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.
Valves with larger flow capacities need pilot stages to boost the power necessary to shift the larger spools. The electromechanical methods used for this staging are torque motors, force motors, and proportional solenoids.
Torque motors, Figure 7, are electromechanical rotary machines whose rotational travel is restricte - often to less than one or two revolutions - and are nearly always used for a piloting function. They are fitted with permanent magnets as the major flux source with the flux paths arranged to form a force bridge. Their limited rotation allows the armature to be mounted on a stiff flexure spring rather than bearings, although there is one known proprietary exception that uses a soft spring. The stiff spring and lack of bearings virtually eliminate hysteresis caused by bearing restriction.
Incoming current creates a second set of magnetic fluxes that unbalance the force bridge and results in net torque. The torque causes angular rotation until the flux-induced torque equals the counter-torque of the flexing spring plus any external load. An important characteristic of the torque motor is that the direction of rotation is affected by the direction of current through the coil. The electromagnetic field caused by the current is compared to the field of the permanent magnet in the magnetic bridge circuit and rotation ensues in a commensurate direction.
In the final valve assembly, the torque-motor armature is connected to a flapper sitting between two opposed nozzles, a jet pipe, or a swinging wand or blade. These last two steer a fluid stream, Figure 8, branch B. Basic operating principles and conceptual construction of flapper-nozzle and jet pipe servovalves are indicated in Figures 9 and 10, respectively. Torque motors almost exclusively pilot servovalves, and usually requires less than one watt of power to fully operate although that is not a hard-and-fast rule.
Torque from the torque motor of a jet pipe servovalve steers the jet to one receiver or the other, unbalancing spool end pressures. Movement of the main spool continues until the feedback spring between the main spool and jet forces the jet pipe back to near null. Main spool position then is commensurate with coil current.
The flapper-nozzle has two different implementations: the one already mentioned is the stiff design, wherein the force due to the impinging nozzle flow is small in comparison to spring and torque-motor forces. In soft designs, the torque motor and nozzles are deliberately sized so that nozzle effluent causes a significant force on the flapper. One argument concludes that this design is more tolerant of certain contamination problems. The argument goes like this: when the two fixed orifices are open fully and unclogged, the unpowered flapper will center due to a combination of fluid-momentum force acting on the flapper, restoring force on the light spring, and magnetic force in the torque motor.
Should one of the nozzles or fixed orifices become partially blocked, reduced effluent flow produces less force on the blocked side of the flapper. The flapper then moves toward the clogged bridge leg until reduced force from the opposite receding nozzle equals the diminished flow force from the partially blocked nozzle. Current input to the torque motor then causes the flapper to move about a shifted neutral, but the pressure does not go to a hard-over level. The main spool might not shift fully in one direction, however.
The swinging wand, Figure 8 path D, has a mechanical-to-hydraulic interface that is proprietary. The versions of this interface include:
- dual nozzle where the two fluid streams are deflected off the outside edges of the wand, and
- single nozzle, where a single fluid stream passes through a central hole in the wand.
Consider the dual-nozzle version, Figure 11. The two fluid streams issuing from the source side of the pilot head are collected in opposing receiving ports. When a current into the torque motor causes the wand to swing, one receiving port experiences a rise in pressure while the other experiences a pressure reduction. As in the case of the jet pipe and flapper-nozzle pilots, the resulting difference in pressure shifts the valve's main spool.
The single-nozzle version has a hole laterally bored and centrally located in the wand such that the single fluid stream issuing from the single nozzle must pass through the hole. When the wand is centered, equal pressures are collected in the two receiving ports. A current into the coil causes the wand to shift and the fluid stream is deflected off the inside edge of the central hole resulting in different pressures being collected in the two receivers. The resulting differential pressure between the two receiving ports shifts the main spool.
This swinging-wand design has a supply pressure limitation in that the pilot head must be sized for a particular supply-pressure range. If flow issuing from the nozzle(s) is excessive, fluid momentum force acting on the wand can pin it against the receiver side, locking it there. Installing an orifice - matched with the supply pressure and the needs of the pilot stage - in series with the nozzle side, remedies this problem. An approximately 2:1 change in supply pressure is possible with a single orifice.
Force motors are the linear equivalent of torque motors in that they also have permanent magnets inside. Therefore the direction of motion depends upon the direction of input current, Figure 12. There is only one manufacturer of force motors in the U. S.: Fema Corp., Portage, Mich. The two permanent magnets each create attractive forces, each urging the armature toward it, but nominally offsetting one another when centered. Additionally, a stiff centering spring prevents either of the natural regenerative attractive forces from pulling the armature either way.
When a current is applied in the direction shown in Figure 12, the resulting electromagnetic fields act to strengthen the magnetic fields by increased flux density in air-gaps B and D while at the same time weakening the fields in air-gaps A and C. The resulting force moves the armature and poppet to the left. State-of-the-art force-motor design produces a maximum of about five pounds of stall force, about 0.02-in. of travel (no load) at about 5 watts of power.
Proportional solenoids are about 30 years old and are manufactured by a number of companies world-wide. Some market their solenoids to U. S. industry, while others supply themselves. All competing products have similar performance specifications. State-of-the-art proportional solenoid design yield these approximate typical specifications:
- maximum force, 20 lb
- proportioning travel, 0.10 in. current, 12-V coil, 1.5 to 2.5 A, and
- power, 15 to 25 watts. Figure 13 indicates approximate construction detail of a proportional solenoid. The secrets to success lie in forming the proper trapezoidal air gap dimensions and in keeping stiction low.
Figure 14 shows a representative force-displacement curve for a proportional solenoid. Its unique characteristics are a region of relatively constant force as the armature changes position, plus relatively linear changes in force for changes in solenoid current, both performance goals sought by the solenoid's designers. It is probably true that neither the constancy of force nor the linearity with current are as important as the manufacturers would claim. Things incorporated external to the solenoid substantially affect performance of the total valve: use of pressure feedback or armature-position feedback, for example. Furthermore, the enormous versatility of the hardware and software of modern computers makes control and linearization a fairly straightforward task.
Some differences between proportional solenoids and force/ torque motors are apparent in the specifications while others are not. Torque/force motors require lower current levels. Proportional solenoids:
- require much higher electrical input power than their motor counterparts
- produce substantially greater mechanical travel than motors
- produce higher levels of force
- produce higher levels of stiction
- operate with greater hysteresis, and
- generate force in a direction independent of current direction. Therefore, to make a 4-way directional valve operate requires two proportional solenoids but only one force/torque motor.
All of these factors make proportional-solenoid drive electronics more complex than that necessary for force/torque motors. Proportional-solenoid power requirements have caused manufacturers of proportional-valve drive electronics to adopt pulse width modulation (PWM) as the power-output method of choice. The major reason for the use of PWM is to handle the high-output power required of the solenoid without overburdening the power-output transistors.
There is a second benefit of using PWM: if the PWM frequency is sufficiently low, it automatically provides a mechanical dither that helps minimize stiction-induced hysteresis. In some valves the effects of dither can only be described as dramatic when looking at the reduction in hysteresis. The correct dither frequency must be determined after the valve is designed. Furthermore, the frequency selected must be a compromise between propagating the dither pulsations imperceptibly into the hydraulic circuit and yet achieving sufficient reduction in stiction. A low frequency helps the stiction problem but if too low, the user of the hydraulic system can feel the pulsations.
U.S. industry uses PWM frequencies from about 33 Hz to about 400 Hz. At least one European manufacturer uses 40 kHz and receives no dither effects whatsoever. Their amplifier supplies dither with a separate on-board dither generator. There is an advantage to this method: dither power remains constant throughout the modulation range, whereas when relying on the PWM frequency for dithering, the dither power varies with the amount of modulation. There is none at the 0% and 100% modulation points, but maximum at 50% modulation.
Summary of pilot-operated valves
Figure 15 shows a family tree of all electrically modulated, continuously variable pilot-operated valves. It is a peculiarity of the U.S. hydraulic-valve manufacturing industry that each terminus of the tree also tends to define a specific manufacturer's product. For example, the A/C/E/I/N/V path rather accurately describes the servovalve built by Sauer-Danfoss's Controls Div. in Minneapolis. (Note that its mate, U, has no supplier.) In contrast, though, the A/C/E/J/O path is well populated. That is where the products of Moog, Eaton/Vickers, Bosch-Rexroth, Parker Hannifin's Dynamic Valve, and others have congregated.