Laser Tachometer
martintech
Posts: 6
Hi!
I use a optical laser tachometer in my work. It has 2 modes: optical and direct contact (ie rubber wheel) though it seems to use a laser in both modes (when measuring with direct contact you add an adapter to it).
I wonder how this work and what sensors to buy for creating such device.
When measuring with the optical mode you apply a reflective tape on the wheel, then you aim the laser at the reflective tape and the rpm shows in the LCD.
What type of device do you need to read laser beams, same "wavelength as the transitter? (I would prefer a directlink to a sensor/part).
What type of laser diode to use.(I would prefer a directlink to a sensor/part).
Is the laser beam different from an IR beam just because it has a different wavelength or?
I know·when I play with the IR transmitter/receiver from parallax that I can make a detector when it reflects when my hand comes close but as I understand it a laser beam wont work that way and thats why you need a reflective tape.
Although I could use IR transmitter/receiver I would like to go with the laser. (and as a can tell the laser is more precice and that the reflective tape wont reflect IR light)
How does a reflective tape work? Does it reflect some type of light, not IR but laser?
As I see it the procedure is like this:
1. The laserbeam is aimed at the wheel with the reflective tape.
2. When the laser hits the reflective tape on the wheel it reflects the laser back to the device.
3. A laser receiver sensor sense the laserbeam and the microcontroller adds 1 to the variable.
4. Some math is·done to calculate the RPM and then displays it.
Thanks for your help.
I use a optical laser tachometer in my work. It has 2 modes: optical and direct contact (ie rubber wheel) though it seems to use a laser in both modes (when measuring with direct contact you add an adapter to it).
I wonder how this work and what sensors to buy for creating such device.
When measuring with the optical mode you apply a reflective tape on the wheel, then you aim the laser at the reflective tape and the rpm shows in the LCD.
What type of device do you need to read laser beams, same "wavelength as the transitter? (I would prefer a directlink to a sensor/part).
What type of laser diode to use.(I would prefer a directlink to a sensor/part).
Is the laser beam different from an IR beam just because it has a different wavelength or?
I know·when I play with the IR transmitter/receiver from parallax that I can make a detector when it reflects when my hand comes close but as I understand it a laser beam wont work that way and thats why you need a reflective tape.
Although I could use IR transmitter/receiver I would like to go with the laser. (and as a can tell the laser is more precice and that the reflective tape wont reflect IR light)
How does a reflective tape work? Does it reflect some type of light, not IR but laser?
As I see it the procedure is like this:
1. The laserbeam is aimed at the wheel with the reflective tape.
2. When the laser hits the reflective tape on the wheel it reflects the laser back to the device.
3. A laser receiver sensor sense the laserbeam and the microcontroller adds 1 to the variable.
4. Some math is·done to calculate the RPM and then displays it.
Thanks for your help.
Comments
I have used lasers with LDR, the laser I used was the cheapest one I could find ( I used this pens that’s 630mn @ less than 1Mw ), it worked perfect, at that time I use to feed one side of an OP-AMP ( LM741 ) and it was used to trigger an alarm system.
I think that the response from the LDR will be too slow for this application, you may need a phototransistor. There are many in the market , look at www.alldatasheet.com and try to find any phototransistor , by looking at its data sheet you will find if that responses to the wavelength that you are trying to achieve.
about the reflective tape, as far as I know , is just a tape with a polyacrilate that reflects everything that goes for it , it does not change any wavelength ( but this is as far as I know OK )
That´s great project to be done , very usefull
I´ll try find more information ( may be do some tests ) for you.
Hope I can help
Ricardo Amaral
The IR LED is designed to throw light everywhere. Thus an IR-Decoder doesn't have to be very 'focussed' to get the light. A Laser Diode is designed to throw a very narrow beam of light. Thus the 'beam' needs to be targed on something reflective, so the beam bounces back to the detector.
An RPM counter gets a pulse when the beam is reflected by the reflective tape.
Perhaps this has with how many watts the laser are at and probably a white wall reflects better then a black wall.
For this Laser Tachometer-project it seems resonably that I have to vary the output of the laser so that it does not reflect back to the detector at a dark shaft without a reflective tape (That means the reflective tape should·be pretty good reflecting back the laser beam).
Perhaps it has to do with different types of·optical sensors/detectors with different sensitivity.
I´m not looking for any fansy laser diode, just a cheap one that has some potential. (like the ones we have in our pens).
Thanks for your time
So a phototransistor has a faster response time and is the way to go I guess. Is there any other alternatives for the optical receiver but the phototransistor and photodiode?
If I read at this site a photodiode is better to use regarding response time:
http://www.imagineeringezine.com/ttaoc/detector.html
"The photodiode/transistor connection dramatically slows down the otherwise fast response time of the diode inside. Most phototransistors will have response times measured in tens of microseconds, which is some 100 times slower than similar PIN diodes. Such slow speeds reduce the usefulness of the device in most communications systems. "
"Many circuits that have been published in various magazines, have specified "photo transistors" as the main light detector. Although these circuits worked after a fashion, they could have functioned much better if the design had used a different detector. From the list of likely detectors, only the silicon "PIN" photodiode has the speed, sensitivity and low cost to be a practical detector."
But it also says:
"Also notice that the device's response falls off sharply beyond 1000 nanometers, but has a more gradual slope toward the shorter wavelengths, including the entire visible portion of the spectrum. In addition, note that the device's response drops to about ½ its peak at the visible red wavelength (640 nanometers)."
Is there any chance that I can find a photodiode for 630 nm wavelength.
What type of optical receivers do you think a Laser Range Finder use?
Thanks for your help!·
Post Edited (martintech) : 7/12/2005 9:33:31 AM GMT
You can put the reflective tape on and get a good reflection from it....but you'll probably still get some reflection off of the shaft which might not 'trigger' for you.
IF the environment is dark, I'd use a photocell connected to a comparator (adjusted to not trigger on 'shaft' reflections)....however, if you were to turn the lights on in the room/closet/hood/enclosure....while it was running, it might trigger then too!
If you were to modulate it....I wouldn't see a problem....but it would depend on how fast your shaft was turning (how long the laser reflected off of the tape....)
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Steve
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
I think I will go with a photodiode with a filter to suit the wavelength my laser diode beam. (typicably 635-650 nm it seems).
Someone knows for sure if a beam that reflects of a reflectiva tape changes wavelength?
Hmmm...just thinking in my head (as opposed to that OTHER place haha)
if: wavelength = speed of light / frequency....
doesn't the speed of light change as it passes through more dense mediums (like in to water...) and wouldn't this then change the wavelength of the light?
Just curious (this is almost another thread topic...hope I don't hijack this thread by accident!)
I know at our Radar site (weather radar) we get beam bending when the humidity is outrageous (LIKE TODAY...WHEW!). So there are some neat FX....but was just wondering about the speed....I think it does....smarter ppl, pls chime in! haha
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Steve
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
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Steve
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
how do I do to insert this data sheet here ? ( so everyone could have it )
well anyway , this is at www.alldatasheet.com
about the photodiode, I agree, that’s probably the best choice.
Ricardo Amaral
How can I use CD4017 as a frequency counter? (will I have to have a timer chip?)
I would like an·IC that counts how many switches from low to high is done under 1 sec. 1,3 Mhz will do it.
How is that possible with a decade counter as CD4017? How can I read it with the BS2?
Thanks
Post Edited (martintech) : 7/13/2005 6:44:41 PM GMT
One Big problem I had , and spent a lot of time to fix , was that the 555 wasn’t given me a good, always positive, pulse , and I though about interference on the power supply and every different stuff until get to it , the solutions was simply put a diode and a resistor keeping all non signal to Zero volts. I think that the BS is going to give a good clear pulse, but anyway , think about using at least a 10K resistor to keep the non-signal on Zero.
Ricardo Amaral.
keep us posted about your project, i really would like to know how did you solved out
Ricardo Amaral.