ATOS electro-hydraulic proportional control basic principle

1 What is the electro-hydraulic proportional control?

Electro-hydraulic proportional control means that hydraulic output parameters are adjusted according to the electrical input signal. This is an ideal combination of a hydraulic system and an electronic system that can be used in an open-loop or closed-loop control system to achieve fast, stable, and precise control of various movements. This kind of control is necessary for modern modern machines and factories.

The electro-hydraulic system is an integral part of the fully automated chemistry department. Information such as data, control, and alarms can be transmitted from the electro-hydraulic system to the centralized electro-hydraulic system via the fieldbus, or from the centralized electrical control system, in a concise manner.

The proportional electro-hydraulic control system has the following advantages over the electromechanical system: inherent overload protection function, self-adaptive force, quick action response, self-lubrication of the system, easy to realize stepless speed change, variable force and variable torque, energy storage function, high power Density, long life and high reliability.

2 What is a proportional valve?

The core of the electrohydraulic control is the proportional valve, which can regulate the pressure P and flow Q according to the input signal provided by the CNC machine. The proportional valve must be used in conjunction with an electronic amplifier that supplies an appropriate current to the proportional valve solenoid according to the input signal. The proportional solenoid converts the current into a mechanical force acting on the spool and overcomes the return spring: As the current increases, the output force of the solenoid increases, the return spring is compressed, and the spool begins to move. The proportional valve can be single stage or pilot type, with or without pressure/position sensor. The proportional valve with sensor provides better adjustment accuracy. For the proportional valve of the pilot structure, the pilot valve adjusts the pressure and flow acting on the main valve.

When the circuit fails, the return spring resets the valve spool to the neutral position according to the structure of the valve to ensure the power-off protection function. That is, in the absence of input signals or failure of the electrical system, this system configuration ensures that the system will not cause damage. Power failure protection can be achieved directly through a proportional valve or through a series of actions of a set of valves.

3 control loop

Today's industrial equipment is multi-axis, and more and more proportional valves provide electro-hydraulic control. The axis motion can be open loop or closed loop, depending on the accuracy required in practical applications. In many applications, the motion cycle does not require a high degree of precision to be available for open loops, but in applications where actuator positioning is required each time, a closed loop must be used.

Open-loop motion control

Axis control is achieved by inputting a reference signal to the proportional valve amplifier.

No feedback on adjusted hydraulic parameters

The accuracy of the open-loop control system depends on the quality of the hydraulic system, especially the quality of the proportional valve and the amplifier.

Closed-loop motion control

Axis control is achieved by inputting a reference signal to a closed-loop controller. The closed loop controller can receive the feedback signal of the actuator sensor. According to the need of PID control, the controller compares the input feedback signal and transmits the processed difference to the proportional valve for calibration adjustment.

Compared with the open-loop control, the closed-loop control accuracy is much better, because of the existence of feedback, it is not easy to be affected by the external environment.

In short, the higher the overall quality of the hydraulic system, the higher the axis control accuracy.

4 Proportional Valves and Amplifiers

ATOS's proportional valves are spool type and cartridge type. They can be divided into 3 types according to different functions:

* Pressure Control Valves: Proportional Relief Valves and Proportional Pressure Reducing Valves Proportionally Adjust System Pressure Based on Input Signals

* Four-way directional valve: proportionally controls the flow and direction of the actuator according to the input signal. These valves are used in open-loop and closed-loop control systems to control actuator direction, speed and acceleration;

* Flow control valve: 2-way or 3-way, with pressure compensation, control system flow, this flow is independent of user load.

5 typical electrohydraulic technical terms

Repeat accuracy : The maximum difference between a series of hydraulic parameter values ​​obtained after the same command is sent multiple times to the proportional valve under the same hydraulic and electrical conditions. Rereading accuracy is calculated as a percentage of the maximum value of the hydraulic parameter being adjusted.

Spool cover : valve neutral closed to open

Safety position : The safety function of the spool in the event of a power failure

Linear spool : linear relationship between valve adjustment and input signal

Parabolic linear spool : precise control of small flow rates

Drive current : current output by the electronic amplifier to the valve solenoid

Bias current : static bias current value superimposed on the input signal to compensate for the valve core being covered

Flutter frequency : Adjust the pulse frequency of the electronic amplifier current to minimize the hysteresis of the valve

Gain adjustment : Set the valve adjustment amount according to the maximum value of the input signal

Ramp time : the time required for the valve to regulate when a step signal is input