A humanoid robot is being developed, intended to operate robustly in a typical day-to-day human environment. Its ability to react quickly to stochastic disturbances, especially its ability to avoid falling, termed stability in this context, is of prime importance. The choosing of a suitable actuator that affords such stability is seen as a challenge in the current stage of the humanoid project. A novel method for comparing various actuators in terms of humanoid stability is investigated, to aid with this choice.
A common method for comparing actuators is the use of low-level actuator parameters, e.g. power density. With a low-level parameter, a specific actuator property can be compared between multiple actuators. To incorporate relevant properties of all actuators, a large set of actuator parameters is required. This becomes even more challenging when comparing a heterogeneous collection of actuators, e.g. muscle and hydraulics. Besides subjectiveness in parameter definitions and comparisons, the multidimensional set of actuator parameters needs to be related to a one-dimensional measure of humanoid stability. As low-level parameters and high-level humanoid stability are not directly relatable, subjective weighting factors are traditionally employed to reduce dimensionality.
Step time is introduced as a one-dimensional criterion for comparing different actuators. The ability to make fast movements is a critical component in defining humanoid stability. Therefore, by comparing step times of different actuators in a fixed test case, a measure of humanoid stability is obtained. The step movement represents a simplified motion of the lifting of a foot, in which essential elements of humanoid motion are incorporated.
A framework is developed, with which step times of Human Muscle (HM), ElectroMechanical Actuators (EMAs) and Hydraulic Actuators (HAs) are computed. While EMAs are often thought to have limited potential for humanoid applications, step times are similar to those of muscle. Supplemented with recent examples of successful EMA implementations, step time demonstrates the potential of EMAs. Furthermore, HA capability is also demonstrated, as step times can obtain lower values than those of HM and EMA.
Step time as developed in this thesis appears to be a powerful tool for comparing vastly different actuators. It provides a one-dimensional measure related to humanoid stability, evaluated directly for each specific actuator.