STUDY OF DOMINANT PERFORMANCE-CHARACTERISTICS IN ROBOT TRANSMISSIONS

Six different transmission types suitable for robotic manipulators wer e compared in an experimental and theoretical study. Single-degree-of- freedom mechanisms based on the different transmissions were evaluated in terms of force control performance, achievable bandwidth, and stab ility properties in hard contact tasks. Transmission types considered were (1) cable reducer, (2) harmonic drive, (3) cycloidal disk reducer , (4) cycloidal cam reducer, (5) ball reducer, and (6) planetary/cyclo idal gear head. Open loop torque following error, attenuation and phas e lag, and closed loop bandwidth and stability margin were found to be severely dominated by levels of inertia, stiffness distribution and v ariability, stiction, coulomb and viscous friction, and ripple torque. These aspects were quantified and shown to vary widely among all tran smissions tested. The degree of nonlinearity inherent in each transmis sion affected its open and closed loop behavior directly, and limited the effectiveness of controller compensation schemes. Simple transmiss ion models based on carefully measured transmission characteristics ar e shown to predict stability margins and achievable force-control band widths in hard contact to within a 5 to 15 percent error margin.