the effect of speed differences on rolling load distribution.
Dynamic load adjustment: The system monitors the torque or current of both motors in real time and uses a closed-loop control algorithm, such as PI regulation, to adjust motor output and compensate for mechanical transmission errors. For example, when the upper roll carries a higher load, the system can reduce its torque command and increase the torque command for the lower roll to bring the loads back into balance.
Master-slave synchronization: A master-slave control strategy can be used, with the upper roll assigned as the master axis and the lower roll as the slave axis. Their operation is coordinated through speed or torque synchronization.
Uncoiler and coiler tension control
The uncoiler and coiler handle strip unwinding and winding. Their tension control directly affects strip stability during rolling. Excessive tension or significant fluctuations can interfere with strip movement and affect rolling quality. The control system therefore needs to establish tension reference values according to material thickness, width, and rolling pass, then apply dynamic compensation during operation.
Tension control methods
Uncoiler and coiler tension control can be divided into indirect tension control and direct closed-loop tension control, depending on the detection method and control principle.
Indirect tension control uses an open-loop torque control mode. It is suitable for applications where coil width and specific gravity remain fixed and system data is accurate. This method does not require tension sensors, but it relies on coil diameter calculation and rotational inertia estimation.
Direct closed-loop tension control mainly uses two feedback methods:
Load cell feedback: A tension sensor directly measures strip tension and sends the measurement to the control system. The system uses this feedback to correct the torque reference value. Accurate coil diameter calculation is required to match the torque output to the required tension.
Tension roll position feedback: The position signal of a tension roll is used as the feedback value, while the drive remains in speed control mode. This method is suitable for applications requiring high tension control accuracy.
Tension control compensation functions
In addition to tension measurement and regulation, the control system must account for changes in coil diameter, rotational inertia, and mechanical transmission conditions.
Coil diameter calculation: The system calculates the coil diameters of the uncoiler and coiler in real time. These values are used for torque compensation and tension control.
Rotational inertia compensation: The system adjusts acceleration and deceleration torque according to changes in coil diameter to reduce sudden tension variations during speed changes.
No-load compensation: This function compensates for the effects of mechanical friction, transmission backlash, and other nonlinear factors on tension control.
Tension control accuracy requirements
The control system must meet separate accuracy requirements under steady-state and dynamic operating conditions. The specified design values are as follows:
| Control parameter | Interstand tension | Coiling tension |
|---|---|---|
| Steady-state tension accuracy | ≤1.0% | ≤±2.0% |
| Dynamic tension accuracy | ≤±2.0% | ≤±4.0% |
Main drive system selection
Selection of the main drive system requires consideration of the mill's load characteristics, speed range, and starting and stopping frequency. The motor, gearbox, and supervisory control system must also be compatible.
Motor selection
AC variable-frequency motors are generally preferred for the main drive. They have low dynamic inertia, which allows faster acceleration and deceleration response. They are also relatively easy to maintain because they do not require carbon brush replacement.
Motor selection should be based on the actual load characteristics of the mill. Peak torque and overload capacity need to be checked to ensure that the motor power matches the required load.
Gearbox ratio and speed matching
The gearbox ratio must be determined according to the motor speed and the required mill operating speed. During the design process, the gearbox supplier and mechanical design team should work together to optimize the transmission ratio. The aim is to keep the motor operating within its efficient range while meeting the mill's speed requirements and frequent starting and stopping cycles.
Reversible cold rolling mills typically operate at relatively low speeds but require repeated starting, braking, and direction changes. Motor and gearbox selection should therefore account for these operating conditions rather than relying solely on rated speed.
Control system compatibility
The drive control system must be compatible with the supervisory control system, such as a PLC. The communication protocols need to be checked during system design to ensure real-time data exchange for speed, torque, tension, and equipment status.
Drive control method selection
Vector control and direct torque control are two commonly used motor control methods for reversible rolling mills. They differ in control principles, dynamic response, and application requirements. Selection should be based on the mill's operating conditions.
Vector control
Taking Siemens control systems as an example, vector control separates the motor torque and flux components to achieve control performance similar to that of a DC motor.
This method is well established and provides stable operation. It is suitable for rolling mill drive systems that require a wide speed range and moderate dynamic response.
Direct torque control
Taking ABB control systems as an example, direct torque control regulates motor flux and torque directly without coordinate transformation.
It provides accurate current control and fast dynamic response, with response times reaching the millisecond range. This makes it suitable for reversible rolling mills that require high control accuracy and rapid starting and stopping.
Selection criteria
The choice between these methods depends on the rolling process, dynamic performance requirements, and equipment cost. Vector control is suitable for a wide range of conventional drive applications. Direct torque control is more appropriate for applications with demanding dynamic response and control accuracy requirements.
Dynamic response optimization
A reversible cold rolling mill must accelerate, decelerate, and change direction repeatedly between rolling passes. If the drive control system cannot respond quickly enough, strip tension may fluctuate and affect strip running stability. Acceleration, deceleration, and reversal therefore require specific control strategies.
Acceleration and deceleration control
The control system should use smooth acceleration and deceleration curves to prevent sudden changes in strip tension. Feedforward compensation can also be applied to adjust torque commands in advance according to the required acceleration or deceleration, offsetting the effects of inertial loads on system operation.
Reversal control
When the mill changes rolling direction, the control system uses rapid braking followed by reverse startup. The control sequence should minimize the interruption between the two rolling directions.
The braking resistor capacity and regenerative energy feedback strategy must be selected appropriately to handle the energy generated during braking and reduce unnecessary heat loss.
System redundancy and operational reliability
Motors, variable-frequency drives, and other key components directly affect the mill's operating continuity. Depending on system requirements, critical equipment can use hot standby or parallel configurations to reduce the risk of a complete shutdown caused by a single-point failure.
Fault diagnosis should also be included in the control system design. Sensors can collect operating data such as vibration and temperature, allowing the system to monitor mechanical and electrical faults in real time. When an abnormal condition is detected, the corresponding protection measures, such as emergency braking, can be triggered.
Energy efficiency and equipment maintenance
Energy losses occur during frequent acceleration, deceleration, and direction changes. Appropriate energy recovery and maintenance strategies can help reduce operating energy consumption and improve maintenance planning.
Regenerative energy feedback
During braking, the motor generates regenerative energy. An energy feedback system can return this energy to the power grid, reducing energy losses during braking and lowering overall system energy consumption.
Preventive maintenance
Operating data from key components such as motors and gearboxes can be collected and analyzed to assess their condition and estimate service life. This information provides a basis for maintenance planning. Scheduling preventive maintenance according to operating data helps reduce unplanned downtime compared with waiting for component failures before carrying out repairs.
Custom precision machining for industrial equipment
The drive control system of a reversible cold rolling mill involves motor drives, gear transmissions, and mechanical connections. Some non-standard mechanical components require custom machining to meet specific equipment structures and assembly requirements.
Hansheng Automation provides drawing-based custom precision machining for non-standard mechanical components. Customers can submit their design drawings for manufacturing according to the specified requirements. Our machining services cover more than rolling mill and transmission equipment. We also manufacture precision components for tobacco and packaging machinery, filling equipment, yacht equipment, and remote-controlled racing cars. Dimensions, materials, and machining processes are determined according to the supplied drawings and component requirements.
