The schematic drawing in Figure 21.7 shows the general arrangement for a typical servocircuit that accurately controls cylinder position. When a cylinder must quickly go to many different locations with an accuracy of less than ±0.020 in., a servocircuit is the best way to control it.

Figure 21.7












Notice that the hydraulic power unit has a pressure-compensated pump with accumulators. This arrangement holds constant pressure and has ample volume for short bursts of high flow. Without the accumulators, there is a sharp pressure drop when a cylinder starts moving. Fixed-displacement pumps and accumulators work also, but the power unit shown here is best overall.

Place the servovalve as close as possible to the actuator (preferably attaching it directly to it). Use rigid piping when the valve cannot mount directly on the actuator. Flexible lines between the servovalve and the actuator can negatively affect the accuracy and stability of the circuit.

Always install pressure filters in the lines to the servovalves. One pressure filter after the pump might be sufficient when the power unit is close to the valves. Separate filters are advisable when there is some distance to the servovalves. Use a cleanliness level of 1 to 5 µm in a servocircuit. Even normal pump-wear contamination quickly plugs orifices and sticks spools in most servovalves. Do not use a bypass-type pressure filter in a servovalve circuit. Even with a 125-psi bypass spring, contaminated fluid can get around the filter during a normal cycle. It is better to shut the machine down with a clogged filter than with a contaminated servovalve and a dirty filter.

Because the cylinders in this arrangement must stop accurately at many different locations, the circuit includes a feedback transducer at the cylinder rod. When the PLC commands the cylinder to go to a certain location, the PLC sends a signal to the servovalve control card. The servo control card sends an output to the servovalve that starts the cylinder moving. As the cylinder moves, the feedback transducer constantly sends position information to the servo control card. When the cylinder approaches the predetermined position, it slows and stops within a few thousandths of an inch of that location every cycle. Because electronic hardware controls the speed and position of the cylinder, fluid viscosity, load, pressure drop, or machine friction have no effect. The control card modifies the sevovalve shift to offset external or internal system changes as long as the actuator has ample power to overcome them.

In essence the electronics modify servovalve output according to actual actuator movement to get the desired accuracy. A servovalve controls oil flow as a 4-way directional valve would, but it has the ability to change flow continuously. Response time of the servovalve to the electronic controllers' changes is the important thing. Less-expensive, more dirt-tolerant servovalves offer less-accurate control.

With the circuit in Figure 21.7, cylinder positioning at any location within its stroke is attainable with repeatable accuracy to thousandths of an inch.

Servovalves for accurate control of position and velocity
The schematic diagram in Figure 21.8 shows servovalves controlling the velocity and position of a cylinder. The cylinder in this circuit has position and speed control, while the hydraulic motor only has speed control. All previous information about hydraulic power unit type, valve location, and filters applies to this circuit — or any other servo application.

Figure 21.8












The cylinder in this circuit has accurate positioning as does the cylinder in Figure 21.7, but this cylinder has controlled speed as well. A milling operation requires accurate speed control but also may need depth control. When fast accurate positioning at multiple locations is important, use a servovalve.

When the PLC sends a signal to start the cylinder moving, it smoothly ramps up to any speed desired. A servovalve allows for accurate velocity change anywhere within the stroke when the controller calls for it. At the end of stroke, the cylinder decelerates rapidly and smoothly to an accurate stopping position, without shock. Again, the servovalve performs the 4-way function while the electronic controls change speed and position. The servovalve must respond fast enough to follow the controllers' outputs or the cylinder position and/or speed will not match the machine requirements.

The hydraulic motor in Figure 21.8 must turn at a constant rate regardless of load or changes in pressure drop or fluid thickness. Even with a pressure- and temperature-compensated flow control, motor speed varies as pressure changes. Internal slippage in the motor is greater at higher pressures, so speed decreases even with constant input flow.

With a servovalve feeding the hydraulic motor and a feedback device giving the servocontrol card continuous speed information, motor speed is consistent. The only time motor speed varies is when it stalls at relief valve pressure.

As before, electronics handles all input and modifications to get the desired speed. A servovalve controls oil flow as a 4-way directional valve does, but it also has the ability to change flow as needed. It is the response of the servovalve to the electronic controllers' changes that is most important. Less-expensive, more dirt-tolerant servovalves have less accuracy.

With the circuit in Figure 21.8, cylinder speed is fast, and the cylinder stops in a precise position without shock. The hydraulic motor maintains the set speed regardless of load or input fluctuations — until it stalls from lack of torque. All motions are repeatable.