Let's start at the beginning.
Exactly HOW are you going to control 5 motors and 1 solenoid valve?
What kicks them off, what stops them? Pushbuttons, HMI/SCADA, time-of-day?
Do you plan to have feedback to let the PLC 'know' if they're running/open, or are you going to assume that if the output is on, the device must have responded as designed (typically a bad assumption, but some folks cut costs anyway they can).
What kinds of safeties do you have (E-stops, etc.) that the PLC needs to know about to make proper decisions?
Do you have a budget, or are you expecting the beancounters to pay for whatever you recommend?
Where are you, geographically? Who are the biggest PLC distributors in your area. These people are going to be your biggest friends, or your biggest impediment to service/performance/cost/your job? (All this assumes that this isn't just a veiled attempt by a student to get us to do his homework for him).
Standard Answer #1:
Start by making a complete I/O list, including anything you need to communicate with. Double check that I/O list with a DETAILED sequence of operations. Pretend you are the operator whose job it is to use the equipment. How are you going to tell the PLC what you want it to do? What information do you need from the instruments to make your own decisions? Be sure to include when things DON'T work, as well as when they do (alarms (light/horn), etc)
Polish your I/O list until it's right, and then think about the future. What else would someone like to control with it? A 6th motor? A second valve?
Standard Answer #2
The I/O list determines the size of the PLC. Most PLC manufacturers have a range of PLCs to fit most any I/O count / budget.
Selecting a brand is a personal choice, and there is no one answer. Everyone has their personal favorites (and most hated) based on what they're used to, how they think, what their budgets are like, and what kind of service the local rep gives them.
There is no right answer. And while some here might guess at your real I/O count and make recommendations, when it comes down to it, they are just guessing that what's right for them is going to be right for you. Sometimes we guess right, sometimes not.
But you've got some more homework to do, student or not. Make that I/O list. Contact the local distributers (they'll tell you which is the right solution for your application).
