Force feedback is being used as an interface between humans and material handling equipment to provide an intuitive method to control large and bulky payloads. Powered actuation in the lift assist device compensates for the inertial characteristics of the manipulator and the payload to provide effortless control and handling of manufacturing parts, components, and assemblies. The use of these Intelligent Assist Devices (IAD) is being explored to prevent worker injury, enhance material handling performance, and increase productivity in the workplace. The IAD also provides the capability to shape and control motion in the workspace during routine operations. Virtual barriers can be developed to protect fixed objects in the workspace, and regions can be programmed that attract the work piece to a certain position and orientation. However, the robot is still under complete control of the human operator, with the trajectory being determined and commanded using the judgment of the operator to complete a given task. In many cases, the IAD is built in a configuration that may have singular points inside the workspace. These singularities can cause problems when the unstructured trajectory commands from the human cause interaction between the IAD and the virtual walls and fixtures at positions close to these singularities. The research presented here explores the stability effects of the interactions between the powered manipulator and the virtual surfaces when controlled by the operator. Because of the flexible nature of the human decisions determining the real time work piece paths, manipulator singularities that occur in conjunction with the virtual surfaces raise stability issues in the performance around these singularities. We examine these stability issues in the context of a particular IAD configuration, and present analytic results for the performance and stability of these systems in response to the real-time trajectory modification of the human operator.
Handling heavy and bulky loads in manufacturing settings is an ongoing problem in achieving flexible and re-configurable assembly operations. One current trend is toward using powered heavy lift assist devices to augment human decision making skills with the mechanical muscle necessary to move and position large work pieces. As advanced automation has made it possible to apply power to multiple axes of a lift assist device, a need has arisen to implement a more natural control interface for the operator. One method is to use sensors to measure the force applied by the operator as the motion command to the powered assist device. This approach allows an untrained operator to easily guide the work piece using natural hand motions. In this research, we explore the use of multiple powered axes in a lift assist device to enhance and increase the material handling capability of the human operator. Input forces from the operator are measured and translated into joint actuation commands using a micro-controller. The automated control system is used to augment the workspace of to include virtual walls and attraction locations that can guide the operator as the work piece is moved. Closed loop control issues arise from the three coupled, nonlinear systems meeting at the manipulator input handle--the human, the powered lift assist device, and the digital computer control application. We present analytic and experimental comparisons of the performance of this class of human strength enhancing devices. Stability of the various control approaches is considered from a theoretical standpoint and compared to experimental results. The combination of theory and experiment are used to provide boundaries to the allowable performance of these types of machines.
In the approach presented in this research, a six DOF industrial manipulator is used as the master device to provide haptic feedback to the operator. In order develop effective constraints between the motion of the slave and master, a virtual manipulator concept is developed that couples the actual robotic kinematics with the constraints of the simulated slave manipulator. The position and velocity errors between the actual and virtual mechanisms are used to develop an optimal impedance controller that constrains the motion of the master in all directions that are orthogonal to the allowable motions of the slave. This approach allows the use of a conventional industrial manipulator as an effective haptic display device.
A recent progression of heavy lift assist device is to place the human operator closer to the end effector to provide close coupling of the operator input and the payload. This close coupling of the human for control and the power of a heavy lift assist device provides improved accuracy with ease of handling in the case of heavy and bulk objects. However, collisions with obstacles may still occur in a crowded manufacturing environment due to the large work piece inertia characteristics, inappropriate motion command from the operator and inattention or fatigue of the human operator. In this research, a fictitious force field is assigned to each obstacle in the workspace. As a work piece moves closer to an object, an impedance force is calculated and combined with the control forces, in order to prevent collisions. In addition, a set of impedance fields are developed and applied that associate desired trajectories with the layout of the workspace. Thus, the force fields guide the work piece to achieve advantageous orientations and positions during the material handling operation. This includes adjustment of the height of the work piece for placement on tables, orientation to preset positions, and optimizing the configuration of the lift assist robot during motion. Experimental results show that this approach to augmentation provides the operator with a natural and effective interface to the heavy lift assist device.
Assembly operations require high speed and precision with low cost. The manufacturing industry has recently turned attenuation to the possibility of investigating assembly procedures using graphical display of CAD parts. For these tasks, some sort of feedback to the person is invaluable in providing a real sense of interaction with virtual parts. This research develops the use of a commercial assembly robot as the haptic display in such tasks. For demonstration, a peg-hole insertion task is studied. Kane's Method is employed to derive the dynamics of the peg and the contact motions between the peg and the hole. A handle modeled as a cylindrical peg is attached to the end effector of a PUMA 560 robotic arm. The arm is handle modeled as a cylindrical peg is attached to the end effector of a PUMA 560 robotic arm. The arm is equipped with a six axis force/torque transducer. The use grabs the handle and the user-applied forces are recorded. A 300 MHz Pentium computer is used to simulate the dynamics of the virtual peg and its interactions as it is inserted in the virtual hole. The computed torque control is then employed to exert the full dynamics of the task to the user hand. Visual feedback is also incorporated to help the user in the process of inserting the peg into the hole. Experimental results are presented to show several contact configurations for this virtually simulated task.
Heavy lift assist devices are an important part of manufacturing facilities that involve large, heavy or bulky material. Many devices are available that provide lift but not motive force augmentation. In these devices, the physical strength of the operator is used to move and position the work piece. Due to large work piece inertial characteristics, inertial contributions from the lift device itself, and misuse of the assist manipulator, injuries may still occur. In this research, an approach is presented that provides reduced-authority actuation to the motive joints of the lift device that allows for augmentation of the human motion forces, provides a means of correcting injurious ergonomic interactions, and allows for high rate energy dissipation for payload trajectory control and emergency situations. The approach is to provide low torque input controlled by operator hand motions. These hand motions move the payload under a centralized trajectory generation scheme that uses modulated braking commands to impose motion constraints, such as object avoidance, resonance attenuation and ergonomic trajectory enhancement. The system is implemented in an virtual reality robot simulator that allows for the investigation of dynamic characteristics prior to the prototype stage.
Robot manipulators are the natural choice for transmitting computer-generated dynamic forces from a virtual environment to a human user immersed in that environment. Control schemes for this type of interactive interface are extensions of the techniques used in teleoperation systems, with the forces applied between the master and the human generated by the dynamic model rather than measured from sensors on the slave. Stability problems arise when the robot inertia and dynamics are coupled to the complex and time-varying human dynamic characteristics. In this work, a new local feedback error control technique is applied to an interface robot to allow motion only in free directions in the virtual space. This virtual force interface does not require the use of an end effector force transducer. As in all local control schemes, the dynamic characteristics and cross-coupling effects of the manipulator are neglected, so that actual motion may deviate slightly from the desired trajectory during high speed operation. Experimental results are presented showing the use of this haptic device moving within a cubic space representing a fish tank. Motion of the hand controls the motion of virtual fish within the tank, and as the fish contact the walls of the virtual tank, the human feels the hard surface of the tank walls. Stability for this scheme is based on the stability of position error feedback for the manipulator.
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