How can I detect metal contact on u shaped electromagnet and register that contact to an Arduino?

Thread Starter

Michael Campbell

Joined Oct 25, 2015
36
Hi Guys,

I am using an electromagnet to hold a loaded metal plate in place but I'd like to also use it to simply detect the plate's presence and register that detection on an Arduino. For the detection, I don't actually want to hold it in place. Is this possible? Any tips on how I can achieve this? I have a little image below to paint the picture.

BOrwwGr.png
http://i.imgur.com/BOrwwGr.png

Thanks much!
 
You could imagine the plate as a switch and use the arduino to detect the continuity of it. If it was high, the circuit formed by the plate would be closed and the plate would be in contact. If it was low, the plate would not be in contact with anything.

What do you want to use this for and what is the plate doing?
 

Thread Starter

Michael Campbell

Joined Oct 25, 2015
36
Oh yes, of course.

I want detection to take place at the point of touching, with maybe a couple mm of tolerance. As close as I can get. I do want it to be sensitive enough to detect it in a fraction of a second though as it may only be there for that long.

10 volts
1 amp
Not sure about amp-turns, but it's about 1 lb of copper coil (2 coils), 24 gauge, roughly 400 wraps.
 

Thread Starter

Michael Campbell

Joined Oct 25, 2015
36
You could imagine the plate as a switch and use the arduino to detect the continuity of it. If it was high, the circuit formed by the plate would be closed and the plate would be in contact. If it was low, the plate would not be in contact with anything.

What do you want to use this for and what is the plate doing?
I have a small spring loaded door that is articulating down. At certain points I want to hold it down, but other times I just want to know if came down. Can an arduino detect continuity through the coils? And is it possible to detect presence of only a few microseconds?
 

GopherT

Joined Nov 23, 2012
8,012
Hi Guys,

I am using an electromagnet to hold a loaded metal plate in place but I'd like to also use it to simply detect the plate's presence and register that detection on an Arduino. For the detection, I don't actually want to hold it in place. Is this possible? Any tips on how I can achieve this? I have a little image below to paint the picture.

http://i.imgur.com/BOrwwGr.png

Thanks much!
Are both coils in your image turned the same direction? That us, will both end of the U be North poles of a magnet or will there be one N and one S?
 

Thread Starter

Michael Campbell

Joined Oct 25, 2015
36
What is the DC resistance of each coil? What voltage is powering the two coils (connected in series as I see it).
Not sure about resistance, but's 24 gauge wire and I'm running 10-12 volts through it to actually hold it. I can change that amount for detection only. And yes, this is connected in series.
 

#12

Joined Nov 30, 2010
18,186
I think you are missing the fact that magnetic circuits change suddenly and strongly at the moment of contact. This requirement to detect the plate when it is NOT contacting is complicating the situation. For that we need to work with, "how much". Is it really important to detect the plate when it is NOT contacting? How far away (2mm). Is that distance merely air? etc.
 

Thread Starter

Michael Campbell

Joined Oct 25, 2015
36
I think you are missing the fact that magnetic circuits change suddenly and strongly at the moment of contact. This requirement to detect the plate when it is NOT contacting is complicating the situation. For that we need to work with, "how much". Is it really important to detect the plate when it is NOT contacting? How far away (2mm). Is that distance merely air? etc.
I prefer it to register only on contact. I only included some tolerance just in case, as I'm not familiar with this application.
 

GopherT

Joined Nov 23, 2012
8,012
Not sure about resistance, but's 24 gauge wire and I'm running 10-12 volts through it to actually hold it. I can change that amount for detection only. And yes, this is connected in series.
Ok, can we assume 50 feet of wire in each magnet? That is 100ft in total and 2.4 ohms, giving 5 amps of current.

So, 60 watts (5A x 12V) of power into those coils. Hope you don't mind the heat. The plastic bobbins will likely melt.

Now, if you have 1000 feet of wire, then you have 24 ohms and 0.5 amps (6 watts). Some heat but the mass of 500 feet of 24 gauge wire should be ok.

We are getting there. We just need some context here.
 

GopherT

Joined Nov 23, 2012
8,012
I prefer it to register only on contact. I only included some tolerance just in case, as I'm not familiar with this application.
As #12 is say, we can start detecting as the bar approaches but it will be a small effect, then, in the last fractions of a mm, the effect should go up quickly. Telling the difference between a few tens of micrometers will be difficult differentiating from true contact.
 

Thread Starter

Michael Campbell

Joined Oct 25, 2015
36
Ok, can we assume 50 feet of wire in each magnet? That is 100ft in total and 2.4 ohms, giving 5 amps of current.

So, 60 watts (5A x 12V) of power into those coils. Hope you don't mind the heat. The plastic bobbins will likely melt.

Now, if you have 1000 feet of wire, then you have 24 ohms and 0.5 amps (6 watts). Some heat but the mass of 500 feet of 24 gauge wire should be ok.

We are getting there. We just need some context here.

I didn't count honestly, as I'm just experimenting but it's gotta be on the order of 300 + ft. It certain is getting warm though. Here's a pic:

pbNzjzK_800x600.jpg
http://i.imgur.com/pbNzjzK.jpg
 

Thread Starter

Michael Campbell

Joined Oct 25, 2015
36
As #12 is say, we can start detecting as the bar approaches but it will be a small effect, then, in the last fractions of a mm, the effect should go up quickly. Telling the difference between a few tens of micrometers will be difficult differentiating from true contact.
Let's just go with absolute contact. The only potential challenge would be it potential presence for only a few microseconds (or is that milliseconds).
 

Thread Starter

Michael Campbell

Joined Oct 25, 2015
36
Material lift magnets typically require a bi-polar control, depending on the weight of the lifted item, it may need a reverse pulse to de-mag.
BTW you do not require laminated core with a DC electromagnet.
Max.
Interestingly, when I wrap a fresh electromagnet, it does works great, however after a bunch of abuse, it does seem like it needs a reverse pulse de-mag. I'm not sure what's happening.. is it short circuiting somewhere?

Are you saying I don't need laminated cores? Or are you asking?
 

GopherT

Joined Nov 23, 2012
8,012
Interestingly, when I wrap a fresh electromagnet, it does works great, however after a bunch of abuse, it does seem like it needs a reverse pulse de-mag. I'm not sure what's happening.. is it short circuiting somewhere?

Are you saying I don't need laminated cores? Or are you asking?
You are making a permanent magnet.
 
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