Basics of PLC Analog Signal Processing: Making Data “Speak”

Hello everyone, I am Ouyang. Today, let’s talk about analog signal processing in PLCs. In automation control, analog signals act like the “senses” of devices, helping us accurately perceive and control various physical quantities. Mastering analog signal processing will make your PLC programs more flexible and powerful. So, let’s dive into this interesting topic together!

What is an Analog Signal?

We need to clarify what an analog signal is. Simply put, an analog signal is a signal that can change continuously, such as temperature, pressure, flow rate, etc. Unlike switches that have only two states of on and off, it can take any value within a certain range.

For example, imagine you are adjusting the temperature of an air conditioner. You can set the temperature to 20℃, 20.5℃, 21℃, etc., which is the characteristic of analog signals.

How Does PLC Process Analog Signals?

The processing of analog signals in PLCs mainly involves three steps: acquisition, conversion, and processing.

1. 1. Acquisition: The PLC obtains analog signals from sensors through the analog input module.

2. 2. Conversion: The analog signals are converted into digital signals, a process known as A/D conversion (Analog to Digital).

3. 3. Processing: The converted digital signals are computed and processed in the PLC program.

Let’s demonstrate this process with a simple piece of code:

// Read temperature value from analog input channel 0
LD %IW0
// Multiply the read value by 0.1 to convert to actual temperature
MUL 0.1
// Store the result in the temperature variable
ST %MD10

// If the temperature exceeds 30 degrees, start the cooling system
LD %MD10
GT 30.0
ST %Q0.0

In this example, we read the temperature value from the analog input channel, convert it to the actual temperature, and then control the cooling system based on the temperature value.

Scaling Analog Values

Tip: In practical applications, we often need to scale analog values to convert the raw data from sensors into meaningful physical quantities.

For example, a pressure sensor outputs a current signal of 4-20mA, corresponding to a pressure range of 0-100Bar. We need to convert the digital value read by the PLC into the actual pressure value. Here is a simple scaling formula:

Actual Value = (Max Value - Min Value) * (Current Value - Min Input) / (Max Input - Min Input) + Min Value

Let’s implement this scaling process with code:

// Read raw value from analog input
LD %IW0
// Subtract the minimum input value
SUB 4000
// Divide by the input range
DIV 16000
// Multiply by the pressure range
MUL 100.0
// Store the result in the pressure variable
ST %MD20

// If the pressure exceeds 80Bar, trigger an alarm
LD %MD20
GT 80.0
ST %M0.0

Analog Output Control

In addition to reading analog signals, PLCs can also output analog signals to control devices. For example, we can use a PLC to control a variable frequency drive to adjust motor speed.

Here is a simple example that shows how to adjust motor speed based on production line speed:

// Read production line speed
LD %MD30
// Map speed value to a range of 0-100%
DIV 10.0
// Limit output between 0-100%
LIMIT 0.0, 100.0
// Convert result to analog output format and output
MUL 327.67
TRUNC
ST %QW0

Notes: When using analog outputs, pay attention to the range and resolution of the output values. Different PLCs and output modules may have different specifications, so be sure to consult the relevant manuals.

Filtering Analog Signals

In practical applications, analog signals may be affected by interference, leading to unstable readings. In such cases, we can use simple filtering algorithms to smooth the signals.

A commonly used method is moving average filtering. We can continuously read multiple data points and take the average as the final result. Here is a simple implementation:

// Read new analog value
LD %IW0
// Subtract the oldest data
SUB %MD100
// Add to the sum
ADD %MD104
// Update the sum
ST %MD104
// Calculate average (assuming we stored 10 data points)
DIV 10
// Store the filtered result
ST %MD108

// Update historical data (here we need to cyclically move the array, simplified)
LD %MD100
ST %MD101
LD %MD101
ST %MD102
// ... continue moving other data
LD %IW0
ST %MD100

Conclusion

Today we learned the basics of PLC analog signal processing. We understood what analog signals are, how PLC processes these signals, and some common processing techniques such as scaling, output control, and filtering.

Remember, analog signal processing is a very important part of PLC programming, as it helps us control various industrial processes more precisely. With more practice, you’ll find that processing analog signals is not difficult at all, but can make your PLC programs stronger and more flexible.

I have a small assignment for you: try writing a PLC program that reads data from a temperature sensor (range 0-100℃), performs filtering on the data, and then controls a heater (turning it on when the temperature is below 50℃ and off when above 60℃). I believe that through this exercise, you will gain a deeper understanding of analog signal processing.

Keep it up, and soon you will become an expert in processing analog signals! I am Ouyang, see you next time!