Have problem with detecting magnetics field on a rotating disc - Page 1
Forum section: electronics / repair

author: Chriss
author_rank: Frequent Contributor
timestamp: November 30, 2019, 11:22:50 pm
content: Hi!I'm repairing an E-bike where a disk shape element is broken.On the disk is mounted 6 magnets which are traveling in front of a sensor during the ride.The disk is mounted on the shaft of the bicycle pedal shaft.So, whenever the pedals are moving the shaft is rotating and the disk is rotating too with his magnets on it and theyrunning away in front of a sort of sensors.Cos that 4mm thick and 75mm diameter plastic disk part is not accessible to buy, I decide to make it and put the magnetsin place as they are on the original disk.Now, I did everything perfect as the original part and I have some trouble.when I rotate the pedals, the controller did not respond correct to the rotating.As long as the pedals are rotating the controller should activating the motor in the front wheel, but it stops and start and stops and start... etc... no continuous running as it was before.The distance between the sensor and magnets are exact the same as with the oem part.Everything is the same, except one thing maybe.I didn't worried about the magnet polarity when I reassembled my new disk.Probably I mixed up the polarity of the magnets on the disk and now maybe some magnets are showing his S but other N polarity tothe sensor.Could this be a problem?I think the polarity of the magnet is not a matter to detect the magnetic field, but I'm not sure.What you think about this?Thank you.

author: shakalnokturn
author_rank: Super Contributor
timestamp: December 01, 2019, 12:15:04 am
content: That would be a problem... Not the polarity in itself of course but if you have not replaced them in an alternating sequence.Find a small magnet to test with. The magnets on the disk must form a N-S-N-S.... sequence, if you have reassembled giving something like N-S-N-S-S-N-S-N-N-S... It's very likely the controller won't be happy with it.

author: Chriss
author_rank: Frequent Contributor
timestamp: December 02, 2019, 08:21:47 pm
content: Ok, today I had access to another OEM disk from another bike, just to test.I used a small magnets to check how are the magnets oriented on the disk.What I found out was unnatural for me.All 6 small magnets on the disk was oriented like N-N-N-N-N-N on one side of the disk and the opposite polarity on the other side.So, how can this work in this way?I don't understand now how does it work now.For me it is somehow logically asshakalnokturnwrote, does they should be set up in an alternating sequence.But today I found out they are not set in an alternating sequence.Any explanation maybe?I'm confused now...Thank you.

author: DaJMasta
author_rank: Super Contributor
timestamp: December 02, 2019, 11:50:48 pm
content: Can you monitor the output of the sensor? What's the clearance between the magnet and the sensor like? Have you tried rare earth magnets instead of standard ceramic or iron ones?Could be the sensor is dead (would be visible on the output), could be that the magnetic field from your replacements isn't big enough to trigger the sensor at the distance required. If it's a hall effect sensor or similar, orientation shouldn't matter - it will be able to detect a changing magnetic field in either case.

author: m3vuv
author_rank: Super Contributor
timestamp: December 03, 2019, 12:10:23 am
content: just saying the rotation maters with these,if you fitted it backwards it wont work,shope the output of the hall sensor as well.

author: shakalnokturn
author_rank: Super Contributor
timestamp: December 03, 2019, 12:47:49 pm
content: I was assuming it was an inductive sensor.I suppose that if all the poles are identical on one side it is using a hall sensor and in that case it may only be able to use one magnet polarity.

author: SMdude
author_rank: Frequent Contributor
timestamp: December 03, 2019, 08:30:40 pm
content: The magnets on the disk are set like that to make sure you really are pedaling the bike. Otherwise anyone could set up a small signal generator and make the bike work without pedaling. Or you could sit with the pedals in the right spot and just rock them back and forward. So by having the disk set ++++---- the controller knows that the pedals are really being rotated and will provide drive.

author: m3vuv
author_rank: Super Contributor
timestamp: December 04, 2019, 04:32:07 am
content: it wont trigger from both rising or falling edge,it needs to be going in the right direction,i fitted mine on the front wheel hub with the wires extended,only had to move the wheel a tad for it to trigger then,turning the pedals all the time got boring,with it on the wheel i didnt have to!.

author: Chriss
author_rank: Frequent Contributor
timestamp: December 06, 2019, 03:40:06 pm
content: Problem solved. I just rotated the disk to the opposite side and everything is fine.