Alexander Winkler

Dynamic force/torque measurement using a 12DOF sensor

This article presents an algorithm for dynamic force/torque measurement and robot load identification using the so called 12DOF sensor to measure forces/torques and linear/angular accelerations. The basic equations of dynamic forces and torques arising during robot motion and acting on the end-effector were worked out. To be able to perform the experiments suitable robot system based on a six axes articulated manipulator was constituted. For this system also the appropriate software was developed. The load parameters of the tool were determined during particular motion sequence of the robot and the values were compared with values from CAD software. The compensation of dynamic forces and torques is verified using the experimental robot system and the results are presented.

Force-guided motions of a 6-d.o.f. industrial robot with a joint space approach

This article deals with the interaction between humans and industrial robots, more specifically with the new design and implementation of an algorithm for force-guided motions of a 6-d.o.f. robot. It may be used to comfortably teach positions without using any teaching pendant or for some assistance tasks. For this purpose, from readings of the force/torque sensor mounted in the robot wrist, the gravity forces and torques first have to be eliminated. To control the robot in joint space, it is then convenient to transform the external force and torque values from Cartesian space into joint space using the manipulator transposed Jacobian. This is why with the present approach the Jacobian matrix of the robot used was calculated. Now, from the computed joint torques, suitable position commands of the robot arm can be generated to obtain the desired behavior. A suggestion for this desired behavior is also included in this article. It is based on the impedance control approach in joint space. The proposed algorithm was implemented with the standard Stäubli RX90B industrial robot.

An approach to compliant motion of an industrial manipulator

Abstract This article presents an approach to robot force control for a compliant motion task with a commercial robot system. It consists of two parts: First is the algorithm for fast and sure environment contact detection. Second is the hybrid position/force control algorithm for contour following of an unknown surface. It contains the estimation of gradient of environment surface at contact point and the adaptation of working velocity. The approach is implemented in the robot controller and its functionality is shown on the drawing example task. The results of new algorithm are compared with results of the standard force control functions.

Position feedback in force control of industrial manipulators - An experimental comparison with basic algorithms

This paper investigates simple position based force control algorithms for an industrial robot. A lot of approaches to robot force control published in the previous three decades are difficult to implement in commercial robot controllers. This fact is one reason why force controlled robots rarely appear in industrial applications. On the other side force control may improve some robot tasks like automated assembling or material processing. Therefore, is seems to be necessary to concentrate on simple controller structures on the field of robot force control which are easily realizable and adjustable. For this purpose in this paper linear standard control algorithms have been selected and implemented with a real robot. One proposed structure is the proportional controller with positive position feedback which dealt probably here for the first time and shows very good control behavior. Some proposals to parameterization of different controllers are given and their performance with respect to contact detection, impact and response to setpoint changes is shown and compared by practical experiments.

NOVEL JOINT SPACE FORCE GUIDANCE ALGORITHM WITH LABORATORY ROBOT SYSTEM

Abstract This article presents the implementation of a new algorithm of force guided motions with a six axis articulated robot frequently used in research laboratories. This new approach is based on the idea of impedance control in joint space and it is implemented on a digital signal processor-based robot controller. It allows an intuitive force guidance of the robot by taking the gripper by the hand. Robot may be also guided over the singularities in this way. Behaviour of the robot is thus freely programmable in the wide range.

Possibilities of force based interaction with robot manipulators

One way of interaction between a human and a robot manipulator is the interaction via forces and torques. We will call it also force guidance. For this purpose the human acts on the robot arm or on the robot end-effector. From the interaction forces and torques than a suitable motion of the robot is generated. This kind of human robot interaction may be useful e.g. for the comfortable teach-in process. Commonly, positions and orientations of the robot tool are taught by the operator using the manual control pendant. With the keys on this device he or she moves the robot in joint or in task space. To improve the usability of the robot, some manual control pendants are additionally equipped with a more intuitive teach in device. It is called 6D mouse or space mouse (Hirzinger & Heindl, 1986). For further optimization of the teach-in process another way to move the robot would be force guidance. It will be shown that it is possible, with some differences, both in joint or in task space. Force based human robot interaction can be seen as a special kind of active robot force control (Zeng & Hemami, 1997). To perform this, the robot has to be equipped with a force/torque sensor (Gorinevsky et al., 1997). Usually this sensor is mounted in the robot wrist and it measures forces and torques in all Cartesian directions. The cost of such a 6D F/T sensor can exceed 10% of the price of a low payload six axes articulated robot. For that reason it should be searched for an alternative possibility of force/torque measurement. One idea is to estimate the interaction forces and torques from the joint motor currents. For this purpose an algorithm is presented and verified with experiments. Besides the kinematics of the robot motion during human robot interaction also its dynamics is important. For its representation the so called target or desired impedance behaviour will be defined as the relationship between interaction forces/torques and the velocity components of the robot motion. The simplest desired impedance behaviour is the behaviour of the mass damper system. Moreover, there are some more variants and additional features, e.g. the intuitive collision avoidance which will be described in this article. Apart from the desired impedance behaviour selected and parameterized by the operator the dynamics of the robot system has been respected. It depends on the access level of motion generation. Commonly, the robot motion is controlled by the trajectory generator. However, some robot systems permit the direct access to the position or velocity control loops which is favourable in all kinds of robot force control.

Traded and shared vision-force robot control for improved impact control

Many automated manufacturing processes require robots to interact with environment and to perform force/torque interaction such as mechanical assembly. Importantly, impact forces occur while robot and environment are in contact. Robot manipulators and control systems can experience instability or poor control performance after impacting with an environment. In this paper we combine vision and force feedback in shared/traded vision-force control to reduce the impact forces and to increase the performance of robot, by calculating the distance between robots end-effector and the environment and reducing the speed according to it. Experimental results are presented to illustrate the performance of robot in two cases with and without vision feedback and to compare these results with simulation.

Intuitive Collision Avoidance of Robots Using Charge Generated Virtual Force Fields

This article presents an approach to intuitive collision avoidance of handoperated robots. Hand-operations are realized by means of zero-force impedance control using force/torque sensor. Impedance control is then combined with the method of artificial potential fields exerting force on the end-effector. This force can be felt by human, who acts on the manipulator and cooperates with the robot. The way to generate potential fields in this work is based on virtual electrical charges placed on the obstacle surfaces. In comparison with other approaches this method is quite universal and can be flexibly used for all forms of obstacles. It may be favourable to implement an additional artificial damping field to prevent hurting obstacles in the case of higher end-effector velocities. It is also possible to use this approach with moving obstacles. In this situation the charges would be placed dependent on sensor information provided e.g. by camera.

Robot visual servoing using the example of the inverted pendulum

In this paper the control of the well known inverted pendulum will be performed by using visual servoing algorithms. The pendulum will be mounted on the flange of an articulated robot arm and it will be observed by 2D-camera. The inclination angle of the pendulum will be computed in the real time with the help of image processing algorithms. In order to swing-up the pendulum, a special algorithm will be designed to generate the coordinated robot motions. Whereas, the balancing of the pendulum will be performed by a state space controller. Both algorithms will be implemented in the standard robot controller with a relatively high cycle time. Furthermore, the calculation of the inclination angle of the pendulum by the camera, which is integrated in the closed loop controller, instead of an angular transmitter will make the successful achievement of the task more difficult. All the proposed algorithms are verified successfully in the practical experiments.

Handing-over model-free objects to human hand with the help of vision/force robot control

Handing-over objects from robot to humans is essential step to perform different tasks especially those requiring physical interaction between the robot and the human, e.g. service robots can help elderly, blind and disabled people or human-robot teamwork could work together in factories. This work will propose a new robot system which combines visual servoing and force control in order to hand over model-free objects from undefined place to human hand. This work will present: 1. vision algorithms which help the robot system to detect and to segment the objects without any information about their model. 2. The possibility for visual tracking of human hand with the help of Kinect camera. 3. The importance of fusion vision and force control in order to ensure the safety during the human-robot interaction. 4. How the robot will deliver the objects to the human hand with the help of vision and force control. This work will be supported with experimental results.