UBC Theses and Dissertations
Computer control of a hydraulic press brake Lane, John Alexander
Hydraulic press brakes are widely used in industry to form sheet metal into various shapes using bending operations. When the press brake is controlled by a Computer Numerical Control (CNC) unit, the desired bend can be achieved without the use of mechanical gages or manual adjustments, leading to increased flexibility and accuracy in the manufactured components. In this thesis, a single axis hydraulic press brake is retrofitted for dual axis CNC control. The ram is positioned using two parallel hydraulic cylinder type actuators. One servo-valve and amplifier is dedicated to each actuator so that the motion of each axis can be independently controlled by the CNC unit. Each actuator is instrumented with linear optical encoders which provide feedback for closed-loop servo position control. The hydraulic system is modified to provide constant pressure fluid power to the servo valves. An accumulator is used between the pump and the valves to suppress pressure fluctuations. Pressure transducers are integrated into the ports of the actuator to monitor the pressure during the operation of the press. The dynamics of the hydraulic servo system, including the valves, actuators and the ram are modeled. A non-linear model, which includes the influences of piston position, is presented. A simplified model is shown to be adequate provided that the practical ranges of the piston position is considered. The mathematical model is used to experimentally determine the dynamics using parametric identification techniques. The well-damped system is approximated by a first order system with substantial delay between the servo-valve amplifier command and the actuator piston motion. A readily available CNC system is retrofitted to the press brake and a delay compensating pole placement digital control system is developed and implemented for the independent control of two actuators. The performance of the system is evaluated for a series of forming operations. This thesis provides basic guidelines for the design and analysis of hydraulically actuated CNC presses.
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