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This robot is a single-RCX biped with the ability to turn within its own footprint. It's also my first foray into doing a 'tough-looking' robot, as opposed to one that is purely functional.

Turning works by advancing the robot until the leg that leads the turn is in front (e.g., the right leg is forward for a right turn). To make a right turn, the robot takes its weight on the right foot (left leg off the ground) and turns the right ankle turntable about 15 to the right; it then takes half a step back onto the left foot, raising the right foot so that it can straighten the right ankle. It then moves its weight forward onto the right foot again, turns the ankle another 15, and so on. This procedure can be repeated until the robot has turned the desired amount. The advantage of turning in this way is that the robot turns pretty much within its own footprint -- no requirement for maneuvering in large-diameter circles.

Biped_II, 3/4 view


The lower (belly) motor drives a worm-24t gearbox. The axle from the 24t gear connects to two cams, one for each leg, offset 180, lifting and moving the legs alternately to provide a walking action.

The two motors mounted high on the rear supply power to rotate the large Technic turntables which form the ankles; one motor for each ankle. The power is transferred to the ankles via Znap flexible axles; for each leg, one link runs from the motor to a small double-bevel gear at the 'knee' and drives a medium double-bevel gear which is flexi-linked to the worm screw at the back of the 'ankle'. Having heavy legs is really the death of chicken-walker robots like Biped_II because the legs tear off after 10 or so paces -- the Znap flexy-drive links allowed me to put all the weight in a well-braced position at the center of the robot, rather than having the weight of the motors down at the ankles.

Biped_II, motor placement (rear view)

The position of the moveable elements is monitored by three rotation sensors: one each for the ankles and one for the drive motor. The ankle sensors each have a 24t gear which is driven by an 8t gear on the same axle as the worm gear that turns the ankle turntable. The drive motor sensor has a pair of 12t gears driven by the 24t gear that is connected to the walking cams.
Biped_II, ankle details
Biped_II, side view I tried to make the legs as light and stiff as possible -- the other problem with 'chicken-walkers' is that the legs tend to sag inwards so that, as the rear foot comes forward, it catches on the heel of the foot in front. The lower part of the leg is mostly composed of single-bend liftarms pinned together whilst the 'thighs' are Technic bricks, cross-braced with plates to make box-section girders. The ankles are large Technic turntables; the feet are Technic bricks, cross-braced with double-bend liftarms and stiffened with 2-stud wide plates at the trailing edge of the heel. These plates also have the effect of tilting the whole robot slightly forward which improves the balance and makes it look more 'poised for action.'


The robot runs a simple NQC program that interprets messages from the LEGO remote control:

  • message 1 starts a turn to the left;
  • message 2 toggles forward motion on and off (i.e., the first message 2 starts the robot walking in a straight line, the next message 2 makes the robot stop, the next message 2 starts the bot walking in a straight line again, and so on);
  • message 3 starts a turn to the right.
Check the code for more details. [I must admit, I had fun making the robot as menacing-looking as I could. Hope you like it.]

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