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A kind of asymmetric digital valve with generalized pulse code modulation control is researched. By using different encoding methods, the area ratio of throttle valve forward and reverse throttle can be adjusted. The same valve can adapt to the requirements of asymmetric cylinder with different cavity area ratio. A general principle of generalized pulse code modulation coding is proposed. Combining with experiment, the control strategy and control method of the system are studied. An effective control method for the generalized pulse code modulation hydraulic position servo system is obtained. Introduction Asymmetric hydraulic cylinders are widely used in hydraulic position / force servo systems. Due to the difference in the area of ​​action between the two chambers of the cylinder, the speed characteristics and the dynamic characteristics in the forward and reverse direction are different, resulting in the difference of the dynamic and static characteristics in the positive and negative directions of the system. Asymmetric valve control asymmetric cylinder can effectively reduce the sudden change of pressure and dynamic and static properties of the asymmetry. Common non-symmetrical valve throttle window area gradient ratio of a fixed value, the need to be used with the cylinder design, poor interchangeability, manufacturing process complexity, the valve cost increase. Generalized pulse code modulation (GPCM) valve with a certain number of different flow throttle base components, low price, strong anti-pollution ability, according to the needs of the system can be flexibly to change the throttle throttling Area and encoding, get different flow. The author of GPCM hydraulic servo control theory has been studied in this paper GPCM digital valve asymmetric cylinder pressure and flow characteristics of the study. 1 GPCM valve control cylinder system 1.1 System Description GPCM valve by a four-way directional control valve and a set of throttled cells, each cell throttle area according to a certain modulation rules set by the pulse control signal to control their Open and close the state, the combination of different total throttle area, oil return throttling speed control system, so as to achieve the purpose of the control system flow, the flow control principle shown in Figure 1. In the figure, Q1 and Q2 are respectively the cylinder rodless cavity and rod cavity pressure oil flow, m3 / s; ps is the system pressure, Pa; Qs is the system flow, m3 / s; pr is the valve outlet pressure, Pa; Qr is Valve exit flow, m3 / s; A1, A2 are the cylinder rodless cavity and rod cavity cross-sectional area, m2; p1, p2 are the cylinder rodless cavity and rod cavity pressure, Pa; m is the system equivalent mass, kg. 1.2 GPCM coding rules When asymmetric hydraulic cylinder piston in different directions, due to the asymmetric acting area on both sides of the piston, at the same speed, flow through the valve throttling unit group is not the same. GPCM valve flow control for the directional valve plus back to throttle mode, only in one direction flow control role. Where Q is the flow of the GPCM valve in the rodless cavity oil return, m3 / s; Cd is the flow coefficient; Ni is the pulse code value; S0 is the throttle base area, m2; Δp is the orifice throttle pressure drop , Pa; Ï is the liquid density, kg / m3. Due to the different effective cross-sectional area of ​​the two chambers of the asymmetric cylinder, the flow of the GPCM valve is not the same at the same speed when the asymmetrical cylinder piston is running in the opposite direction. Ignoring hydraulic cylinder and valve leaks and assuming that the hydraulic fluid is incompressible gives the pistons the same speed as if symmetrical coded flow control was used and the same control input would give a different speed during control so that the hydraulic cylinder The symmetry of the piston movement is affected, especially in the multi-cylinder system requires synchronous movement, the system movement is not coordinated, reducing control performance. GPCM servo control system can use encoding method to make GPCM valve flow asymmetric valve, which can effectively reduce the asymmetric cylinder left and right asymmetric movement of the system control performance. Left and right motion speed equal to the corresponding encoding rules for the hydraulic cylinder retraction stroke encoding value is out of stroke encoding value of A1 / A2 times, you can guarantee the symmetry of asymmetric hydraulic cylinder speed. The asymmetric cylinder two-chamber effect area ratio is approximately 1: 2, which brings convenience to asymmetric cylinder pulse code control. Control, the output pulse corresponding to the left one can reach the output requirements. The stability of GPCM system is analyzed theoretically and experimentally by using nonlinear control theory. The minimum throttling base area of ​​throttling primitive of GPCM control valve is deduced. S0 is the minimum throttling of valve control when cylinder piston rod extends and retracts After the flow rate is determined, the maximum valve control flow rate is determined according to the system requirements. GPCM Valve Control The minimum throttling flow rate, known as the GPCM valve resolution, is the minimum incremental control that changes the valve's control flow. Generally, when the electro-hydraulic servo system is in the normal working state, the valve resolution has little effect on the system operation. However, when the system is operating at low speed, the valve resolution has a great influence on the dynamic performance of the system. When the system is at low speed, due to small changes in the amount of the valve, which also changes in the input signal is also small, this time, the valve resolution must be considered, in general, small flow servo valve to distinguish the ability of the input signal Servo valve for large flow. Therefore, the GPCM electro-hydraulic servo system to take the variable gain valve solution hydraulic cylinder in the case of high-speed or constant speed movement, the valve showed a high gain, when the hydraulic cylinder at low speed, the valve was low gain in order to achieve high resolution, To achieve high-speed and high-precision control of a combination of purpose. Accordingly, the following GPCM coding rules are determined: The minimum amount is determined. The flow rate of the first few throttling units of the valve is arranged in a binary scale so as to obtain a higher resolution and achieve the required control performance. 2 control strategy GPCM valve position servo system in addition to the hydraulic servo system inherent nonlinear characteristics, but also due to the use of pulse modulation control, with discontinuous flow changes, the system of high precision control difficult, the system is not easy to model and related parameters It is difficult to determine accurately, making all kinds of control methods based on the mathematical model of the controlled object can not effectively solve this control problem. This paper presents a new control method applied to GPCM hydraulic servo control system. The GPCM servo positioning system response process is divided into three stages: â‘ rapid start, the system speed increases from zero to maximum, the position deviation decreases rapidly; â‘¡ deceleration operation, when the movement reaches a certain range, in order to prevent overshoot began to reduce the operation Speed; â‘¢ positioning to maintain steady state has a strong anti-interference ability. The three stages correspond to the valve flow from large to small, beginning with a larger combined flow to get a fast response, as the deviation decreases, the valve flow gradually reduced to the specified position to maintain output flow to zero. Using a single control algorithm is difficult to achieve high-speed, high-precision control system requirements, so the use of a combination of three control methods, respectively, corresponding to the three phases of the system to implement the control response. The controller control algorithm of the final design is as follows: â‘ When the displacement error | e |> ε1, Bang-Bang control; â‘¡ displacement error ε1> | e |> ε2, PID control; â‘¢ displacement error | e | ≤ ε2, fuzzy control. Where ε1 and ε2 are thresholds for handover control measurement. In the start-up phase, the system uses Bang-Bang control to quickly adjust the performance, so that the system can quickly reach the deceleration positioning process. During the deceleration process, the PID control has good dynamic performance adjustment function, which can effectively eliminate and reduce overshoot, Finally, the use of fuzzy control, can easily achieve asymmetrical pulse code output, to achieve precise positioning. EXPERIMENT Based on the above theory, the prototype of GPCM valve was designed. The output flow of the control valve was composed of 6 throttling cells. According to the requirements of control accuracy and response speed, the flow orifice of 6 throttles was determined according to formula (5) Diameter of 0.2mm, 0.3mm, 0.4mm, 0.7mm, 1.2mm, 2mm. It consists of asymmetric cylinder GPCM position servo system, the system control block diagram shown in Figure 3, the hydraulic system of the main parameters A1 = 1.256 × 10-3m2, A2 = 8.76 × 10-4m2, m = 30kg, ps = 7MPa. The control algorithm is implemented by computer, which can automatically switch between algorithms conveniently and adjust the parameters of each controller conveniently during debugging. The step response system of the position servo system under different control modes only uses the experimental results of the PID control. Because the output of the controller is small near the specified position, the valve is often operated in the dead zone. When the valve works in the dead zone , The hydraulic cylinder stops moving until the output of the controller exceeds the dead zone due to the error integral action. The valve suddenly opens and the cylinder accelerates, which usually causes large overshoot, oscillation, long transition time and low control precision. The fuzzy controller is used in the positioning stage. The output of the controller can quickly compensate for the non-linearity of the valve dead zone and effectively overcome the influence of the dead zone to improve the control accuracy, as shown in Figure 4b. System response to the square wave input signal test curve shown in Figure 5. The results show that asymmetric cylinder in the opposite direction of the control characteristics are basically symmetrical, to achieve the control target. 4 Conclusion (1) GPCM valve flow coding rules can be based on the system control accuracy and response speed requirements to determine the minimum throttling flow and control accuracy, and the speed and the integrated flow related. GPCM electro-hydraulic servo system to take variable gain valve program, the first few throttle flow into a binary ratio, after a few in accordance with the total flow demand. (2) The GPCM valve can realize the function of asymmetric valve by changing the pulse code value, and obtain the different positive and negative throttle area ratio. (3) The combination of Bang-Bang control, PID control and fuzzy control is feasible for GPCM valve-controlled asymmetric cylinder servo system, and does not need to know the nonlinear characteristic parameters of the system, and has strong practicability.
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