What Is Analog and Why Use It
An analog signal represents an amount that can vary across a range. Temperature, pressure, tank level, and motor speed are familiar examples. A programmable logic controller (PLC) uses those measurements when a simple on/off answer does not tell it enough.
Think of a tank
A high-level switch answers, “Has the liquid reached this point?” Its answer is TRUE or FALSE. A level transmitter answers, “How full is the tank?” Its reading might be 25%, 47.5%, or 80%.
Both signals can be useful on the same tank. The switch gives a specific yes/no indication; the measurement lets the program and operator see how the level changes across the working range.
What you will learn
Explain the difference between an on/off indication and an analog measurement.
Recognize the measurement, target, and output command as different values.
Describe why scaling, limits, alarms, and process control use analog values.
Why measure the amount?
Imagine filling that tank. A full-level switch can tell the PLC when to stop a pump. A level measurement also lets an operator see how much space remains, choose a different filling target, or watch whether the level is rising as expected.
The same idea applies to an oven's temperature or a pipe's pressure. Knowing the actual amount supports more useful displays, adjustable alarms, and control decisions.
| Question | Useful information |
| Is a part at the sensor? | A discrete TRUE/FALSE indication |
| How hot is the oven? | A temperature, such as 72.4 °C |
| How much pressure is in the pipe? | A pressure, such as 3.2 bar |
| How full is the tank? | A level, such as 47.5% of capacity |
How the measurement reaches the program
The path is physical quantity → sensor or transmitter → PLC input → value in the program. An electrical analog connection can carry a current or voltage; the input module represents its reading as a digital number.
The program needs to know what that number means. Converting a reading into familiar engineering units, such as °C, bar, or % level, is called scaling. The PLC samples the signal with limited resolution, so its stored reading changes in finite steps even though the physical quantity can vary continuously.
The earlier Analog Signals: Hello Real World lesson explains the transmitter, current loop, and stored value in more detail. These lessons follow that value through scaling, useful decisions, and commands back to the process.
Keep three jobs separate
| Value | Its job | Tank example |
| Measurement | Report what is happening | The tank is 47.5% full. |
| Target or setpoint | State what is wanted | Fill the tank to 80%. |
| Output command | Request an action | Ask an adjustable inlet valve for 30% opening. |
These percentages describe different things. A 30% valve command does not mean the tank is 30% full, and a command does not prove the device actually moved. The measurement tells the program what the process is doing.
An analog output carries a varying command back toward a device, such as a valve position request or a drive's speed request. Ordinary switching logic still decides things such as whether operation is allowed.
What comes next
You will turn input signals into engineering units, recognize an invalid reading, choose alarm and operating limits, and scale output commands. Later, the proportional-integral-derivative (PID) lessons use measurements and targets to decide how much output to request.
Before continuing, explain which value tells you what is happening, which says what you want, and which asks the equipment to act. Then start Scale a 4–20 mA Signal.
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