Its pilot stage uses a torque motor driving a nozzle-flapper or jet pipe rather than a proportional solenoid, and the main spool is position-fed-back to the pilot. That construction gives far faster response and far less hysteresis than proportional hardware can achieve.

Response measured in a few milliseconds, near-zero hysteresis, and a critically lapped spool with no dead band around centre. Where a control loop must be both fast and accurate, those are the requirement rather than refinements - and no proportional valve substitutes for them.

Aerospace and flight control test rigs, materials testing machines applying precise force profiles, steel mill roll positioning, injection moulding control, and vibration and simulation tables. The common factor is a fast closed loop where accuracy is the product. In each case the accuracy of the control is itself the product being sold.

Because the pilot stage has extremely fine passages that a single particle can block. Servo systems typically demand the tightest cleanliness codes in hydraulics along with dedicated filtration, and a servo valve installed on an ordinarily filtered system will fail - which is the most common reason they disappoint.

Rarely. It costs several times a proportional valve of similar capacity, demands filtration most systems do not have, and delivers performance most machines cannot use. For ordinary variable control a proportional valve is the correct and far cheaper answer. Specifying one usually buys performance the machine cannot exploit.

One where a small pilot stage positions the larger main spool, since the torque motor cannot move a large spool directly. Nearly all servo valves of useful capacity are two-stage, and the feedback between stages is central to the accuracy the valve achieves.

Principally maintaining the fluid cleanliness they require, since almost all servo valve problems are contamination. Null adjustment and periodic performance verification are the other items, and repair is normally a specialist rebuild rather than an on-site task. A spare valve is often held where downtime matters.