Attitude control of a hopping robot: a feasibility study

Transcription

1 Attitude control of a hopping robot: a feasibility study Tammepõld, R. and Kruusmaa, M. Tallinn University of Technology, Estonia Fiorini, P. University of Verona, Italy

2 Summary Motivation Small mobile rovers. Benefits and limitations. Past work on hopping robots Methods for attitude control Proposed approach of a hopper with attitude control Results on feasibility Conclusions

3 Motivation of this work Hopping mobility: could overcome obstacles pass rough terrain would give good visibility during the hop allows for smaller and more mobile robot can complement bigger rovers But... hopping trajectory is unpredictable landing can be damaging a small robot would have limited visibility on the ground Therefore in this work we propose an attitude control system that would overcome these limitations

4 Smaller rovers for planetary Planetary exploration with wheeled rovers Desirable to make smaller rovers Cheaper transportation Additional scouts robots Small wheeled rovers would have less mobility Small hopping robot would be more mobile exploration

5 Past work on hopping robots Hopping investigated in different contexts: Search & Rescue Surveillance in security and military Locomotion in general and in the context of biomimetics 7g hopper from EPFL The Moon Car (1959) NASA-JPL hoppers CSA Mars hopper Soviet Phobos 2 mission Japan s MINERVA lander Soviet hopper MINERVA JPL hoppers CSA Mars hopper

6 Past work on hopping robots Possibilities of landing protection: Reinflatable airbags surrounding the robot Elastic cage: propulsion and landing protection in one. Both limit accessibility of instrumentation to the environment Jollbot from University of Bath Elastic cage design from University of Verona.

7 Example of hopping motion

8 Example of hopping motion

9 Attitude control Controlling the landing attitude allows optimized design and cushioning of the mechanical structure Attitude control during flight allows directing the camera/instrumentation towards area of interest

10 Methods for attitude control Gas thrusters Use fuel that must be carried and will run out Angular momentum exchange devices using electrical power Reaction wheels Momentum wheels Control moment gyroscopes Variable speed control moment gyroscopes. Very interesting, but new and complex system Can an attitude control system be implemented with the small mass and volume budget of a small hopping robot?

11 Proposed approach 2nd generation hopper developed in JPL Simulate an attitude control system with 2 reaction-wheels to control orientation of a robot s axis during flight Theoretical mission on Earth s Moon Due to spring energy, the 1.7 kg robot had hop distance 22 m and hop duration 5 s 5 second to take an image of a distant object and make a rotation to land safely In an ideal case, the unactuated axis would be principal axis of inertia

12 Design parameters A reaction-wheel with more moment inertia will provide more reaction Need to compromise : Moment of inertia Mass Maximum RPM Power consumption Chosen actuator parameters: Moment inertia equivalent of a steel disk 2mm thick, with a diameter of 90mm and mass of 80 grams RPM, 100 W maximum

13 Attitude control with 2 RWs In the special case of zero initial angular momentum, 2 reaction-wheels can give full attitude control Due to possible disturbance torques during take-off, zero initial angular momentum cannot be assumed. With non-zero angular momentum, only axis perpendicular to the wheel axes is controllable Spin-axis control law makes use of w-parameterization that describes direction of a vector with two variables Kinematic equation for the w-parameterization: Control law (by Sungil, Youdan, 2001) : w 1 = ω 3 w 2 + ω 2 w 1 w 2 + ω w 1 2 w 2 2 w 2 = ω 3 w 1 + ω 1 w 1 w 2 + ω 2 2 (1 + w 2 2 w 1 2 ) T w1 = I 2 ω 2 ω 3 + h w2 ω 3 + k 1 w 1 + k 2 ω 1 T w2 = I 1 ω 1 ω 3 h w1 ω 3 + k 1 w 2 + k 2 ω 2

14 Simulation goals A simple maneuver from worst-case attitude error Unactuated axis is principal axis of inertia Unactuated axis is not principal axis of inertia Realistic hopping maneuver with attitude tracking and attitude maneuver before landing

15 Robustness towards initial angular momentum System with ideal inertia Robustness better towards angular momentum in some axes. Non-ideal inertia Longer convergence and more irregular robustness towards initial angular momentum. Times to rotate from 179 degrees error angle to less than 5 degrees error with different initial angular velocities.

16 Attitude tracking and landing After take-off, the attitude control will start to aim an axis of the robot to the target (black box). Predetermined time before landing, it will rotate to landing attitude. Energy to accelerate the RWs: 180J. Equivalent of only 7mAh from 7.4V battery. maneuver

17 Attitude tracking and landing maneuver

18 Conclusion and future work Hopping robots are a viable alternative to wheeled rovers We presented a feasible solution to improve functionality and survivability of a hopping robot Designing these small, but powerful reactionwheels and integrating them to a hopping robot is the challenge of this approach