Proportional Valve Creep at Zero Command
A hydraulic axis uses a 0-10 V proportional directional valve and amplifier; 10 V commands 100% rated flow. The valve datasheet lists 8% deadband and 2% hysteresis. The PLC analog output is commanded to 0 V, yet the cylinder creeps at roughly 4% speed. A handheld DMM reads 0.75 V at the amplifier input terminals and 0.00 V at the PLC output terminal. The signal cable is 30 m long, shielded, with the shield grounded at both the PLC cabinet and the valve cabinet. A 15 kW VFD for the pump motor is mounted 0.5 m from the signal cable, and the pump motor and valve amplifier share a cabinet grounding bus. The machine has run for a year without this symptom; the cable was recently rerouted during a cabinet modification. The question is whether the valve is defective or the command signal is corrupted.
1. Calculate the command percent represented by 0.75 V and decide whether the valve should be closed. Why might a DMM mislead?
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Command percent = 0.75 / 10 x 100 = 7.5%. The valve deadband is 8%, so on paper the spool should still be closed. But 7.5% is only 0.5% below the deadband edge, and with 2% hysteresis plus a shifted null the valve can crack open. A high-impedance DMM can also read an induced or floating voltage that disappears under load; measure with a 250 ohm burden or at the PLC input with the amplifier connected. The fact that the PLC terminal reads 0.00 V while the amplifier reads 0.75 V points to a voltage drop or ground offset in the cable, not to a command from the program.
2. Diagnose the likely source of the offset and the creep. The shield is grounded at both ends and a VFD is nearby.
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The shield grounded at both ends creates a ground loop, and the nearby VFD couples common-mode noise onto the signal. A single-ended analog input converts that common-mode voltage into an apparent command. Check the actual potential between the PLC 0 V and the amplifier 0 V with a meter; if it is about 0.75 V, the offset is a ground-potential difference. If shorting the amplifier input to its own 0 V still reads 0.75 V, the amplifier is offset. Creep occurs because the spool is at the edge of deadband and the null has shifted; vibration and oil temperature then move it enough to pass flow.
3. Specify the corrections and a safe validation procedure.
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Reroute the signal cable away from the VFD and motor cables, use a shielded twisted pair with the shield grounded at the PLC end only, and use a differential input or a signal isolator. Bond the cabinets to a single ground bus and separate power and signal wiring. Calibrate the valve null and deadband, and if needed use a 4-20 mA command, which is more noise-resistant. For validation, lock out the actuator, command 0, 25, 50, and 100%, and measure spool travel or actuator flow. Verify zero flow at 0 V, correct direction, failsafe on wire break, and relief valve operation.