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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?

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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. –  Mathew Foscarini Jun 18 at 18:23
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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 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. –  Mathew Foscarini Jun 18 at 18:34
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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 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 at 15:07

14 Answers 14

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.

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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 at 8:04
    
@SteveJessop yup that's the idea. –  Andy aka Jun 19 at 8:51
    
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 at 13:25
    
@laindir It has to be done carefully of course. –  Andy aka Jun 19 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. –  hoosierEE Jun 19 at 15:40

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.

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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 at 9:15
    
@JohnU sure.. retro-reflective tape and visible or IR illumination at the camera. –  Spehro Pefhany Jun 19 at 9:58
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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 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 at 16:31
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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 at 12:16

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).

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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.

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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 at 0:35
    
@horta like I said, the accelerometer at the end of a fpc, so on the outer edge. –  Passerby Jun 20 at 0:37
    
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 at 2:34
    
@Samuel feel free to give your own answer –  Passerby Jun 20 at 2:36
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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 at 2:45

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.

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I've looked into this because I'm planning on building one of these: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 at 0:06
    
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 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: analog.com/static/imported-files/data_sheets/ADXRS649.pdf –  swineone Jun 23 at 22:50

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.

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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 at 15:26
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@Barodapride ehow.com/… for the concept, and any microcontroller+accelerometer tutorial for the implementation –  Sparr Jun 19 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 at 0:38
    
+1 This is by far the most sensible solution, low power, light weight, easy to examine the data. @Barodapride Check out lpl.arizona.edu/~rlorenz/frisbee/MSTfrisbee.pdf –  Jason C Jun 21 at 17:19
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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 at 22:04

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.

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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 at 0:37
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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. –  user1512321 Jun 20 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 at 17:21

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.

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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.

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This is pretty clever, but it would be difficult in the day time. –  Jason C Jun 21 at 17:31

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.

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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

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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.

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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 at 9:13
    
f = m·Ω²·r for centripetal force, ie force increases proportional to the square of RPM. –  jwpat7 Jun 22 at 14:05

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.

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