Accelerometers & Gyros for N00bs

[Vikesrock]

Originally Posted by Trevor_Decker

is 100 g better then 1 g or is it the other way around?

I think the confusion revolves around what you mean by better. Do you mean easier to measure? Or which is greater?

100 g is a much greater acceleration than 1 g. If you are talking about an option between a sensor with a range of 1 g and a sensor with a range of 100g then it matters what you are measuring and what kind of accuracy you need.

For a given resolution (in bits) a sensor with a larger range will have larger steps (less accurate for small changes in acceleration). A sensor with a smaller range may be saturated if the acceleration is larger than the range (5g of acceleration applied to a 1g sensor).

[Chris Hibner]

Originally Posted by Trevor_Decker

is 100 g better then 1 g or is it the other way around?

It depends on the application (i.e. what are you using it for?).

If you're using the accelerometer for an anti-lock brake system, you would want to use a low-g range, like a 1.5g accelerometer, since the largest acceleration you should see under braking is about 1g, (the extra 0.5g is because you'll want a little room for vibrations and noise so you don't get distortion in your filters, but that's a more advanced topic).

If you're using the accelerometer for a side-impact airbag crash sensor, you'll want to use an accelerometer in the 250g range since you mount it right next to the impact and you can see accelerations well above 100 g's.

The idea is that you pick your sensor by determining (from physics or tests) what is the largest acceleration you will see. You should then select a sensor that can measure with just a little more range than this maximum.

[Trevor_Decker]

Originally Posted by Chris Hibner

It depends on the application (i.e. what are you using it for?).

If you're using the accelerometer for an anti-lock brake system, you would want to use a low-g range, like a 1.5g accelerometer, since the largest acceleration you should see under braking is about 1g, (the extra 0.5g is because you'll want a little room for vibrations and noise so you don't get distortion in your filters, but that's a more advanced topic).

If you're using the accelerometer for a side-impact airbag crash sensor, you'll want to use an accelerometer in the 250g range since you mount it right next to the impact and you can see accelerations well above 100 g's.

The idea is that you pick your sensor by determining (from physics or tests) what is the largest acceleration you will see. You should then select a sensor that can measure with just a little more range than this maximum.

So if I wanted to use an accelerometer to tell how fast a robot is moving (constantly adding/ subtracting the acceleration from the speed) , I would want a 2g accelerometer since the field is 54 feet in length, which divided by 32.174(earths gravity) = 1.678 which rounds up to 2.

would that be correct?
How accurate is a 2g accelerometer?

Thanks again for the help!

[Tristan Lall]

Originally Posted by Trevor_Decker

So if I wanted to use an accelerometer to tell how fast a robot is moving (constantly adding/ subtracting the acceleration from the speed) , I would want a 2g accelerometer since the field is 54 feet in length, which divided by 32.174(earths gravity) = 1.678 which rounds up to 2.

Time for some dimensional analysis: 54 ft ÷ 32.174 ft/s² = 1.678 s². Since g is not given in s² (seconds squared), you probably shouldn't rely on that calculation.

Constant acceleration of 2g means that for every second the robot moves, its velocity increases by 2 × 9.81 m/s = 19.62 m/s = 64.35 ft/s. I can't think of any FIRST robot that can sustain that sort of acceleration for any meaningful length of time. (So it turns out that that might be sufficient for measuring your robot's acceleration due to its own drivetrain, but for different reasons than you suggested.)

How fast is your robot, and approximately how long does it take before it reaches its top speed? That will help you find your average acceleration. You'll want a sensor that can handle that range for sure, but like Chris said, if you're depending on this sensor to measure anything but the idealized performance of your drivetrain—for instance, stopping due to hitting things on the field—you'll need to expand the range by some unknown amount.

Originally Posted by Trevor_Decker

How accurate is a 2g accelerometer?

The accuracy of the device itself will be published on its datasheet, provided by the manufacturer (or sometimes the distributor of the part). It's usually a function of the device's temperature, so many incorporate a temperature sensor.

[Chris Hibner]

Originally Posted by Trevor_Decker

So if I wanted to use an accelerometer to tell how fast a robot is moving (constantly adding/ subtracting the acceleration from the speed) , I would want a 2g accelerometer since the field is 54 feet in length, which divided by 32.174(earths gravity) = 1.678 which rounds up to 2.

would that be correct?
How accurate is a 2g accelerometer?

Thanks again for the help!

As a side note, if you want to know how far your robot has travelled, you're much better off using encoders. With that being said, I have 10 years of engineering experience in control systems using intertial sensors (inertial sensors = accelerometers and gyros), so I enjoy talking about this stuff. So, here goes the answer to your question...

Is your robot slip limited? In other words, if you put your robot on the carpet facing against a wall and you apply full power, do the wheel slip or do the motors stall?

If the robot is slip limited, the highest acceleration that the robot can experience under its own power is the coefficient of friction between the wheels and the carpet (if you don't believe that, go through the math/physics, if you wish). Therefore, if your coefficient of friction is 1.5, you can accelerate up to 1.5g. Once again, you'd want to pad this to account for vibration and noise, so you might want to select a 3g accelerometer. Why pad for vibration and noise? See the "VERY IMPORTANT" section below.

If the robot is torque limited (wheels don't ever slip), then you need to calculate the force applied to the carpet by all of the wheels. You can do this by either going through all of the gear-train calculations, or just put a scale against the wall as you drive into it. Then use Newton's handy-dandy 2nd law (Force = mass*acceleration) to solve for the maximum acceleration.

VERY IMPORTANT: If you EVER saturate the accelerometer, your velocity and distance caculations will be forever wrong after that point. For example, let's say you are using a 3 g accelerometer and your robot bumps into something and experiences 4 g's for 0.1 seconds. Your accelerometer can only measure 3 g's, so for 0.1 sec you think you're accelerating 3 g's when in fact you're actually accelerating 4 g's. Your velocity calculation will then be off by 3.2 ft/s and your position calculation error will grow to infinity (at a rate of 3.2 feet per second - see the connection?). Not a good situation. There is a lot of engineering work that goes into solving these types of problems on antilock brake systems and traction control systems - a lot more time than you have in the 6 week period, which is why I would suggest encoders.

[Trevor_Decker]

Originally Posted by Chris Hibner

As a side note, if you want to know how far your robot has travelled, you're much better off using encoders.

My problem with encoders is that if the wheels slip then the robot will think that it has moved more than it has.

I am not to concerned about the acceleration being greater then the accelerometer max, since I plain to get one that is between .5 g and 1 g greater then the hiegst acceleration the robot should be moving at, also the date from the accelerometer will probably only be used during the autonomies period, so it only really matters for the first 30 seconds. that being said it would be nice for it to work the whole time, Hopefully giving the drive an on screen birds eye view of the robots position.

[Bomberofdoom]

Accelerometers and Gyros have other applications than for the drivetrains:

In the 2007 game, where at the end-term of the match (20 seconed before end), you would have to lift up two of your other alliance robots, our team has developed a jack system to lift the two robots.
The robot had two "wings" (flat plates from each side that would open up). In the inner corners of the robot were 4 poles that were attached to a motor (The robot. I'm afarid the good pictuers are gone, but it shows most of the robot). These poles were designed to be pushed downwards (more exactly, rotated downwards) when the motors where activated. At some point all 4 poles would reach the ground, and start pushing the ground, which, thanks to Newtown's third law (If object A exerts a force F on object B, then object B exerts an equal and opposite force –F on object A - http://www-istp.gsfc.nasa.gov/stargaze/Snewton3.htm), would result in the lifting of the robot itself, with the two other robots on each of the plates (overall lifting 2 robots "plus one more").

The major problem we had in our design is that we had only a total of 2 of the main motors we planned to use to rotate the poles, so we were forced to use 2 lesser powerful motors, which would result in that two poles would push the robot earlier than the two others, resulting in the tilting o the two other robots on our plates and thier fall or our poles braking from the pressure of 180 KG.

We resolved on using a 2-axis accelerometer to measure the the tilting to each direction - forwards/backwards and sideways. By integrating the change in each axis, we were able to come up with a mathematical algorithm for sending the correct speed to each motor, in order to compinsate for any tilts that would might have been caused from the varying dimension and weight of the alliance robots.

I've seen a team use a Gyro for thier robot's arm, to measure the rate of change in the arms movements to know what its current position is (by integeration, as described before me).

Note that usually a potentiometer is more effective and easier to use than a Gyro for this kind of arm positioning application. The potentiometer revolves with the arm, and as it revolves, it chances the resistance inside of it, and therefore the voltage that is recieved from it changes also. If the potentionmeter is constant, you get a constant voltage of that position. If your position would change, so would the voltage - even if you would stop in a new position, it would send the voltage for that position, instead of giving you the "rate of change" like the Gyro - no need for integration here in order to know what is your position.

Hope I gave a good lesson.

[Chris Hibner]

Originally Posted by Trevor_Decker

My problem with encoders is that if the wheels slip then the robot will think that it has moved more than it has.

That's an easy problem to solve - use dummy wheels. In other words, put a wheel on the end of a trailing arm that is lightly spring loaded toward the floor. Hook up the encoder to this wheel. Then if the drivetrain slips, you don't have to worry because this wheel is just an unpowered dummy wheel along for the ride. We've been doing it that way since 2003 and it works perfectly every time. Last year we used two dummy wheels - one on each side of the drivetrain (the dummy wheels were in the same line as the drive wheels). By doing this you can also eliminate the gyro. I'll let you do the math to figure that out.

[ptan]

Originally Posted by ManicMechanic

Can anyone explain to a not-very-techie-type person (think "stay-at-home mom drafted as technical mentor/coach") how these 2 sensors work, and give some instances of when you would find them useful?

I'm surprised no one mentioned this yet:

An accelerometer is a Wii controller.