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| To designate a proportional valve schematically, the familiar digital directional control valve symbol in the illustration above is augmented with two straight exterior lines outside the valve envelope that run parallel to the valves long axis. These additional lines indicate that the spool has the ability to assume and maintain any intermediate position over the full range of spool travel. |
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| Fig. 1. First-stage configurations for nozzle flapper and jet-pipe valves. |
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| Fig. 2. The linear force motor often is used to drive the spool of high-performance valves directly. An alternative is to use one or two proportional solenoids to drive the spool. |
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| Fig. 3. When the flapper nozzle pilot section (a) is drawn in schematic form, (b), it is obvious that a bridge circuit exists. By moving the flapper, restrictions Ra and Rb change in opposite directions. This unbalances the bridge and causes the spool to move against its centering springs. |
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| Fig. 4. The 4-way spool valve has four individual lands that vary in unison as the spool shifts - two lands open while the other two close. When drawn in schematic form, it is clear that the four lands constitute a bridge circuit, and spool movement unbalances the bridge one way or the other to cause a reversal in load flow. |
High-performance valves are usually classified as either servo or proportional, a distinction that gives an indication of expected performance. Unfortunately, this classification tends to generalize and blur the true differences between various valve styles. Selection depends on the application, and each valve has merit when it comes to controlling pressure or flow.
Traditionally, the term servovalve describes valves that use closed-loop control. They monitor and feed back the main-stage spool position to a pilot stage or driver either mechanically or electronically. Proportional valves, on the other hand, move the main-stage spool in direct proportion to a command signal, but they usually do not have any means of automatic error correction (feedback) within the valve.
Confusion often arises when a valve's construction resembles a proportional valve, but the presence of a spool position feedback sensor (usually an LVDT) boosts its performance to that rivalling a servovalve. This reinforces the concept that designers and suppliers should use common terminology and focus on the performance reuirements of the particular application at hand.
Typically, proportional valves use one or two proportional solenoids to move the spool by driving it against a set of balanced springs. The resultant spool displacement is proportional to the current driving the solenoids. The springs also center the main stage spool. Repeatability of the main-stage spool position is a function of the springs' symmetry and ability of the design to minimize nonlinear effects of spring hysteresis, friction, and machining tolerance variations.
Servovalves
The term servovalve traditionally leads engineers to think of mechanical feedback valves, where a spring element (feedback wire) connects a torque motor to the main-stage spool. Spool displacement causes the wire to impart a torque onto the pilot-stage motor. The spool will hold position when torque from the feedback wire's deflection equals the torque from an electromagnetic field induced by the current through the motor coil. These two-stage valves contain a pilot stage or torque motor, and a main or second stage. Sometimes the main stage is referred to as the power stage. These valves can be separated primarily into two types, nozzle flapper and jet pipe, Figure 1.
The electromagnetic circuit of a nozzle flapper or jet-pipe torque motor is essentially the same. The differences between the two lie in the hydraulic bridge design. A hydraulic bridge controls the pilot flow which, in turn, controls the main-stage spool movement. In a nozzle flapper, the torque produced on the armature by the magnetic field moves the flapper toward either nozzle depending on command-signal polarity. Flapper displacement induces a pressure imbalance on the spool ends which moves the spool. In a jet pipe, the armature movement deflects the jet pipe and asymmetrically imparts fluid between the spool ends through the jet receiver. This pressure imbalance remains until the feedback wire returns the jet pipe or flapper to neutral.
Historically, jet pipe and nozzle-flapper servovalves have competed for similar applications that require high dynamics. Typically, better first-stage dynamics gives the nozzle flapper better overall response, whereas improved pressure recovery of the jet/receiver bridge design gives the jet-pipe motors higher spool driving forces (chip-shearing capability). Both valves require low command currents and therefore offer a large mechanical advantage. Motor current for these style valves is typically less than 50.0 mA. Note that these servovalves are also proportional valves, because spool displacement and flow are directly proportional to the input command.
Direct-driven valves
Direct-driven valves, unlike hydraulically piloted two-stage valves, displace the spool by physically linking it to the motor armature. These valves usually come in two basic varieties, those driven by linear force motors (LFM) and those actuated by proportional solenoids. Within these two general classifications, the valves can be separated into proportional and servoproportional. The distinction is based on the use of a position transducer to provide spool position feedback. Servoproportional valves must incorporate closed-loop spool position feedback to increase repeatability and accuracy necessary for high-control applications. Typically, servoproportional, direct-driven valves have an overall lower dynamic response than hydraulically piloted two-stage valves with the same flow characteristics. This is usually due to the large armature mass of the LFM or solenoid and the large time constant associated with the coil, which is a function of the induction and resistance of the coil.
Unlike hydraulically piloted servos, direct-driven valve performance does not vary with changes in supply pressure. This makes them ideal for applications where pilot flow for first-stage operation is not available. Direct-driven valves also tend to be viscosity insensitive devices, whereas nozzle-flapper and jet-pipe valves work best with oil viscosity below 6,000 SUS. However, most direct-driven valves cannot generate the high spool driving forces of their hydraulically piloted counterparts.
Like the torque motor used in the nozzle flapper/jet pipe servos, the LFM allows for bidirectional movement by adding permanent magnets to the design and therefore making the armature motion sensitive to command polarity. In the outstroke, the LFM must overcome spring force plus external flow and friction forces. During the backstroke to center position, however, the spring provides additional spool-driving force which makes the valve less contamination sensitive. Magnetic-field forces are balanced by a bidirectional spring that lets the spool remain centered without expending any power.
Unlike the LFM, the proportional solenoid is a unidirectional device. Two solenoids oppose each other to achieve a centered, no power, fail-safe position. When a single solenoid is used, holding the spool at midstroke requires a continuous current to balance the load generated by the return spring. This makes the design less energy efficient than its LFM or a dual-solenoids counterpart. During a power loss, the LFM and dual proportional solenoid designs fail to a neutral position and block flow to the load, that is the piston. When a single solenoid design loses power, the spool must move through an open position that tends to cause uncontrolled load movements.
Multistage valves
All of the aforementioned designs can be used to create a multistage hydraulic valve. The approach for each design is specific to the application requirements. Usually, most designs do not exceed three stages. Mounting a nozzle flapper, jet pipe, or direct-driven valve onto a larger main stage satisfies most requirements for dynamics and flow. Sometimes, the jet-pipe valve is used in a multistage configuration where the mechanical feedback of a traditional jet pipe is replaced with electronic feedback. This servojet style has pilot characteristics of a typical jet pipe. Depending on the required control, many multistage valves close a position loop about the main stage using a linear variable differential transducer. This device monitors the spool position. In case of hydraulic power loss, springs on opposite sides of the main stage spool return it to a neutral position.
Hydraulic system design
To choose the proper hydraulic valve for a specific application, designers must consider specific application and system configurations. Supply pressure, fluid type, system force requirements, valve dynamic response, and load resonant frequency are examples of the various factors affecting system operation.
Hydraulically piloted valves are sensitive to supply pressure disturbances, whereas direct-driven valves are unaffected by supply pressure variation. Fluid type is important when considering seal compatibility and viscosity effects on performance over the system's operating temperature range.
Total force requirements must include all static and dynamic forces acting on the system. Load forces can aid or resist, depending on load orientation and direction. Forces required to overcome inertia can be large in high-speed applications and are critical to valve sizing.
The load resonance frequency is a function of the overall travel stiffness, which is the combination of the hydraulic and structural stiffness. For optimum dynamic performance, a valve's 90° phase point should exceed the load resonant frequency by a factor of three or more.
The valve's dynamic response is defined as the frequency where phase lag between input current and output flow is 90°. This 90° phase lag point varies with input signal amplitude, supply pressure, and fluid temperature so comparisons must use consistent conditions.
A closer look
Viewing the principal internal parts of a flapper-nozzle servovalve, Figure 3, it should be clear that a torque applied from a torque motor to the flapper arm, say in the clockwise direction, moves the flapper closer to nozzle A and tends to close it.
Concurrently, the flapper moves away from nozzle B to allow more flow through it, so the net result is a rise in pressure Pa and a drop in pressure Pb. The pressure difference, Pa - Pb is felt across the two ends of the main valve spool, driving it to the right and creating communication from port P to port B, and from port A to port T.
A 4-way directional control valve is represented in Figure 4. When the valve spool moves to the right, Rp to a and Rb to t open while Rp to b and Ra to t close. Fluid flows from the valve's A port to the load and returns via B port to tank. Left spool movement opens Rp to b and Ra to t so that fluid flows from the valve's B port to the load, returning to tank via A port.
