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Patrick Themessl posted on April 7, 2008, 5:18 pm
Hello,
we are participating in a competition held by a local university where
each of the participants got a circular hexapod robot that has to be
trained to run as fast and as smooth as possible. The smoothness of the
motion is measured by a little cup located in a larger cup that is fixed
to the hexapod, where the inner cup is filled with water and has a scale
on it. So far so good... However, we are facing a problem here that has
already cost us quite a lot of time:
Whenever one of the three-leg pairs of the tripod gait we implemented
hits the floor, the whole body of the hexapod suffers quite a bump that
spills a lot of water, which we want to avoid. We have done the servo
linearization and the hexapod calibration very, very exactly and are
able to set the footpoint to almost any point with an accuracy of less
than a millimeter, but there are still those annoying bumps when
walking. We have tried to just make the part of the movement where the
legs are set down to the floor slower, but then the deceleration and
acceleration in horizontal direction causes another impulse on the water
cup and spills almost the same amount of water than the bump we are
avoiding with it.
And to be honest, I have spent so much time in front of this f****** AVR
IDE in the last few days that I think I'm going to go crazy right now,
so I would really, really appreciate any suggestion we can get. This
concern is also quite urgent, as the competition takes place this
thursday! Please help!
Thanks in advance for your replies!
Yours sincerely,
Patrick Themessl.
P.S.: Sorry if my English isn't perfect, I'm Austrian. Smile
we are participating in a competition held by a local university where
each of the participants got a circular hexapod robot that has to be
trained to run as fast and as smooth as possible. The smoothness of the
motion is measured by a little cup located in a larger cup that is fixed
to the hexapod, where the inner cup is filled with water and has a scale
on it. So far so good... However, we are facing a problem here that has
already cost us quite a lot of time:
Whenever one of the three-leg pairs of the tripod gait we implemented
hits the floor, the whole body of the hexapod suffers quite a bump that
spills a lot of water, which we want to avoid. We have done the servo
linearization and the hexapod calibration very, very exactly and are
able to set the footpoint to almost any point with an accuracy of less
than a millimeter, but there are still those annoying bumps when
walking. We have tried to just make the part of the movement where the
legs are set down to the floor slower, but then the deceleration and
acceleration in horizontal direction causes another impulse on the water
cup and spills almost the same amount of water than the bump we are
avoiding with it.
And to be honest, I have spent so much time in front of this f****** AVR
IDE in the last few days that I think I'm going to go crazy right now,
so I would really, really appreciate any suggestion we can get. This
concern is also quite urgent, as the competition takes place this
thursday! Please help!
Thanks in advance for your replies!
Yours sincerely,
Patrick Themessl.
P.S.: Sorry if my English isn't perfect, I'm Austrian. Smile
cadcoke4 posted on
Re: Hexapod tripod gait is bumping too much!
In robotics, it is generally good to have an acceleration/
deacceleration curve for all movements. If you had some success
making a more gentle curve for the up/down motion, then perhaps you
also need to add this for the front/back movement.
By the way, servos often have their own internal acceleration/
deacceleration curves. The best servos allow you to adjust this curve
(by some sort of digital interface). There is also some "dead space"
where the servo won't attempt to move. I've also played with cheap R/
C servos and been able to adjust some of its "tightness" by changing
the rate at which I send pulses to them. Normally, a pluse lasting 1
to 2 ms is sent every 20 ms. But, by increasing or decreasing the
20ms value, you can adjust how quickly the servo acts to correct its
position.
Joe Dunfee
Re: Hexapod tripod gait is bumping too much!
thanks for your reply.
I will try varying the frequency of servo pulsing and see if this has an
effect.
The robot is actually not one produced by Lynxmotion, I was just looking
up "robotics forum" in Google and this is where I was led and because of
the fact that their hardware seems to be very similar to the hexapod we
have, I thought I'll take a chance asking. An image of the hexapod we
have can be found here:
http://forum.hexapod.at/forum/galerie/bilder/bild-99.jpg
Hardware modifications are unfortunately not allowed, so we have to live
without any feedback.
At the moment, I am thinking that one of the main problems we are having
is that the CPU speed of the µC located at the board is just too slow.
When working with an incrementation of 1 for each cycle of the gait
sequencer loop, the hexapod seems to stay relatively stable, but much
too slow. The only way to make it go faster is incrementing by 2 for
each loop cycle, as there is no artificial delay in our code anywhere
and the µC is operating at its limit, but then (especially in the leg-up
and leg-down sections) the movement gets too unexact. To solve this, we
tried to pre-calculate the servo angles for steps in each direction we
want, but then the program gets too large to fit on the µC.
Another question: Does anyone know a web site where a tripod gait that
does not stop the body of the hexapod for changing the stance legs is
explained in detail? I think I am having some conceptual errors in my
code, too.
Thank you very much for your help!
Patrick.
cadcoke4 schrieb:
Rich Webb posted on
Re: Hexapod tripod gait is bumping too much!
Try one leg at a time, in the sequence (using DIP numbering, 1 = left
front) 1 - 5 - 3 - 6 - 2 - 4 ... You still have your triplets (1, 5, 3
and 6, 2, 4) but now you have six small taps instead of two kalumps.
8-)
--
Rich Webb Norfolk, VA
John Nagle posted on
Re: Hexapod tripod gait is bumping too much!
This person also asked the same question on a Lynxmotion support
forum, at
http://www.lynxmotion.net/viewtopic.php?p9015&sid 6fc52e8197b373af7d723a668215c3
The discussion there is more informative. He's apparently using a Lynxmotion
CH3-R robot:
http://www.lynxmotion.com/Category.aspx?CategoryID 1
This is a nice piece of machinery, but has no useful feedback.
For over $700, there should be more sensors on the thing, enough to
do what Rod Brooks was doing twenty years ago with his insect
robots. But no. There's no force feedback signal for the
servos. There don't even seem to be foot/ground contact
switches. It's a dumbbot.
The typical programming approach is to put a table of leg position values
into an Excel spreadsheet and load those onto the robot's CPU.
Given that you have no useful info about what the legs are doing,
that's probably as good as it gets.
John Nagle
Gordon McComb posted on
Re: Hexapod tripod gait is bumping too much!
Lynxmotion
John is correct ... feedback is the answer here. Yes, you can always
deaccelerate at the point of motion where the leg is most likely
approaching the ground, but this is a bit wasteful of the hardware. But,
following John's link to the Lynxmotion forum I get the feeling hardware
mods are not allowed, as the student groups get the use of the robot,
and the next tgroup may not appreciate the hacks made to it. So that
means going into the servos and tapping out the motors for EMF feedback,
or maybe even attaching some rudimentary whisker sensors, may not be
allowed.
This has me thinking what overall use the exercise might have. If it's
purely a programmatical problem, then it's all in the gaits and
accel/deaccel curves. But it seems to me this is like trying to build a
fuel efficient car using steam. The robot will never do as well than one
with even simple feedback.
I once experimented with the servo controller from NetMedia that touts
force feedback using unmodified servos (unmodified for rotation and
unmodified for picking off the EMF from the motor). It worked well
enough that the concept behind how it works could probably be used here
to achieve a substantial improvement for this application, without
guessing at how the robot should walk - i.e., the robot will decide that
in actual motion. And this approach would not require hardware
modification.
-- Gordon
Re: Hexapod tripod gait is bumping too much!
Lynxmotion
Exactly. This is what irks me about most robotics hardware for hobbyists
- totally inadequate feedback. This is teaching kids the Wrong Stuff.
If you want to deal with the real world, you have to have considerable feedback.
This is NOT like programming a computer, where what you command happens
reliably.
The pathetic thing is that even simple R/C servos have position and
current info available within the servo, yet so few of them bring it out
to where a controller can read it.
The Dynamixel servos have real feedback and a two-way digital
interface, but are more costly than R/C pulse-coded servos.
Bioloid, Troxxen, and Robotis robots now use them. Slowly,
the hobbyist robotics industry is getting enough volume that
the components become affordable.
John Nagle
Re: Hexapod tripod gait is bumping too much!
However it would be counter-productive to add this to all the hardware
because there is no one generic use of robotics components. It would
generally be overkill, for example, to provide feedback (and therefore
cost) for servos used on the typical line-follower. For kids price is
paramount; you don't want to foist $300 kit of fancy parts when $100
will do.
To me this is an integration issue, but it won't get better any time
soon. The Robotis servos are nice, but I bet the company is still
working through the R&D cost. It's not like hobby servos where the R/C
market - which does not require feedback - is MANY time larger than
amateur robotics.
The ideal product is the one with a base model at an affordable price,
and options that can be added on. It's a wonderful idea, but who has the
money to create such a product line for what is ultimately a market of a
few tens of thousands of potential buyers. Too rich for my blood!
-- Gordon
Re: Hexapod tripod gait is bumping too much!
The Linxmotion 6-legged thing is over US$700. At that price,
there should be decent sensing.
Things are happening faster than you think. Take a look at
"http://www.robots-dreams.com" for what's going on in Japan.
Some very interesting robotic toys will be shipping for Xmas 2009,
too.
For some things, especially electronics, integration is cheaper
than add-ons. The extra silicon is cheaper than the connectors and
wiring for the accessories.
John Nagle
Re: Hexapod tripod gait is bumping too much!
You're still missing the point. For R/C servos, 99% of the intended
buyers would have no use for such a thing. The manufacturer is wasting
his money, even if it's only incremental. There is no business sense in
that, and this is business. They couldn't care less about the state of
the art of robotics. (And those that do charge a lot for those servos,
if you've ever priced them. Beyond what most of the kids you referred to
a message ago can afford.)
-- Gordon
Re: Hexapod tripod gait is bumping too much!
Some of Hitec's newer servos supposedly talk a "proprietary HMI protocol",
which supposedly allows daisy-chaining. There's a document on this
("http://hitecroboticsusa.com/Tony%20information/HMI%20Protocol.pdf" )
but it's almost useless. Somebody finally reverse-engineered the things
and produced a document:
http://robosavvy.com/Builders/i-Bot/HSR8498HB%20Servo.pdf
which at least describes how to talk to the thing via a serial
port at 19.2KB.
Much to the annoyance of robotics people, the Atmel ATmega8 in the
thing has been set to "mode 3, further programming and verify
disabled", so you can't look at or replace the firmware.
John Nagle
Si Ballenger posted on
Re: Hexapod tripod gait is bumping too much!
Lynxmotion
I think you guy's insistance on feedback is a little over done
for this application. If the bot were moving over uneven ground
then leg position feedback might be of use, but probably not in
this case. Perhaps you could desribe the benefits that would make
it worth while in this instance. As for the $700 bot without
sensors, start making one for $700 that is laden with sensors and
get rich! ;)
D Herring posted on
Re: Hexapod tripod gait is bumping too much!
Don't underestimate the benefits of force-based (as opposed to
position-based) control. A couple classic examples where
force-feedback dominates are writing and drilling/grinding.
With force feedback, this guy could have moved the legs into position
and then lowered them under force control, thus setting an upper bound
for the acceleration. Instead, he had to lower them "slowly enough"
that it "usually worked".
That said, heavily-geared motors (e.g. hobby servos) are horrible for
force control due to (a) friction (b) backlash and (c) the large
downconversion of the geartrain. Real torque motors run nearly
direct-drive, but they're big, heavy, pricey, and use high voltages.
So we make due with hobby servos instead.
- Daniel
Re: Hexapod tripod gait is bumping too much!
This is why I suggested a simple feedback via the serial servo
controller. It's a coarse guide, but in my experience it at least can be
used for determining if a bot is trapped against a wall and otherwise
not able to sense this because of its sensor arrangement. The NetMedia
controller is the only commercial one I've seen with this feature
built-in, but there might be others. With this type of device force
feedback is basically free, except for the cost of programming.
-- Gordon
pogo posted on
Re: Hexapod tripod gait is bumping too much!
of the participants got a circular hexapod robot
http://www.lynxmotion.net/viewtopic.php?p9015&sid 6fc52e8197b373af7d723a668215c3
Lynxmotion
It doesn't really invalidate the points you are trying to make, but the original
poster is not using the CH3 - just one similar in
configuration. (see the 1st reply to cadcoke ... )
fwiw ... jcd
Re: Hexapod tripod gait is bumping too much!
http://www.lynxmotion.net/viewtopic.php?p9015&sid 6fc52e8197b373af7d723a668215c3
If you have to do this without feedback, probably the thing to do is
to put in an IK solver and control the endpoints of the feet.
John Nagle
Re: Hexapod tripod gait is bumping too much!
of the participants got a circular hexapod robot
By "IK solver" do you mean Inverse Kinetics ? I've never tried any of that at
all, so I'll ask this: given the limited hardware,
would it involve a look-up table or would it need to calculate everything
*completely* for every step ?
Just a thought...
thanks!
JCD
Re: Hexapod tripod gait is bumping too much!
Inverse kinematics; given the end point (foot position), calculate
the joint angles.
Here's a simple two-link 3D IK solver in Java, with a demo
applet. The first joint has 2DOF, the second joint has 1DOF,
which, I think, is what your robot has.
http://mrl.nyu.edu/~perlin/gdc/ik/talk.html
Doesn't even need any trig functions.
Once you have an IK solver, you can treat six legs as two
triangular supports, each of which is moved as a unit.
John Nagle
Re: Hexapod tripod gait is bumping too much!
Since they can't modify the hardware there is no way to tell the foot
position other than "guessing" where it should be at given the
instantaneous pulse position sent to the servo, and waiting X for the
servo to travel there (and hoping it does).
I don't think this solution gives them much more than they had to begin
with for their particular problem, frankly, and at the cost of more
coding for what they programmed their robot to have (one gait).
-- Gordon
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