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What is a proportional control valve?

Proportional control valves provide variable hydraulic outputs proportional to an electric input signal in direction, flow or pressure. Electromagnetic output force of the solenoid is proportional to the current flowing through the coil.

Proportional control valves are used in a variety of applications to control the flow or pressure of fluids. Proportional valves are used in a variety of applications, such as controlling the flow or pressure of fluids. A proportional valve can be either directional or non-directional. Proportional valves are widely used in hydraulic systems and produce variable hydraulic outputs proportional to an electric input signal in direction, flow or pressure. .In hydraulic systems, a proportional valve consists of an actuator and a two-port or three-port valve.

A proportional valve is a valve that regulates the flow of fluid based on some input signal. A proportional valve consists of an actuator and a two-port or three-port valve. The actuator is usually a piston or diaphragm with a rod attached to it. The rod is connected to the stem at one end and pivots in the center of the two-port or three-port.

A proportional control valve is a type of valve that can be used to control the flow of a fluid by using an electric input signal. The electric current is converted into a magnetic field which interacts with the coiled wire in the valve, which in turn changes the flow and pressure of the fluid. There are many different types of proportional valves, but they all function in this same way.

In order for a proportional control valve to work properly, it needs an input signal. This input signal can be either analog or digital and it determines how much pressure is required for the desired output. The coil inside of the proportional control valve changes its shape when there is an electric current running through it, which then moves two plates inside the valve to allow more or less fluid through.

The proportional control valve is one of the most powerful tools for a company’s production line. It can hold from 4-5% to 100% open value and uses different inlet flow rates and electric input rates to adjust the nozzle’s speeds.

Force generated by a solenoid coil=Current (I) x Number of windings

Ohms Law,  Volts = Current x Resistance  V = I x R    

Watts = Voltage  x Current  P = V x I

Heat rise in the coil should be considered in terms of getting the right output. You can ask your PWM driver supplier whether it will compensate for the increase in resistance as temperature rises so ensure the maximum current draw is calculated for the highest temperature and lowest voltage condition.

As an example Parker EXPRO driver compensates for this.

Parker EXPRO

Basic Schematic

Basic schematic of a Proportional valve

Comparison of Proportional valve to Servo Valve

A Proportional valve, also known as a variable displacement valve or rocking-lever control valve, is an automatic valve which adjusts the opening or closing point with the drive pressure whereas a servo valve can regulate its opening and closing through small incremental changes within some range.

Proportional

  • Proportional Solenoid relatively high power (approx. 30 W+) for direct actuation of valve spools and compression springs.
  • Nominal filtration 10 micron
  • Typical response 8Hz to 50 Hz depending on valve type.
  • Operated at low pressure drop, approx. 10 bar.
  • Hysteresis in open loop 3% – 6%,
  • Positive overlap – dead zones around null (zero) position.
  • Relatively low cost due to utilization of standard components and production manufacturing.

Servo

  • Mostly directional valve functions. Flow control is automatically incorporated. Torque motor relatively low power (approx. 0.1-0.3 W) applied to a baffle plate amplifier.
  • Nominal filtration 3 micron
  • Typical response 200Hz +
  • Operated at high pressure drop, approx. 70 bar.
  • Hysteresis approx. 0.1% due to high power at the pilot stage and by position control of main stage.
  • Zero overlap due to high manufacturing precision. No spring centering.
  • Relatively high cost due to precision manufacturing of components and testing.

Hysteresis

All spool valves suffer from friction between the spool and valve bore. Frictional forces are not constant and can be affected by manufacturing tolerances, wear, fluid viscosity, contamination and thermal expansion.

The solenoid force has to overcome this variable friction force which leads to an effect known as hysteresis.

Hysteresis is a percentage of the maximum rated input signal comparing the rising and decreasing output figures.

Spool position feedback can be used  to reduce the effect and also dither from the valve driver.

Proportional Solenoid Valve Hysteresis

PWM Driver With Current Feedback

As current passes through a coil, heat is generated. The resistance of a solenoid coil increases as temperature increases. The amplifier produces a certain output voltage for a given input voltage which generates a current in the coil, however, if the coil resistance changes then the current will also change, therefore the valve setting will also change.

Coil power could be controlled directly using a rheostat (a high power potentiometer) for instance, but power is wasted, heat is generated, and valve performance will suffer without dither and current feedback as provided by a well engineered amplifier.Specifically, the valve performance will not be repeatable nor will the hysteresis be within published tolerances. So, for maximum energy efficiency and operational performance, a proportional amplifier is highly recommended.

Some advantages of using the Sun proportional amplifier are:

  • Sun amplifiers include an internal closed loop current feedback feature which maintains consistent current to the solenoid coil. As electrical power flows through the copper windings of a coil, the copper heats up, which increases the resistance of the copper to current flow. As solenoid force is directly related to coil current, if current varies, then force and valve performance will also vary. The internal closed loop control on current ensures consistent operation and good repeatability.
  • Traditionally, proportional valve amplifiers are tuned with a screw driver turning a potentiometer. With Sun amplifiers, the tuning is accomplished via an infrared (IR) adapter and either a hand held programmer (HHP) or the Sun HydraulicsAmplifier Set Up software package for PCs. There is no need for a screw driver, digital volt meter or multimeter(DVM), or oscilloscope. The proportional amplifier, once set and initially powered, remembers the necessary information to control the valve. The HHP or PC cable can then be removed and stored away. The combination of the IR adapter plus HHP or software, creates a precise and easy to use“electronic screw driver”.
Embedded amplifier ready for setup

What is PWM (Pulse With Modulation)?

A proportional control valve is a simple mechanical system consisting of:

– a mass that sets the open state of the orifice (output setting);

– an air spring that sets the pressure at which flows therefrom;

– an electric input signal, PWM, pulse width modulation with hysteresis loop so that it behaves like an effective “shockless” damper on changeovers. This can be done by circuit feedback and/or any other type of pressure release available on this particular make or type.

  • Smaller power requirements for electronic controllers.
  • Smaller electronic components – less power wastage.
  • Faster valve response time.
  • Can be directly interfaced with digital (microprocessor) controllers.
PWM Duty Cycle

Frequency Response

Bode plots are used to characterize the dynamic performance of a valve.  These are normally for Servo products but increasingly mobile OEM’s are asking for this data for proportional valves.

Bode Plots

Proportional Valve Types

  • Relief Valves
  • Pressure Regulating  Valves
  • Directional Valves
  • Flow Regulator Valves

Proportional Pressure Relief

Proportional Pressure Relief Symbol

Control pressure in a circuit relative to input current, either.

  • Increasing pressure with increasing current or
  • Decreasing pressure with increasing current (Fan Drive Circuits).
  • Reducing valves to follow

Pilot line for ventable relief valve

Controlling system pressure electrically in small & large flow circuits.

Remote pump unloading

Proportional Directional Valves

Example Circuit
  • Provide both the direction of movement of an actuator as well as a controlled flow to and from the actuator.
  • A compensator can be used in front of the directional valve for improved control
  • Up to 38 LPM and 350 bar
  • Manual Override option available

COILS

Coils
  • Super Coil in ½” and 5/8” ID  – Fits most original Parker valves
  • Integral Deutsch Connector meets IP69K requirements
  • ½” ID COIL = CC Coil in 14W AND 19W Versions
  • 5/8” ID COIL = CA Coil and in 18W AND 28W Versions
For more details: https://www.mcs-hydraulics.com

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