Motor suggestions for PLC trainer?

I am planning to build some PLC trainer units for a class I am teaching. I'd like to have a small motor as one of the output devices, but I can't decide which would be the best option:

1. A small brushed DC motor with a driver circuit that would be speed-controlled from an analog output from the PLC. The motor would turn a small disk which would be sensed every revolution by a prox sensor.

2. DC stepper motor and driver.

3. DC brushless servo motor with amplifier.

By the length of my description, you can probably guess I am leaning toward option 1. My goal here is to provide the most thorough learning experience. A regular DC motor would give them practice with analog outputs, and the prox sensor (a rudimentary encoder) would allow them to calculate the current RPM or even implement a basic PID routine for motor speed.

Steppers seem to be used less and less these days as the price of servos continues to go down, so I'm not sure if that would be a wise investment for a trainer. However, I think the price of a full-blown servo and amplifier would be way out of my price range. (I'm looking to spend less than $75 for the motor and related hardware.)

Any suggestions or opinions would be appreciated.

Thanks,
John

P.S. On a sidenote, does anyone have any experience getting educational discounts from Rockwell? I am with a legitimate institution (state-run vocational school) but we have had a hard time jumping through all the hoops to get authorized for the discounts. My rep keeps telling me that there are "deep deep discounts" for education but he won't give me any idea of actual pricing yet.

Hi:

I would use options 1 and 3. So your students can work with old technology (DC drive and prox) and new ( servo drive).
Finally they'll get the best of both worlds.

william

This guy has surplus DC drives and motors who is thinking of scraping them.

Brian.

Gotta read threads more carefully, I don't suppose >100 hp fits your training scheme!

oopps.

Brian.

If you want to consider adding PID training you might think about having the motor operate a fan - it could blow on a small heated element.

kolyur said:

P.S. On a sidenote, does anyone have any experience getting educational discounts from Rockwell? I am with a legitimate institution (state-run vocational school) but we have had a hard time jumping through all the hoops to get authorized for the discounts. My rep keeps telling me that there are "deep deep discounts" for education but he won't give me any idea of actual pricing yet.

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I see McNaughton McKay is in Mansfield, have you been talking to them or A-B direct?

I used/did Alaric's idea in school. Analog out to a cheap VFD driving a motor with business cards taped to the shaft as a fan, and a thermocouple hosed-clamped to a soldering iron. Another analog output went to a chart recorder that monitored iron temp.

As said before, you could skip the VFD and use a PID loop.

Mike

Honestly, in real life your students are more likely to use AC drives and VFDs than DC drives.

It might cost a little more but you can get small drives fairly reasonably from AutomationDirect.com. Plus AB is supposed to be releasing some new small drives that are supposed to be quite reasonable.

jstolaruk said:

I see McNaughton McKay is in Mansfield, have you been talking to them or A-B direct?

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Our distributor is Rexel but I have had to deal with Rockwell direct for the educational discounts. And I am about ready to pull my hair out... my experience with them has been less than pleasant. I assume this is because there is no profit on educational sales.

OZEE said:

Honestly, in real life your students are more likely to use AC drives and VFDs than DC drives.

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This is very true, but my primary goal is teaching PLC concepts. If a student understands how an analog output can control a DC motor, that translates very easily to a VFD. So basically I'm looking for the cheapest motor/drive combination I can get.

I find that steppers are easy to come by, just find a broken printer and rip it out. Thats what I do for my low budget hobbies.
I also find that steppers seem to still be the drive of choice for CNC machines, which are actually not my area of speciality but are something that I am becoming interested in.
If budget is a problem, I would choose option 1.
I would then bribe some students with some extra marks to get them to build a driver for option 2.
Finally, once these are shown to be successful, you can then try for some additional funding for option 3 and 4.
I know you didn't list an option 4, but a VFD with analogue or bus control would be a nice to have.

FINALLY got a quote back from Rockwell with some educational pricing for my trainers... I thought I'd post the multipliers we were given (% of list price) in case anyone's curious.

Operator interface - 0.94

PLC hardware/cables - 0.40

Power supplies - 0.30

Software (RSLogix, PanelBuilder32) - 0.15


I don't really understand why the HMI multiplier is so high... at work we get no discount off list for HMI products even though we pay less for everything else.

Alaric said:

If you want to consider adding PID training you might think about having the motor operate a fan - it could blow on a small heated element.

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put some reflective tape on the fan blades and you could have a crude encoder. So then you could do a PID and monitor speed.