## 3 Changes Made to e-David
### Brush Holder  
A major design issue was the construction of an adequate tool holder. The robot should be able to yield a large variety of brushes, from very fine round No. 2 up to larger No. 24 brushes. It should also be possible to integrate brushes that require maintaining a certain orientation, like square brushes. These tools should be held with high precision and should not move under normal painting forces. The design goal for the brush holder was to provide the ability to allow repeatability of \(\pm\)0.1 mm between strokes. In the previous system (see Figure 3a), the brush was wrapped in tape and placed into a holding tube, which interfaced with the robot's pneumatic gripper. The friction between the tape and the tube held the brush in position. Due to the low rigidity of this setup, the brush would deflect by \(\pm\)2 mm under normal loads. Over time the tape would deform due to the forces applied and other effects like water getting into it. This added a long-term drift of the brush from the TCP up to \(\pm\)5 mm, which made precise painting very difficult. Furthermore, preparing a new brush was a very messy process, involving many trial fits before an acceptable result was reached. As brushes are a consumable item, preparing them for use with the robot must be a quick and repeatable operation. However, brushes are not precision-made. They come in shapes which are ergonomic to hold for humans, but challenging for robots. Designing a gripper would be time-consuming and is outside of the scope of our research. Based on this a prototype holder has been designed and tested (see Figure 3b). We replace most of the structure of the brush with a part that can be precisely held by the machine and that eliminates play from the robot/brush handle interface. The brush is cut to a specified length and center drilled. An aluminum adapter is press-fitted into the hole, so no play is possible. The adapter fits into a brass bushing on the robot end-effector and is held in place using a magnet. Due to the magnetic retention, the holder allows tool changes without the use of pneumatics or other active parts. By avoiding these, we minimize the complexity of the painting set up and increase reliability. Tools are clipped on a holding rack, which engages the groove in the tool adapter. This allows the robot to overcome the magnetic force and pick up another tool. We have measured a final tolerance of \(\pm\)0.05 mm at the brush ferrule, indicating that the setup is rigid. As the metal parts do not bend during normal operations, no long-term tool deflection has to be considered, aside from the actual bristles deforming. The entire holder can be seen in Figure 3.  
Figure 2: e-David plotters. Figure 3: e-David brushholder development. The second iteration of the holder will use a non-self-locking tapered adapter. Using tapers for tool holding is common in machining [23] and will make tool changings more reliable. A cross section of the next design can be seen in Figure 4.  
Through the upgrade of the brush holder, we partially achieve the goal of _improved precision_ in stroke placement. Now that the brush is held with minimum play, the flexible bristles of the brush remain as an element which is difficult to control. During painting we observed that brushes deform gradually, i.e., their tips move off the center axis of the brush. Robotic systems are traditionally modeled to consist of rigid bodies, called links, with moving joints between them [24]. As a consequence, in most robotic arm systems the TCP is also assumed to be rigidly attached to the last link [25]. For robotic painting, we use the brush tip as the TCP. However, the compliant tool exhibits both temporary and longer lasting deformation, which must be considered. Data on brush deformation is hard to find. Publications about approximating the physics of brushes do exist, which focus on simulating painterly effects in digital painting [26; 27; 28]. These systems do not allow parametrization for a physical brush or consider long term bending of the bristles.