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Thread: Measuring position of an accelerating ball using light gates

  1. #1 Measuring position of an accelerating ball using light gates 
    Forum Masters Degree thyristor's Avatar
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    Hi!
    For a science project I need to accurately measure the position vs time of a ball in accelerating linear motion at intervals of about 5mm. The most straightforward solution seems to involve a high-speed camera captioning the motion of the ball next to a ruler. However I do not possess such a camera with sufficient frame rate and resolution, and to acquire one would be too expensive.

    I therefore thought of using several light gates in a row along the direction of motion of the ball. By knowing the distance beteween the light gates and recording the time at which the ball passes each light gate, one would be able to obtain values of position vs time. The lightgates in themselves constitute no problem, as I have found the following product KTIR0921DS at Kingbright Europe: Products to be sold by a local electronics store at a reasonable price. However, what I do not know is how I should feed them, how I should collect the signal, and how I could directly transfer the data to a computer for facilitated analysis.

    Thanks for any relevant input on this issue!


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    The circuit for the light gate is the easy part.
    Here is the data sheet for your photo-interrupter.
    KTIR0921DS pdf, KTIR0921DS description, KTIR0921DS datasheets, KTIR0921DS view ::: ALLDATASHEET :::

    Here is a data sheet for a similar device, which shows a circuit design with voltage and resistor values.
    http://www.nkkswitches.com/pdf/ds-bTiltSwitch.pdf

    I don't know of a cheap way to get the signals into your computer. You will need something like a multi-channel analog input device, I guess.
    Analog I/O | F&eIt series Isolated Analog Input Module for USB ADI12-8(USB)GY
    Maybe somebody else will have a better idea.


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    Forum Masters Degree thyristor's Avatar
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    Thanks a lot for your help, Harold! Yes, I figured that building a system to directly the data into a computer would be harder and more expensive. At school we have a lot LabQuest and LabQuest Mini units (Interfaces > Vernier Software & Technology) for collecting data from Vernier probes. Do you think it would be possible to somehow use these ? One of the compatible Vernier probes is actually a light gate, however one far too wide for my intentions, so I thought that maybe it would be possible connect my own light gate to LabQUest and somehow trick it into thinking that the Vernier LabQuest was connected.
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    Quote Originally Posted by thyristor View Post
    Thanks a lot for your help, Harold! Yes, I figured that building a system to directly the data into a computer would be harder and more expensive. At school we have a lot LabQuest and LabQuest Mini units (Interfaces > Vernier Software & Technology) for collecting data from Vernier probes. Do you think it would be possible to somehow use these ? One of the compatible Vernier probes is actually a light gate, however one far too wide for my intentions, so I thought that maybe it would be possible connect my own light gate to LabQUest and somehow trick it into thinking that the Vernier LabQuest was connected.
    Sure, that would work. One of the available labquest sensors is a voltage probe, and it says you can connect up to 6 probes to the labquest. Just wire up the light gate with the appropriate voltage source and resistors, then take the alligator clips on the voltage probes and connect from the collector to emitter of each light gate. If you had more than 6 light gates, you could even connect them to multiple labquests, connecting the last input of one in parallel with the first input to the next for indexing the time scale.

    One problem you might run into is the sampling rate. Depending on how fast your ball is moving, how big it is, etc, it will just make a narrow spike in voltage. There is a possibility your computer software might miss the spike, because it is not continuously reading the voltage but is just sampling it at some frequency. If you have a problem, increase the sampling rate as much as possible and that might work.
    Last edited by Harold14370; December 27th, 2011 at 11:00 AM.
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    Quote Originally Posted by thyristor View Post
    Hi!
    For a science project I need to accurately measure the position vs time of a ball in accelerating linear motion at intervals of about 5mm. The most straightforward solution seems to involve a high-speed camera captioning the motion of the ball next to a ruler. However I do not possess such a camera with sufficient frame rate and resolution, and to acquire one would be too expensive.

    I therefore thought of using several light gates in a row along the direction of motion of the ball. By knowing the distance beteween the light gates and recording the time at which the ball passes each light gate, one would be able to obtain values of position vs time. The lightgates in themselves constitute no problem, as I have found the following product KTIR0921DS at Kingbright Europe: Products to be sold by a local electronics store at a reasonable price. However, what I do not know is how I should feed them, how I should collect the signal, and how I could directly transfer the data to a computer for facilitated analysis.

    Thanks for any relevant input on this issue!
    Can you not just record the ball's motion on a digital camcorder, with a ruler in the background ? You know the camcorder's framerate ( I think something like 20fps is achievable with ease ), so when you import the movie into your PC and play it back frame-by-frame ( plenty of freeware for this I believe ), you can measure exactly where the ball was at a given time ( remember, the frame rate is known ! ). Maybe I am missing something here, but this seems much easier than all the complicated electrical work you are proposing.
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    Markus has a good suggestion. The frame rate on my camcorder is 30 frames per second. You could put a digital clock in the image as well so you can precisely verify the timing, a good way to verify that the frame rate matches reality. You could place narrow beams of light facing the balls spaced a centimeter apart so you could precisely time when the ball entered each beam. This eliminates the problem of having to account for perspective. (Please do it in the metric system, so much better). You can import the video into a number of programs then step through frame by frame.
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  8. #7  
    Forum Masters Degree thyristor's Avatar
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    Quote Originally Posted by Harold14370 View Post
    Quote Originally Posted by thyristor View Post
    Thanks a lot for your help, Harold! Yes, I figured that building a system to directly the data into a computer would be harder and more expensive. At school we have a lot LabQuest and LabQuest Mini units (Interfaces > Vernier Software & Technology) for collecting data from Vernier probes. Do you think it would be possible to somehow use these ? One of the compatible Vernier probes is actually a light gate, however one far too wide for my intentions, so I thought that maybe it would be possible connect my own light gate to LabQUest and somehow trick it into thinking that the Vernier LabQuest was connected.
    Sure, that would work. One of the available labquest sensors is a voltage probe, and it says you can connect up to 6 probes to the labquest. Just wire up the light gate with the appropriate voltage source and resistors, then take the alligator clips on the voltage probes and connect from the collector to emitter of each light gate. If you had more than 6 light gates, you could even connect them to multiple labquests, connecting the last input of one in parallel with the first input to the next for indexing the time scale.

    One problem you might run into is the sampling rate. Depending on how fast your ball is moving, how big it is, etc, it will just make a narrow spike in voltage. There is a possibility your computer software might miss the spike, because it is not continuously reading the voltage but is just sampling it at some frequency. If you have a problem, increase the sampling rate as much as possible and that might work.
    Thank you for your suggestions. However I do not quite see how connecting multiple Lab Quests in the way that you proposed would work. It seems to me that the software would not be able to register the individual spikes from all the light gates.
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    Forum Masters Degree thyristor's Avatar
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by thyristor View Post
    Hi!
    For a science project I need to accurately measure the position vs time of a ball in accelerating linear motion at intervals of about 5mm. The most straightforward solution seems to involve a high-speed camera captioning the motion of the ball next to a ruler. However I do not possess such a camera with sufficient frame rate and resolution, and to acquire one would be too expensive.

    I therefore thought of using several light gates in a row along the direction of motion of the ball. By knowing the distance beteween the light gates and recording the time at which the ball passes each light gate, one would be able to obtain values of position vs time. The lightgates in themselves constitute no problem, as I have found the following product KTIR0921DS at Kingbright Europe: Products to be sold by a local electronics store at a reasonable price. However, what I do not know is how I should feed them, how I should collect the signal, and how I could directly transfer the data to a computer for facilitated analysis.

    Thanks for any relevant input on this issue!
    Can you not just record the ball's motion on a digital camcorder, with a ruler in the background ? You know the camcorder's framerate ( I think something like 20fps is achievable with ease ), so when you import the movie into your PC and play it back frame-by-frame ( plenty of freeware for this I believe ), you can measure exactly where the ball was at a given time ( remember, the frame rate is known ! ). Maybe I am missing something here, but this seems much easier than all the complicated electrical work you are proposing.
    In principle, it would be possible. However there are some problems with this method. First of all you have to consider the parallax distortion of the picture. Indeed it is possible to compensate for this by measuring how the apparent length of the ruler changes over the screen. Despite this, there is still the problem of determining when the ball passes a certain point along its track; MeteroWayne's suggestion with the light beams could resolve this issue.

    Finally, the frame rate of the camera is a problem. At 30 fps, the uncertainty in time is too great to make accurate measuements of a ball travelling with a speed of, say 1 m/s. It is possible to increase the frame rate, and I will have access to a high speed camera able to capture at 1000 fps. At this framerate however, the resolution is really bad, and the ball is difficult to see.
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    Quote Originally Posted by thyristor View Post
    Quote Originally Posted by Harold14370 View Post
    Quote Originally Posted by thyristor View Post
    Thanks a lot for your help, Harold! Yes, I figured that building a system to directly the data into a computer would be harder and more expensive. At school we have a lot LabQuest and LabQuest Mini units (Interfaces > Vernier Software & Technology) for collecting data from Vernier probes. Do you think it would be possible to somehow use these ? One of the compatible Vernier probes is actually a light gate, however one far too wide for my intentions, so I thought that maybe it would be possible connect my own light gate to LabQUest and somehow trick it into thinking that the Vernier LabQuest was connected.
    Sure, that would work. One of the available labquest sensors is a voltage probe, and it says you can connect up to 6 probes to the labquest. Just wire up the light gate with the appropriate voltage source and resistors, then take the alligator clips on the voltage probes and connect from the collector to emitter of each light gate. If you had more than 6 light gates, you could even connect them to multiple labquests, connecting the last input of one in parallel with the first input to the next for indexing the time scale.

    One problem you might run into is the sampling rate. Depending on how fast your ball is moving, how big it is, etc, it will just make a narrow spike in voltage. There is a possibility your computer software might miss the spike, because it is not continuously reading the voltage but is just sampling it at some frequency. If you have a problem, increase the sampling rate as much as possible and that might work.
    Thank you for your suggestions. However I do not quite see how connecting multiple Lab Quests in the way that you proposed would work. It seems to me that the software would not be able to register the individual spikes from all the light gates.
    Here is a way you can connect multiple light gates to one input. This will give you a single waveform with voltage spikes from each light gate in series. Two lightgates are shown, but any number may be connected in the same fashion. You will need a diode for each light gate to make an "auctioneer" circuit. The output will be the highest voltage of the connected light gates.

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