Hydraulic Flow Control
Flow control can be achieved by one of two quite distinct methods: hydrostatic or hydrodynamic. In a hydrostatic transmission Row is controlled by varying the swash angle of a variable displacement pump or motor. An electro-hydraulic servo valve is often used to perform this function, with positional feedback taken from the pintle. In a velocity control system, a signal derived from an actuator-driven tacho can be used either as an alternative or additional feedback signal. In a position control system, the major feedback signal is derived from an actuator-driven positional feedback transducer, with the swash angle feedback providing the minor feedback loop. A block diagram of a hydrostatic transmission employing a variable displacement pump is shown in Fig. below.
Variable delivery pump block diagram.
For unidirectional velocity control systems, a ope side of centre swash angle control is sufficient, whilst a bidirectional velocity control system requires an additional directional valve or an over-centre swash angle control. A hydrostatic position control system invariably requires an over centre swash angle control.
The majority of variable displacement pumps and motors are of the vane or piston type, although units of different design are on the market.
In hydrodynamic now control a throttling valve converts the pressure energy at the inlet to kinetic energy at the outlet port. Valves can be of two three- or four-way construction, the conventional usage to describe valves containing two, three or four external ports. The four basic circuit configurations:
(1) meter-in
(2) meter-out
(3) meter-in/meter-out
(4) bleed-off
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