Skippy Project Videos and Animations

This page collects together some of the videos and animations we have made.  Some animations are available both as a video for you to play directly and as a Matlab .mat file that you can play using the function showmotion in Spatial v2, which will give you a much better viewing experience.  To download the .mat file, click where you see the icon SV2.

Skippy Hops Higher with a Ring Screw   (Aug 2024)
This video follows on from the one below, and compares Skippy's hopping height when fitted with a ring screw versus a ball screw in the drive train.  Not surprisingly, it hops higher with the ring screw thanks to its higher maximum speed.  In this experiment Skippy achieves a 34cm hop, measured as the rise of the centre of mass from lift-off to apex.  The novelty here is that this is the first time that a ring screw has been used in any robotic device.  For more details see publication 35.
Skippy Balancing, Hopping and Falling Over   (Oct 2023)
This video, prepared by Antony and Federico, shows Skippy balancing and hopping in a vertical plane, and falling over a lot.  A shoe in the shape of a blade is fitted to Skippy's foot, so that Skippy is effectively balancing on a knife edge and can therefore use a planar balance controller.  The video first shows Skippy balancing, making a small hop in place, recovering its balance, and making another hop in place.  It then shows Skippy making a two-hop sequence in which the first hop is in place and the second is a travelling hop.  The rest of the video then shows Skippy falling over again and again.  This last part shows how robust Skippy really is: it has now crashed and fallen many dozens of times without suffering any damage.  Everything that Skippy does in this video is accomplished using a single actuator.
Making a Double Backflip Using a Springy Leg   (June 2023)
This animation, prepared by Juan, shows a planar double pendulum with a springy leg get up off the floor, balance, crouch, launch into a double backflip, control its motion in flight, land, absorb the energy of landing, and come to rest in a balanced position—all of it accomplished using only a single actuated joint.  It was the climax of Juan's Ph.D. studies, which focussed on the problem of balancing and hopping with a springy leg, including techniques to absorb energy from the spring on landing, and to damp out vibrations in the spring while balancing.  It is a clear improvement on Morteza's original work shown below.  For more details on Juan's work on this topic see publications 21, 23, 27 and 31.
Control of Absolute Motion While Balancing   (Dec 2021)
This animation shows a planar triple pendulum balancing on a knife edge while making its tip follow a trajectory shown in green.  The novelty here is that the robot is controlling the absolute motion of its tip, using an operational-space motion controller, and using the null space of the motion task to balance; but both the operational space and the null space involve the orientation of the bottom link about the knife edge, which is the robot's passive degree of freedom, so the robot has both an underactuated operational space and an underactuated null space.  This animation is an excerpt from Roy's presentation at ICAR 2021.  To see the complete presentation click here; and for more details see publication 26.
Tippy Balancing in 2D Using the Crossbar   (Feb 2018)
This video shows Tippy balancing while simultaneously following a command signal that specifies the motion of the crossbar.  You can see the command signal in the left part of the video.  The balance controller includes a balance offset observer that measures the difference between true and estimated balanced configurations so that the controller can improve its estimate.  You can see this happening during the first few seconds, and again near the end of the video when the robot is being pushed.  The command signal asks Tippy to make some very fast movements, and you can see that Tippy responds very quickly.  You can also see it leaning in anticipation of the command signal.  Finally, the video shows Tippy resisting and recovering from large external disturbances.  For more details see publication 17.
Bend-Swivel Balance Control in 3D   left: (2012) SV2, right: (2016) SV2
These animations show what bend-swivel balance control looks like.  In this strategy, the bend angle is controlled by a 2D balance controller, and the task of the swivel controller is to keep the bend plane vertical and control the overall heading of the robot.  The animation on the left shows Morteza's original implementation; and the one on the right is a more recent implementation by Roy on a Tippy-like robot.  Both keep their balance very well, but neither tracks motion commands accurately while swivelling.  The likely reason is gyroscopic forces.  See publications 6 and 8.
Leaning in Anticipation   left: (2015) SV2, right: (2016) SV2
The one on the left was Roy's first demonstration of leaning in anticipation.  It was presented at talk 1, and shows a simple bending and straightening movement performed twice without leaning in anticipation, and then twice more with leaning in anticipation.  The graph of the response is here.  The one on the right begins with a sequence of movements performed without leaning in anticipation, and continues with a similar sequence performed with leaning in anticipation and much higher feedback gains, resulting in much faster, sharper movements.  Gains this high would be completely impractical without leaning in anticipation.  See publication 14.
Balancing in Combination with Other Motions   (2015)   SV2
This is the animation that Roy showed at Int. Symp. Robotics Research (see publication 10), and again in talk 1.  It is the first animation to show the balance controller operating together with a motion controller.  It is also the only animation to show a branched kinematic tree; and it is the last animation that Roy made before working out how to do leaning in anticipation.  The animation begins with three bending and straightening motions.  In the first, the balance controller operates the lower joint; in the second it operates the upper two joints; and in the third it operates all three.  In the remainder of the animation, the balance controller operates only the lower joint.
Morteza's Great Leaps Forward   (2012)
The animation on the left shows Morteza's balance controller performing single hops beginning and ending in a balanced upright configuration.  The robot is based on the Acrobot, as defined here.  The sequence of actions is more complicated than it appears.  First, the robot must maintain its balance in the initial position.  Then it must crouch down and lean forward (deliberately losing its balance), and then push with the right speed and timing in order to reach the right lift-off momentum (linear and angular) as the foot leaves the ground.  Then it has to control its foot trajectory during flight in order to land on the right spot.  Finally, it has to recover its balance after landing, and return to an upright configuration.  And all of this is done using just one actuator.  See publications 4 and 6.  The animation on the right, which Morteza never published, shows the same movement, using the same control system, but on a robot with a springy leg.  It shows that the balance controller can cope with the large disturbances caused by a spring that the control system doesn't know is there.  Although never published, this was a pioneering result.