A simple technique for measuring rotation speed in rpms using an oscilloscope.



Measuring rpm with an Oscilloscope

In a related article I recently published, I showed how to adapt an old drill to a new battery... Since most new batteries are higher voltage, the rotation speed normally increases. I wanted to measure the rotation speed of the drill before and after the conversion. Professionals probably have dedicated tools for this, but since I don't perform this type of measurement very often, I tried a simple approach using an oscilloscope. This article shows how this is done.

The magnetic approach

The approach I used involved placing a small neodymium magnet in the metallic head of the drill "chuck" (Figure 1). Neodymium magnets (such as these ones from Amazon) are surprisingly strong and won't normally be dislodged by the drill's rotation (though care should be taken if the drill's rotation is very high).   



 Figure 1 - The Neodymium Magnet

A as the drill rotates, the magnetic field will vary as the magnet nears a given stationary point. This can induce a small but measurable current in an external inductor. If that inductor is connected to an Oscilloscope, you can then easily measure the rotation period. It's as simple as that! For the inductor/detector I used a small transformer from my parts bin. This is a telephone type transformer (I believe I took it from an old modem) and thus is responsive to the low frequencies involved (Figure 2). I haven't experimented with other transformers, but I suspect many transformers commonly found in power supplies or audio equipment could also be used.


Figure 2 - Signal Transformer used as Detector

My "test setup" is depicted in Figure 3. The oscilloscope probe connects to one end of the transformer while the other end is placed near the drill. When the drill is turned on, a small, periodic signal can be observed in the oscilloscope. Setting the scope's time base to about 50 ms should be a good starting point.


 test setup

Figure 3 - Test Setup

Figure 4 shows the transformer output for the "low-speed" mode in my drill. To calculate the rpms, simply divide 60 (seconds in a minute) by the period of the waveform (176 ms = 0.176 s in this example). The measurement in Figure 4 suggests a rotation speed of about 341 rpm. 


Figure 4 - First Speed easurement: 176 ms = 60/0.176 = 341 rpm

The drill's own manual rates it at 460 rpm max for this range (see Figure 5), so the measurement is plausible, though a little lower than specified.



 Figure 5 - Drill Specifications

Next I tried the drill's second (high-speed) setting where the manual specifies 1375 rpm maximum. As shown in Figure 6, I measured  60/0.050 = 1200 rpm in this case which seems quite  reasonable.



Figure 6 - Second Speed Measurement: 60/0.050 = 1200 rpm

So far, the measurements were made using the specified nominal voltage of 13.2V. As explained in the article, I increased the drill's operating voltage significantly, to about 18V. Figure 7 shows the resulting measurement in the "high-speed" setting, this time with the drill running at 18V.


Figure 7 - High-Speed @ 18V: Speed = 60/0.035 = 1714 rpm.

As expected, the speed increased significantly, jumping to 1714 rpm from an initial measurement of 1200 rpm. Notice that the increase in rpm (1714/1200 = 1.43) is not too far from the increase in voltage (18/13.2 = 1.36) though I don't know enough about how these motors work to assert whether this is the expected behavior. 

So, in summary, this is a simple and quick way to measure rpms when all you have around is an Oscilloscope, a couple of spare magnets and a transformer. Hope you found this method useful.

Comments, questions, suggestions? You can reach me at: contact (at sign) paulorenato (dot) com