motor speed sensing

[Jaja]

Hi, i'd like to sense the speed of a motor by pointing a light source to
it and capturing the reflected signal from it. The motor is enclosed but
there are a couple of holes at the bottom of the case to which i could
attach a reflecting film or something like that.

Any suggestions?

 

[Jerry G.]

Look at the DT-207L laser tachometer. This is a very precise unit. I doubt
you can build one yourself at home, but you will find this interesting.

http://www.davis.com/showpage.asp?L3ID=457

--

Jerry G.
=====


"Jaja" <jaja@simon.net> wrote in message news:c95hhu$c9g$1@news.bnl.gov...
Hi, i'd like to sense the speed of a motor by pointing a light source to
it and capturing the reflected signal from it. The motor is enclosed but
there are a couple of holes at the bottom of the case to which i could
attach a reflecting film or something like that.

Any suggestions?

 

[JeffM]

sense the speed of a motor by pointing a light source to it
and capturing the reflected signal from it.
Jaja

Is this a 1-time deal?
Will you need an unsupervised (no human) technique?
Will this be used for feedback/control?

If not, see Jerry's post and
http://groups.google.com/groups?&threadm=f8b945bc.0310111233.67a6315e@posting.google.com
(This is an optical technique, but not reflective. It requies a
human.)

 

[Jamie]

it depends./
if your only looking for absolute motion sensing to ensure
the unit is fully stopped there are a couple of ways, i have
done them both.
on AC sync type when you turn off the contactor it will spin down
thus giving you a regenerative output of AC voltage. not much current
how ever but enough for electronics to know if the unit is at near stop.
i use a TL082 chip with some zener input clipping to protect it.
its a comparator circuit and will detect down to 0.002 volts of output
which is not much of a rotation at all in the motor. the reason why
we need this?, is because if the direction was to change on the motor we
must make sure it is at near stop, or in our case you have some other
unit that tends to randomly fire off a clutch that is connected from the
output of this motor thus causing an unexpected jerk in machine parts if
the operator happens to turn off the unit and restart it before the motor
spins down.
the other way was for us to use an LED and PHoto Detector both side
by side aimed close to a shaft point. the rotation of the shaft caused the
IR to get modulated. we made what you may call a AM detect or a slop
detector on the signal, that can be done using an OP-amp with both
inputs decouple via a resistor and small cap on the other side to delay
the remote input ..
this will generate a pulse output when surfaces change just alittle.
you can then run this into a one shot timer to hold the output on..


Jaja wrote:

Hi, i'd like to sense the speed of a motor by pointing a light source to
it and capturing the reflected signal from it. The motor is enclosed but
there are a couple of holes at the bottom of the case to which i could
attach a reflecting film or something like that.

Any suggestions?