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Electromagnet analog control

Electromagnet analog control

Electromagnet analog control

I need to control the force of 3 electromagnets wired in parallel to pick up steel discs that are piled up in a stack.
Discs range from 4" to 42" in diameter of various thicknesses (0.075" to 0.25"). 1/2 lb to 85lbs. About 30 different format total.
One stack is composed of only identical discs (diameter, thickness, finish) up to 50 pcs for small sizes.

I'm designing the machine that will pick up the discs.
The goal is to pick up only one disc at a time and thus my need to adjust the voltage that will power the electromagnets. I'll set them up to produce a bit more power than needed to lift one disc but less power than for 2.
The PLC running the system will "know" which discs are next and will have the settings for each disc pre programmed (50% power for medium disc, 85% for large discs for example).
Note: I will setup 3 sets of 3 magnets to match different diameter/force requirements. PLC will control which set is needed at which power.

My PLC (P2000 Automation Direct) can output a 0-10V signal to control the power needed.
Strongest electromagnets used are 24VDC, 0.37A, 420N 8.9W each.

I did some research and was surprised I couldn't find a power supply or a standalone MOSFET Board I could use with 0-10V input to power the EM 0-24VDC. I'm looking for 2A power supply min to have some buffer.

I found a complete machine that does exactly what I need and they are dealing with the 0-10V signal to 0-24VDC output with a custom proprietary board. ;(

I have been toying around the idea of using a DC motor drive that will receive the O-10V input and output a PWM signal to the EM.
What do you guys think?
Any other ideas?
It's a 1 off project.

Thanks a bunch!

RE: Electromagnet analog control

Now you're talking. I'd definitely go with PWM. Much more efficient which means way less waste heat to deal with.

It also means you get to use a junky unregulated 24ish-Vdc supply that will take a beating-and-keep-on-giving.

PWM that out thru the magnets. Include flybacks which will cause a general averaging of the PWM into something akin to an analog magnetic field.

Look at Solid State Relays - the DC variety.
dc ssr1

dc ssr2

Crydom DC SSR AppNote

Automation Direct SSR that includes DC outputs (three in the middle of the table)

Keith Cress
kcress - http://www.flaminsystems.com

RE: Electromagnet analog control

Thx Keith for you reply and info.
Ok for PWM.

You mentioned the SSR.
The PLC doesn't have a PWM function per say but it can be done in the ladder in few rungs (time proportional control). What period would you recommend? 50ms?
I'll get the SSR and wire a diode to protect from the inrush current (following your Crydom link do I need a fast recovery diode in my case?).

What are your thoughts of using the DC drive (PWM) that will receive the analog 0-10V from the PLC as input? Is it a viable option or overkill? Might be easier to implement.

Thank you,

RE: Electromagnet analog control

I'd probably start around 5ms. Nice thing is this should be an easy project to tweak while commissioning. Regular diodes should suffice.

Not clear what DC PWM drive you're referring to.

As for using a motor drive. Small drives like those used in RC and quad-copter like stuff would probably work but you can't use the ones for brushless motors as they need to see three phase things back from the motor. You'd have to use one for a brushed motor. Also, they look for that RC-servo signal which is digital in nature. Simple but digital. It's easy to come up with. Often they're rated at 20A some to 80A and they're really cheap. You could snag one and experiment.

Brushed ESC

A BUNCH more

Keith Cress
kcress - http://www.flaminsystems.com

RE: Electromagnet analog control

That might work as long as you actually get the analog-in version or module. Call their tech support and ask them.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Electromagnet analog control

It does,
Thanks for the help Keith.
I ordered the SSR and I am sourcing the diode right now.
DC driver ordered as well as option B.

RE: Electromagnet analog control

Not done yet, but here is a quick update.

1st method:
Using the Drive made to run DC motors works well with the electromagnets. The PWM does the job fine. I just use the potentiometer of the drive to test the electromagnets in parallel under 0V-24VDC. I shouldn't see any issues using the 0-10V output from the PLC to replace the potentiometer and run the drive remotely from the PLC.

2nd method:
I tried with the PLC as well running few lines to output the PWM signal.
One variable takes care of the period, another one the ON/OFF time withing that period in %.
Keith, you mentioned starting at 5ms. It seems it's going to be too small for the PLC to keep up and have a clean signal.
I'll probably have to go 50 or 100ms. I'm testing with regular outputs at this point on a P2000. I might have to use a fast output usually dedicated to running steppers to pilot the SSD.
I'll try next week.

One thing I might have to do is reverse the voltage to get rid of any remanence. I'll run more tests as I'd like to avoid that; it just adds another thing to deal with.

I'll update as more tests are done.

RE: Electromagnet analog control

I'm a little late but if you look on Ebay for "PWM motor control" you'd find lots of little boards of various types that take an analog input signal and produce a PWM output.

I'd suggest controlling current, not voltage, as current is what actually creates the magnetic field so keeping the current constant would keep the magnetic field constant even when the coil resistance changes with temperature.

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