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Engineering Guide > Choosing the Right 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% open provides a room temperature of 25ºC. 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 function 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 function 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 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 Figure 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 Figure 2. The Linearity of a valve is related to how close to a straight line the graph of travel vs signal is – see Figure 3. All of these factors will affect the accuracy of a valve and its ability to control your process.
 Valve selection
Before selecting a valve for proportional control you will need some basic information about your application. Are you controlling pressure, flow, temperature or level? What are the maximum and minimum pressures that the valve is likely to experience and the flow range; also whether your set point will be static, dynamic or ‘step
by step’?
A static set pointis generally fixed with infrequent changes. Typical applications are: spot welding — where a valve is used to control pressure to a cylinder that applies the pinching force, and the pressure is set dependant on the material type and thickness being welded and will remain fixed until the machine is set up for a different material, and; leak testing — where a fixed pressure is applied to a component to check for soundness, and the pressure is generally fixed for a specific type of component, but with provision for adjustment when the application is changed. A dynamic set point changes at a high frequency. Typical applications are: pressure control valvesconnected to cylinders operating a flight simulator, where signals from the simulator control system constantly adjust the pressure in the cylinders to move the simulator cabin, and; material testing, where the pressure output of a valve is steadily increased until the material under test fails. A ‘step by step’ set pointchanges, but at a low frequency. Typical applications are:filling systems 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, and; the control of gas to a burner on a cooker, which requires different flow rates of gas dependant on the amount of heat required.
   Other factors need taking account of when selecting the right proportional valve and many decisions will need to be made in conjunction with the suppliers application experts. One will be the type of operator required – proportional solenoid, pulsed solenoid or piezoelectric element. In addition, valves may be pneumatically operated and controlled by a positioner.
Proportional solenoids (eg in the Sentronic valve from ASCO Numatics) 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. A common operator type, it tends to give short response times and has an extremely low hysteresis. Pulsed pilot valves use small solenoid valves to load and unload pressure in a control chamber, allowing the valve to open and close.
Piezoelectrically operated valves(eg ASCO Numatics Piezotronic valve) are ideal when very low power consumption is required. Their highly compact size makes them ideal for use in portable and table top machines such as medical equipment, measuring systems and gas analysers. Power consumption is so low that they can 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 which acts as a pilot enabling air to enter and vent from a diaphragm or a cylinder, enabling the much larger valve to operate.
Another consideration may be the way settings are adjusted, eg a set pressure or a set flow rate. As already mentioned, the set point can be static, dynamic or ‘step-by-step’. On some valves the set point can be adjusted by sending an analogue control signal over the power cables or by using fieldbus communications. Others have a local interface and some (eg the ASCO Numatics SentronicD) enable you to communicate via a PC and an RS 232 interface, enabling a greater degree of tuning and optimisation to take place.
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