Optimization free control and ground force estimation with momentum observer for a multimodal legged aerial robot

TL;DR

A control framework is proposed that bypasses heavy onboard computers by using an optimization-free Explicit Reference Governor that incorporates external thruster forces from an attitude controller that incorporates external thrusters from an attitude controller.

Abstract

Legged-aerial multimodal robots can make the most of both legged and aerial systems. In this paper, we propose a control framework that bypasses heavy onboard computers by using an optimization-free Explicit Reference Governor that incorporates external thruster forces from an attitude controller. Ground reaction forces are maintained within friction cone constraints using costly optimization solvers, but the ERG framework filters applied velocity references that ensure no slippage at the foot end. We also propose a Conjugate momentum observer, that is widely used in Disturbance Observation to estimate ground reaction forces and compare its efficacy against a constrained model in estimating ground reaction forces in a reduced-order simulation of Husky.

Figures (10)

• Figure 1: Husky fitted with the Electric Ducted fans. Each leg is actuated with 3 BLDC motors with ELMO motor drivers
• Figure 2: Depiction of the robot and HROM parameters. $\bm p_{h,i}, \bm p_{t,i}$ and $l_i$ are the locations of the hip joint, thruster position and the leg length of the $i^{th}$ leg/location
• Figure 3: Flowchart of Control and estimation framework. The momentum observer has access to the control input from thruster attitude controller and the state of the simulation
• Figure 4: Explicit Reference Governor (ERG) manipulates the applied reference states $\bm x_w$ to be as close as possible to the desired reference $\bm x_r$ without violating the constraint equation $\bm h_w\geq 0$. For the full algorithm refer to sihite_optimization-free_2021 from Sihite et al.
• Figure 5: Model used in the ERG controller to estimate the ground reaction forces. The ERG then manipulates the reference to make sure the new applied reference is within the constraint admissible set