Hello, I have a question regarding a project I am working on.
I am working on a Speed sensor, that would measure the RPM of a DC motor. My approach is to use a hall effect sensor. Now I have successfully gotten accurate RPM readings (around 6600 RPM which I verified with an optical tachometer). However this test was done by putting magnets on a spinning device such as a drill or the shaft of the motor. My problem is that I wanted to make a little test rig in which I can run a speed test for about an hour automatically and log the data into excel (which also works fine for me). The issue is that no hall effect sensor I have used can detect RPM through the motor's shield (armature). My rig will have the shaft covered and unfortunately I cannot place magnets on the shaft because of that.
Google Microchip's app. note AN893, see if it worst to try:
Sensorless RPM Measurement
Low-cost RPM measurement can be performed with a
BDC motor by measuring the back EMF voltage from
the motor (see Figure 3). The BDC RPM is directly
proportional to the back EMF voltage. Since a BDC
motor can be modeled as an inductive load, the voltage
across the motor is equivalent to the inductance multi-
plied by dI/dt. In this application, a 12V, 9600 max RPM
BDC motor was used. To measure the back EMF
voltage, turn the modulated FET “off.” This will cause
the current to flow in the opposite direction. After
initially shutting off the FET, dI/dt must stabilize before
taking the measurement. In order to use the PICmicro ®
microcontroller A/D converter, the measured voltage
must be between 0V and V DD . Since the back EMF
voltage can be between 0V-12V, a voltage divider
circuit is used to scale the back EMF voltage between
0V and V DD . Using Microchip's MSP6S26
Programmable Gain Amplifier (PGA), a gain of 1 is
used for buffering the scaled voltage that is being
measured by the PIC16F684 A/D channel (see
Equation 8 for calculating RPM).