Circuits with proportional throttle valves
The circuits in Figures 14-11 and 14-12 control acceleration and deceleration of an actuator. Electronic signals to these circuits also can vary the speed of the actuators infinitely.
A proportional throttle valve in the pump line of Figure 14-11 controls flow to a standard solenoid valve. This circuit is good for resistive loads only because it meters fluid to the cylinder. To reduce energy waste, use a load-sensing pump and sense the line between the proportional valve and the directional valve. Load sensing lets the system operate at lower pressures during most of the cycle. Load sensing also makes the circuit pressure compensated.
The proportional throttle valve in Figure 14-12 meters flow out of the tank line of a standard solenoid valve. This circuit is good for over-running loads because it meters fluid from the cylinder. CAUTION: The directional valve may see pressure as high as twice the pump compensator setting. Make sure this pressure does not exceed its tank line rating. Allowing the throttle valve to shift abruptly in this meter-out circuit could result in detrimental shock. Use a proportional control card with adjustable ramps for this application.
If the cylinder must set without creep, use a counterbalance valve. A throttle valve has internal leakage and may not be able to prevent cylinder drift. A counterbalance valve in this circuit must have an external drain. Backpressure at the counterbalance valve outlet modifies the pressure setting of an internally drained valve. (See Chapter 5 for a full explanation of counterbalance circuits.)
Typical conventional valve circuit with resistive load
A horizontally mounted cylinder typically requires force at all times to stroke. This cylinder configuration is known as a resistive-load application. Heavy loads at fast operating speeds usually require a means of acceleration and deceleration for smooth operation. One way to control acceleration in these circuits is to shift a standard open-center solenoid valve to extend the cylinder and let excess pump flow relieve to tank during acceleration. A small pressure spike and some heat generation take place during this part of the cycle, but otherwise cylinder start up is smooth. The schematic diagram in Figure 14-13 shows a double pump in a hi-lo circuit that operates this way. Figure 14-14 shows the circuit with a closed-center valve and a pressure-compensated pump. This arrangement eliminates some of the pressure spikes and reduces heat generation, but is more expensive.
When the cylinder approaches the end of its stroke, a limit switch unloads the high-volume pump of the hi-lo circuit, decelerating the cylinder as quickly as friction on the machine members allows. When the cylinder slows to the speed of the low-volume pump, it continues to the end of stroke at a velocity low enough to eliminate most of the shock. (In this application, a cylinder with standard cushions will eliminate virtually all shock.)
Figure 14-15 shows another shock-free deceleration circuit. Here a pressure-compensated bleed-off flow control dumps excess flow from a single fixed-volume or pressure-compensated pump. Deceleration is still as fast as the friction of the machine dictates. Secondary speed is adjustable to meet any requirement.
Another option for decelerating a load is to specify a cylinder with longer than standard cushions that have a tapered flow cutoff. Always specify load, pressure, and speed when ordering tapered cushions. Tapered cushions are very effective for machines that have fixed working parameters. If the load constantly changes, tapered cushions are only effective over a narrow range of the change.