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Unread 06-02-2017, 12:28
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Recommended gyros that can keep up with fast rotation?

We have one of the ADXRS450 from FIRST Choice, and it works pretty well overall. Works out of the box, drifts a little but not terribly. The issue is we'd like to code a few "automagic" buttons to go to specific angles to face certain field elements (we already have the math worked out, and rotating the robot very slowly works), but the ADXRS450 is limited to 300 degrees/second before it can't keep up with how fast you're turning. What gyros does your team have / recommend? Is this a pipe dream? We could turn slower of course, but could still get hit while turning and end up losing the gyro again... thoughts, CD?
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Unread 06-02-2017, 16:01
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Re: Recommended gyros that can keep up with fast rotation?

https://www.sparkfun.com/products/10612 is one we've used in the past with some success. We did have occasional issues with it dropping out, and i'm not sure if it was the gyro itself, or the I2C libraries/hardware on the RIO.... But, for a fast robot, this might work decently for you!
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Unread 06-02-2017, 16:04
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Re: Recommended gyros that can keep up with fast rotation?

Pigeon IMU from CTRE. 2000 degrees/s and it has integration of an accelerometer and gyro so you get a drift similar to what you'd find on other top-of-the-line IMU's like the Nav-X. Even better is that you can connect it over the can bus with 2 wires rather than mucking about with I2C or any other wiring scheme. You can also wire it in through a ribbon cable to one of your motor controllers if you are using Talon SRX's.

http://www.vexrobotics.com/pigeon-imu.html
http://www.ctr-electronics.com/gadge...le-pigeon.html

We used the analog kit-op-parts gyros up through 2014. In 2015 we went to the same one that 971 uses. Did it again in 2016. The Pigeon took a lot of the headache out though.

Last edited by Tom Line : 06-02-2017 at 16:09.
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Unread 06-02-2017, 20:10
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Re: Recommended gyros that can keep up with fast rotation?

How fast does your robot rotate?
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Unread 07-02-2017, 10:35
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Re: Recommended gyros that can keep up with fast rotation?

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How fast does your robot rotate?
Depends on how fast we tell it to.. Honestly though, pretty fast if we spin at full power.

Really though, even if we slow it down to cap at 300 degrees / second, there's the possibility of getting pushed around a bit by someone playing defense or something and losing track of where you're facing that way. I'd like the drivers to be able to turn quickly in teleop, but still have quick and easy "go to this angle" buttons for quick alignment.
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Unread 07-02-2017, 12:04
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Re: Recommended gyros that can keep up with fast rotation?

Your robot does probably turn pretty fast at full power, but you never need to turn more than 180degrees to snap to a location - can your robot really accelerate to 300 deg/s AND decellerate back to zero in 180degrees of motion?

I don't think I've ever seen a FRC robot that can do that.

Do you have data that shows that you are hitting that limit?
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Unread 07-02-2017, 13:20
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Re: Recommended gyros that can keep up with fast rotation?

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Really though, even if we slow it down to cap at 300 degrees / second, there's the possibility of getting pushed around a bit by someone playing defense or something and losing track of where you're facing that way. I'd like the drivers to be able to turn quickly in teleop, but still have quick and easy "go to this angle" buttons for quick alignment.
The issue here isn't going to be the rate then. The issue of drift over the course of a match can't just be overcome with a "better" gyro. You are always going to have noise in a gyro system. A gyro is very good at measuring rotational acceleration, however things like vibrations, linear accelerations, and electrical noise can be read from the sensor as rotational acceleration, causing drift.

An IMU will help with this issue, it will take inputs from other sensors (accelerometers, other gyros, magnetometer) on the same chip, and use them to filter the gyro signal to help minimize the drift. The NAV-X, Pigeon IMU, and ADIS16448 are all used by FRC teams successfully, and are pretty well documented. I think you will have a lot more luck with one of these IMUs than you would trying to find a better quality gyro. Even with an IMU there will be some drift over the course of a match. Inertial sensors in general do much better at tracking short term relative movements than they do at tracking position or orientation over a period of time.
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Unread 07-02-2017, 13:31
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Re: Recommended gyros that can keep up with fast rotation?

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The issue here isn't going to be the rate then. The issue of drift over the course of a match can't just be overcome with a "better" gyro. You are always going to have noise in a gyro system. A gyro is very good at measuring rotational acceleration, however things like vibrations, linear accelerations, and electrical noise can be read from the sensor as rotational acceleration, causing drift.
This is true. However, with good calibration, you can get the drift down to ~2-3 degrees over the course of a match.

That's likely enough to snap to a rough position - and if you want to use that for shooting, you'll need either vision or manual alignment anyways, so not being exactly exact isn't a problem.
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Unread 07-02-2017, 13:50
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Re: Recommended gyros that can keep up with fast rotation?

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This is true. However, with good calibration, you can get the drift down to ~2-3 degrees over the course of a match.

That's likely enough to snap to a rough position - and if you want to use that for shooting, you'll need either vision or manual alignment anyways, so not being exactly exact isn't a problem.
You have gotten drift to only a couple of degrees with just a gyro? Or with an IMU?
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Unread 07-02-2017, 14:52
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Re: Recommended gyros that can keep up with fast rotation?

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You have gotten drift to only a couple of degrees with just a gyro? Or with an IMU?
We're just using a gyro (the one on the WCP Spartan board, the ADXRS453), running onboard the RoboRIO at 200hz. After our fairly long calibration sequence (~45 seconds, to the dismay of our autonomous programmers ) we've been getting around 2 degrees per match of drift.
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Unread 07-02-2017, 15:26
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Re: Recommended gyros that can keep up with fast rotation?

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We're just using a gyro (the one on the WCP Spartan board, the ADXRS453), running onboard the RoboRIO at 200hz. After our fairly long calibration sequence (~45 seconds, to the dismay of our autonomous programmers ) we've been getting around 2 degrees per match of drift.
Yep, the 453 is an excellent gyro. Though I don't think you really need 45 seconds to calibrate it.

It's rare that the actual rotation rate of the robot by itself will saturate the gyro. What's more likely is that collisions will. A short period of saturation can throw a gyro off by a substantial amount.

That can usually be solved by giving the drivers the ability to rezero, or by making your calculations relative to something else, like a camera angle.
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Unread 07-02-2017, 21:18
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Re: Recommended gyros that can keep up with fast rotation?

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Yep, the 453 is an excellent gyro. Though I don't think you really need 45 seconds to calibrate it.
Oh yeah, it's definitely overkill. We got that value last year, when our calibration logic was a little touchy, and just haven't changed it since then. I've been meaning to tweak it to see if I can get it to a lower value without sacrificing accuracy, so thanks for the reminder .
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Unread 07-02-2017, 23:00
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Re: Recommended gyros that can keep up with fast rotation?

We use a navX-MXP from Kauai Labs. It works pretty well I think, though we didn't use it last year so it hasn't gotten a whole lot of competition use.
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Unread 07-02-2017, 23:59
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Re: Recommended gyros that can keep up with fast rotation?

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We use a navX-MXP from Kauai Labs. It works pretty well I think, though we didn't use it last year so it hasn't gotten a whole lot of competition use.
Agreed. Nothing will come close to comparing with the navX for FRC use.

It's increadibly easy to set up, small, and has accuracy beyond belief. Gyros we have used in previous years all had some slight drift and we're some what complicated to use.

First use out of the box we were able to drive around blind for a little and go back to where the gyro thought was 0 to within an inch or 2.

Would highly recommend the navX, only slight con is that it takes up a USB port.
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Unread 08-02-2017, 15:51
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Re: Recommended gyros that can keep up with fast rotation?

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Agreed. Nothing will come close to comparing with the navX for FRC use.
I'm curious what data and performance testing you've done on competitive products to make that statement. There are a number of IMU's on the market other than that navX. Have you tested them?

I am in no way disparaging the NavX. We've played with it and it works very well. However, I'm an engineer and I try not to make statements that product A is better than product B unless I have seen a marked difference in some measurable.

Last edited by Tom Line : 08-02-2017 at 15:54.
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