Variance in Turn Degrees with VEXCode IQ Blocks

[Amy_Anderson]

All items below were attempted with the Clawbot - built precisely following directions from VEX.

1. When you set up the drivetrain and check the box for gyro and choose the port, VEXcode no longer shows you the blocks for gyro in the sensing category. It also doesn’t make you put in any measurements – just gear ratio and wheel size. When I tell it to turn right 90 degrees with this configuration, it isn’t consistent with the turns. Sometimes the turns are at 88 degrees, sometimes they are at 92. Is it even using the gyro at all? Or is it determining 90 degrees based on the wheel size and gear ratio?

2. If you leave the gyro off the drivetrain in configuration and include it separately by adding it as a separate component, then the gyro sensing blocks appear. When I tell it to turn right 90 degrees with this configuration, it only turns 77 degrees in either direction. I checked the track width measurement on the drivetrain and everything is correct.

3. The only way I could get it to turn an accurate 90 degrees consistently is by including a command that says…
   Wait until “Gyro Heading in Degrees” > 90
   Stop Driving

Is there a reason for the inconsistencies in turn rotation using the drive commands when the gyro is included in the drivetrain config? I know it is only off a couple of degrees each time, but that makes a big difference when the robot is travelling a longer distance.

[matt_e]

Amy, did you ever find out the issue? We are having the same issues. Wondering why they gave the ability to add it to drivetrain. Not much information out there on this. Thanks

[tfriez]

Gyro based turns will always have some variance to them due to momentum of the robots and other external factors (surface friction, wheel friction, etc.). Not to mention the initial alignment of the robot as the “zero point”.

In the examples above, the gyro is waiting until it’s greater than 90 degrees (I.e. 90.1 or higher) or less than 270 (I.e 269.9 or below) and then is instructed to begin stopping the motors.

The drivetrain functionality is built and configured to stop the gyro directly at the desired heading (by slowing the motors down as it approaches the desired heading) which may be causing the observed “under rotation”.

For gyro-less turns, the wheelbase and track width parameters (as of 1.08) are used to estimate the number of motor encoder degrees needed to rotate the robot. These values will be particular per robot design but will also suffer from additional issues such as wheel slippage that may affect accuracy.