Technology of Proportional Valve and Servo Valve
Category：2018-08-25 Read：4109second
In the past 25 years, great progress has been made in the realization of automatic continuous control of pressure and flow in fluid systems. Electro-hydraulic servo valve was invented as an advanced technology solution for motion control in the late 1930s, but the cost is very high. In the mid-1980s, as an alternative solution to electro-hydraulic servo valves, proportional valves with feasible technology and reasonable price were introduced. This paper will explore the technology used in proportional valves and servo valves, and try to explain what kind of valves should be used in a specific application.
If continuous pressure or flow control is not strictly required for the operation of mechanical equipment, preset pressure or flow function can be achieved through a set of valves. The valves are connected by oil circuit and controlled by solenoid valves. For example, if three specific pressure values are required, two pilot relief valves can be used in parallel to the vent port of a pilot relief valve. The second two pilot relief valves and the first one are separated by a normally closed two-way electromagnetic reversing valve. Three pressures can be controlled by controlling two solenoid directional valves separately. But what if stepless pressure control is to be achieved? Or if the increase or decrease of pressure is required to follow a specific flow rate, or if the rate of change is not fixed, how can the designer do it?
Until the servo valve was invented, if the pressure of the actuator needed to be changed to change the force or moment, then the machine operator needed to rotate the adjusting handle, or adjust the connecting rod, or other mechanical input methods to change the set value of the valve. If there is a need to change traffic, a similar approach may be adopted. The manual control of the valve is rather unstable. Mechanical control of valves may be more stable and repeatable, but flexibility is not enough when there are different adjusting speed requirements.
In the 1980s, before the emergence of microprocessors, most of the electrical equipment control systems did not develop very well. Because most of the machine equipments with electrical automation adopt relay logic control, the sequential operation strategy of the equipment can not be easily changed. Relays are digital or switching devices. With the invention of microprocessor and PLC, the designers of mechanical equipment have opened a door to control diversity. The operation sequence of the equipment is no longer relay hard wire connection. Although Boolean operation is logically feasible in relay control, it is inconvenient, even quite difficult, expensive and time-consuming. The introduction of PLC and proportional valve greatly expands the diversity of control requirements of mechanical equipment designers.
The first proportional valve on the market is what we often call "open-loop" control valve. Compared with the mechanical feedback (MFB) servo valve, there is no feedback connection between the coil assembly and the spool. Since there is no feedback loop between instruction input and valve output, the feedback loop is "open" rather than "closed". In contrast to servo valves, in order to improve the performance of proportional valves, manufacturers install linear displacement sensors (LVDT) on the spool to sense the change of spool position. The output signal of LVDT is fed back to the amplifier. The amplifier calculates the deviation between the theoretical position and the actual position of the spool, and then changes the output of the coil to make the spool position reach the expected value corresponding to the input. These improved performance proportional valves are defined as "closed-loop" control proportional valves. Because feedback is electrical rather than mechanical, it is defined as "electrical feedback" (EFB).
How does the proportional valve work
An electrical signal is transmitted to the amplifier and then the proportional valve coil is controlled. Since most of the power supply signals are lower than the current signals required to work on the coil, the input signals must be amplified. This function requires an amplifier. The amplifier can be mounted on the valve body, or OBE (onboard electronics), or remotely installed, separated from the valve. Electrical input signals can come from different sources, such as potentiometers controlled by machine operators, lever, or PLC.
The amplifier drives the coil with a current signal. When the current flows through the coil, it generates electromagnetic force, which causes the armature to move in the coil. The force on the armature drives the spool, so flow control, pressure adjustment, or direction control, or relief pressure adjustment of the seat valve are obtained. The spool or cone valve of the slide valve is offset by the spring. Therefore, the force on the coil is balanced with the spring force.
Many proportional direction control valves, as shown in Figure 1, have two solenoid coils at both ends of the valve. Proportional direction control valves provide direction and flow control. This special valve contains LVDT. Basically, the dual-coil proportional valve is developed based on the standard switching solenoid directional valve. The main difference between the direct switching valve and the direct proportional valve is that:
1) The center spring of the proportional directional valve is harder than that of the general on-off directional valve.
2) The electromagnet of proportional directional valve can produce more force than that of common switching directional valve.
3) Proportional directional valves always use DC DC electromagnets.
4) Switch and proportional valve bodies are always the same (most manufacturers do), but the spool is somewhat different.
A) Proportional valve spool design for flow control
B) For proportional valves of the same specification, the spools are designed differently to achieve different flow ranges
C) Proportional valve spool has throttle port, when different electrical input can provide different flow range
5) Direction-controlled proportional valve can provide 1:1 or 2:1 flow rate ratio to control the hydraulic motor and double-rod cylinder, or the effective area ratio of 2:1 cylinder.
If continuous pressure or flow control is not strictly required for the operation of mechanical equipment, preset pressure or flow function can be achieved through a set of valves. The valves are connected by oil circuit and controlled by solenoid valves. For example, if three specific pressure values are required, two pilot relief valves can be used in parallel to the vent port of a pilot relief valve. The second two pilot relief valves and the first one are separated by a normally closed two-way electromagnetic reversing valve. Three pressures can be controlled by controlling two solenoid directional valves separately. But what if stepless pressure control is to be achieved? Or if the increase or decrease of pressure is required to follow a specific flow rate, or if the rate of change is not fixed, how can the designer do it?
Until the servo valve was invented, if the pressure of the actuator needed to be changed to change the force or moment, then the machine operator needed to rotate the adjusting handle, or adjust the connecting rod, or other mechanical input methods to change the set value of the valve. If there is a need to change traffic, a similar approach may be adopted. The manual control of the valve is rather unstable. Mechanical control of valves may be more stable and repeatable, but flexibility is not enough when there are different adjusting speed requirements.
In the 1980s, before the emergence of microprocessors, most of the electrical equipment control systems did not develop very well. Because most of the machine equipments with electrical automation adopt relay logic control, the sequential operation strategy of the equipment can not be easily changed. Relays are digital or switching devices. With the invention of microprocessor and PLC, the designers of mechanical equipment have opened a door to control diversity. The operation sequence of the equipment is no longer relay hard wire connection. Although Boolean operation is logically feasible in relay control, it is inconvenient, even quite difficult, expensive and time-consuming. The introduction of PLC and proportional valve greatly expands the diversity of control requirements of mechanical equipment designers.
The first proportional valve on the market is what we often call "open-loop" control valve. Compared with the mechanical feedback (MFB) servo valve, there is no feedback connection between the coil assembly and the spool. Since there is no feedback loop between instruction input and valve output, the feedback loop is "open" rather than "closed". In contrast to servo valves, in order to improve the performance of proportional valves, manufacturers install linear displacement sensors (LVDT) on the spool to sense the change of spool position. The output signal of LVDT is fed back to the amplifier. The amplifier calculates the deviation between the theoretical position and the actual position of the spool, and then changes the output of the coil to make the spool position reach the expected value corresponding to the input. These improved performance proportional valves are defined as "closed-loop" control proportional valves. Because feedback is electrical rather than mechanical, it is defined as "electrical feedback" (EFB).
How does the proportional valve work
An electrical signal is transmitted to the amplifier and then the proportional valve coil is controlled. Since most of the power supply signals are lower than the current signals required to work on the coil, the input signals must be amplified. This function requires an amplifier. The amplifier can be mounted on the valve body, or OBE (onboard electronics), or remotely installed, separated from the valve. Electrical input signals can come from different sources, such as potentiometers controlled by machine operators, lever, or PLC.
The amplifier drives the coil with a current signal. When the current flows through the coil, it generates electromagnetic force, which causes the armature to move in the coil. The force on the armature drives the spool, so flow control, pressure adjustment, or direction control, or relief pressure adjustment of the seat valve are obtained. The spool or cone valve of the slide valve is offset by the spring. Therefore, the force on the coil is balanced with the spring force.
Many proportional direction control valves, as shown in Figure 1, have two solenoid coils at both ends of the valve. Proportional direction control valves provide direction and flow control. This special valve contains LVDT. Basically, the dual-coil proportional valve is developed based on the standard switching solenoid directional valve. The main difference between the direct switching valve and the direct proportional valve is that:
1) The center spring of the proportional directional valve is harder than that of the general on-off directional valve.
2) The electromagnet of proportional directional valve can produce more force than that of common switching directional valve.
3) Proportional directional valves always use DC DC electromagnets.
4) Switch and proportional valve bodies are always the same (most manufacturers do), but the spool is somewhat different.
A) Proportional valve spool design for flow control
B) For proportional valves of the same specification, the spools are designed differently to achieve different flow ranges
C) Proportional valve spool has throttle port, when different electrical input can provide different flow range
5) Direction-controlled proportional valve can provide 1:1 or 2:1 flow rate ratio to control the hydraulic motor and double-rod cylinder, or the effective area ratio of 2:1 cylinder.