Where to place Hall Effect Sensors.

[Moloch]

I know there is already a thread on this topic, but I need a more clear answer. Where exactly is the best place to put the Hall effect sensors? And what is the best way to mount them?

[zdeswarte]

Originally Posted by Moloch

I know there is already a thread on this topic, but I need a more clear answer. Where exactly is the best place to put the Hall effect sensors? And what is the best way to mount them?

All I know is that they need to be pretty close to the gears without touching them. They also need to be near the gear teeth so that the sensor senses the variations between tooth and valley. My team abandoned using those because we don't know enough about them but thats what we figured out. Its probably going to be more of an off season project than anything.

[Moloch]

Do they have to be close the the teeth on the wheels or in the transmission?

[zdeswarte]

Originally Posted by Moloch

Do they have to be close the the teeth on the wheels or in the transmission?

I would do it close to the wheels- its closer to the actual output that you'll be looking for. The bigger the teeth and valleys the better results you'll get, and I would bet that the wheel sprockets are the best place to put them for this reason. Either way works though, it just depends on your bot.

[Moloch]

Thinking about putting on extra wheels for the soul purpose of counting teeth. This will prevent skid and spinning. Not sure how accurate or phesable this will be though.

[Leav]

Originally Posted by Moloch

Thinking about putting on extra wheels for the soul purpose of counting teeth. This will prevent skid and spinning. Not sure how accurate or phesable this will be though.

not a bad idea.

the hall effect sensors need to sit really really close to the gears, about 3 mm, right in front of the point where the teeth change from peak to valley.

a good indication would be to move them closer by hand, until you can feel the magnet inside them pulling the you towards the gear.

in order to position them accurately, i would suggest that you use an oscilloscope, that way you have a visual and real-time reference , so you can decide exactly where to put them.

[Mark Pierce]

Originally Posted by Moloch

Thinking about putting on extra wheels for the soul purpose of counting teeth. This will prevent skid and spinning. Not sure how accurate or phesable this will be though.

This is a good idea, but the measuring wheels need to be sturdy enough to handle side loading when the robot turns and such. My experience when we attempted this was the weight budget and design time the mechanical guys left us with was not enough to make sturdy wheels.

[Moloch]

What about the wheels slipping? How do we count that into effect? Any advice is welcomed openly.

[Leav]

Originally Posted by Moloch

What about the wheels slipping? How do we count that into effect? Any advice is welcomed openly.

wheel "slippage" can only occur on a wheel which offeres some resistance to turning (i.e. a powered wheel or one with a brake turned on et cetera), so if you used a wheel that was not powered to count revolutions, that wheel would not slip therefor no problem.....

also, you were given an acceleromneter, so you could tell if the acceleration measured by the hall effect sensors (you would need to calculate this) is different than the one you get from the accelerometer, you would know you are slipping.....

calculating the difference between the accelerometer and the hall effect sensors would be difficult, since they do not automaticly have the same units..... though now that i think about it.... you could just use psudo units....

-Leav

[Holtzman]

One thing that also should be considered is WHICH gear you place the hall effect on. Placing them on a wheel sprocket will give you a lower resolution than if you place it on a gear higher up in your gearbox. This is because your wheel sprocket spins much slower than a gear in your gearbox. 1114 did some experimenting with this in the preseason and found we needed to place it on the first stage of our(AndyMark) gearbox to get the resolution required.

[Kevin Sevcik]

As a note for rookie programmers, though, more resolution inevitably means more counts per second. This means more interrupts per second, which can horrendously slow down your main loop if you start getting too many interrupts too quickly. So you need to strike a balance, as in most engineering problems.