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Several of the 'Environmental Test Chambers' which I have come across during the 'Stress Screening' of electronic subsystems were fitted with 'Circular Chart Recorders' which recorded process parameters such as temperature, humidity and pressure-altitude on a circular chart as a real time record of the overall test plan.

Recently my computer Hard Disk failed and I opened it up with a view to see if something could be made out of the basic parts.

The similarity of construction between this HDD and a circular chart recorder immediately struck me:

  1. Both have a circular platter which rotates at a predetermined speed
  2. While the HDD records data on the magnetic material of the platter the chart recorder writes on chart paper using a suitable marking mechanism
  3. The moving arm which carries the writing mechanism is operated in both cases by an electromechanical system

Keeping this in mind I set about trying to modify the HDD to a circular chart recorder.

In the steps ahead I will bring out:

  1. How I considered using the spindle-motor itself for the chart drive
  2. Finding this unsuitable, how I experimented with an unipolar stepper motor from an old HP printer
  3. How I experimented on various methods of adding a spring onto the electromechanical arm so as to convert it into a D'Arsonval galvanometer
  4. And how I added a clutch-pencil tip to the end of the moving arm to write on the chart paper

Finally I realized that although the concept is good, several practical problems came in the way of attaining a working system. I document these findings and suggest possible ways ahead.

I would appreciate any feedback to get this project working!

Step 1: Spindle Motor for Chart Drive?

I disassembled the spindle motor and examined its mechanical and electrical characteristics.

I used a small compass to mark out the N/S poles of the annular magnet within the spindle. I found 4 pole pairs.

The spindle motor is basically a brush-less motor and the winding is arranged as a 'Y' with 3.3 Ohms between terminals.

When used as a stepper I found that there are 6 X 4 = 24 steps possible. However because of the 'Y' configuration and the center-tap not being brought out micro-stepping would not be possible.

It is therefore not possible to use this motor for the chart drive as the rotation resolution would be very poor.

<p>Great idea. Years ago I toyed with making a chart recorder, but that was before the advent of Arduino, and your presentation here has me excited to try again.</p><p>Have you considered making a strip chart recorder rather than a circular one? A small one could use a roll of adding machine paper for the chart paper, and would not be a problem to change, and also could record for several days before needing changing. </p><p>Also, check into how micro-barographs of the mid-1900s wrote on the chart paper. They used a rather primitive fountain pen that needed very little pressure in order to write, was easy to load with ink, and was mounted in the vertical. (In the late 1900s, they began using small, redesigned felt tip pens.)</p>
Thanks for the suggestions! The fountain pen approach may be suitable. I am attempting to put the pen mechanism in a closed loop which should enhance the accuracy and also the drive capability.
Great suggestion, thanks! Developing on your idea I will try to use the timing motor removed from an old washing-machine timer. This should provide the required drive and the resolution of the circular movement would be finer than the stepper.
<p>Suggestion on the &quot;drive&quot; motor for the rotating chart: Instead of a stepper -- or maybe even a geared stepper, what about a quartz clock movement? 1 rev per hour or 1 rev per 12 hours ... considering the number of hard drives and CDs that have been converted into clocks, it seems a possibility! Just don't fit the hands, and adapt the clock to drive the platter?</p>
<p>Great project ....</p>
How about a rubber ring around the rim of the platter retention plate on the spindle to keep the paper in place? Just slide on and off to change the paper.
<p>Yes that is a workable solution, but it may need a custom fabricated rubber ring. Currently I am considering 4 small clips to clip the paper to the platter. The difficulty is that they need to be very narrow.</p>
And I understand your desire to not move away from the HDD parts for the marking arm... it does look superb as is.
Absolutely great Instructable!

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Bio: I am a retired Electronic Systems Engineer now pursuing my hobbies full time. I share what I do especially with the world wide student community.
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