The hydraulic control system is composed of hydraulic control components and transducers. These components typically include hydraulic control valves, pumps, etc.
Hydraulic control technology is a crucial aspect of automatic control technology and is known for its unique features, prominent benefits and indispensable role.
This technology represents electromechanical fluid integration and is demonstrated in the electrical-hydraulic control system, which uses dynamic and negative feedback systems. The hydraulic control system is a dynamic system that integrates mechanical, electrical and hydraulic systems.
Hydraulic control technology has been widely adopted in various industries, including equipment manufacturing, automotive, aerospace, weapons, metallurgy, shipping, medical engineering and more.
Open-loop hydraulic control and closed-loop hydraulic control
Just like the electromechanical control system, the hydraulic control system can be divided into open-loop and closed-loop control.
To illustrate the difference between the two, we will use machine tool motion beam control as an example.
The motion beam of the machine tool is a common control object and serves as the bench body of the machine tool. It is mounted on the slide guide of the machine tool body.
Different machine tools have varying performance requirements for the motion beam. For example, the surface grinder's motion beam requires only constant horizontal reciprocating motion and does not require precise control of its displacement.
On the other hand, NC machining center or CNC milling machine motion beam is used for precision feeding motion, and its motion displacement must be precisely controlled to ensure proper machining quality.
In the case of the electro-hydraulic press brake, the ram movement distance also requires high precision to maintain consistent bending angles and avoid reduced bending effects.
To better understand the difference between open-loop and closed-loop hydraulic control, we will use the motion beam of the machine tool as a controlled object and build three common hydraulic control systems using electromagnetic directional valves, electromagnetic proportional directional valves and electro-hydraulic servo . valves as main control components.
1.1 Hydraulic control system built with electromagnetic directional valve.
The hydraulic control scheme can be used for the horizontal reciprocating work table of a typical surface grinding machine, as shown in Figure 1.1.
This hydraulic control system uses an electromagnetic directional valve.
A four-position, three-position electromagnetic directional valve serves as the control unit, and a travel switch or proximity switch provides instructions. An electrical relay forms a logical computing network.
This configuration allows logical operation of the control signal and power amplification, generating enough electromagnet to control the electromagnetic directional valve.
The electromagnetic directional valve core has three positions: left, middle and right, and can control the circuit breaker and oil switch.
Each valve port has only two states, fully open and fully closed, which classifies the electromagnetic directional valve as a hydraulic electromagnetic switching valve.
The electromagnetic directional valve can only start and stop the operation of the oil circuit to control the moving beam, but cannot adjust the speed of the moving beam.
To regulate the speed of the moving beam, a butterfly valve is installed in the hydraulic control system to realize throttle control.
By adjusting the valve opening, the throttle pressure difference can be regulated, which changes the flow of oil back to the tank and indirectly adjusts the input and output of hydraulic oil to the hydraulic cylinder, ultimately changing the speed of the valve. beam.
Fig. 1.1 Diagram for adopting electromagnetic directional valve for control system
The speed of the moving beam can only be controlled by the butterfly valve and cannot be controlled by electrical control. This results in sudden changes in speed and significant vibration of the beam.
The principle of the hydraulic control system using the electromagnetic directional valve is shown in Figure 1.2.
The control signal is generated by the shift switch and is a logical control unit (0 or 1).
The relay network performs a logical operation on the control signal and amplifies the power supply to the corresponding electromagnet, causing the corresponding valve core to move.
This results in the three positions of the valve core changing from left to right and generates hydraulic control flow, which drives the hydraulic cylinder and moves the motion beam of the machine tool.
Fig.1.2 Diagram for adopting an electromagnetic directional valve control system
The hydraulic control system composed of an electromagnetic directional valve and relays can only generate simple control instructions.
The control signal is unidirectional and only flows in the direct direction of the controlled object.
This control system is an open loop control system.
The response time of the control instruction to the controlled object depends on the response time of each component in the signal transmission path.
However, because the control instruction signal is simple, there is no problem with the control system not tracking the output instruction signal.
If a component is disturbed and produces a false movement, the system cannot automatically correct or compensate for the error.
1.2 Hydraulic control system built with proportional solenoid valve.
Proportional electromagnetic directional valve is a kind of high-performance and high-price electromagnetic hydraulic valve.
For motion beam control that requires higher performance, such as numerical control surface grinders (where precise control of worktable displacement is not required), a proportional solenoid valve can be used as a control unit to form a hydraulic system low impact and low vibration. control system, as shown in Figure 1.3.
Fig.1.3 Diagram of adopting proportional electromagnetic directional valve for control system.
The proportional hydraulic valve uses an electrical signal to control the valve core for gradual movement.
Therefore, to control the gradual change in valve opening, the pressure drop and flow rate of the proportional hydraulic valve can be adjusted by changing the relationship between flow and control signals.
The program controller generates electrical signals to control the moving beam, allowing gradual change in electrical signals to control and adjust the speed of beam movement. This results in a smooth change in the speed and direction of beam movement with minimal impact.
The principle of the hydraulic control system using the proportional electromagnetic directional valve is shown in Figure 1.4.
The control signal is generated by the program controller and is an analog control signal (continuous electrical signal) that is amplified by a proportional amplifier to control the corresponding proportional electromagnet of the proportional solenoid valve.
This produces a continuously adjustable displacement and constantly changing hydraulic pressure to control the oil flow and drive the oil cylinder, thus realizing the movement of the machine tool beam.
In the hydraulic control system using proportional electromagnetic directional valve, although a degree controller can be used to output a continuous gradient control command signal, the control signal is unidirectional and only flows in the direct direction of the controlled object. This is an open loop control system.
The command system can output a continuous gradient signal, and the system output can track the command signal, but the tracking accuracy is low and the response speed is slow and depends on the response time of the signal transmission components.
Errors caused by interference cannot be compensated automatically.
Fig.1.4 Diagram of adoption of proportional electromagnetic directional valve for control system.
1.3 Hydraulic control system built by electro-hydraulic servo valve.
The movement of the NC machining center work table is a critical part of the machining process, requiring high precision and fast response speed.
In this case, an electro-hydraulic servo control system can be used, with an electro-hydraulic servo valve serving as the control unit.
The electro-hydraulic servo valve is a high-performance hydraulic control unit with precise control and fast response speed, but it is expensive.
Electro-hydraulic servo valve is often used in closed-loop electro-hydraulic control systems, where the controlled object can be temporarily driven by open-loop control mode.
The hydraulic control system of the machine tool motion beam using an electro-hydraulic servo valve is shown in Figure 1.5.
The machine tool is equipped with a displacement sensor, which detects the position of the moving beam, generates a position voltage signal, and inputs the signal into the electronic control device after amplification.
Fig.1.5 Diagram for adopting electro-hydraulic servo valve for control system.
The control device compares the current beam position voltage signal of the machine tool with the voltage signal of the control instruction to generate the offset voltage signal.
The offset signal is a continuous analog voltage that accurately and in real time reflects the difference between the machine tool beam position and the control instruction (the desired beam position).
The deviation signal is amplified by the proportional amplifier, controlling the displacement of the torque motor in the electro-hydraulic servo valve and high-precision, high-dynamic control valve core.
This generates the hydraulic flow and pressure necessary to drive the hydraulic cylinder movement and move the machine tool beam.
The movement of the beam is detected by the displacement sensor and sent to the electronic control device, forming a closed-loop control signal. This control system is known as closed-loop control.
The control process described above is shown in Figure 1.6.
The system is a closed-loop control structure.
In a closed-loop hydraulic control system, there is not only the direct control effect of the controller on the controlled object, but also a feedback effect from the controlled object to the controller.
The closed-loop control system features high precision, fast dynamic response and automatic compensation for external interference.
Fig.1.6 Diagram for adopting electro-hydraulic servo valve for control system.
1.4 Open-loop control vs closed-loop control
Open-loop hydraulic control and closed-loop hydraulic control are two types of basic hydraulic control methods.
1. Open-loop hydraulic control.
The open loop control system, which utilizes common hydraulic valves and proportional hydraulic valves, has significant technical overlap with the hydraulic transmission system as they often utilize similar types of hydraulic components and circuits.
The performance of the open-loop hydraulic control system largely depends on the performance of the hydraulic components.
Accuracy in the open-loop system is influenced by the accuracy of each component, and the system response is directly linked to the response of each component.
The open-loop hydraulic control system cannot control or compensate for changes in system output resulting from external disturbances or internal parameter variations.
In terms of design, the open-loop hydraulic control system is simple in structure and requires stability, making the analysis, design and installation of the system relatively simple. You can also benefit from the experience and knowledge gained in designing hydraulic transmission systems.
The main difference between the open-loop hydraulic control system and the hydraulic transmission system is in their focus. The open loop hydraulic system is typically used in conditions that require low precision control, minimal external disturbance, small changes in internal parameters, and allow for slow response time.
In conclusion, the open-loop hydraulic control system is a basic, feedback-free control method. The controller only has control over the single direction of the controlled object and there is no reverse effect from the controlled object to the controller. Any errors caused by interference cannot be compensated for automatically.
Due to the low accuracy and slow response of the open-loop control system, it is generally not recommended to use a servo valve, which has high requirements for working conditions and is expensive and high-performance, in the construction of an open-loop control system. loop control.
2. Closed-loop hydraulic control.
The closed-loop hydraulic control system generally employs an electro-hydraulic servo valve or a direct drive valve (DDV) as the control unit.
Electro-hydraulic servo valves and direct drive valves are high-performance hydraulic control components that feature closed-loop feedback control systems, leading to high accuracy and fast response speeds.
Closed-loop hydraulic control system is also known as hydraulic feedback control system, which operates based on the feedback principle.
The basic concept of feedback control is to eliminate or reduce deviations through the use of offsets.
The feedback control system works by comparing information about the controlled object detected by the feedback unit with the control instructions from the system instruction unit, producing a deviation signal.
This deviation signal is amplified and used to drive a high-power hydraulic control valve, which in turn controls the hydraulic actuator and the controlled object.
The closed-loop hydraulic control system forms a closed loop, making it more complicated to analyze, design, and commission than open-loop systems. However, its high control precision and strong anti-interference ability make it a worthwhile investment.
Closed-loop control (feedback control method) allows the construction of a control system with high precision and anti-interference ability, even if the hydraulic components used have lower precision and weaker anti-interference ability.
Furthermore, existing hydraulic components can be utilized to achieve better control system performance and control effects through closed-loop control.
Feedback control offers advantages that cannot be achieved through open-loop control.
