control valve

What is a proportional control valve?

The proportional control valve can be considered as the combination of the hydraulic valve actuation and the electronic control. So basically a hydraulic valve that is electrically operated by proportional solenoids are classified as proportional valves. The proportional valves can be used to do the variation of the flow or pressure according to the electrical input, we must make sure that the electronics are linked to this valve very carefully. A proportional valve is used in places where the pressure and flow must be changed continuously. So by using a proportional valve we can replace the multiple fixed flow valves and pressure valves in places where acceleration and deceleration control is needed.

By using this valve in a hydraulic system we can reduce the components in hydraulic circuitry and also can increase the accuracy and efficiency. The electronically controlled proportional valve could control the flow according to the input current it receives.

What are the parts of a proportional valve?

  • Proportional solenoid
  • Metering area it is the place where the valve spool is located
  • Electronic position feedback device

The proportional valve can control the cylinders or the hydraulic motors in applications where it needs precise speed control like acceleration and deceleration. The proportional flow control valves are pressure compensated so that they can reduce the flow variations that are caused by the changes in the inlet and the outlet pressure. In proportional valves, proportional solenoids are used to drive the spool of the valve.

The proportional valve spools are designed with V notches so that they can secure the control over the fluid flow, speed of the actuator, and also the control over the direction of the fluid flow by a single valve. So we can control the direction of the flow and the actuator speed by the same valve. The spool won’t shift all the way all at once and due to this, the proportional valves can control the acceleration and deceleration of an actuator.

The electronic part of the proportional valve will control the solenoids according to the indicated nominal value. A controlled buildup and drop of the controlled pressure are ensured. All the analog values (nominal values, measured values) will be digitized and they will be used to calculate the set values.

How does a proportional valve work?

The operation of the proportional valve starts when it would receive a signal from an external device such as PLC, computer, etc. These control devices would transmit the electrical signals to the valve’s driver card and this would send a current signal to the valve’s solenoid. The spool would shift because of the electromechanical force and thus a flow path will be created from the pump to the actuator port. The spool movement is controlled by the solenoid actuators.

If the input command signal is high then the current to the valve solenoid will be high too and because of this high flow could occur from the valve. The major feature of the proportional valve is that all the elements are proportional so because of this any change in the input current would proportionally affect the other steps. So according to the input current changes could happen in shifting of the valve spool, flow path size, the amount of fluid flows through the valve, and also the actuator movement.

So when the spool shifts then its motion is checked by a position feedback transducer. This signal will be transferred to the driver card and this signal will be compared to the input signal from the controller. If the signals don’t match then, the driver would vary the position of the spool till the signals match.

What are the types of proportional control valves?

  1. Proportional direction control valves
  2. Proportional flow control valves
  3. Proportional pressure relief valves
  4. Proportional pressure reduction valves

Proportional direction control valves

The directional proportional valves are used in many hydraulic applications. There are certain types of proportional DCV without any spool feedback for manual and open-loop control applications. These valves could be direct or pilot operated, this type of directional control valve has good accuracy and it would operate properly in many closed-loop applications.

Proportional flow control valves

This type of control valves has an electronically adjusted orifice with pressure compensation. The pressure differential across the throttling orifice is controlled by a compressor that is integrated into it. This would keep the flow, or actuator speed constant when the load forces change.

Proportional pressure relief valves

These valves are used for remote pressure setting by electrical means and in which the increase and decrease in pressure are influenced with respect to time and in this way the pressure can be varied, by means of the electrical signal corresponding to the requirement of the process. These valves would limit the hydraulic pressure in a hydraulic circuit and it would adjust the set limit by electronic means. During the process of pressure-relieving, the excess flow is returned to a hydraulic reservoir tank so that the machine efficiency can be improved.

Proportional pressure reduction valves

By controlling the downstream pressure in a branch circuit, the proportional pressure reducing valve would adjust the set limit by electronic control. This valve can reduce the effect of pressure changes due to varying flow rates.

What is a hydraulic proportional valve?

The proportional valves are used in the hydraulic circuits because these valves are capable to vary the flow volumes many times. This valve can do the infinite position of the spool, this type of valves will be very useful for hydraulic circuits that require more than one speed of actuators. The proportional valves are used in hydraulic circuits where we need to change the flow or pressure to reduce the lunge. This type of hydraulic valve solenoid will move the spools according to the voltage which will be applied to the proportional solenoids and these signals are created by the PLC or by a rheostat. These valves do the acceleration and deceleration by changing the spool’s shift time. The operation of the proportional valves is done by using electrical power and this would be DC power.  

What is the difference between a servo valve and a proportional valve?

The servo valve is different from a proportional valve so it can’t be used in place of a proportional valve.  A servo valve is a closed-loop system, it has feedback built into a system. The feedback is a section of the closed-loop system, and it would monitor the machine or process and it would send a response to the valve that controls this process. In a proportional valve, there is no built-in feedback mostly it is controlled by the operator or there could be a fixed setting for the valve. The servo valves are highly accurate while the proportional valves are not much accurate when compared to the servo valves. The response time of the servo valve is also better than the proportional valve. The servo valves are used in places where high accuracy and high dynamic response are required, while a proportional valve is used for any directional or flow control purpose.

The major disadvantage of a servo valve, when compared to a proportional valve, is that the servo valves are really costly. Due to the complexity of the servo valve, it would need more auxiliary equipment, mostly electronics. The proportional valves don’t need this much electronics, the servo valves are dirt sensitive too.

How to select a proportional valve?

  • We must consider the valve size and flow rate
  • We must consider the sensors natural frequency
  • If it is possible to use a small valve for the desired application then it would be better
  • Flow medium
  • Operating pressure
  • Ambient temperature
  • Repeatability
  • Mounting position

What are the advantages of proportional valves?

  • Good switching process
  • It can continuously control the variable values
  • It has controlled switching transition and because of that pressure peaks can be prevented
  • Good service life
  • Low cost

What are the applications of proportional valves?

  • Cranes
  • Hydraulic steering system
  • Lifting devices
  • Pressure regulators
  • Dialysis machines

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