XRP Servo 1 vs 2 differences in behavior

Technical Motors
1

Hi Folks,

DevilBotz 2876 has been using the XRP robots for our 2nd Annual MiniFRC competition. Students have noticed differences in behavior between the Servo 1 and Servo 2 ports.

We have tried both XRP WPILib FW v1.0.1 and FW v1.1.0 and they both have the same behavior.

We aren’t even simulating/running any code on the XRP, so this is just after initial boot up.

We are testing using both 180 servos and continuous (360) FS90R servos. Servo 1 port seems to work as expected for both. However, Servo 2 does not behave the same way. E.g. at bootup, when both ports are set to angle=90/position=0.5 (i.e. 1500us duty cycle), the same motor behaves differently. I used a cheap digital scope and confirmed the duty cycle is 1500us for both PWM signals, so am at a loss on how to debug further. Perhaps there’s a signal noise issue?

I took a video of the behavior: XRP servo 1 and 2 differences - Google Drive

2

I have noticed that my XRP robot motors seem to run at different speeds, causing the robot to drift.

3

Interesting point. We also noticed the left and right drive motors don’t quite spin at the same speed. I looked at the XRP FW and it looks like all 6 motors (4x DC and 2x Servo) are all driven using PWM.