1.5 Governor valve relays - part 2
The function of the servo-valve is to provide an output flow rate of hydraulic fluid proportional to the input current. The input current, derived by the governor servo-amplifier is applied to the coils of a force motor thereby deflecting the armature and drive arm mounted in a flexure tube. The high pressure supply is filtered and supplied via orifices to the boost chambers and at either end of the spool.
The fluid from each boost chamber is cross-connected via porting to the reverse ends of the servo-spool. It travels down the centre of the spool and returns to low pressure via the drain port. The deflection of the drive arm will block one or other of the nozzles and create a high pressure in one boost chamber and a low pressure in the other, thus creating a force to move the servo-spool until the pressures are equalised. The spool deflection is thus proportional to the drive arm deflection which is itself proportional to the current. In the diagram, the servo-spool is shown in the central position with the control ports blanked-off. This is referred to as the 'null position'. When the spool moves, one control port is opened to the supply port and the other opens to the drain port. The flow rate is proportional to the spool deflection.
When connected as a position controller, as shown diagrammatically, the null position of the spool corresponds to the condition of equal 'demanded' and 'actual' positions. Any discrepancy with the position measured by the LVDT causes a current to flow, resulting in a hydraulic fluid flow to the power piston until the required position is achieved.
To avoid stiction of the valve it is usual practice to superimpose a 'dither' current on the servo-valve amplifier output. This is a medium frequency (1 kHz) low amplitude current which plays no part in the overall position control, but keeps the servo-valve spool in a 'live' condition, the effect being to vibrate it by a negligible amplitude at the 'dither' frequency.