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'[EE]: stepper motor in TemUR microrobot'
2002\07\08@231817 by Irek Rybark

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Hi all!

I don't know how to begin... Perhaps this is not even [EE] question but
since I know that you guys have huge experience and thousands of projects
behind let's give a shot.  Here we go... It is a long story.

Last months I was tinkering with very small (it is nice to think 'micro' but
I know what 'micro' means) mobile robots.
The heart of the creatures is PIC of course!
The first robot [TeamUR the First] was moving pretty well however I was not
very happy with the precision of its suspension. Two DC motors with worm
gear run it.
http://teamur.netfirms.com/tmr001/tmr001.htm

The second robot [TeamUR the Second] was supposed to be a [my!]
technological breakthrough because I decided to go for the stepper motors.
I was hoping for more precise motor control.  The first stepper motor
control attempts were very promising.  Since I am electronics engineer it
was easier for me to build a robot and test it rather than try to calculate
all this mechanical crap like torque, friction etc.
http://teamur.netfirms.com/tmr002/tmr002.htm
It looks that I was wrong.  I put a sprocket directly on the motor's shaft
and unfortunately the torque was not enough big to move it.  Of course the
first mistake I made was the size of this sprocket.  Replacing with the
small one helped a lot.

And here comes the disappointment.  The robot should have a nickname "Jerky"
because of how it moves!  I suspected that dynamic (continuous) vs. static
(single step) stepper motor control is different but what is happening with
it when the stepper runs below 10 steps per second is heart breaking!  The
whole robot jerks probably because the motor is not enough precise (16 steps
per revolution) and makes too BIG STEPS.  Sometimes the motor gets stuck and
robot turns around.
I know that this is cheap surplus motor but at this point of time I am not
that big robot "enthusiast" to pay $$$ per motor to some Swiss company.

The second problem is that the motors suck current from battery like fresh
orange juice.

1.      Do you think that gearhead is necessary in such cases?
2.      Where can I find an explanation of the dynamics of the stepper motors
without going for LONG studies?
3.      How to decrease power consumption and increase torque, which are
contradictive goals?
4.      Is there a source of SMALL (diameter << in.) & CHEAP (price << $50)
stepper motors?
5.      Do you have any suggestions regarding using stepper motors in such
construction?

I am not afraid to take the robot to my "closet junkyard" and build a new
one but I would like to avoid this type of probably basic mistakes in the
future.

Let me apologize if you think that it should be [OT] - I can change the
prefix.
I appreciate any feedback.
Irek

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2002\07\08@235009 by Andrew Errington

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Hello,

Neat robots!

Problem number 1: Jerky

You have your sprockets the wrong way round. It looks like your driving
gear is 1.5" diameter, which means that each step drives the chain
about 0.3".  The small ones look about 0.5", so if your motor uses them
then each step is smaller (maybe 0.1").  Also, you should research the
techniques of half-stepping, which should allow you to achieve 32
'steps' per revolution.  There is another technique called
microstepping, but it is more difficult to implement.

Problem number 2: Power consumption

Unfortunately, the coils in the stepper motor are just lengths of wire,
with a fairly low resistance, so the current is high.  If you keep the
current turned on you can hold the stepper motor in a fixed position,
however, if your motor will not be subject to external forces (and the
robot is pretty small and light weight) then you could maybe try pulsing
the coils to make the motion, and then leaving them off when you know
the motor has reached the next step position.  I have not tried this, it
is only an idea.

Problem n:

Other small stepper motors can be found in floppy disk drives.
Vibrating pager motors are small (but they are not stepper motors)
Increasing torque can be done with a gearbox or series of rubber bands
and pulleys.
That's it for now.

Oh, did I say, neat robots!

Andy

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2002\07\09@005459 by Irek Rybark

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Andy:
thanks for the response!

> -----Original Message-----
> From: pic microcontroller discussion list
> [spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU]On Behalf Of Andrew Errington
> Sent: Monday, July 08, 2002 11:48 PM
> To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
> Subject: Re: [EE]: stepper motor in TemUR microrobot
>
>
> Problem number 1: Jerky
>
> You have your sprockets the wrong way round. It looks like your driving
> gear is 1.5" diameter, which means that each step drives the chain
> about 0.3".  The small ones look about 0.5", so if your motor uses them
> then each step is smaller (maybe 0.1").  Also, you should research the
> techniques of half-stepping, which should allow you to achieve 32
> 'steps' per revolution.  There is another technique called
> microstepping, but it is more difficult to implement.

Yes, originally the driving gear was ca. 1". The purpose of this approach
was to cover (hide) the motor.  Very early I realized, that this is not
clever since I have low-precision motor. I've changed the sprocket. It
helped but not too much. See:
http://teamur.netfirms.com/tmr002/movement_problem.htm

Half stepping is something I would like to try... I am afraid that
microstepping requires way more complicated circuit.


{Quote hidden}

Another yes. This was my initial idea, to put 754410' enable low after
pulsing for enough long to make a step. This way for the most of time the
motors are not powered. So far to make the robot moving I disabled H-bridge
enable pin pulsing.

> Problem n:
>
> Other small stepper motors can be found in floppy disk drives.

I tried to use stepper motors from floppies but the current constructions
require addtional support (ball bearing) at the end of the shaft. See:
http://teamur.netfirms.com/test/test_floppy_disk_drive.htm

> Vibrating pager motors are small (but they are not stepper motors)
> Increasing torque can be done with a gearbox or series of rubber bands
> and pulleys.

Yeah... I was hoping that the stepper motors can give me hardware simplicity
combined with relative precise control. There must be somewhere a mistake...
I am not going give up yet!

> Oh, did I say, neat robots!

Thanks once again!
Irek

>
> Andy
>
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2002\07\09@050531 by Roman Black

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Hi Irek, shame about the jerky problem. You really
need more gearing, like a 1:5 pair of gears, as you
probably won't get the performance you want with 1:1
connecting the stepper to the chain, even with your
small cog.

Steppers *will* vibrate quite a bit, and even with the
right gearing you might find that because you have hard
shiny chains it will still not get precise location on
the ground. It is fairly important to have good gripping
rubber between the stepper motor and the ground. :o)
My little stepperbot here:
http://www.romanblack.com/deskbot.htm
has 1:7 gearing on the steppers, and rubber tyres.
Without the rubber tyres it slides and does not drive
in a straight line or get accurate distance etc.
Even with the rubber tyres it loses/gains 1mm in every
50 or so, without rubber tyres it is more like 20mm
error out of every 50!! Skid city.

Another thing to think about is the h-bridges, they are
normally 3v to 4v (saturation) when turned on, so you
may only be getting 4v on your motor and losing 4v on
the bridge. Better to use 4 fets and drive the motor
unipolar (has half the coil resistance and 10x less
loss on the drivers).

You can get tiny stepper motors in any 3.5inch floppy
drives, dead drives are available everywhere. The best
advice I can give is a redesign using at least 5:1
gearing, *rubber* tyres, and low saturation drivers.
:o)
-Roman


Irek Rybark wrote:
{Quote hidden}

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2002\07\09@063037 by Irek Rybark

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Yeah!
Roman, thanks for the professional feedback.  I was hoping that there will
be someone with *robotic* experience.

> {Original Message removed}

2002\07\09@142651 by Peter L. Peres

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On Mon, 8 Jul 2002, Irek Rybark wrote:

>It looks that I was wrong.  I put a sprocket directly on the motor's shaft
>and unfortunately the torque was not enough big to move it.  Of course the
>first mistake I made was the size of this sprocket.  Replacing with the
>small one helped a lot.

You can measure the required torque/force using a piece of string a
plastic bag (to be filled with coins) and a smooth-edged table. Refer to a
physics textbook (for school 10th grade or less) for howto.

>And here comes the disappointment.  The robot should have a nickname "Jerky"
>because of how it moves!  I suspected that dynamic (continuous) vs. static
>(single step) stepper motor control is different but what is happening with
>it when the stepper runs below 10 steps per second is heart breaking!  The
>whole robot jerks probably because the motor is not enough precise (16 steps
>per revolution) and makes too BIG STEPS.  Sometimes the motor gets stuck and
>robot turns around.
>I know that this is cheap surplus motor but at this point of time I am not
>that big robot "enthusiast" to pay $$$ per motor to some Swiss company.

You can add viscous dampers or friction brakes. The latter are easier to
DIY. A simple one: tie a short bit of string to a short piece of elastic,
wrap the string (not the elastic) once round the axle and fix the two ends
of your brake to the chassis. Adjust the elastic tension as required. This
is a sort of rope brake (dictionary term for this type of device). The
string should be wide (shoelaces etc). It brakes more in one direction
than in the other.

>The second problem is that the motors suck current from battery like fresh
>orange juice.

You need to look into current chopping stepper controllers imho, with
current settable by the CPU.

>1.      Do you think that gearhead is necessary in such cases?

Must have.

>2.      Where can I find an explanation of the dynamics of the stepper motors
>without going for LONG studies?

Ha. Try to get a copy of B.C.Kuo's book on stepper controls.

>3.      How to decrease power consumption and increase torque, which are
>contradictive goals?

See above at current chopper.

>4.      Is there a source of SMALL (diameter << in.) & CHEAP (price << $50)
>stepper motors?

Pass.

>5.      Do you have any suggestions regarding using stepper motors in such
>construction?

Avoid them. You can get servos that small or hack your own using some
small motors and wakeup clocks or windup toy gearboxes.

>I am not afraid to take the robot to my "closet junkyard" and build a new
>one but I would like to avoid this type of probably basic mistakes in the
>future.

Do the basic physics on your robot ('coefficient of friction' without
motors, weight -> required starting torque reduced to motor torque by
gears) etc. Then it will be easy ;-)

Peter

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2002\07\10@043154 by Roman Black

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Irek Rybark wrote:
>
> Yeah!
> Roman, thanks for the professional feedback.  I was hoping that there will
> be someone with *robotic* experience.

It's only professional if you pay! ;o)

> Ok. Then dreams about attaching stepper directly to the sprocket are really
> unreal?
> With the gearing the whole thing becomes too big for me.  I was thinking
> about building a robot with the dimensions below 1". The robots currently
> have ca. 2" and in the next generation should shrink rather than grow... :o)

Your motors had tiny metal cogs, if you find a larger
plastic gear that matches, and glue it to wheels with
rubber tyres you may get 3:1 or 4:1, and have tyres.
simple and still small, maybe smaller than the chain
setup you have now. The extra torque from re-gearing
makes all the difference. :o)


> Now I am aware that one of the problems is this "hard shiny chain" I was so
> proud of.  Mechanically there is no way to have this chain *not skidding* on
> the floor. There is no mechanical part, which can accumulate step's energy
> and smoothly release it.  Probably a drive made with pulleys and rubber band
> should help a lot. What do you think?

Not really. Part of the problem is the acceleration
and deceleration of the bot, when stopping and starting.
You *really* need rubber if you want decent traction,
unless the whole bot runs on a rubber tabletop.

> > My little stepperbot here:
> > http://www.romanblack.com/deskbot.htm
>
> This is a real beauty! :o) Especially impressive for me is that your robot
> "has enough torque to push a full coffee cup without slowing, in fact it
> will push about 1kg across a smooth table".

Yep it's quite torquey, the 7:1 gearing and heavy
bot weight make it like a little tractor. But again,
without the rubber tyres (only plastic wheels) it
won't push nothin'.

It really sounds like you want to make these bots
small, the best advice is to look at what the REAL
experts are doing, like the "ants";
http://www.ai.mit.edu/projects/ants/

For tiny bots you can get tiny "RC servos" which
have a motor and gearbox built in. They can be modded
to give continuous rotation. Try the RC (radio control)
web sites. Rubber wheels are a must. :o) You can also
add a tiny infrared sensor to the gearboxes, giving
you position feedback to the PIC at 32 pulses per wheel
rotation etc. Little servos are going to use 10x less
power than steppers, are smoother, more torque, etc etc.

One way that appeals to me is the "angled motor direct
drive" concept. You use tiny pager motors, with a tiny
rubber wheel not much bigger than the shaft, say 2mm shaft
and 4mm rubber "wheel" like a rubber grommet around the
motor shaft itself.

The gearing is provided mainly by the tiny size of the
wheel. The 2 motors are angled at about 45 degrees to
give some ground clearance. This gives decent start
torque and good speeds too. The secret is to put a
tiny IR encoder wheel on the back end of the motor,
and use good PWM control of motor voltage, giving
good slow speeds, good high speeds, good traction, good
energy efficiency, good positional feedback etc etc.
See the basic motor configuration of the Beam photovore;
www.robotbooks.com/beam_solar_robot_kits.htm
and improve it with a couple of optical encoder wheels
from a mouse. :o)
-Roman

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2002\07\10@125004 by Brendan Moran

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> It really sounds like you want to make these bots
> small, the best advice is to look at what the REAL
> experts are doing, like the "ants";
> http://www.ai.mit.edu/projects/ants/

The other goal of ants was to make a robotic comunity.  You'll note that
that link is part of the "retired robots" section at MIT.  I found "Gaak the
predator robot"
education.guardian.co.uk/higher/engineering/story/0,9840,740719,00.ht
ml
to be a more up to date AI experiment (;

--Brendan

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2002\07\10@220956 by Irek Rybark

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Hi all!

Roman wrote:

> Your motors had tiny metal cogs, if you find a larger
> plastic gear that matches, and glue it to wheels with
> rubber tyres you may get 3:1 or 4:1, and have tyres.
> simple and still small, maybe smaller than the chain
> setup you have now. The extra torque from re-gearing
> makes all the difference. :o)

This is worth to try!  Sounds like amnesty for TeamUR the Second.  It will
be saved for later.


>> There is no mechanical part, which can accumulate step's energy
>> and smoothly release it.  Probably a drive made with pulleys and rubber
band
>> should help a lot. What do you think?
>
>Not really. Part of the problem is the acceleration
>and deceleration of the bot, when stopping and starting.
>You *really* need rubber if you want decent traction,
>unless the whole bot runs on a rubber tabletop.

I understand.  To move, the robot needs a support (good ol' Newton!) but the
friction is not everything. You must agree that after while of skidding the
robot should start moving but nearly it didn't.  It looks for me that there
is micro-acceleration and micro-deceleration of rotation of the stepper's
shaft (in my case: without gearing).  I think that even with the rubber
wheels this micro deceleration can be drastically decreased when the rubber
band stretches "on step" and releases the energy between steps. It makes the
whole movement smoother.  I tried it with the "shiny chain" and the rubber
band.  The band really makes a difference.


>It really sounds like you want to make these bots
>small, the best advice is to look at what the REAL
>experts are doing, like the "ants";
>http://www.ai.mit.edu/projects/ants/

I saw The Ants.  Love them but look at this PIC bot:
http://chanhak.com.ne.kr/robot.html/


>For tiny bots you can get tiny "RC servos" which
>have a motor and gearbox built in. They can be modded
>to give continuous rotation. Try the RC (radio control)
>web sites.

Yep!  As I wrote in response to Peter's feedback: I am already experimenting
with one purchased some time ago in my local hobby store: Hirec HS-55 servo
"it looks very promising. Dim. 7/8"x5/8"x3/8", very light comparing to the
currently used steppers, A LOT of torque: 18oz.in!."


>Rubber wheels are a must. :o) You can also
>add a tiny infrared sensor to the gearboxes, giving
>you position feedback to the PIC at 32 pulses per wheel
>rotation etc. Little servos are going to use 10x less
>power than steppers, are smoother, more torque, etc etc.

What you are saying is that I should leave the electronics controlling the
servo and send the pulses?  This is a very good idea!  As I saw servo
hacking somewhere in the Internet I need to replace the potentiometer with
resistors to force continuous rotation.  But if I will stop pulsing the
servo should remain in the same position.  Is it true?


> One way that appeals to me is the "angled motor direct
> drive" concept. You use tiny pager motors, with a tiny
> rubber wheel not much bigger than the shaft, say 2mm shaft
> and 4mm rubber "wheel" like a rubber grommet around the
> motor shaft itself.

Solar bugs...  I tried this approach on the very beginning.  I am not a big
fan of biocore. I am rather PIC-er.  Unfortunately looks for me that this
type of propelling tends to be somewhat unpredictable because of high
rotation speed of DC motor.  Besides I would like to give to my robots an
ability to climb some object since the are supposed to be desktop robots and
my desk is never empty :o)

Thanks for your time!  I am sure that I will run in some more problems soon
(with PIC for example) and it will be an opportunity to let you guys know
how is going.

Irek
http://teamur.netfirms.com

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2002\07\10@223238 by Irek Rybark

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Peter:
Thanks for the feedback. I am sorry that I did not reply quickly: for some
reason your post couldn't reach my mailbox but luckily I found it in list's
archive...


>You can measure the required torque/force using a piece of string a
>plastic bag (to be filled with coins) and a smooth-edged table. Refer to a
>physics textbook (for school 10th grade or less) for howto.

Sure, but the tricky part is an answer to the question: what next? It is
rather difficult to evaluate how much torque I need to move the robot when
it exists on a paper only (or even worse: in my mind). There are way too
many variables... :o(


>You can add viscous dampers or friction brakes. The latter are easier to
>DIY. A simple one: tie a short bit of string to a short piece of elastic,
>wrap the string (not the elastic) once round the axle and fix the two ends
>of your brake to the chassis. Adjust the elastic tension as required. This
>is a sort of rope brake (dictionary term for this type of device). The
>string should be wide (shoelaces etc). It brakes more in one direction
>than in the other.

Sounds simple but I am afraid that it won't last long in my
“micro-construction” :o).


>>1.      Do you think that gearhead is necessary in such cases?
>
>Must have.

This is sad part... :o(


>>2.      Where can I find an explanation of the dynamics of the stepper
motors
>>without going for LONG studies?
>
>Ha. Try to get a copy of B.C.Kuo's book on stepper controls.

Will try.

>>5.      Do you have any suggestions regarding using stepper motors in such
>>construction?
>
>Avoid them. You can get servos that small or hack your own using some
>small motors and wakeup clocks or windup toy gearboxes.

This is probably the best possible advice. I am just trying to use hacked
Hirec HS-55 servo and it looks very promising. Dim. 7/8"x5/8"x3/8", very
light comparing to the currently used steppers, A LOT of torque: 18oz.in!.


>Do the basic physics on your robot ('coefficient of friction' without
>motors, weight -> required starting torque reduced to motor torque by
>gears) etc. Then it will be easy ;-)

Those equations look simple in "physics textbook".  Of course that I can try
to make estimates...

Once again: thanks!
Irek

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2002\07\10@224101 by Irek Rybark

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Hi all!

Brendan wrote:

> The other goal of ants was to make a robotic comunity.  You'll note that
> that link is part of the "retired robots" section at MIT.

I am thinking about building a group of semiautonomous desktop robots.  I
want to give a PC ability to play a role of mother-brain. The mother could
show a goal and the TeamURs should proceed avoiding obstacles and solving
minor problems.
Unfortunately as more time I spend playing with this crazy project as better
I see the difficulties...



> I found "Gaak the predator robot"
> education.guardian.co.uk/higher/engineering/story/0,9840,74
> 0719,00.html
> to be a more up to date AI experiment (;

This is really funny story. I've read somewhere more detailed relation.
Unfortunately it was just accident, lack of attention rather than AI.  But
you never know... :o)


Irek
http://teamur.netfirms.com

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2002\07\11@055310 by Roman Black

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Irek Rybark wrote:

> >It really sounds like you want to make these bots
> >small, the best advice is to look at what the REAL
> >experts are doing, like the "ants";
> >http://www.ai.mit.edu/projects/ants/
>
> I saw The Ants.  Love them but look at this PIC bot:
> http://chanhak.com.ne.kr/robot.html/


Wow! Some very nice tiny robots and ALL are using
PICs! I noticed just about all the tiny rubber wheels,
pulleys motor/gearboxes etc are all standard VCR
(or walkman etc) parts. That page is really worth
looking at! :o)
-Roman

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