Motor Speed Sensing

hello, have question regarding project working on.

i working on speed sensor, measure rpm of dc motor. approach use hall effect sensor. have gotten accurate rpm readings (around 6600 rpm verified optical tachometer). test done putting magnets on spinning device such drill or shaft of motor. problem wanted make little test rig in can run speed test hour automatically , log data excel (which works fine me). issue no hall effect sensor have used can detect rpm through motor's shield (armature). rig have shaft covered , unfortunately cannot place magnets on shaft because of that.

any ideas? possible?

thanks in advance!

ps: feel free ask questions.

google microchip's app. note an893, see if worst try:

quote

sensorless rpm measurement
low-cost rpm measurement can performed a
bdc motor measuring emf voltage from
the motor (see figure 3). bdc rpm directly
proportional emf voltage. since bdc
motor can modeled inductive load, voltage
across motor equivalent inductance multi-
plied di/dt. in application, 12v, 9600 max rpm
bdc motor used. measure emf
voltage, turn modulated fet "off." cause
the current flow in opposite direction. after
initially shutting off fet, di/dt must stabilize before
taking measurement. in order use picmicro ®
microcontroller a/d converter, measured voltage
must between 0v , v dd . since emf
voltage can between 0v-12v, voltage divider
circuit used scale emf voltage between
0v , v dd . using microchip's msp6s26
programmable gain amplifier (pga), gain of 1 is
used buffering scaled voltage being
measured pic16f684 a/d channel (see
equation 8 calculating rpm).

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