Andata Details

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Andata
I designed this robot for a navigation contest which took place during the 9th Italian Legofest. Each robot had to start from a given point, go straight for a short distance until touching a wall, turn itself 180° and go back more precisely as possible to the starting point. The rules were very detailed, but the major points were that rotation sensors were forbidden and that the robot had to actually turn, it couldn't simply reverse its motion. The initial distance from the starting point to the wall was 1m, and we performed five rounds with increasing distances, adding 1m at each new round up to a final distance of 5m.

"Andata" is the first of the two robots I prepared for the contest. Though it did not win the contest, it performed very well, reaching the final round and proving itself always very precise and very reliable. On the maximum distance of 5m, after the round trip it was no more than 4-5cm far from the reference point.

It can be considered a differential drive, though of a special kind. There is only one drive motor, which powers the right wheel. When the robot is required to go straight, a simple mechanism keeps the left wheel coupled to the right one, so they travel at exactly the same speed. This solution makes the robot go perfectly straight.
When the robot needs to turn, a group of pneumatic cylinders disengages the mechanical coupling, and at the same time brakes the left wheel. Doing so, the robot can turn pivoting around its locked left wheel.

For the contest, the robot needed only a limited supply of compressed air, so I adopted the solution of pre-loading the tank with a manual pump.

The long red axle protruding from the front of the robot has the purpose to close a touch sensor when the robot hits the wall.

In this picture you can notice the simple motorized valve switch at the back of the robot, and the braking system for the left wheel. When the pneumatic cylinder pushes the gear rack agains the 24t gear, the wheel gets locked.

With the RCX removed, you can see some of the gearings, and the touch sensor activated by the red axle.

Two small pneumatic cylinders operate the switch mechanism. When the cylinders are extended (as in this photo), an idle axle mounting two 27t gears engage two 40t gears conected to the wheels, thus keeping them mechanically linked.

On the contrary, when the cyilinders are retracted the idle axle comes apart from the 40t gears and the left wheel becomes uncoupled from the right one and the motor. At the same time, a third small cylinder activates the brake, pushing a rack gear against a 24t gear mounted on the main axle of the left wheel.

Looking at the robot from the bottom, you see how power is trasmitted to the wheels through a double reduction stage (3:1 * 5:1 = 15:1). You can notice also that the intermediate axle is used to rotate the disc that measures the travelled distance (top left in the picture).

As the rules forbade rotation sensors, I used a black&white disc in front of a light sensor to count rotations. This method proved very effective and absolutely reliable.