Plotter - in its first practical incarnation
It all started 1986 with these stepper motors and interface that my older brother Markus built, and left with me after coming home on a Christmas break from university back in 1986. He even suggested I build a plotter, although I was very skeptical of the idea at first.
One of the key components was the 'straight line drawing code'. This sounds simple, but really isn't that simple. I had a magazine article from the 'transactor' magazine explaining how to do it, but I couldn't understand it. Besides, the algorithm was targeted at pixels, not line segments. So, not understanding that article, I designed my own from scratch. Reading the article afterwards, I was able to make sense of it then, although I felt that where they differed, mine was better.
Note that the above output is proportionally spaced. The 'word processor', I used, which was 'Paperclip 64', did not support proportional spacing. But the font that I designed (using a font editor I wrote for the purpose) was proportionally spaced. So I wrote my own text formatter which could read paperclip files, do the printer formatting. Paperlip was not a WYSIWIG sort of word processor, so parsing its files was very straightforward. The tools were very limited back then, but so were the objectives.
The above output is very messy, as it was with an early version of the mechanism, and I was running the plotter at 5 characters per second. Yes, 5 characters! That's pretty fast for any plotter, although real plotters at least write neatly.
I also optimized the pen up/down mechanism. My drum was not perfectly cylindrical, and it deviated as much as 3mm in radius as a function of x and y. This meant I had to keep the pen a fair distance from it, and then it would hit the paper with too much momentum when the solenoid was powered up. This would then cause the pen to bounce a bit as it hit the paper, and that could be seen in the output. I improved the mechanism by adding a 'feeler' next to the pen. The 'feeler' essentially pushed the pen forward to within 0.5 mm of the paper at all positions on the drum. It was a clever mechanism.
In its glory, the plotter could write at 10 characters per second, in a font that was about 1.5 mm tall, but barely legible! I was constantly pushing the edge, and often having breakdowns.
My regular size font also became more legible, even at 5 characters per second. I never did optimize out some of the resonance the font caused with the newer mechanism at the highest speed. The above sample is digitized from a letter, where I cared more about speed than print quality. Fact is, it took as long to print the letter as it took to write it!
Doing this sort of hack in Northern Ontario did have its downside though. For one thing, If I didn't have the part, I just plain didn't have it. If it wasn't in my junk inventory, then maybe Radio Shack had it, but that always cost lots of money, and I had to wait until the next time my mom drove to Sault Ste Marie, which was about 80 km away.
On optimizing the electronic drivers, I used a 68 ohm series resistor in series with each stepper coil, Unfortunately, I didn't have 68 ohm power resistors. Despite a cooling fan, I soon burned out all my half watt 68 ohm resistors. The 1/4 watt resistors from radio shack had a tendency to burn out at the multi watt loads I was driving them with, I ganged them up in fours, and the cooling fan in front of the resistors, I was able to drive several watts in to the 1/4 watt resistors. I also had a cooling fan to keep the solenoid cool, as it was not designed for the duty cycles I was using it with.
One of the science teachers, Mr Barret, however, did have appreciation for this sort of stuff, and knew it wasn't easy. I told him him everything about it. I was so glad to find an appreciative and understanding soul to tell about this cool hack of mine.
Other related pages on my website:
