By Bob Wojcik
For more information, please visit Factors affecting proportional valve performance.
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 1940s. 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.
A throttle valve in the tank line of a conventional solenoid valve controls actuator speed in a meter-out configuration. The actuator cannot run away with a throttle valve at this location. Make sure the directional valve can withstand any backpressure in the tank line that is greater than the circuit produces.
If you are looking for more details, kindly visit Comparing analog and digital proportional valves.
One throttle valve in the main pump line can vary the speed to one actuator or several that cycle at different times. This type of circuit is less expensive but requires a more-complex electrical control circuit.
The throttle valve configuration in Figure 14-10 gives infinitely variable flow. Adding the hydro stat module to the pump line keeps the pressure drop across the orifices constant. With a constant pressure drop, flow does not fluctuate. Because the 4-way valve never sees reverse flow, both flow paths can supply the circuit. Either flow path has a nominal pressure drop at a specified flow. This arrangement gives twice rated flow without excess pressure drop or heat.
The parallel flow path module comes with all flow paths internally drilled and sized to keep pressure drop to a minimum. This module is available in D03 and D05 sizes for flows up to approximately 50 gpm.
Use proportional control valves to reduce shock and give a finer degree of control to circuits that do not require extreme position accuracy, or repeatable speed and force.
Proportional valves restrict flow to and from an actuator. They work best with a pressure-compensated pump in a closed-center circuit. An accumulator in the circuit enhances cycle response time and protects the pump from pressure spikes. Systems that use proportional valves usually require a heat exchanger because energy waste is higher with this type circuit.
The following sections describe a few more circuits — with some pointers for using proportional valves in several applications. Always remember to size the valves for maximum flow and pressure drop to get optimum response and repeatability from the circuit.
READ MORE: Advanced Electropneumatic Positioning Achieves Dynamic Force Control
The circuits in Figures 14-11 and 14-12 control acceleration and deceleration of an actuator. Electronic signals to these circuits also can vary the speed of the actuators infinitely.
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