Analog signals are widely
used in industrial automation for measuring important process variables such as
temperature, pressure, flow, level, speed, and pH. Unlike digital signals,
which have only two states, analog signals continuously vary over a range of
values and provide accurate process information to the Programmable Logic
Controller (PLC). However, one of the most common challenges faced by engineers
and technicians is dealing with noisy analog signals.
Signal noise can create
unstable readings, inaccurate measurements, unexpected alarms, and poor process
control. In severe cases, electrical noise may even cause equipment shutdowns
or product quality problems. Understanding the causes of noisy signals and
implementing proper corrective measures are essential for ensuring reliable
operation.
Understanding Analog
Signals
An analog signal represents a continuously changing electrical
quantity.
Common industrial standards include:
·
0-10 VDC
·
±10 VDC
·
1-5 VDC
·
0-20 mA
·
4-20 mA
Among these, the 4-20 mA signal is the most widely used because it
offers excellent noise immunity and long-distance transmission capability.
Figure 1. Typical Analog
Signal
20 mA
│
│ /
│ /
│ /
│_____/____________
4 mA
Process
Value
The
PLC converts these electrical signals into engineering units for monitoring and
control.
What Is Signal Noise?
Signal noise refers to unwanted electrical disturbances superimposed
on the desired analog signal.
Instead of receiving a stable value, the PLC sees fluctuating
readings.
Figure 2. Ideal and
Noisy Signals
Ideal Signal
──────────────
Noisy Signal
~~~~~≈≈~~~~≈≈~~
Even
small disturbances can affect process accuracy.
Symptoms of Noisy
Analog Signals
Typical symptoms include:
·
Fluctuating display values
·
Unstable process control
·
Oscillating PID loops
·
False alarms
·
Sudden spikes
·
Inconsistent sensor readings
·
Erratic trends
·
Poor product quality
These symptoms often confuse operators and maintenance personnel.
Common Sources of Noise
Electromagnetic
Interference (EMI)
Electromagnetic
fields generated by electrical equipment can interfere with analog signals.
Common
sources include:
·
Variable Frequency Drives
(VFDs)
·
Contactors
·
Transformers
·
Welding machines
·
Motors
·
High-current cables
Figure 3.
Electromagnetic Interference
Motor Cable
│
Electromagnetic
Field
│
Analog Cable
│
PLC Input
EMI is one of the
leading causes of unstable measurements.
Radio Frequency
Interference (RFI)
High-frequency
devices generate radio waves that affect sensitive circuits.
Examples
include:
·
Wireless transmitters
·
Mobile phones
·
Inverters
·
Radio equipment
These
disturbances may create random spikes in the signal.
Improper Grounding
Grounding problems can
create voltage differences that introduce noise into the system.
Figure 4.
Ground Loop
Sensor
│
Ground A
│
Voltage Difference
│
Ground B
│
PLC
Ground loops are common causes of measurement instability.
Long Cable Runs
Long cables act like
antennas and can pick up unwanted electrical signals.
Problems increase with:
·
Distance
·
Nearby power cables
·
Poor shielding
Long cable installations
require careful design.
Damaged Shielding
Shielded cables are
designed to reject noise.
However, damaged
shields or improper termination reduce their effectiveness.
Consequences include:
·
Signal fluctuations
·
Random spikes
·
Communication problems
Loose Connections
Poor electrical
connections create unstable resistance.
Typical locations
include:
·
Terminal blocks
·
Junction boxes
·
Sensor connectors
·
PLC terminals
Intermittent contact
produces erratic readings.
Why 4-20 mA
Signals Are Preferred
Current signals offer several advantages.
High Noise Immunity
Current is less
affected by voltage drops.
Long Distance Capability
Signals can
travel hundreds of meters.
Wire Break Detection
A reading below 4 mA
indicates wiring failure.
Figure 5. 4-20 mA
Transmission
Transmitter
│
4-20 mA Loop
│
PLC Analog
Input
These
benefits make current loops ideal for industrial environments.
Voltage Signals and
Noise
Voltage signals are more sensitive to interference.
Common voltage ranges include:
·
0-10 V
·
±10 V
Voltage drops and electrical noise can easily affect these signals.
Therefore, current signals are generally preferred for long
distances.
Effects on PID Control
Noisy signals create unstable control loops.
Figure 6. Effect on PID
Control
Sensor Noise
│
▼
PLC PID Controller
│
▼
Valve Oscillation
│
▼
Unstable Process
The
controller continuously reacts to false changes, causing unnecessary movement
and reduced efficiency.
Cable Routing Practices
Proper cable routing minimizes interference.
Recommended Practices
·
Separate analog and power
cables.
·
Avoid parallel routing with
motor cables.
·
Cross power cables at right
angles.
·
Use cable trays appropriately.
·
Maintain adequate spacing.
Good wiring
practices improve measurement accuracy.
Shielded Cable
Installation
Shielded cables help reject electrical noise.
Figure 7. Shielded Cable
Outer Shield
=============
Signal Wire
-------------
The shield
captures interference before it reaches the signal conductor.
Proper Grounding
Techniques
Grounding is essential for noise reduction.
Guidelines
·
Use single-point grounding.
·
Avoid multiple ground paths.
·
Ground shields correctly.
·
Maintain low resistance
connections.
Proper grounding improves
system stability.
Signal Filtering
Modern PLCs provide digital filtering functions.
Filtering removes unwanted fluctuations.
Common methods include:
·
Moving average filters
·
Low-pass filters
·
Exponential filters
·
Time averaging
Figure 8. Filtering Process
Noisy Signal
~~~~≈≈~~~~≈
│
Filter
▼
Smooth Signal
────────────
Filtering
improves measurement stability.
Analog Input
Module Configuration
Incorrect module settings may cause inaccurate readings.
Important parameters include:
·
Input type
·
Sampling rate
·
Resolution
·
Scaling values
Proper configuration ensures accurate signal conversion.
Isolation Techniques
Signal isolators electrically separate circuits.
Benefits include:
·
Elimination of ground loops
·
Improved noise immunity
·
Increased safety
Figure 9. Signal Isolator
Sensor
│
Isolator
│
PLC
Isolation is
particularly useful in harsh environments.
Ferrite Cores
Ferrite cores suppress high-frequency interference.
They
are commonly used on:
·
Sensor cables
·
Communication cables
·
Power cables
These components help reduce electromagnetic disturbances.
Diagnostic Tools
Engineers commonly use:
|
Tool |
Application |
|
Multimeter |
Voltage and current measurement |
|
Clamp Meter |
Current verification |
|
Oscilloscope |
Waveform analysis |
|
Signal Generator |
Calibration |
|
Loop Calibrator |
4-20 mA testing |
|
Insulation Tester |
Cable health |
These tools simplify troubleshooting.
Troubleshooting
Procedure
Figure 10. Signal
Noise Troubleshooting
Unstable Reading
│
▼
Check Wiring
│
▼
Inspect Shielding
│
▼
Verify Grounding
│
▼
Measure Signal
│
▼
Apply Filtering
│
▼
Confirm Stability
A systematic approach helps identify the root cause quickly.
Preventive Maintenance
Regular inspections reduce noise-related problems.
Recommended Practices
·
Tighten terminals periodically.
·
Inspect cable shields.
·
Clean electrical panels.
·
Verify grounding systems.
·
Check sensor calibration.
·
Replace damaged cables.
·
Maintain wiring documentation.
Preventive
maintenance improves reliability and reduces downtime.
Industry
4.0 and Smart Signal Monitoring
Modern automation systems employ:
·
Intelligent transmitters
·
Digital sensors
·
Self-diagnostics
·
Predictive maintenance
·
Wireless monitoring
These technologies enhance measurement accuracy and simplify
troubleshooting.
Conclusion
Noisy
analog signals are among the most common challenges in industrial automation.
Electrical interference, grounding problems, improper wiring, and environmental
factors can all contribute to unstable measurements. Such disturbances affect
process accuracy, control performance, and equipment reliability.
By
applying good engineering practices—including proper grounding, shielded
cables, filtering techniques, isolation methods, and regular
maintenance—engineers can significantly improve signal quality and ensure
dependable operation. In modern automation systems, reliable analog
measurements are essential because every control decision ultimately depends on
the accuracy of the information received by the PLC.