Signal drift is one of the most frustrating issues in industrial automation.
It won’t trip the system, won’t raise alarms, and won’t obviously fail — it simply “moves a little” over time.
A slow slope appears in the trend, the analog value gradually shifts, and the PID loop starts behaving strangely.
Engineers may spend days chasing the drift—replacing instruments, checking programs, measuring terminals—only to conclude:
“It’s drifting again.”
Signal drift is subtle, persistent, and often overlooked. It doesn’t cause immediate failure, yet it silently degrades the entire control system.
This article explains — in true engineering terms — why drift happens, why it’s hard to diagnose, how to quickly pinpoint the cause, and how to permanently eliminate it.
These are not theoretical concepts, but practical insights gained from real industrial experience.
1. What Exactly Is Signal Drift?
Most instrument failures are sudden:
If it’s broken, it stops. If there’s an alarm, it shows. If there’s a fault, it trips.
But drift is different.
It is a slow, continuous deviation:
A few milliamps off today
Several percentage points off in a week
Control deviation in a month
An unreadable trend curve in six months
The difficulty lies in this:
Drift is rarely caused by a single component — it is the accumulated effect of slight changes along the entire signal chain.
A typical analog measurement chain includes:
Instrument → Transmitter → Cable → Junction Box/Terminal → Isolator → PLC AI → Power Supply → Grounding → Ambient Environment → EMC Interference
A small deviation in any single part can gradually accumulate into drift.
This is the “mysterious” nature of drift.
2. What Causes Signal Drift?
Although the symptoms vary, most root causes fall into the following categories.
2.1 Ground Potential Difference — The Most Common Hidden Cause
If the PLC and instrument do not share the same reference ground, even a few tens of millivolts difference can cause the analog signal to shift.
Typical symptoms:
Value jumps when motors or pumps start
Long cable runs amplify the drift
Installing an isolator immediately stabilizes the signal
If you see drift that “just doesn’t make sense,” start with grounding and reference potential.
2.2 Electromagnetic Interference (EMI)
In environments with:
Large VFDs
High-power motors
Welding machines
Switching power supplies
analog signals often experience continuous noise coupling — not enough to fail, but enough to create unstable small fluctuations.
Typical trend:
Tiny saw-tooth noise, micro-oscillations, or non-smooth curves.
Around 40% of drift issues are related to EMI.
2.3 Cable and Terminal Problems
This is extremely common in older plants:
Moisture inside cables
Oxidized terminals
Broken shielding
Increased impedance
Aging insulation
Any of these can cause gradual signal attenuation or instability.
2.4 Transmitter’s Natural Drift
Sensors such as:
Pressure transmitters (diffused silicon)
RTDs
Thermocouples
Level transmitters
Flow transmitters
all exhibit natural drift due to:
Temperature variation
Membrane stress
Material aging
Mechanical fatigue
All transmitters drift — the only difference is how fast.
This is not a quality issue; it is physics.
2.5 Loose Terminals
One of the simplest yet most frequently overlooked causes.
Vibration-prone areas such as:
Pump rooms
Compressor rooms
Blower areas
can cause terminal screws to loosen over time.
2.6 Unstable 24 VDC Power Supply
An underrated root cause.
Ripple, load fluctuation, voltage drop, or shared grounding can all introduce drift into analog loops.
In many cases where no other cause is found, engineers finally replace the power supply — and the drift disappears.
2.7 Temperature Effects
Temperature changes affect:
Sensor sensitivity
Cable resistance
Transmitter internal electronics
Terminal contact resistance
Semiconductor characteristics
These small variations accumulate into long-term drift.
If temperature fluctuates significantly, drift becomes unavoidable without compensation and calibration.
3. How to Make the Signal Stable Again?
Despite its complexity, drift always comes down to one underlying concept:
Stability
Stable zero/reference
Stable grounding
Stable power supply
Stable cable integrity
Stable EMC environment
Stable sensor performance
Improving just one link reduces drift slightly;
improving the entire chain eliminates it structurally.
Here are the most effective engineering practices:
3.1 Use Signal Isolation — Stabilize the Reference
Isolation is the industry-proven method to eliminate:
Ground potential differences
EMI coupling
Multi-point grounding issues
Loop inconsistencies
In practice, 70% of drift issues can be solved by proper isolation.
3.2 Improve Wiring and EMC Design
Separate signal and power cables
Use shielded cables (shield grounded one side only)
Avoid coiling cables
Install EMC filters near VFDs
Use independent cable trays for power vs. signal
Many “mysterious” drifts are simply wiring problems.
3.3 Ensure Cable and Terminal Reliability
Use industrial-grade shielded cables
Replace aged or moisture-exposed cables
Re-tighten terminals regularly
Avoid water ingress in junction boxes
3.4 Ensure Power Supply Quality
High-quality 24 VDC supply is essential.
Symptoms of poor power:
Trend shifts with load change
Multiple loops drifting simultaneously
Values unstable during startup
3.5 Calibrate Instruments Regularly
Drift is a natural process.
Only calibration can bring the measurement back to its designed accuracy.
Critical sensors (pressure, temperature, level) should have annual or semi-annual calibration plans.
3.6 Tighten Every Terminal Screw
Simple but crucial.
Loose terminals account for a surprisingly high percentage of random drift cases.
4. What Does Drift Really Tell Engineers?
Drift is not a failure — it is an early warning.
It often indicates:
Grounding issues
Aging cables
Degraded EMC environment
Sensor aging
Unstable power supply
Temperature influence
Environmental changes
The older the system, the more frequent the drift.
The more stable the system, the less drift you’ll ever see.
Engineers who understand signal drift understand the health of the entire automation system.