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.
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.
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.
Before this article was published, when I added R10, I sent the editor an updated pdf file and for some reason, the first pdf file was published. I have resent the corrected file twice and it has not been posted. R10 is a 22k resistor connected between Q2's drain and ground. Please email me if you would like the updated pdf file.
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.
Please email me if you want to build this gadget and don't have the means to program a microcontroller. If you live in the US, I'll send you a programmed PIC for the cost of the blank PIC and estimated postage (unless that practice gets out of hand). Just let me know whether you intend to build the full-sized or the in-line version of the control, so I know whether or not to disable the overload timer. I'm sorry that I have no PCB (printed circuit board) artwork to give to you for either version. Due to the circuit's simplicity, a PCB layout should be very easy to do. I find it easier to hand-wire something than to make a PCB if I'm only making one copy of it. Hand wiring also gives me more packaging flexibility. If you create a PCB layout for it, please share it.
My lifelong interest in motor speed regulators (also called governors) began in high school in the 60's, when I had a cassette recorder, a portable record player and later, an 8-track tape player, all with faulty mechanical governors. I eventually figured out how the cleverly designed Philips (Norelco) governor circuit worked. Most manufacturers wouldn't pay Philips for the rights to use their patented circuit. I repaired cassette recorders and 8-track tape players for other people by bypassing the mechanical governor and adding my version of the Philips governor circuit. The simple two-transistor circuit was easy to build and tuck away inside the set. Nowadays, cassette recorder motors have built-in governor chips.
Back in 1987, I designed a "bang-bang" motor speed regulator using a 555 timer and was disappointed to discover that it was no more efficient than a linear regulator. For a fixed speed and load, the current draw remained constant with changes in the power supply voltage. The motor got warm instead of the output transistor. The 555 timer turned the motor on for a fixed period of time whenever the motor's BEMF dropped below a certain level. The hardware configuration was very similar to that used in this gadget. The patent application referenced in this article explains "bang-bang" motor control in much more detail. BTW, the aforementioned mechanical governors are examples of "bang-bang" motor control.
This gadget is not just about motor speed regulators, it's also about patents. I worked for 5 years in a R&D (research and development) center, where I developed the motor control algorithm and where we worked with patents every day. I had been a member of an inventor's club for about 15 years before that. There may be a lesson here for other gadget makers who hope to patent their inventions. I hope to spark some conversation on the subject of patents, as well as the gadget itself.
This discussion of the patent process is remarkably similar to what I experienced while doing engineering technical consulting for large corporations.....right down to the description of the witnesses and the many co-inventors.
I was routinely asked to evaluate patents for useful information. Often this evolved into equally simple ways in which the whole patent could be subverted.
Without offering any moral judgment, I'll simply note that my engineering career spanned enough years to see what was once ethically questionable becoming a common and valued engineering practice.
It may be at a patent was never as much of a strong protection in reality as it was in folklore. How could it be wit such a low level of international support?
My advice to startups is not to assign too much importance to patents. Lack of a patent should not be the deciding factor. It may be that more businesses failed to get started from fear of not having a patent than ever failed from patent challenges. In my opinion, the real value of the patent process is in the way that it promotes education and advances knowledge.
I would like to see the patent process continue evolving away from it's legal and protective roll and more towards becoming a vehicle for recognizing unique achievement and distributing new ideas.
I believe that when my former employer closed the facility where I worked before the final step in the patent process could be completed, they effectively served my invention to the sweeper manufacturer on a silver platter. The timing is right and the patent office is the first place someone would look, who was looking for a cheap motor speed regulator for a consumer product. Even if the sweeper manufacturer intended to develop their own motor control, they almost certainly would have done a patent search first in order to avoid creating a costly patent infringement. But then, something like this probably happened: "Hey, check this out! Here's just what we need, and for some reason it didn't get patented. It's now in the public domain and a prior-art search has already been done. Wow! Let's grab it!"
I'm glad the patent application failed. I wouldn't have gotten any money from it anyway, but at least now, people are able to use my idea. My former employer would have used it in a very limited application, if at all. It's common practice for big companies to patent ideas that they're not going to use and then sit on them to prevent competitors from using them. That's probably why they tried to patent my invention, even though they didn't really want or understand it. If they really wanted it, they would have assisted me somehow in getting witness signatures on the disclosure form (explained later) when I couldn't find someone, rather than let it become public domain a year later. The purpose of the invention was to replace a piece of mechanical hardware with software, saving some recurring cost in a mass-produced product. This proved to be much more complex than just the speed regulator.
The make and model number of the floor sweeper that I believe uses my speed regulator algorithm was not given out in order to avoid potential problems from the manufacturer for publicly disclosing details of their design. Anyone interested can contact me individually for that information. If you like, I'll send you a JPG copy of the hand-drawn schematic and a PDF copy of the user manual (or the link to it). It's a good example of another use of the speed regulator algorithm, and how to interface with it. For example, the floor sweeper uses a pushbutton to sequentially select one of three fixed speeds and has three speed-indicating LEDs, using just an 8-pin PIC. I don't have access to the sweeper's code-protected software, but I used a digital oscilloscope to observe its operation.
I did not give out the name of my former employer, or what the invention was that employed my speed regulator algorithm, in order to avoid potential problems from them for publicly disclosing "company confidential information". Instead, I made reference to a public document that contains that information. The patent application also supports my claim that I invented the speed regulator algorithm. The patent lawyers did a "prior-art" search and would have discovered if someone had patented or tried to patent it before me. For those not familiar with patent searches, here is the link to the aforementioned document:
Yes, it measures the back EMF after momentarily disconnecting the motor. This scheme would not work with a universal motor. You need to use IR compensation for that kind of motor. The analog version of this gadget, referenced in the article, uses IR compensation. A scaled-up version of that circuit would work fine with a universal motor, like the Dremel Moto-tool.
Also, why do you need to have extremely high speed for working on hearing aids. A Dremel moto-tool runs extremely fast without boosting the voltage. I'm afraid to do delicate work at high speed. Do you know something I don't? High speed would melt plastic parts.
The invention disclosure form is a document produced by my former employer's legal department to meet the requirements of the patent office. Besides the signatures of the inventors, the document requires the signatures of two "witnesses" to whom the invention is disclosed. Since I was already laid off when the form was emailed to me, I was unable to find qualified witnesses to sign it. I still have the pre-filled, confidential document on my hard drive.
I've signed many such witness forms for co-workers. I would have been given a copy of the related patent application to read and understand. My name would not have been on any of those patent applications, as a witness may not be a co-inventor. I am a co-inventor in 12 patents. The patent for the motor speed regulator would have been the second one, of which I would be the sole inventor. My co-workers were all mechanical engineers.
Gotta love those PIC projects that you can not only build at home - but are extremely useful in day to day applications. I really like your project - the improved torque with lower speed capability makes it very versatile. Cool gadget!
Regarding your patent discussion- I have found that every time I thought about patenting something (personally, not as part of a corporate patent) it simply wasn't worth it. The burden of prosecution if someone infringes on your patent falls on the patent holder and there is no way I could go against a team of corporate lawyers if someone stole my design.
It's a shame to have to say it, but IMO, patents are no longer a viable option for the individual, except in rare cases. I just wanted to stir up some conversation on the subject and see what came out of it. I also wanted to tell my story about what happened to the patent on my invention, which is being used in this gadget.
I looked into it with my portable trail obstacle small business (no one makes portable trail obstacles and hubby and I do all of the design work). We ended up just using a trade mark symbol for the phrase "portable trail obstacles" which would give us some ownership rights depending on what state you are in...and we could have done a patent pending for a small fee without actually going through the trouble of getting a patent - but as you said, it would be whoever had the deepest pockets would have control. Just not worth pursuing for a business run out of a tiny home office. Also, people can tweak their design enough for a patent to be issued to their design even if it was originally copied from yours...
One of the things we did at the R&D center was break other people's patents. We worked with the lawyers to find loopholes to use ideas that we wanted. This was not the fault of my former employer, but is common practice in big business.
So, yes Nancy, we did exactly what you're talking about. We would make some small change to get around the patent.
Andy, the small changes you describe to get around patents doesn't seem to work in the world of smartphones. There is a great number of patent suits in that world. Some of the suits are quite successful at banning companies from selling their products in a number of countries.
Nancy, I agree PIC projects are really cool and fun to build. In reading Andrew's Build document, the assembly language code used to monitor and control the motor is quite typical of Microchip. All of their microcontroller software reference design documents illustrates the target application with assembly code. Here's a link for a cool PIC Lab Development kit including the PIC10F microcontroller.
What a cool looking kit, mrdon - that looks like a lot of fun. Thanks for the link, it makes me want to start playing with my PIC stuff again. Another thing I love about using Microchip products is the technical support that is available, their extensive documentation of their PICs, and their forum community. I have gotten some excellent help when I was stumped on a project that I was contracted to do and wound up making a great friend and partnering on the project with him that (speaking of patents) was eventually patented by the company we wrote the code for.
Sorry to hear that armorris - that has not been my experience. I am not saying you did this, but what I did find is that there is little patience on the forum for questions that are just thrown out there without evidence of the person trying to solve the problem themselves first. I did notice that when I went into detail as to what I had already tried and where I was stuck with code examples - plenty of folks jumped in with some great suggestions. There are also different areas that help localize where you should post to get the best chance of receiving knowledgeable help. You might want to give it another try - or maybe jump on it to help someone else out ;)
Thanks, Nancy. I'll take your advice next time I need help. When I needed help, I didn't actually ask for it. I tried to find the answer from comments that were already posted, but the archive is just so huge, and I do not know how to sort through them in any useful way.
I saw plenty of people willing to give advice for a fee.
For a fee???? Times have changed...but I am willing to bet there are still plenty of folks who love what they are doing and love sharing their knowledge with others. I needed some help with my last project and had four different people offering very intelligent suggestions that helped me resolve the issue I was having - and they never asked for compensation.
I just visited the PICLIST for the first time in about 2 years. I can't find any sign of somebody asking money for help. I see that it is under new managment from when I last visited it. I don't know where I got that idea. It must have been for some big-name consultants like (but not necessarily) Myke Predko. I guess the big-name consultants probably have to ask a fee to prevent being swamped with questions. I'm sure that there are plenty of lesser known wizards that are willing to help. I'll be less reluctant to go to the PICLIST for help next time I need it. Maybe I'll even try to help someone else, since I have acquired a fair amount of experience with PICs over the years. I do not consider myself an expert, however.
When it comes to learning PIC microcontrollers, Mike Predko is an excellent teacher. I have several PIC microcontroller books by Mike and his writing style make developing microcontroller applications a breeze. His books also provide solutions on debugging software errors as well.
In 1979 I designed an inexpensive PWM speed controller for use with a 12 volt DC motor. The control kept the torque at maximum because of not having a voltage drop, other than the Vce of the switching transistor. And the design was not patentable, so I was home-free there as well. It use a 556 dual timer chip with one section as an oscillato9r and the other section as a comparator for the ramp waveform and the setpoint voltage. One intermediate transistor buffer stage and it drove a 100 volt 5 amp switching transistor, which I don't recall the number. The BOM cost, except for the control pot, was just under $5. Probably all of the parts are still available, and there is probably a much better output transistor available today. Possibly even a FET of some type.
Mrdon, the motor controller had one section of a 556 generating a triangle wave from 1/3 Vcc to 2/3 Vcc, the other section used a potentiometer to vary the reference for the second section that is used as a comparator. The signal from the comparator feeds a small transistor that drives a TO-3 power transistor.
The circuit to generate the PWM signal came from an applications note. So it is really not anything that complex. I may be able to find a copy of the circuit someplace in my archives, but that may take a while.
William K. Thanks for your response. Was your device a controller or a regulator? In other words, how did you sense the motor speed? I can build a DC motor controller with a 555 timer, a pot, a MOSFET and a few small components. Such a device would not be a speed regulator as it would have no feedback.
OK, it was a variable speed drive. What we used for feedback was the effect of the pump delivery rate, which was controlled by the motor speed. It was not clear that any feedback was used with the tool speed controller, either. Small tools seem to work better if they do slow as the load increases.
The other thing is that a motor controlled with PWM may not have as much speed drop as the load increases, since PWM control does not increase the source impedance.
Cool that someone did this, but if I'm not mistaken these types of tools are readily available for purchase. So NOTHING NEW HERE. Can't we find more interesting and unique gadgets to feature in this segment. Maybe I'm just being a curmudgeon.
I have not seen a drill motor that is so easy to hold in the hand that has decent speed regulation and torque. I have a Dremel Moto-Tool that is great for big jobs, but it's awkward to use for delicate work. The Chinese mini-drill is dirt cheap and with my speed regulation technique, a piece of useless Chinese junk has been turned into an indespensible tool. I stand by its usefullness.
I own a cordless, three-speed electric floor sweeper that has my speed regulation system in it, so somebody else has found my invention useful. IMO, there is no cheaper way to regulate the speed of a small DC motor. I helped a guy in India tune the software program to run a larger motor for a CNC machine he was building. It was working for him, the last time I talked to him.
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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