Proportional and servovalves
Infinitely variable directional control valves
The directional control valves discussed so far in this series have all been configured to either pass full flow or completely block flow. The only way to decrease flow through these valves is by adding flow controls or by mechanically limiting movement of an internal part.
The first infinitely variable valve available was the servovalve. Internal flow-modifying parts could be moved to any position at any rate, so output from any port could be varied at will. (Some call these valves infinitely variable 4-way flow controls.) The main problem with servovalves was (and still is) that they require very clean fluid to keep them operating effectively. Fluid from a standard well-maintained hydraulic circuit contains enough contamination to cause most servovalves to fail in a matter of minutes or only last a few hours at best. This meant that the original servovalves were tried and removed from many machines that needed precise control but not at the perceived cost of cleaning up the hydraulic oil.
Why use infinitely variable valves?
Some actuators must move at a precise speed, stop at a close-tolerance position, or produce a very accurate force to perform the work for which they were designed. With the proper input signals and feedback devices, proportional or servovalves can make an actuator perform any or all these functions flawlessly.
Rolling mills turn out sheet consistently to a tolerance of ±0.0005 in. at sheet speeds of 2000 to 5000 feet per minute. Hydraulic cylinders controlled by servovalves maintain the proper force and position the rolls precisely from feedback signals sent by sensors that measure metal thickness, cylinder force, and position. Airline pilots train in simulators moved by hydraulic cylinders so precisely that the pilots get the feel of landing gear raising and locking in position. Even entertainment rides use servovalves to make passengers think they are in 20-ft waves when they are actually in an enclosed articulated room in a shopping mall.
For less precise movement, there are proportional valves that mimic the output of servovalves but respond more slowly. They are less expensive than servovalves and more contamination tolerant, so they have replaced cam valves and other mechanical devices used to get smooth motions.
Hydraulic proportional directional control valves
The symbol and cutaway view in Figure 12-1 represent a direct-acting proportional valve that handles flows as high as 10 to 30 gpm in D03- or D05-size valve interfaces. Proportional valves use the same interface standards as NFPA and ISO directional valves so they can be installed in a circuit without having to change the piping.
Physically, proportional valves appear the same as their on/off solenoid counterparts. The big difference is in the way their solenoid coils perform. Proportional coils operate on DC current and produce varying force with varying voltage. The symbol shows the solenoid slash in the operator box with a sloping arrow through the slash. This indicates the solenoid has variable force that moves the spool more or less as voltage increases and falls. The other indication on the symbol that shows the spool is infinitely variable is the parallel lines down both sides of the boxes. Proportional valves operate similarly to manual valves, but they use electronics instead of hand power.
To eliminate flow lag from spool overlap, most manufacturers cut vee notches or use some similar method that allows some flow to pass as soon as the spool moves. Vee notches also give smooth flow buildup until the spool moves through the land overlap.
Proportional valves only have two center configurations (as shown by the symbols in Figure 12-1). This means that pressure-compensated pumps with accumulators normally power circuits with proportional valves. The circuits are pressurized at all times to produce fast response from an actuator when motion is called for. (A pressure-compensated circuit also wastes the least amount of energy when throttling flow.)
The valve in Figure 12-1 depends on a certain voltage to move the spool a certain distance to pass a certain flow. This works reasonably well, but is not accurate over a broad range of pressures, flows, and temperatures. Most valves of this design are used to smoothly accelerate and/or decelerate an actuator. The spool is electronically controlled to shift over a period of time to increase flow at a controlled rate. Spool-shift speed can be controlled electronically as it opens and closes to give smooth acceleration and deceleration. Spool-shift distance can also be limited electronically to set a maximum speed when required.
To give better spool control, a linear variable-displacement transducer (LVDT) is added to the basic valve. The cutaway view and symbol in Figure 12-2 represent a direct-acting valve with an LVDT. Feedback from the LVDT tells the electronic controller the spool’s position and makes sure it goes to the same place when it receives the same signal. With this arrangement, the spool always shifts to an exact location and opens the same size orifice so it can pass the same flow when pressure drop and viscosity stay the same. Control of flow is more accurate with an LVDT but pressure drop does not stay constant and viscosity often changes throughout the day so speed variations are still apparent. Such flow variations caused by system pressure fluctuation can almost be eliminated by the addition of a hydrostat module in port P as shown in Figure 12-3.
A hydrostat is simply a pressure-reducing valve set to hold downstream pressure in a 100- to 150-psi range. However, a hydrostat has a pilot line from a shuttle valve that reads downstream pressure at ports A and B, then feeds it back to the bias-spring end of the spool that controls the 100 to 150 psi. The function of a hydrostat is to maintain a constant pressure drop across the spool orifice so flow stays constant regardless of changes in system pressure. (For a thorough understanding of how pressure compensation works, see the pressure-compensated flow control valve section in Chapter 13.)
With the addition of a hydrostat, actuator speed is controlled as accurately as possible without a closed-loop electronic circuit that reads speed and modifies spool position. Closed-loop electronic circuits are used with proportional valve systems, but they only give nominal control. When accurate control is required, use servovalves with closed-loop electronics.
The symbol and cutaway view in Figure 12-4 is for a proportional valve that only controls flow. Such valves are commonly called throttle valves because they are not pressure compensated unless a hydrostat module is added.
Basic operation is identical to the proportional control valves just discussed. The only difference is they have a single solenoid and may be piped with dual flow (as shown). Dual-flow piping allows a given size valve to pass twice the volume at the same pressure drop. It can be used with any 4-way directional control valve with one precaution: the valve must be capable of handling maximum system pressure in its tank port. Many wet-armature valves will not operate at full rated pressure at their tank port. Check the supplier’s catalog to see what maximum tank line pressure is allowed. Air-gap solenoid valves and solenoid pilot-operated valves with external drains normally allow full rated pressure in the tank port.
The circuit in Figure 12-5 shows a possible use for a proportional throttle valve. The vertical-down acting cylinder with a platen needs speed, acceleration, and deceleration control. This could be done with a 4-way proportional valve, but the circuit uses an inline or screw-in cartridge valve that is not directly replaceable. Adding a proportional throttle valve to the tank line of the present 4-way circuit can give the required control without extensive piping changes. The circuit is shown using a single flow path for low volume. A dual flow path setup (like the one in Figure 12-4) would allow as much as twice the flow. As stated earlier, make sure the 4-way directional control valve can accept tank-line backpressure without damaging it or causing a malfunction.
If the cylinder had a resistive load, the proportional throttle valve could be placed in the pump line of the 4-way as a meter-in flow-control circuit. A meter-in circuit would not damage the directional control valve or cause it to malfunction. The circuit in Figure 12-5 could have a counterbalance valve in the rod-end line to make it resistive. (See Chapter 14 for counterbalance valve operation and applications.)
The platen would start to move at a controlled rate when the 4-way valve shifts and the proportional throttle valve is signaled to open slowly. The shift time for the throttle valve determines the acceleration time, while shift travel distance determines maximum cylinder speed. Cylinder speed would be infinitely variable to match any production need.
Near the end of the stroke, a slowdown limit switch would signal the proportional throttle valve to start shifting back to its closed position. The proportional throttle valve would close at a controlled rate and flow from the cylinder would be retarded smoothly. When the cylinder slows sufficiently, it contacts the end-of-stroke limit switch and the 4-way directional control valve shifts to center to stop it.
The circuit in Figure 12-5 would only hold position if some retract signal was applied when stopped. This is due to internal leakage by the spool of the 4-way directional control valve and the proportional throttle valve. Another option would be to use a float-center directional control valve and a counterbalance valve.
Proportional valves for flows higher than 25-to-30 gpm use solenoid pilot-operation similar to conventional directional control valves. A small pilot-operated valve receives a signal and then sends hydraulic oil to proportionally move a larger control spool that controls actuator movement.
The cutaway view and symbol in Figure 12-6 depict a typical solenoid-pilot valve arrangement. A reducing valve module, between the pilot operator and the pilot-operated valve, keeps maximum pilot pressure below 200 psi. A proportional Input to one of the coils on the pilot operator directs flow to the spool of the pilot-operated valve and shifts it against a spring. As pressure against the spool increases, it shifts farther and sends more flow to the actuator. Feedback signals from both spools tell the electronic controls that the command has been carried out. A vee-notched spool allows flow to increase at a smooth rate so actuator speed is consistent throughout the speed range.
Ports for internal or external pilot X or drain Y provide options for these control lines to meet a particular requirement.
The complete symbol is shown in Figure 12-6. (For the simplified symbol, see Chapter 4.)
Flows up to 200 gpm are common for a D10-size proportional valve. For higher flows, use slip-in cartridge valves (discussed in Chapter 11) with proportional operators.