Thermobot: towards semi-autonomous, thermographic detection of cracks

Robot path planning

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  all relevant areas on the part’s surface have to be covered by acquiring images from these viewpoints;
  
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  when passing through the viewpoints the part must not collide with the robot, the camera or other devices in the workspace of the robot;
  
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  all the viewpoints must be within the reach of the robot.
  

These conditions apply to all robot-based inspection systems that include camera as the main sensor system. Thermography, however, has one additional constraint that is quite difficult to fulfil. In order to do a proper analysis of the heat flow, each position on the part has to be recorded in several images at specific time intervals. Therefore an additional requirement is that

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  the path of the robot has to follow a specific trajectory, that ensures that the viewpoints are reached at a particular time.
  

It should be noted that the motion of the robot has to be continuous and not a start/stop motion as is possible e.g. in standard machine vision applications.

This set of boundary conditions still allow a large (actually infinite) number of different solutions for the sequence of viewpoints that is chosen for the particular inspection task. Even if the restrictions of the robot’s kinematics are fully considered, there is still a wide range of different paths. Therefore, an additional optimization criterion is required that reduces the set of possible solution to (ideally) a single one. In many real-world applications this optimization criterion corresponds to

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  the time required for inspecting all areas on the part should be minimal.
  

While this optimization criterion may sound trivial it poses a number of challenges. Estimating the time required for following a complex path in 6 degrees of freedom requires detailed knowledge about the dynamical behaviour of the robot as well as detailed consideration of the thermodynamic processes that are involved.

Figure 2. Process points on small section of a crankshaft with a small outward pointing normal vector.

Tests on objects of different complexity included parts with an essentially flat surface that have only low curvature as well as more complex parts such as the crankshaft shown in the figure above. Path planning was able to generated useful paths for both of these test parts. The current implementation of the path planning just tries to find a feasible path for the robot that complies with the above mentioned boundary conditions. It does not yet use an optimization criterion to e.g. reduce the time needed for the inspection task. For inspecting the relevant regions on a whole crank shaft a total time of a few minutes is required, but there is still substantial potential for optimization so that cycle times below 1 minute can be expected.