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Hardware Hacking

Measuring Acceleration/Speed for Small Vehicles? 55

Posted by Cliff from the better-than-a-cop's-speedometer dept.
stampfli_brit asks: "I am the captain of rowing at my local club, and was looking for a way to improve training. We have an NK SpeedCoach. However, I was looking for something that could measure the speed of the shell several times a second, more like this [pdf]. Any ideas on a cheap system using IC accelerometers or GPS, that could get this kind of information, log it for an hour or two, then dump it to an Apple Mac for analysis?"
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Measuring Acceleration/Speed for Small Vehicles? More

Measuring Acceleration/Speed for Small Vehicles?

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  • Or rather the acceleration sensor in it?
  • Not exactly what you're looking for, but it would work:

    Either delegate people from your team, or hire some local person (IE: College student looking for a little extra cash) to point a radar gun at the boats, and have the radar gun output to something.
  • Acceleration or speed? If this is the combination Calculus/Rowing club, you can figure one from the other. Otherwise, it's much easier to pick what you want ahead of time.

    • Actually you can't. Acceleration is a vector and speed is a scalar. Speed is the magnitude of velocity. Additionally, the derivative of speed is not the magnitude of acceleration, but rather the magnitude of the tangential component of acceleration.

      /nitpick ;)
  • One of the standard GPS messages is VTG (track made good and heading).

    The message provides the current speed and heading.

    A standard (>$200) GPS receiver should give this information. You will need a more expensive one if you want to save the data and sync it to your computer.

    If you want a real cool rolls royce solution you could contact my company - we'll write PDA solution for you.
    http://www.sapien.com.au/ [sapien.com.au]

  • GPS Handheld (Score:2, Informative)

    by hillg3 ( 656728 )
    Most handheld GPS devices out these days offer this functionality. You can dump raw NMEA data to a PDA/LAPTOP as well if the built-in speed logging isn't sufficient. Take a look around Garmin [garmin.com] and Magellan [magellangps.com]'s web sites for a starting point.

  • How about.... (Score:3, Informative)

    by Seraphim_72 ( 622457 ) on Monday April 25, 2005 @07:34PM (#12342503)

    How about one of these [vernier.com] attached to one of these [vernier.com]? Together you are looking at about $500 but compared to the alternative ... Dig around on thier site they even have accelerometers [vernier.com] and other neat stuff. I have used thier software and hardware it's good stuff and the people at the company are friendy and helpfull.
    • The OP wanted to use a Mac -- not sure if Vernier supplies software for MacOS? I know they are pretty open about their devices' specs, though. The older versions went through the COM ports, but I think the newer versions use USB.

      I'm also not sure what kind of accuracy you get on the x(t) and v(t) by integrating off of the a(t) the acceleremeter gives (haven't had a chance to play with one yet). You'd have lots of oscillations in the a(t) due to the oars cycling, right?

      We measure accelerations in physics l

      • Their Graphical Analysis comes in a Mac flavor, and (IIRC), you can even use LabPro with Linux (assuming you have a USB->serial converter dongle handy).

      • I'm also not sure what kind of accuracy you get on the x(t) and v(t) by integrating off of the a(t) the acceleremeter gives (haven't had a chance to play with one yet). You'd have lots of oscillations in the a(t) due to the oars cycling, right?

        You can integrate a(t) once to get v(t) and twice to get x(t), but I wouldn't recommend it. I've tried it (not with a boat) and it just plain doesn't work very well. At all. Period. Even with very high quality accelerometers and a very high acquisition rate, yo

    • I know a guy who uses these to measure power output of an engine in a car. Put in the accelerometer, stand on the gas pedal, compute the g forces, and work the numbers backwards. From what I hear, the problem with using very sensitive accelerometers is that they can pick up stray noise such as the moon passing overhead, so you'll need to be careful.
  • serial accelerometer [sparkfun.com]

    Scroll down, and they have accelerometer you can just plug into a serial port. You'd want to have a laptop on board, and some simple software to interface with the serial port...but serial port interfacing isn't to hard, so this might work for you.

    oh, and it's like $45 or so.....
    • Thanks! This looks the most useful.

      Any ideas on where to buy a similarly cheap data logger for serial? Or software for a PDA to do the same thing?

      • :-/ not really
        Yes, it would be convenient to have a handheld data logger that you could dump later....but I think that would just add to the complexity of using something like this device.

        I think the simplest solution would be to plug it into a laptop, and either capture the data with a serial terminal (which are easy to get/use on any system) then dump it to some kind of software and plot it....or write something to intercept the data in real time and plot it. The latter would probably be the most usef
    • You don't want a laptop on board a racing shell. Trust me on this one.

      The only way it would work is if it was very light and very waterproof, or better yet, if it floats and is waterproof.

      I'm surprised nobody mentioned this, although geeks who row are probably a rarity.

  • Oh, that's easy! (Score:3, Interesting)

    by NewWazoo ( 2508 ) <bkmatt AT gmail DOT com> on Monday April 25, 2005 @07:53PM (#12342671) Homepage
    All you need is a string, a weight, three poles, and a protractor.

    Make a crossbar out of the poles, and mount the protractor to it, with 90* facing downward. Tie the weight to the string, and hang the lot right next to the protractor. Align the contraption such that the length of the protractor runs parallel with the keel of the boat.

    With some (very) simple fizzix, you've just built an accelerometer.

    B
  • Roughly how fast do you normally go? GPS speed becomes less reliable the slower you go because of positional error. 15-20 foot error can be fairly substantial at low speeds.
    • while in a crew shell you travel about 4 to 5 m/s that is about 10 mph
    • That depends on the receiver. The Rockwell/Conexant/NavMan/SiRF receivers used in the DeLorme Earthmate are (at least were in the serial version; don't know about the new USB ones) quite sophisticated. They use a separate receiving chain for the Doppler data, so the velocity solution is independent of the position solution. The error ellipse on a zero velocity point was only about 0.1 m/s in radius. That's plenty good to get a nice velocity on a crew hull. They only return data 1 point per second, but

  • Tricky (Score:4, Informative)

    by linuxwrangler ( 582055 ) on Monday April 25, 2005 @08:10PM (#12342819)
    This can be a lot harder than it seems at first. In the yachts I race on we have some top-of-the-line instrumentation. Sensors feed GPS info, wind speed/direction, speed-through-water, compass-heading, etc. into the instrumentation computer (black box) and on to the various displays on-deck as well as into a pc for tactics and analysis.

    It's very useful info but you also learn rather quickly that it's full of errors. Consider that the wind speed and direction is usually read from a anemometer/vane at the top of the mast (often on a small pole to attempt to keep it out of the wash of the sails). Speed through the water is done ultrasonically or with a paddle wheel. All these direct measurements are converted to things like true wind speed, true wind direction, velocity made good and so on.

    Damping is a huge problem. You may think that several readings a second is good but as the boat rides over waves and heels to varying degrees all of your measurements will be off. Wind due to the mast swinging (doesn't take much heel to really move things around 7 stories up - I've been there many times), boat speed due to water-flow differences at different heel angles (some boats average readings from multiple sensors to combat this issue), and even magnetic heading due to compass-dip if the compass sensor isn't gimballed or otherwise compensated. Essentially you become limited to short-term running averages of the readings.

    You don't have quite as many inputs but still need to be concerned with yaw and tilt of the shell, turbulence from the oars and so on. Boat motion will be especially problematic if you are trying to directly measure acceleration.

    I suspect that an appropriately designed towed speed sensor might get you the data you desire (though the data-quality will still be sketchy). Perhaps a small propellor/chopper-disk/optical sensor. You could probably build one for a few dollars. The problem then will be to convert the pulses to something you can use.

  • Normally I'm a big proponent of GPS units. I think they are very cool, and they have a lot of inherent functionality.

    For this task they are probably a poor choice. Typical GPS unit uses algorithms for determining speed which will filter out minor changes, or delay their appearance in the data.

    They do a very good job of giving an accurate speed when they are neither accelerating nor decelerating. (more accurate than your average speedometer in good conditions). But it is unlikely the data will show any det

  • Why? (Score:4, Informative)

    by Zebra_X ( 13249 ) on Monday April 25, 2005 @08:37PM (#12343075)
    Several times a second? What can you garner from that kind of resolution in speed? Perhaps you are looking to analyze the impact of the rudder on the boats performance?

    Having been a varstiy cox for a top D1 rowing team I've never needed such information to make my boat faster.

    The speed coach is a very good device. Short of rebuilding it, you will not find anything that will come close to it's level of accuracy (or durability). If you're looking for a device that can dump to a PC (presumably Mac as well) have a look here [nkhome.com]. It's not that expensive and comes with all of the hardware that you would have to build yourself.

    Your only other option is to find an impeller, and get a data logger and make it waterproof. You'll also need a display and a C compiler to drive the impeller -> display conversions.

    I guess the question is how much is your time worth?

    • Re:Why? (Score:2, Informative)

      by stampfli_brit ( 878877 )
      Several times a second: in order to measure changes in the force profile of the stroke.

      Example: If the cox calls "send the boat off the finishes", to see what effect that has on the force curve, and boat speed.
  • My crew club is having another problem (which I know all crews are subjected to). The steel ballbearings in the slide wheels rust out, and when you have 4 wheels for 8 seats you have 32 wheels rusting. We end up replacing the shotty we find about every two weeks. The old slide solution didn't involve ballbearings, but it sucked. It involved rolling axles attached to the wheels, a sliding plate, and another set of rollers below the actual seat. It jams all the time, but it never rusted out. What would anyone say is a more appropriate mechanical solution? Thanks for not modding this off-topic, because you don't see crew on /. very often...
    • Go back to slide plates. In all my years (5) of competitive rowing, the only times I ever lost a seat that was connected via a slide plate was when I wasn't paying attention to my technicque. The few times I did, it was because I was putting too much weight onto one side of the seat (which will throw off the set of the boat anyways), or too much pressure on the front edge of the seat (which leads to an inefficient stroke.

      So my advice - go back to what works. Slide plates have very few moving parts, and do
    • Stainless steel bearings.

      Seriously, why is this even a question?

    • there are sealed bearings with grease fittings. Some modification may be required, and it will probably end up spewing some grease which is nasty to clean up, but it will solve your corrosion problem. It's one solution to protecting the bearings on boat trailers.
  • ... but not for cheap as far as i know. we have played with several imus and a couple accelerometers. all the reasonably cheap ones have enormous drift rates, and the rest all seem to be $1500+.

    that said, once you have the sensor, you can talk to it pretty easily using a standard serial interface.

  • More than Accel (Score:4, Informative)

    by BSDevil ( 301159 ) on Monday April 25, 2005 @10:06PM (#12343762) Journal
    Correct me if I'm wrong, but you're looking for something that will do everything a SpeedCoach does, but you want a better resolution, and you want it (presumably) for less money.

    My immediate thought would be to go grab a few Vernier sensors (as someone else mentioned) and a TI-92, and do it on that. However, NK has spend a whole lot of time and cash making sure their impellers work perfectly for what they're supposed to do, and produce the minimum drag possible. I know if I was rowing at that level (I rowed in HS for a while), I wouldn't want some homemade impeller giving me excess drag.

    Upon further though, why would you want that much resolution in terms of speed? The SpeedCoach measues at the midpoint of each stroke, as the power phase is finishing. If you want something more specific than that (maybe to analyze for the effect of small changes in stroke shape or recovery speed), I would think that you'd be using a tank anyways, and you could do something with video for that (not needing an impeller at all).

    Also, think about this: most national teams use SpeedCoaches for their training. If it's good enough for them, why do you need more prescision?

    I think you need to rethink your problem. If you're measuring for the impact of rudder usage on speed, you can do that with models in a flow tank (go talk to your hydrodynamics departement). If you're doing stroke geometry, use an erg (or a tank, a videocamera, and a few Vernier probes). I can't think of a good reason for needing that much prescision in an on-water situation.
    • The SpeedCoach is good for measuring general speed. What it does not show is how small technique changes alter the power curve of a stroke.

      I want the ability to be able to compare the power curve on the water to that on the erg. And see how specific changes in the stroke make the boat slower / faster, and compare that to the rowers' perceptions.
      • What if you mounted force transducers on your rowing stays. Lots of work, I know, but then you could plot the application of force over time. You could see how well the team is in sync, how well the left and right halves are balanced and so on. It would be independent of wind. You could compare individual rowers, and their performance over time.

  • One option (Score:3, Informative)

    by stienman ( 51024 ) <adavis@@@ubasics...com> on Monday April 25, 2005 @10:35PM (#12344039) Homepage Journal
    If you want to do precision inertial measurements then cheap GPS will not work well.

    A good system would be GPS coupled with an accelerometer and microcontroller. Use the GPS for absolute positioning, and to calibrate the accelerometer. Use the accelerometer for speed and position updates as quickly as you need them.

    It would probably take about 40-60 hours of work to develop and test the concept, and probably cost about $200 per prototype unit. Thereafter one could probably make them for $100 apiece. Assuming a cheap engineer at $60/hr you're looking at $2,800 to $4,000 for the first 2 prototypes.

    If you want to go the cheap and quick route, you can skip the GPS and simply use an accelerometer. One such gizmo is here [myplace.nu]. This person created a simple acceleration meter that analyzes a few car parameters by monitoring acceleration during a 0-60 test. This doesn't match up directly with what you need, but with some work you can take this and make it do what you're looking for.

    -Adam
  • http://memsense.com/ [memsense.com]
  • I keep a pair of fuzzy dice hanging from my rear view mirror.
  • Okay, the easiest is to average the output from an array of accelerometers and put them into a joystick port on a laptop (mac probably won't work) because these can double as a 0-5V ADC. The next level of coolness will work with a mac. Average the accelerometors and use it to control the gain of a JFET. use a schmidt trigger (trust me, 555s are too noisy for anything) to occilate at some frequency. Put this into the audio in of the mac and use supercollider to write a program to exam the amplitude of th
  • I use a Garmin G76 GPS to record my glider flights. It takes a breadcrumb trail, and can be set to do it pretty damn frequently, at least 1/sec I am pretty sure. You can then download the trace via a serial port to your computer for analysis.

    It's very simple to use.

  • GTech-Pro (Score:3, Informative)

    by Rob Parkhill ( 1444 ) on Tuesday April 26, 2005 @11:34AM (#12348497) Homepage
    I don't know if this would work for you, but check out the features on the GTech-Pro RR [gtechpro.com].

    It's a set of precision accelerometers designed for car racing, but it might give you enough detail that you can use the output to suit your needs.

    Looks like it only works with a PC, but for $300 you can't have everything.

  • Also remember that most cheaper/oem GPS modules only output 1 pulse per second on a serial line. This may infact not be anywhere near the frequency that you require.
  • You might want to mount a corner-cube reflector on the boat and use a laser doppler velocity measurement system. Your like would be made simpler if you could row directly away from the laser. This would move all the electronics off the boat and allow reasonably high-precision and high-sample rate measurments.
  • There are several inexpensive (under $200) data loggers available. For example, the HOBO [onsetcomp.com], or something from the Datalogger Store [microdaq.com]. Dataq [dataq.com] also sells inexpensive A/D converters, but you would have to take a laptop along on the rowing shell to record the data, Their stand-alone data loggers are a bit more pricey ~$400), but use SD memory and have high resolution and high storage capacity. You can get free samples of accellerometers from Analog Devices [analog.com].

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