Meter-in control refers to the use of a flow control at the inlet to an actuator for use with actuators against which the load is 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 systems where such velocity variations are undesirable a pressure compensated flow control valve (PCFCV) can be used. This valve will maintain aconstant delivery flow providing that the pressure drop is greater than its mini­mum controlled level that is usually in the region of 10 to 15 bar.

Figure 1 shows a typical system in which the flow 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 as shown in Figure 2.