In this article, Dave Minors, product support manager with ASCO NUMATICS, looks at some of the factors to consider when choosing a proportional valve

Proportional valves are suited to a wide variety of applications in machinery, basic processes and simple or complex, hydro-pneumatic systems.  They provide a cost effective and compact solution for controlling flow or pressure at relatively low flow rates.  There are many different designs and styles of proportional valves for varying operating pressures and flow rates, with either electric or electro-pneumatic actuation.

Process control is a specialist field and like many other specialisms it has developed a ‘language’ of its own that is alien to many outsiders.  These outsiders may include engineers who have the responsibility to specify valves to control pressure or flow within machines they are designing, or in simple processes they have developed.  A basic understanding of control terminology is essential if an engineer is to select the correct valve for an application.

Open and closed loop control - what does it mean?

Imagine a room with a central heating radiator fitted with a control valve.  Open loop control describes how the markings on the valve are used to control room temperature.  For example, experience may have shown that 30 percent open provides a room temperature of 25º C. So this is where the valve is set to maintain that temperature. However, if it is a sunny rather than a cloudy day and the room heats up, there is no feedback mechanism to close the valve and maintain the temperature to take account of the change in circumstances.  Closed loop control adds that feedback.  By adding a room thermostat the temperature in the room would be monitored and a signal provided to close the radiator valve as the required temperature is reached thus maintaining the temperature at the set level.

PID Controller

In the example used to explain closed loop control the thermostat acts as a simple controller, comparing the room temperature to a set point and opening and closing the valve as required.  Most industrial type controllers provide more sophisticated control and incorporate PID (Proportional, Integral and Derivative) functionality to improve the control accuracy.

The Proportional (P) action will decide how much to change the valve position after it determines the temperature, and therefore the error. In a proportional only controller the valve will be told to open until the room temperature is reached and it will then be told to close.  There will often be an offset where the room temperature never actually matches the set point. 

The integral (I) action accelerates the change towards the set point and also eliminates any offset.  Because the rate of change is greater the temperature will often overshoot the set point and then have to come back down again. 

The derivative (D) function notes the temperature is changing, and how fast, it then anticipates further change and alters the rate of change accordingly.  This gives a ‘soft’ landing at the desired temperature.

Hysteresis, dead band and linearity.

Often referred to in the same breath, hysteresis and dead band are different.  When related to a valve, hysteresis is the difference between the valve position on the upstroke and its position on the down stroke at any given input signal.  For it to be true hysteresis the valve will be moving at all times. See fig 1.  Hysteresis is most often caused by a high degree of static friction within the valve. 

Dead band on the other hand is when there is no movement, it generally occurs when the valve changes direction.  If you can imagine a linkage with a slot and a pin.  As the actuator is moving in one direction the pin is pressed against the side of the slot, moving the valve. When the actuator changes direction, the pin has to press against the other side of the slot before the valve will change direction.  If there is significant play in the slot there will be a period when the valve does not move, see fig 2.

The linearity of a valve is related to how close to a straight line the graph of travel vs. signal is. See fig 3.

All of these factors will affect the accuracy of a valve and its ability to control your process.

Valve selection

Before you can select a valve for proportional control you will need some basic information about your application. Are you controlling pressure, flow, temperature or level?  You will also need to know the maximum and minimum pressures that the valve is likely to experience and the flow range.  Another question that needs to be answered is whether your set point will be static, dynamic or ‘step by step’.

A static set point is one that will generally be fixed with infrequent changes.  Typical applications would be spot welding where a valve is used to control pressure to a cylinder that applies the pinching force.  The pressure will be set dependant on the material type and thickness being welded and will remain fixed until the machine is set up for a different material.  Another application could be leak testing where a fixed pressure is applied to a component to check for soundness.  The pressure will generally be fixed for a specific type of component but has provision for adjustment when the application is changed.

A dynamic set point is one that changes at a high frequency.  A typical application could be pressure control valves connected to cylinders operating a flight simulator.  Signals from the simulator control system constantly adjust the pressure in the cylinders to move the simulator cabin.  Another application could be material testing where the pressure output of a valve is steadily increased until the material under test fails.

A ‘step by step’ set point is one that changes but at a low frequency.  Typical applications could be a filling system where, as the container being filled approaches the required fill level, the flow rate is adjusted down so that the final filling happens at a lower rate.  Another application could be the control of gas to a burner on a cooker.  This application would require different flow rates of gas dependant on the amount of heat required.

There are other factors that need to be taken into account when selecting the right proportional valve for your application.  Many of these decisions will be made in conjunction with the suppliers application experts.

One of these factors will be the type of operator that you require.  Proportional valves can be operated by different means such as proportional solenoids, pulsed solenoids or piezoelectric elements.  In addition the valves may be pneumatically operated and controlled by a positioner.

Proportional solenoids adjust valve position based on a varying voltage across the coil.  The higher the voltage the greater the magnetic current and the more the valve spindle moves.  Valves such as the SENTRONIC valve from ASCO Numatics use a proportional solenoid.  A common operator type, it tends to give short response times and has an extremely low hysteresis.

Another operator type is a pulsed pilot valve.  These use small solenoid valves that load and unload pressure in a control chamber, allowing the valve to open and close.

Piezoelectrically operated valves are ideal when a very low power consumption is required.  Their highly compact size makes piezoelectric operated valves ideal for use in portable and table top machines such as medical equipment, measuring systems and gas analysers.  The valves have very low power consumption and can even be operated by batteries or solar cells.

When looking at larger capacities and larger flow rates it may be necessary to move to a valve fitted with a positioner.  The positioner acts as a pilot, enabling air to enter and vent from a diaphragm or a cylinder which enables the much larger valve to operate.  Valves such as these offer a wide flow range in a compact size.  These valves can be fitted with a positioner such as the COMPACT or POSIMATIC positioner.  The COMPACT positioner, as its name suggests is used when space is at a premium.  The POSIMATIC positioner is easily programmable by four data entry keys, a pop up menu and an alphanumeric LCD screen.

Another consideration when selecting a valve may be the means by which you want to adjust the setting, whether that is a set pressure or a set flow rate.  As mentioned previously, your set point could be static, dynamic or ‘step by step’.  Some valves enable you to adjust the set point by sending an analogue control signal over the power cables or by using some sort of fieldbus communications.  Others have a local interface and some enable you to communicate with the device via a PC and an RS 232 interface. Having a PC interface often enables a far greater degree of tuning and optimisation to take place, enabling you to get the most out of your process.

It was only ever the intention with this short article to introduce proportional valves and to discuss some of the factors that must be considered when purchasing them.  There are many companies supplying proportional valves and to the uninitiated, the selection of the best one for your application can be a challenge. By selecting a supplier with a broad range of technologies, sizes and operator types, together with the expertise to specify them, no matter what the application, flowing media or conditions, you can be guaranteed that you will get exactly what you need - rather than something that is good enough. 

ASCO Numatics
Skelmersdale
Lancs. WN8 9PG

Can be contacted on

Tel: 01695 713600
Fax: 01695 713633