Plain-English Summary
Analog signals carry changing values, not just on/off status. Pressure, temperature, level, speed reference, valve command, and drive feedback may use analog signals such as 4–20 mA current loops or 0–10 VDC voltage signals.
Why It Matters
Analog problems can cause wrong readings, unstable control, poor speed, bad pressure, false level, drifting temperature, bad valve position, and machine behavior that looks random. The device may be working while the scaling, wiring, grounding, or PLC interpretation is wrong.
Field Rule of Thumb
An analog number is not useful until you know what it represents: signal type, sensor range, engineering units, scaling, wiring, power, input card type, and actual process condition.
Walt - Simple Man Takeaway
A number on the screen is not proof. Know what the sensor range is, what the signal means, and whether the machine reality agrees with the display.
Core Formula / Concept
A 4–20 mA signal uses current to represent a measurement or command. Often 4 mA represents the low end and 20 mA the high end. A 0–10 VDC signal uses voltage to represent a measurement or command. Scaling converts the electrical signal into engineering units such as PSI, gallons per minute, speed percentage, level, or valve command.
Worked Example
A tank level reads 50% on the HMI, but the tank is nearly full. The transmitter sends 4–20 mA and is ranged for 0–120 inches, but the PLC scaling was set for 0–240 inches. The signal is present, but the interpretation is wrong.
Common Mistakes
- Assuming the signal type without checking the actual device.
- Ignoring scaling.
- Ignoring device range.
- Measuring in the wrong place.
- Ignoring electrical noise, shielding, grounding, and cable routing.
- Replacing the transmitter before checking the real process condition.
First Checks / Troubleshooting Flow
- Make the area electrically safe.
- Identify device and signal type.
- Confirm device range and engineering units.
- Confirm power supply and wiring.
- Check signal at the device when practical.
- Check signal at the PLC/input card when practical.
- Confirm input card type and configuration.
- Confirm PLC/HMI scaling.
- Compare displayed value to real process condition.
- Check noise, shielding, grounding, cable routing, moisture, and loose terminals.
- Document original settings before changing scaling.
Walt says STOP! - Safety First
Make these checks prior to proceeding.
Stop when analog signals control speed, pressure, heat, level, flow, dosing, valve position, or motion; live testing is required; a wrong value could create unsafe behavior; or scaling changes are suggested without documentation. Do not change analog scaling to make a bad process look good.
Source Notes / References
This page is original Dingfelder practical field guidance. Verify controlled requirements against drawings, OEM documentation, current standards, site procedures, customer requirements, and qualified authority where applicable.