PLC is a crucial and basic component of the automation system of any industrial process, using a PLC in your process will help increase productivity, reliability, and performance of your process. But these benefits can only be achieved if an adequate selection of the PLC was made. Choosing the PLC that best fits your application will help your process reach its designed targets.
In this article, we will talk about how to select a PLC for your application.
Contents:
PLCs are control devices used to automate and control machines and industrial processes. They are used in various industries such as manufacturing, chemical processing, and transportation to control and monitor industrial processes.
PLCs can be programmed using ladder logic or other programming languages via many programming software and platforms depending on the PLC brand you use. PLCs are designed to withstand harsh environments and operate reliably for long periods of time.
PLCs are an important component of automation because they provide a reliable and efficient way to control industrial processes and machines.
Some of the benefits a PLC provides are:
These benefits and many more are why PLC is very important for automation.
As we said before, all the benefits and advantages of using PLCs in your automation system will only be achieved if a suitable PLC was chosen for the application.
Failing to choose the PLC that best fits your process requirements will not only cause the loss of some of these benefits but also might be a cause for hindering your whole process.
There are many factors that should be considered when choosing a PLC for your system, some of these factors are:
For example, if you want to control a conveyor system, you may need a PLC with a high number of I/O points to handle the many sensors and actuators used.
You may also need a PLC that supports high-speed counting to monitor the conveyor’s speed accurately.
If you are controlling a complex process with multiple steps, you may need a PLC with a higher processing power to execute the program efficiently.
However, if your process is relatively simple, you may be able to use a PLC with less processing power.
For example, if you are controlling a process that requires temperature monitoring, you may need analog I/O modules to handle the temperature sensors.
If you are controlling a process that requires motion control, you may need I/O modules that support pulse width modulation (PWM) to control the speed of the motors.
Your process will most likely have many components that need to communicate with each other; most likely your PLC will need to communicate with most if not all of them.
You need to make sure that the PLC can communicate with the needed partners in your process.
If your PLC will operate in an environment with high levels of dust or moisture, you may need a PLC with a higher protection rating, such as IP67 or NEMA 4X.
If your PLC will operate in an environment with high temperatures, you may need a PLC with a built-in fan or a larger heatsink.
You may compare different models from vendors such as Siemens, Allen-Bradley, or Mitsubishi Electric (or other brands, in this article we will talk about these 3 PLC brands). You may also consider factors such as the availability of spare parts and accessories, the cost of the PLC, and the level of support provided by the vendor.
These are just some factors that you can consider, depending on your application some of these points can be very critical for your or none at all, also other factors might be of interest.
Here we will discuss two PLC-based projects for easy understanding of the concept with 3 PLC manufacturers.
Suppose that you need to control a process that involves mixing and heating chemicals in a laboratory.
You have the following requirements:
Based on these requirements, you could consider several PLC models from different vendors, such as:
The Siemens S7-1200 PLC supports several channels of analog inputs and analog outputs, as well as Ethernet communication and high-speed pulse outputs.
Siemens S7-1200 has a compact size and a built-in web server for remote access. It is suitable for small to medium-sized applications and has a moderate cost.
The Allen-Bradley CompactLogix 5370 PLC supports also several analog inputs and analog outputs channels, as well as Ethernet communication and high-speed pulse outputs.
CompactLogix 5370 has a compact size and a modular design for easy expansion. It is suitable for medium to large-sized applications but has a higher cost.
The Mitsubishi Electric FX5U PLC can support needed analog inputs and analog outputs, as well as Ethernet communication and high-speed pulse outputs.
Mitsubishi Electric FX5U has a compact size and a built-in web server for remote access. It is suitable for small to medium-sized applications and has a lower cost.
Those 3 PLCs can be used with your application; it is up to you to choose which one will be more suitable. Of course, this is just an example, many more models from these brands and other vendors can be used with this application.
Let’s consider another example. Suppose that you need to control a large-scale manufacturing process that involves multiple machines and systems.
You have the following requirements:
Based on these requirements, you could consider several PLC models from different vendors, such as:
The Siemens S7-400H PLC supports up to 2,048 I/O points and has high processing power, redundancy features, and support for multiple communication protocols. It is suitable for large-scale applications and has a high cost.
The ControlLogix 5570 PLC supports up to 32,000 I/O points and has high processing power, redundancy features, and support for multiple communication protocols.
The ControlLogix 5570 has a modular design for easy expansion and is suitable for large-scale applications. It has a high cost.
The Mitsubishi Electric iQ-R PLC supports up to 4,096 I/O points and has high processing power, redundancy features, and support for multiple communication protocols.
Mitsubishi Electric iQ-R has a modular design for easy expansion and is suitable for large-scale applications. It has a moderate cost.
Look, even with this many advantages of the PLCs, sometimes a PLC will not be the best choice for your application, here are some examples of when a PLC is no longer suitable for your application:
If your application has become too complex for the PLC to handle, you may need to consider other solutions.
For example, if you need to control a complex robotic system with multiple axes of motion and vision-based sensing, a PLC may not be the best choice.
If your application is likely to grow and change over time, you may need to consider other solutions that can be easily scaled and expanded.
A PLC may not be the best choice if you need to add a large number of I/O points or support additional communication protocols.
If your application requires special functionality that is not available in a PLC, you may need to consider other solutions.
For example, if you need to perform advanced motion control or high-speed data acquisition, a dedicated motion controller or data acquisition system may be a better choice.
Cost is always a consideration. If a PLC is too expensive for your application, you may need to consider other solutions that are more affordable.
The alternative to a PLC depends on the specific requirements of your application. Here are a few examples of alternative solutions:
A PC-based control system can provide a high level of processing power, advanced graphics capabilities, and support for multiple communication protocols.
These systems use software to perform control functions and typically require specialized I/O modules or interface cards to interface with sensors and actuators.
A distributed control system (DCS) consists of multiple controllers that communicate with each other over a network.
Each controller is responsible for a specific portion of the application, and the system as a whole can provide a high level of redundancy and fault tolerance.
A programmable automation controller (PAC) combines the features of a PLC and a PC-based control system.
These controllers provide advanced programming capabilities, support for multiple communication protocols, and a high level of processing power.
If your application involves advanced motion control, a dedicated motion controller may be a better choice than a PLC.
These controllers provide advanced functionality for controlling servo and stepper motors and are designed for high-speed, high-precision motion control applications.
In summary, there are several alternative solutions to a PLC, and the best choice depends on the specific requirements of your application.
Let’s say you are developing a high-speed packaging machine for a food processing plant. The machine needs to handle multiple packaging formats and materials, and it requires high-speed motion control and precision positioning of the packaging components. In addition, the machine needs to collect data from various sensors and cameras to ensure quality control and traceability.
In this case, a PLC may not be the best choice because it may not be able to handle the high-speed motion control requirements and the advanced vision-based sensing needed for quality control. A dedicated motion controller and a PC-based control system may be better suited for this application, as they can provide the necessary processing power and advanced functionality.
Also, if the machine needs to be integrated with other plant systems, such as an enterprise resource planning (ERP) system or a manufacturing execution system (MES), a PLC may not be the best choice. In this case, a distributed control system (DCS) or a programmable automation controller (PAC) may be a better choice, as they can provide the necessary connectivity and data management capabilities.
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