INSTRUMENTATION & CONTROL

Types of hydraulic control system
There are two types of hydraulic control system:-
1. Pump controlled system
2. Valve controlled system

Pump controlled system
This is a first class of hydraulic control system which usually consist s of single variable -stroke pump and a fixed stroke motor. Control of the motor is exercised by vary the amount of oil delivered by the pump. This is accomplished by mechanically changing the pump volume or stroke as shown in fig. The pump control system is a quite simple and is used when there is only one motor to be controlled. As shown in fig. 1, the motor is the usual rotary form. In certain applications a linear actuator is a required

Valve controlled system
Although fast acting accurate servo valves are compensate ,they result in a lighter more flexible system and relatively inexpensive fixed stroke pump pulse pressure regulator is compare to the several expensive variable stroke pumps required by independent pump controlled systems, The cost of valve controlled operation is usually less.

THE BALL PUMP

The balls are forced in and out radially by the cam plate and hydraulic pressure, thus displacing fluid in the case of a pump, or forcing rotation in the case of motor. the inlet and outlet ports are inside the balls race. The stroke may be made variable by allowing the cam plate eccentricity to be adjusted. In motor operation, high pressure fluid is admitted at the port marked in and forces the balls radially outward. This may be accomplished only by rotary motion of the ball race. Thus the problem leakage due to ball wear or eccentricity is serve in the ball pump.

SPOOL TYPE PILOT VALVE

The spool type valve shown in fig.5 is one of the several spool type valves are classified as four ways valves. In four ways valves there are four orifices that are critically spaced with respect to one another. It will be noted that these valves involves no basic modification in operation nor simplification of manufacturing. the number of critical diminution remains the same as with the valve in fig.5.