I’m an industry service engineer for food packaging machines and not an automation specialist, however can give you few hints.
For all automation systems to work, you need to first have a clear and detailed mechanical plan with all of details finalized. When you accomplish that, you need to specify the motions involved, e.g.: linear or rotary. This lets you have in mind the number and types of motors and actuators you need(servo, ac single phase, ac 3 phase, pneumatic actuator).
For each and every motors you will 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 being set out. This is often level sensors, flow sensors, proximity switches and other devices when needed. The reason why i’m stating out this routine is always to enable you to define the specifications necessary for your control system hardware requirements. All PLC manufacturers layout their product line-up based on system complexity.
Most PLC hardware comes as reconfigurable rack chassis. Basically you will find the CPU which is the master brain that is supplemented with I/O device that could be slotted in like cards. Additional complex systems which needs servo motor may 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 obtain the necessary hardware and software needed. You may want additional hardware essential for for fancy touchscreen display HMI, line automation and internet-based diagnostic and asset monitoring functions. That’s how a guy with mechanical background can approach complex automation problems.
The solutions varies depending on different manufacturer offering specifically if you use beckhoff based systems. The best way to start can be to work with existing machines so that you learn the basics. Go obtain a few catalogs from reputable manufacturers to understand what the market can give. I usually suggest people to go through Omron catalogues. They also have a totally free automation online course that may educate you on the child steps needed.
You have to be able to design complete PLC systems: architecture design, hardware specfications and selection, logic narratives, logic programming, connection drawings. Everything. Perhaps you simply need extra training about the more knowledge about each bit of it technology, on how to program or properly connect them, but it is not too difficult, a great mechanical engineer should probably excel for this as any other engineer. The main part of control system design would be to see the process you are likely to control and also the goals you wish to achieve.