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Closed Loop Control of Hydraulic Actuators in AGC Systems

The closed loop operation of hydraulic actuators are a significant advance over electromechanical, screw-down actuators used in the automatic gauge control (AGC) systems of strip rolling mills. They attain three important benefits often sought by milling managers:

¨      High power capability

¨      High accuracy

¨      High stiffness

Traditional roll gap control systems suffer from long response times, above average maintenance and inaccurate gauge or roll gap control. Hydraulic automatic gauge control (HAGC) systems overcome these inherent disadvantages with a simple system design implementing closed loop, computer controlled, hydraulic down/up cylinders that control the gap between the rolling mill’s work rolls. The result is not only the precise control of a steel sheet’s thickness, but also the possibility of high production rates.

Hydraulic Automatic Gauge Control System Components

A HAGC system generally consists of the following components:

¨      Hydraulic Cylinders

¨      Servo Valves or Control Blocks

¨      Hydraulic Power Units (HPUs)

¨      Transducer, Pressure or Position

¨      Electronic Controller

Hydraulic or roll force cylinders are installed between the top and bottom back up rolls to provide the force to increase or decrease the position or pressure to maintain the desired roll gap that maintains sheet thickness tolerances. Hydraulic cylinders can either be short- or-long stroke. Short stroke hydraulic cylinders are used with mechanical screw-down actuators to enforce roll gap controls, while long-stroke hydraulic cylinders eliminate the need of mechanical actuators. Hydraulic cylinders can be constructed as an integrated unit consisting of the cylinder, transducer and control block.

The electro-hydraulic servo valve or control block receives an input from the electronic controller to increase or decrease the fluid flow into or out of the hydraulic cylinders. The servo valve typically has a maximum hysteresis of about one percent, which enables fast responses and a tightly controlled roll gap. The transducer senses the pressure or position of the gap and feeds an electrical signal proportional to this condition to the electronic controller.

The electronic controller receives inputs from the system components to generate a signal to drive the servo valve. Finally, the hydraulic power unit provides the fluid power used by the entire HAGC system. While HPUs vary in design, they typically consist of a reservoir or tank, electric pump drive motor, relief valves, internal piping and an adequate volume of fluid to operate the system.

Description of Closed Loop Operation

The closed loop operation of the HAGC controller makes possible the tight gauge control permitting a fast dynamic response capable of adapting to system perturbations such as, varying levels of strip hardness, mechanical changes in the mill stand, or geometric deviations of the roll. The controller can be operated in either a position or pressure mode to control either the pressure or the position of the hydraulic up/down cylinder. The controller compares the transducer's position or pressure signal to the roll gap setpoint to produce an error signal. This error signal is converted into an output that drives the servo valve, which opens or closes to provide the necessary fluid power to drive the hydraulic cylinder up or down to maintain roll gap control.