In our previous discussions, we mastered the “Discrete” world—the 1s and 0s of pushbuttons and proximity sensors. But the real world doesn’t just turn on and off. Temperature rises and falls, pressure fluctuates, and tanks fill gradually.
To capture this reality, we use Analog Signals. For the automation student, this is where the “Digital Chain” becomes a high-fidelity instrument. If you want to feed an AI model in 2026, you don’t just need to know if a motor is running; you need to know exactly how much current it is drawing and the precise temperature of its bearings.
1. The Contenders: Voltage vs. Current
There are two primary ways we transmit analog data across the factory floor: 0-10VDC and 4-20mA.
The 0-10VDC Standard (Voltage)
- The Logic: The sensor output is directly proportional to the variable (e.g., 0V = 0%, 10V = 100%).
- The Weakness: Voltage is highly susceptible to Electromagnetic Interference (EMI). Furthermore, because of the resistance in long wires ($V=IR$), the 10V sent at the sensor might only be 9.5V by the time it reaches the PLC. This results in inaccurate data.
The 4-20mA Standard (Current)
- The Logic: The sensor regulates the current in the loop.
- The Strength: Current is the same at every point in a series loop, regardless of wire length. It is also much more resistant to electrical noise.
- The “Live Zero”: Why 4mA instead of 0mA? Because if the PLC sees 0mA, it knows the wire is broken. If it sees 4mA, it knows the sensor is healthy but at its minimum value. This is a critical safety feature.
2. The Math: Scaling and Resolution
A PLC doesn’t “see” 4-20mA. It sees a digital number. This process involves two steps:
Step 1: Analog to Digital Conversion (ADC)
The PLC’s input card converts the electrical signal into a “Raw” integer. A common 15-bit resolution card will turn a 4-20mA signal into a value between 0 and 32,767.
Step 2: Scaling
As the engineer, you must scale that raw number back into “Engineering Units.” If a pressure sensor (0-100 PSI) sends 12mA, the PLC sees roughly 16,384. You must program the logic to understand that 16,384 means 50 PSI.
3. The 2026 Mandate: Data Fidelity
In the Intelligence Economy, “close enough” is no longer acceptable. AI-driven predictive maintenance relies on the Resolution of your analog signals.
A low-resolution signal (8-bit) might only tell you the temperature in 1 deg C increments. A high-resolution signal (16-bit) allows the AI to see micro-trends of 0.001 deg C, detecting a failing bearing weeks before a human technician could ever feel the heat.
4. Summary: The Student’s Checklist
- Prefer 4-20mA for any distance over 10 feet or in electrically “noisy” environments.
- Use Shielded Twisted Pair (STP) cable to protect the signal integrity.
- Check your Resolution: Ensure your PLC card can capture the fine details your AI models will eventually require.
Lab Exercise: Connect a 4-20mA signal generator to your PLC. Change the output by 1mA increments. Record the “Raw” values in the PLC. Is the relationship perfectly linear? Calculate the “Slope” (m) of your scaling equation (y = mx + b).
Analog Signal Lab
Visualizing the bridge between physical physics and digital intelligence.
1. The Scaling Workbench
Adjust the Physical Variable below to see how the sensor translates the world into an electrical pulse, and how the PLC perceives it.
Loop Current
12.00mA
4mA (0%) to 20mA (100%)
PLC Raw Data
16384
15-Bit Integer (0 – 32767)
Signal Linearity
Scaling_Equation:
Value = ((Raw / 32767) * 100)
Why Current (mA) Wins
Comparison of signal accuracy over distance. Voltage (0-10V) drops as cable resistance increases, while Current (4-20mA) remains constant.
The “Live Zero” Advantage
“If a wire snaps in a 0-10V system, the PLC sees 0V and thinks the process is at its minimum. In a 4-20mA system, a snapped wire results in 0mA—which triggers an ‘Out of Range’ alarm. This is why safety-critical systems always prefer 4-20mA.”
EMI Rejection
High-impedance current loops ignore the magnetic noise from VFDs and motors.
Resolution
A 16-bit card provides 65,536 steps, enabling precise AI modeling.
The Math of Measurement
Sensor Range
0 – 100°C
PLC Scaling
0 – 32767
By mastering this mapping, you provide the “Clean Data” required for 2026 industrial intelligence.