Reducing Drift in KOP Gyroscope

[TogetherSword8]

I have done some digging into why our autonomous code didn't work last year and discovered the drift that I found was referenced in some other parts around this forum, and i think it is in all KOP Gyroscopes.

Does anyone know the best way to reduce this drift besides make its mount completely level? because I cannot find any other way, and I don't know how to go into it to see if there is any settings to change.

Thanks for any help or ideas

[Joe Ross]

What is the magnitude of the drift you are seeing?

[nathan_hui]

The quick and almost useless answer is sensor fusion/Kalman filters (It's useless because it's not a simple answer).

Are you just using the gyroscopes? Gyroscopes are inherently susceptible to drift (There's no perfect sensor for any application, only the most optimal). You theoretically can use a set of accelerometers to zero out the gyroscopes continuously, and as another set of data to measure robot orientation. Could also use your wheel encoders to tell you when you're stopped.

See http://stackoverflow.com/questions/1...lerometer-data for more information, or search Kalman Filters, Sensor Fusion.

[sur]

Originally Posted by nathan_hui

Are you just using the gyroscopes? Gyroscopes are inherently susceptible to drift (There's no perfect sensor for any application, only the most optimal). You theoretically can use a set of accelerometers to zero out the gyroscopes continuously, and as another set of data to measure robot orientation.

An accelerometer won't give you heading. If the acceleration on your robot is mainly due to gravity, then you can look at the magnitude of acceleration in each axis to measure tilt. You could use a magnetometer to measure heading and combine it with the gyro, but you might have to make sure you don't get noise from the motors. See http://robotics.stackexchange.com/qu...be-characteriz for some information on magnetic shielding to prevent this.

[z_beeblebrox]

What about averaging the heading from multiple gyros?

I heard from a member of 1717 that doing this helped them a lot.

[nathan_hui]

Originally Posted by sur

An accelerometer won't give you heading. If the acceleration on your robot is mainly due to gravity, then you can look at the magnitude of acceleration in each axis to measure tilt. You could use a magnetometer to measure heading and combine it with the gyro, but you might have to make sure you don't get noise from the motors. See http://robotics.stackexchange.com/qu...be-characteriz for some information on magnetic shielding to prevent this.

True. It won't give you heading. But it will tell you when you're not moving. And that's the key to preventing the gyros from drifting. If you're not moving, the gyro shouldn't be moving either. If you have two accelerometers and you know the distance between them, you can calculate the amount of turn. Magnetometer may or may not work - you're in an environment with way too many motors and ferromagnetic materials (i.e. screws). Shielding also only does so much. In the end, you'll probably end up running a software filter over the data anyways.

[Ether]

Originally Posted by nathan_hui

True. It won't give you heading. But it will tell you when you're not moving.

An accelerometer does not tell you when you're not moving. It tells you when you are not accelerating.

[sur]

Originally Posted by nathan_hui

True. It won't give you heading. But it will tell you when you're not moving. And that's the key to preventing the gyros from drifting. If you're not moving, the gyro shouldn't be moving either. If you have two accelerometers and you know the distance between them, you can calculate the amount of turn. Magnetometer may or may not work - you're in an environment with way too many motors and ferromagnetic materials (i.e. screws). Shielding also only does so much. In the end, you'll probably end up running a software filter over the data anyways.

This thread seems to suggest that the HiTechnic sensor is pretty accurate.

But to answer OP's question more directly, I don't think you should be seeing much drift during a single match. The gyro should be mounted as level and close to the center of the robot as possible, since it measures rotation around the axis perpendicular to its board:

The angular rate sensor (gyroscope) using Analog Devices part number ADW22307 detects angular changes about the board’s top surface axis.

from http://www.andymark.com/product-p/am-2067.htm.

[TogetherSword8]

Originally Posted by sur

But to answer OP's question more directly, I don't think you should be seeing much drift during a single match.

The problem that we had this year was it drifted prior to the match, and our initial driving based on the gyroscope angle was off, because it drifted a different amount depending on the time from robot placement to when they actually started the match.

I have thought about putting in a software cancel, would something like that be a if encoders are not moving, keep angle the same, and count the accumulation in the code instead of letting the gyro do it itself, and reset the gyro every iteration? That is the only way i could think of doing it.

[Ether]

Originally Posted by TogetherSword8

The problem that we had this year was it drifted prior to the match, and our initial driving based on the gyroscope angle was off, because it drifted a different amount depending on the time from robot placement to when they actually started the match.

Simply reset the gyro at the start of the match.

count the accumulation in the code instead of letting the gyro do it itself

The gyro itself does not do any accumulating. That is done in the FPGA, which probably does it much more effectively that you could do in your code.

Some teams have added a manual reset function that the driver can activate by simply pressing a button when the bot is at the zero position any time during a match.

[Gdeaver]

Our team is looking at using a gyro on the bot next year. The gyro we are looking at using walks down .01 degrees every 2 to 3 seconds. How does this compare to other gyro's teams have used? At begin autonomous just zero the gyro accumulator.

[otherguy]

Originally Posted by Gdeaver

Our team is looking at using a gyro on the bot next year. The gyro we are looking at using walks down .01 degrees every 2 to 3 seconds. How does this compare to other gyro's teams have used? At begin autonomous just zero the gyro accumulator.

That's similar to what I have seen.
If you consider the duration of the autonomous period (15 sec.) and assume a drift of 0.01 degrees every 2 sec. (the worst case of what you reported) you're going to accumulate a total drift of 0.075 degrees. If you can't handle that kind of error, you're doing something wrong.

A good rule of thumb to follow with the gyros, reset/zero them any time you know you are stopped (beginning of autonomous). This will reduce the effect the drift has on your reported heading.
Also, make sure that your sits still after you turn it on (make sure your drive team understands this). The gyro is calibrated during this time and movement will affect the magnitude of drift you see. Try shaking it around when power is applied to the cRIO and see what happens.

[Ether]

Originally Posted by otherguy

If you ... assume a drift of 0.1 degrees every 2 sec. (the worst case of what you reported)

Typo. He reported .01 degrees every 2 (or 3) sec.

[stevethetinker]

We (1288) have used gyros in some years. The stability of the KOP gyro was not an issue. They have been good enough for the autonomous period. The 2010 robot had a gyro, but was not used in competition. I have since been working with that robot developing a better drive system. The gyro drifted enough to be a problem. It would drift 10 degrees or more in just a minute or two, which was a big problem for field-oriented drive controls. I replaced it with a new gyro from Andymark and the drift is much less. I'd say your 0.01 degree per 2-3 seconds is good for this technology. As noted in other posts, these gyros are temperature sensitive. There is an on-chip temperature sensor that you can access through another analog input. The hard part is measuring the temperature-drift equation. Once you have that you can compensate for most of the drift.