Choosing the appropriate controller to use for an automation project is a decision an automation engineer would have to make many times during their career. The following factors would help you make the optimum choice for your projects.
NEW OR EXISTING SYSTEM
For a new project, the engineer has more flexibility especially with regards to selecting a PLC Brand. An engineer can pick a controller they’re familiar with that can perform the required task provided it is within the client’s budget. In an already existing system with installed PLCs, it would be easier to work with a PLC from the same manufacturer as the installed ones to enable easy integration into the control system.
TYPE OF APPLICATION
Even though controllers can be adapted for use in any system of choice, some are more suited to certain industries than others. The Siemens S7 300 was designed for use in the manufacturing industry usually involving a lot of digital signals while the s7 1500 was designed for use in the process industry which involves a greater use of analog I/Os.
NUMBER, TYPE AND LOCATION OF I/Os
PLCs are generally priced according to the maximum number of I/Os that can be supported by the CPU. A good practice is to pick a PLC that can support at least 20% more I/Os than needed for an application. This keeps the cost low while giving room for future expansion. If remote I/Os are required, then a PLC that supports remote I/Os must be used. Applications requiring analog control would also require a controller that can perform that function.
COMMUNICATION
Another factor to be considered is the ability of the controller to communicate with other devices in the automation network. An example is a system where the existing devices communicate using Modbus TCP/IP over a network. A PLC that is to be added to the system should also support Modbus TCP/IP protocol.
SPECIAL MODULES OR FEATURES
Certain applications for example motion control have certain software and hardware requirements which must be supported by the controller. Any application involving process control must use a controller that offers PID Loop Programming capability. The number of loops to be controlled further affects the choice of controllers as most PLCs have a maximum number of loops that can be supported.
CLOSEST BRANDS (REGION)
Its important to state that the controllers for machines and plants are often chosen based on the closest brands to the OEMs. An European machine is more likely to have a Siemens controller installed, an American machine is likely to have an Allen Bradley controller while Omron/Mitsubishi controllers are commonly found in machines built in Asia . This is because control system engineers for companies in these regions are more likely to be familiar and knowledgeable in handling these brands.
There are more other factors that can be considered depending on the project but the above should help you make a reasonably informed decision during a project.
