Servo: A system consisting of several devices which continuously monitor actual information (position, velocity), compares those values to desired outcome and makes necessary corrections to minimize that difference.

Slew: In motion control, the portion of a move made at a constant non-zero velocity.

Sequence: The order of a series of operation or movements, To drive flow of accomplish a subsequent operation or movement.

Sequence Valve: A pressure operated valve which, at its setting, diverts flow to a secondary line while holding a predetermined minimum pressure in the primary line.

Serial Data Transmission: A form of digital data transmission in which the data bit of a binary word are processed or transmitted one at a time. Slower than parallel data transmission, but does not require separate circuitry for each bit.

Servo Mechanism: A mechanism subjected to the action of a controlling device which operates as if it were directly actuated by the controlling device, but capable of supplying power output many times that of the controlling device, this power being derived from an external and independent source.

Servo Valve: A valve which modulates output as a function of an input command, A flow valve.

Single Acting Cylinder: A cylinder in which hydraulic energy can produce output as a function of an input command., A follow valve.

Slip: Internal leakage of hydraulic fluid.

Solenoid: An electromechanical device that converts electrical energy into linear mechanical motion. Used to actuate directional valves. May be air gap or wet armature type.

Strainer: A coarse filter.

Static Torque: The maximum torque available at zero speed.

Step Angle: The angle the shaft rotates upon receipt of a single step command.

Stiffness: The ability to resist movement induced by an applied torque. Is often specified as a torque displacement curve, indicating the amount a motor shaft will rotate upon application of a known external force when stopped.

Stroke: The length of travel of a piston or plunger, To change the displacement of a variable displacement pump or motor.

Subplate: An auxiliary mounting for a hydraulic component providing a means of connecting piping to the component.

Suction Line: The hydraulic line connecting the pump inlet port to the reservoir or sump.

Sump: A reservoir.

Synchro: A rotary electromagnetic device generally used as an AC feedback signal generator which indicates position. It can also be used as a reference signal generator.

Synchronism: A Motor rotating at a speed correctly corresponding to the applied step pulse frequency is said to be in synchronism. Load torques in excess of the motor's capacity (rated torque) will cause a loss of synchronism. The condition is not damaging to a step motor.