Revolutionizing Hydraulic Control
Since the late 19th century, when the rapid development of electrical engineering brought tremendous changes to industry and society, hydraulic engineers have been expecting to control hydraulic valve systems electrically. The ability to control hydraulic valve operations through electrical means represented a significant leap forward in industrial automation.
Early in the 20th century, switching solenoids emerged, enabling electrical control but only in binary states - either fully open or fully closed. These devices couldn't provide the fine control necessary to regulate spool position and, consequently, the opening size of valves. This limitation restricted the precision with which hydraulic systems could operate.
The fundamental principle that magnetic field strength is proportional to the current through a coil suggested that the force generated by an armature-sleeve assembly should be proportional to the current. This principle held the promise to control hydraulic valve mechanisms with unprecedented precision by regulating electrical current.
Technical Challenges in Early Designs
The challenge with conventional solenoids was that their output electromagnetic force, as previously mentioned, depended heavily on the air gap - in fact, it was inversely proportional to it. Since the air gap is determined by the armature's stroke, as the armature stroke (valve spool displacement) changes, the air gap changes, and the electromagnetic force changes accordingly. This made precise control of spool displacement extremely difficult, hampering efforts to accurately control hydraulic valve operations.
Figure 1: Illustration of how air gap affects electromagnetic force in conventional solenoids
In the 1940s, classic servo valves were invented, such as flapper-type servo valves, which could regulate spool displacement with high precision. While these valves offered excellent performance, their intricate and complex construction resulted in high manufacturing costs. This expense prevented their widespread adoption and limited the advancement of electrical control in hydraulic systems that need to control hydraulic valve mechanisms efficiently.
The high cost and complexity of servo valves created a significant barrier to the broader implementation of electro-hydraulic control systems. Industries needed a more economical solution that could still provide the precision required to control hydraulic valve operations effectively.
The Invention of Proportional Solenoids
The 1970s marked a significant breakthrough with the invention of proportional solenoids - electromagnets whose output force remains essentially unchanged regardless of armature stroke. These innovative devices were far simpler in construction than classic servo valves and much more cost-effective to manufacture.
This combination of simplicity, lower cost, and reliable performance enabled proportional solenoids to quickly gain widespread adoption, facilitating a revolutionary advancement in the ability to control hydraulic valve systems electrically. For the first time, precise electro-hydraulic control became accessible to a wide range of industries.
Like switching solenoids, proportional solenoids consist of coils, armature-sleeve assemblies, and other components. The coils used in proportional solenoids can be identical to those used in switching solenoids, with some manufacturers offering interchangeable coils across their product lines. The composition of the armature-sleeve assemblies is also generally similar to those in switching solenoids, but with critical design modifications that enable their unique performance characteristics.
Proportional Solenoids for Pressure Control Valves
Armature-sleeve assemblies used in pressure valves differ from those in flow valves. As mentioned in section 3.3, the spool displacement in pressure valves varies with flow rate, and the key to regulating pressure is controlling the force acting on the valve spool. To effectively control hydraulic valve pressure, engineers developed specialized proportional solenoid designs optimized for this specific application.
Figure 2: Conventional relief valve with spring mechanism
Figure 3: Electro-proportional relief valve with solenoid control
In conventional non-electric relief valves (Figure 2), a spring regulates the force acting on the spool. Electrical control is achieved by replacing this spring with a proportional solenoid, allowing precise regulation of the force applied to the spool via a push rod by controlling the electrical current. This approach enables accurate control hydraulic valve pressure settings through simple current adjustments.
Relief valves require only small spool displacements. When spool movement is restricted to a very small range, the corresponding air gap changes minimally, which has little effect on the electromagnetic force. Consequently, the armature-sleeve assembly for these applications does not require specialized design features beyond those needed for basic proportional control.
In proportional solenoid assemblies designed for electro-proportional relief valves, the sleeve is typically made of stainless steel with moderate magnetic permeability. Axially, due to the sleeve's small cross-sectional area, only a small number of magnetic flux lines pass through it. Radially, the larger area allows most of the magnetic flux to pass through, creating a closed magnetic circuit between the armature and pole piece. This configuration generates the electromagnetic force that is transmitted to the valve spool via the push rod, allowing precise control hydraulic valve pressure by regulating the input current.
Practical Example
For instance, Hydraforce's non-electric direct-acting relief valve RV08-20 can handle a maximum flow rate of 22 L/min, while their similarly sized electro-proportional direct-acting relief valve TS08-20 has a maximum flow rate of only 4 L/min. This difference is due to the limited spool displacement in the proportional version, which is less than half that of the conventional valve. This example illustrates the design trade-offs when implementing proportional solenoids to control hydraulic valve functions.
Figure 4: Proportional solenoid assembly for electro-proportional relief valves (1-Push rod, 2-Pole piece (stop), 3-Coil windings, 4-Sleeve, 5-Guide bushing, 6-Armature)
Proportional Solenoids for Flow Control Valves
When proportional solenoids are used in flow control applications such as throttle valves and directional control valves, two significant challenges must be addressed to effectively control hydraulic valve performance:
- How to maintain consistent output force even with relatively large spool displacements (and thus larger air gap variations)
- Since solenoids can only regulate electromagnetic force, how to convert this force into precise displacement control
The following discussion focuses primarily on this type of proportional solenoid, which is specifically designed to control hydraulic valve flow rates with high precision across a range of operating conditions.
1. Displacement-Force Characteristics
The key innovation in these proportional solenoids is the strategic placement and shaping of magnetic shunts (also called flux rings). This design creates a portion where the armature and sleeve radially overlap, allowing some magnetic flux lines to pass radially through the sleeve, creating a closed magnetic circuit.
Figure 5: Magnetic flux line illustration in proportional solenoids (a) Structural diagram (b) Detailed view - 1-Pole piece, 2-Magnetic shunt, 3-Coil windings, 4-Guide bushing, 5-Armature, S-Air gap
As the armature moves, the radial overlap between the armature and pole piece increases. This causes more magnetic flux lines to pass radially through the sleeve and fewer to pass axially through the air gap from the armature end face. Consequently, even though the axial magnetic resistance decreases with armature movement, the axial electromagnetic force remains relatively constant. This design creates a "flat region" in the force-displacement characteristic where force remains essentially unchanged as the armature moves within the working range, which is critical to accurately control hydraulic valve flow rates.
Figure 6: Force-displacement characteristic of a switching solenoid (F-Electromagnetic force, S-Armature displacement/air gap)
Figure 7: Force-displacement characteristic of a proportional solenoid showing the flat region
The shape and position of the magnetic shunt are critical factors in determining the stroke-force characteristics. Tests have shown that different magnetic shunt configurations (angles) result in significantly different stroke-force characteristics, which directly impact the ability to control hydraulic valve performance consistently.
Figure 8: Influence of shunt geometry on stroke-force characteristics (Thomas) (A-Pole piece, B-Coil, C-Magnetic shunt, D-Limiting plate, F-Electromagnetic force, S-Armature stroke/air gap, δ-Idle stroke, S-Working stroke)
With this design, the output electromagnetic force can be precisely controlled by regulating the input current, providing a consistent force output across the working range of the solenoid. This consistency is essential to reliably control hydraulic valve positioning and flow rates in various operating conditions.
2. Converting Force to Displacement Using Springs
To convert the proportional force output of the solenoid into precise displacement control, the electromagnetic force is opposed by a spring. The valve spool assumes a position where the electromagnetic force balances the spring force, creating a stable equilibrium point. This arrangement allows the spool to move to and maintain a position proportional to the input current, enabling accurate position control by regulating the current supplied to the solenoid.
Figure 9: Current-force characteristics of a proportional solenoid at different currents (i-Current, F-Electromagnetic force, S-Armature displacement/air gap)
Figure 10: Force balance determining spool displacement (F-Electromagnetic force, S-Spool displacement, i-Control current)
This force-balance principle is what enables proportional solenoids to provide the precise positioning control necessary to regulate flow rates in hydraulic systems. By maintaining a linear relationship between input current and spool position, these devices allow engineers to accurately control hydraulic valve performance across a wide range of operating conditions, from simple on/off applications to complex proportional control scenarios.
Advantages of Proportional Solenoids
Precise Control
Enable accurate proportional control of hydraulic systems, allowing for precise regulation of pressure, flow, and direction. This level of precision was previously only achievable with much more expensive servo valve technology.
Cost-Effective
Offer superior performance compared to standard solenoids at a fraction of the cost of servo valves, making advanced hydraulic control accessible to a wider range of applications and industries.
Simplified Design
Have a simpler construction than servo valves, resulting in increased reliability and reduced maintenance requirements. This simplicity also makes them easier to integrate into existing systems.
Linear Characteristics
Provide consistent linear performance across their operating range, simplifying system calibration and control algorithm development when implementing them to control hydraulic valve functions.
Fast Response
Offer rapid response times to current changes, enabling dynamic control of hydraulic systems. This responsiveness is crucial for applications requiring quick adjustments to maintain system stability.
Versatile Application
Can be used in various hydraulic components including pressure valves, flow valves, and directional control valves, providing a unified approach to control hydraulic valve operations across entire systems.
Applications of Proportional Solenoids
The ability to precisely control hydraulic valve functions using proportional solenoids has revolutionized numerous industries, enabling more efficient, accurate, and responsive hydraulic systems. Some key application areas include:
Industrial Machinery
In injection molding machines, proportional solenoids control hydraulic valve systems that regulate clamping force, injection speed, and pressure, ensuring consistent part quality. Similarly, in metal forming equipment, they provide precise control over pressing forces and speeds.
Mobile Hydraulics
In construction equipment like excavators and cranes, proportional solenoids enable smooth, precise control of boom, arm, and bucket movements. Agricultural machinery uses them for accurate control of planting, harvesting, and material handling operations.
Automotive Industry
Proportional solenoids are used in automatic transmissions, active suspension systems, and hydraulic braking systems. They provide the precise control necessary for smooth gear shifts, adaptive damping, and anti-lock braking functionality.
Aerospace Applications
In aircraft systems, these solenoids control hydraulic valve mechanisms for flight control surfaces, landing gear actuation, and thrust reversers. Their reliability and precision are critical for safe aircraft operation.
Material Handling
In hydraulic lifts, forklifts, and conveyor systems, proportional solenoids enable smooth acceleration and deceleration, precise positioning, and safe load handling by providing accurate control over hydraulic flows and pressures.
Renewable Energy
Wind turbines and solar tracking systems use proportional solenoids to control hydraulic valve operations that adjust blade pitch and panel orientation. This precise control maximizes energy capture while protecting equipment from excessive loads.
Conclusion
The development of proportional solenoids represented a significant milestone in hydraulic control technology. By providing a cost-effective means to precisely control hydraulic valve functions with electrical signals, these devices have enabled widespread adoption of electro-hydraulic control systems across numerous industries.
The innovative design of proportional solenoids, particularly the strategic use of magnetic shunts, allows them to maintain consistent force output across a range of armature positions. When combined with spring mechanisms to convert force to displacement, they provide the precise, linear control necessary for modern hydraulic systems.
As industrial automation continues to advance, the role of proportional solenoids in enabling precise, efficient control of hydraulic systems will only grow. Their unique combination of precision, reliability, and cost-effectiveness makes them indispensable for applications requiring accurate control hydraulic valve operations.