I’m a field service engineer for food packaging machines rather than an automation specialist, but i can provide few hints.
For all those automation systems to operate, you must first use a clear and detailed mechanical plan with all of details finalized. If you accomplish that, you need to specify the kind of motions involved, e.g.: linear or rotary. This lets you have in mind the number and types of motors and actuators you’ll need(servo, ac single phase, ac 3 phase, pneumatic actuator).
For 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(similar to conveyors, liquid tank level control pumps or rollers).Servo motors need Servo drivers to manage their precise movement.
These are generally your output devices, you will want your input devices to become put down. This is level sensors, flow sensors, proximity switches and other devices as required. The main reason i’m stating out this routine would be to enable you to define the specifications required for your control system hardware requirements. All PLC manufacturers layout their product line-up depending on system complexity.
Most PLC hardware comes as reconfigurable rack chassis. Basically there is an CPU the actual master brain which can be supplemented with I/O device that could be slotted in like cards. Additional complex systems which needs servo motor could 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 workout you IO devices list, then have the necessary software and hardware needed. You may want additional hardware required 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 based on different manufacturer offering especially if you use beckhoff based systems. A sensible way to start may be to develop existing machines so you learn the basics. Then go have a few catalogs from reputable manufacturers to understand the marketplace is offering. It’s my job to suggest visitors to go through Omron catalogues. They also have a totally free automation online course that may 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 you just need additional training for the information each bit of equipment, on how to program or properly connect them, but it’s not brain surgery, an excellent mechanical engineer should probably excel about this just like any other engineer. The main element of control system design would be to understand the process you are going to control and also the goals you want to achieve.