In Electromechanical Displays, Part 1: The Nipkow Disk, the Nipkow disk was considered as a possible low-cost, mechanically-simple, perspicuous display like that of oscilloscopes. The Nipkow disk geometry was worked out in some detail and it is unattractive. The search for a better scheme continues in this second part.
A variation on the Nipkow disk is the “Nipkow drum”, made by spinning a steel can such as the one shown for New Zealand butter. It has a radius of R = 5 cm and a useable height of 6 cm. A spinning drum has the advantage over a Nipkow disk of linear rather than rotational motion in the horizontal dimension. The curvature of the can must still be corrected like that of a CRT with a curved screen. This is a lesser problem than nonlinearity in both dimensions of the screen. The disadvantage over the disk is that spinning a can is somewhat more mechanically involved than spinning a disk, but not by much. The most difficult task is to center the motor shaft at the center of the can to avoid wobble in its rotation. While various kinds of motors could be used, readily available, low-cost motors are spindle motors from hard disk-drives. These motors are nearly ideal for the application because they are designed to spin platters similar to the can, as shown below.
The screen dimensions of a Tek 454 oscilloscope are 0.8 cm/div or Y = 6.4 cm and X = 8 cm. For the butter can,
The arc angle swept out for the display is
and has 0.75 cm/div, slightly smaller than the 454 screen. The angle is less than for the disk because of the distance-amplifying effect of curvature projection. The line spacing at a resolution of 10 lines/div is 0.75 mm. The pixel holes in the can should be 0.75 mm in diameter, or about 30 mils, corresponding to a 28 mil # 70 drill bit.
A mechanical sketch of the Nipkow drum assembly is shown below. The display circuit board is floated above (and envelops) the spinning can with four corner standoffs, and is cabled to the motor-drive board below it. A LED board is suspended into the can from the display board, as shown. Only the can and motor shaft rotate. Each LED is optically separated by baffles and each illuminates one of N sectors of lines. By processing N sets of lines concurrently, the same line limitation that applies to Nipkow disks is remediated.