Open continuum robotics-one actuation module to create them all

Experiments on physical continuum robot are the gold standard for evaluations. Currently, as no commercial continuum robot platform is available, a large variety of early-stage prototypes exists. These prototypes are developed by individual research groups and are often used for a single publication. Thus, a significant amount of time is devoted to creating proprietary hardware and software hindering the development of a common platform, and shifting away scarce time and efforts from the main research challenges. We address this problem by proposing an open-source actuation module, which can be used to build different types of continuum robots. It consists of a high-torque brushless electric motor, a high resolution optical encoder, and a low-gear-ratio transmission. For this article, we create three different types of continuum robots. In addition, we illustrate, for the first time, that continuum robots built with our actuation module can proprioceptively detect external forces. Consequently, our approach opens untapped and under-investigated research directions related to the dynamics and advanced control of continuum robots, where sensing the generalized flow and effort is mandatory. Besides that, we democratize continuum robots research by providing open-source software and hardware with our initiative called the Open Continuum Robotics Project, to increase the accessibility and reproducibility of advanced methods.

Towards Scalable Strain Gauge-Based Joint Torque Sensors

During recent decades, strain gauge-based joint torque sensors have been commonly used to provide high-fidelity torque measurements in robotics. Although measurement of joint torque/force is often required in engineering research and development, the gluing and wiring of strain gauges used as torque sensors pose difficulties during integration within the restricted space available in small joints. The problem is compounded by the need for a scalable geometric design to measure joint torque. In this communication, we describe a novel design of a strain gauge-based mono-axial torque sensor referred to as square-cut torque sensor (SCTS), the significant features of which are high degree of linearity, symmetry, and high scalability in terms of both size and measuring range. Most importantly, SCTS provides easy access for gluing and wiring of the strain gauges on sensor surface despite the limited available space. We demonstrated that the SCTS was better in terms of symmetry (clockwise and counterclockwise rotation) and more linear. These capabilities have been shown through finite element modeling (ANSYS) confirmed by observed data obtained by load testing experiments. The high performance of SCTS was confirmed by studies involving changes in size, material and/or wings width and thickness. Finally, we demonstrated that the SCTS can be successfully implementation inside the hip joints of miniaturized hydraulically actuated quadruped robot-MiniHyQ. This communication is based on work presented at the 18th International Conference on Climbing and Walking Robots (CLAWAR).

Stabilization of the Fast Modes of a Flexible-Joint Robot

In this work the robot is assumed to be an open kinematic chain with only revolute joints. Each joint is modeled as a linear torsional spring. The model equations consist of two coupled dy namic systems, one representing the usual rigid body or slow dynamics and the other the fast dynamics introduced by the joint flexibility. The model presented in this article is in aform that brings out the influence on the fast subsystem dynamics of the rigid body parameters and the robot geometry. The model clearly shows the effect that link and drive parameters have on the dynamics of the fast subsystem.

It is shown that under certain assumptions there exists a decen tralized velocity control law that asymptotically stabilizes the fast subsystem dynamics. In general this control law is gain scheduled. For sufficiently small drive inertias there always exists a fixed de centralized control law that will asymptotically stabilize the fast dynamics. This is true even for large drive ratios. For sufficiently large drive inertias it may not be possible to use a fixed decentral ized control law. Under certain conditions a gain-scheduled velocity feedback law can be designed to give attractive pole damping factors. Some examples are given to illustrate these ideas.

Design of a Hollow Hexaform Torque Sensor for Robot Joints

This work describes the design of a new one-axis torque sensor. It achieves the conflicting requirements of high stiffness for all six force and torque components, high sensitivity for the one driving torque of interest, and yet very low sensitivity for the other five force/torque components. These properties, combined with its donut shape and small size, make this sensor an ideal choice for direct-drive robotic applications. Experimental data validate the basic design ideas underlying the sensor’s geometry, the finite element model used in its optimization, and the advertised performance.

Static Friction Effects During Hard-on-Hard Contact Tasks and Their Implications for Manipulator Design

In this article, we present the results and implications of our experimental study into the effects of static, nonlinear, fric tion during hard-on-hard contact tasks where making a stable contact is problematic. Experiments have been conducted with a PUMA 560 manipulator to address the hard-on-hard con tact stability problem and to stress the importance of damping (active-velocity feedback, and passive-nonlinear friction) for hard-on-hard applications typified by robotic drilling. Experi mental results reveal that during hard-art-hard contact, active damping is inoperative because of quantization, leaving only passive damping to stabilize the system. However, passive damping characteristics vary between joints. Most importantly, the last three joints of the manipulator, which are more af fected by contact force disturbances than others. lack sufficient passive damping, which we argue should match their respec tive joint stiffnesses to achieve contact stability. Therefore, we conclude that manipulator joints with low passive damping should have high-resolution encoders for velocity feedback. These are required to stabilize a force control system making a hard-on-hard constrained contact. The lower the joint friction, the higher the resolution required. This constitutes the main contribution of this article to manipulator design.

Human Interface, Automatic Planning, and Control of a Humanoid Robot

This paper presents an integrated robotic system consisting of human interfaces, motion- and grasp-planning algorithms, a controller, a graphical simulator, and a humanoid robot with over 60 joints. All of these subsystems are integrated in a coordinated fashion to enable the robot to perform a commanded task with as much autonomy as possible. The highest level of our system is the human interfaces, which enable a user to specify tasks conveniently and efficiently. At the mid-level, several planning algorithms generate motions of the robot body, arms, and hands automatically. At the lowest level, the motor controllers are equipped with both a position controller and a compliant motion controller to execute gross motions and contact motions, respectively. The main contributions of our work are the large-scale integration and the development of the motion planners for a humanoid robot. A hierarchical integration scheme that pre serves the modularities of the human interfaces, the motion planners, and the controller has been the key for the successful integration. The set of motion planners is developed systematically so as to coordi nate the motions of the body, arms, and hands to perform a large variety of tasks.

Frequency-Domain Consequences of Low-Velocity Friction: The Nonminimum-Phase Behavior of Geared Transmissions

We make several experimentally based observations that enhance the understanding of the dynamics of geared robot drives. We show that: (1) at low velocities, a gear transmission can behave as a nonminimum-phase system; (2) this nonminimum phase behavior is intimately related to the dynamics of friction at gear-tooth contact; and (3) by virtue of compliance in bearings and supports, gear drives do not have a constant transmission ratio over all frequencies, but in stead show step changes in transmission ratio. These findings guide the development of a new dynamic model of a gear transmission that captures these salient characteristics and makes explicit the different torque-transmission modes. We remark on the implications of these findings for robot force control.

Robust Control of Robot Manipulators Based on Joint Torque Sensor Information

This article is concerned with robust control of robot manip ulators in the case where joint torque sensors are available. First, we derive a dynamic equation of the manipulator with joint torque sensors that explicitly expresses a nonlinear mul tivariable structure. This dynamic equation makes it possible to construct the control systems of the manipulators with joint torque sensors based on a similar method used in the conven tional case without the sensors. Second. based on this dynamic equation, we propose a robust trajectory control scheme that achieves the specified tracking accuracy in the presence of modeling error, including the modeling error of actuator sys tems. The proposed method fully exploits joint torque sensor information to compensate the uncertainty of link and load parameters. Furthermore, an illustrative simulation result is given to show the effectiveness of the proposed control method.

Joint friction estimation for walking bipeds

This paper proposed a new approach for the joint friction estimation of non-slipping walking biped robots. The proposed approach is based on the combination of a measurement-based strategy and a model-based method. The former is used to estimate the joint friction online when the foot is in contact with the ground, while the latter adopts a friction model to represent the joint friction when the leg is swinging. The measurement-based strategy utilizes the measured ground reaction forces (GRF) and the readings of an inertial measurement unit (IMU) located at the robot body. Based on these measurements, the joint angular accelerations and the body attitude and velocity are estimated. The aforementioned measurements and estimates are used in a reduced dynamical model of the biped. However, when the leg is swinging, this strategy is inapplicable. Therefore, a friction model is adopted. Its parameters are identified adaptively using the estimated online friction whenever the foot is in contact. The estimated joint friction is used in the feedback torque control signal. The proposed approach is validated using the full-dynamics of 12-DOF biped model. By using this approach, the robot center of mass (CoM) position error is reduced by 10% which demonstrates the effectiveness of this approach.

A Kinematic Approach to Determining the Optimal Actuator Sensor Architecture for Space Robots

Autonomous space robots will be required for such future missions as the construction of large space structures and repairing disabled satellites. These robots will need to be precisely controlled. However, factors such as manipulator joint/actuator friction and spacecraft attitude control thruster inaccuracies can substantially degrade control system performance. Sensor-based control algorithms can be used to mitigate the effects of actuator error, but sensors can add substantially to a space system’s weight, complexity, and cost, and reduce its reliability. Here, a method is presented to determine the sensor architecture that uses the minimum number of sensors that can simultaneously compensate for errors and disturbance in a space robot’s manipulator joint actuators, spacecraft thrusters, and reaction wheels. The placement and minimal number of sensors is determined by analytically structuring the system into “canonical chains” that consist of the manipulator links and spacecraft with force/torque sensors placed between the space robot’s spacecraft and its manipulators. These chains are combined to determine the number of sensors needed for the entire system. Examples of one- and two-manipulator space robots are studied and the results are validated by simulation.

Simplification of manipulator dynamic formulations utilizing a dimensionless method

This paper presents a new approach for simplifying dynamic equations of motion of robot manipulators by using a nondimensionalization scheme. With this approach the dynamic analysis is done in a nondimensional space. That is, it is required to establish a dimensionless coordinate system in which the dynamic equations of motion of manipulators are formulated. The characteristic parameters of the manipulators are then defined by choosing proper physical quantities as basic units for nondimensionalization. Within the nondimensional space the Lagrange method is applied to the manipulator to obtain a set of general dimensionless equations of motion. This dimensionless dynamic formulation of manipulators leads to an easier way to simplify the dynamic formulation by neglecting insignificant terms using the order of magnitude comparison. The dimensionless dynamic model and its simplified version of PUMA 560 robot are implemented using the proposed approach. It is found that the simplified dynamic model greatly reduces the computation burden of the inverse dynamics. Simulation results also show that the simplified model is extremely accurate. This implies that the proposed nondimensional simplification emethod is reliable.

A control architecture for a mobile heavy-lift precision manipulator with limited sensory information

Mobile robotic manipulators can augment the strength and dexterity of human operators in unstructured environments. Here, the control system for a six degree-of-freedom heavy-lift mobile manipulator for lifting and inserting payloads on the deck of a ship is described. The robotic hardware and the application present several control challenges, including structural resonances, high joint friction that varies with time, limited sensors for measuring the joint friction, complex interaction with the environment, tight tolerances for the insertion tasks, lack of bilateral force feedback of the contact forces, and ship motions. The control system enables an operator to perform insertion tasks using feedback of tactile clues of the manipulator position, and reduces the effects of friction with a combination of sensor-based, adaptive, and model-based methods of friction compensation. The control architecture is validated in simulation and on a laboratory manipulator.

Feedback Linearized Joint Torque Control of a Geared, DC Motor Driven Industrial Robot

Implementation of advanced model based robot control algorithms necessitates that joint actuators be capable of generating com manded joint torques. This capability permits compensation of gravity torques, for example. Relatively recent work reported in the robotics literature has focused on development of load torque sensing and control of robots actuated with permanent magnet DC motors and harmonic drive gear reducers. This development in troduces the possibility of the retrofit of a large class of small- to midsized industrial robots with advanced model based controllers, which require that the commanded torque signal be reproduced at each robot joint. In this paper, the authors establish theoretically that the direct application of computed torque control to a geared, DC permanent magnet actuated robot with local joint load torque controllers does not lead to decoupled dynamics, as is the case with direct drive robots. The resultant dynamics are not decoupled due to the interaction of the computed torque control with the inner joint torque control loops. To achieve decoupled system behavior, a modified computed torque control law is developed that leads to decoupled system dynamics. Experimental results are presented that compare the modified computed torque control with the stan dard computed torque control. The experiments are conducted on a commercial 6-DOF robot that has been retrofitted with joint torque control on the first three joints. Tests have been conducted at a con trol update rate of 1000 Hz. The experimental results illustrate that the modified computed torque controller performance is somewhat better than the conventional computed torque control approach.