Digital Human Motion Planning of Operation Sequences Using Optimal Control of Hybrid Systems

S. Björkenstam, P. Mårdberg, M. Roller, J. S. Carlson. DHM2020 : Proceedings of the 6th International Digital Human Modeling Symposium, 31 August-2 September, 2020, 2020, vol. 11, pp. 115-120.


In IPS-IMMA the operation sequence planning tool offers an easy and powerful way to construct, analyze, and simulate sequences of human operations. So far, the simulations created using this tool have been quasi-static solutions to the operation sequence. In this paper we present new functionality for motion planning of digital human operation sequences which also takes the dynamics of the human into consideration. The new functionality is based discrete mechanics and optimal control, and will be seamlessly integrated into to the IPS-IMMA software through the operation sequence planning tool. First, the user constructs an operation sequence using the operation sequence tool in IPS-IMMA. The operation sequence is then converted into a discrete optimal control problem which is solved using a nonlinear programming solver. Finally, the solution can be played back and analyzed in the graphical interface of IPS-IMMA. In order to obtain physically correct solutions to complex sequences consisting of several consecutive and dependent operations, we view the digital human as a hybrid system, i.e. a system containing both continuous and discrete dynamic behavior. In particular, the optimal control problem is divided into multiple continuous phases, connected by discrete events. The variational integrators used in discrete mechanics are particularly well suited for modelling the dynamics of constrained mechanical systems, which is almost always the case when considering complex human models interacting with the environment. However, special care must be taken in order to maintain good results when connecting several dynamic phases with discrete events. To demonstrate this new functionality, we model and solve several industrial cases, with particular focus on cases where the dynamics of the system plays an important part in the solution.

Photo credits: Nic McPhee