The basics of electrohydraulic valves are easy to understand: they are electrically operated valves that control how hydraulic fluid is sent to actuators. However, to apply electrohydraulic valves for efficient and effective hydraulic systems, designers must consider several factors. This article will explore seven key design considerations for applying electrohydraulic valves.
On/off vs. proportional valves
On/off valves are basically the on/off switches for hydraulic systems. On/off valves are typically used in applications where precise position or speed control are not required. Parker offers a first-class line of on/off directional control valves under the VW series. Proportional valves offer more variable control of flow rates for hydraulic systems. These valves are typically used in applications where more control is required beyond a standard directional control valve. A few applications that call for variable flow rate control where proportional valves shine include wind turbine pitch control, wood processing, machine tools, metal forming. If specific timing and/or positioning is required, think proportional.
Onboard electronics vs. offboard electronics
Determining whether a valve with onboard or offboard electronics is the best choice requires an in-depth evaluation of the application. Generally, onboard electronics are used to localize control at the valve and simplify the wiring at the controller. Offboard electronics are often utilized in areas with high vibration and temperatures that can reduce the performance of the electronics.
Driving an offboard electronics valve requires use of an electronic module, such as a series PWD00A-400, which can be configured to custom parameters such as desired solenoid drive current and ramp rates. Onboard electronics valves can be commanded directly with a standard command including 4-20 mA or ±10 VDC and flow for the same level of customization.
Open-loop vs. closed-loop control
There are two control options for hydraulic systems: open-loop and closed-loop. In general textbook terms, an open-loop system cannot compensate for any disturbances that alter the driving signal of the controller. Closed-loop systems do not have this shortcoming; disturbances in the system are compensated for by measuring the output response and comparing it to the input. If there is an observed difference (known as an error signal), the error is fed back to the controller to adjust the output to the desired value.
For example, FB series valves are operated by proportional solenoids but do not internally close the loop around the spool. They are used for acceleration and metering applications.
Valves like the FC and FP series close the loop internally around the position of the spool but can be integrated into a closed-loop system. The error in the system is measured by a transducer, i.e. a position or speed sensor on an actuator, a pressure transducer, or a flow meter for even greater accuracy. Electrohydraulic servo valves operating in closed-loop control systems are designed to use low power and mechanical feedback to provide precise control.
Parameters that must be considered when specifying open and closed loop control systems include:
- Hysteresis: the difference in measured output between increasing and decreasing command
- Step response: the time required from initial command to when the valve stabilizes at the desired output
- Frequency response: the maximum speed at which a valve can operate with accuracy
- Internal leakage: bypass flow inherent to spool valves due to mechanical clearances
- Flow capacity: the amount of fluid that can pass through the valve
Proportional valve spools are typically rated for a nominal flow at a differential pressure of 10 bar, while servo valve spools are typically rated for a nominal flow at a differential pressure of 70 bar. Equal metering spools provide symmetrical flow to each work port. This can be useful when driving a motor or a double-rod cylinder with equal effective areas. Equal metering spools will result in reduced speed during retracting of a single-rod cylinder due to the differential area between the rod and the piston.
Ratio spools provide asymmetrical flow between the work ports. The most commonly used is a 2:1 ratio design. When used to drive a 2:1 ratio cylinder, for example, equal speed will be achieved between extension and retraction of the cylinder due to the matching of the imbalanced areas. It is recommended to size proportional valves as small as possible to control the load. To maintain control, back pressure against the load must be exerted at all times. A general rule of thumb is to select a spool that will use 90-95 percent of the maximum flow rating. Selecting a spool with too large of a flow capacity can result in instability of the system.
When selecting the compound for the elastomeric seals in any directional control valve, consult the manufacturer’s resources for fluid and compound compatibility information such as in the Parker O-Ring Handbook. Standard industrial applications using mineral oil will typically use a nitrile seal, which is also recommended when controlling water-glycol. Applications involving elevated temperatures or less commonly used fluids may utilize one of many grades of fluorocarbon seal. When in doubt, consult the factory for assistance in selecting a seal compound.
Regenerative circuits and hybrid functions
A regenerative circuit routes fluid evacuated from the rod end of a cylinder back to the piston end instead of to the tank for accelerated extension of the actuator. Use of a regenerative circuit can allow a system designer to use a smaller pump to achieve design requirements when rapid movement is needed in only one direction. Parker offers regenerative valves directional control valves for on/off and proportional control (R flow code) which allows system designers to achieve regenerative function without the need to add additional valves to the circuit.
Hybrid regenerative valves offer system designers the ability to enable regenerative control through a separate electrical signal than the command signal. When utilizing regenerative control, force is sacrificed for speed. The hybrid function (Z flow code) enables designers to select between standard hydraulic function to build force and regenerative function to accelerate the load quickly.
Mounting configurations for electrohydraulic valves are guided by NFPA/ISO standards. D03, D05, D07, D08 and D10 series designations will indicate compliance with the standards. Parker valves that are D07 or larger are pilot-operated. Pilot-operated valves are more stable across a wide range of flows and enable systems to operate with greater flow capacity. Often, the hydraulic pilot pressure used to control the main stage spool provides greater force than that of a solenoid on a directional drive valve, resulting in more predictable performance for the user.
There are many resources available to design engineers when specifying components for systems: reference sheets, calculators, configuration tools and more. However, there is no substitute for experience and deep application knowledge. Take advantage of the “been there” and “seen that” experience that your supplier’s applications engineers can offer. Not only do they understand how the components their company offers perform, they have the benefit of having helped others in the same situation correct design mistakes and they have problem-solved for some of the most unique applications. More information is available in our EH catalog or Contact us.
Article contributed by Matthew Davis, product sales manager; Mitch Eichler, applications engineer; and Tom Gimben, product sales manager, Hydraulic Valve Division, Parker Hannifin Corporation.