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Bob the Janitor

Bob the JanitorA biped walking mechanism with shifting center of gravity. Features head, arms and a broom that move as it walks.


Completion date: 21/06/2016
Power: electric (Power Functions)
Dimensions: length 27 studs / width 17 studs / height 25 studs
Weight: 0.4 kg
Suspension: none
Propulsion: 1 x PF L motor geared 14.4:1
Motors: 1 x PF L motor

Everyone has met Bob the Janitor at least once. Bob usually populates schools and offices, it is usually seen on its own and doesn’t form herds, and it feeds on shoeprints recovered from the floor as well as on schoolchildren’s fear. He has a strong territorial instinct and may be aggressive if his freshly cleaned floor is trespassed. Interestingly, his age appears to be constant but advanced, although all attempts at carbon dating have been unsuccessful. Theories abound about how Janitor is often actually older than the building he maintains and how he gets an extra year added to his lifespan for each scene he causes against the trespassers.

The distinguishing features of a Janitor include the Broom of Doom, a sleepy yet vigilant and always suspicious look, a mandatory mustache, a cigarette that never burns out, and a belly used to store gathered shoeprints for later consumption. Thus it is possible to visually estimate the effectiveness of any Janitor by the size of his belly.

The Janitor’s mating habits and methods of breeding remain unverified, mostly due to an underdeveloped sexual dimorphism (it’s extremely difficult to tell males from females), as well as due to the fact that sightings of more than a single specimen within the same territory are exceedingly rare. No one has actually seen Janitor’s offspring either, which led some to believe than instead of mating, this species relies on cloning.

On a serious note, this typical biped walker with shifting center of gravity was a result of my studies on the biped walking mechanisms. Its rocking side-to-side motion appeared amusing to me, so I decided to make use of it and give the walker some personality.

The actual mechanism was pretty simple and consisted of a single PF L motor driving a transverse axle using a worm gear. The axle drove two opposite cranks that caused movement of the two legs – the legs were simple parallel linkages and the cranks went between the links of each leg, forcing it to move forward and backward. As there was no upward/downward movement, a tilting was used in order to walk. The same crank that moved each leg controlled the angle of its foot using a simple link. During the walking cycle, when a leg was put down, the crank pulled the link tilting the foot, so that the entire walker tilted to the side of that leg. This took the weight off the other leg which could then move forward. The resulting motion was similar to shuffling, with both feet kept very close to the ground at all times.

The actual mechanism was located in the center of the walker, with a battery in the back and two weighted bricks acting as battery’s counterweight in the front. The walker was a relatively fast one for a biped, thanks to moving the legs and tilting by large angle, which also required large feet to retain stability. The rocking side-to-side motion produced by the tilting of the feet was quite severe and in fact produced small shocks that would often change direction of the walker moving on a bare floor. To prevent this, I have added small wheels with rubber tires in front and at the back of each foot. Still, the walker had a tendency to slowly veer round, which was probably caused by the vertical motor at its center of gravity, quickly spinning in one direction. I guess that two motors, each spinning in opposite direction to the other, would not cause such an effect. All the other movements – of the head, arms and the broom – relied solely on the rocking motion and gravity.



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  1. Sariel
    June 23rd, 2016 at 12:48 | #1

    Thanks. I don’t have instructions but I believe it’s simple enough to be built from the photos.

  2. Kathryn
    June 23rd, 2016 at 03:28 | #2

    I think this is so cute I love it can you give instructions on building it

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