Making a cardboard tank part 1 – Chassis and tracks

I like cardboard, hot glue and tanks. I guess it was only a matter of time before this happened.

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I started out by making a small box which would serve as the body of the tank. Then i created some wheels using empty toilet paper rolls and some round cardboard pieces. The tracks were made by cutting thin strips of cardboard, and then removing one side, exposing the “curly” part of the in middle of the cardboard. The method is described here. The drive wheels were covered with the same cardboard strips that make up the tracks. This ensures good friction between tracks and drive wheels. The motors initally used were 5V stepper motors. They were driven using an AVR ATMEGA 162 Microcontroller unit (MCU) with a stepper controller circuit.

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The body with one idler wheel (right) and a stepper motor (left) attached

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Half-finished tracks.

 

2014-03-07 21.24.40

Tracks and wheels finished

The electronics

The electronics

Things seem to be working:

 

The finished (fail)version with stepper motors. The driving wheel falls off because it is badly attached to the stepper motor shaft.

The stepper motors did not have enough torque to drive the tracks, so i decided to try out some 9 gram servo’s modified for continuous rotation.

The servo's gears

The servo’s gears

The servo has an internal regulator which takes its setpoint from a PWM (pulse width modulation) signal from the ATMEGA. The feedback of the regulator comes from a potmeter which is used to measure the angle of the servo through varied resistance. By using two identical resistors, we trick the regulator into believing that the servo is in the middle position at all times. A setpoint at 0 degrees makes the servo spin in one direction, a setpoint of 180 degrees makes is spin the opposite way. The servos proved to have enough torque for the tracks, but they require quite an amount of current. A guide to this mod can be found here.

Tricking the servo

Tricking the servo

Servo mod complete

Servo mod complete

I installed the servos with axels made out of some old pens. The axels were attached to typical servo mounts using two-component epoxy. I also installed a 9V battery to make the tank able to drive around. It is programmed to just test out the six different running modes (forward, backward, turn right/left and spin right/left). As i’m writing this i realize that i also should have implemented turning left/right while going backwards. While “turning” the tank moves one track while the other stands still. While spinning it turn both tracks, but in opposite direction, effectively rotating in place.

The tank running on 9V. I don’t remember if the twitching is caused by tracks getting stuck or a software bug.

The tracks were initially made by gluing two layers of cardboard together. These tracks proved to be a bit too stiff, and introduced too much friction to the system. I chose to remove the outer cardboard layer, and therefore losing the curly side with good friction. To ensure enough friction between the floor and tracks i decided to add some road wheels that “push” the tracks downward. The road wheels axels are made with paper clips, this provided some degree of suspension. They were attached to the underside of the tank using super glue and gaffa tape.

Road wheels

Road wheels

Road wheels assembled

Road wheels assembled

The tank running with road wheels intalled, and a 12v power supply. The 9v battery did not deliver enough current to let the servos run at max power.

Future work

  • Rotation turret with gun. I may include some auto targeting based on sound or IR detection.
  • Remote control. I plan to use the same RF equipment as in my home automation post.
  • Autonomous control. Install range detection and/or camera to make it able to navigate by itself.
  • Quadrotor setup for aerial maneuvering. I will also implement a regulator which makes it able to hover in place.
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