Servo and servoproportional valves control pressure or flow, and ultimately, force or velocity. Unlike simple directional valves, they can maintain any position between fully open in one direction or the other.
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.