Pneumatic proportional and servo directional control valves
Since the late ‘80s, several companies have been controlling air cylinders with open- and closed-loop proportional or servovalve circuits. The difference between the air valves in these circuits is how fast they respond. Most proportional valves have a sealed spool that controls direction and flow so the valves tend to hang up and jump. Pneumatic servovalves often have spools with metal-to-metal fits that float on bypass air.
The valve shown in Figure 12-8 is sold as a proportional or servo directional control valve for hydraulic or air circuits. Controlling the amount of air and which direction it goes is not a problem but the compressibility of air creates some giant hurdles to overcome.
Proportional valves usually control only acceleration, deceleration and/or speed because these circuits do not include feedback transducers. It is very easy to get smooth acceleration and deceleration with high speed in between without other controls or shock absorbers to stop the load mechanically.
Adding feedback transducers to a proportional air circuit can provide servo-like control for light loads -- such as those found in pick-and-place applications. However, a proportional valve is usually not responsive enough for exacting part placement or speed control. For very accurate control, a servovalve with feedback transducers can give close-tolerance positioning (with light loads), repeatable velocity control, and very accurate holding force.
Pneumatic proportional and servovalves are not a replacement for electromechanical or servo hydraulics, but they have price advantages over both systems. When the loads are light and cost is a factor, they are worth a look.
General information for hydraulic infinitely variable valves
- The symbol for proportional and servovalves shows a 4-way, 3-position function and the valve can move to each of the positions. However, the parallel lines along the sides of the symbol indicate the valve does not have to shift all the way all at once. These valves can shift into straight or crossed arrows in any proportion from 0 to 100%. They are infinitely variable and can pass any flow desired.
- servovalves are always 3-position, all-ports-blocked center condition, as shown by the symbols in Figures 12-8 through 12-11.
- always size proportional or servovalves for high pressure drop. Proportional valves should have 200- to 500-psi pressure drop at full flow. Most servovalve manufacturers rate their valves at 1000-psi pressure drop at full flow. This means the valve may look physically small for a given flow in relation to conventional valves. It also means most servo and proportional valve circuits require a heat exchanger to deal with excess wasted energy.
- always mount the valve as close as possible to the actuator ports. Any piping between the valve and the actuator holds extra fluid that can make the system softer and less responsive. This is especially important on air-powered circuits.
- never use hose between the valve and the actuator. If isolation is necessary, mount the valve on the moving part and use flexible lines for supply and return.
- use in-line pressure filters at the supply to each servovalve or bank of valves to protect the valve from contamination in the pump and piping.
Specific Information for pneumatic infinitely variable valves
- use air at the highest pressure possible that does not exceed component or plumbing limits. This is usually as high as 250 psi.
- size the valve to flow just enough to produce the maximum desired actuator speed. Oversize valves produce erratic control because a small spool movement gives more flow than required.
- use an actuator with the largest area practicable so the load moves with a low pressure difference. Note: the larger the cylinder, the more air it consumes so operating cost escalates.
- pneumatic servo circuits do not work well when outside forces push against the actuator. The actuator tries to resist, but force buildup is slow in comparison to electromechanical or electrohydraulic systems.
Typical servo circuits
Figure 12-14 shows schematic drawings of three typical servo circuits. In the figure, each type circuit controls a different actuator, but any actuator could have more than one type of control.
The typical power unit for a servo system is a pressure-compensated pump with an accumulator or accumulators. A servovalve must always have a ready supply of fluid because no matter how fast it reacts, without an immediate supply of fluid the system will be sluggish. The pressure-compensated pump may be at full pressure when no actuators are moving, but its flow is zero. Adding accumulators assures that there is no wait for the pump to come on stroke before the actuator receives flow. (A full discussion of how accumulators work and how they are applied is in Chapter 16.)
Again, pressure filters in the lines to the servovalves make sure they receive clean oil. One filter could be sufficient for multiple valves when the valves are close to each other. The reason for pressure filters in the valve lines is the pump constantly produces contamination particles that will shut down flapper-type servovalves.
Cylinder 1 is in a position loop. It can be placed at a precise location repeatedly within ±0.0005 in. A programmable logic controller (PLC) sends a signal to the summing amplifier’s control card. The signal passes on to the valve-driver card and then to the valve coils. This signal shifts the servovalve to start cylinder movement at a set rate. The linear potentiometer sends position feedback to the summing amplifier and modifies valve position to find and maintain a certain position. Often, position control is paired with speed control to accelerate the actuator to a certain speed, then decelerate and stop it at the desired position.
Cylinder 2 is in a force loop. A certain size cylinder operating at a given pressure produces a given force. This force can be calculated by multiplying area times pressure, but the result is not exact. Friction from seals and between external machine members can reduce this force by a few pounds or more on an operating machine. When an exact force calculation is required, a servovalve-controlled cylinder that has a load cell for feedback can keep forces within 1/2% with ease. The summing amplifier sends a signal from the PLC and feedback from the load cell modifies the valve position to exactly match the input signal to generate the desired force.
The hydraulic motor is in a speed loop that maintains the motor’s rpm when the fluid viscosity, pressure, or load changes. A rotary device called an encoder constantly sends rpm information back to the summing amplifier to open or close the servovalve as needed. Just as a cylinder does, a hydraulic motor will slow when the load increases, when fluid gets thinner due to temperature increases, or when system pressure fluctuates as other actuators move. If the encoder sends a reduced-rpm signal back, the servovalve opens to let more fluid in. If the hydraulic motor tries to speed up, the servovalve closes enough to maintain the set speed.