Portable Tachometer for spinning Helicopter Rotor

frasenci

· Hey you

I just got my BS2 with homework board and been toying around with leds and servos. My real , serious proyect is to make an optical tachometer to measure the RPM on a flying RC Helicopter ( 1400 - 1600 RPM 2 blades ) . I got the software somewhat designed but still looking for the correct hardware - chips - etc to count the blades passing ,.... any help out there ???

by the way ,... TWO THUMPS UP Parallax ,... Basic Stamp a GREAT product and Concept.

Thank you

Francisco

Tim Marks

There is a fellow who sells them new for $45 on ebay. It would be hard to design a cost effective BS2 solution to beat that price.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=4678&item=3830941989

AlWilliamsAWC

A lot of folks build stuff for the adventure of it. I have a friend who has been building a switching power supply for 2 years. He has wound the transformers and all that. He could certainly have bought one by now, but he has learned a lot.

I don't have a lot of constructive advice about the tachometer. The Stamp can resolve pulses using PULSIN. The size of your pulse will depend on the speed and the arrangment you use to sense the blade (and probably the width of the blade). If you need something that runs all the time, I know quite a few of our PAK-VII·chips are used for tachometry. The advantage is they read data all the time even when the Stamp is busy doing something else.

Al Williams
AWC
http://www.awce.com
·

frasenci

Thanks Tim , I already own one of similar design but not laser ( Hobby ) .... the question is : ? i want it to be put onboard de heli and make data readins during flight ... !!! anyway , nice gadget you showed me.

And thanks Al ,.... impressive line of products you got on your web ,... ! I want them ALL !!! and it is not a joke ... still want to go back there and verify everything once more. ( Already donloaded all the docs you offer ) ... You are right about your friends example ,... i am one of those ..., simple as that !!!!

The problem i have is what photransistor or the like to use ,to get the pulses signal............, i already thoght about your coprocessor,....of course.

I am in Chile but still find your prices very well put , congrats.

Franciso
Chile

AlWilliamsAWC

I would think you would not want anything to unbalance your blades, so magnetics are out. Unless you balanced the magnets. Then you could use a hall effect sensor to watch the magnets go by. This has the advantage of not being subject to ambient light problems. A photocell or phototransistor could watch the blades go by if they were dark, but you might have trouble with flying at night.

Probably the best solution would be to beam some modulated IR up at the blades and watch for it to bounce back. There are some all in one IR demodulators/detectors used in IR remote controls that are inexpensive or that can be salvaged from old remote controlled gear. Look up IROD or IRODS on the Web and you'll see what I mean. This should work day or night and be immune to stray light hitting the sensors. I'm not sure how good the dynamic range is if you are hitting it with full daylight and then a brief IR burst. Another idea, if you are onboard, is to not look at the blades, but instead look at the rotor shaft, where you could be "inside" and out of the ambient light. That would make things much simpler to do optically. You might even get away with magnetic there since I doubt you would imbalance the shaft. In fact, if you were going to do that, you could actually put a small wheel around the shaft like the one in a mouse. The wheel would have one or more slits to pass light and you would count the speed at which the slit passed. That would allow you to have very good slow resolution, but I doubt you really need that.

Maybe someone with practical experience will pipe up with some better suggestions.

Al Williams
AWC
* Floating point math for the Stamp
http://www.awce.com/pak12.htm
·

frasenci

Right !!! there is place " inside " the Heli where i got acces to a gear that rotates with the shaft .... The sensor could be shielded from light in that place. I got access from one side only ) looking upwards ). That gear is solid and the sensor could be mounted as near as a pair mm from it. Question : model - name of this kinda sensor .. i guess something could sense IR reflected on a tape or pie design on the gear ( white - black ).. would that be fast enough reaction ??

later i will investigate on sensors

Thank you for your comments

DntGvaSht

Where I work I use an optical tachometer on a dialy basis, it consists of a light source, a detector ,beam splitter, and a focusing lens.·· A peice of reflective tape is placed on the axle of the motor or gear in question, and the device's light beam is aimed at the tape while the axle is spinning.· This allows for a very broad range of angles and distances for measuring.

Hope this helps

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http://68.11.58.106:69/ircchat2/jicra-1.2.2/index-js.html

Ken Gracey

Francisco,

If you can attach a magnet somewhere this approach could work really well:

http://www.parallax.com/html_pages/robotics/machining/RPM_display.asp

You could use one magnet on any rotating part. I can attest that it works really well for my milling machine.

Sincerely,

Ken Gracey
Parallax, Inc.

frasenci

just answering is a lot of help man !!! thanks. Since this is only a hobby project , all ideas will enhance my experience. I used to see some the like in a Ship building yard a time ago.

greetings

Francisco

frasenci

Thanks Ken ....EXACTLY what I needed ,... will see if that Hall effect sensor is stocked here in Chile. ( And if it is not too high priced )

I wil let you know

And I want to say it again : Wonderfull product and concept your Basic Stamp !!

so long

Francisco

frasenci

Just want to ad the rest of the story ...... Once I can read the RPMs with the stamp ,..what I really want is to control the motor throttle servo to hold constant RPMs under changing loads of the machine....

Thats it

hu

AlWilliamsAWC

I agree magnetic is the way to go if you can do it.

But to answer your question on the optics, there are a few sensors that would do what you want. A phototransistor is probably your best bet.· They also make "optointerruptors" that have an LED and phototransistor (and maybe some logic) with a slot for your piece to move between. They also make reflective optointerruptors. But you can always just make your own from an LED and a phototransistor.

Regards,

Al Williams
AWC
Control 8 servos at once!
http://www.awce.com/pak8.htm

KenM

Please correct me / educate me.......I have often had trouble with optointerrutors used outdoors. In bright sunlight there are wavelengths equal or very close to the same wavelength of the opto device?

So even with the beam blocked by the interrupting medium, the phototransistor does not turn off. This has often occured even when attempting to put the phototransistor recessed in a cavity to shield it from ambient light.

Are there specific optos that are less sensitive to ambient sunlight? Or am I imagining something wrong and there is another possibe reason for the photointerruptor not working in bright sunlight?

Ken

Al Williams/AWC said...
I agree magnetic is the way to go if you can do it.

But to answer your question on the optics, there are a few sensors that would do what you want. A phototransistor is probably your best bet.· They also make "optointerruptors" that have an LED and phototransistor (and maybe some logic) with a slot for your piece to move between. They also make reflective optointerruptors. But you can always just make your own from an LED and a phototransistor.

Regards,

Al Williams
AWC
Control 8 servos at once!
http://www.awce.com/pak8.htm

AlWilliamsAWC

Yes, that's why I was saying it would be best to do optical inside a housing to protect against ambient light. Or use modulated IR. The only problems with modulated IR are that a) they dynamic range of the sensor would have to be very good, and b) the response time of the sensor may not be fast enough. But if the interruptor were inside the housing shielded from ambient light, it should work very well.

Not to say that magnetic would not work also and avoid the light problem alltogether.

Regards,

Al Williams
AWC
Easy RS232 Prototypes
http://www.awce.com/rs1.htm

KenM

Thanks Al.

frasenci

yes Al , putting that little magnet near shafts center ,there is no unbalance to overcome. You where right on the first place. I changed my mind , opto seems to be more complicated.

And I definitely need that Coproc. , the one with A/D an comparator.....must figure out how to bring one home.

so long

Thank you all guys, never thoght such a forum could be this productive.
sorry my poor english , thats all there is to it.



Francisco
Naval Mech. Engineer
Chile

frasenci

Einverstanden !!!!

We all agree , I will go Hall - Magnetics.

Francisco

Ken Gracey

Francisco,

If you are in Chile I can forward the parts to our distributor Rambal and he can get them to you. These components cost only a few dollars in the U.S. and I have some extra samples at home. All I need to do is put them in the next order from Rambal and you will have them. I imagine he'd charge you a few dollars or so. Let me know if you want me to do this for you.

Sincerely,

Ken Gracey
Parallax, Inc.

frasenci

he Ken ,... now that is a neat offer !!!!
a few dollars ??? how much is a few for you ??? .... The answer is yes , of course . Hope Rambal people do not want to take advantage ...

By the way , I went there ( about 500 km away from my city ) , just to talk with them. I did it as soon as i knew they where Parallax distributors for Chile. I wanted to know if they could support me in starting an educational program in my city based on the hardware you make. This proyect is something i wanted to do for many years now and the Parallax Stamp and its Concept made such a good impression on me that i think now it is the time to fullfill that dream. I think that this activity ( micros programming and the like ) is an excellent choice for youngsters with technical apetite and the best way to keep them away from too much TV , solo Game playing on Pc , etc.

If you can do what you are offering , i will be delighted , just let me know when the time has come.

greetings
Francisco

Ken Gracey

Francisco,

Send me your shipping address kgracey@parallax.com and I'll send the parts directly to you, bypassing Rambal to keep this simple. I'll use USPS shipping. No charge for the parts.

Did Rambal agree to support your efforts with education? I sure hope so and if not I'll have to talk with Jose!

Ken

frasenci

Ok Ken, I will do that.

As for Rambal supporting me ,... they did agree , yes .... ,..... partly,.......

I bought an "BS2 Homework board" from them for my personal use and headed back home. Did not get anything else only because i am not a rich man and must plan carefully to make this kinda things. I wanted to test the Stamps potential and Programming environment myself . IT IS EXACTLY HOW YOU ARE ADVERTISING IT !!!!! Bravo !!!

I guess it is not easy to do better for them...... consider the fact that some crazy day an unknown fellow comes out of the clear blue sky saying he wants to teach Basic Stamps.... we talked for about 1 hour and I must say it was a pleasant meeting though.

I wil come back to you via mail.


Thanks for your interest and supportive , helping attitude.

a thankfull Francisco
Chile

"Steamboat Ed" Haas

· Rats! the link:
http://www.parallax.com/html_pages/robotics/machining/RPM_display.asp
· ...is dead! Anyone got an alternate source of info??

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························ Hacking the Trailing Edge!
················http://www.nmpproducts.com/intro.htm
················---Decks a-wash in a sea of words---

Ken Gracey

Ed, it's still here: http://www.parallax.com/Default.aspx?tabid=420

Ken Gracey
Parallax Inc.