Proportional control valves are infinitely variable but they are neither highly responsive nor capable of handling minute flow changes rapidly and accurately. On the other hand, servovalves easily meet both of these requirements . . . but at a cost. They are more expensive than proportional valves, they require super-clean fluid, and they need extra electronics to exploit their full capabilities.

The three common servovalve types are flapper, jet pipe, and mechanical. Each design has advantages as far as operation accuracy, leakage, contamination tolerance, and price. They range in flow capacity from less than 1 gpm to more than 1000 gpm. Most manufacturers make valves that operate at 3000 psi, but some offer valves at 5000 psi.

The main difference between proportional and servovalve circuit design is that servo systems have a method of feedback that assures that the actuator is doing what the controller tells it to do. A super-simple form of servo control would be a backhoe operator moving manual valves to cause a bucket to move toward him at a given rate. Feedback from the operator’s eyes would tell his hands when and how far to move the levers to give more or less flow to maintain the action he wants. Other familiar mechanical feedback examples are hydraulic driven power steering and hydraulic power brakes on a vehicle.

The circuit in Figure 12-7 is an example of a working circuit with mechanical feedback that controls a hydraulic press. The operator needs to have a feel for the motion of a platen as it cuts through some tubing. Originally this was done with an arbor press, but it was hard for the operator to keep up with production due to the physical exertion. Now all the operator has to do physically is overcome the spring force of the manually controlled mobile directional valve to make the platen move. As the platen moves, the directional valve body also moves, so the operator has to keep moving the lever to advance or retract the platen. Notice the mechanical link between the valve body and the cylinder rod that moves the valve body at the same rate the cylinder rod moves. The operator now has a hydraulic force multiplier that gives some feel to what he is doing.

The reason for using a mobile-type valve is because those valves have less spool overlap and the spool has notches cut in it. The notches pass a small flow almost immediately when the spool moves. That flow increases in proportion to spool movement.

Most industrial applications use feedback from electronic linear, rotary, or force transducers. A transducer is a device that produces an electrical signal in direct relation to a position, force, or speed.

Linear potentiometers work for short strokes (12 in. or less). Longer strokes require a device such as a Temposonics transducer. In either case, these devices feed a precise position or speed indication back to an electronic controller.

For rotary motion, an encoder or similar device that produces multiple pulses per revolution sends a signal about rpm or angle of rotation to the controller.

When information about force is required, a load cell sends the data to the controller.

With these very accurate feedback devices and a fast-response servovalve, an actuator’s position, speed, and/or force can be repeatedly established within an extremely close range. Electronics provides the accuracy while hydraulics provides the force via a super-responsive servovalve.

The cutaway view and symbol in Figure 12-8 show a less-responsive but more contamination-tolerant servovalve. There are other mechanical ways of driving the spool. The valve in Figure 12-9 uses a rotary drive and an eccentric to move the spool left or right to an infinite number of positions. Because the drive is quite strong and there are no orifices to clog, this valve can operate with fluid that meets ISO Code 4406 20/16/13.

Notice the difference in design between spools in proportional valves and servovalves. Most proportional valves use spools with overlap and some sort of notches that pass flow while moving out of overlap. A servovalve has no overlap or underlap of the spool lands to the body lands. (One manufacturer calls it “Critical lap” because all points blocking fluid cannot move without passing flow.) This spool design makes the valve very responsive (as well as very expensive and prone to above average bypass). Servovalve spools and bodies always come in matched sets because of their close fit and four points of land-to-land match.

The rotary-drive eccentric valve pictured in Figure 12-8 has fast, controllable spool movement from a rotary drive that incorporates a feedback loop. When the drive receives a signal to move the spool to pass a certain flow, a position feedback output sends a signal back to the controller when the motion is complete. There is still feedback from the actuator that what was commanded is happening, so spool position can be changed via the electronic feedback and controller as necessary.

Several factors determine when a given input will not produce the desired actuator output. The main factor is actuator load. As load changes, input force must change -- by allowing more or less fluid into the circuit. Fluid viscosity also has an effect, so the flow path must be reduced as viscosity lowers and enlarged as viscosity rises. Then there is system pressure. As pressure fluctuates, flow across the spool orifice changes. The higher the pressure drop, the greater the flow. Because a servovalve circuit has feedback from the actuator, it can adjust flow or pressure to match system changes continuously.

Figure 12-9 shows another mechanically driven servovalve. This setup works for cylinders and hydraulic motors, but must be directly attached to the actuator (as shown) or driven by it with a toothed belt and pulleys. This is a very contamination-tolerant valve arrangement because the stepper motor is quite strong and there are no small orifices to clog. The spool has no overlap or underlap so any movement immediately initiates fluid flow to and from the cylinder. The piston and rod cannot rotate so the feedback screw turns as the piston extends or retracts.









As the stepper motor turns, a threaded rod inside the threaded spool moves the spool to direct fluid to extend or retract the piston. When the cylinder piston moves, the feedback screw turns the spool back on the threaded rod to counteract the stepper motor shift. When the stepper motor turns, the piston moves at a speed proportional to the stepper motor’s rpm. When the stepper motor stops, the piston catches up and stops also. Manufacturers claim a tolerance of ±0.001 in. repeatability. If an external force tries to push or pull the piston out of place, the feedback screw shifts the spool and fluid starts resisting movement.

From the foregoing explanation, it is easy to see how this valve – when attached to a hydraulic motor -- would give an exact number of turns and repeatedly cause the motor to stop at exactly the same place. This will happens even if the hydraulic motor has internal leakage. The only time the actuator gets out of place is when it cannot overcome the load and stalls. The stepper motor continues to receive pulses, but it also stalls when the spool shifts all the way. The stepper motor received a signal that should have placed the actuator at a certain distance but the actuator did not get there because of insufficient force. When the valve reverses, it starts its motion from the wrong point and will overshoot home position as it returns. A limit switch at the home position can alert the controller that there is a problem when the actuator overshoots -- and prevent the machine from producing scrap.

The jet-pipe servovalve pictured in Figure 12-10 also tolerates contamination due to a control orifice that is large enough to pass large particles. Pilot oil is tapped off the system fluid inlet, sent through a coarse filter, and on to the jet pipe that terminates in an orifice. The orifice outlet is centered over the inlet of two passages that terminate at each end of a critical-lap spool. Flow into these passages puts equal pressure on both ends of the spool as the feedback wire holds it centered. Current signals to the coils cause the armature to rotate and shift more of the output of the jet pipe to one passageway than the other. Pressure increases on one end of the spool and decreases on the other end. As the spool shifts, it starts to pass flow to the actuator at a rate set by the input electrical signal.

When the jet pipe shifts to the left, the spool moves to the right. At the same time, the feedback wire also moves to the right, pulling the jet pipe nozzle back to center and stopping spool movement. A given input to the coils electromechanically shifts the armature that moves the jet pipe. This moves the spool hydraulically and forces the jet pipe back to center mechanically through the feedback wire.

A measured electrical input to a servovalve produces a fixed flow output, similar to a proportional valve. This control alone does not give much better control than a proportional circuit even though the valve is more responsive. There is still no compensation for viscosity or pressure changes that can cause the actuator’s speed to fluctuate. To overcome this problem, some sort of electronic feedback from the actuator is necessary. The feedback signal through an electronic circuit board modifies the signal to the servovalve to make the actuator perform as planned. Actually, the electronics do the work as long as the valve can respond quickly enough to keep everything working at the correct rate. This means the spool must be free enough to move easily without excessive bypass. Anytime the spool moves, it should pass flow to and from the actuator.

The valve in Figure 12-10 is considered a 2-stage valve. The first stage is electronic and receives an electronic input signal, while the second stage is fluid powered by a hydraulic signal.

The jet-pipe servovalve depends on clean oil for long trouble-free operation -- not as clean as the requirement for the flapper-valve design discussed next, but clean enough to prevent the jet-pipe nozzle from clogging. It is obvious that once nozzle flow is retarded enough or stops, the valve loses all ability to control flow to the actuator.

The most responsive and accurate servovalve design is the flapper valve, shown in Figure 12-11. This design is the least tolerant of contamination because it depends on very small orifices for fast response with minimal wasted energy. It is called a flapper valve because the element that holds equal pressure on both ends of the spool at rest reminds one of a flapping device. It is a 2-stage valve with an electronically controlled torque motor as the first stage and a pilot-operated spool as the second stage. As in all servovalves, the spool has no overlap or underlap that would make it sluggish or bypass a lot of fluid unnecessarily.

Fluid from the pump inlet is tapped off through rather-coarse filter elements, passes through orifices past both ends of the spool, goes on to nozzles, and out to the return line. The orifice diameters are slightly larger than the nozzle diameters, so there is a pressure buildup at both ends of the spool. A feedback wire attached to the flapper terminates in a ball end that sits in a very close-fit slot in the spool. A sleeve around the spool can be moved left or right by a null adjustment to align the spool and body lands perfectly when the valve is first installed. (Usually null adjustment is only required at startup of the valve.)

The null adjustment usually is a hexagonal wrench fitting attached to an eccentric pin located in the sleeve slot. With the null adjustment centered, turning it one round moves the sleeve from center to full right, back to center to full left, and back to center. If the valve cannot be nulled within one rotation of the null adjustment, replace it and send it in for repair. This usually indicates a clogged orifice or nozzle controlling one end of the spool.

Unplug the electrical supply to the valve before setting null. Start the pump and watch for actuator movement. If the actuator moves, loosen the null lock screw and carefully turn it. Observe whether the actuator slows or picks up speed. A nulled valve stops actuator movement because the forces on both sides are equal. High-flow 3-stage valves cannot be nulled to the point of stopping an actuator due to the piloted spool slipping by the stop-flow position as the pilot operator is adjusted. When null is set, lock the null screw and reattach the electrical plug.

Turning the null screw with the electric plug detached is one way of moving an actuator manually. This might be done to prove the valve is working properly and the problem is electrical.

When the torque-motor coils receive a current signal, the armature rotates clockwise or counter-clockwise and pushes the flapper closer to one nozzle and farther away from the opposite one. This allows pressure to increase at one end of the spool and decrease at the other. The spool then starts to move away from the higher pressure. If the armature turns clockwise, pressure builds on the left end of the spool and it moves to the right, as shown in the left cutaway view of Figure 12-12.







As the spool moves to the right, it also drives the feedback wire to the right. The feedback wire is strong enough to overcome armature force and pull the flapper back to center. After the flapper centers, pressure is equal on both ends of the spool and it stops. More current to the coils causes more rotation and additional spool shift until the feedback wire again centers the flapper.

From the foregoing explanation, it is obvious why this valve needs clean oil. If an orifice or nozzle clogs, the spool shifts all the way to one end and the actuator moves until it runs into a resistance it can’t overcome. Also, the spool must start shifting at a very low pressure drop across it to keep response high. Contaminated fluid can cause sticking and require high differential shifting pressure that makes spool movement erratic.

For flows above 60 to 80 gpm, a 3-stage servovalve is required. It consists of a small 2-stage pilot operating a large pilot-operated spool, as depicted in Figure 12-13. The 2-stage valve operates as just explained, but its output goes to move a pilot-operated spool in the third stage to a precise position to control high flow to large actuators.

An LVDT signals the electronic control circuit that the pilot-operated spool is where it was signaled to go. After receiving that position signal, the 2-stage valve shifts to no flow or whatever flow it takes to keep the pilot-operated spool in place.

A 3-stage valve also depends on feedback signals from the actuator to modify the input signal when the action is not in compliance with the command. This makes 3-stage valves very accurate controllers of large cylinders.