Using parallelism in bufferless production lines can improve productivity, with significant productivity gains achieved with crossover. However, including crossover in the line implies additional material-handling requirements that may reduce the availability of the system. This paper examines if parallel systems with crossover between the stages are more productive than parallel systems without crossover between the stages, when one considers the availability of the additional material handling required for the crossover. The minimum material-handling availability necessary for inclusion of crossover is determined for a given parallel line's configuration such that productivity can be maximized.Note to Practitioners-Two approaches in configuring parallel manufacturing lines are currently being used in industrial plants. These have been characterized as the Japanese approach of parallel independent cells of serial operations, and the European approach of a serial line with each operation being duplicated in parallel. The European approach has a productivity advantage over the Japanese approach when considering machine failures within each operation. However, the European approach requires more material handling which increases the configuration complexity and can reduce productivity. A math model is developed to determine which approach is best for a given line design when line length is defined by process planning and line balancing, and line width is determined by throughput requirements. The analysis is limited to cell configurations that do not use buffers internal to the cell.

A blind traveler walking through an unfamiliar environment and a mobile robot navigating through a cluttered environment have an important feature in common: both have the kinematic ability to perform the motion, but they are dependent on a sensory system to detect and avoid obstacles. The paper describes the use of a mobile robot obstacle avoidance system as a guidance device for blind and visually impaired people. Just as electronic signals are sent to a mobile robot's motion controllers, auditory signals can guide the blind traveler around obstacles, or alternatively, they can provide an "acoustic image" of the surroundings. The concept has been implemented and tested in a new travel aid for the blind, called the Navbelt. The Navbelt introduces two new concepts to electronic travel aids (ETA's) for the blind: it provides information not only about obstacles along the traveled path, but also assists the user in selecting the preferred travel path. In addition, the level of assistance can be automatically adjusted according to changes in the environment and the user's needs and capabilities. Experimental results conducted with the Navbelt simulator and a portable experimental prototype are presented.

This paper introduces error eliminating rapid ultrasonic firing (EERUF), a new method for firing multiple ultrasonic sensors in mobile robot applications. EERUF allows ultrasonic sensors to fire at rates that are five to ten times faster than those customary in conventional applications. This is possible because EERUF reduces the number of erroneous readings due to ultrasonic noise by one to two orders of magnitude. While faster firing rates improve the reliability and robustness of mobile robot obstacle avoidance and are necessary for safe travel at higher speed (e.g., V>0.3 m/sec), they introduce more ultrasonic noise and increase the occurrence rate of crosstalk. However, EERUF almost eliminates crosstalk, making fast firing feasible. Furthermore, ERRUF's unique noise rejection capability allows multiple mobile robots to collaborate in the same environment, even if their ultrasonic sensors operate at the same frequencies. The authors have implemented and tested the EERUF method on a mobile robot and they present experimental results. With EERUF, a mobile robot was able to traverse an obstacle course of densely spaced, pencil-thin (8 mm-diameter) poles at up to 1 m/sec.>

A new robot path planning methodology, called obstacle accommodation, has been introduced for robots in cluttered environments. This methodology does not attempt to avoid physical contact between the robot and obstacles. Instead, it controls the contact to prevent damage to the robot. Obstacle accommodation represents a new robotic paradigm in which obstacles can contact robots at unspecified positions, i.e., a contact point can be at any point on any link. Obstacle accommodation requires analysis in kinematics, motion planning, dynamics, and control. This paper deals with the development of the kinematic constraints and with motion planning under these constraints. We begin by providing a general formulation of the motion constraints due to contact with obstacles. Next, a new inverse kinematics is presented that provides joint motion for robots under contact constraints. Finally, the new motion planning algorithm for robot motion in a cluttered environment is provided. The motion planning algorithm is verified by two examples, one on a linkage robot and the other on a mobile robot.>

The NavChair assistive navigation system is being developed to meet the needs of multiply handicapped people who are unable to operate available wheelchair systems. The NavChair Project was conceived as an application of mobile robot obstacle avoidance to a power wheelchair. During the course of this project, the vector field histogram (VFH) method has been adapted for use in human-machine systems and the shortcomings of the wheelchair platform have been overcome. This paper briefly reviews the VFH method, describes interesting aspects of its application to a power wheelchair, and presents an experimental evaluation of system performance. Finally, unresolved problems of obstacle avoidance in human-operated vehicles are discussed.>

This paper describes the design and implementation of a model-reference adaptive motion controller for a differential-drive mobile robot. This controller uses absolute position information to modify control parameters in real time to compensate for motion errors. Robot motion errors are classified into internal and external errors. A cross-coupling control method is used to compensate for the internal errors that can be detected by wheel encoders. The adaptive controller provides compensation for external errors. The adaptive controller is analyzed, and its stability and convergence are discussed. Experiments are conducted to evaluate the control system and the results show significant improvements over conventional controllers.>

A blind traveler walking through an unfamiliar environment and a mobile robot navigating through a cluttered environment have an important feature in common: both have the kinematic ability to perform the motion, but are depended on a sensory system to detect and avoid obstacles. This paper describes the use of a mobile robot obstacle avoidance system as a guidance device for blind and visually impaired people. Just like electronic signals are sent to a mobile robot's motor controllers, auditory signals can guide the blind traveler around obstacles, or alternatively, they can provide an "acoustic image" of the surroundings. The concept has been implemented and tested in a new traveling aid for the blind, called the Navbelt. Experimental results of subjects traveling with the Navbelt in different surroundings are presented.>

Abstract-The paper presents the design and motion planning for a mechanical snake robot that was built at the University of Michigan. The structure of the robot enables it to move without wheels. It is constructed of a series of articulated links, each one with a motor and linear solenoid. Although each link has only one motor, this structure allows the body configuration to be easily controlled thereby enabling the robot to move in very cluttered environments. The motion planning system provides the robot with a basic motion pattern that can be easily modified for different tasks and environments. The mechanical snake does not avoid obstacles on its way, but rather “accommodates ” them by continuing its motion towards the target while in contact with the obstacles. With our design and motion planning, each link has a different number of degrees-of-freedom in each motion stage, providing the robot with great adaptability even during contact

The authors categorize different types of noise and discuss methods for rejecting each type of noise. They introduce a method called error eliminating rapid ultrasonic firing (EERUF). EERUF combines different noise rejection techniques and optimizes them for rapid firing. EERUF almost completely rejects crosstalk. Its unique noise rejection capability allows multiple mobile robots to collaborate in the same environment, even if their ultrasonic sensors operate at the same frequencies. For each noise category methods are described to identify and reject the resulting errors. These individual rejection measures were combined into one error rejection method which was then combined with a fast firing algorithm. The resulting combination was EERUF. The EERUF method was implemented on a mobile robot and experimental results are presented. With EERUF, a mobile robot was able to traverse an obstacle course of densely spaced, pencil-thin poles at up to 1 m/s.>

A method that determines the absolute position of a mobile system with a hybrid optoelectronic processor has been developed. Position estimates are based on an analysis of circular landmarks that are detected by a TV camera attached to the mobile system. The difference between the known shape of the landmark and its image provides the information needed to determine the absolute position of the mobile system. For robust operation, the parameters of the landmark image are extracted at high speeds using an optical processor that performs an optical Hough transform. The coordinates of the mobile system are computed from these parameters in a digital coprocessor using fast algorithms. Different sources of position estimation errors have also been analyzed, and consequent algorithms for improving the navigation performance of the mobile system have been developed and evaluated by both computer simulation and experiments.>

Based on a rigorous mathematical analysis, the authors present a systematic overview and a critical discussion of the inherent problems of potential field methods (PFMs). The authors previously (1989) developed a PFM called the virtual force field (VFF) method. Much insight has been gained into the strengths and weaknesses of this method. Four distinct drawbacks with PFMs are identified. Because of these drawbacks, the authors abandoned potential field methods and developed a new method for fast obstacle avoidance. This method, called the vector field histogram method, produces smooth, nonoscillatory motion, while sampling time and hardware are identical to those used in the VFF method.>

A real-time obstacle avoidance method for mobile robots which has been developed and implemented is described. This method, named the vector field histogram (VFH), permits the detection of unknown obstacles and avoids collisions while simultaneously steering the mobile robot toward the target. The VFH method uses a two-dimensional Cartesian histogram grid as a world model. This world model is updated continuously with range data sampled by onboard range sensors. The VFH method subsequently uses a two-stage data-reduction process to compute the desired control commands for the vehicle. Experimental results from a mobile robot traversing densely cluttered obstacle courses in smooth and continuous motion and at an average speed of 0.6-0.7 m/s are shown. A comparison of the VFN method to earlier methods is given.>

Histogramic in-motion mapping (HIMM) is introduced as a new method for real-time map building with a mobile robot motion. HIMM represents data in a two-dimensional array, called a histogram grid, that is updated through rapid in-motion sampling of on-board range sensors. Rapid in-motion sampling results in a map representation that is well-suited to modeling inaccurate and noisy range-sensor data, such as those produced by ultrasonic sensors, and requires minimal computational overhead. Fast map building allows the robot to use immediately the mapped information in real-time obstacle-avoidance algorithms. The benefits of this integrated approach are quick, accurate mapping and safe navigation of the robot toward a given target. HIMM has been implemented and tested on a mobile robot. Its dual functionality was demonstrated through numerous tests in which maps of unknown obstacle courses were created, while the robot simultaneously performed real-time obstacle avoidance maneuvers at speeds of up to 0.78 m/s.>

The method described, named the vector field histogram (VFH), permits the detection of unknown obstacles and avoids collisions while simultaneously steering the mobile robot toward the target. A VFH-controlled mobile robot maneuvers quickly and without stopping among densely cluttered obstacles. The VFH method uses a two-dimensional Cartesian histogram grid as a world model. This world model is updated continuously and in real time with range data sampled by the onboard ultrasonic range sensors. Based on the accumulated environmental data, the VFH method then computes a one-dimensional polar histogram that is constructed around the robot's momentary location. Each sector in the polar histogram holds the polar obstacle density in that direction. Finally, the algorithm selects the most suitable sector from among all polar histogram sectors with low obstacle density, and the steering of the robot is aligned with that direction. Experimental results from a mobile robot traversing a densely cluttered obstacle course at an average speed of 0.7 m/s demonstrate the power of the VFH method.>

A tour plan generator (TPG) specifically adapted for mobile robots is described. The TPG computes the itinerary of a tour passing through a number of locations. One typical constraint is that a single-armed mobile robot is usually unable to carry more than one object at a time in its gripper. This constraint requires the TPG to generate a tour in which each pick-up location is visited immediately prior to the corresponding drop-off location. A model that makes it possible to reduce this problem to a special case of the traveling salesman problem with asymmetric cost matrix and nonEuclidean distances is introduced. Another constraint in many mobile robots is the need to periodically visit a home location. The TPG introduced here automatically creates multiple subtours such that a predefined maximal length for each subtour is not exceeded. The algorithm obtains near-optimal solutions with short computation times by combining different heuristic tour-construction rules into a heuristic team approach.>

A novel technique for the remote guidance of fast mobile robots has been developed and implemented. With this method, the mobile robot follows the general direction prescribed by an operator. However, if the robot encounters an obstacle, it autonomously avoids collision with that obstacle while trying to match the prescribed direction as closely as possible. This novel implementation of shared control is completely transparent and transfers control between teleoperation and autonomous obstacle avoidance gradually. The method, called teleautonomous operation, allows the operator to steer vehicles and robots at high speeds and in cluttered environments, even without visual contact. Teleautonomous operation is based on the virtual force field (VFF) method, which was developed for autonomous obstacle avoidance. The VFF method is especially suited to the accommodation of inaccurate sensor data and sensor fusion, and allows the mobile robot to travel quickly without stopping for obstacles.>

A mobile system with a hybrid opto-electronic processor has been studied. The position estimation is based on analysis of landmarks being detected by a TV-camera attached to the mobile system. The difference between the known real shape of the landmark and its image provides the information necessary for determining the relative position of the mobile system to the landmark. The parameters of the landmark image are extracted at high speed using an optical processor that performs an optical Hough transform, while the coordinates of the mobile system are computed from these parameters in a digital coprocessor using fast and simple algorithms. Different sources of errors have been analyzed, and algorithms to improve the performance of the mobile system have been developed and evaluated by computer simulation.>

A real-time obstacle avoidance approach for mobile robots has been developed and implemented. It permits the detection of unknown obstacles simultaneously with the steering of the mobile robot to avoid collisions and advance toward the target. The novelty of this approach, entitled the virtual force field method, lies in the integration of two known concepts: certainty grids for obstacle representation and potential fields for navigation. This combination is especially suitable for the accommodation of inaccurate sensor data as well as for sensor fusion and makes possible continuous motion of the robot with stopping in front of obstacles. This navigation algorithm also takes into account the dynamic behavior of a fast mobile robot and solves the local minimum trap problem. Experimental results from a mobile robot running at a maximum speed of 0.78 m/s demonstrate the power of the algorithm.>

A mobile robot system, capable of performing various tasks for the physically disabled, has been developed. To avoid collision with unexpected obstacles, the mobile robot uses ultrasonic range finders for detection and mapping. The obstacle avoidance strategy used for this robot is described. Since this strategy depends heavily on the performance of the ultrasonic range finders, these sensors and the effect of their limitations on the obstacle avoidance algorithm are discussed in detail.>

A general algorithm for identification of deterministic rapidly time-varying parameters is introduced. The new algorithm is based upon applying a linear combination of the equations determined by the ordinary least-squares method. With this algorithm there is no need to modify the covariance matrix or to use forgetting factors in order to accurately follow the time-varying parameters.

Robotics for engineers , Robotics for engineers , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

A software interpolator which is comprised of linear and circular interpolations is compared with its hardware counterpart and with other circular interpolation methods. The software interpolator and the feed-rate control are contained in the numerical control (NC) program of a computer numerical control (CNC) system and enable a contouring control of the machine tool in any required feed-rate.