Activity-Bot + Boe-Bot Tank Tread Kit

toothmouth

I recently purchased the Activity-Bot kit and decided it needed a little help traversing rougher terrain. The Boe-Bot Tank Tread Kit (http://parallax.com/product/28106), seemed like what I needed. The Activity-Bot kit is supposed to be able to accept Boe-Bot add-ons and accessories, however, I was having some trouble getting the tank treads to work with the Activity-Bot.

The encoders and stock wheels need to be removed so the tank tread kit can be attached to the chassis. The high-speed servos do not perform correctly without the encoders. The encoders also need the stock wheels to function.

Has anyone else encountered this problem? I suppose I could use some setup with basic servos, but I really like the control the Activity-Bot's high-speed servos/encoders provide. Any advice would be appreciated.

Thanks!

Chris Savage

This is kind of a double-edged thing. On one hand the ActivityBot is totally compatible with the Tank Treads in the sense that the chassis is the same as the BOE-Bot and the servos can handle this modification no problem. The only issue is as you described. You cannot use the encoders as they normally work because they depend on the wheels, which are removed to facilitate putting the tank tread kit on. So you would lose encoder functionality unless you made some modifications to the hardware and/or code.

Andy Lindsay (Parallax)

The abdrive library is designed to work with wheels and encoders. It can be used without the encoders, but only if you have done the ActivityBot calibration procedure with the wheels and encoders first (http://learn.parallax.com/activitybot/calibrate-your-activitybot). If you have performed that calibration, all you have to do is add a call to drive_feedback to disable encoder feedback and make the servos run open loop. Here is an example:

/* Forward 1 second.c */

#include "simpletools.h"
#include "abdrive.h"

main
{
  drive_speed(0, 0);
  drive_feedback(0);
  drive_speed(128, 128);
  pause(1000);
  drive_speed(0, 0);
}  

Note: The ActivityBot calibration is different from the Boe-Bot calibration. It's all code driven and does not require you to manually center the servos with a screwdriver. It instead finds the center, and also builds a linear interpolation table that relates servo signals to actual measured speeds.


If you never have and never will use wheels and encoders, it's probably best to start with the Boe-Bot style calibration. For that, you'll run center code and adjust the pot in each servo with a screwdriver so that it stays still:

/* Center.c */

#include "simpletools.h"
#include "servo.h"

int main()
{
  servo_speed(12, 0);      // Stay still
  servo_speed(13, 0);
}

When that's done, your navigation code might look like this. Note that servo_speed has to be the reverse direction for the right (P13) wheel to result in forward motion.

/* Forward 1 second with servo lib.c */

#include "simpletools.h"
#include "servo.h"

int main()
{
  servo_speed(12, 100);      // Forward
  servo_speed(13, -100);
  pause(1000);
  servo_speed(12, 0);      // Stop
  servo_speed(13, 0);

}

erco

There's clearly an opportunity here for custom tank wheels which work with the stock Activitybot encoders, assuming that it's physically possible given the geometry of the wheels, encoder location, and tread length. Some tank tread lovin', 3D printin' Solidworks guru might beat Parallax to it.

Chris Savage

erco, I was just discussing this with Stephanie yesterday. I'm not sure there's an easy way to make this happen, but we're open to suggestions. :nerd:

erco

Chris Savage wrote: »

we're open to suggestions. :nerd:

I suggest you dare Duane Degn to do it, 'cuz DD clearly has the fours D's (drive, desire, dedication and discipline) to jump onto a project and run with it. Just look what he did here: http://forums.parallax.com/showthread.php/151868-Cheap-8x8-LED-Displays