This model is based on the Sup3r Car of Daniele Benedettelli described in his fantastic book “The LEGO MINDSTORMS EV3 Laboratory”. The vehicle is equipped with two MS-EV3 Large Motors each driving one of the two rear wheels, one MS-EV3 Medium Motor to steer the front wheels, a MS-EV3 Color Sensor mounted on the front side, and the MS-EV3 IR Sensor which also gives an overall evil-looking appearance. Here, I propose two slight changes to the model which improve the vehicle maneuverability:
- A 3:1 ratio gearbox on the steering medium motor to improve the steering accuracy;
- A constraint on the shaft of the steering medium motor to reduce the transmission backlash.
The model has been built using the Lego Mindstorm EV3 31313 retail set.
Improving the Sup3r Car steering accuracy
The original model uses two Angular Wheels driven by the medium motor to steer the two front wheels; the gear ratio from the motor to the wheels is 1:1; this means that one degree of rotation of the medium motor corresponds to one degree of rotation of the wheels. The medium motor has a built-in encoder with a resolution of 360 pulses per turn, so the idea is to improve the steering accuracy by modifying the transmission adding a 3:1 gearbox. In this way, the accuracy obtained becomes 1080 pulses per turn; of course a steering speed reduction comes in as a side effect, but it is not really something that you will notice given the slow speed of the vehicle.
Substituting the two Angular Wheels with a Conical Wheel z12 on the medium motor shaft, and adding a Double Conical Wheel z36 on the front wheels turning assembly, will give 3x steering accuracy. See this picture for details
The space needed for the z36 wheel will force you to mount the color sensor out of the vehicle frame, and I admit spoiling a bit the elegance of the original model.
Reducing the backlash on the steering gears
If you put a rod in the medium motor and apply a small radial force with one of your finger, you can easily feel that it sags a bit under the strain; because the torque that the medium motor applies to the wheels produces a radial reaction, the sagging causes the motor shaft to adsorb part of steering power, thus, especially for small angular displacement, the motor turns without actually turning the wheels.
Simply use a slightly longer shaft, namely a Cross Axle 3M, passing through a Cross Block 3M to a dramatically reduce the shaft oscillation under the strain; this way almost all the motor torque is transmitted to the wheels.
Only the backlash between the Conical Wheel z12 and the Double Conical Wheel z36 gear remains, and that I think it’s pretty structural given the material of which the gears are made. After all, it’s a toy…shouldn’t it be considered as such?
And to see the Sup3r Car in action, check here !
