Introduction

• It should have 6 wheels, 4 of them steered.
• The maximum number of wheels should be motorized.
• A maximum of sensors should capture the environment.
• A sensor arm should move out to scan objects.
• The robot should receive commands from an operator terminal via Bluetooth.

The drive

In early project stages, I had ambitious aims regarding the drive. The robot should run across small obstacles. For this I considered drives as the Rocker Bogie Suspension like the drive mechanics of the original Mars Rover. However, it turned out, this was very difficult for a number of reasons: Lego Technic bricks have a large mechanic tolerance, thus tend to be unstable in large constructions. More important: for the original Rocker Bogie drive I would need one motor for every wheel and another motor for steered wheels. These are too many motors for a single construction. Even if I did not incorporate the Rocker Bogie, 6 wheels lead to a number of problems. If we steer, left and right wheel should have different steering angles. Actually, for different distances to the middle wheels, even the angles of each of the 4 steered wheels should be different. This is very difficult to realize with a single motor. As a result, my construction steers all 4 wheel to the same angles. Even though not exact, this does not lead to any steering problems. I also had to give up the goal to motorize all 6 wheels. Having a single motor for each side, we would need 4 cardan shafts as 4 wheels are steered. But only three were available. As a result, 4 of the 6 wheels were motorized, what required 2 cardan shafts. Experiments showed that this was the minimum. With only 2 motorized wheels, the construction does not always move stable. To be precise, 2 steered and 2 fix wheels should have differend speeds when driving curves. This would require 4 motors, 2 motors and 2 differential gears or 1 motor und 3 differential gear All these variations were refused due to limited space in the construction. As a result I used one motor for each side without any differential gear. Driving curves, the outer wheels have a higher speed, however, the 2 outer wheels have the same speed. The resulting friction was acceptable and does not lead to any mechanical problem.

The sensors

• turning left and right;
• pitching up and down in order to get distance values above and below the horizon.

The result

• Two motors for driving foreward and backward (one on each side), 2 cardan shafts, no differential gear.
• One motor to steer the 4 steered wheels.
• One motor to turn the sensor head left and right.
• One motor to pitch the sensor head up and down.
• One motor to deploy the sensor arm.
• Two touch sensors to detect obstacles in driving direction (front and rear).
• One distance sensor to detect small obstacles in driving direction.
• One distance sensor to create a distance map (all directions).
• Two sensors on the sensor arm (e.g. Touch and light sensor).

• we need 6 motors
• we need 6 sensors
• as each brick only can control 3 motors and 4 sensors we need 2 bricks.