NXT Sensor Guide

Some sensors

This is a list of some available sensors for the LEGO NXT

Acceleration Sensor, Gyro and Compass 

One very common question about LEGO MINDSTORMS NXTs is: "What's the difference between a Gyro sensor, an Acceleration (or tilt) sensor, and a Compass sensor.  This page will attempt to explain how each is different, and how they can be used.

Feature	Acceleration	Gyro	Compass
Function	read acceleration in three axis	read rotation speed in one axis	read direction
Data Axis
Where to buy? Several companies sell sensors.  This is one source.	Acceleration	Gyro	Compass

Notes

(1) standard note 
 

Common Questions:

Here are some answers to questions posted on nxtasy forums by our friend and NXTpert Brian Davis and also myself

Q: If I'm correct, the acceleration sensor only measures when the sensor is, well, accelerating!  Right?

A: More exactly, it determines when the internal sensor is experiencing an acceleration relative to the outer casing.

Q: Can I use an acceleration sensor to balance my robot?

A: No.   Actually, surprisingly, this doesn't work as well as you might like. The problem is when the robot is falling over, the acceleration sensor is not returning the angle the robot is at, but the acceleration it is under, which depends on the angle, as well as the angular velocity, as well as the angular acceleration. Building a LEGway style robot using both ground-sensing elements (and no, there is no "super-resolution" mode for the US sensor that I'm aware of) as well as a tilt sensor might allow hill climbing (something significantly more difficult), but I suspect the biggest limitation is going to be the speed of the software response. To get LEGway to balance well takes a "tight control loop" (i.e., very rapid and accurate correction).

Q: Is a Gyro the same as a one axis acceleration sensor?
A: Well... no. Not at all . The gyro sensor measures how fast it is rotating (or turning) in around one axis, but that's it. That's not an acceleration like the accelerometer measures. How to explain it...

Put yourself in an elevator with a compass sensor, a gyro sensor, and a 3-axis accelerometer. With the elevator at rest on the 13th floor, one axis of the accelerometer reads '200' (about 1 G), while the other two are very close to zero. the compass sensor reads something like '90' (it just happens, in this example, to be pointed just about due east), while the gyro reads '0' (it isn't turning).

Now spin the robot rapidly (but smoothly, not speeding up or slowing down) in place. The accelerometer will still read about '200' (down), as none of the linear accelerations on it have changed. But the compass sensor will start changing rapidly as the direction it points changes... '90' becomes '110' which becomes '130' etc, until at '359' it suddenly "drops" to '0' again and starts counting up. The gyro sensor on the other hand will show some constant value, like '15', as the unit rotates at 15 degrees per second. But it's not linearly accelerating.

OK, now cut the elevator cable. The accelerometer now reads '0' in all directions. Why? I mean, you are certainly accelerating (towards the bottom of the shaft in this rather touchy case). But the sensor within the accelerometer is now accelerating exactly the same as the casing to which it's attached, so it can't detect this, and reads zero. The compass still works, giving you which way you are facing relative to magnetic north. And the gyro still works, because it's measuring the rotation rate about one axis. Neither of these depend on an external acceleration.

I do suggest you step out of the plummeting elevator just before it hits the bottom

Q: Can I use an acceleration sensor on a bot going a steady speed to tell whether or not I was flipped over or on a hill?

Actually, yes. If the robot is moving at a steady speed, then there is no acceleration on it, and the only thing the accelerometer will report is the gravitational acceleration on the internal sensor itself. If that's all "down", then the robot is moving flat. If it's "half down and half forward" (i.e., two of the readings are about 140), then the robot is at a 45° angle, or if the reading is "about 200, in the opposite sense that it normally is", you know the robot is flipped over like a turtle on its back.

Q: Will a pendulum always point down, as long as you let it be (not touch it or anything) and if the "counterweight" (the thing that is at the bottom of the pendulum ) is heavy enough?

A: Well... no. Although if it's any help, I have to re-teach this to college students as well.

In a *static* situation, where things are not accelerating, then a pendulum at rest hangs straight down, and the mass it has makes absolutely no difference (in fact, the mass makes no difference to *gravitational* accelerations of the pendulum, either - the result is the period of a pendulum is independant of the mass, something you can verify to yourself the next time you swing on a swing with your parent or child). But in the case in question there are certainly accelerations (other than gravity) to consider.

Think about it this way: I seal you in a little room on the top of a long pole that is hinged on the lower end, and you have a little plumb bob that is hanging straight down. Now, I let the pole tip to the side, toppling over. Does the plumb bob always remain pointed at the ground, or at the floor of the little room, or somewhere else? Well, I know that the instant before the little room hits the ground it is moving straight down, in "free fall", so at least at that moment, the plmb bob would not be "pointing" down - it would be floating, or drifting, in the little room.

When the pole had only tipped 45°, for instance, my hand that is holding the plumb bob would be moving diagonally towards the ground - in fact, it would be *accelerating* diagonally towards the ground, while the pendulum bob would be accelerating downward (due to gravity) as well as to the side (becasue of the string connecting the bob to my hand). The situation actually gets quite complicated.

This gets back to why an acceleration sensor is not the best way to balance a LEGway-style platform. After all, what is a pendulum but a different kind of acceleration sensor, but a very complex one to deal with.

Q:What if the acceleration sensor was mounted very close to the ground? (for a balancing robot) Wouldn't that negate some of the additional accelerations that you mentioned? I guess my theory is that if you mount the acceleration sensor at the pivot point of the "inverted pendulum", it should only tell you the angle it is currently at, right? Of course the pivot point is where the tires are touching the ground, and you can't mount it there, but you could mount it very close, no?

A: Many of the above examples hold true when the sensor is placed near the "center of mass" of the robot.  If the sensor is in a different location, many other (confusing) forces may come into play, making it even more complex.

Q: What can you measure with an acceleration sensor:

A: Things Brian has done with an accelerometer...

• Made a pedometer (counting footsteps)
• Logging accelerations while swinging (& jumping off)
• Playing on a merry-go-round
• Recording accelerations on rides at Disneyworld
• Measuring free-body rotations during free-fall
• Finding your way (or the robots way) up hills
• Having a robot that can flip itself back upright when it detects that it is upside down
• Using it for a no-moving-parts control stick for remote control robots
• Datalogging a hula-hoop
• Probably lots of other things I'm not remembering off-hand

The HiTechnic accelerometer is still one of my all-time favorite sensors, because it can do so much... but you have to understand how it works in order to take advantage of that. And the sad fact is (speaking as a physics teacher here) people, even intelligent, well-educated, "aged" people like myself, usually haven't spent enough time thinking about the differences between things like position, location, velocity, speed, linear acceleration, angular acceleration, and angular position (or the difference between rotation and revolution, or... dang, I think I need to go back to teaching again, the summer's been too long ).

Other Sensors (incomplete list)

Mindsensors Sensors

• Lattebox servo controller
• Bluetooth GPS
• The many custom sensors people have created

"LEGO Certified sensors"

Some sensors are listed as "LEGO Certified".  That means they pass LEGO's strict quality control standards.  These include current limits and safety standards.

Converter cables (this needs more information)

Other Team Hassenplug NXT stuff