Physical selector switch PLC wiring

Hi everyone,

I have a small machine that has been well-received on the manufacturing floor and 'they' would like to use it for multiple product lines. This means the ladder logic needs to behave slightly differently for each "mode"...different numeric constants, slightly different operation, etc. I would like to avoid installing a touchscreen and I was wondering about selector switches.

Is there a proper/best/common way to use a selector switch with a PLC? For example, if I want to toggle between 8 different "modes", would I:

1) Use an analog module and 8 resistors to create 8 different voltage levels?
2) Get an additional digital input module and give all 8 positions their own digital input?
3) Use three digital inputs as three 'bits' for 8 unique logical states?
4) I'm far from experienced in the field of PLCs...is there a piece of hardware out there that simplifies this task?

All suggestions welcome. I believe technically all of these options will work but I'm looking for the most reliable or common way of doing this. Sorry if this has been covered numerous times but my searches aren't helping...probably my lack of understanding & terminology.

Thanks!

Hi!

I would go for option 3.

I would also consider the cost of what you are trying to do. Programming hours don't come cheap, a simple HMI, or even a PC hosted application like AdvancedHMI would make it easier to switch and change. You could use recipes.

What PLC do you have? And how big are the differences. Is it just a difference in values, or do you have a difference in IO as well?

I would probably go with "3) Use three digital inputs as three 'bits' for 8 unique logical states?".

The only thing I would think about before axing a small HMI is what are the chances that some settings will have to be tweaked from time to time as it is moved from machine to machine?

dpmanthei,

I also vote for option 3.

PLUS,
I would also add a subroutine for each module to keep them separate.
this would mean more than likely reworking the outputs, but in the long run,
it's time well spent in my opinion. I did this for 16 different production fixtures
and it worked great and by defining each routine, you make it easier for maintenance to debug.

regards,
james

any change of product code require password to prevent unexpected operation

HMI the way to achieved.

Hey thanks a lot for all of the replies!

boneless: I'm using the IDEC MicroSmart Pentra. At this time, the differences are only numerical values. It's an automated spring tester that compresses a spring with a stepper, gearbox, rack, and pinion and measures force with a load cell. The target force and speed of the stepper are different for each product, but the process of measuring the spring and shim underneath it is always the same.

TWControls: I agree that you lose some flexibility. However, my company is trying hard to better-control the assembly process to ensure our products are consistent regardless of the technician, time of day, or batch. For this reason, we strongly encourage technicians to come to engineering for any changes, no matter how small. So in this case, I actually prefer that the system isn't TOO flexible because we are trying to carefully document any and all changes on the production floor.

Mcquade: I like your subroutines idea. Initially, each program will behave the same and it might seem tedious. However, as soon as one product requires a fundamental change to the process, it can greatly complicate the ladder. Going the subroutine route will probably save me from doing a complete re-write down the road!

osmanmom: Passwords are used elsewhere in the facility, but in this case we are safeguarded by our specifications. Our spring shim tolerances are on the order of 5-10 microns so an incorrect mode selection will be caught immediately as the required shim size will be well out of spec.

Use option 3, and encode the bits with whatever plug you use to connect the control to different machines.

Go with option 3, and use a thumbwheel or pushwheel switch. The wires may have to be soldered but it is only one component you have to worry about. The output of the thumbwheel will correspond to a binary code, so you can bring it into 4 inputs and interpret those to be the number selected on the thumbwheel. If you get a single digit wheel it will have enough for 10 settings (digits 0-9), which will give you room for future expansion.

For $200 you can put a simple Nematron Optimate HMI on the machine (available through Automation Direct).

I have a standard panel template that I use for all the various kinds of "stick it" and "push it" type single operation machines that can be found in many different varieties of manufacturing. They all revolve around the Micrologix 1100. The panels are identical but there is space for a few components to customize to the application as needed and the same basic box can be used on many different kinds of machines. I use the Optimate for parameter setting, mode selection, etc as it gives me an inexpensive customizable interface for simple machines that don't require a GUI.

I recommend this approach because it is highly adaptable and I find that having a cookie-cutter design for the simple stuff keeps me from being getting bogged down and lets me stay focused on the larger projects. When someone says "I need controls for a machine that inserts a slug into a die" I can pull up the template wiring diagrams and order everything from a standard bill of material. Its a huge time saver and I can get back to my main projects. The maintenance techs like it because they have something familiar on very different machines and the operators like it because the different machines have the same basic intuitive operation.

I'm not OK with option 3 as selector switches will eventually malfunction.

Option 2 is good, because in case of malfunction of selector switch you will know what to do. Also, my advice is to take 9-position selector switch (with "0" position).

Depending of PLC, DI card is around 140$ an 9-position selector switch is around 25$, and it is very easy to program and later trace the program.