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My goal is to measure the velocity of a moving object that has several different changes in velocity throughout the actuation.

I am planning on using a phototransistor to detect black and white lines that are taped to a moving object that has a peak velocity of 25 ft/s (7.6 m/s). To get the granularity required to pick up the changes in velocity, the black and white lines have to be around .1" (2.5 mm) or smaller.

Is a phototransistor capable of differentiating between such small lines? (I am assuming there is a minimum focal window).

Is there a better way to approach this?

I am quite ignorant when it comes to electronics so apologies in advance!

I was looking into CNY70 and TCRT5000 for no reason other than that they seem to be frequently used in white/black line detection in commercial robotics.

It doesn't appear their datasheets have the half angle like that of the SFH 313 FA.

  • I am measuring the velocity of essentially a 1" x 10" metal rectangle (overly simplified) that moves along a single axis .
  • The phototransistor can be mounted within .005" (.13 mm), perpendicular to the moving mass.
  • Measurement frequency will be approximately 10kHz.
  • To achieve the velocity, there is some inherent heat (<150F) and vibrations (frequency and amplitude unknown, but can be dampened if necessary).
Null
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Nicholas Myers
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    The phototransistors I have are mostly completely unfocussed. If you are using ones with some optical system built in, post the datasheet so we can see what they are. Otherwise, you are probably left with designing suitable optics. –  Feb 18 '22 at 16:23
  • You need field of view data. SFH 313 FA has 10 degree half angle. If you told us the setup we could help more. Close? Far? Linear trajectory? – DKNguyen Feb 18 '22 at 16:31
  • Thank you both! I was looking into CNY70 and TCRT5000. It doesn't appear their datasheets have the half angle like that of the SFH 313 FA. I am measuring the velocity of essentially a 1" x 10" metal rectangle (overly simplified) that moves along a single axis . The phototransistor can be mounted within .005" (.13 mm), perpendicular to the moving mass. Measurement frequency will be approximately 10k Hz . To achieve the velocity, there is some inherent heat (<150F) and vibrations (frequency and amplitude unknown, but can be dampened if necessary). – Nicholas Myers Feb 18 '22 at 17:04
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    Please [edit] your question to add all those details in. Links to datasheets are a plus. – Mat Feb 18 '22 at 17:34
  • Normally you would use a lens to image the pattern onto the detector. The advantage of this is that you don't need to put the detector so close to the object since with the right magnification you could have arbitrarily high resolution even at a long distance. That said, if you can put the detector close enough for it's native viewing angle, that approach would also work. – user1850479 Feb 18 '22 at 17:47
  • If you're that close (0.13mm) I don't think it will be a problem. If you're that close you could even darken the ambient and use a tight laser to illuminate a very small area even if the photodetector has a wide FOV. – DKNguyen Feb 18 '22 at 17:52
  • How do you intend to illuminate the lines? – Bruce Abbott Feb 25 '22 at 19:30

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