Dalong Gao

A Friendly Beast of Burden: A Human-Assistive Robot for Handling Large Payloads

This article presents a novel robotic assistive device for the handling of large payloads. The design of the robot is based on the application of the following fundamental mechanical principles: inertia is minimized, a parallel closed-loop cable/belt routing system is used to kinematically decouple the transmission from fixed actuators and to the end-effector, and variable static balancing is used to minimize the actuation forces required for vertical motion. As a result, the device requires only low power, thereby improving safety, and can be operated manually, even in the event of a power failure (with minimum backup power for brake release). A novel force/torque sensor is also introduced along with a control algorithm based on variable admittance that provides a very intuitive interface for physical human-robot cooperation. Finally, a full-scale prototype integrating all of the above concepts is presented.

A New One-Sided Joining Process for Aluminum Alloys: Friction Stir Blind Riveting

Friction stir blind riveting is a new joining process for one-sided joining (compared with the two-sided access required for, for example, self-piercing riveting) of aluminum alloys, which eliminates the need to predrill a hole for rivet insertion. A blind rivet rotating at high speed is brought into contact with the workpieces, thereby generating frictional heat between the rivet and the workpiece, which softens the workpiece material and enables the rivet to be driven into the workpieces under reduced force. Once fully inserted, the blind rivet is upset using the internal mandrel (as in a conventional blind riveting process) to fasten the workpieces together. Our study showed that friction stir blind riveting process can be carried out over a wide range of operating parameters. The resulting joints show consistent strength under tensile load with minimal influence of changes in operating parameters. The robustness of the process against variations in operating conditions shows that the process can be carried out without high-end equipment and without requiring precise initial setup. It also suggests that the process is feasible for rapid joint fabrication in volume production. Further study revealed superior static and fatigue strength from the friction stir blind riveting process, when compared with conventional spot welding, which suggests potential for reduction in the number of joints required in a structure.

Stable and Intuitive Control of an Intelligent Assist Device

Safety and dependability are of the utmost importance for physical human-robot interaction due to the potential risks that a relatively powerful robot poses to human beings. From the control standpoint, it is possible to improve safety by guaranteeing that the robot will never exhibit any unstable behavior. However, stability is not the only concern in the design of a controller for such a robot. During human-robot interaction, the resulting cooperative motion should be truly intuitive and should not restrict in any way the human performance. For this purpose, we have designed a new variable admittance control law that guarantees the stability of the robot during constrained motion and also provides a very intuitive human interaction. The former characteristic is provided by the design of a stability observer while the latter is based on a variable admittance control scheme that uses the time derivative of the contact force to assess human intentions. The stability observer is based on a previously published stability investigation of cooperative motion which implies the knowledge of the interaction stiffness. A method to accurately estimate this stiffness online using the data coming from the encoder and from a multiaxis force sensor at the end effector is also provided. The stability and intuitivity of the control law are verified in a user study involving a cooperative drawing task with a 3 degree-of-freedom (dof) parallel robot as well as in experiments performed with a prototype of an industrial Intelligent Assist Device.

Improved stability of haptic human–robot interfaces using measurement of human arm stiffness

Necessary physical contact between an operator and a force feedback haptic device creates a coupled system consisting of human and machine. This contact, combined with the natural human tendency to increase arm stiffness to attempt to stabilize its motion, can reduce the stability of the system. This paper proposes a method to increase stability on demand while maintaining speed and performance. Operator arm stiffness is not directly measurable, so controllers cannot typically account for this issue. The causes of arm end-point stiffness are examined as related to system stability, and a method for estimating changes in arm stiffness based on arm muscle activity was designed to provide a robotic controller with additional information about the operator. This was accomplished using electromyograms (EMGs) to measure muscle activities and estimating the level of arm stiffness, which was used to adjust the dynamic characteristics of an impedance controller. To support this design, the correlation between EMGs and arm stiffness was validated experimentally. Further experiments characterized the effects of the designed system on operator performance. This showed increased stability and faster, more accurate movements using the compensating system. Such a system could be used in many applications, including force assisting devices in industrial facilities.

Measurement of Muscle Stiffness to Improve Stability of Haptic Human-Robot Interfaces

Haptic devices require physical contact between operator and machine, using force feedback and creating a coupled system. Human contact reduces stability due to the response of human operators to stiffen the arm to stabilize the system, leading to a less stable system. Controllers cannot account for this, as operator stiffness is not measurable. This research examined the decreased stability due to increased operator arm stiffness and designed a system to compensate by providing the controller with additional information about the environment. Operator arm stiffness was estimated by measuring muscle activity using EMGs, then the dynamic characteristics of an impedance controller were adjusted according. The design is discussed and experimentally validating, showing increased stability and higher performance. Based on the results, an advanced probabilistic model of operator actions is explored for its applicability to enhance the system. Such a system could be used in many applications, including force assisting devices in industrial facilities.Copyright

Closed-Loop Transmission Routings for Cartesian SCARA-Type Manipulators

SCARA-type manipulators are widely used in industry. SCARA motions can be produced by serial SCARA arms or by Cartesian rail-mounted mechanisms, such as in overhead cranes. This paper focusses on the latter mechanisms which are referred to here as Cartesian SCARA-type manipulators. When such mechanisms are actuated, the motors are generally moving, which increases the inertia of the system. In this paper, several closed-loop cable/belt routings are proposed in order to allow the actuation of Cartesian SCARA-type manipulators from four actuators fixed to the ground. Then, the kinematic analysis of these routings is performed, which reveals that decoupled actuation is possible and leads to great advantages regarding the power needed. Finally, preferred routings are selected and an example of application is given.Copyright