rpm

[fritz]

Hi,
Perhaps someone might be of help? I am building a my own router table
that will have a 3HP router motor rated at 23,000 rpm. I have a speed
controller that will vary the motor speed but of course there is no
actual speed readout. Depending upon the diameter of the bits one must
lower or raise the speed of the motor.
My desire is to build a sensor and display that would read out the
rpm. I am good with a soldering iron and in the past have built a few FM
radio, and other kits. Perhaps someone could point me in the direction
of a kit that might do the job or perhaps find a schematic that I could
build. I was thinking that the sensor would be some kind of light sensor
that would 'see' a white line painted on the chuck of the router?
I searched the newsgroup for 'rpm' or 'speed' and came up blank. I
also searched google with mixed results.
Any help would be greatly appreciated!


fritz

 

[Dan Hollands]

Building a tachometer with readout is a bit of a challanging task

Why not use a laser strobe tackometer:

See Pocket Tach 99 (PT99) for $150
http://www.monarchinstrument.com/Pocket%20Laser.htm#Pocket

Thats probably cheaper - and much much quicker than you can buy the parts
and build one

Dan

--
Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606
dhollan3@rochester.rr.com
www.QuickScoreRace.com
"fritz" <nofritzb@noisland.net> wrote in message
news:Kr6dnXpgsNlRSzXeRVn-jg@wavecable.com...

Hi,
Perhaps someone might be of help? I am building a my own router table
that will have a 3HP router motor rated at 23,000 rpm. I have a speed
controller that will vary the motor speed but of course there is no actual
speed readout. Depending upon the diameter of the bits one must lower or
raise the speed of the motor.
My desire is to build a sensor and display that would read out the rpm.
I am good with a soldering iron and in the past have built a few FM radio,
and other kits. Perhaps someone could point me in the direction of a kit
that might do the job or perhaps find a schematic that I could build. I
was thinking that the sensor would be some kind of light sensor that would
'see' a white line painted on the chuck of the router?
I searched the newsgroup for 'rpm' or 'speed' and came up blank. I also
searched google with mixed results.
Any help would be greatly appreciated!


fritz

 

[Jasen Betts]

On 2005-12-21, fritz <nofritzb@noisland.net> wrote:

Hi,
Perhaps someone might be of help? I am building a my own router table
that will have a 3HP router motor rated at 23,000 rpm. I have a speed
controller that will vary the motor speed but of course there is no
actual speed readout. Depending upon the diameter of the bits one must
lower or raise the speed of the motor.
My desire is to build a sensor and display that would read out the
rpm. I am good with a soldering iron and in the past have built a few FM
radio, and other kits. Perhaps someone could point me in the direction
of a kit that might do the job or perhaps find a schematic that I could
build. I was thinking that the sensor would be some kind of light sensor
that would 'see' a white line painted on the chuck of the router?
I searched the newsgroup for 'rpm' or 'speed' and came up blank. I
also searched google with mixed results.
Any help would be greatly appreciated!

and RPM measuring device is called a "tachometer".
a sdimilar device called a "frequency counter" will
give you a result exactly 60 times lower.

there are off-the-shelf and kit versions of both. good luck.

PS. There may be a tachometer for motorcycle applications that can
read upto 23000.

as for the sensor, a line on the shaft, or some sort of magnetic sensor ...

Bye.
Jasen

 

[fritz]

Jasen Betts wrote:

and RPM measuring device is called a "tachometer".
a sdimilar device called a "frequency counter" will
give you a result exactly 60 times lower.

there are off-the-shelf and kit versions of both. good luck.

PS. There may be a tachometer for motorcycle applications that can
read upto 23000.

as for the sensor, a line on the shaft, or some sort of magnetic sensor ...

Bye.
Jasen

Jasen,
Thanks for the info! I will look into both of the things you suggest.
If anything works out I will put it back here at the newsgroup.

f.

 

[Don Kelly]

----------------------------
"fritz" <nofritzb@noisland.net> wrote in message
news:Kr6dnXpgsNlRSzXeRVn-jg@wavecable.com...

Hi,
Perhaps someone might be of help? I am building a my own router table
that will have a 3HP router motor rated at 23,000 rpm. I have a speed
controller that will vary the motor speed but of course there is no actual
speed readout. Depending upon the diameter of the bits one must lower or
raise the speed of the motor.
My desire is to build a sensor and display that would read out the rpm.
I am good with a soldering iron and in the past have built a few FM radio,
and other kits. Perhaps someone could point me in the direction of a kit
that might do the job or perhaps find a schematic that I could build. I
was thinking that the sensor would be some kind of light sensor that would
'see' a white line painted on the chuck of the router?
I searched the newsgroup for 'rpm' or 'speed' and came up blank. I also
searched google with mixed results.
Any help would be greatly appreciated!
---------

A white line and a photocell can be made to work at 1 pulse/revolution- feed
it into a counter. Add more lines for more counts/ per rev. A lamp shining
through a film circle with alternating black and clear areas (i.e. a
negative) and a photocell will work (1 transistor amp may be needed). The
number of areas can be adjusted to give multiple pulses per revolution but
at the speed and needed accuracy, this won't be needed. Alternatively, glue
a small magnet on the shaft and pick up magnetic pulses.

I have done this at lower speeds- a disk giving 60 pulses/revolution, on a 1
second gate will give a 1 second count good to +/- 1 rpm. Select the number
of pulses to give a convenient scale factor. (at 24000 rpm, 6 pulses per
revolution give a count of 2400Hz -scale by 10 and you have your speed
measurement to a satisfactory accuracy.

HP also made a pulse tachometer - may be still available and price?
--

Don Kelly @shawcross.ca
remove the X to answer

fritz

 

[fritz]

Don Kelly wrote:

---------
A white line and a photocell can be made to work at 1 pulse/revolution- feed
it into a counter. Add more lines for more counts/ per rev. A lamp shining
through a film circle with alternating black and clear areas (i.e. a
negative) and a photocell will work (1 transistor amp may be needed). The
number of areas can be adjusted to give multiple pulses per revolution but
at the speed and needed accuracy, this won't be needed. Alternatively, glue
a small magnet on the shaft and pick up magnetic pulses.

So I assume that if the line is reflective enough, the light source
could be right beside the photocell. The distance between the photocell
and the line would be about 1.3 inches but that could be ameliorated by
encasing the photocell in a small tube with a focus on the line. A
magnet wouldn't work because one has to use a wrench on the collet to
remove the bit.

I have done this at lower speeds- a disk giving 60 pulses/revolution, on a 1
second gate will give a 1 second count good to +/- 1 rpm. Select the number
of pulses to give a convenient scale factor. (at 24000 rpm, 6 pulses per
revolution give a count of 2400Hz -scale by 10 and you have your speed
measurement to a satisfactory accuracy.

HP also made a pulse tachometer - may be still available and price?

I would use one or two lines so that should be well within the ability
of a photocell to capture.
Thanks for the verification that it can be done. I'll start looking
at photocells after the holidays. Later I may need some help in getting
from the photocell output and the counter out to the display. Thanks
again for the input.

f.

 

[Jasen Betts]

On 2005-12-23, fritz <nofritzb@noisland.net> wrote:

Don Kelly wrote:
---------
A white line and a photocell can be made to work at 1 pulse/revolution- feed
it into a counter. Add more lines for more counts/ per rev. A lamp shining
through a film circle with alternating black and clear areas (i.e. a
negative) and a photocell will work (1 transistor amp may be needed). The
number of areas can be adjusted to give multiple pulses per revolution but
at the speed and needed accuracy, this won't be needed. Alternatively, glue
a small magnet on the shaft and pick up magnetic pulses.

So I assume that if the line is reflective enough, the light source
could be right beside the photocell. The distance between the photocell
and the line would be about 1.3 inches but that could be ameliorated by
encasing the photocell in a small tube with a focus on the line. A
magnet wouldn't work because one has to use a wrench on the collet to
remove the bit.

if the shaft is shaped to accept a wrench it's be perfect for a variable
reluctance pickup... it could be positioned behind the shaft where it
doesn't get in the way of the wrench
______
/ _____) _
______/ //~~~\ |o|
wrench //shaft\|~|~|~|___ sensor
______ \\ /|_|_|_|
\ \\___/ |o| mounting
\ ~~~~~) ~ ----- bracket
~~~~~~

shown with a hexagonal shaft but it'll work with round a shaft with two flat
sides or with one or more notches etc too, it'll just give a different number
of pulses per rotation.

"reluctors" are used in electronic speedometer pickups etc once can
probably be had for little cost from an automotive wrecker

I had a car that had one on that sensed the flywheel's (ring gear) teeth
it was only connected to the diagnotic socket.



Bye.
Jasen

 

[fritz]

Jasen Betts wrote:

I've got bad news, it might not work - the magnetic field from the motor
might be stronger than the differences caused by the shaft aproaching and
retreating..

I just remembered similar sensors are also used in antiskid breaking
systems ...

but If it works it'll be immune to sawdust and the like....

Bye.
Jasen

Well, I'll just start with a photocell and see how it goes. If that
doesn't pan out I'll try this.
Thank you!

f.

 

[Don Kelly]

"fritz" <nofritzb@noisland.net> wrote in message
news:ksmdnbNsEdpD4S3enZ2dnUVZ_v-dnZ2d@wavecable.com...

Jasen Betts wrote:

I've got bad news, it might not work - the magnetic field from the motor
might be stronger than the differences caused by the shaft aproaching and
retreating..

I just remembered similar sensors are also used in antiskid breaking
systems ...

but If it works it'll be immune to sawdust and the like....

Bye.
Jasen

Well, I'll just start with a photocell and see how it goes. If that
doesn't pan out I'll try this.
Thank you!

f.
----------

How much space do you have? This can limit options. The machines that I
used a photocell on were 5HP units and space, away from couplings, wasn't a
problem. There a negative of a picture of a circle with black and white
sectors and about 6 inch diameter worked well. the light source and
photocell were mounted in a U bracket over the film which was put between
two sheets of 2mm plexiglass. Alignment as needed for a reflected signal
wasn't critical as one was picking up the direct beam through clear areas on
the film. It was necessary to amplify the output signal but the technology
has advanced since then (early 80's) so better light output/sensing may not
need the amplifier. It is no big deal to add one. The magnetic approach may
still work but may need some shielding. Tin can around all but the pickup
side. Even a shaft bump and a capacitive pickup can work.
--

Don Kelly @shawcross.ca
remove the X to answer
----------------------------

 

[fritz]

Don Kelly wrote:
---

How much space do you have? This can limit options. The machines that I
used a photocell on were 5HP units and space, away from couplings, wasn't a
problem. There a negative of a picture of a circle with black and white
sectors and about 6 inch diameter worked well. the light source and
photocell were mounted in a U bracket over the film which was put between
two sheets of 2mm plexiglass. Alignment as needed for a reflected signal
wasn't critical as one was picking up the direct beam through clear areas on
the film. It was necessary to amplify the output signal but the technology
has advanced since then (early 80's) so better light output/sensing may not
need the amplifier. It is no big deal to add one. The magnetic approach may
still work but may need some shielding. Tin can around all but the pickup
side. Even a shaft bump and a capacitive pickup can work.

Mr. Kelly,
I figured that a picture is worth a thousand words so I put up
some photos and layout here:

http://ballistic.org/wood/router.htm

Check it out.
Thanks.

f.

 

[Jasen Betts]

On 2005-12-30, fritz <nofritzb@noisland.net> wrote:

Don Kelly wrote:
I am a bit concerned about the distance and angle between source and pickup.
What are you using as a source? Can you shroud the pickup so that less
ambient light hits it? However, about all you can do now is to test it- then
make modifications.

The source I figured would be the smallest halogen 12v bulb inserted
inside an Aluminium 1/4" tube with the bulb at the base of the tube and
the light exiting about 3/4" later. This would shield the pickup from
the light near it. Then the photocell would also be inserted into a tube
and positioned deep within it so that this would add to the light
shielding. This would effectivley shield the photo cell from the lamp as
well as most ambient light.

if you can paint the inside of the sensor tubes matt black (or use black
plastic tubes) that'd help, otherwise the sensor will get reflected light.
a high-intensity LED might work better than an incandescent lamp - often
they produce a focussed beam, and are certainy more immune to vibrations)

As you say, all I can do is build it and test it. Several years ago
I bought a computer addon digital storage oscope (PCS64i) which I
haven't tried out but it should give me a good look at the photocell out.
Thanks for all the suggestions. It's up to me now to find the
appropriate photocell,

the motor is probablly too fast for a photo-resistive cell (like CdS cells)
a photo-transistor or photo-diode is probably the way to go. use shielded
cable. it's more important that the photo-sensor only see the shaft than
that the light only shine on the shaft, if there's ambient light shining on
the shaft what harm can it do?

Bye.
Jasen

 

[fritz]

Jasen Betts wrote:

if you can paint the inside of the sensor tubes matt black (or use black
plastic tubes) that'd help, otherwise the sensor will get reflected light.
a high-intensity LED might work better than an incandescent lamp - often
they produce a focussed beam, and are certainy more immune to vibrations)

I had thought of painting the inside but I also had thought of
unshrunk, heat shrinkable, tubing. A high intensity LED I hadn't.

the motor is probablly too fast for a photo-resistive cell (like CdS cells)
a photo-transistor or photo-diode is probably the way to go. use shielded
cable. it's more important that the photo-sensor only see the shaft than
that the light only shine on the shaft, if there's ambient light shining on
the shaft what harm can it do?

23,000 rmp is a little less than 400pulses/ps. Is that too fast? I

thought that I would do a trial with the LED and one of the radio shack
generics hooked up to the little scope. See what I get. After the new
year....

thanks
f.