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I'd like to make a little device I can stick onto a Frisbee that could measure stats such as RPM/speed of the Frisbee when thrown. Would this be realistically possible?

I was looking into near-field communication to transfer the data from the Frisbee into a smartphone, but it's just an idea. It seems like it would be difficult to make something small enough to not affect the flight of the Frisbee itself. Can anyone think of any ideas for this?

Peter Mortensen
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Barodapride
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  • All rotational force would be tangent to the circular path of the Frisbee. I imagine this tangential force to be proptional to the RPM in some way, so that could be something to look at. Any device that measures air speed would either be quite heavy or introduce significant resistance through friction. The immediate idea that comes to mind would be a painted dot on the device and the use of a ground camera that captures the motion. – sherrellbc Jun 18 '14 at 18:20
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    I'd put a light sensor on the frisbee pointing outwards, and see what kind of data it collects when spinning. I bet there is a correlation between rate of changes and RPM. – Reactgular Jun 18 '14 at 18:23
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    @MathewFoscarini, I like that idea. I was thinking of experiments using accelerometers and force sensors to see if some relationship exists between force and RPM. If you took your idea and put the Frisbee in a controlled environment in which a fixed bright light was on a wall, then the light sensor response could be used to determine how many times per second (scaled to per minute for RPM) that the fixed light is detected. – sherrellbc Jun 18 '14 at 18:26
  • @sherrellbc if the frisbee has two light sensors at polar ends to each other, then you just need to time the difference between peaks. This is how they measure the speed of cars using strips on the road. – Reactgular Jun 18 '14 at 18:29
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    If you decide to use on-board sensors, do you need the data in real-time? You could reduce the electronics (and power requirements) by storing the data in the micro's RAM and then downloading it after the flight is over. – tcrosley Jun 18 '14 at 18:30
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    I think putting a pattern on the frisbee (maybe just one line) and filming it will be a much easier route to some realistic data. – Wouter van Ooijen Jun 18 '14 at 18:30
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    You could ducktape an iPhone under the frisbee and record all the sensors using a simple app. This will at least tell you if gyroscopes have any chance of work. I'd use your girlfriends phone... tell her it's for science. – Reactgular Jun 18 '14 at 18:34
  • I'm not looking for real-time data - downloading afterwards would work fine. But it seems like it'd be tricky for the circuit to know when a throw begins and ends. – Barodapride Jun 18 '14 at 18:37
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    @Barodapride, If you go with something like a light sensor that is controlled by an MCU you could simply set 10 second (arbitrary example) intervals in which valid data is collected. You could have a blinkning LED or something that indicates when a valid sampling period has begun. Depending on the size of your MCU's EEPROM you could store multiple "runs" of data. You then just download the EEPROM and analyze the data perhaps using another program written by you. – sherrellbc Jun 18 '14 at 18:55
  • It seems like it'd be unreliable to use a light sensor because you'd have to be in a room with a fixed light source as you said. Maybe it would only work on bright sunny days. – Barodapride Jun 18 '14 at 19:21
  • @Barodapride "But it seems like it'd be tricky for the circuit to know when a throw begins and ends." An accelerometer would take care of that. – tcrosley Jun 18 '14 at 19:54
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    Whatever you put on the frisbee will have to be _very_ light (a few grams at most) to avoid significantly impacting its flight, though you can reduce the effect by distributing the weight symmetrically between opposite sides of the disc. – David Z Jun 18 '14 at 23:28
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    @DavidZ +1 ...putting anything on the disc will alter its flight in mutliple ways--some obvious (balance, asymmetric drag), some subtle (altered moment of inertia). Discs used in competition have requirements not just on total mass and size, but also on how that mass is distributed. More mass on the outside of a disc means it will spin longer and more stably--adding sensors may mean you're measuring your specific set up and not the original situation. – laindir Jun 19 '14 at 15:07
  • @Barodapride: How often are you in a location with a mobile light source? – davidcary Jun 20 '14 at 02:59
  • I just realized how many upvotes Mathew got for his comment about color patterns and filming. *I said that exact same thing on my second post of this question!* Dang, bad luck. – sherrellbc Jun 25 '14 at 02:40

14 Answers14

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Off the top of my noggin here's an idea. Build a small tone generator and tiny speaker onto the frisbee with the speaker around the edge (balanced mass on the other side of course).

When you spin the frisbee you could record the sound being made and it will have doppler features depending on how fast it is spinning. If you can analyse the data recorded you should be able to tell the rotational speed.

Andy aka
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    As a bonus, you don't even need to measure the amount of doppler shift. The frequency at which the tone varies is enough. – Steve Jessop Jun 19 '14 at 08:04
  • @SteveJessop yup that's the idea. – Andy aka Jun 19 '14 at 08:51
  • Won't placing anything on the edge of the disc multiply the impact it has on its moment of inertia? Aerodynamics may be an issue too. – laindir Jun 19 '14 at 13:25
  • @laindir It has to be done carefully of course. – Andy aka Jun 19 '14 at 14:38
  • You don't need to place it at the edge, just point it toward the edge. Most small speakers have some degree of directionality. – Alex Shroyer Jun 19 '14 at 15:40
  • I'm not sure about this; but won't the comparatively low RPM of a Frisbee against it's linear velocity mean that it would be hard to distinguish between the two different causes? – Thomas Russell Jun 19 '14 at 21:00
  • Linear velocity detracts from doppler+ and doppler- the same I reckon – Andy aka Jun 19 '14 at 21:35
  • Put the microphone at either end of the flight, and now the linear velocity doppler is a common mode effect and cancels out. It does become an amplitude effect with distance, of course. I bet the guts of a musical greeting card provide everything you need to try it... – RBerteig Jun 20 '14 at 00:30
  • But the added weight will influence the measurement, i.e. slow down the frisbee. – Geert Goeteyn Jun 20 '14 at 15:42
  • @GeertGoeteyn frisbees weigh X grams and the added stuff will weigh somewhat less than X grams therefore to equalize you can remove weight from the frisbee to accommodate the weight increase. Considering that there is no universal standard that dictates how heavy a frisbee should be, is adding a few grams a big deal? – Andy aka Jun 20 '14 at 15:45
  • @Andyaka but as the speaker needs to be mounted on the edge of the frisbee for the best doppler artifacts to be observed; you'd have to remove more mass uniformly over the entire frisbee than the mass of the speaker system (we can use the parallel axis theorem to determine the effect on the moment of inertia will \$\sim m r^{2}\$, where \$m\$ is the mass of the speaker system and \$r\$ is the radius of the frisbee)? – Thomas Russell Jun 21 '14 at 11:18
  • This seems like an interesting idea but overly complicated and prone to errors; sound from other sources, wind, etc. can affect the outcome and a significant amount of effort would be spent attempting to analyze the data. An accelerometer should be sufficient, and with low power consumption (and therefore less battery weight) and light weight. [This guy already did it.](http://www.lpl.arizona.edu/~rlorenz/frisbee/MSTfrisbee.pdf) – Jason C Jun 21 '14 at 17:07
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The RPM of a frisbee is something like 10rps, so you could spray paint a black and white pattern onto the frisbee and video it. A 60fps (120 fields per second) camcorder should be able to capture it quite reliably.

For night time, mount a tiny superbright LED and mechanically balanced lithium cell.

Spehro Pefhany
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    For night time, a small piece of reflective tape on the frisbee and IR illuminators on the camera. – John U Jun 19 '14 at 09:15
  • @JohnU sure.. retro-reflective tape and visible or IR illumination at the camera. – Spehro Pefhany Jun 19 '14 at 09:58
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    In addition to being the simplest, of the answers submitted so far, this one also has the least impact on the disc's flight characteristics. +1 – laindir Jun 19 '14 at 15:09
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    The vast majority of 60fps cameras do not actually capture 120 fields per second. You will get blurring or warping depending on the sensor type so it may not be easy to reliably calculate on anything but a highend or highspeed camera. – JamesRyan Jun 19 '14 at 16:31
  • For best results, through at or away from the camera. A bright light source at the camera and retroreflective tape on the disk probably works as long as the ambient light is dim enough that you can resolve the flashes. You will want a camera designed for higher frame rates if you can get it for the extra temporal resolution. – RBerteig Jun 20 '14 at 00:33
  • @RBerteig Usually the high frame rate cameras are advertised as being capable of "slow motion" (record at high frame rate) – Spehro Pefhany Jun 20 '14 at 00:38
  • @SpehroPefhany yup. Unless they are truly high frame rate and sold for research, where they don't need to explain what they are good for. 10000 frames per second can be had, but not on my toy budget. The pro sports field has lots of interesting slo-mo cameras, some of which are surprisingly inexpensive that would likely suit this. Anything designed to watch your golf swing will be plenty fast enough, for instance. 60 frames per second *is* shockingly slow for motion studies, but 240 is often enough and not that hard to build. – RBerteig Jun 20 '14 at 00:42
  • My answer is similar to yours, except I suggest using a strobe instead of a video camera. You get faster frame rates, and potentially cleaner images. –  Jun 20 '14 at 16:25
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    I think actually capturing the flight with the camera at sufficient quality without changing angle or distance too much might be non-trivial - frisbees can move quite fast. – Paul Paulsen Jun 21 '14 at 12:16
  • Capturing flight data with a camera will be difficult. Consider the length of a frisbee's path. If you throw a frisbee 50 feet, you either need to track it with a camera by hand (automated tracking would be way beyond the scope here) and ensure that you are always zoomed in enough to get a good read, or have a sufficiently high resolution camera to maintain good precision when the frisbee is farther away. This assumes a camera with a good high-speed shutter to reduce motion blur as well. You're forgetting the camera has to *see* the frisbee through its flight path. – Jason C Jun 21 '14 at 17:10
  • (I.e. you have a very large viewing volume to cover, not to mention having to compensate for the angle of the frisbee, and implement the analysis software to turn irregular "curly" paths across the frame into RPM readings - certainly doable but also not trivial.) – Jason C Jun 21 '14 at 17:12
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There is a paper describing a method for doing just this here: Flight Dynamics Measurements on an Instrumented Frisbee. The techniques worked well; I would use this as a starting point.

Essentially it was a microcontroller (BS2IC) and a 2-axis accelerometer (ADXL202), mounted to the center of the frisbee with silicone glue, and then balanced by the batteries (CR2032's) which were mounted with tape. The hardware was specifically chosen for its low power consumption.

A small switch was mounted near the edge to permit easy activation near the moment of throwing, again to reduce power consumption, and also because the space for data logging is limited:

enter image description here

My own idea on reading your question was to mount a 1-axis accelerometer / pressure sensor near the edge of the frisbee and measure the centrifugal force, although mounting near the edge can make balancing more challenging. The 2-axis accelerometer is not a big step up and you end up getting a lot more data out of it.

You could use a low-power near-field radio system such as Zigbee (or Bluetooth; which is less convenient for initial setup, and requires at least the addition of a button or some other logic to help make pairing usable but natively supported by many devices, e.g. your smart phone) to stream data to a nearby device, or you could log data on the microcontroller and retrieve it later.

In any case, that paper got some interesting data. In particular, note the author's observation that direct data can be more easily extracted during steady periods of accelerometer readings:

enter image description here

That graph gives a nice view of initial wobble becoming steady during flight, as well as the centrifugal force.

It doesn't really seem necessary to overcomplicate it with light / sound sensors; although if you do, I would log / transmit raw data and do the actual processing on the receiving device to limit power consumption and microprocessor performance requirements, at the expense of increased memory requirements (for logging).

Jason C
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Two small SMD ICs, one an accelerometer, the other a gyroscope, on opposite ends of an FPC, with a microcontroller, EEPROM, and coin cell battery equally balanced in the midddle of the frisbee. Minimal weight and air friction. Have the microcontroller log the IC's output to the EEPROM. The combination of the graphed accelerometer and gyroscope output will give you an approximation of speed and rpm.

FPC isn't needed; some thin custom PCBs with a magnet wire connecting them would work too. We are talking grams and ounces at most anyway.

For wireless access, a microcontroller or SoC with Bluetooth low energy would be great. Look at the Texas Instruments Sensor Tag for a complete development kit that has a BTLE enabled SoC and I²C accelerometer and gyroscopes as well as iPhone/Android app examples powered by a single CR2032 coin cell Hell, you could take the sensor tag, remove the red casing, and tape it to the frrisbee and the rest is pulling data from the free app.

Peter Mortensen
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Passerby
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    If other comments are right, and a frisbee spins at 10Hz, with a 0.1m radius, that would give you 40G=(2*pi*0.1*10)**2/0.1/9.8 Accelerometer would need to come in from the edge unless they've come out with better/stronger accelerometers, and the amount of math and the drift of the sensors would make this one of the more difficult methods. – horta Jun 20 '14 at 00:35
  • @horta like I said, the accelerometer at the end of a fpc, so on the outer edge. – Passerby Jun 20 '14 at 00:37
  • Why the accelerometer at all? You'd be measuring the sum of centripetal force and gravity vectors. How does that give you a good measure of RPM or linear speed? – Samuel Jun 20 '14 at 02:34
  • @Samuel feel free to give your own answer – Passerby Jun 20 '14 at 02:36
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    The reasonable solutions seem well represented, so there is little need. There is also no need to be defensive, I was questioning a possible flaw in an *idea*, not in you. – Samuel Jun 20 '14 at 02:45
  • It was mentioned elsewhere that an issue with gyro's is finding any that work in the ~10 rev/s range (there are a few, but not many); an interesting alternative would be to try using a (+/- 100G) accelerometer at known distance from the center to backout the rpm from centripetal force. – CoderTao Jun 20 '14 at 09:33
  • @Samuel see Jason C's answer – Passerby Jun 22 '14 at 16:37
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Just throwing an idea out there, no idea how feasible it is: MEMS gyroscopes are small, cost-effective, low-power sensors capable of measuring angular speed. As for measuring speed, assuming you mean linear and not angular speed, then the only thing I can think of that's not extremely complex is using a GPS module like this one. Given two consecutive positions, and knowing when each measurement was made, you can easily calculate linear speed.

swineone
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    I've looked into this because I'm planning on building one of these:http://www.youtube.com/watch?v=1n6ZmwzSL0Y but gyros aren't fast enough. The only thing that would work are digital compasses which is what the MIT and Embry Riddle guys used – slebetman Jun 19 '14 at 00:06
  • That GPS option isn't bad; it might be a little heavy but power consumption is only 56mA. A low-power 30-60mA microcontroller with that could run off a pair of CR2032's for about 2 hours, which is room to spare. User-interface may be an issue, though - you'd want a way of determining if the GPS was locked or not so you didn't waste throws - perhaps just a little green LED or something. – Jason C Jun 21 '14 at 17:18
  • This may well be out of your price range, but it can measure up to 50k dps, which works out to 8300 RPM, which I assume is enough: http://www.analog.com/static/imported-files/data_sheets/ADXRS649.pdf – swineone Jun 23 '14 at 22:50
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You can measure rotational speed with an accelerometer oriented radially. Acceleration 'outward' from the center of the frisbee indicates rotation (or the frisbee being at an angle, but you can average that out). Since you know the distance from the center to the accelerometer, it's a simple rotational acceleration calculation.

Sparr
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  • This seems like the simplest solution, where can I learn how to wire up an accelerometer and measure the data? – Barodapride Jun 19 '14 at 15:26
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    @Barodapride http://www.ehow.com/how_12093135_use-accelerometers-measure-angular-velocity.html for the concept, and any microcontroller+accelerometer tutorial for the implementation – Sparr Jun 19 '14 at 17:00
  • @Sparr obtaining the velocity accurately will be the challenge. Drift, and a constantly changing velocity vector from the accelerometers point of view will be difficult to resolve. Actually, a stopwatch and distance measuring tool may be a very good way to obtain the average velocity. While rotation may be best done through the accelerometer. – horta Jun 20 '14 at 00:38
  • +1 This is by far the most sensible solution, low power, light weight, easy to examine the data. @Barodapride Check out http://www.lpl.arizona.edu/~rlorenz/frisbee/MSTfrisbee.pdf – Jason C Jun 21 '14 at 17:19
  • If the frisbee is flying perfectly level the acceleration should be a smooth decay with no RPM component. If the frisbee is at an angle the component of gravity away from the normal would modulate the signal detectably. – KalleMP Nov 13 '14 at 09:45
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    @KalleMP you can calculate the rotational speed from a single acceleration sample, based on the distance from the center of the frisbee to the sensor. – Sparr Nov 14 '14 at 22:04
  • @Sparr Ahh, now I get it, you will measure the centripetal acceleration and not the frequency of the oscillation, time for me to keep quiet now. – KalleMP Nov 20 '14 at 12:42
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Many of these should be able to be made very light and small:

An accelerometer that points at some angle between radial and vertical will have a varying gravity component as it spins. Even a radial one will have this except when the flight is exactly horizontal.

A light sensor will in most cases also do the same. As it transits between ground features or ground to sky and back it will see brightness changes and these will have a correlatable rotation speed component.

An LED transmitting radially with a modulation frequency on it will be able to be detected at substantial distance electronically. You can look for modulation or could video it and look for the LED signature in frames (arguably harder).

If you provide a stationery modulated source that illuminates it you can place a detector on the Frisbee. If it is acceptable for another person to help with tracking the beam can be tighter and they can site by eye to track it. A simple post and circle site or two rings will probably allow a say 30 degree beam to be held on the frisbee thereby much increasing the signal level.

RF dfing should be viable.

Air pressure forces probably vary at a point on the peripheral as the frisbee spins while in transit. A pressure sensor with a port on the rim should see a repeated pattern.

Russell McMahon
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The hardest part, optically, is targeting a frisbee in flight. Aside from that you can do the whole thing optically.

  • Since the frisbee's moment of rotation doesn't change in flight we can safely assume that the quickest rotation will occur just after the user launches the frisbee. So to target the frisbee you can focus on the user as he/she launches the toy.

  • Do the experiment at night, with a dark frisbee with a painted thin strip around the outer rim and a thick white blob on one part of the outer rim. Build yourself a strobe lamp (lots of DIY available online, or rent one).

  • Borrow someone's DSLR, set it to Bulb mode (or 30 second shutter). Use a low ISO and a very small aperture as you want depth of field and low gain.

  • Find a friend with a good arm and willingness to throw a frisbee a hundred times for the sake of science.

  • Play with the strobe frequency settings, and the camera settings.

  • Calculate the frequency. Remember that Nyquist theorem puts an upper bound the maximum frequency you can measure.

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    This trades frame rate of a camera for pulse rate of a strobe. Adding something (a fan blade?) with known motion to the image as a reference will help pin down your measurement errors. Use a retroreflector for the "blob" for best SNR. – RBerteig Jun 20 '14 at 00:37
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    +1 for replacing the blob with a retroflector The advantage of the strobe is that you can get a much higher frame rate (off the shelf anyway) and probably cleaner images, since off the shelf DSLR aren't meant for this. –  Jun 20 '14 at 16:22
  • "Calculate the frequency" is far more tedious and error prone than you're letting on. I'm assuming the goal of this project was to also have some fun in the mean time. – Jason C Jun 21 '14 at 17:21
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A camera with a properly painted frisbee is a good low-tech solution. However if you insist on an electronic solution then one of the few things that would work is to use a digital compass.

I've looked into this problem because I'm planning on building a monocopter (see: http://www.youtube.com/watch?v=1n6ZmwzSL0Y) and I've found that gyros don't have a fast enough rate to do it. It's not the problem of the rate of data but a more basic one of the maximum angular velocity measurable by the gyros as stated in the datasheets. Most gyros (in fact, all the ones I've looked at) will simply report something like 0xffff all the time when attached to something like a frisbee.

Digital compass however don't have this problem because it's not measuring angular velocity but absolute position/heading. In fact, the successful monocopters like the MIT and Embry Riddle ones use digital compass for orientation.

Another solution I've considered is a light detector. Something like this: https://www.sparkfun.com/products/9768. Then simply look for the brightest spot and assume it's the sun and time the duration between bright spots to get the time of a single rotation.

slebetman
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Put some light sensor with a narrow field of view + chip that can detect the light pulses received and their interval. Filter the received light for some frequency to avoid spurious detection, and make the light source match that frequency.

Download the data from the disc.

woliveirajr
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Search for "Frisbee Cam". They used a vane to keep the camera from rotating. The same idea could be used to hold an optical senor from turning so it could count marks going by to measure rpm.

BrianK
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The TI cc2541 bluetooth gyroscope could be used to to this. Only $25 and it only weighs an ounce or two. http://www.ti.com/tool/cc2541dk-sensor

Raydot
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You can use a accelerometer like MMA8451 and add a HC06 Bluetooth Module and a small microcontroller to it. The RPM is easy to calculate.. You read out the force along the Z-Axis! Now you have your centrifugal force. This force is simply increasing proportional to the RPM.

Vladimir Cravero
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Stefan
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  • Hi Stefan, welcome to EE! I removed the greetings part from your answer because this board adheres to a Q&A format instead of a forum format. Please have a look around, search for great answers and read them to grasp what the idea is. – Vladimir Cravero Jun 20 '14 at 09:13
  • f = m·Ω²·r for centripetal force, ie force increases proportional to the **square** of RPM. – James Waldby - jwpat7 Jun 22 '14 at 14:05
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I would consider going for a light sensor behind a slit as a part of an oscillator fed into a simple rf carrier - you could receive the signal and the variation of the tone would give you the number of times per second that the brightest light passed the slot. You could also consider using a DF antto check the position for the Frisbee giving you rotational and absolute velocity.

Steve Barnes
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