Robot Guidance for Part-to-Part Variation

One typical issue to deal with in various robotic applications is the variation from part to part of the location of the fixture holding the part the robot is operating on; another typical case is simply the variation of (the location of) the part itself within the fixture. So, while robot calibration can accurately compensate, at the launch of the production line, for errors in the ‘metrological chain’ formed by the robot, the end-effector, the fixture, and optionally external axes and/or a positioner, that metrological chain is ‘broken’ the minute the part and/or the fixture move relative to their initial location during the calibration  process.

‘Robot Guidance’ is the logical way to address such situations where a robot’s final resting position needs to be accurately located relative to the part – despite the varying location of each such part – with less concern of the path leading to that final resting position. A typical example in the automotive industry is the assembly of a door within a door opening on a car body. The DynaGuide™ system provides such a Robot Guidance solution.

Based on the measurement of some datum (or ‘reference’) points on the part, the location of that part relative to the robot coordinate frame is adjusted accordingly. The measurements can be provided by different types of sensors, depending on many factors, such as the kind of datum feature on the part to measure, the allowed cycle time, the sensor’s cost, its ruggedness to withstand plant floor conditions, etc. Furthermore, such sensor can be either mounted at the end of the robot (typically then a single sensor is carried by the robot to each datum feature sequentially), or mounted fixedly on the floor (multiple sensors are then typically used, each sensor measuring one datum feature).

The necessary measurements are then performed first during the initial setup (to establish a ‘reference’ measurement), and then repeatedly in between production cycles (each time measuring the effective difference in location relative to the initial ‘reference’ measurement). The difference calculated each time is generally under the form of a 6DOF transformation matrix, used to update the ‘User Frame’ on the robot controller accordingly (alternate methods exist).

Robot-Cell Calibration (see “Absolute Accuracy Robot-Cell Calibration”) should still be performed during the initial setup of any Robot Guidance application. This guarantees that the robot will truly move by the amount (i.e. the 6DOF transformation) calculated based on the measurement data: without calibration and depending on the magnitude of the calculated transformation matrix, the robot will otherwise not move accurately enough to the desired location. Another benefit of calibration is to minimize the possibility of collisions with surrounding equipment (especially in tight areas); it will also be invaluable in the case of a possible crash, where a quick re-calibration minimizes the production downtime (when replacing a robot-mounted sensor for example).

 

The DynaGuide system is a highly sophisticated software package, integrating several key technologies: it can interface with multiple measurement sensors, automatically communicates with virtually any robot controller, incorporates robot-cell calibration, interacts with standard PLC’s, and can populate customer’s data servers. ‘Scripts’ (which have been pre-defined or can be created from scratch) define what sensor data to use for what purpose, what calculation to perform based on the measurement data, what tolerance level to accept, what sequence of operations to follow, etc. The DynaGuide system includes both a Robot Guidance solution as well as an Adaptive Robot Control one (see “Adaptive Robot Control for Ultimate Accuracy”).

 

 

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