Michael R. Zinn

A New Actuation Approach for Human Friendly Robot Design

In recent years, many successful robotic manipulator designs have been introduced. However, there remains the challenge of designing a manipulator that possesses the inherent safety characteristics necessary for human-centered robotics. In this paper, we present a new actuation approach that has the requisite characteristics for inherent safety while maintaining the performance expected of modern designs. By drastically reducing the effective impedance of the manipulator while maintaining high-frequency torque capability, we show that the competing design requirements of performance and safety can be successfully integrated into a single manipulation system.

Playing it safe [human-friendly robots]

We have presented a new actuation concept for human-friendly robot design, referred to as DM/sup 2/. The new concept of DM/sup 2/ was demonstrated on a two-degree-of-freedom prototype robot arm that we designed and built to validate our approach. The new actuation approach substantially reduces the impact loads associated with uncontrolled manipulator collision by relocating the major source of actuation effort from the joint to the base of the manipulator. The emerging field of human-centered robotics focuses on application such as medical robotics and service robotics, which require close interaction between robotic manipulation systems and human beings, including direct human-manipulator contact. As a result, this system must consider the requirements of safety. To achieve safety we must employ multiple strategies involving all aspects of manipulator design.

Mechanics Modeling of Tendon-Driven Continuum Manipulators

Continuum robotic manipulators articulate due to their inherent compliance. Tendon actuation leads to compression of the manipulator, extension of the actuators, and is limited by the practical constraint that tendons cannot support compression. In light of these observations, we present a new linear model for transforming desired beam configuration to tendon displacements and vice versa. We begin from first principles in solid mechanics by analyzing the effects of geometrically nonlinear tendon loads. These loads act both distally at the termination point and proximally along the conduit contact interface. The resulting model simplifies to a linear system including only the bending and axial modes of the manipulator as well as the actuator compliance. The model is then manipulated to form a concise mapping from beam configuration-space parameters to n redundant tendon displacements via the internal loads and strains experienced by the system. We demonstrate the utility of this model by implementing an optimal feasible controller. The controller regulates axial strain to a constant value while guaranteeing positive tendon forces and minimizing their magnitudes over a range of articulations. The mechanics-based model from this study provides insight as well as performance gains for this increasingly ubiquitous class of manipulators.

A new actuation approach for human friendly robot design

Many successful robotic manipulator designs have been introduced. However, there remains the challenge of designing a manipulator that possesses the inherent safety characteristics necessary for human-centered robotics. In this paper, we present a new actuation approach that has the requisite characteristics for inherent safety while maintaining the performance expected of modern designs. By drastically reducing the effective impedance of the manipulator while maintaining high frequency torque capability, we show that the competing design requirements of performance and safety can be successfully integrated into a single manipulation system.

Towards closed loop control of a continuum robotic manipulator for medical applications

Robotic catheters are gaining widespread use in the medical community for cardiac, neurological and other surgical interventions. However, many of the catheters used in these procedures exhibit non-linear behavior, and thus present many difficulties in implementing effective open-loop control. Systems such as this have been shown to benefit from closed-loop control, however very little investigation has been done into 3D closed loop control of this class of manipulators. Initial investigations by the authors have shown greatly improved positioning accuracy and response with closed-loop control based on feedback from an electromagnetic localization sensor. This paper describes the control approach and experimental results, and provides a robotic catheter system overview.

An evaluation of closed-loop control options for continuum manipulators

Continuum manipulators are gaining widespread acceptance in commercial robotics, particularly in the medical field, where their compliance allows a large benefit for patient safety. However, this compliance also makes precise position control of these manipulators quite difficult. This paper presents two closed-loop control implementations applied to a small scale continuum manipulator. These implementations are both based on manipulator tip position feedback from an electromagnetic sensor. The command tracking and disturbance rejection properties of the two control implementations are shown to be approximately equivalent, and provide improved position control when compared to open-loop control, without sacrificing system stability.

Large Workspace Haptic Devices - A New Actuation Approach

Large workspace haptic devices have unique requirements, requiring increased power capabilities along with increased safety considerations. While there are numerous haptic devices available, large workspace systems are hampered by the limitations of current actuation technology. To address this, the Distributed Macro-Mini (DM2) actuation method has been applied to the design of a large workspace haptic device. In this paper, the DM2 method is described and we present experimental results which demonstrate its effectiveness. Finally, the control design is presented along with a discussion of the unique challenges associated with its robustness.

Effect of Compliance and Travel Angle on Friction Stir Welding With Gaps

This paper presents an investigation of the effects of friction stir weld tool travel angle and machine compliance on joint efficiency of butt welded 5083-H111 aluminum alloy in the presence of joint gaps. Friction stir welds are produced with a CNC mill and an industrial robot at travel angles of 1 deg, 3 deg, and 5 deg with gaps from 0 mm to 2 mm, in 0.5 mm increments. Results indicate that the more rigid mill resulted in higher joint efficiencies than the relatively compliant robot when welding gaps greater than 1 mm with a 3 deg travel angle using our test setup. The results also show that when gaps exceed 1 mm welds made with a travel (tilt) angle of 5 deg are able to generate higher joint efficiencies than welds made with a travel angle of 1 deg and 3 deg. Based on tool geometry and workpiece dimensions, a simple model is presented that is able to estimate the joint efficiency of friction stir welds as a function of gap width, travel angle, and plunge depth. This model can be used as an assistive tool in optimizing weld process parameters and tool design when welding over gaps. Experimental results show that the model is able to estimate the joint efficiency for the test cases presented in this paper.

Actuation Methods For Human-Centered Robotics and Associated Control Challenges

In recent years, many successful robotic manipulator designs have been introduced. However, there remains the challenge of designing a manipulator that possesses the inherent safety characteristics necessary for human-centered robotics. In this paper, we describe recent developments in low impedance actuation that have allowed for improvements in the safety characteristics of human-centered manipulators. In addition, the control challenges unique to the use of low impedance actuation are discussed along with possible control strategies for their successful implementation.

A modeling approach for robotic catheters: effects of nonlinear internal device friction

Recently, robotic surgery systems using passive flexible catheters have been developed for minimally invasive surgical applications – such as in the treatment of atrial fibrillation – where catheter control in the open chamber of the heart is required. The soft, atraumatic construction of these devices help reduce injury to delicate cardiac structures while providing a means of tool placement and control. To provide kinematic and control relationships, various models of continuous catheters have been developed. However, these approaches cannot explain the nonlinear behavior of the catheter when the effect of internal friction is considered. In this paper, we describe a lumped-parameter modeling approach which directly accounts for the effects of internal device friction. The nonlinear model is validated against experimental results from a prototype robotic catheter and is shown to correctly predict the variations in curvature and path-dependent instantaneous behavior observed. Finally, the validated model is used to investigate and describe a set of non-ideal catheter motions observed in practice. Graphical Abstract