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Date:	Tue, 4 Mar 2003 21:07:23 PST8
From:	Sheridan George <s-geo@usa.net >
Reply-To:	scoug-help@scoug.com
To:	scoug-help@scoug.com
Subject:	SCOUG-Help: Drive stiction ?

Content Type: text/plain

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Steven Levine wrote:

> The term stiction is a commonly used term when referring about the
> frictional force that needs to be overcome to get an object at the rest
> position to the moving state, either rotating or straight line. A Google
> search for "bearing stiction" will get you plenty of hits.
>
> Stiction is nasty when doing servo control because it's basically
> non-linear.

Permit me to butt in and be a bit nit-picky here.

Stiction (static friction) is not non-linear. It is very linear. It resists a linear force (or
non-linear force for that manner) in lock step until the object moves. At that instant static
friction ceases to exist and dynamic friction takes over. It's that difference in the coefficients of
static and dynamic friction that drives servo designers nuts.

BTW, dynamic friction by definition must be less than static friction. If it is not than nothing moves
because static friction has complete sway. At least until one gets into the very non-linear world of
the irresistible force vs. the immovable object.

Sheridan

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