I was recently called to an automotive plant to help troubleshoot a press that would not lower. In order to lower the press, a two stage proportional valve was used to initially allow a fast down speed, then a deceleration speed once it closed on the part. 22.6 volts was supplied from the power supply to the amplifier mounted on the valve, as shown in the picture. When the command was given to lower, a DC voltage of 3.5 volts was input into the amplifier to drive the valve (see picture). The amplifier converts the input voltage into a current signal which is applied to the pilot valve coil. The pilot valve should then shift and port external pilot pressure to shift the main spool. As the main spool shifts, an LVDT (linear variable differential transformer) electrically indicates the amount of shift. When operating properly, -3.5 volts should be fed back from the LVDT. Notice in the picture that the LVDT voltage is 00.4. This most likely meant that the main spool was not shifting. I then moved the selector switch in the bottom left-hand corner to “internal” setting where the valve could be driven with the box. The command voltage was increased to 10 volts by rotating the knob (potentiometer) on the box. The LVDT signal never changed. This indicated that the problem was one of the following;
- The pilot valve was bad and not shifting.
- The main spool was stuck and not shifting
- The LVDT was bad
- There was no pilot pressure to shift the main spool
- The pilot drain line was plugged which would prevent the main spool from shifting.
The mechanic I was working with removed the end covers of the main spool and could physically move the spool with his hand. He removed the pilot valve from the main spool and we observed no spool movement while driving it with the test box. This confirmed that either the amplifier was not sending a current signal to the valve, the valve solenoid was bad or the pilot spool was stuck. It took quite some time to locate a “new” valve, but once installed the press operated normally.
When troubleshooting proportional valves, a test box is the best troubleshooting tool you can use. The company I was working for liked it so much that they bought the box!