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I’m a field service engineer for food packaging machines and not an , but i can give you few hints.
For all automation systems to work, you must first have a clear and detailed mechanical plan with all details finalized. When you do so, you must specify the type of motions involved, e.g.: linear or rotary. This allows you to know the number and types of motors and actuators you need(servo, ac single phase, ac 3 phase, pneumatic actuator).
For each motors you may need relay contactors (for single speed discrete/on-off type motors like blower fans and liquid pumps), VFD for speed controllable ac 3-phase motors(more like conveyors, liquid tank level control pumps or rollers).Servo motors need Servo drivers to control their precise movement.
These are your output devices, then you need your input devices to be set out. This can be level sensors, flow sensors, proximity switches and other devices as needed. The reason i’m stating out this routine is to allow you to define the specifications needed for your control system hardware requirements. All PLC manufacturers layout their product line-up based on system complexity.
Most PLC hardware is sold as reconfigurable rack chassis. Basically you have the CPU which is the master brain which is supplemented with I/O device that can be slotted in like cards. Additional complex systems which needs servo motor will have servo card to connect with servo driver, communication bus cards like CAN-BUS, PROFIBUS and DEVICENET and sensor cards for special sensors like RTD temperature sensors and level sensors.
So work out you IO devices list, then get the necessary hardware and software needed. You may need additional hardware needed for for fancy touch screen HMI, line automation and online diagnostic and asset monitoring functions. That’s how a guy with mechanical background can approach complex automation problems.
The solutions may differ based on different manufacturer offering especially if you use beckhoff based systems. A good way to start will be to work on existing machines so that you learn the basics. Then go get a few catalogs from reputable manufacturers to understand what the market has to offer. I always suggest people to go through Omron catalogues. They also have a free automation online course which will teach you the baby steps needed.
You should be able to design complete PLC systems: architecture design, hardware specfications and selection, logic narratives, logic programming, connection drawings. Everything. Perhaps all you need is some additional training on the specifics of each piece of equipment, on how to program or properly connect them, but it is not rocket science, a good mechanical engineer should probably excel on this as any other engineer. The most important aspect of control system design is to understand the process you are going to control and the goals you want to achieve.