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Useless  Bot - Our biggest one yet!

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Overhead view of the propulsion base

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Close-up view of the remote control pendant assembly

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Light module from the Extreme Creatures set including the drive motor and drive belt system

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Front view of the touch sensors and third wheel assembly

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Close-up of the head assembly

Useless Bot was another project to determine how big and obnoxious we could make a robot using the Lego Mindstorms system. For this project we combined pieces from the Invention, Discovery, and Extreme Creatures sets to make a truly big robot.

Many of the concepts for this design were based on lessons we learned from building Big Bot. Of greatest importance was the technique we used to build the vertical sections using the minimum required pieces to create a solid structure. The head of Useless Bot is also similar to Big Bot in that we used tubing to provide the illusion of fullness while creating little moment arm at the top of the structure.

To improve the stability of this model, we chose to purchase a green Lego base assembly. This allowed us to create a large wheelbase for the robot that made it much more stable on uneven surfaces. The motors and RCX brick are attached directly to the base assembly and structural members provide additional axle support near the wheels. The axles are coupled to the motors using the axle extenders. In a preliminary version we used the large tires but the result was that the robot was much too fast to allow a comfortable walking pace while following it. We subsequently replaced the large tires with the racecar style wheels that slowed the model down to a more comfortable walking pace. Numerous pieces were attached to the base along the centerline and edges parallel to the direction of motion to improve the rigidity of the drive assembly. The front wheel assembly is a freely rotation third wheel design - all steering is provided by the motion of the drive wheels.

From the onset of the design process we chose to make this model a remote-controlled robot instead of an autonomous design. This was due to the design complications of placing sensors along the vertical section of the body to detect collisions above floor height. The remote control pendant is shown in the detail picture and consists of two touch sensors mounted with levers sandwiched between two large flat pieces. The levers are held in the open position by using rubber bands. Steering is accomplished by pressing only one of the levers while forward motion requires the closure of both touch sensor switches.

The vertical section is similar in design to the Big Bot vertical section. Axle cross member support was left out on this design. We added the LED light array from the extreme creatures set and mounted the motor to drive the array in such a way as to provide the necessary support at the middle of the vertical section. Both the lights and the drive motor are connected to the same outlet of the RCX brick, so that when the lights are on, the drive motor also is on to cycle the order in which the lights come on.

The head assembly is also much like the Big Bot design as mentioned earlier. We added a few extra pieces to make antennae and a nose for the robot. Pieces of flexible tubing provide additional structural support. We also fabricated two faux arm assemblies. The arms don't have any gripping or holding ability but are simply there to improve appearance.

In our initial trial runs of Useless Bot we found that the vertical section had a large amount of forward and back sway on acceleration and deceleration. Two pieces of green tubing were used to add support to solve this problem, but at this height the sway is merely reduced rather than eliminated. Another problem we encountered was finding enough room inside the house to make a complete turn without difficulty. Useless Bot can turn fairly quickly, but once an obstacle was encountered, there was no way to back up. One possible solution would have been to add a reverse to the remote control pendant, but there were only two long wires available for the remote control system. Therefore, we added touch sensor bumpers at the front on each side. When a collision occurs, the instructions from the remote are ignored and the robot backs up for two seconds to allow space to complete the desired turn. This solution resolved the problem.

Programming for the RCX brick was extremely simple. Each of the touch sensors in the pendant is checked for closure. When one closes, the related drive motor is turned on. The third sensor input is dedicated to the collision sensors. When a collision is detected, the motor direction for all motors is reversed for two seconds - this allows space to complete a desired turn or to retreat from an obstacle.

Performance of the model was extremely good. As mentioned earlier, it maintains a pace near normal human walking speed. It successfully operates on carpeting, but performs even better on tile floors and cement areas outdoors. The vertical sway problem does not seem to affect the structural integrity of the model except in extreme cases of collision or large bumps - some of the pieces pulled loose from the base in these situations, although we merely pressed them back into place and continued with operation. There was no large-scale disintegration of the structure even in these circumstances. The small racecar style wheels did not provide much ground clearance at the rear of the drive chassis, perhaps inch. The small ground clearance did prove to be a problem at time when the model was operated outdoors on uneven pavement. In most cases it did prove adequate, but a little larger tire would have improved the design.

The large wheelbase made this model extremely stable and there were no instances of tipping over in the several hours we operated it. Useless Bot was the first model that visited neighbors several houses away under its own power. The neighborhood children enjoyed driving Useless Bot on the sidewalk - for many of them this was their first opportunity to operate a functional robot. We really had fun with this design and this was another robot that wore out its batteries during the operational phase because we were having so much fun with it.