Sunday, July 8, 2012

The Monster (2003)

An incomplete Lego construction that I'm documenting shortly here because it spent so much time gathering dust before I finally took it apart. The pictures aren't great, but such is life.

It was Lego monster truck/dune 2x4 buggy of sorts. It was relatively large, and as such, was quite underpowered by the Mindstorms motors. This was much to my chagrin, but the exercise in designing and building it was very entertaining.

It sported 4-wheel independent suspension built using Lego suspension arm parts, ending in sports car / F1 Silver Champion wheels. The drivetrain featured a twin-engine, twin-transmission system, so I guess it would be more appropriate to say there were two independent drivetrains. Each rear wheel was controlled by a separate motor. The trains were mirror images of each other, with each transmission supporting two reduction ratios (1:3 and 1:1 to the wheels). The gear ratio was locked manually before operation. I considered using a micromotor as an operator, but 1) I was missing ports on the RCX, and 2) after seeing how little torque I was getting at the wheels, the wind in my sails faltered.

Underside - you can see the steering motor at the very bottom, with the pair of transmissions just above. 
Rear of the vehicle, with both transmissions.
The transmissions and shifting levers (the grey axle terminators).  The propulsion motors are up top, and the steering motor is at the bottom right.
Another view of the same.  Transmission is in alternate gear.
Perhaps the coolest part of the monster was the steering system, and it's a shame I have no great pictures of it. The steering servo was at the very rear of the car, and driveshafts were routed through chassis all the way to the steering pinion. (You can see this on the underside pic above.) It was a standard sliding rack and pinion system that was assembled to work at the steep strut angle. I had to shorten the rack from the standard suspension setup in order to get the pivot points lined up properly and all the links operating in parallel instead of as just some weird four-bar linkage.

There was a light sensor encased in the chassis right behind and below the rack. It sensed the light level in a closed chamber that opened onto the underside of the steering rack. On the rack was a series of white and black 2x1 flat bricks that sealed the chamber; thus, as the rack moved back and forth, the proportion of black and white bricks that was exposed to the chamber varied, affecting the amount of light that bounced around inside the chamber. The inside of the chamber was fitted with aluminum sheeting to maximize the ability of the light sensor to detect the movement of the steering rack. The RCX monitored the varying light levels; after a calibration process, it was able to determine how the wheels are angled from the light level.

I also built a handgun-style controller for the vehicle, complete with analog bi-directional trigger (did you know the Lego touch sensors are actually pressure-sensitive?) and an analog steering wheel that uses the same principle as the steering: a light sensor on a coloured strip. There was an RCX on the remote that sent signals to the RCX on the vehicle; commands were multiplexed using a small protocol I threw together.

Low-res view of the unmolested remote.
You can sort of see the two facing pressure sensors, used to detect fore and aft tilt on the trigger.  The light-detection mechanism for steering was unfortunately busted at this point - I had dropped the controller during a move and not bothered to put it back together properly.
Another view of the same.

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