Over the past decades industrial robot manip- ulators have expanded throughout the automation landscape with their high degree of repeatability and accuracy. For safety reasons these robots are not allowed to share their workspace along with humans, which limits the study of various tasks that can be performed by a human-robot collaboration. To circumvent this limitation, a new kind of variable stiffness series elastic actuator (or vSEA) is proposed as a joint actuator. This vSEA has an ability to servo-control its joint stiffness, and provide a measure of the instantaneous joint torque. These unique abilities play a crucial role in an intelligent robot control design that can scale down the risk factors in a field where humans and robots collaborate. The intrinsic soft joint in vSEA provides a higher bandwidth of compliance to better mitigate the effects of collision. This article dissects the use of the vSEA joint in a robot manipulator, and its potential use in a human- robot collaborative workspace.Various studies have shown that soft joints play a crucial role in mitigating impact related injuries. With an appropriate control scheme, a rigid joint can also be compliant, However, the extent to which this is possible is&nbsp;limited&nbsp;due to the control loop, and the inertia tensor of the manipulator. These properties can also be used in exoskeleton&nbsp;and other related applications.The peak mean contact pressure and the total transferred energy are discussed as the dominant parameters of bruise tolerance by blunt impact in order to obtain safety criteria for robot design in human-robot interaction. The results show that there is no capillary damage if the peak mean contact pressure is less than 1.3&nbsp;M P a&nbsp;and the total transferred energy is less than&nbsp;87&nbsp;kJ/m2.A series spring in an actuator is not absolutely necessary to determine the torque that is being transferred. It is well known that the torque produced by a motor is proportional to the magnitude of current flowing through it, thus the torque can be computed by measuring the current flowing through the motor, and comparing it with a state observer. While this is a very simple solution, practically speaking this approach suffers from several drawbacks. One such drawback is the poor toque resolution. The robotic joint actuator has a high gear ratio gear box. This provides a great mechanical advantage, however fluctuations in the motor current due to an external disturbance are reduced by the gear ratio. In other words, the problem is lack of current sensing/control resolution when masked by the high gear ratio. The least significant bit of current control/sensing at the motor is multiplied by the ratio. Thus, relying on the motor’s current results in a poor torque resolution.The fundamental soft joint of a manipulator limits the maximum impact force it can exert. The impact force can be controlled by adjusting the stiffness. The ability to adjust the stiffness allows a robot adapt its behavior during its operation. The proposed vSEA coupler prototype is capable of adjusting its stiffness to meet a given collaborative work (refer to Section II for more details) requirement. By adjusting the stiffness and the operating joint speed, a manipulator can prioritize among one of the aspects out of accuracy, safety and compliance.

Variable Stiff Revolute Joint for Compliant Robot

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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