How To Optimise PLC Scan Time

Scan cycles run continuously in your PLC: first, it reads every input’s status, then executes whatever program logic is contained, and then uses that information to update its outputs. 

The reason it’s so important is that this cycle governs the PLC’s rate of response to any changes in your system’s input signals. The faster it responds, the more efficiently it can control its output devices. The rapidity of response in high-speed automated systems is essential.

What’s The Best Way To Optimise Scan Time?

You can make a start by refining your PLC program, simplifying logic operations and removing accumulated clutter like unnecessary instructions. For example, you can implement more efficient, straightforward logic in place of complex nested loops and redundant checks. You can organise the PLC program more effectively using techniques like function block diagrams or ladder logic.

Grouping, Subroutines And Functions

You can also reduce scan time by grouping related tasks together, which leads to faster execution and a smoother flow of operations. It minimises the amount of time required for the PLC to switch between its program’s different parts, so it can handle, for example, all its input processes in one go before switching to the outputs. 

You can also optimise your PLC effectively by establishing subroutines and functions. These are repeatable blocks of code that you can use throughout your program to reduce duplication. You’ll organise your code more efficiently and simplify debugging. You first need to define these blocks in your program using the assigned input and output parameters. Once you’ve executed the initial blocks of code, you can then program them to repeat for various time-sensitive control events like process monitoring or motion control. Less critical functions and lower-priority tasks are generally programmed for continuous execution. 

Optimising Your Logic

Logic optimisation can also help reduce your PLC scan time. This means that you employ only the most appropriate and efficient instructions, data types and logic operations for your application. For instance, you can replace complex loops with timers and counters and simplify your logic expressions with boolean algebra. Try replacing floating-point numbers with integers and substituting word-level operations for bit-level operations. If you use a bit to block a rung, it’s better to put it early in the rung. Streamlining your PLC logic in this way means that it needs to perform fewer instructions, calculations and operands, which should minimise the memory usage and scan time of its program.

Another way to optimise your logic is to stagger the different parts into continuous and intermittent operations. You’ll usually only need to run a minimal amount of continuous logic on every single scan, whereas you’ll have other parts that you run at intervals. This means you can create a simple scheduler to separate the two different types of operation so they're not all running simultaneously. This will significantly reduce your scan times.

Using Arrays And Data Tables

A third way you can optimise your PLC scan time is to use arrays and data tables. These help you manage large amounts of data, by storing in sequential order multiple values of each data type. These include data such as alarms,  setpoints, sensor readings, HMI tags and historical data. Once you’ve accessed the array or data table the first time, you only need pointers or indexing to access the individual values.

Arrays also make memory operations more efficient, as they allow you to manipulate sizeable blocks of contiguous memory using only one instruction. This means you can copy an array of nine items to another equally-sized array much faster than copying nine individual tags to nine other tags.

Comments And Documentation

Comments and documentation have no direct effect on your PLC’s scan time and memory usage, but they are crucial for making your code readable and maintainable. They represent text annotations to your code that explain its logic, functionality and purpose. They’ll guide you and other programmers when you need to understand,  troubleshoot or modify your code more quickly and simply. In this way, both yourself and the original programmer will be prompted to remember the steps taken previously instead of having to learn the logic all over again. When you employ comments and documentation, it’s necessary to use common conventions and standards, such as clear language, consistent formatting and descriptive names.

Code Testing And Debugging

Testing and debugging your code is an additional technique for optimising your PLC scan time. These processes help you identify and fix bugs and errors in your code, ensuring that it works as it’s supposed to. That means it meets the requirements of the operation for which it’s programmed and performs optimally. You’ll need some testing tools and debugging methods, that may include:

• Simulation software

• Error logs 

• Watch tables

• Breakpoints

• Online monitoring

Testing and debugging your code will eliminate faulty or unnecessary code that can overload or slow down your PLC. It should also reduce its memory usage scan time.

Control, Efficiency And Quality

You might have thought that scan time was a small detail, but in the field of industrial automation, it’s a big deal. By implementing these strategies to optimise your PLC scan time, you’ll speed up your PLC operations and make them more efficient. This is essential for maintaining the reliability and effective performance of automated systems. Reducing the scan time provides you with tighter control, helping to maintain efficiency and quality.

Shorter scan times make your system more immediately responsive, but you should bear in mind that this can also put a burden on the PLC hardware. You may even need to upgrade your PLC to achieve the critical balance between load and speed, but when you get it right your system will run at its best. Once you’ve unlocked the secret to optimised scan times, your operations will run smoothly, efficiently, and at their most effective.