ASSEMBLING THE TRACKS

Although the basic VEX kit contains several different wheels, the PackBot is a tracked vehicle, which gave us a great opportunity to try out the new VEX tank tread kit. The kit consists of two long tracks comprised of snap-together pieces. These plastic pieces allow the builder to make the track longer or shorter simply by adding or removing links. Also included are two pairs of custom toothed rollers that can be used as idlers, or to transmit drive power from a motor to the track via the familiar square shafts used in the rest of the system. The rollers are positioned at the front and back of each side, and a tensioner (also included) goes somewhere in the middle to pick up the slack.

The first step in our build was to construct the tracks and connect them in pairs. We needed two pairs of tracks- one for the front (equivalent to the real PackBot’s “flippers”) and one for the back, or base of our PackBot. We dutifully followed the assembly instructions, opting for the longest tread we could make. One of the toothed rollers was connected to a motor, while the other was allowed to spin freely.

Figure 1. A close-up of the toothed roller and track during assembly

Figure 1 shows a close-up of the toothed roller and chain. We quickly assembled one pair for the front tracks, and another pair for the rear tracks. A completed side is pictured in Figure 2.

Figure 2. A completed tank tread assembly with drive motor attached

A light framework connected the left and right sides of each pair of tracks. Performing a brief bench test, we confirmed that moving the control stick forward made both treads roll forward, while backward caused them to roll back. During turns, one tread rolls forward while the other rolls backward, allowing the robot to turn on a center point.

MOUNTING PIVOT SERVOS

We knew that the iRobot PackBot’s front and back tracks were joined together with a common pivot, which was something we would also have to duplicate. The PackBot has the ability to lift its front tracks in order to climb over obstacles and debris. Our PackBot had to perform the same duties, so we immediately decided to use servos for their ability to hold a commanded position, which would be helpful in keeping a track elevated.

We installed four servos to try and lift the front tracks, which had become somewhat heavy by this point. Two servos were fastened to the front tracks, while two more were fastened to the back half. By having the two front-track servos mounted on the inside pointing out, and the two back-half servos mounted outside pointing in (as shown in Figure 3), we could capture the shafts coupling them as well as double the travel of a single servo alone.

Figure 3. Completed front and rear tracks with servos forming the common center joint.

This idea required cutting down the square-drive shafts to a custom size, which is easy to accomplish with a small abrasive chop saw or a hacksaw. Though this mounting system seemed a bit loose, we continued with it for proof of concept.

The servos and drive motors were plugged into the controller and programmed appropriately, although we could have really used some long extension cords for the front drive motors. We made our own by cutting the VEX connectors and soldering them to long servo leads.

The drive system used a single stick “arcade style” drive configuration, while the servos had to be ganged together with Y-adapters to get enough signal channels. Using the programmable mixing included in the radio, we were able to get one channel mixed into another with inverted polarity, since two servos were facing forward, and two backward.

DRIVE TEST

	Figure 4. Grant replaces the servos with a motor and gear system.

Everything was looking pretty good until our first drive test. First of all, we noted that the tracks worked great in the straight forwards or backwards situations, but as soon as we attempted to turn, they stopped. Our first fear was that there was too much tuning friction, since the robot employed tracks with a large surface area instead of wheels, and the overall length was much more than the width. However, we noticed that the robot seemed to have ample turning power at random times, and sometimes the front tracks would stop turning, and sometimes the back. Upon closer inspection, we found that the single-supported square-drive shafts were slipping out of the clutches in the drive motors. Though we followed the assembly instructions exactly, we discovered that without a secondary support, the tension of the track along with the additional sideslip force during turning caused the shafts to flex and pull out of their sockets. Easily enough, we added another rail running on the outside of each track, thus making all axes double-supported, and preventing any further slipping problems.

Next, we tried to lift the front tracks and found that the servos were fighting each other, and only turning each other’s shafts, without lifting the robot. By fixing one problem and adding the necessary support rails, we created another one by making the tracks heavier. If we helped the front track and pushed it up a little, the servos were able to limp the heavy track up, but with nowhere near the power that we needed. At this point, we decided to replace the servos with gears and a motor, as shown in Figure 4.

We also noticed a major problem with the pivot axis. Originally, we created two identical tracks of equal width: one for the front, and one for the back, with a pivot in the middle. When we attempted our first lift, we found that there was ample clearance to lift the front track, as shown in Figure 5, but when it was lowered (as in the case of climbing down off of a debris pile), the front and rear tracks crashed into each other (see Figure 6), which was a major design flaw. Well, it’s back to the drawing board with a few changes. Fortunately, with the VEX system, major system changes like this only took an hour or two.

Figure 5. Our PackBot has ample clearance lifting the front tracks up.

Figure 6. The tracks crash in the first prototype when the front tracks are lowered.

Continued
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