NXT Roadster

 

I’ve spent far too much time on this…

One of the things I wanted to do with the NXT before I even bought it was to build a working Lego car with proper rack and pinion steering, a differential to split power to the rear wheels and an engine that moved proportionally to the car’s speed.

My first attempt started with the 1:10 scale Ferrari F1 car (view on Peeron) and I tried to bodge the NXT and motors into it…

It was ok for a first attempt, but it looked rubbish and didn’t work very well. The design of the F1 car simply wasn’t strong enough for the weight and torque of the motors and the engine in its original position was completely obscured by the NXT brick. The engine also had the drive to the rear wheels going through it and the crank was very prone to coming apart.

I abandoned the idea for a while until I was reinspired by this post at The NXT Step Blog. It seems someone else had managed to build a very nice looking coupe from the NXT kit and the same F1 car. I started by building a rolling chassis with the features I wanted (proper steering, differential and working engine). The photo below shows it during disassembly.

 

I took some liberties by using other parts that I still have from before my Lego “Dark Ages”, notably the chain drive for the engine.  The engine is driven by a separate motor to the drive motor to reduce the stress on the crank.

The sensors used are the ultrasonic sensor at the front for collision avoidance, and a touch sensor at the back as a bumper. Additionally there is a sound sensor at the back. I couldn’t think what to do with that so it acts as emergency stop if I shout at it!

The bodywork started off as an attempt to cover up the workings, but gradually the thing started to turn into a sportscar.  The end result is below.

I created two programs in NXT-G for the roadster, one to automatically drive around and one to be controlled by my mobile phone using the Lego NXT Mobile application. The latter is much more fun and I’ve added various embellishments to it like a Ferrari badge on the screen, an engine startup sound and a horn.

Both programs use the same startup calibration routine for the steering, which turns slowly to the maximum extent in both directions until the motors stall, then uses the number of degrees between the two extremities to set the mid-point and maximum deviation for the motor. The steering is then controlled using a MyBlock to request a signed percentage value, with right being positive and left being negative.

The engine speed is controlled in a similar way, with a MyBlock that takes a percentage power and applies that to the rear wheels and runs the engine proportionally between idle and 100%. The engine can also be stopped, in which case the engine power MyBlock does nothing.

You can see video of the car in action under Bluetooth control below.

More photos…