Hydraulic Meter In: Meter-in control refers to the use of a flow control at the inlet to an actuator for used with actuators against which the load in opposition to the direction of movement. For a meter-in-circuit that uses a simple adjustable restrictor valve selection of the DCV to create extension of the actuator will cause flow to pass through the restrictor into the piston end of the actuator. The required piston pressure, Pp, will depend on the opposing force on the actuator rod. With a fixed displacement pump delivering a constant flow, excess flow from the pump will be returned to tank by the relief valve at its set pressure. PSmax. Consequently, the available pressure drop, PSmax-Pp will determine the flow delivered to the actuator for a given restrictor opening.
With this system the flow, and hence the actuator velocity, will vary with the load force. for system where such velocity variations are undesirable a pressure compensated flow control valve (PCFCV) can be used. This valve will maintain a constant delivery flow proving that the pressure drop is greater than its minimum controlled level that is usually in the region of 10 to 15 bars.
Fig. Meter-in Control for Actuator
Figure shows a typical system in which control is bypassed with a check valve for reverse operation of the actuator. If the load force varies considerably during operation, there will be transient changes in actuator velocity at a level that depends on the mass of the load.
For example, when the load force suddenly reduces, the piston pressure will reduce but at a rate that is dependent on the fluid volume and its compressibility and the mass of the load. During the period that the pressure is greater than the required new value, the actuator will accelerate and, as it does so the piston pressure will fall. The pressure can then fall below the new level and deceleration results and damped oscillations can occur.
Hydraulic Meter Out: For overrunning load forces and/or those with a large mass, meter-out control is used where the actuator outlet flow during its extension passes through the restrictor as shown in the circuit.
The flow control operates by controlling the actuator outlet pressure at the level required to oppose the forces exerted on the actuator by the load and by the piston pressure which is the same as that of the pump. This prevents cavitations from occurring during transient changes arising from load force variations or due to force that act in the same direction as the movement (i.e. pulling forces).
This system can, however, cause high annulus pressures to occur from the intensification of the piston pressure together with the pressure created by pulling forces. Further, when compared to meter-in, the rod and piston seals have to be capable of withstanding high pressures that may require a higher cost actuator to be used.