Figure 13. Initial version of the motor and gear system for raising the tracks.

As is common in the early prototyping stages, the first test of the new gearing was by no means a hands-down winner. For one thing, the double-motor system with a 1.7:1 ratio wasn’t strong enough to lift the front track on its own, which had grown much heavier. Even more troubling was the fact that the gears made a horrible stuttering noise when we tried to lift the tracks. We first thought the machine-gun noise was due to the pivot joint failing. Temporarily removing the drive motor, we tried the joint and it seemed rock-solid. Upon closer examination of the meshing of the gears, we saw that the axles were flexing under load, causing the gears to slip out of mesh.

The quick fix for this problem was to add a bar with two bearing blocks set at the exact pitch distance of the two gears, joining their axles for a perfect mesh. In the VEX world, this is quite easy, since the gears have been carefully chosen to work at the exact distances of the holes in all of the rails. Adding this bar immediately cleared up the gear skipping problem, but we would have to add additional gearing to get more torque.

Another great benefit of this prototyping system is that a gear change takes only a matter of minutes. If we had to perform a similar operation on a real-world robot, it would take literally hours to try the next ratio. This would involve making calculations, setting up a mill, boring holes, pressing bearings, putting everything back together. With the VEX system, we just moved the motor down to the next set of holes and put a 12-tooth gear on the shaft. As shown in Figure 14, we kept the 36-tooth gear as an idler, which simply transmitted the torque without changing it, so that we wouldn’t have to change the spacing near the joint. This modification made the torque higher with a 5:1 ratio, but still not quite optimal.

Figure 14. Next version of the track-raising gear system with idler.

After a test of the lifting capability, we found that the front track lifted pretty well, but sank back down when power was lost. Unlike a servo, the motor/gear combo doesn’t automatically try to hold a commanded position, so we needed another minor adjustment. There was still plenty of speed to burn, which meant that we were able to add a second stage to the gear system, shown in Figure 15, which not only increased the torque, but also slowed down the movement of the tracks.

Figure 15. Final version of the track-raising gear system with two stages.

As a side benefit, the greater ratio would also help hold the tracks in place when power was removed. The output of the first stage would transfer power by driving the same shaft as the input to the second stage. We used a 12-tooth gear to drive a 60-tooth gear for a 5:1 ratio on the first stage, and then went back to driving the 36-tooth gear for the second stage, which in turn meshed with a 60-tooth gear for a 1.7:1 ratio. This made our overall ratio a comfortable 8.5:1, yielding a good balance of speed and torque for the lifting. As the first day drew to a close we were finally ready for the big test.

CLIMBING TEST

For the final step of day one, we took our PackBot to the main staircase at our headquarters, M5 Industries. We raised the tracks in anticipation of the first step, and the motors lifted them beautifully. As we advanced towards the step, we changed the angle of the front tracks to give us more leverage, which worked great for getting up onto the first step, as shown in Figure 16.

	Figure 16. Our PackBot attempting to climb its first steps.

To get to the second step, however, we had to flex the body almost straight, which caused the plastic tracks to slip on the smooth front edges of the stairs. Quite a bummer, but we realized that the relative smoothness of the tracks gave almost no traction on the equally smooth step corners.

In a moderate debris pile test (i.e. driving over Jamie’s legs—see the video clip), our PackBot performed well and had plenty of climbing power, as the front track provided leverage for the back track. While the stair climbing was a failure, the debris pile was a complete success, given only a single day’s work.

STRIPPED GEAR BLUES

Programming the robot to read the ultrasonic sensor and raise the front tracks was a snap using the VEX EasyC environment. Building from the included sample routines, we were able to quickly write a program to test the ultrasonic sensors and download it to the microcontroller. Though the optical encoder had not yet been installed, we went ahead and tested the program. When a hand was placed in front of the ultrasonic sensor to simulate an approaching barrier, the robot responded by raising the tracks as expected. We used a short “WAIT” statement to run the lifting motors for a brief period of time, so that the tracks wouldn’t slam into the body and strip the gears.

Unfortunately, we didn’t count on the robot interpreting multiple triggers, and the tracks continued to rise, slamming into the body and stripping the 60-tooth gear on the first stage, which is a situation we had hoped to avoid. Upon removal, we found that the square hole in the gear had failed, allowing the gear to spin freely, as shown in Figure 17. Just goes to show that even experienced builders need to be careful. Deciding that this was indeed an excellent situation for a limit switch, we wasted no time in installing one as well as the shaft encoder, so the robot would know exactly where its front track was at all times.

Figure 17. Failure of the square hole in the gear with shaft.

The limit switch (see Figure 18) was mounted on the front track, and had a corresponding rail mounted on the rear track to bump into and trigger the switch. When the limit switch is activated, the robot knows to stop the motors, thus preventing damage to the system.

Figure 18. Here the limit switch on the front track is activated by contact with a flat bar
mounted to the rear track.

The optical shaft encoder required a bit more work to install because the case was so large, and ended up needing another 60-tooth gear to mesh with the 12-tooth gear on the motor shaft (see Figure 19). Unfortunately, since this represents a gear change, we lose resolution on the optical shaft encoder that’s proportional to the gear ratio. Given more time, we could have come up with a system of idlers, but this resolution seems to work fine for the application.

Figure 19. Optical shaft encoder mounting system.

The next test with the shaft encoder and limit switch caused the 60-tooth gear to fail again. This time, we realized that the hub of a single gear wasn’t enough to transfer all the torque required to lift the tracks. Due to space constraints, the first stage had only a single gear, while the next stage had double gears. No problem. We extended the space between mounting rails and added a second 60-tooth gear, as shown in Figure 20. Another 10 minutes later, and we were able to perform another test. This time, the new system didn’t fail.

Figure 20. Gears were doubled on the first stage of the track-raising
mechanism to provide more strength.

As a side note, the VEX system also offers another solution in the form of a nifty black plastic plate that has a square hole made for the square shafts. The mounting holes in the plate will line up perfectly with the holes in a gear, and by joining the plate and gear with a few standoffs, you can couple the gear to the shaft in a very robust way.

Figure 21. Obstacle-climbing sequence.

CONCLUSION AND NEXT STEPS

Future refinements have been planned, including modifications to the tracks to improve their traction on stairs, and the addition of more drive motors for increased climbing power. Once we have the traction to climb stairs manually, we also plan to add a routine in the program to deal with autonomously climbing stairs, which requires a different sequence of events than climbing over a single obstacle. Finally, a pan/tilt system for the camera built with VEX servos will expand our ability to assess our PackBot’s immediate surroundings. The best part is that with the VEX system, modifications can be completed easily, and numerous versions can be quickly tested until the best configuration is found. The pieces are well designed and their ease of use promotes creativity in the problem solving process. We’ve found the system to be highly adaptable and accessible for the robotics enthusiast from beginners to advanced builders and look forward to seeing what the rest of you can do with this versatile system.

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