How To Specify a PLC - Choosing The Right PLC Guide

The controller is the core of any automated system, and must be able to give a reliable performance for many years. You don't want to be looking at an expensive upgrade because your PLC was under-specified and the system performance compromised. Nor do you want to over-specify the project requirements, and end up with lots of unnecessary features. Don't commission your PLC too soon in the system design process, but wait until you have a more precise idea of what it will need to do.

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When deciding how to specify a PLC, these five tips will ensure that you always get exactly the controller you need, customised to the exact requirements of your application. In this way, you'll get the maximum performance return on your investment.

1. System Size
2. Functionality and Flexibility
3. Performance
4. Connectivity
5. Security

It will help to consult with experts before deciding on any particular make, model or platform to ensure that the system will perform to expectations.

Obviously, the capacity of your PLC must be dictated by what you need it to do, so you need to begin with an assessment of how many I/O connections you'll require. Modern PLCs vary widely, from the micro PLC with a maximum of 300 I/O, to the large modular devices that handle thousands. If you need only a low I/O count, then you'll get exceptional performance from a micro system, but unless you're running a complex operation, a mid-range compact device might suit you better.

Today's modular PLCs combine familiar programming with many more advanced options which can provide an ideal solution. These include extensive combinations of function sets and I/O counts which can be customised to your individual application. Bear in mind also that you may wish to add elements to your system in the future, so you should allow some capacity for expansion.

The most significant expenditure in system development nowadays is no longer hardware, but programming time. Designers need to develop a highly functional system in the least possible time, and that requires both hardware and software to be highly flexible. You'll want to be able to move seamlessly between a variety of programming languages, whether this is from one design project to another, or within a single project. You'll also want programming tools that allow you to design a system in a virtual environment, and pre-commission it before committing to any expensive physical commissioning time. Many of the best programming tools for PLCs now include powerful pre-coded function blocks, which will save you further time in development.

The performance capability of your PLC relates directly to your application, with more complex processes like packaging requiring a much higher-speed processor. There may also be connectivity and motion control options, and even simpler operations like temperature control can require the PLC to monitor or control numerous analogue I/O.

Calculate your instruction set to ensure that your PLC has sufficient power, perhaps including PID control and maths, as well as all the sensory control options now available. Don't cut it too close, either: you must be sure of having enough juice to run your applications, as well as any potential future upgrades.

Connectivity is one of the most important aspects of knowing how to specify a PLC, and your requirements must be carefully assessed. PLCs used to be designed as stand-alone systems and you may still want to do this, perhaps because it would be cheaper or more secure. Even compact PLCs, though, can now provide improved connectivity to a substantial number of open and proprietary network protocols. With sufficient connectivity you'll have the additional benefits of remote access, which provides monitoring flexibility and can vastly improve maintenance. You'll also be able to gather data sets which can help optimise system performance and reduce downtime.

Your PLC will be connected to the internet for many important monitoring and control processes, but this does leave you open to the threat of cyber attacks. PLC code, classified as intellectual property, is also being cloned and pirated by unauthorised users, so you need to be absolutely sure about the security of your suppliers. They should offer robust security, both within the PLC itself and within its programming tools, as well as providing fast and reliable customer support and a substantial warranty.

If you follow these golden rules on how to specify a PLC, you'll be able to avoid many of the common pitfalls in configuring your system.

Choosing the right Siemens S7-1200 PLC for your project can be challenging, but with the right guide, it can be easy. If you have read our previous S7-1200 PLC 101 article, there are multiple models to choose from, including expansion modules.

This article will provide a step-by-step guide on choosing the right Siemens S7-1200 PLC and expansion modules for your project. We’ll also go over a small project example using the step-by-step guide.

Part 1: Step-by-Step guide

1. Determine the project requirements

The first step in choosing the right S7-1200 PLC is determining your project requirements.

Some questions you can ask yourself are:

– What is your application about?

– How many I/Os are you going to be needing?

– What devices are you going to be communicating with?

– Is this going to be a safety application?

– Is it a time-sensitive application?

This first step will give you an overall picture of your project needs.

2. Identify the necessary functions

The second step would be to identify the necessary functions for your application.

For example, do you need to control speed and direction? Will you need an emergency stopping function? Do you need the PLC to provide diagnostics? Maybe you need to maintain the flow rate using pressure sensors.

Doing these first steps will narrow down the types of models to consider.

3. Choose S7-1200 model

The third step is choosing the right S7-1200 model and the expansion modules and boards if required. With your functions and requirements laid out, you can select the CPU model, expansion module, and boards to reach your requirements.

Take note of the I/Os already integrated into the CPU module and the max number of modules and boards the CPU can take. If you need a list of the CPU and I/O module options, check the Siemens catalog for S7-1200 central processing units.

4. Verify the compatibility

With the fourth step, you need to verify the compatibility. This means you get the correct software and devices compatible with your S7-1200 PLC.

All S7-1200 PLCs will need STEP 7, Basic Edition to start programming. Check this link for the list of STEP 7 Software. Be sure to filter the Edition drop-down to Basic.

5. Consider future expandability

The fifth step is to consider future expansions on your S7-1200. Make sure to know your I/O point limit and expansion limit on the model you choose. Should you anticipate adding more I/Os into your system, your S7-1200 will be ready to handle the extra I/O points and modules.

A good general rule is adding about 15-20% more IOs than what your project currently needs.

6. Evaluate the cost

The sixth step is evaluating the cost of your S7-1200 PLC and the associated components. Are the components within your budget and provide good value for the features and performance?

You can do an estimated cost breakdown on your selected components to get an idea of the value. Remember to take into account the smaller components such as the power supply, cables, and connectors.

7. Consult with experts

The final step is consulting with experts. If you need a second opinion or technical support, the RS engineering team and Technical Solutions Center can provide guidance on the S7-1200 PLC selection, configurations, and programming.

Part 2: Example – Industrial Conveyor System

Now that we’ve gone over the step-by-step guide, let’s go ahead and do a simple example using the guide by selecting the right S7-1200 PLC for an industrial conveyor system project.

1. Determine the project requirements

First, let us determine the project requirements.

– The industrial conveyor system will require the PLC to control and monitor the movement of products on the conveyor, such as boxes, packages, or pallets.

– The PLC will need approximately 12 digital inputs, 12 digital outputs, 4 analog inputs, and 2 analog outputs.

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– It will need to communicate with a human-machine interface, or HMI, over Profinet.

– It will also need to provide diagnostics information to maintenance personnel.

2. Identify the necessary functions

Second, let’s identify the necessary functions for this application. The PLC must be able to perform the following functions:

– Control the speed and direction of the conveyor motor using a variable frequency drive, or VFD, based on the input from a speed sensor and a position sensor.

– Stop the conveyor in case of emergency using a safety switch and a safety relay.

– Control activation of other devices, such as pneumatic actuators, based on the position of the products on the conveyor.

– Communicate with an HMI over Profinet to display status information and receive commands.

– Provide diagnostic information to maintenance personnel, such as fault codes, maintenance alerts, and sensor readings.

3. Choose S7-1200 model

Third, we can now select the right S7-1200 model. Based on the project requirements and functions, we can select the following S7-1200 PLC model:

– The 1212C CPU will provide 75KB memory capacity and a max of 2 expansion modules, giving it sufficient memory and expansion for the application. We will choose the version that needs DC Power and has transistor outputs (6ES7212-1AE40-0XB0).

– The 1212C CPU will already have 8 digital inputs, 6 digital outputs, and two 0-10V analog inputs. We would need to add some expansion modules to reach our I/O requirements.

– The first expansion module is the SM 1223. We will choose the version with 8 digital inputs and 8 digital outputs with transistors (6ES7223-1BH32-0XB0).

– The second expansion module is the SM 1234. We will choose the version with 4 analog inputs and 2 analog outputs (6ES7234-4HE32-0XB0). This module can work with either voltage or current signals.

– The S7-1200 supports Profinet protocol, enabling communication with the HMI.

4. Verify the compatibility

With the CPU and expansion module chosen, we can now move into the fourth step, verify compatibility.

The S7-1200 will be compatible with the Siemens TIA portal software. You will need the Step 7 Basic software. It will also communicate with the HMI through Profinet.

5. Consider future expandability

Fifth, we consider future expansions. If we need to add more sensor inputs or drive an extra device, we know our system’s total I/Os.

Our total I/O points are 16 digital inputs, 14 digital outputs, 6 analog inputs, and 2 analog outputs. That leaves us with an extra 4 digital inputs, 2 digital outputs, and 2 analog inputs. We also have space to add a board for more I/Os.

6. Evaluate the cost

Sixth, we look at our complete S7-1200 system and evaluate the cost. We’ll need to consider the following parts:

– CPU 1212C ($$$)

– SM 1223 Expansion module ($$)

– SM 1234 Expansion module ($$)

– STEP 7 Basic Software ($$)

– Power Supply ($)

– And Wiring, terminals, and cables ($)

7. Consult with experts

The final step is consulting with an expert. You might need more guidance on selecting the right S7-1200 PLC or have questions about it. The RS engineering team and Technical Solutions Center can assist with S7-1200 PLC selection, configurations, and programming.

Conclusion

In this article, we have learned that following these steps can make the selection process easier for an S7-1200 PLC that best fits our project, providing reliable and efficient operation. Specifically, we learned to

– Determine your project requirements and necessary functions

– Choose the right S7-1200 CPU and expansion modules and boards

– Check to verify compatibility

– Consider future expandability

– Evaluate the cost of the system

– And consider consulting with an expert on your selection or application

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