Dynamic behavior of servo control systems under several operation conditions

Moving heavy parts at high speeds with precise positioning is a production goal of many companies. Several researchers dealt with this challenge and proposed different studies. Achieving this task requires a system which is powerful enough to accelerate or deccelerate the heavy load quickly. Also, it requires high accuracy control tools. These tools should have fast response and good stability under changeable working conditions. In this work a servovalve-controlled-hydraulic motor driven positioning system is modeled and its sensitivity to different operation conditions changes was studied. The positioning system consisted of a square rail bearing linear table (Parker 41803ST) with an accuracy of 0.001 inches and maximum travel span of 36.5 inches. The power screw was driven by a high speed hydraulic motor (Parker M4-015) which provided the system with up to 6 inches per second linear speed for the 36.5 inch span. The motor speed was controlled by a BD15 Parker servovalve. The position of the moving table is monitored using an absolute encoder to feedback position information to the control system. A LASER distance sensor (LDS 80/10 of Dynavision) was used to detect the workpiece with a +/−0.001 inches accuracy. The analog output of this sensor was fed into the control system to correct the table position until it reached the desired operating point. The EASY5 was used to build a dynamic model of the system. The hydraulic system was modeled and simulated using EASY5 which had predefined hydraulic components models in addition to the ability of defining new ones. EASY5 model made it possible to study the dynamic behavior of the system under varied conditions of the entrained air, servovalve frequency, motor displacement, conductor length, motor leakage, mass and temperature changes. A servovalve-controlled hydraulic motor-driven positioning system was built and a dynamic model of the system was built using EASY5. Eigenvalues sensitivity analysis showed that fluid line between the hydraulic motor and the servovalve is the most influential factor on system stability. As a matter of total time to reach the final point, performance was at: (1) 0.2% entrained air compared with 0.5% and 1%. (2) Servovalve frequency, time delay for 1Hz was increased to 1.38 sec. compared with 1.28 sec. for 50 Hz. (3) Using 0.1in3 displacement motor reduced total time although it increased time delay compared with the 0.15in 3 and 0.3in3 displacement motors. (4) Shorter conductor reduced the total time compared with the longer lines. Also, increasing tubing length increased the pressure in motor outlet port. (5) Increasing motor leakage increased the total time to reach the final point. (6) Increasing the working mass with about 65% increased total time with about 30%. (7) In EASY5 model, replacing PID controller with PI increased settling time more than 50%. (8) Adding Smith predictor to the PID reduced settling time with about 11%. (9) When it was added to the PI controller, settling time was reduced by about 18%