[Dprglist] Compass heading using gyroscopes?
Karim Virani
pondersome64 at gmail.com
Sat May 16 19:19:34 PDT 2020
Hey Murray,
In my earlier forays into robotics I was similarly fixated on compass
heading. Even for indoor situations. I've been down that road and learned a
bit through my failures. In a purely pragmatic way I would say give up now.
Learn from the pain of those who have traveled before you. Turn back from
this mire.
I haven't noticed anything that indicates whether you are thinking indoor
vs. outdoor. But simply by the scale of the robots you are working on I'll
assume indoors. Indoors is an untrustworthy environment for working with a
mems compass or any kind of compass really. Even if you have it on a mast.
This is why I advise turning off the compass of the BNO055. All it can
really do is inject uncertainty into the system. Transient
anomaly filtering is usually good. But what if your robot comes to rest in
an anomaly or just spends some time there? Can you predict how that will
challenge the sensor's fusion? Do you have map of the anomalies in your
space? Can you take the time to build one?
Magnetic heading is also pretty much not helpful indoors. How exactly does
it help you to navigate knowing which way the robot is pointed vs. the
magnetic north and south poles of the planet? Heading is important, but one
based on an arbitrary local coordinate system is more than enough. And from
that point a good imu and good odometry are enough to carry you a long way.
Get these capabilities thoroughly implemented and you'll have a good sense
of where you are relative to your starting point. This still has nothing to
do with navigation, just relative location history.
Over the longer term (minutes vs. seconds), correction of drift can happen
by referencing known landmarks or using a positioning system. Methods and
technologies abound for various environments. Have fun spending money
exploring these - I do. But I wouldn't recommend going down that road until
I had decent location history implemented and thoroughly tested.
It didn't appear as if David's RCAT used any of the sonic landmarks in the
room to correct drift during the run? It does look like it used the corner
it started in to calculate the final X & Y drift. If it achieved that
accuracy without correction on carpet over that duration, then he has an
extremely dialed in solution. Kinda like we expect, but that doesn't make
it any less impressive. I'd like to hear if a compass was used, necessary
or even helpful in that test.
Outside is different. Especially for drones and ships. Even with a goodly
mast, I'd still be cautious with a compass on a small rover - though modern
IMU's have fairly good secret sauce for filtering anomalies. And outdoors
"compass" heading is quite relevant. Though latitudes, longitudes, headings
and bearings are relative to the current rotational axis of the earth
(maybe/probably still measured celestially - let's not get into a
precession discussion) and not the faster drifting magnetic axis. But hey,
we are outside. Why not look into GPS at this point? Most code libraries
tagged as "navigation" kind of assume they are being used in an outdoor
context and with access to GPS data.
Every bit of the universe is moving with respect to every other bit. The
point is we can arbitrarily choose the coordinate system that has the most
practical value and support behind it for our application. Indoors you
don't need to worry about great circles and the obliquity of the earth.
Most of us are happy to just get around that next corner. It's a different
problem and can have a simpler solution. Indoors, figure out where you are
and then calculating heading to where you want to be is just Trig.
Now with all that said. Ignore my earlier advice. Don't be the burnt (or
should I say sodden) and hopeless victim of the compass like I am. Be its
ruler. Become the first person to use indoor magnetic anomalies to improve
a SLAM algorithm...
Cheers, Karim
ps. strike everything I've said and do what you want. These are just the
random mutterings of an itinerant dabbler and madman.
On Sat, May 16, 2020 at 4:36 PM David Anderson via DPRGlist <
dprglist at lists.dprg.org> wrote:
> Howdy DPRG,
>
> Maybe it's not clear that the video of RCAT I showed at the monthly
> meeting is actually a superset of the four corners contest methodology to
> which Doug refers. Sometimes things are not as obvious as I think they
> are. To wit:
>
> The four corners contest is actually the University of Michigan Bench Mark
> (UMBmark) odometry calibration method developed by J. Borenstien.
>
> http://www-personal.umich.edu/~johannb/Papers/umbmark.pdf
>
> The concept is to drive around a large square, clockwise and
> counter-clockwise, while tracking the robot's position with odometry, and
> stop at the starting point and measure the difference between the stopping
> point and the starting point. This shows how much the odometry is in error
> and in which direction, and allows calibration of the odometry constants
> and also the potential difference in size between the two wheels of a
> differentially driven robot. The DPRG uses this calibration method as a
> contest.
>
> Here is a paper I wrote about how I used the UMBMark to calibrate the RCAT
> robot as an example:
>
> http://www.geology.smu.edu/dpa-www/robo/rcat/calibrate.html
>
> The video of RCAT I showed earlier this month does the same thing except:
> 1) the route taken is MUCH longer than a 10 foot square (hence the errors
> should be larger), 2) the route taken involves many, many turns, not just
> four nice ninety degree turns as in the contest, so again the errors should
> be much larger, 3) the route taken is not symmetrical, is different for
> each run, and is not known in advance, 4) the robot itself measures the
> error between the starting and ending point using it's sonars, as
> Borenstien suggests in his paper, and 5) the robot corrects its position
> and pose for the next run each time after measuring those errors.
>
> As I have opined before, robot contests are easy compared to the real
> world!
>
> So the robot is turned loose to do whatever tasks it has been assigned, in
> the case of the video it was to do perimeter following, and at the end of
> three minutes or whatever the timeout is set for, it stops what it is doing
> wherever it is and enables navigation to seek its way back to the starting
> point, or to wherever it thinks the starting point now is.
>
> With those behaviors the RCAT robot can, after a three minute run as seen
> on the video, return to within a few inches of the starting point
> repeatedly. This means that the locations that the robot is tracking
> during it's run are accurate to within a few inches or less FOR THE ENTIRE
> RUN.
>
> In all humility this is virtuosic perfomance from that little robot, which
> may not be apparent if one does not understand what it is doing and what
> the implications are.
>
> cheers,
>
> dpa
>
>
>
> On 05/15/2020 02:18 PM, Doug Paradis via DPRGlist wrote:
>
> Murray,
> John's points are very valid. You can get a feel for the accuracy
> needed on an indoor robot by marking a point on your robot and the floor of
> your test space. Travel a square that is 375 cm (~12 ft) on side, with a
> total travel distance of 1500 cm (48 ft), and mark where your robot ends on
> the floor using the mark on the robot, and measure the distance it is off.
> This is a short description of a DPRG Four Corner contest (see:
> https://www.dprg.org/wp-content/uploads/2018/09/four_corners-201804303.pdf).
> Typical performance a winning robot is sub - 12 mm, the 2nd and 3rd place
> robots sub - 75.mm. Often in the past only encoders were used for
> navigation (i.e., no gyro).Now usually a gyro is used for heading. Several
> years ago John S. used a unique sonic trilateration system to achieve a
> 17.5 mm error, at the time a club record. Similar competitions by other
> clubs have about the same outcomes.
>
> Regards,
> Doug P.
>
> On Fri, May 15, 2020 at 4:36 AM Murray Altheim via DPRGlist <
> dprglist at lists.dprg.org> wrote:
>
>> Hi John,
>>
>> A one degree error may be two inches after ten feet, but I'll be reading
>> that heading continuously so over that ten feet wouldn't things tend to
>> balance out?
>>
>> I don't disagree but I think you may be missing my ultimate point: that
>> even with low accuracy a robot will still have "a behaviour" and that is
>> plenty good enough for my purposes, which are not competitive but rather
>> the exploration of an indoor environment using a Behaviour-Based System,
>> basically following along the research path set by Rodney Brooks et al.
>>
>> I am aware of the difference between resolution and accuracy, and having
>> both a higher resolution and a higher accuracy would improve the robot's
>> ability to navigate accurately -- certainly -- and I'm hardly against
>> that; it's just that I'll be satisfied with much less. As I described
>> previously, something akin to a determining which point on a 16 or 32-
>> point compass rose is fine. If I can get better accuracy I might explore
>> navigating according to a bearing but I have no requirement for that at
>> this point. Maybe in the future... it's an interesting journey either way.
>>
>> Cheers,
>>
>> Murray
>>
>> On 15/05/20 8:00 pm, John Swindle via DPRGlist wrote:
>> > A quick note: A one-degree error is two inches after ten feet. Not
>> > good enough for small robot navigation unless something is constantly
>> > providing better pointing accuracy. Even for the contests, if the robot
>> > depends on that amount of error, it would miss the goal in 6 Can, might
>> > hit the cone in Square Dance, etc. Outdoors, the accuracy has to be a
>> > lot better. I think you will be very unhappy with 11.25 degree
>> > resolution. And be careful that resolution and accuracy are not the
>> > same thing.
>> >
>> > Later,
>> > John Swindle
>>
>> ...........................................................................
>> Murray Altheim <murray18 at altheim dot com> = =
>> ===
>> http://www.altheim.com/murray/ ===
>> ===
>> = =
>> ===
>> In the evening
>> The rice leaves in the garden
>> Rustle in the autumn wind
>> That blows through my reed hut.
>> -- Minamoto no Tsunenobu
>>
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