I am trying to identify the friction model of the system. There are
several model of friction taking the streabeik effect into
consideration. suppose i choose any model. for example say Tustin
model, how do i find its parameters. Well while capturing the
experimental data(for a given force measure velocity), the streabeik
effect is there only near zero(very small velocity) region. How do i
define this low velocity. At lower velocity(where velocity is obtained
by numerical differentiation of position), the signal noise ratio is
low. Can you kindly help me out with some suggestion or pointer to
some useful literature where i can get more insight into this.
Bwa ha ha ha ha ha ha ha!
Oh, er. Sorry.
"The Control Handbook", William Levine, Ed., 1996, CRC Press, has a good
article on friction modeling and compensation. I also have a note on my
website -- http://www.wescottdesign.com/articles/Friction/friction.html
-- which is more about tried-and-true methods of controlling motors
rather than any attempts to accurately model friction.
If you have a "normal" motor control problem to solve I _strongly_
suggest that you just design your controller to operate in a friction
environment, and that you make it as robust as possible. Friction is a
very slippery effect to pin down (pun intended); you'll find that even
if you can characterize it exactly today it's sensitive to so many
factors that it'll be different tomorrow. It's best to just design your
system to be robust to it's vagaries.
The nice thing about all that is it frees you from having to accurately
model it -- just design your controller with a lot of margin, and be
done with it. And keep in mind that while I'm giving this advise I have
spent days sweeping a system with a control systems analyzer to get it's
_exact_ linearized transfer function, because that _was_ something that
was fairly well controlled, and could be profitably used in design.
Niclas Granqvist wrote:
You can do something simple or perform complex designs for friction.
My experiences, regardless if you do something dirt_simple, or
complex, is to monitor the temperature, so and to ensure that your
system is not headed for catastrophic failure. Temperature monitoring
is very simple, and can be use for testing the implementation as well
as for PM (preventative maintenance) sheduling. Things with friction,
fail, it's just a question of when they fail, not if.
Thanks to all.
my question was more on identification..so that i can have a
simulation study of the practical system. i read some literature on
idetification of friction but i am not convinced, they sound me too
Well, friction is a nonlinear phenomenon and that means
that you will have to work some more.
Is this a school project or an application?
1) Gather the data. Also gather validation data
to try the model on.
2) Basically you need to settle on a few model structures to try out.
3) Identification is not hard with todays fast PC:s.
A lot of people buy matlab and use sophisticated tools.
If you only have a few parameters then you can do with much less.
I have survived with a C-compiler and a couple of for-loops+numerical
and trying all possible combinations and then saving the interesting
The PC could be running a day, but it doesn't matter. Eventually the
correct results come out.
4) When you have the parameters then plot model input/output
in time and look with your own eyes if it seems to work.
If you are not happy go to 2)
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.