Hall Effect Sensor.

[Al Skierkiewicz]

Mike,
I can't get to the page you are listing.
So for the meantime, think of this. You are able to get two pulses from two seperate sensors. As the gears turn, one sensor has an output slightly different in time from the other. (You are positioning them that way for this demo.) Each outputs exactly the same info, (error checking) and if you determine which sensor outputs first for a particular direction then you will always know that you are traveling in that direction. When the pulses change places you have reversed direction.

[Greg Ross]

Originally Posted by Al Skierkiewicz

When the pulses change places you have reversed direction.

Al, there's a problem with your graph. The way you've drawn it. Each sensor is always low whenever the other sensor goes high. Without keeping track of time intervals between pulses, it won't be possible to tell the direction of rotation.

To reliably get direction information, you would need to position the sensors just far enough out of phase that the pulses overlap as illustrated below. Then when you see one of the lines go high, you can just check the other one, and immediately know the direction.

[jzampier]

With regards to sensors in general:

Buy good sensors, but make sure you consider their context of operation:
Example: Last season I used optical sensors with white lines on output
sprockets to count wheel revolutions. Didn't get me very far because the
robot wheels slipped horribly. I probably would have required some kind of
accelerometer to determine forward velocity independantly.
(Think like this: If i know how fast I'm going forward and my wheels are
going faster than that, they don't have full traction)

Also, consider what you are trying to measure, because integrating
sensor values introduces a large amount of noise into the system.
(Little bits of noise, added up over a 'long time')

I'd avoid getting fancy compasses or other such devices. But If you want to
read about them... look here:
Honeywell Magnetic Sensors

If you are interested in the Hall Effect, grab a physics book. I believe
Fundamentals of Physics by Haliday, Resnick, and Walker covers the Hall Effect.

[Al Skierkiewicz]

Originally Posted by gwross

Al, there's a problem with your graph. The way you've drawn it. Each sensor is always low whenever the other sensor goes high. Without keeping track of time intervals between pulses, it won't be possible to tell the direction of rotation.

To reliably get direction information, you would need to position the sensors just far enough out of phase that the pulses overlap as illustrated below. Then when you see one of the lines go high, you can just check the other one, and immediately know the direction.

Greg,
Thanks for the correction. I need to stop trying to rush a post before I leave work. You don't get a chance to go back and look at what you've done.

[Bob Steele]

Has anyone come up with a good way to mount the Hall Effect Sensors?
We are having a devil of a time figuring out a good way...

Can anyone offer a suggestion?

thanks

[Al Skierkiewicz]

Originally Posted by Bob Steele

Has anyone come up with a good way to mount the Hall Effect Sensors?
We are having a devil of a time figuring out a good way...

Can anyone offer a suggestion?

thanks

My recommendation is to mount the sensor on a piece of circuit board stcok. Radio shack has a variety of prototyping circuit boards that are prepunched and can be easily cut to size. Make the board larger than needed for the sensor. Make a small bracket from thin aluminum stock with a slot for adjustment. Mount with a screw, near the gear of interest and adjust as needed.

[Karthik]

Originally Posted by Bob Steele

Has anyone come up with a good way to mount the Hall Effect Sensors?
We are having a devil of a time figuring out a good way...

Can anyone offer a suggestion?

thanks

Here's a picture of how Team 60 mounted their tooth counter last year.

http://www.firstrobotics.uwaterloo.c...etrains025.jpg

[emersont49]

Originally Posted by Karthik

Here's a picture of how Team 60 mounted their tooth counter last year.

http://www.firstrobotics.uwaterloo.c...etrains010.jpg

I've read the sensor documentation as well as many of the threads and still have one question.

Can the sensor sense the larger gear in this years gearbox or does it need to sense the sproket teeth?

[Al Skierkiewicz]

Originally Posted by Karthik

Here's a picture of how Team 60 mounted their tooth counter last year.

http://www.firstrobotics.uwaterloo.c...etrains010.jpg

Nice!

[KVermilion]

Hey,

Does anyone know where we can source more ats660s? Allegro seems to have ditched their remaining surplus of the soon to be obsolete sensors on FIRST. Very gracious and professional.

Amiclament.

Right now were are looking for a healthy alternative...

[bobtheblob]

Is the Hall Effect Sensor included with the Kit an ATS660 or ATS665? The KOP lists it as a 665 but the specification sheet on the FIRST website is for the 660. The two models have different placements for the Hall IC element within the sensor. This means the 660 needs to be mounted so the end faces the teeth, while the 665 is mounted so the side faces the teeth. It would be helpful to know which model it is without having to test it by experimenting. Can anyone clear up this contradiction?

Distributors for the sensors can be found through the allegro website. A company called NU Horizons electronics sells the ats660, but the time to deliver looks like it is 16 weeks. Newark in one sells ats665 sensors at $4.30 each.

Thanks
Bobby Sakurai
Team 159

[KVermilion]

Judging from the pictures, it is a 660, but the 665 is much easier to use (less external circuitry) and has a much more convient side mount.

Oh yeah... and Nu Horizons has a large lead time (194 days IIRC)

[SteveO]

Does anyone know a reliable method to track your displacement using these guys? Our idea is to have something similar to a small idler/roller attached to a sprocket on the ground with positive only motion (perhaps one for forward motion and one for reverse) to eliminate the problems associated with wheels slipping/object contact etc. Any other ideas?

[Andrew]

Originally Posted by Bob Steele

Has anyone come up with a good way to mount the Hall Effect Sensors?
We are having a devil of a time figuring out a good way...

Can anyone offer a suggestion?

thanks

We made a small mounting piece which we have registered on the inside of the KIT gearboxes. I am waiting for a connector so that I can test it. The sensor faces the big output gear. I'll post up drawings, p/n's, mods to the gb cover, instructions, and output of the sensor once I have tested it. I don't want to encourage teams to make the mods until I'm sure it works.

[Sparks333]

Originally Posted by Al Skierkiewicz

Mike,
I can't get to the page you are listing.
So for the meantime, think of this. You are able to get two pulses from two seperate sensors. As the gears turn, one sensor has an output slightly different in time from the other. (You are positioning them that way for this demo.) Each outputs exactly the same info, (error checking) and if you determine which sensor outputs first for a particular direction then you will always know that you are traveling in that direction. When the pulses change places you have reversed direction.

Al,
I was looking over the allegra datasheets (links up above... Thanks Caleb Fulton!) and on Page 9, it describes something regarding polarity and direction. I think its internal circuity makes it so when you are facing a tooth going one direction, it's high, but when you are going the other direction, it's low. If your gear teeth are fairly far apart, you would have to measure the duty cycle to see which way it's turning... not sure how it would be done, but it may be possible with a single sensor to tell direction.
Here's what I read. Please correct me if I'm wrong.

Output Polarity: The output of the device will switch from off to on as the leading edge of the target passes the sensor in the direction indicated (look at the datasheet for the pic), which means output voltage will be low when the unit is facing a tooth. If rotation is in the opposite direction (again, see URL for datasheet), the output of the device will switch from on to off as the leading edge of the target passes the sensor, which means that the output voltage will be high when facing a tooth.

http://www.allegromicro.com/datafile/0660.pdf , pg.9