Dongjun Shin

A hybrid actuation approach for human-friendly robot design

Safety is a critical characteristic for robots designed to operate in human environments. This paper presents the concept of hybrid actuation for the development of human- friendly robotic systems. The new design employs inherently safe pneumatic artificial muscles augmented with small electrical actuators, human-bone-inspired robotic links, and newly designed distributed compact pressure regulators. The modularization and integration of the robot components enable low complexity in the design and assembly. The hybrid actuation concept has been validated on a two-degree-of-freedom prototype arm. The experimental results show the significant improvement that can be achieved with hybrid actuation over an actuation system with pneumatic artificial muscles alone. Using the manipulator safety index (MSI), the paper discusses the safety of the new prototype and shows the robot arm safety characteristics to be comparable to those of a human arm.

Design and Control of a Bio-inspired Human-friendly Robot

The increasing demand for physical interaction between humans and robots has led to an interest in robots that guarantee safe behavior when human contact occurs. However, attaining established levels of performance while ensuring safety creates formidable challenges in mechanical design, actuation, sensing and control. To promote safety without compromising performance, a human-friendly robotic arm has been developed using the concept of hybrid actuation. The new design employs high-power, low-impedance pneumatic artificial muscles augmented with small electrical actuators, distributed compact pressure regulators with proportional valves, and hollow plastic links. The experimental results show that significant performance improvement can be achieved with hybrid actuation over a system with pneumatic muscles alone. In this paper we evaluate the safety of the new robot arm through experiments and simulation, demonstrating that its inertia/power characteristics surpass those of previous human-friendly robots we have developed.

Air muscle controller design in the distributed macro-mini (DM 2 ) actuation approach

Recently, on the base of distributed macro-mini actuation approach (DM2), a new robotic manipulator with hybrid actuation, air muscles-DC motor, has been developed. Among existing actuators, the hybrid actuation employs air muscles because they represent an advantageous tradeoff of performance and safety, due to their power/weight ratio and inherent compliance. The air muscles, however, are limited in bandwidth and their behavior is highly nonlinear. In order to overcome these limitations, the paper presents a torque control strategy based on a pair of differentially connected force-controlled air muscles. This controller was implemented and evaluated on a single joint testbed, first by itself and then as macro component into the Macro-Mini control strategy.

Capacitive skin sensors for robot impact monitoring

A new generation of robots is being designed for human occupied workspaces where safety is of great concern. This research demonstrates the use of a capacitive skin sensor for collision detection. Tests demonstrate that the sensor reduces impact forces and can detect and characterize collision events, providing information that may be used in the future for force reduction behaviors. Various parameters that affect collision severity, including interface friction, interface stiffness, end tip velocity and joint stiffness irrespective of controller bandwidth are also explored using the sensor to provide information about the contact force at the site of impact. Joint stiffness is made independent of controller bandwidth limitations using passive torsional springs of various stiffnesses. Results indicate a positive correlation between peak impact force and joint stiffness, skin friction and interface stiffness, with implications for future skin and robot link designs and post-collision behaviors.

Design methodologies of a hybrid actuation approach for a human-friendly robot

Determining design parameters is often a challenging procedure, especially in human-friendly robot design due to competition between robot safety and performance. Presenting an analytical model of hybrid actuation for human-friendly robot development, this paper proposes design methodologies to improve performance factors such as range of motion, payload, and acceleration while maintaining the safety factor of effective inertia. The optimized parameters for various design requirements have been provided for 1DOF and 2DOF applications. Comparison between current design parameters and the optimized parameters for a current platform shows the performance improvement. In future work this research will be extended to systems with higher degrees of freedom.

Analysis of torque capacities in hybrid actuation for human-friendly robot design

A formidable challenge in the development of human-friendly robots is to simultaneously achieve desired levels of performance and safety. To address this issue, a hybrid actuation concept has been proposed, combining large, low impedance actuators and small, high-frequency actuators. However, the determination of design parameters remains a challenge, as stiffness and electrical motor torque capacity simultaneously affect both the control performance and the safety of the manipulator. Using analytical models of the hybrid actuation system, we propose a methodology to achieve a combination of low impedance and high control bandwidth. The optimized parameters are verified and compared with previous ones through simulation and experimentation.

Variable radius pulley design methodology for pneumatic artificial muscle-based antagonistic actuation systems

There is a growing interest in utilizing pneumatic artificial muscles (PAMs) as actuators for human-friendly robots. However, several performance drawbacks prevent the widespread use of PAMs. Although many approaches have been proposed to overcome the low control bandwidth of PAMs, some limitations of PAMs such as restricted workspace and torque capacity remain to be addressed. This paper analyzes the limitations of conventional circular pulley joints and subsequently proposes a design methodology to synthesize a pair of variable radius pulleys to improve joint torque capacity over a large workspace. Experimental results show that newly synthesized variable radius pulleys significantly improve position tracking performance in the enlarged workspace.

Circular Pulley Versus Variable Radius Pulley: Optimal Design Methodologies and Dynamic Characteristics Analysis

Human-centered robotics has received growing interest in low-impedance actuations. In particular, pneumatic artificial muscles (PAMs) provide compliance and high force-to-weight ratio, which allow for safe actuation. However, several performance drawbacks prevent PAMs from being more pervasive. Although many approaches have been proposed to overcome the low control bandwidth of PAMs, some limitations of PAMs, such as restricted workspace and torque capacity, remain to be addressed. This paper analyzes the characteristics and limitations of PAMs-driven joints and subsequently provides an optimization strategy for circular pulleys (CPs) in order to improve joint torque capacity over a large workspace. In addition to CPs, this paper proposes a design methodology to synthesize a pair of variable radius pulleys (VRPs) for further improvement. Simulation and experimental results show that newly synthesized VRPs significantly improve torque capacity in the enlarged workspace without loss of dynamic performance. Finally, the characteristics of CPs and VRPs are discussed in terms of physical human-robot interaction.

A new hybrid actuation scheme with artificial pneumatic muscles and a magnetic particle brake for safe human-robot collaboration

Interest in the field of human-centered robotics continues to grow, particularly in utilizing pneumatic artificial muscles (PAMs) for close human–robot collaborations. Addressing the limited control performance of PAMs, we proposed a hybrid actuation scheme that combines PAMs (macro) and a low-inertia DC motor (mini). While the scheme has shown significantly improved control performance and robot safety, a small DC motor has difficulties in handling the large stored energies of the PAMs, particularly for large changes in initial load due to PAM failure. In order to further improve robot safety, we develop a new hybrid actuation scheme with PAMs (macro) and a particle brake (mini). This design allows for a higher torque-to-weight ratio and inherently stable energy dissipation. Addressing optimal mini actuation selection between a motor and a brake, and a control strategy for PAMs and a brake, we conducted comparative studies of hybrid actuations with (1) a DC motor and (2) a brake for concept validation. Experimental comparisons show that the hybrid actuation with PAMs and a brake provides higher energy efficiency for control bandwidths under 2 Hz, and more effective reduction of large impact forces due to the brake’s high torque capacity and passive energy dissipation.

Instantaneous stiffness effects on impact forces in human-friendly robots

Joint stiffness plays an important role in both safety and control performance, particularly in human-friendly robots using artificial pneumatic muscles. Due to the limited control bandwidth of pneumatic muscles, stiffness characteristics and their effects on safety in the frequency domain should be taken into account. This paper introduces the concept of instantaneous stiffness and validates its model with the Stanford Safety Robot (S2ρ. The potential effects of instantaneous stiffness on safety is explored through experimental comparison of peak impact accelerations under various impact conditions. Instantaneous stiffness demonstrates different effects on the impact acceleration depending on impact velocity and controller gain. Finally, the paper discusses the stiffness characteristics as a guideline for design and control to improve the robot safety while maintaining the control performance.