I’m a field service engineer for food packaging machines instead of an automation specialist, but i can give you few hints.
For many automation systems to work, you have to first have a clear and detailed mechanical plan effortlessly details finalized. Whenever you achieve this, you should specify the motions involved, e.g.: linear or rotary. This lets you understand the number and types of motors and actuators you need(servo, ac single phase, ac 3 phase, pneumatic actuator).
For each motors you might 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(similar to conveyors, liquid tank level control pumps or rollers).Servo motors need Servo drivers to manage their precise movement.
These are your output devices, you’ll need your input devices to get determined. This can be level sensors, flow sensors, proximity switches and also other devices as needed. The main reason i’m stating out this routine would be to let you define the specifications needed for your control system hardware requirements. All PLC manufacturers layout their product line-up determined by system complexity.
Most PLC hardware is sold as reconfigurable rack chassis. Basically there is a CPU which is master brain which can be supplemented with I/O device which can 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 exercise you IO devices list, then obtain the necessary hardware and software needed. You may need additional hardware required for for fancy touchscreen HMI, line automation an internet-based diagnostic and asset monitoring functions. That’s the way a guy with mechanical background can approach complex automation problems.
The solutions varies according to different manufacturer offering especially if you use beckhoff based systems. A great way to start is to focus on existing machines so that you will study the basics. Then go get a few catalogs from reputable manufacturers to understand industry provides. I always suggest people to go through Omron catalogues. There is also a free of charge automation online course that may teach you the infant 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 you simply need additional training around the more knowledge about each bit of equipment, on the way to program or properly connect them, however it is not nuclear physics, an excellent mechanical engineer should probably excel with this because other engineer. The key facet of control system design is always to understand the process you will control along with the goals you wish to achieve.