Hall Effect Sensor.

The 2005 kit came with something a Hall Effect Sensor, after a bit of looking I found the manual for it, it’s actually a ATS665LSG made by Allegro Microsystems. Allegro desribes it as a True Zero Speed, High Accuracy, Gear Tooth Sensor. What does that mean? Is this the “gyro” for this year? And does this work in congunction with the accelerometer. There are two of Hall Effect sensors so it seems we need to use them in twos. Also the Allegro manual mentions gears and gear tooth length. So I’m a bit confused and a bit curious and maybe even a bit excited. Anybody know anything about the Hall effect?

My GUESS is that you would use these to count gear teeth - used for feedback to determine how fast/far your robot moves, or for sensing the motion of anything else using chain/sprocket. However ours were missing when we did our kit inventory …

Okay. A gear-tooth sensor sits in your gearbox and uses a magnetic field in order to sense each specific tooth go by. The Hall Effect is the concept that allows the integrated circuit to take a measurement of the magnetic field surrounding it. (Hall effect compasses are also popular) The amount of time that elapses between the each tooth gives the speed of the rotating gear, which can then be scaled to the speed of the wheel, etc.

This is not a gyro. A gyro returns your angular acceleration (NOT your angular heading… you would need to integrate the acceleration to get that)

Check the Accelerometer bag. They are very tiny things and many teams have found them stuck to the accelerometer so they look like part of it.

Nixterrimus, Is Ken Wittlief still on your team? He should be able to explain the sensors and demonstrate their usage.

This is a neat article I found on calculating heading with a rate gyro.


Not sure if FIRST provides us with this code though, I’ll have to dig deeper into the documentation and report back.

[EDIT] I found this article from a Hall Effect sensor manufacturer about how they work. http://www.micronas.com/products/overview/sensors/index.php [/DIT]

No, sadly Mr. Wittlief’s son graduated and he isn’t a part of the team this year.

Alright so this isn’t a gyro, I’m good with that, but is this what was Demoed at the kickoff where the robot was able to correct its position? Does the hall effect sensor return your angular heading?

I looked at that too. Did you think that it was in English?
:o I thought that a lot of it was out there, I suppose I’m looking for just an intro to the hall effect.

www.google.com , my friend

Gyros measure angular velocity, not acceleration. The first integral of the velocity it the distance, so integrating a gyro provides angular distance, or relative heading.

Using a gyro to measure the robot’s heading is pretty common with FIRST robots. The parts teams used last year are the Analog Devices ADXRS150EB and ADXRS300EB. The only difference is that the 150 can measure ± 150 degrees per second and the 300 measures up to 300 degrees per second.

Lots of posts describing how to do it.

I BELIEVE what was demoed at the kickoff (not 110% sure) was the accelerometer. An accelerometer gives back a reading that tells how fast you are accelerating, i.e. changing speed, in a given direction. Using some programming, you can sense when you are moving and take steps to remedy it. A hall-effect sensor uses a magnetic field to detect a metallic presence, and therefore does not return angular headings. I am pretty sure they put out pulses when a metallic presence is detected. It’s just an FET, in other words.


To clear things up, the HAL effect sensors have NOTHING TO DO WITH ACCELERATION, OR HEADING. They are a type of sensor that detects the presence of ferrous materias. They are designed to be mounted up against(but not touching) the teeth of a gear in a gearbox or other assembly. They will output 1 pulse for each gear tooth that goes by them. If for example you were to connect them to your drivetrain, you could count the number of pulses they output to figure out how far your robot had traveled. It is also possible to use them to determine the speed at which your robot (or some subassembly) is moving.

So this is how FIRST is killing dead reckoning? Hooking these up next to gears tell you how far you’ve gone, the accelerometer could tell you if you were straight and now we can drive the robot any direction and know exactly how far. Right?

So this is how FIRST is killing dead reckoning? Hooking these up next to gears tell you how far you’ve gone, the accelerometer could tell you if you were straight and now we can drive the robot any direction and know exactly how far. Right?

No. To be honest it’s still dead reackoning and you still do not percisely know where the robot is with 100% certainty.

Here are a few tips on the Hall Effect sensors:

These guys simply output a digital signal corresponding to whether or not a gear tooth is in front of them or not, and the polarity of the signal depends on the direction of gear rotation wrt the sensor itself (see the data sheet for more details: http://www.allegromicro.com/datafile/0660.pdf). Think of this device as a one-bit wheel encoder when you program for it.

The air gap, according to the manufacturer, should be 0.5-2.5mm, though they assure us that their adaptive sensing technology can make it work for an even bigger gap, especially with larger gear teeth.

The device has an open-drain output stage, but the digital inputs on the RC have pull-up resistors built-in, so you can directly connect the signal to the RC’s inputs. Try to avoid grounding the signal wire, though, as this would technically be a short-circuit. Don’t worry if you do – the manufacturer says it has short-circuit protection on the output stage.

We are given two Hall Effect Sensors. By using them to count teeth of sprockets on opposite sides of your robot you can calculate the difference in number of wheel rotations, and from that rotation of your robot.

If they are used this way, i believe, they work as a replacement for a gyro, although they would not account for being pushed or otherwise interfered with.

- Toby

Read some of the posts above before making guesses. The Hall effect sensors are easy tachometers that can tell you how fast a gear is rotating and how many gear teeth have passed by the front of it. Using two on the same gear, about 90% apart, can also give you direction information with some simple electronics or software interpretation. The accelerometer can be used only to give an indication of turning. It will give a large output for a bump or fast turn and a small output for a slow turn. Combine the data from all three and you will know that you have traveled “X” inches in some direction relative to your starting direction.
As a caveat, if your wheels are not firmly on the ground, counting gear teeth tell you nothing.

Thanks, I haven’t had a chance to re-check our kit yet, but I see that this is documented in the “Guidelines, Tips and Good Practices” document (page 26 - see attachment). IMO this should have been documented on the kit checklist (to look on the accelerometer). Seems these could be easily missed, even for those who know what they are looking for :o




I can confirm that is the proper spec sheet for the hall effect gear tooth sensor that is included in the 2005 FRC KOP. It is labled Data sheet 27627.107