The Difference Between Proportional vs. Directional vs. Servo Valves

By Bob Wojcik

You will get efficient and thoughtful service from Huade Hydraulic.

Understanding the available technologies in fluid power is imperative in order to design the most efficient, cost effective, and energy saving system. Traditional hydraulic equipment designs used directional control valves almost exclusively.

These directional valves are sometimes referred to as either &#;switching&#; or &#;bang-bang&#; valves and can be used to control flow direction, flow volume and fluid pressure. These valves can be operated by either AC or DC power.

Directional Control Valves

Directional control valves have been commonly referred to as switching valves because they simply direct or &#;switch&#; fluid passing through the valve from the source of flow to one of a selection of available cylinder ports. The flow control variety of valve generally selects an orifice which only allows a specified volume of flow to pass. The specified volume controls the speed of a cylinder or hydraulic motor. Likewise, the pressure control type is used to select a particular pressure setting.

Changing direction, flow or pressure during machine operation with these valves would require a separate individual valve for each direction, flow or pressure desired. The hydraulic circuit would become quite complex very quickly!

Proportional Valves

The technological solution to these more complex circuits has been the development of proportional valves. These revolutionary valves allow infinite positioning of spools, thus providing infinitely adjustable flow volumes. Either stroke-controlled or force-controlled solenoids are used to achieve the infinite positioning of spools.

This variable positioning allows spools to be designed with metering notches to provide flow/speed control as well as directional control functions all in one valve, instead of requiring separate valves for direction and speed. The other major benefit is when the circuit requires more than one speed. The various speeds are achieved by changing the electrical signal level to deliver the flow/speed required. No additional hydraulic components are required! These proportional directional valves are controlled by DC power.

The proportional controls, used with their associated electronic controls, also add the desirable features of acceleration and deceleration. This offers a variety of machine cycles, safely operated at greater speeds, yet with controlled start and stop characteristics. Regulated acceleration and deceleration result in improved machine overall cycle times and production rates.

Servo Valves

The third type of hydraulic directional control technology is the servo valve. Servo valves are not a new technology as servo valves were first used in the s. Servo valves operate with very high accuracy, very high repeatability, very low hysteresis, and very high frequency response. Servo valves are used in conjunction with more sophisticated electronics and closed loop systems. As a result, servo valves are always much more expensive. A proportional control valve system can be used to improve control of most machines without the high price tag of servo control systems.

Quality Hydraulics & Pneumatics, Inc. offers design, products and systems for all three types: directional, proportional and servo valve control systems. The Certified Fluid Power Specialists at Quality Hydraulics will assist you in selecting the best component, with the most efficient and cost effective solution.

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When it comes to Proportional Valves, they are well-suited to a number of different applications in machinery, basic processes and hydro-pneumatic systems.  They provide compact and cost effective solutions for controlling pressure or flow at low flow rates.

With so much differentiation and sources of supply, it is important to ensure you are picking the correct proportional valve to suit your application. This article will describe the important considerations you must take into account when selecting the correct proportional valve.

Proportional Valve Set Points

The first things to consider are:

• What are the maximum and minimum pressures that the valve is likely to experience?
• What are the flow ranges the valve is likely to experience?

Static Set Point

Pressure or Flow will be Fixed With Infrequent Changes

Typical Applications:

• 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.
• 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.

Dynamic Set Point

Pressure or Flow will Change at a High Frequency

Typical Applications:

• 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.
• Material testing where the pressure output of a valve is steadily increased until the material under test fails.

Step-By-Step Point:

Pressure or Flow will change but at a Low Frequency

Typical Applications:

• 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.
• 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.

Adjusting Set Points

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 discussed above, the set point can 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
• Some enable you to communicate with the device via a PC and an RS 232 interface (this enables a far greater degree of tuning and optimisation to take place.

Actuation of Operation of your Proportional Valve

Engineers should also consider the type of operation they require. Different operation methods for proportional valves could be:

• Proportional solenoids: valve position is adjusted based on a varying voltage across the coil. The higher the voltage the greater the magnetic current and the more the valve spindle moves. It tends to give short response times and has an extremely low hysteresis.

• Pulsed solenoids: uses small solenoid valves that load and unload pressure in a control chamber, allowing the valve to open and close.

• Piezoelectric elements: 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.

• Pneumatically operated and controlled by a positioner: 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.

The intention of this article is to discuss some of the factors that must be considered when specifying and purchasing proportional valves. For more information, please leave comments below, post a technical question in the Solution Centre or us at [ protected].

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