Cubli

Can we build a 15cm sided cube that can jump up, balance on its corner, and walk across our desk using off-the-shelf motors, batteries, and electronic components?

Overview

The Cubli is a one degree of freedom robot that can jump up and balance on its corner.

The reaction wheels mounted on three faces of the cube rotate at high angular velocities and then brake suddenly, causing the Cubli to jump up. Once the Cubli has almost reached the corner stand up position, controlled motor torques are applied to make it balance on its corner.

In addition to balancing, the motor torques can also be used to achieve a controlled fall such that the Cubli can be commanded to fall in any arbitrary direction. By combining these three abilities — jumping up, balancing, and controlled falling — the Cubli is able to 'walk'.

The power, computation, and control components were not mounted on the prototype to avoid making the first prototype too complex.

Specifications

Height	15
Width	15
Length	15
Controller
IMU (inertial measurement unit )	6

DC motor	3

CUBLI

The design

The other feature is its ability to jump up from a resting position without any external support, not only an interesting concept for the control engineer but also also an appealing demonstration for the general public.

Fig. 2 shows the jumping up strategy of the Cubli. Initially the Cubli, lying flat on its face, will jump about its edge by instantaneously stopping one of its momentum wheels. Once complete, the Cubli will provide an inexpensive, open source test-bed with a relatively small footprint for research and education in estimation and control.

Figure 1: The CAD drawing of the Cubli with covers removed.

Figure 2: The Cubli jump-up strategy: (Left) Flat to Edge: Initially lying flat on its face, the Cubli jumps up to stand on its edge. (Right) Edge to Corner: The Cubli goes from balancing on an edge to balancing on a corner.

Although high angular velocities of the wheels before braking can be reduced by increasing the wheel inertia, i.e., increasing wheel mass since the wheel size is constrained, this was not taken to the extreme since this will result in reduced recovery angles while balancing.

A gear chain between the wheel and the motor was avoided since it would not allow the high angular velocities for jump-up and would add extra weight and volume. Although the balancing maneuver, which requires high torques, was affected by the choice of no gear, the brushless DC motors were still able provide enough torque for recovery angles up to 7 ◦ .

CUBLI

The design

Fig. 3 illustrates the one dimensional prototype that was built to examine the feasibility and develop control algorithms for the Cubli. Similar momentum exchange wheel based inverted pendulum designs, except for the braking mechanism.

Figure 3: Illustration of the one dimensional prototype consisting of a square plastic plate that holds the momentum exchange wheel through the motor at its center. The plate is attached to a bearing at the bottom.

Figure 4: The CAD drawing of the RC servo-based braking mechanism: An RC servo is used to quickly collide a metal barrier (blue) with the bolt head (red) attached to the momentum wheel.

The prototype consists of a square plastic plate that holds the momentum exchange wheel through the motor at its center and the braking mechanism at one of its corners. The dimension of the plastic plate matches the dimension of the proposed Cubli face, and it will be referred to hereafter as the pendulum body. The plate is attached to a bearing at the bottom that gives it a single degree of freedom to pivot around its corner on a horizontal plane.

CUBLI

Overview

The Cubli is a one degree of freedom robot that can jump up and balance on its corner.

Specifications

CUBLI

The design

CUBLI

The design

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