Nonlinear In-situ Calibration of Strain-Gauge Force/Torque Sensors for Humanoid Robots

High force/torque (F/T) sensor calibration accuracy is crucial to achieving successful force estimation/control tasks with humanoid robots. State-of-the-art affine calibration models do not always approximate correctly the physical phenomenon of the sensor/transducer, resulting in inaccurate F/T measurements for specific applications such as thrust estimation of a jet-powered humanoid robot. This paper proposes and validates nonlinear polynomial models for F/T calibration, increasing the number of model coefficients to minimize the estimation residuals. The analysis of several models, based on the data collected from experiments with the iCub3 robot, shows a significant improvement in minimizing the force/torque estimation error when using higher-degree polynomials. In particular, when using a 4th-degree polynomial model, the Root Mean Square error (RMSE) decreased to 2.28N from the 4.58N obtained with an affine model, and the absolute error in the forces remained under 6N while it was reaching up to 16N with the affine model

Momentum-Based Extended Kalman Filter for Thrust Estimation on Flying Multibody Robots

Effective control design of flying vehicles requires a reliable estimation of the propellers’ thrust forces to secure a successful flight. Direct measurements of thrust forces, however, are seldom available in practice and on-line thrust estimation usually follows from the application of fusion algorithms that process on-board sensor data. This letter proposes a framework for the estimation of the thrust intensities on flying multibody systems that are not equipped with sensors for direct thrust measurement. The key ingredient of the proposed framework is the so-called centroidal momentum of a multibody system, which combined with the propeller model. It enables the design of Extended Kalman Filters (EKF) for on-line thrust estimation. The presented approach tackles the additional complexity in thrust estimation due to the possibly large number of degrees of freedom of the system and uncertainties in the propeller model. For instance, a covariance scheduling approach based on the turbines RPM error is proposed to ensure a reliable estimation even in case of turbine failures. Simulations are presented to validate the proposed algorithm during robot flight. Moreover, an experimental setup is designed to evaluate the accuracy of the estimation algorithm using iRonCub, a jet-powered humanoid robot, while standing on the ground

Nonlinear Model Identification and Observer Design for Thrust Estimation of Small-scale Turbojet Engines

Jet-powered vertical takeoff and landing (VTOL) drones require precise thrust estimation to ensure adequate stability margins and robust maneuvering. Small-scale turbojets have become good candidates for powering heavy aerial drones. However, due to limited instrumentation available in these turbojets, estimating the precise thrust using classical techniques is not straightforward. In this paper, we present a methodology to accurately estimate the online thrust for the small-scale turbojets used on the iRonCub - an aerial humanoid robot. We use a grey-box method to capture the turbojet system dynamics with a nonlinear state-space model based on the data acquired from a custom engine test bench. This model is then used to design an extended Kalman filter that estimates the turbojet thrust only from the angular speed measurements. We exploited the parameter estimation algorithm to ensure that the EKF gives smooth and accurate estimates even at engine failures. The designed EKF was validated on the test bench where the mean absolute error in estimated thrust was found to be within 2% of rated peak thrust

Uma proposta para o processo de definição do estoque de segurança de itens comprados em empresas que fabricam produtos complexos sob encomenda A proposal for the process of definition of safety stock of purchased items for make-to-order companies that manufacture complex products

A definição do Estoque de Segurança costuma seguir critérios puramente qualitativos, muitas vezes sem nenhum fundamento ou puramente quantitativos, em que o conhecimento de pessoas-chave não é aproveitado. Com base na experiência prática do autor e com o apoio da literatura, propõe-se neste trabalho um modelo para a definição deste parâmetro com base nestes dois critérios, conjuntamente. Espera-se que ele possa subsidiar as decisões relacionadas à definição do Estoque de Segurança de itens comprados em empresas que fabricam produtos complexos sob encomenda, nas quais este tipo de item responde pela maior parte do custo do produto final. Incertezas existentes nos ambientes interno e externo destas empresas tornam necessário este tipo de estoque em muitas situações.<br>The definition of Safety Stock usually follows only qualitative criteria, which most of the times do not have any conceptual foundation, or only quantitative criteria, which does not make actual good use of the knowledge of essential people involved. Based on the author's practical experience and with the support of the literature, this paper presents a model for the definition of this parameter based on these two criteria, at the same time. It is expected it can support the decisions related to the definition of Safety Stock of purchased items for make-to-order companies that manufacture complex products, in which this kind of item accounts for most of the cost of the final product. Uncertainties in the internal and external environment of these companies make this kind of stock necessary in a lot of situations