Control Considerations in the Design of a Parallel Kinematic Machine with Separate Actuation and Metrology Mechanisms

Machines based on parallel kinematics typically utilize connector geometry to establish the kinematic state of the moving platform. Most active applications require the connectors to be both prime movers and sensors. In some applications, the connectors are passive, with a metrology function of determining moving platform position and orientation. A methodology is developed that generates optimized parallel kinematic mechanism geometry based on design criteria. This methodology is applied to two parallel kinematic mechanisms to develop a high payload machine that has direct manual input for motion command. The deign criteria utilized for objective function development are directly related to control issues. The actuation frame is developed with load leveling between actuators in mind, and the metrology frame is designed with accuracy of determination of position and orientation in mind. An overview of the machine’s control architecture is presented. Several control behaviors are detailed. A control behavior in which the operator provides the motion command directly is developed, and concerns with respect to command determination and sensitivity are discussed. Introduction, Motivation An application of parallel geometry is proposed. An operational mode of the proposed device is the augmentation of a manual assembly task. An example would be aiding the fitting of a transmission to an engine block. It is desired that the resulting machine cause no damage to surrounding structures. An operational mode to meet this requirement will incorporate force control of the machine. A load is acquired by the machine, gravity effects are compensated for, and then the operational mode is effected that controls the mechanism to move away from external loadings. These loading can be from an operator’s direct input, contact with the environment, or a combination of both. The machine is to be utilized in a research setting. The application of parallel geometry in simulators and amusement rides is routine. Other attempts, for example in machining and robotics, at utilizing parallel geometry based mechanism have met less success. The study of parallel mechanisms is driven by a lack of knowledge in the field and by failures in various applications. The resulting knowledge can be utilized to implement a machine based on parallel geometry that avoids previous failure modes and improves on the applicability of the geometry. A design methodology is developed that utilizes and extends existing kinematic analysis of the parallel mechanism to achieve design goals. This methodology is applied to the design problem derived from the application to arrive at a machine incorporating the methodology’s results. The resulting machine will provide a test bed for further research in the area of parallel kinematic mechanisms. Several unique considerations have been made in the design specification to allow a more useful resultant machine. These include separation of the actuation and metrology functions of the machine, inclusion of load cells in connectors, utilization of open hardware in the controller development, and use of commercial off the shelf components (COTS) when possible. The control methodology utilized to achieve the desired machine performance characteristics is developed considering the unique elements that comprise the machine. The separation of the actuation and metrology functions between two mechanisms requires substantial computations to determine the pose of the machine at any given instant. Force measurement is via load cells in the actuation frame, so applied force must be calculated.