DIY Tachometer of micro motors?

[SandyEggoJake]

Confession:  I like model trains.  And tools. 
Tools for model trains?  Triple score!

Hence my interest in n scale truck reamers and testing of such https://www.therailwire.net/forum/index.php?topic=43185.0

But another of my (too many) projects has me pondering a DIY tachometer to measure RPMs of motors.   Open to suggestions!

I've found at least two paths:

A) electrical tach: use an oscilloscope to calc RPM based on the Commutation Spikes (https://www.precisionmicrodrives.com/tech-blog/2011/06/08/using-dc-motor-commutation-spikes-measure-motor-speed-rpm)

 and

B) optical tach: Use an IR sensor to count passing of a target to calc RPMs (http://www.instructables.com/id/Simple-Motor-Speed-Tester-Tachometer/)

The optical tach is straightforward, but this version is a bit ghetto (tinfoil on an armature?).  Would require me to uninstall motors from locos to test.  So I'm currently leaning to the other... to make a simple scope. 

I was thinking of buying one of these cheapo DIY kits ($20 DSO138), which seems like it would do the job.  But might take a few days to get and I'm thinking I could use stuff on on hand to build one better suited for this task.  So looking at something I could employ using an arduino, an android based mobile phone or both.   Along with my well stocked supply of salvaged and new components, of course.

There are several options for both:

The Arduino based solution would use a 8-bit AVR RISC-based microcontroller running at 16 MHz (my rarely used OSEPP UNO R3 Plus). 

The Android would use one of my many Samsung phones, and freeware from several android based scopes.

I've looked as several online options using these devices and both seem doable with parts on hand and modifiable to give RPM output.  But before I reinvented the wheel here, I thought I would toss this out to TRW hivemind.  I sure I'm not the first down this track...



 

[mmagliaro]

Far be it for me, of all people, to suggest "not building it all yourself"   

But one of these:
https://www.ebay.com/itm/Professional-Digital-Infrared-Photo-Tachometer-RPM-Motor-Speed-Gauge-Tach/292326598007?epid=1711978748&hash=item441005e577:g:i6UAAOSwGPxaBwZn

is only about 20 bucks.  I have one very similar to this model that I bought years ago, and it works great.
On a bare motor shaft, I can color the whole thing with black Sharpie, and then scratch off one clean silver spot,
then train the IR beam on the shaft and it will measure the RPMs.   I also have a simple black cardboard disk with a piece
of shiney tape on it in one spot that I can tack onto the shaft with a dot of Walthers Goo;  shoot the light at the spinning disk and it measures the RPMs.

[SandyEggoJake]

A worthy optical tach option, Max.  Thx!

Obviously useful for bench testing free spinning motors plucked from a chassis. And I'm thinking one nice feature could be shooting gears to get their RPM, for which the electronic tach wouldn't work directly (you'd need to do the ratio math).

But I'm kind of interested to test motor RPM both under load (assembled loco) as well as free spinning.  Even pondering a stripped down electronic tach package stuffed in a gondola with WIFI to the Android device, to allow for "on layout" RPM reading.  I know... perhaps crazy / useless?

[peteski]

Since model trains do not use variable gear ratio transmissions, the motor's RPMs are directly related to the wheel's RPMs and to the speed of the model traveling on the track (unless the wheels or the drive train is slipping).  I don't see any need to know the RPMs of the motor on a loco in motion (the speed and/or the wheel RPM's already give you motor's RPMs indirectly).

But from a "cool gadget" perspective, it would be a cool gadget to own.

[Doug G.]

Handy to speed-match locos in DC.

Doug

[nkalanaga]

Doug:  Very true, but that can still be done directly using the wheels, at least on diesels, as most of them SHOULD have the same wheel diameter.  As long as the wheel RPMs are the same, the speed should be the same.

For steam it wouldn't work, directly, from the wheels or shaft, as one also has to factor in the wheel diameter.  That would require calculating the actual RPMs for the desired speed, and would be easiest to do on the wheel.

[peteski]

Handy to speed-match locos in DC.

Doug

Why would monitoring the motor RPMs be any different for speed matching than the wheel RPMs, or the model's speed (as I mentioned in my previous post)?

And to Mr. Kalanga: why would wheel diameter make or brake the speed matching process?  Different manufacturer models might have different gear ratios (even if the wheel diameter is the same).

[GhengisKong]

Far be it for me, of all people, to suggest "not building it all yourself"   

But one of these:
https://www.ebay.com/itm/Professional-Digital-Infrared-Photo-Tachometer-RPM-Motor-Speed-Gauge-Tach/292326598007?epid=1711978748&hash=item441005e577:g:i6UAAOSwGPxaBwZn

is only about 20 bucks.  I have one very similar to this model that I bought years ago, and it works great.
On a bare motor shaft, I can color the whole thing with black Sharpie, and then scratch off one clean silver spot,
then train the IR beam on the shaft and it will measure the RPMs.   I also have a simple black cardboard disk with a piece
of shiney tape on it in one spot that I can tack onto the shaft with a dot of Walthers Goo;  shoot the light at the spinning disk and it measures the RPMs.

That would probably work well for a chuff cam stand in as well.

[Doug G.]

I guess I was thinking of having two motors and seeing if their RPMs were the same. Given the same friction, etc. in the rest of the drive train, the speeds of the two (or more) locos should be the same.

Or, in reverse, say you had two locos that ran at different speeds. You could remove the motors from them and at least test them to see if they run at the same RPM at different voltages. If they're different, you have found the problem. If the same, look elsewhere.

I realize these things could be done using different methods (listening to motor pitch, for instance) but for those interested in tachs and such, it would be useful. We used a tachometer where I used to work manufacturing 8-Tracks for Ford if there were problems with motors or related parts.

Doug

[SandyEggoJake]

BTW, while I'm still exploring Android based electronic tachs, for those with Arduinos, one super cheap option for an optical tach is the Arduino IR sensor...$1.45 plus shipping.

https://www.ebay.com/itm/New-Digital-Tachometer-Speed-Module-Sensor-for-Arduino-UNO/382043372608?_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D1%26asc%3D20160908110712%26meid%3D75ee627dc44d4972a578b5ced26f5f96%26pid%3D100677%26rk%3D1%26rkt%3D30%26sd%3D222188582104&_trksid=p2385738.c100677.m4598

[nkalanaga]

Peteski:  For a steam loco, the diameter makes a big difference.  Take two 2-8-2s, the GN O7 and the D&RGW K36.

The O7 had 69 inch drivers.  At 60 mph, they would be turning just over 292 RPMs.  They could easily pull a train at 60, especially on the plains.

The K36 has 44 inch drivers.  At 60 mph, they would have to turn a little over 458 RPMs.  They weren't supposed to go that fast, but did, occasionally, between Alamosa and Antonito.

So, just knowing the motor RPMs of two models, with different driver diameters, wouldn't be enough to match the speeds.  You also have to know the diameter of the wheels.

With diesels, most first and second generation American diesels had 40 inch wheels, so knowing the RPMs would be enough there.  As long as two models had the same RPMs, they would be traveling at the same speed.

[peteski]

Peteski:  For a steam loco, the diameter makes a big difference.  Take two 2-8-2s, the GN O7 and the D&RGW K36.

The O7 had 69 inch drivers.  At 60 mph, they would be turning just over 292 RPMs.  They could easily pull a train at 60, especially on the plains.

The K36 has 44 inch drivers.  At 60 mph, they would have to turn a little over 458 RPMs.  They weren't supposed to go that fast, but did, occasionally, between Alamosa and Antonito.

So, just knowing the motor RPMs of two models, with different driver diameters, wouldn't be enough to match the speeds.  You also have to know the diameter of the wheels.

With diesels, most first and second generation American diesels had 40 inch wheels, so knowing the RPMs would be enough there.  As long as two models had the same RPMs, they would be traveling at the same speed.

Of course, the motor's RPMs at a certain speed are directly related to the driver/wheel diameter and the gear ratio of the model.
You missed my point which is why would we need to monitor motor RPMs in the first place. The drive-train between the motor and the track is fixed.   I don't see any need to know the motor's RPMs when speed matching locomotives.  All we care about is the actual speed of the model traveling on the track and the speed step of the DCC throttle.  I couldn't care less about the motor's RPMs.

[nkalanaga]

OK, I misunderstood you, and I agree.  Knowing the driver RPMs is all we need.  What's inside the loco is irrelevant.  In fact, it doesn't even have to be a motor.  If someone can build a DCC-controlled live steam model, setting the speed table would work the same.

My point was that, no matter where you measure the RPMs, that that isn't enough to determine the speed.  One also has to determine the actual speed, at a given RPM, for that model, and that depends on the driver diameter.  One can't say that, because the wheels, or motor, are turning at the same speed on two models, the two will run at the same speed, unless the wheels are the same diameter.

[narrowminded]

Far be it for me, of all people, to suggest "not building it all yourself"   

But one of these:
https://www.ebay.com/itm/Professional-Digital-Infrared-Photo-Tachometer-RPM-Motor-Speed-Gauge-Tach/292326598007?epid=1711978748&hash=item441005e577:g:i6UAAOSwGPxaBwZn

is only about 20 bucks.

I had used a similar non contact one from a customer's shop that is basically like that but a pistol grip.  For the few times I needed it or just wanted to confirm RPM's, I had a way.   In that shop it is used primarily for setup RPM and inspection of machine tools.  When I checked that link and then looked at the other equivalent links, the same model was available from a vendor in GA for $10 and change, including shipping.   At that price I had no choice and ordered one.   For the stuff I've been doing it will be very convenient to have that tool in the box, available on a whim.  Thanks, Max.

[SandyEggoJake]

Ten & change?  Rip off!   

How 'bout $8.60 w/ shipping, direct from CHI-na?

Just ordered one as well, as I'm unsure the electrical method using an oscilloscope will work on brushless motors.   If when I can find a plan I like and get that build done, I plan compare to electric method to the optical method for RPM measuring on a bench.