Abstract
Cable-driven transmissions are used widely in robotic applications. However, design variables and parameters of this kind of transmission remain under study, both analytically and experimentally. In this paper, an experimental test bench to evaluate the behavior of medium-low power pulley-cable transmissions is presented. The design of the test bench allows manipulating variables such as dimensions, external load, speed, and cable tension. The system consists mainly of a brushless direct current (DC) motor, two load cells to measure the mechanical reactive force in the motor, two dismountable pulleys, two drums, a perforated disk, and several masses that provide the load and the inertial load, and electronic modules to control the speed and position of the pulley. Special attention was paid to the calibration of the load cells, focused in compensating the effect of creep. Validation tests were carried out in order to evaluate the device design. Next, pilot experiments were performed to estimate the friction behavior in the transmission. Preliminary results suggest that the friction in the transmission is largely governed by the friction behavior of the bearings.
Highlights
A mechanical transmission is used to transmit power over distance and change torque or speed from input to output
When developing robotic systems that aim towards physical human–robot interaction, it is desired to count on devices that have a high open-loop back-drivability
Further evaluation of the transmission operating conditions is required to establish a suitable friction model. These first experiments made it possible to evaluate the test bench and establish a baseline for the experiments to be carried out. These first experiments were conducted at a constant speed and no payload, the test bench is intended to be used to evaluate friction behavior which includes some additional factors that could contribute to friction fluctuation, such as temperature, transmission ratio, and payload
Summary
A mechanical transmission is used to transmit power over distance and change torque or speed from input to output. When developing robotic systems that aim towards physical human–robot interaction (pHRI), it is desired to count on devices that have a high open-loop back-drivability In applications such as exoskeleton construction, pinion-based articulations prove to be difficult to back-drive, requiring a variety of force sensors that translate the operator intentions to the machine, and enabling an ergonomic interaction with it [5,6]. Previous works on cable-driven transmissions [28,29,30,31,32] have involved the use of test benches developed for each research project, but the majority of these systems have issues that need to be considered in more detail to evaluate friction. Some differential features of this new test bench from the previous ones are: (i) the ability to evaluate the effect of more than one turn of cable wrapping, (ii) the chance to change input shaft and pulley to evaluate different transmission ratios and possible materials, with and without groves, and (iii) here there is no transmission behind the cable transmission
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