【正文】 ulator. Then decreasing the height setting value, the balance of force and position of lifting cylinders is broken. The actual lifting force is smaller than the total weight of the workpiece, gripper, and its support. So the workpiece moves down with the gripper. The decent velocity of the workpiece is determined by the change rate of the height setting value. When the workpiece touches the lower die, the pressure in lifting cylinders will change greatly for the workpiece begins to be supported by the lower die. At this moment, keeping the height setting value constant, then the force and position of lifting cylinders es to a new balance. (c) Comply with the Workpiece During forging, when the workpiece is pressed by the upper die, extra force is applied on the gripper by the deformed workpiece. The force causes an increase of the load in lifting cylinders. Meanwhile, the servosystem is trying to keep the lifting force constant. As a result, the gripper moves down with the workpiece and applies the minimum reaction force on the deformed workpiece. When press is over, the upper die rises, and the external force exerted on the gripper disappears. The workpiece is lifted up to the original position by a constant lifting force. Then the manipulator carries the workpiece to feed. The next press is ready. As discussed above, hybrid force/position control is the core principle in forging manipulator control. Figure 2 illustrates the hybrid control system that incorporates these ideas. Force control is the base of the control loop. In the balanced state, expect that the actual lifting force is equal to the weight of the workpiece, gripper, and its support, smaller than the force setting value. The error of force should be pensated by another control variable. The negative feedback of error of position is added to the control loop. When the actual lifting position is lower than the height setting value, the feedback of error of position is zero, and the gripper moves up under a constant lifting force. When the actual lifting position is higher than the height setting value, the negative feedback of error of position works, and the actual lifting force decreases until force and position tends to balance. Figure 2: Construct of the hybrid control system. Owing to the newly designed mechanism of the forging manipulator which is decoupled as discussed in Section 2, the motions of side shifting, tilting, damping, and lifting are independent, and any one of the motions has no effect on the others. The control of the forging manipulator is simplified and only related to lifting cylinders. During the whole period of forging process, front sideshifting cylinders, rear sideshifting cylinders, and tilting cylinders keep their positions all the time. The deformation of workpiece in horizontal direction is absorbed by damping cylinders during the process of inpress. The pliance happens when the deformation resistance is larger than the setting value of pressure relief valve in damping hydraulic circuit. The reaction force in horizontal direction almost keeps constant that depends on the property of the relief valve. 4. Pressure/Position Control Method of Hydraulic Servosystem A block diagram of the overall control scheme is shown in Figure3. The system is posed of controller, power amplifier, servovalve, cylinder, pressure sensor, and position sensor. Figure 3: Block diagram of hybrid pressure/position control system. Hybrid pressure/position control is very useful in manipulator lifting control where plunger cylinders are used. Based on this method, the intelligent control of forging manipulator is realized. 5. Analysis of Intelligent Control . Automatic Identification of Pressure Value Settings As shown in Figure 4, when the workpiece is grasped by the manipulator, the hybrid pressure/position control method is used, and the pressure and position mand values are set with original ones of P0 and X0. The workpiece is lifted up with a constant force created by hybrid control loop with mand pressure P0. When the position mand value X0 is exceeded, the negative position error feeds back to the pressure control loop, the actual pressure in system begins to decrease and will stop decreasing at the weight balance pressure. In equilibrium, the workpiece stops moving, and the actual lifting force equals the total weight of the workpiece, gripper, and its support. Record this actual balance pressure and use it as the pressure value settings in pliance control in forging process. Figure 4: Illustration of the measurement of workpiece. . Automatic Identification of the Placement Height of Workpiece As shown in Figure 5, supposing the workpiece is staying at a position