[Dprglist] Compass heading using gyroscopes?
David Anderson
davida at smu.edu
Sat May 16 14:36:27 PDT 2020
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 <http://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 <mailto: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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