The Skippy project began in 2010 when Morteza Azad joined the Australian National University as a PhD student, and Roy spent a one-month sabbatical at ISIR in Paris, courtesy of Vincent Hayward. Morteza provided the energy and enthusiasm to get things going, while Roy's sabbatical gave him the chance to figure out how balancing really works, viewing it as a physical process rather than an exercise in control theory. Over the next four years, Morteza implemented planar balancing, both on a sharp point and on a rolling contact; single hops beginning and ending in a balanced configuration; and the first example of bend-swivel balance control in 3D.
That's an impressive amount of progress for a single PhD, but it was all done in simulation. The real challenge was to make a physical robot that could do all this stuff, and that became possible when Roy joined IIT in 2014. IIT has all the necessary resources and expertise to create world-class robotic devices, and it is a sensible place to try and create Skippy.
The Skippy project got off to a good start at IIT, but then suffered a serious setback: the first student on the project (name withheld) turned out to be a very unfortunate choice. He held back progress for years, then wrote a dishonest PhD thesis in which he claimed that his own technically incompetent design was Skippy. Team productivity more than doubled after he left.
In 2016 a balancing machine called Tippy was designed and built (by name withheld) for the purpose of carrying out a series of experiments to test Roy's balance controller in practice. By late 2017 the machine was operational, but we quickly discovered that it was much too wobbly to be physically capable of the performance we were looking for. By clamping the mechanism in a specially-made stiffening brace we were able to obtain acceptable results, for the first experiment only, which were eventually published in 2019.
By the summer of 2018, the project had been dragged so far off course that Roy decided to 'hit the reset button' and restart the project in the correct direction. The first step in a correctly executed Skippy project is the parametric design of Skippy, which solves the following problem: in the beginning we do not have a design, but only a list of performance objectives (hop high, hop far, balance, somersault, etc.) and the task is to find a design that meets all of the objectives. The solution is to perform a multi-objective design optimization, in which the software searches a space of design parameters looking for designs that meet the objectives. The result is a pareto front of designs that meet every objective, from which we can pick the one we like best, and make it.
Antony performed this task using a commercial product called modeFRONTIER, and got his first result in August 2019. He then proceeded to improve upon it, and this work forms the centrepiece of his PhD project. See publications 18, 19 and 20. Thanks to Antony, we now have good values for the most important design parameters, and can proceed directly to the detailed design stage (i.e., the CAD models).
Meanwhile, Roodra was doing lots of useful things, including electronics, parts selection, and shock-testing of components and subassemblies, so that we can be sure that Skippy will survive a crash landing from a substantial height. But he was also doing good research in the areas of balancing and shock propagation. On balancing, he made a significant improvement in Roy's balance control theory by showing how it can be generalized to allow control of passive degrees of motion freedom in addition to active ones. The most important consequence is that it lets robots balance while controlling the absolute positions and orientations of their limbs, which is essential if they want to interact with objects in their environment. He also worked with Justin Yim on balance controllers to allow Salto-1P to make accurate launches, and to balance after landing. On shock propagation, he employed the concept of centre of percussion to design a leg that minimizes the propagation of mechanical shock from the foot to the torso. See publications 16 and 17.
Another important issue that must be studied is how to balance in the presence of spring-loaded passive motion freedoms. Skippy will have both a spring-loaded ankle and a spring in series with its main motor, and both springs are essential for Skippy to reach its performance objectives; so we need to know how these springs affect balancing performance, and how to make Skippy balance skilfully in the presence of these springs. This is the main topic of Juan's PhD studies, and publications are on the way.
And finally, the newest member of the team, Federico, has been making the CAD models of Skippy's parts, and has started building Skippy itself. It won't be long now....