Plot Clock Alpha

Plot Clock Alpha

Calibration Test

This simple maneuver moves each arm of the PlotClock exactly 90 degrees from the origin angle, bumps a touch sensor to reset its internal angle and then moves to its origin position. This is a key action, since it allows the arms to start from nearly any starting position and then calibrate to a known baseline position.

Initial Test

My first attempt to producing a digit from a fixed position. It performed quite admirably for the first first plot, but distinctly drew an unwanted line from the origin position to top left start of the two.

Mapping digit positions

The next step is laying out exactly where each digit will be drawn. This took quite a few attempts to get correct, since there was some discrepancy between the origin position and the scaled digit locations. Took a few hours to correct, but the end result is remarked improvement. Notice I added a Pen up and Pen down feature to mitigate unwanted lines too!

Writing the time!

After I mapped each digit from 0 to 9 I simply had to scale it using a fixed value and then make the arms move to each position in segmented chunks. I am starting to see the limitations of affixing the arms directly to the NXT servos now... the residual jitter causes the pen to jerk when it moves along the Y-axis of the grid. I have a feeling that I will need to redesign exactly how the robot moves or implement something to make the relative motor angle less inaccurate. Fetching the time and mapping into a HH:MM format was trivial.

Considering the next step

Plot Clock Alpha Upon evaluation and review, there a few key limitations I will need to address the next version (Beta).

  • Movement Jitter - The obvious problem with the writing is the jitter or "noise" generated by the drawings arms causing friction with the pen and the drawing surface. The obvious approach is to make the arms heavier, but this would require additional frame support to reduce vertical jitter.
  • Reduce pen-tip to surface length - After testing the pen at different drawing lengths I found that the closer the pen is to the drawing surface is then the neater the overall line would becomes. This is certainly evident in Heberlein PlotClock since the arms position the pen within militmeters to the drawing surface.
  • Angle translation accuracy - The biggest problem with the initial Alpha design is the inaccuracy of the NXT servos. The LeJOS documentation explains that the movement functions are within a -2 to +2 degree accuracy. Therefore the 4 degrees in rotation inaccuracy can translate up to 20mm variation in at the draw tip! The limitation here is the servos' tachometers which do not provide an accurate reading after successive runs. The best way to solve this is a mechanical gear translation which will be discussed in the Beta development.

Plot Clock Alpha

Continue to PlotClock Beta -->

Website templatesDont click me