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Measuring oscillation of a multi tool Answered

I have a harbor freight oscillating multitool that is advertised as oscillating between 10k and 20k oscillations per minute. In reality to speed control goes from 0 opm to an unknown value. I’m trying to set the tool to oscillate at 35 Hz. What is the best way to measure the frequency of the tool and set it to 35 Hz?

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A strobe light will do it. flash the strobe 35Hz and when the tool is at 35Hz it will look as if it is standing still.

Also harmonics of strobe frequency, e.g. 70 Hz, 105 Hz, 140 Hz, the tool will look like it is standing still.

35 hz = 2100 oscillations per minute. (about 1/5th of the 10k in your description of the tool)

if 10,000 oscillations per minute is the lowest setting, thats 167 Hz minimum.

If you don't own a strobe light, you *could* use a photodiode and matched phototransistor.

circuits for building detection circuits using photodiodes and photo transistors are easily found using google and other search engines.

I am naively guessing the tool you are asking about is HFT part # 63111, the one seen on this product page,

https://www.harborfreight.com/variable-speed-oscil...

and in this pdf manual for the same,

https://manuals.harborfreight.com/manuals/63000-63...

Anyway, the reason I am wondering about documentation, is in the hope it might give us some insight into how its moving parts are, uh... driven. I mean, some details about what makes it go.

On page 11 of the manual linked above there is an exploded-view diagram of all the little parts that make up this machine, and you should look at this, for to follow along with the explanation I am going to write about how I think this thing works.

Essentially what you have here, is a universal motor with fast turning shaft, in the body of the tool. Then it looks like that shaft somehow makes a 90-degree turn, and goes into the little oscillating mechanism, that makes the blade go to and fro.

Regarding the electrical parts, the what is called, "speed control", parts 33 and 38 in that diagram, I am guessing that is triac-based chopper circuit, essentially the same thing as a triac-based lamp dimmer. There is little triac that gets triggered (turned on) at some specific time every half cycle, of the AC waveform.

https://en.wikipedia.org/wiki/TRIAC#Application

Essentially what this gives you is an output waveform with less RMS voltage, due to the fact that, a lot of the time, the triac is turned off completely.

So, you know, it is not like a true speed control. It is more like a variable voltage knob. It is a way to give the universal motor less voltage, and this naturally results in less speed.

Anyway, I think that is a good overview, of how this tool works. Or how I believe it works.

Regarding the question of how to sense how fast it is oscillating, maybe a small magnet and a pickup coil? Maybe there is a place where the shaft could be painted black, with a single white stripe, so an optical sensor could "see" the flicker of that stripe spinning by?

From there I guess you could build some kind of control circuit, to feed back from the speed signal. This might be a job for a phase-locked loop,

https://en.wikipedia.org/wiki/Phase-locked_loop

Or just make a sensor that gives an analog voltage proportional to speed, e.g. by making a small square-shaped pulse for every count, and then feeding those pulses to an integrator.

By the way, that universal motor might be happy running with DC, or even PWMed DC, if you think that might be easier for whatever design you want for controlling it .