The rate of flow, and therefore the speed and position of the actuator - not just its direction.

A conventional directional valve is a switch. It connects a cylinder port to supply or to exhaust, fully. The cylinder then moves at whatever speed the flow, the load and any fitted restrictors allow, until it hits its end stop.

A proportional directional valve meters. Its element takes an intermediate position, partially opening the path, so the flow is controlled continuously. A small signal gives a trickle and slow movement; a large one gives full flow and fast movement.

Because flow can also be reduced to zero at the mid position, the actuator can be stopped part way - and with feedback from a position sensor, held there.

So the valve turns a cylinder from a two-position device into something that can be commanded to a speed and a position, which is a different class of capability.

Closed-loop position control of a pneumatic actuator, requiring a proportional valve, a position sensor and a controller.

The three components work together. A position transducer on the cylinder continuously reports the piston's actual position. The controller compares that with the demanded position and calculates an error. The proportional valve meters air to reduce that error, admitting to one side and exhausting the other.

The loop runs continuously, so the actuator is driven towards the demanded position and held there against disturbance.

What makes it demanding is the compressibility of air. The controller must handle a system with a spring in it, which requires more sophisticated tuning than a hydraulic or electric equivalent - typically with damping terms and sometimes with pressure feedback as well as position.

Many suppliers offer the valve, sensor and controller as a matched package with commissioning software, which is generally the practical route rather than assembling components.

Because the air between the valve and the piston behaves as a spring, so the system oscillates rather than moving crisply to position.

In a hydraulic cylinder the fluid is effectively incompressible, so moving the valve moves the piston almost immediately and the relationship is stiff. In an electric actuator the coupling is mechanical and stiffer still.

In a pneumatic cylinder, admitting air compresses the volume already present before the piston moves. That volume acts as a spring between the valve and the load, and any spring with a mass on the end of it has a natural frequency and will oscillate.

So a naively tuned pneumatic position loop hunts around the target rather than settling on it, and increasing the gain to sharpen the response makes it worse.

The remedies are careful tuning with damping, keeping the volume between valve and cylinder as small as possible by mounting the valve on or near the actuator, and using controllers designed for the problem. It also means performance degrades as tube length increases.

When pneumatic properties are wanted and full electric performance is not required - light mass, high force for the size, tolerance of stall and impact, and hazardous areas.

An electric actuator gives better positioning accuracy, better repeatability and much easier control, and for most positioning tasks it is the right answer.

Pneumatics keeps some advantages. The moving mass is low, so acceleration is high. Force density is excellent, so a small cylinder produces a large force. It tolerates being stalled, jammed or impacted without damage, where an electric actuator's drive would fault or its screw would be damaged. It works in wet, washdown and explosive environments with no protection. And it is compliant, which suits handling delicate objects.

So servo pneumatics suits applications wanting moderate positioning accuracy with those properties: handling, tensioning, force-controlled assembly, and axes that must survive collisions.

Where accuracy is the priority, use an electric axis.

Moderate - good enough for handling and process work, and well short of an electric servo.

Achievable accuracy depends on the valve, the sensor, the controller tuning, the load and how much volume sits between valve and piston. Under good conditions a well-tuned system positions repeatably enough for pick-and-place, tensioning and process positioning.

What degrades it is everything that makes air springy: long tubing, large cylinder volumes, varying load, and friction in the cylinder that produces stick-slip near the target.

Friction is the underrated factor. A cylinder's seal friction means small correction signals produce no movement at all until the force builds enough to break free, and then it moves too far - which shows up as a system that will not settle within a small band.

Low-friction cylinders are used for this reason in servo applications, and they make a substantial difference to what the loop can achieve.

Clean dry air to a fine standard, and the valve mounted as close to the actuator as possible.

A proportional directional valve holds a small element at a precise intermediate position. Contamination causes sticking and hysteresis, which in a closed loop appears as poor control rather than an obvious fault - so filtration to the manufacturer's standard, locally to the valve, is essential. Check whether lubricated air is permitted; often it is not.

Mounting position matters as much as air quality here. Every millimetre of tube between the valve and the cylinder adds compressible volume, which softens the spring, slows the response and makes the loop harder to tune. Mounting the valve directly on the cylinder, or immediately adjacent, transforms achievable performance.

That is a design decision that has to be taken early, because it constrains where the valve can physically go and how it is protected - and it cannot be corrected later by tuning.

It depends on the valve's fail behaviour, and on a positioning axis the consequences are more significant than on a switched one.

Some proportional directional valves close to a blocked centre on loss of signal, trapping air on both sides of the piston and holding the actuator roughly in place - though air is compressible and an external load will move it. Others go to an open or exhausted centre, letting the actuator float or drift under load.

Which is safe depends on the machine: a vertical axis that floats will fall.

So the fail state must be chosen from the risk assessment, and on any axis where a falling or drifting load is hazardous, the proportional valve should not be the only protection. A separate mechanical brake, a lock, or a pilot-operated check valve arrangement is used to hold the load independently.

As with proportional regulators, the controlling element is not a safety device and should not be treated as one.