Measures to improve the accuracy of speed control

     (1) the minimum speed threshold speed stable small flow control valve is an important technical parameter is the minimum stable flow, when the hydraulic system is higher in the speed accuracy requirements, should choose the minimum stable flow of small hydraulic valve. For example, a spinning machine hydraulic system (local) as shown in Figure 2-19, the working procedure is a fast forward rewind a workpiece. The problem of the system is that the slow feed speed of the workpiece can not be adjusted, and it can not reach the required 1cm/min or so. The cause of the problem is that the minimum steady flow (10.6/min) of the speed regulating valve 1 (QDF-B20H-S type one-way speed control valve) is too large. The improved hydraulic circuit is shown in figure 2-20. The main improvement measures are the installation of two higher precision one-way control valve 2FRM5 (B and C valve in the figure), the range of adjustment is 0.2~15/min, which can meet the requirements of slow feed speed of the workpiece. At the same time, parallel with a two bit four way solenoid valve D, used for fast forward. In addition, the system is also provided with a hydraulic control one-way valve E and F to improve the positioning accuracy of hydraulic cylinder, and the one-way valve 2-19 is shown in Figure 2 to diversity valve A to realize flow control in two directions, the fast forward and rewind synchronization accuracy are higher.

    (2) use the brake valve to eliminate the interference of the movement speed of the external load; in the steel rolling line, the steel coil turnover machine is used to overturn the steel coil. In the turning process, when the center of gravity of steel coils and turnover frame across the center of rotation, due to gravity steel coil, there is a tendency to drop down, may cause the loss of control and caused a serious impact, so the hydraulic system of steel coil turning machine to set the speed to prevent special hydraulic valve - control brake valve.

Fig. 2-21 is a hydraulic loop diagram of a steel turnover machine, 1 of which is a brake valve. The working principle of the brake valve is that when the reversing valve 3 is in position B, the pressure oil enters the rod chamber of the hydraulic cylinder through the one-way valve 2 to realize the load turn. At the beginning of the flip stage, as the winder rolls up with a steel coil, it is still a lifting process, so the cylinder of the hydraulic cylinder must have a certain amount of pressure in order to turn the turnover machine over. At this time, the oil pressure of the oil pipe in the rod cavity is also transmitted to the control piston on the left of the brake valve, so that the brake valve is in a state of opening and throttling, and the turnover machine rotates slowly. When rotated to a certain angle, turning machine and coiled steel center across the rotary fulcrum, the coil weight to fall automatically, drives the hydraulic cylinder to move fast, then the rod chamber pressure decreased rapidly, brake valve in the spring under the action of the main valve heart left, the orifice was gradually reduced, and the one-way the valve closed die, thus limiting the speed of piston movement. Throttle mouth close to death, as the return of oil throttling role increases, there is the bar cavity pressure has picked up, it will still be the throttle valve opening a certain degree of opening, so as to ensure that the machine always rotates slowly until it is in place.

Other similar applications can also be provided with a brake valve to eliminate the interference of the external load on the speed of movement.

(3) the pressure compensator control proportional directional pressure difference between proportional valve can change the direction of movement of actuating element, can adjust its velocity, but only proportional valve throttle speed control function (Fig. 2-22), it will be with the load changes due to the speed fluctuation, the hydraulic equipment unstable movement. The pressure compensator can ensure that the pressure difference at the throttle orifice of the proportional reversing valve does not change with the load, and is always a constant, thus improving the speed regulating characteristics and the control accuracy of the proportional system. The pressure compensator enables the resolution of the proportional directional valve to rise from 1:20 to 1:300, and the flow characteristics are very good throughout the range of pressure variation (Figure 2-23).

Pressure compensator according to its installation form is divided into two types of imports and exports.

Figure 2-24 shows a steel coil trolley moving ratio system. The weight of the coil car is 10934, the maximum load is 30000kg, and the system pressure is 14Mpa. The hydraulic motor drives the car to walk on the track. It requires stable operation and good braking performance.

In order to ensure the stable operation of the steel coil car, an outlet pressure compensator is adopted to adjust the flow in the process of advancing and retreating. In order to ensure stable braking of the car, the back pressure valve 3 is set on both sides of the hydraulic motor. The system is characterized by the speed can be automatically controlled, and the movement is stable, good braking performance, shorten the time to the middle, improve the efficiency of the equipment, but also convenient maintenance.

(4) closed loop feedback control is adopted. In order to control the speed of hydraulic system accurately, closed loop feedback control method can be adopted. Generally, the pressure sensor or velocity sensor can be used as the detection and feedback element, and the detected signal is fed back to the computer. The computer controls the flow valve to achieve the best control.

For high-end hydraulic elevator speed curve to achieve the ideal, often using feedback control, the controlled flow accurately follow the input signal to the nonlinear change of hydraulic elevator acceleration and deceleration control accurately.

   The hydraulic elevator speed control loop using electro-hydraulic proportional variable pump is shown in figure 2-25. When the elevator l.- is in line, the bidirectional flow sensor 1, the electro-hydraulic proportional amplifier and the electrohydraulic proportional variable pump form the loop of the flow accurately tracking the given signal. When the elevator is running, the speed of the elevator is controlled by the opening of the electro-hydraulic proportional throttle valve, and the speed limiting shut-off valve 2 is used to prevent the elevator from falling rapidly.

      The hydraulic elevator speed control loop with flow rate electrical feedback is shown in figure 2-26. Double flow sensor 3, an amplifier and a proportional flow valve 1 and 2 constitute two flow electrical feedback loop through the control of the main valve opening of the heart size, then control the import hydraulic cylinder flow, the lift curve according to predetermined change.