Three-port flow control valve
Three-port flow controls are mainly used in fixed-volume pump circuits to save energy. (See the load-sensing pump circuit explained in Chapter 8.) If 20 gpm of fluid enters the Inlet and the flow control is set at 12 gpm, 8 gpm goes to tank as wasted energy. With a conventional relief valve setup, pressure between the pump and flow control would be maximum. With the 3-port flow control, pressure in this portion of the circuit is whatever it takes to move the actuator plus bias-spring force. (Bias-spring force is usually 70 to 125 lb.) An outlet pressure of 200 psi gives a pressure of 270 psi between the pump and the flow control. All fluid going to tank is discharged at 270 psi, not 2000 psi. This takes place because the sensing line sends feedback to the pressure-control side of the relief valve, allowing it to open at load pressure plus bias-spring force. Pressure between the pump and flow control constantly changes with load variations. When the load requires more than the maximum-pressure adjustment setting, the relief valve opens and sends all pump flow to tank at maximum pressure.
A 3-port flow control is only effective with one actuator -- or one actuator at a time. It would not be useful on a pressure-compensated pump circuit because a load-sensing circuit for this type pump would save even more energy. (See Chapter 8 for a load-sensing circuit with a pressure-compensated pump.)
Proportional flow control valves
Figures 13-4 and 13-5 show cutaways and symbols for proportional flow control valves that can electronically remotely control flow through a PLC or other controller. There are many different designs of valves and controllers that control pneumatic or hydraulic fluid. The design in Figure 13-4 uses a modified 2-way pilot-to-close poppet with a drilled pilot passage to send inlet fluid behind it. A light spring holds the poppet closed when there is no pressurized fluid at the Inlet.
The armature controls a small normally closed poppet and shifts the signaled amount to let fluid behind the pilot-to-close poppet leave faster than the pilot passage can supply it. This causes a pressure imbalance that lets the pilot-to-close poppet open enough to give the correct fluid flow. The flow rate is infinitely variable and can be controlled from a variety of inputs.
The valve in Figure 13-4 opens from a given signal but may not always repeat a set flow from the same input. The feedback LVDT added to the valve in Figure 13-5 assures that the pilot-to-close poppet always shifts the same amount so it has the same size flow opening. However, pressure or viscosity changes still affect actual flow, so a hydrostat is necessary when exact flow repeatability is required. Many manufacturers make valves with a built-in hydrostat for pressure compensation.
Meter-in flow control circuits
Figure 13-6 provides a schematic drawing of a meter-In flow control circuit restricting fluid as it enters an actuator port. Meter-in circuits work well with hydraulic fluids, but can give erratic action with air. Note that the cylinder is horizontally mounted, which makes it a resistive load. Meter-in flow controls only work on resistive loads because a running-away load can move the actuator faster than the circuit can fill it with fluid.
The left-hand circuit in Figure 13-6 is shown at rest with the pump running. Notice that the check valves in the flow controls force fluid through the orifices as it enters the cylinder and lets fluid bypass them as it leaves.
The right-hand circuit depicts conditions as the cylinder extends. The directional control valve shifts to straight arrows and pump flow passes through the left-hand flow control to the cylinder cap end at a controlled rate. Fluid leaving the cylinder rod end flows to tank without restriction. The cylinder extends at a reduced speed (in a hydraulic circuit) until it meets a resistance it can’t overcome or it bottoms out. With the non-compensated valve shown, speed can vary as pressure fluctuates or viscosity changes.
While the cylinder is in motion, pressure at PG1 reads the setting of the relief valve or pump compensator. The pressure at PG2 reads whatever it takes to move the load at any point in the cycle. Pressures at PG3 and PG4 only read tank-line backpressure as the cylinder extends.
It is obvious that if the cylinder had an external force pulling on it, it would extend rapidly. Because fluid enters the cap end at a reduced flow rate, a vacuum void would form there until the pump had time to fill it.
Meter-in flow controls can have a problem in pneumatic circuits. When fluid is directed to the cylinder cap end, pressure at PG1 immediately rises to the regulator setting. However, pressure at PG2 starts at zero and increases slowly. Until pressure at PG2 rises enough to generate breakaway force, the cylinder does not move. At breakaway pressure, the cylinder extends quickly and expanding air may cause it to lunge. Often, the lunge forward moves the piston ahead of the incoming air and pressure drops back below the breakaway level so the piston stops. Pressure starts to build again and the lunge/stop scenario continues to the end of stroke. The meter-out circuit discussed next is always the best choice to control air cylinders.
The circuits in Figure 13-7 show applications where a meter-in circuit is the only choice for both pneumatics and hydraulics. On the left in Figure 13-7, a single-acting pneumatic cylinder is mounted with the rod vertically up. The only way to control extension speed is via a meter-in flow control. When retraction speed must be controlled as well, a meter-out flow control also is necessary.
The cylinder pictured on the right in Figure 13-7 is extending to perform an operation prior to retracting or starting the cycle of another actuator. A signal to continue the cycle can come from a pressure switch or a sequence valve. Either of these devices can be set to give an output at any pressure. Usually they are set 50 to 150 psi below system operating pressure for hydraulics, or 5 to 15 psi lower for air. The reason for meter-in flow control is that pressure between the flow control and the cylinder normally stays low until the cylinder contacts the workpiece. At work contact, the resulting pressure buildup switches these pressure-actuated devices and starts the next sequence. Always remember: a pressure switch or sequence valve does not directly indicate that the actuator has reached a physical position. They only indicate that pressure has reached a predetermined setting . . . not why it has.
Other circuits that require meter-in flow controls are the load-sensing pump circuits in Chapter 8.