Mike Grauer's Final Project Report for Bio Inspired Robots Fall 2006

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My robot is modeled on a polar bear, although the lurching and maladroit gait of my creation does little justice to the grace with which that largest of surviving Ursine comports itself. I opted for a four legged creature as it would be simpler to balance than a two legged one, and would require fewer parts than a six legged one. My overall design is fairly simple, with a single drive train and motor that powers all four legs and the offsets of the leg attachments to the body providing the body with stability.

My simulation model was created using the MSC Adams View software, essentially I created an abstracted virtual model of Adams geometric primitives based loosely on the proportions of the Lego pieces in the physical model. My virtual model evolved over time to finally approximate my physical model reasonably well. The rotational speed of the hip gears on the virtual model are closely matched to what I empirically measured on my walking physical model, and the gait of my virtual model is quite similar to the physical one, both in terms of the side to side lurching and the rising and falling of the front and rear body segments. I feel that the virtual model I have created is close enough to the physical model that I can perform parametric tests on the virtual model and feel confident that the findings would apply to the physical model--this idea of simulation and testing the virtual in place of the physical being one of the core ideas in this course. To this end I have actually used my virtual model to find what rotational speed on the hip gears would cause the robot's legs to become unsynchronized, resulting in the robot's loss of leg synchronization and stability, and eventually an overall bodily collapse. This kind of testing can stand in the place of having to try out different motors on the physical robot in order to find the fastest speed sustainable by the robot.

Full robot.JPG

Here is a video of my simulated virtual robot walking, note the similarity in gait to the physical robot.


Robot Design and Rationale

My design goals were simplicity and stability for this robot. By creating a four legged robot I didn't have to worry about correcting for the shifting center of gravity as I would in a two legged robot, and as there were only two cross axles to the central drive train I felt that a single motor could power the robot. In offsetting the legs to four different positions, I thought that I could provide stability to the robot by always having two legs diagonally across from each other touching the ground at the same time. This design also made the power distribution simple by allowing a single rotational power source to supply all four legs at the same time, since the offsets in the legs' positions are enough to keep them in the correct order relative to each other. This meant I could simply turn on the motor and have it power the legs without worrying about any kind of control program, though clearly this also limits the robot to walking forwards or backwards only (depending on the polarity of the motor wires connecting to the 9V battery power source). The 9V battery serves a dual purpose, both providing power and a counterweight to the motor.

The drivetrain consists of a single motor that provides rotational output spinning around the long axis of the robot's body. The motor is connected to a long connector axle with two worm-gears set at fixed distances along the drivetrain, these distances corresponding to the 24T gears which are centered on each of the two leg axles. The two leg axles spin in an axis perpendicular to the long axis of the robot, spinning in what would be the Y axis if the drivetrain is the X axis. As the motor spins the central drivetrain, the worm-gears on the drivetrain spin the 24T gears, which are fixed to the leg axes, and thus spin the leg axes. The hip gears are fixed to these leg axes, and provide four different positions to attach the legs. By attaching the legs in an offset position I keep the stability of the robot intact and keep the drivetrain design simple. When looking at a 24T gear, if the four holes of the gear are positioned so that each one is in one quadrant (upper left = 1, upper right = 2, lower right = 3, lower left = 4), I set the legs at offset positions so that legs diagonally opposed to each other would either be on the ground at similar times (rather than the same times, what I mean here by similar is mostly the same times) or in the air at similar times. So for the description of gear holes above, I could position the front right leg at hole 1, the rear left leg at position 2, the rear right leg at hole 3 and the front left leg at position 4.

Simplified Drive Train example
Description Thumbnail Image (click for full size)
An example of a leg axle system, has an axle with one 24T gear fixed to the axle at the center, and a 24T hip gear fixed at either end of the axle.
Mjg robot gears.JPG
Here the leg axle system is set into a frame of two cross beams set along the long axis, the axle can spin freely while being held in place between the two beams.
Mjg robot axlegearsinframe.JPG
Here a drive train axle with worm gear is attached into cross beams that are set along the same axis as the leg axles, the drivetrain can spin freely while being held in place, as the drivetrain turns, the worm-gear spins the central 24T gear.
Mjg robot drivetrainexample.JPG

Actual Drive Train
Description Thumbnail Image (click for full size)
A view of the drive train exposed, with the right side hip gears removed.
Mjg robot fullviewdrivetrain.JPG
A view of the drivetrain down the long axis of the robot, with the hip gears removed


Mjg robot drivetraintoplongaxisview.JPG
A view of one of the leg axles with the hip gear removed.
Mjg robot hipgearconnectionaxle.JPG
A view of the hip gear with the leg removed (4 different possible positions of leg can be seen).
Mjg robot leggear.JPG

The legs are attached to the hip gears via two rotational joints, one joint connecting the cylindrical leg extender to the hip gear, and one joint connecting the cylindrical leg extender to the leg itself. These cylindrical hip extenders create additional distance of the legs and feet from the body, which provides extra stability. The legs are fixed into the feet of the robot and are also wide for stability--just like the polar bear's wide feet which help reduce pressure by distributing the weight over a larger area, helping to prevent falls through thin ice (of course this is just wild speculation, but it sounds plausible and I wanted to highlight the bio-inspired theme again).

Robot Leg
Description Thumbnail Image (click for full size)
An exploded view of the leg and foot and cylindrical connector.
Mjg robot explodedleg.JPG
A view of the robot leg and foot and cylindrical connector.
Mjg robot leg.JPG
A view of the robot leg and foot and cylindrical connector, connected into a hip gear


Mjg robot legandgear.JPG
A view of the complete leg and foot, attached to the robot body at the hip gear.
Mjg robot motorhidden.JPG

A 9V battery is connected to the motor through the wire, and depending on the polarity of the battery's attachment to the motor, the motor will spin clockwise or counterclockwise. The central axle then spins, and the worm-gears on the central axle spin the central 24T gears on the leg axles. The leg axles rotate the hip gears, which then lift the legs and feet. Each leg is constantly rotating around the leg axle, which means that it is lifting off the ground and moving forward (or backward), which together with the action of the other legs will propel the robot in a direction along the long central axis of the robot.

Here is a video of my physical robot walking, the book behind it is for a frame of reference.


Physical Assemblies

Leg X 4

Leg Assembly Exploded View

Leg assembly.JPG

Leg Assembly

Mjg robot leg.JPG

Leg Assembly Bill of Materials (x4)
Quantity Description Thumbnail Image (click for full size)
4 4x1 (3 hole) beam
MJG LEGO 4x1x3 hole beam.JPG
3 2x1 (1 hole) beam
2x1x1 hole beam.JPG
1 10x1 (9 hole) beam
MJG LEGO 10x1x9 hole beam.JPG
2 cross axle to round connector
MJG LEGO Cross axle to round connector .JPG
1 cross axle to cross axle collar
MJG LEGO Cross axle to cross axle collar.JPG
1 24T Gear

Battery Chamber X 1

Battery Chamber Exploded View

MJG batterychamberexploded.JPG

Battery Chamber

MJG batterychamber.JPG

Battery Chamber Bill of Materials (x1)
Quantity Description Thumbnail Image (click for full size)
1 6x2 (5 hole) plate
MJG LEGO 6x2x5 hole plate.JPG
4 10x1 plate
MJG LEGO 10x1 plate.JPG
2 10x1 (9 hole) beam
MJG LEGO 10x1x9 hole beam.JPG
4 8x1 plate
MJG LEGO 8x1 plate.JPG
5 6x1 plate
MJG LEGO 6x1 plate.JPG
2 2x1 (1 hole) beam
2x1x1 hole beam.JPG
2 4x1 (3 hole) beam
MJG LEGO 4x1x3 hole beam.JPG
1 10x6 plate
MJG LEGO 10x6 plate.JPG

Motor Housing

1 10x6 plate 12 6x1x5 hole beam 2 6x1 plate 12 3x1 plate

Exlpoded View of All Assemblies

Body and legs.JPG

Virtual Model and Physics Based Simulation

Useful Links and Resources

include lego robot book inclue lego robot pdf from handyboard include the adams tutorials