Andrew Morris had a problem with a small rotary tool built in China. The tool fit nicely in the hand and was useful for precision cutting, drilling, and polishing. Yet for delicate work, the tool was in bad need of a speed regulator.
Andrew had developed an analog motor speed regulator back in the mid-1990s, but this time, he wanted the benefits of a digital regulator. The digital version was just as efficient, but it was less expensive to build and easier to assemble. The digital circuit also provided more torque.
Andrew Morris' microcontroller-based DC motor speed regulator brings control for delicate work.
After this article was written, I created an in-line version of the digital speed regulator. Experience with the circuit so far has shown the minimum speed setting to be perfect for my needs. Sometimes it's also handy not to have a control box in the way when not working at a bench or table. The tiny in-line version has no knob, switch, or LED and runs at one set speed. This required disabling the overload timer as there is no quick way to reset it without the on-off switch. The user must take care not to keep the motor overloaded for an extended period of time. The mini-drill will probably get noticeably warm before there is risk of damage.
The circuit was built onto a tiny piece of perfboard, 4 holes by 18 holes in dimension and covered with heat-shrink tubing. Please email me if you would like construction details. My email address is in the article.
You can see in the video that when the mini-drill itself was switched off, the overload LED came on and had to be reset by switching the control unit off and then on with the tool itself on (or switched on within 3 seconds). This is caused by the fact that when the motor is disconnected, the circuit does not see the feedback from the motor and thinks that the motor has stalled. I was aware of this when I designed it, but I didn't see it as an issue until I used it for the first time on a real job, restoring an antique cigarette lighter. It became a bit of an annoyance. The switch on the tool is much more convenient to use than the switch on the control box. I solved the problem after the video was made by adding a resistor (R10) to fool the electronics into thinking that the motor is still connected and running at full speed when the motor is actually disconnected. Due to the extremely low relative resistance of the motor, the resistor is effectively bypassed when the motor is connected and has no effect on circuit performance.
I also changed the software code to boot-up at zero speed, instead of max speed as was the case in the video. You may have heard it start up at high speed and then very quickly come down to the set speed. This "up-to-speed" help was not needed due to the fact that the motor is unloaded at start-up. In the invention for which this algorithm was originally created, the motor was started up under load, requiring this extra boost. This change has no effect on the gadget's performance.
This gadget turns something that's almost useless into something that any gadget maker should find useful. The AC-powered rotary tools are great for heavy work, but this gadget is exactly what is needed for delicate work. The tool fits very nicely in the hand, giving the user excellent control of the drill bit, cutter or polisher. This is critical when replacing tiny surface-mount parts on printed circuit boards (i.e. cutting away old parts), where uncontrolled speed or poor manual control could tear the part off the board, causing damage. In addition to that and many other things, I've also used my older mini-drill and analog speed regulator to polish away oxidation from the tiny switch contacts in my wireless computer mouse. High speed would have torn the delicate contacts right off the tiny switches. With this gadget, the mini-drill has enough torque for most not-so-delicate work as well.
Since this article was written, the digital speed regulator has worked so well for so many projects, that the analog unit doesn't get used anymore. I keep it around since it has a lower minimum speed than the digital unit, just in case. Getting stable performance from the digital circuit at extremely low speeds might be a challenge. I haven't tried it, since the current minimum speed of the digital control has so far been perfect for my needs.
In our third annual contest, Design News and Allied Electronics are going to crown a winner in early 2016 for the best reader gadget submission this year, and once again, you, the readers, are the judges!
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