Which plc to select

Direct wiring and exotic fieldbus cables are being replaced by something that everyone is familiar with; Ethernet. This standard medium for connection is not only easily obtainable from countless outlets, but also enables simple implementation of distributed control without the traditional headaches of creating cable terminations or wiring into terminal blocks. Having an Ethernet port built into the controller makes this simple and cost-effective. Lifecycle Hardware failures can mean a black mark against your company, but not being able to efficiently and inexpensively replace components can mean the end of a relationship.

The benefits of product quality can be problematic to quantify, especially for some machines where the expected lifecycle of other more costly components can necessitate an upgrade or replacement machine well before the controller will likely fail. In simple terms, the controller should not be the weak link in the machine. The CPE also has a suite of built-in advanced security including Achilles Level 2 certification, so it can mitigate security and operational risks to the control system.

The Versamax standard range is pitched at small to medium systems before the RX3i takes over for larger systems.

The S is one of the most feature rich PLC platforms on the market today. In addition to the advanced S, Siemens has the range of industry proven S and S controllers, the latter of which provides a full redundancy option. One of the features of its S7 PLC range is that it can offer two safety runtime groups, which means the infrastructure can be divided into two groups with each section having different control parameters, including priorities and time cycles.

Historically, the Siemens S7 PLC programming environment could be challenging for the uninitiated, however the latest Total Integrated Architecture TIA software suite provides a wide range of programming options that makes it easier than ever to configure and set up not only PLCs, but HMIs, drives and other associated Siemens equipment.

Siemens also offers standard technical support for free. Omron has a wide range of PLC options including rack mount, modular and compact versions. Compared to other PLC vendors, Omron typically provides lower costs options for hardware, software and licenses.

However, when considering a complete system, additional programming time is typically required due to the nuances of the Omron programming environment, so the benefits of lower component costs can be offset by additional engineering costs. Omron PLCs are high performing, flexible, reliable and relatively easy to integrate, although currently only have software-based redundancy. Temperature extremes, for example, may be problematic.

The typical operating range for PLCs is degrees Celsius degrees Fahrenheit , but if your facility's ambient temperature falls outside of that range or has other atypical environmental requirements, you will need to research other potential PLC candidates more thoroughly.

As we've seen so far, inputs and outputs are fundamental to the operation of a PLC. The number of both analog and discrete devices that your system has will impact this decision as well. The answer to this question depends on whether your application will have subsystems that are a long distance from the CPU. Having enough memory is essential too, and it will depend on how many devices you have. Another consideration is the scan time, which is the amount of time it takes for the CPU to perform one cycle of gathering inputs, running the PLC program, and updating the outputs.

Also, program memory for the CPU will be determined by the type of program and instructions you plan to use. The type of communication protocols your system will use is another factor in PLC selection. What kind of networks or devices will the PLC be communicating with? Sometimes PLCs are equipped with communication ports, but others require additional communication modules. Other options are to communicate remotely via Ethernet or to build upon multiple types of communications as your system requires.

Find out what your programming requirements are: will you need basic programming instructions, or will you require special programming instructions? Do you want to use tag name based control, where you assign a specific amount of memory for a certain data type?

Or, are you more comfortable using fixed memory addressing that does the assigning for you. Most of the time they will be in pairs but they might also have a common contact with several outputs. Since the contacts are isolated from the PLC they can control devices that run at different voltages and higher currents.

The exact frequency and pulse width will be decided in the programming. Adding a smoothing capacitor to this output can serve as an analog voltage output, though it might not be as accurate as an analog output. Pulse and PWM are similar, the differences lie in the duty cycle and the shape of the waveform. PWM will have a square wave and an adjustable duty cycle, where a pulse output has a fixed duty cycle and either a trapezoidal or s-curve waveform.

Digital Digital outputs provide a constant voltage when activated. Analog These outputs are either voltage or current based with ranges that can be found on the datasheet, typically 1 - 5V, 0 - 10V, 0 - 20 mA, or 4 - 20 mA. Most of these outputs are not designed to directly control a device but are used to send a signal to another controller for interpretation.