Proportional amplifiers and valves are easy to troubleshoot. If one suspects the valve is bad, one may apply full voltage (6 V dc) or variable voltage (0-6 V dc) from a power supply and bypass the amplifier all together. The only problem that this presents is that it may cause the valve to open 100%, which may cause unwanted machine motion. One could turn the source of hydraulic pressure off prior to this test to render the machine safe. If the valve operates satisfactorily when it's connected directly to a power supply, it's good. If it won't operate when it's connected directly to a power supply, it won't work correctly when it's connected to the valve amplifier.
One can troubleshoot the amplifier by testing the power supply terminals to determine that the correct amount and polarity of voltage is applied. If the correct supply voltage is applied, one can provide an input voltage from 0-9 volts dc. The output signal to the valve should adjust proportionally from 0-800 mA and 0-6 volts. Some amplifiers may not operate if the load (valve) connected to it has an open circuit in the wiring or the solenoid coil. One can determine the condition of the wiring and the coil by taking a current measurement. The valve should draw a small amount of current (1-5 mA) even when the valve is at rest. If voltage is present at the output terminals of the valve amplifier and no current is present, one should suspect an open in the wiring or the solenoid coil. If voltage is present at the valve amplifier terminals and proportional valve terminals and no current is present, one should suspect an open solenoid coil.
Exercise
One is instructed to troubleshoot a proportional valve and amplifier that are used to control the injection pressure on a plastic-injection molding machine. The problem has been listed on the request-for-service tag as: "Valve won't open to build injection pressure." Identify the test points, the amount of voltage, and the type of signal that one would expect to see at each point. Use the amplifier diagram shown in Fig. 1 to help determine a solution.
Solution
Since the service tag states that the valve will not open to build injection pressure, one should suspect that it's not operating at all. One can start at the input or the output section of the amplifier to solve this problem. For this example we will make our first test across terminals 22ac and 20ac where the valve is connected on the lower right side of the diagram. Place a voltmeter across these terminals, or one can place an ammeter in series with either of the terminals. One will need the machine operator to cycle the machine to the step where injection pressure is built up and set the pressure limit to full pressure. Keep the machine on this step while one tests for the voltage or current. The voltage should be approximately 10 volts since the control is set for full voltage. If one is measuring current, the amount of current should be 800 mA. If the voltage is 10 volts and the valve does not move, the electrical system is operating correctly and the problem will be in the hydraulic side of the system. If the voltage is less than 1 volt, the problem will be in the amplifier or the signal being sent to the amplifier from the controller.
If the output voltage from the amplifier is too low, the next check will be at the supply voltage terminals (24ac and 18ac) on the bottom left side of the diagram. The voltage at this point should be 24 V dc with the positive voltage at terminal 24ac. If one doesn't have supply voltage or it's less than 24 volts, you must fix the power supply.
If the supply voltage is 24 volts and the polarity is correct, one may test the voltage as it passes through the amplifier. The first voltage test inside the amplifier can be made across terminals 10ac and 14ac where 9 V dc is available. If one doesn't have voltage at this point, one would change the amplifier card. If voltage is present, proceed to the intermediate test points in the amplifier.
The next place one can test for voltage is at the signal voltage test points at the top right side of amplifier XI. The signal should be approximately 6 V dc at this point. If one doesn't have voltage at this point, one should check the ramp-up switch and set it to no ramp for this test. This will ensure that the maximum amount of voltage is provided to the output of the amplifier. If one doesn't have voltage even after the ramp-up switch is turned off, one has a problem in the amplifier and it must be changed.
Notice that troubleshooting an amplifier requires testing at several terminal points as the voltage moves through the amplifier. One may also remove and replace the amplifier or try a separate power supply of 0-10 volts and test the valve directly without an amplifier. If the valve operates with the external power supply but it won't work correctly with the amplifier, one can replace the amplifier. If the valve won't work with the external power supply, one can focus thier tests on the amplifier and hydraulic system.
Note!
Some amplifiers or controllers have a feature that provides a minimum electrical signal when the controller is requesting full pressure. This is called a reverse-acting valve feature. If the reverse-acting valve feature is selected, the amplifier will provide a minimum electrical signal when the controller requests maximum pressure, and it will produce a maximum electrical signal when the controller requests minimum pressure.
