App note: Measuring RPM from back EMF


Interesting App note from Precision Microdrives on finding RPM using back EMF. Link is here

Measuring the speed of a motor can be an important requirement for some applications. For example DC and gearmotors that are moving loads may require close monitoring or adjustment of the output speed.

External sensors are typically used for speed measurements, such as Hall sensors with a magnet mounted on the shaft, or transoptors with a slotted plate (also mounted on the motor shaft).

These sensors that can give accurate information about motor speed require space and increase cost of parts. Sometimes it’s impossible to add an external sensor to a motor due to space or pricing restrictions.

We can properties of the motor itself for a low-cost RPM measurement, which is based on the back EMF. In fact the back EMF is a very important measurement for haptic feedback chips when driving vibration motors and LRAs.

We’ll explain a sensorless method, where we use an Analogue to Digital Converter which is onboard our microcontroller that generates the PWM drive signal. We haven’t included any additional measurement chips or components.

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1 Comment

  1. Careful, I am pretty sure the first equation in the articel is wrong. It describes the current through a coil as being: I = dV/dt which is clearly wrong (this would mean that there is no current flowing, when you apply a constant voltage, which is wrong). Instead it must say: U = L*dI/dt. However the description what a coil does (limit the rise of a current) is correct in the article.

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