Advancements in automation have pushed forward the development of collaborative robots, which are designed to work together with humans. The majority of actual commercial collaborative robots use geared motors with some kind of torque sensing, and their performance is directly related to how well torque is controlled. However, imperfections in the gear fabrication causes transmission error in the geared motor, producing undesired vibrations which affect the torque control performance. In this study, an integratorless 2DOF (two degrees-of-freedom) torque control scheme is proposed to improve the torque response in such case. The proposed control scheme uses first-order FPIDO (force-position-integrated disturbance observer) which nominalizes and achieves motor side acceleration control, allowing to use the natural mechanical dynamics of the motor side as an integrator instead of implementing it on the control system. This is combined with a derivative-type torsional disturbance observer (DTDOb), which uses torque sensor information to compensate all torsional disturbances and nominalize the torsion torque dynamics, achieving 2DOF (Two degrees of freedom) torque control. The effectiveness of the proposed methodology is verified by simulation and experimental results.

This paper proposes a load-side acceleration control scheme for geared motors affected by unknown backlash and friction. A backlash-deactivated disturbance observer (BDDOb) is proposed to achieve load-side acceleration control while ensuring quick traversing of backlash. The BDDOb works by executing conditional estimation-compensation of load disturbances, and is driven by a backlash detection scheme based on torsion torque sensing which does not require any information on the backlash gap width. In addition, a variable-order force-position-integrated disturbance observer (VOFPIDO) is proposed to quickly compensate nonlinear friction without requiring information of the nonlinear friction magnitude by applying velocity-dependent switching. Experimental results prove the effectiveness of the proposed method.

This paper focuses on single inertialization on the load-side parameter of two-inertia system and employs the equivalent disturbance compensator (EDC). Conventional EDC is designed in case of the plant system without friction; thus, the back-forward drivability control using the EDC is affected by friction. In this paper, we propose the back-forward drivability control using EDC and DOB for single inertialization of two-inertia system. DOB is used to compensate the friction and the EDC is applied to not only improve the back-forward drivability, but also suppress the vibration on load-side output. This proposal consists of basic actuator and force sensor and it is available to general robot actuator that uses current sensor, motor encoder, and force sensor. The effectiveness of proposed back-forward drivability control is verified through the numerical simulation.

Good forward-drivability characteristics are essential for applications such as position, velocity, and force control. Geared motors, usually employed in industrial robots, require lubrication in their gears, and consequently, some form of sealing is needed to avoid leaking. While oil seals prevent leaking, they increase nonlinear frictional effects, which result in stick-slip during velocity zero-crossings, thereby deteriorating the forward-drivability. This paper proposes a stick-slip suppression method based on a velocity-driven N-order stick compensator (VDNSC), which works as an order-variable friction compensator formed by the velocity-driven switching of the internal components of a high order Q-filter. Experiments with torque and velocity control confirm that the proposed method improves the forward-drivability, allowing for more precise motion even under large friction.