Hi,
I am building a chassis for a lawn-mower type of scenario. since I need it to carry 5-7lb of extra weight I figured the rc car chassis wouldn't work for me and I need to use the traditional skid steering chassis. to improve the path precision I was planning to use motors with encoders in the chassis but it appears there is no direct support for those in the pixhawk. are there any plans to add that to the rover in the near future?
reading around I am seeing that the encoder output could still be taken into account via special motor controllers like RoboClaw or Roboteq’s SDC2130. would it give any noticeable difference to use motors with encoders and one of these controllers vs. the regular motor/Sabertooth combo?
or should I be looking for a different chassis (like the wild thumper with 75:1 gears ratio) to achieve the desired high precision path repeatability?
Serg
Replies
@Serg,
Unless there is an ArduPilot Developer willing to add encoder functionality to the ArduRover software, I doubt that it will be added anytime soon.
I have a RoboClaw Motor Controller and I think that you might want to read all about using the encoders on the RoboClaw before deciding to buy one for your project.
If you need highly accurate position knowledge, I would recommend that you consider a RTK GPS to use with the Pixhawk.
Regards,
TCIII AVD
another option I was looking into was one of the 4WD Traxxas Chassis (with a different steering method) but it looks like those aren't really designed to be geared down and carry any additional weight.
is this correct or I am missing something here?
@Serge,
I have a number of the Traxxas 4WD Rovers which include the Slash 4WD and the brushless E-Max 4WD and I do not think that they will meet your requirements.
You really need a Wild Thumper type of chassis, but they are really not designed to carry a lot of weight either. I have built several for some of my clients.
I would investigate a Superdroid 4WD chassis as I have built one, though they are not cheap, but can carry a fair amount of weight.
Regards,
TCIII AVD
thanks again for the quick reply!
another question I had was regarding the finer control over the turning. for my scenario I don't really need it to turn 180 degrees when going back - just an s-curve maneuver to shift orthogonally by ~1.5 feet should suffice. is it something I could achieve with the pixhawk? and I guess this is where the encoders in motors could be really helpful so that it doesn't overshoot/undershoot the shift by much.
@Serge,
One of the problems you are going to have using encoders with a skid steering rover is wheel slip.
When I shifted from the APM2.6 to the Pixhawk1 that could run EKF and now EKF2, there was virtually no undershoot or overshoot when making 90 degree turns and I would expect the same for 180 degree turns. The use of RTK will be the biggest improvement in positional knowledge in my opinion.
Regards,
TCIII AVD
thanks, TC!
I'd start with a Wild Thumper and see if I can get it working, it could end up sufficient for my needs.
By the way, I am seeing pololu carries 12v motors for the thumper which have x1.6 more torque when compared to the 6v motors with the same gear ratio. have you tried using those in one of your build or heard anyone doing that?
Sergey
@Serg,
No, unfortunately I have not tried the 12 vdc motors, but they could definitely work for you if you require more pulling power.
Regards,
TCIII AVD
thanks, TC!
I guess I should have mentioned that I already ordered two rtk-enabled gps receivers from NavSpark (one for the rover and the other for the rtk base station). And compass/gyro would be complimenting gps in achieving smoother path.
what I am trying to figure out is if there is a positional precision difference in skid steering chassis that has motors with and without encoders, and how to take advantage of the encoders if the difference is noticeable.
Sergey