Proportional valve experiments
Suggested exercises and observations
1. Observe the difference in smoothness between the proportional valve and the directional valve operation.
2. Step through the difference pilot control methods used for controlling proportional valves.
3. See how the limits switches can be used to stop a cylinder consistently in the same position, whatever the load on the cylinder.
Direct acting open/close spool
1 A direct acting valve has a solenoid which pushes directly on the end of the spool.
2 A directional valve (black-white) is either fully open or fully closed.
3 Cylinder acceleration is dependent on the pressure available against the actuator load.
4 Increase the load on the cylinder and observe how the actuator judders when the force is too high.
5 Because oil is compressible every hydraulic system can be represented as a simple spring mass system.
6 When the load acceleration force exceeds the system stiffness the cylinder will oscillate.
Pilot operated directional control valve (Click FWD arrow)
11 A valve's spool can be operated via an external pilot pressure supply.
12 The type of valve shown will operate in an identical manner to the direct operated valve although the response times may be different.
Ramp start using orifice and proportional spool (Click FWD arrow)
21 A pilot orifice can be used to control the speed at which the spool moves.
22 A spool with shaped control lands can provide a flow level proportional to its opening.
23 Extend and retract cylinder speeds are different because the pressure area on each side of the cylinder is different.
24 The bore area provides a larger force but moves more slowly because it only has the same supply flow available.
25 With a constant pressure supply, the cylinder speed is dependent on the pressure drop across the valve and the valve opening area.
26 Changing the load will change the pressure drop available at the valve.
Direct acting proportional solenoid
31 A direct operated proportional valve has a spool position equivalent to the electrical control signal provided.
32 Click and drag the slide bar to adjust the control signal.
33 This valve is again direct acting because the proportional solenoid acts directly on the spool.
34 The A and B lines are both under pressure because both supply and tank lands within the valve will generate a pressure drop.
Pilot pressure operated proportional valve
41 The proportional pilot pressure supply may be generated externally by a pressure control valve
42 The pilot operated design performs in an identical manner to the direct operated valve.
43 In this example, the solenoid valve can be used to isolate the pilot pressure supply.
44 Larger proportional valves tend to be pilot operated because the spool forces required tend to be too high to be provided directly by a solenoid valve.
Orifice pressure drop controlled proportional valve
51 Pilot orifice flows are another way of controlling the pressure seen by the end of the spool.
Electrical pilot-operated proportional valve
61 Electrically operated proportional pressure control valves are a common way of controlling larger proportional valve spools.
62 Normally the pilot pressure would be seen by both sides of the spool.
63 This example has been shown with pressure supplied to only one side of the spool and no centre spool position for simplicity.
Limit switches automatically control sequence
71 Using limit switches to sequence the electrical cycle provides a simple automatic control system.
72 The limits are switched when they come into contact with the cylinder.
73 Adjust the cylinder load and observe how the final position of the cylinder varies.
74 Your goal is to adjust the limit switches and set up the control system such that the final cylinder position remains the same whatever the load.
Adjust the limit switches to change the sequence
81 Click and drag the limit switches to re-position them.
82 When the middle limit switches are flashing you can adjust their control setting level by sliding the signal bar.
83 The outside limit switches only control the cylinder stopping position because they always set the valve control signal to zero.
84 When the cylinder speed is at zero the control system automatically sets the valve to full speed in the opposite direction.
85 To ensure that the final cylinder position is independent of load you must set the creep speeds as low as possible.
86 Keeping the duration of the creep speed as short as possible will reduce the cycle times.