Application of Replacing Existing Electronically Controlled Pressure Regulator in Parallel Robot
The concept of parallel robot is based on the Steward platform used for flight simulation driving in 1965, and was proposed by Hunt, a well-known Australian institution professor, in 1978. At present, many experts and scholars have done a lot of theoretical and practical research in this field. Compared with serial robots, parallel robots have the advantages of high stiffness, high load-to-weight ratio, uniform load distribution, no accumulation of position errors, and easy force feedback. It is suitable for occasions with high precision, large load and relatively small working space requirements. At the same time, due to the great improvement in performance and function of pneumatic components, and with the development of science and technology such as electronics, materials, control theory and sensors, electronically controlled pressure regulator control technology has also been rapidly improved. The electronically controlled pressure regulator control system with proportional/servo control valve as the core can reach the high-precision control of continuous change of pressure and flow, and can meet the flexible production requirements of automation equipment. It makes the application of pneumatic drive in parallel robots possible. Taking a typical 6-DOF parallel robot Steward platform as an example, its structure is shown in the figure below. The structure is composed of upper and lower flat joints, and the flat joints are connected by a set of telescopic connecting rods with ball joints at both ends, so that the upper platform can move and rotate in three-dimensional space. Most parallel robots use screw rods or electro-hydraulic servo cylinders as motion actuators. Because the motions between the drive units of the parallel robot cannot be independent of each other, they must maintain a very strict combination relationship. The motion error of any link will lead to control failure. In our study, the hydraulic controlled pressure regulator is replaced by a more tolerant electronically controlled pressure regulator as the transmission medium of the parallel robot actuator. The entire control flow path is shown in the following figure: 1. The extended position of the cylinder is obtained by the analog solver in the upper computer and transmitted to the lower computer. 2. The lower computer converts it into voltage (0~10v) through the D/A card and outputs it to the electronically controlled pressure regulator. 3. The electronically controlled pressure regulator controls size and direction of the opening according to the magnitude of voltage, and outputs the accurate gas flow to cylinder to promote the movement of the piston. 4. The moving position of piston is detected by the displacement sensor, and the feedback voltage of displacement sensor is output to the lower computer through the A/D card; the gas pressure at both ends of the piston is detected by the air pressure sensor, and the feedback voltage is output to the lower computer through A/D card. 5. The lower computer adjusts the control voltage of the electric proportional valve in real time according to the feedback value, so that the cylinder can reach the position accurately. 6. The position signal of the platform is fed back to the upper computer by position sensor, and the position of platform is further corrected by the upper computer. In the entire control process, the focus is on single-cylinder control system. The experimental schematic diagram of the single-cylinder control system is shown in the following figure. For the electronically controlled pressure regulator control system, the state single-cylinder control has better control characteristics; adding PI control in the optimal state feedback can significantly increase the stiffness of system and improve the tracking performance of system. It has good track tracking performance in the frequency range of 0.1Hz to 5Hz. The object of this test is the test platform model of the automobile suspension system, which does not require very high displacement accuracy, and is more sensitive to the speed and acceleration response speed. After the large cylinder is adopted, the load of whole system is greatly increased. The frequency response of system can reach 5~10Hz, which can meet the requirements of the test platform. The 6-DOF parallel platform mainly composed of cylinder and electric proportional valve is adopted, which avoids the difficulty of control caused by the strict motion combination relationship between the six actuators required by hydraulic pressure and screw rod, and ensures the speed and acceleration response sensitivity. Simultaneous use of air cylinders and electronically controlled pressure regulator can greatly reduce costs, noise and pollution.