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.
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.
| Fig. 7. The four nominally equal air gaps of an electromagnetic torque motor each carry equal magnetic flux from the permanent magnets, producing zero net torque on the armature. When current enters the coil, coil-induced magnetic flux adds to or subtracts from the four air-gap fluxes, creating a torque on the armature. Armature movement typically causes a flapper to move, changing resistivity of the two nozzles. |
| Fig. 8. A family tree of torque-motor electromechanical interfaces indicates all of the common piloting methods presently used in industry. |
| Fig. 9. Current entering the torque-motor coil, (a), causes the armature to rotate against a stiff feedback spring. The flapper, attached to the armature, blocks nozzle A and relieves nozzle B, causing pressure PA to rise and PB to fall. This unbalance moves the spool to the left. As the spool moves, (b), the feedback spring, anchored to the spool and the flapper, forces the flapper toward center. Eventually, the flapper and spool reach a position where the flapper is nearly centered, the pressures are nearly equal, and the spool comes to rest at a position commensurate with the amount of torque (coil current). |
| Fig. 10. Current in the torque motor of a jet-pipe servovalve steers a jet nozzle, causing a pressure difference between two collector ports. If A-port pressure is high, for example, the main spool moves to the right. Concurrently, the feedback spring drags the jet nozzle toward center and approximately equalizes collector pressures. Thus, the main spool has been positioned as directed by the coil current. |
| Fig. 11. The swinging-wand pilot stage generates a differential pressure in receiver ports C1 and C2 by deflecting two fluid streams off each edge of the wand. An unseen torque motor moves the wand in proportion to the amount of current. Thus the pressure difference between C1 and C2 is a reflection of coil current. Port pressures are equal, (a), C1 pressure is higher, (b), and lower, (c). |
| Fig. 12. A permanent magnet creates equal fluxes in the four air gaps of electromagnetic force motor that results in net zero force on the armature. Current into the coil in the direction shown, for example, strengthens flux in gaps B and D and weakens flux in gaps A and C. Now there is a net force to the left, pushing the poppet against the nozzle. Through control of force, the current controls output pressure. |
| Fig. 13. The trapezoidal air gap of a proportional solenoid is shaped to create a relatively constant force regardless of armature position when the current is constant. Because there are no permanent magnets, the force is always in one direction (to the left here), regardless of current direction. Thus, bidirectional valves always require two proportional solenoids. |
| Fig. 14. Typical force vs. armature position curves show region of proportional solenoid armature travel where there is relatively constant force at constant current. Valve designers must use the solenoid so the armature operates in this proportional region. With current technology, the region is about 0.10-in. wide. |
| Fig. 15. Family tree of pilot-operated PHEIDs. |