Reply by Michael Plante●December 6, 20092009-12-06
>
>
>graemel wrote:
>
>> Hi,
>> Im a fourth year student and am currently working on a project where I
am
>> attempting to track the position of a racecar in the form of a
strapdown
>> INS. I am trying to use a 2-axis accelerometer(x,y), z-axis gyroscope
and
>> also have GPS coordinates available.
>
>My $.02 to the kindest comments from Tim: if you are planning on using
>MEMS sensors, this won't work. For INS, you need the accuracy better by
>several orders. Consider dead reconing by the steering wheel, compass
>and odometer instead of INS.
Although he didn't say he was going to build it, so cost might not be a
big concern, and he could then avoid MEMS. After all, racecars cost money,
too.
Reply by Tim Wescott●December 5, 20092009-12-05
On Sat, 05 Dec 2009 04:39:09 -0800, Rune Allnor wrote:
> On 5 Des, 06:59, Tim Wescott <t...@seemywebsite.com> wrote:
>
>> as long as the vehicle is undergoing accelerations
>> distinct enough to overwhelm the combined GPS and accelerometer errors
>> then there's sufficient excitation for the Kalman filter to orient
>> itself.
>
>
> ...so this stuff only works if the platform is a panicking spastic
> rabbit on speed...?
>
> Rune
Well, that would certainly allow for cheaper sensors.
--
www.wescottdesign.com
Reply by Eric Jacobsen●December 5, 20092009-12-05
On 12/5/2009 8:04 AM, Vladimir Vassilevsky wrote:
>
>
> Randy Yates wrote:
>> Vladimir Vassilevsky <nospam@nowhere.com> writes:
>>
>>
>>> Rune Allnor wrote:
>>>
>>>> On 5 Des, 06:59, Tim Wescott <t...@seemywebsite.com> wrote:
>>>>
>>>>
>>>>
>>>>> as long as the vehicle is undergoing accelerations
>>>>> distinct enough to overwhelm the combined GPS and accelerometer errors
>>>>> then there's sufficient excitation for the Kalman filter to orient
>>>>> itself.
>>>>
>>>>
>>>> ...so this stuff only works if the platform is a panicking
>>>> spastic rabbit on speed...?
>>>
>>> From MEMS accelerometers that you can buy, with proper compensation,
>>> you can expect error at the order of ~1 cm/s^2.
>>
>> Isn't the required accuracy of the accelerometer dictated by how often
>> the GPS readings are taken, how accurate they are, and the overall
>> required accuracy?
>
> I was thinking in context of the very practical problems of filling the
> gaps in GPS coverage when navigating in urban areas, and a "small scale"
> navigation in the areas of several m. Unfortunately, MEMS inertial
> doesn't seem to be adequate for that type of work.
There are several applications that already exist where successful
systems operate more or less as described, i.e., a MEMS sensor plus a
GPS provides good tracking capability for a race car. Most of these
apps provide accurate-enough traces of the line on the racetrack and
acceleration data to determine lateral g-loads, speed, and forward
acceleration.
There's an app for iPhones/iTouch devices that works reasonably well,
but not quite as well as the units with dedicated hardware.
In fact, the group I race with is now considering using devices just
like that for compliance testing of rules. The rule set we run with
classes cars primarily by power/weight ratios. Weight is easy to
determine by putting a car on a scale as it comes off the track.
Currently power is tested with a portable dynomometer, which is
expensive, not always available, and not hard to defeat by somebody
really intent on cheating. Putting a MEMS/GPS device on a car that's
on track, however, will readily reveal whether the car accelerates
faster than it should for a given ptw ratio. If somebody sandbags when
the device is on the car, the lap time will reveal it.
So these things do already exist for this application and they work
quite well.
The only thing I'd mention is that a three-axis accelerometer is
probably going to be needed over a two-axis, since race cars do roll in
corners, and so do race tracks, some very significantly. The existing
devices use 3-axis MEMS and GPS. Maybe the point of the project is to
figure out how to use the gyro instead of the third axis, in which case,
it does make it harder but it's certainly possible.
Check these out:
http://www.maxqdata.com/http://store.traqmate.com/
Or you can peruse the on-line pages of Grassroots Motorsports for the
ads on similar devices. I'll start you on page 16, where there's a pic
and a paragraph about a very famous car:
http://grassrootsmotorsports.epubxpress.com/wps/portal/grass/c0/04_SB8K8xLLM9MSSzPy8xBz9CP0os3iLkCAPEzcPIwODUFMjA0-TYM9QJ_9AQ39fM_1I_ShznPKWRvohIBMz9SNNTSxMQMxi_UgDEF2gH2loBhbILy1KTtWPTCvKTM1L0S_ITkyqSk2qcnRUVAQAdT9Hzw!!/
;)
--
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.abineau.com
Reply by Vladimir Vassilevsky●December 5, 20092009-12-05
Randy Yates wrote:
> Vladimir Vassilevsky <nospam@nowhere.com> writes:
>
>
>>Rune Allnor wrote:
>>
>>>On 5 Des, 06:59, Tim Wescott <t...@seemywebsite.com> wrote:
>>>
>>>
>>>
>>>>as long as the vehicle is undergoing accelerations
>>>>distinct enough to overwhelm the combined GPS and accelerometer errors
>>>>then there's sufficient excitation for the Kalman filter to orient itself.
>>>
>>>
>>>...so this stuff only works if the platform is a panicking
>>>spastic rabbit on speed...?
>>
>>From MEMS accelerometers that you can buy, with proper compensation,
>>you can expect error at the order of ~1 cm/s^2.
>
> Isn't the required accuracy of the accelerometer dictated by how often
> the GPS readings are taken, how accurate they are, and the overall
> required accuracy?
I was thinking in context of the very practical problems of filling the
gaps in GPS coverage when navigating in urban areas, and a "small scale"
navigation in the areas of several m. Unfortunately, MEMS inertial
doesn't seem to be adequate for that type of work.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Reply by Randy Yates●December 5, 20092009-12-05
Vladimir Vassilevsky <nospam@nowhere.com> writes:
> Rune Allnor wrote:
>> On 5 Des, 06:59, Tim Wescott <t...@seemywebsite.com> wrote:
>>
>>
>>> as long as the vehicle is undergoing accelerations
>>>distinct enough to overwhelm the combined GPS and accelerometer errors
>>>then there's sufficient excitation for the Kalman filter to orient itself.
>>
>>
>> ...so this stuff only works if the platform is a panicking
>> spastic rabbit on speed...?
>
> From MEMS accelerometers that you can buy, with proper compensation,
> you can expect error at the order of ~1 cm/s^2.
Isn't the required accuracy of the accelerometer dictated by how often
the GPS readings are taken, how accurate they are, and the overall
required accuracy?
--
Randy Yates % "Bird, on the wing,
Digital Signal Labs % goes floating by
mailto://yates@ieee.org % but there's a teardrop in his eye..."
http://www.digitalsignallabs.com % 'One Summer Dream', *Face The Music*, ELO
Reply by Vladimir Vassilevsky●December 5, 20092009-12-05
Rune Allnor wrote:
> On 5 Des, 06:59, Tim Wescott <t...@seemywebsite.com> wrote:
>
>
>> as long as the vehicle is undergoing accelerations
>>distinct enough to overwhelm the combined GPS and accelerometer errors
>>then there's sufficient excitation for the Kalman filter to orient itself.
>
>
> ...so this stuff only works if the platform is a panicking
> spastic rabbit on speed...?
From MEMS accelerometers that you can buy, with proper compensation,
you can expect error at the order of ~1 cm/s^2.
VLV
Reply by Rune Allnor●December 5, 20092009-12-05
On 5 Des, 06:59, Tim Wescott <t...@seemywebsite.com> wrote:
> as long as the vehicle is undergoing accelerations
> distinct enough to overwhelm the combined GPS and accelerometer errors
> then there's sufficient excitation for the Kalman filter to orient itself.
...so this stuff only works if the platform is a panicking
spastic rabbit on speed...?
Rune
Reply by Tim Wescott●December 5, 20092009-12-05
On Fri, 04 Dec 2009 17:57:48 -0600, Vladimir Vassilevsky wrote:
> graemel wrote:
>
>> Hi,
>> Im a fourth year student and am currently working on a project where I
>> am attempting to track the position of a racecar in the form of a
>> strapdown INS. I am trying to use a 2-axis accelerometer(x,y), z-axis
>> gyroscope and also have GPS coordinates available.
>
> My $.02 to the kindest comments from Tim: if you are planning on using
> MEMS sensors, this won't work. For INS, you need the accuracy better by
> several orders. Consider dead reconing by the steering wheel, compass
> and odometer instead of INS.
I disagree. Certainly for a gyrocompass-like application where you're
trying to deduce orientation by comparing the rotation axis of the earth
with the gravity vector then even the best MEMS devices don't hack it.
And _most_ certainly the sleazy parts that they put in cars to actuate
airbags won't do for anything reasonable.
But I've got mileage with Kalman filters using GPS input and some of the
better MEMS IMUs; as long as the vehicle is undergoing accelerations
distinct enough to overwhelm the combined GPS and accelerometer errors
then there's sufficient excitation for the Kalman filter to orient itself.
--
www.wescottdesign.com
Reply by Vladimir Vassilevsky●December 4, 20092009-12-04
graemel wrote:
> Hi,
> Im a fourth year student and am currently working on a project where I am
> attempting to track the position of a racecar in the form of a strapdown
> INS. I am trying to use a 2-axis accelerometer(x,y), z-axis gyroscope and
> also have GPS coordinates available.
My $.02 to the kindest comments from Tim: if you are planning on using
MEMS sensors, this won't work. For INS, you need the accuracy better by
several orders. Consider dead reconing by the steering wheel, compass
and odometer instead of INS.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Reply by Tim Wescott●December 4, 20092009-12-04
On Fri, 04 Dec 2009 16:13:48 -0600, graemel wrote:
> Hi,
> Im a fourth year student and am currently working on a project where I
> am attempting to track the position of a racecar in the form of a
> strapdown INS. I am trying to use a 2-axis accelerometer(x,y), z-axis
> gyroscope and also have GPS coordinates available. I have been directed
> to look into Kalman filtering however after much reading I am still
> unsure about how to actually apply this to the data in order to improve
> things.
>
> I would very much appreciate if someone could 'hold my hand' through my
> particular application.
>
> Thanks,
> Graeme
I very much doubt that you have enough information available to you. See
if you can get your hands on another channel of acceleration and two of
gyro.
As for the filter itself -- do the Kalman filter thing: make a model of
the vehicle motion, linearize it, and turn it into a Kalman filter.
You'll need an extended Kalman filter, and the angular representation of
the vehicle state will give you fits -- if you find yourself howling to
yourself and mumbling at the moon that's normal for this sort of thing.
Girls will flock after you -- at least all the ones that are attracted to
guys who walk around campus waving their arms in the air and muttering
gibberish to themselves.
--
www.wescottdesign.com