small motors or linear actuators

"Mike"

There were a couple of other posts on linear actuators this week, take a
look because there are some very nice replies.

Recently I've bought a couple of linear actuators for my project from
Firgelli Auto, but these are too much for your application (30 pounds of
force). They have an affiliated company (firgelly) that is just starting to
commercialize micro linear actuators, and they claim that one of the
applications is for robotic legs (small robots). Search google for firgelly
micro (or mini) linear actuator.

Cheers

Padu

Hi - I saw those discussions, but unless I missed something, they were
all about much larger devices. I'm looking for small! Each leg segment
on my robot is only 6cm long - so the smaller the better!!

I just took a look at the Firgelli ones - and they don't seem like a
very good fit for me. Specifically, their force is a little low (18N
max) and their stroke is incredibly small (2cm). Maybe if they had a
larger stroke I could compensate for their low force with leverage -
but geeze - 2cm isn't alot to work with.

Squiggle motors seem possible as well (http://www.newscaletech.com/
squiggle_overview.html) - but they suffer from having even less torque
than the Firgelli ones. I also have no idea what the pricing on those
buggers is, and calls and voice mails to them have gone completely
unanswered.

-Mike

How about something like this?:
http://www.mutr.co.uk/prodDetail.aspx?prodID —5

Michael

Just to follow up on my own post - I finally got ahold of soembody
about Squiggle motors. Those things run about $600 each!!! That ain't
gonna happen.

-Mike

You might want to try something like this:
http://www3.sympatico.ca/doc/microlinear.jpg

I used silver solder to attach a bit of 6-32 screw to the output
shaft. Doing this was a bit trickery and I don't think it's very
strong.

Might be better to make up a small collar. Thread one
end for the screw and Loctite.  Drill and tap the other end
for a small set screw to handle the geared output.

Geared motors from Solorbotics and other places.

DOC



...

Did you read my post about buying some of those linear actuators you posted
on your site?

Michael

More expensive servos provides better bearings.

* * *

In case when total weight of all servos is significant part of total
robot weight real criterion the main criterion is a factor

   max torque / servo weight

rather than simple max torque because decreasing servo weight is
equivalent of some torque increasing. Such factor for Futaba servos
vary from 23 in to 100 in, and value for your Hitec HS-81MG is

   42 in*oz / 0.67 oz ~= 54

So you can some increase quality by using for example Futaba S3102
(torque 64 in*oz vs current 42 in*oz, weight 0.70 oz vs 0.67).

* * *

Linear actuator from geared DC motor + screw provides much more
better characteristics than servos in case when speed is not an issue,
but this variant requires much more complicated mechanics (for example,
bearings for screw, coupling screw with gear output shaft ad so on);
you need also add encoder (as well as process encoder output).

MicroMo (http://www.micromo.com/ ) provides for example such kind linear
actuator (servo + gearhead + screw+ nut):

   http://www.micromo.com/uploadpk/e_06AS2_MPS.pdf
   http://www.micromo.com/uploadpk/e_0513B_MPS.pdf

   weight:     0.15 oz
   max force:   150 oz
   max torque (for joint rotation range 60 degree): 70 in*oz

You also must add a lot of miniature mechanical parts to embed this
toy into your robot (as well as encoder).

MictoMo provides top quality/reliability/long life, but it is very
expensive way.

Nick

Hi Nick - The Futaba looks like a decent servo. However - I'm looking
to get away from servos. I am just so completely sick of working
around the controllers built into them. I suppose I could just rip out
the electronics and just use the motor + gears - but that is more of a
hack than I would like to do. I would really prefer to have full
control over the control loops governing movement.

The MicroMo parts look like they have potential. I will look into
them.

Thanks!

-Mike

WHAT???!?!?!?!!!! And you call yourself a hobbyist????? ;-)

You shouldn't have to "rip out" anything - it can be very elegantly
disassembled and "repurposed." ;-)

And where else will you find motors and gears that are so ideally
suited to the app, other than where Futaba buys them, and you'd
still have to mount them!

But, then again, I've been a hacker (the good kind) for going on
40 years now, so I like to take old stuff and make new stuff with it. :-)

Cheers!
Rich

Hi Mike, it's not clear why you would want to use "linear actuators"
for a walker. Most people just use servos. Any screw arrangement will
be quite slow. Do you have a picture of your bot?

I use Hitec HS322-HD servos on my walkers now, and although they don't
have ball-bearings, they do have hardened plastic gears and they are
*very* cheap, less then half the HS81 price. The crust crawler, which
is very heavy uses the same HS322's, although it's difficult to
imagine they have enough torque to move a 4# walker, plus 7.5# load,
as advertised.

http://www.crustcrawler.com/products/hexcrawler.php?prod=1

I have a very different design walker, and the legs use of form of
pseudo-linear actuation ... it actually will carry a 3# load.

http://www.oricomtech.com/projects/nico6prj.htm


- dan michaels
===============

Your crawler is very nice, and HexCrawler from crustcrawler.com is nice,
but both uses 2 degree of freedom (and two servo) per leg. This implied
that such creature can't seat down/rise/tilt, can't climb stairs...
Actually such kind of crawlers don't have significant advantages over
tracked bots with 2 motors only. Mike' robot with 3 degree of freedom
per leg can seat down/rise, can climb stairs - of course, if software
is smart enough and robot equipped by powerful actuators. Legs of such
robot can be used also as manipulators or as probes for environment
exploration (must be equipped by tactile sensors, of course).

Nick

I missed any link to Mike's robot, so I don't know what he's doing
specifically.

Also, I'm not sure that going from 2DOF to 3DOF legs is the trick to
climbing stairs. It might be more involved in having a flexible
backbone, to better deal with the transitions top and bottom of the
stairs.

Hi Dan - like Nick said, it's a 6 legged robot with 3DOF per leg.
Total length is about 20cm. Leg segments are about about 6cm long.
Servos are positioned in such a way that there is essentially a ball
joint at the base of the robot (ie a hip) and a single joint (a knee)
6cm away from the hip.

-Mike

So, it's gonna be far too small to climb stairs in any event. Maybe
can scramble over a book, etc. My stiff-legged walker design does not
have enough ground clearance to go over obstacles, but I have thought
about adding hinged segments so the back could flex.

It has about a maximum of about 12cm of ground clearance - thus the
mechanics allow it to go over small stairs.

But this is completely irrelevant and off topic. Stairs are not a goal
for my robot.

-Mike

In case 2DOF legs you are much more restricted in selection of next
point for particular leg; maybe it is possible to realize stairs
climbing using 2DOF legs, but it is much less tricky by using 3DOF
legs. The more DOF - the less restrictions - the more abilities;
stairs is only one example from the long list.

By the way, 2DOF implies gliding of foot over floor at least on
turning; 3DOF provides possibility to avoid gliding. Sometime
it is useful and always is more efficient (less energy dissipated).

It is possible also to use "passive" DOF, i.e. not powered and not
controlled directly (implicitly) but i don't know real samples.

* * *

Mike hexapod:  http://mikael.geekland.org/

Nick

There is defnitely a problem with the usual cantilevered leg designs,
ie where the legs rotate around a point on the body, in that the
distance from the foot to the body varies by a huge amount when a step
is taken. So, either the feet have to slip on the floor, or else the
legs have to undergo some very complicated kinematics. The longer the
step, the worse the problem.

Inverse kinematics are incredibly simple for a well designed 3DOF leg.
Anybody that passed trig in highschool shouldn't have to think too
hard to figure it out. This is the inverse kinematics solution for my
robot's legs:

double hsquare = x * x + y * y + z * z;
double h = sqrt(hsquare);
theta[0] = M_PI - acos((leg2length * leg2length + leg3length *
leg3length - hsquare)/(2.0f * leg2length * leg3length));
theta[1] = asin(z/h) - acos((leg2length * leg2length + hsquare -
leg3length * leg3length )/(2.0f * leg2length * h));
theta[2] = ((x > 0) || (y > 0)) ? atan2(y,x) : 0;

(essentially it's just an application of the law of cosines). If your
leg segments are the same length it's even simpler.

These calculations take, as I recall, about an eighth of a ms to
calculate on a 20 mips 8bit uC. I calculate it after each set of PWM
pulses is sent to the servos.

-Mike

Do you specifically deal with the fact that the distance between the
foot and the side of the body varies by a huge amount during the
course of a step?