🚀 go-pugleaf

An earlier posting says (sorry I don't have the reference):
   I see the next breakthrough in robotics being the 
   introduction of very high accuracy manipulators--
   say an order of magnitude (or more) better than
   any systems of the 80's.  There are plenty of potential
   products that simply cannot be assembled today anywhere 
   outside of a laboratory.

Dan Boehlke (boehlke@sunrise.stanford.edu) replies:
   Very high accuracy manipulators are very difficult to design using
   today's technologies if they are to be of any general use.  The best
   (gp commercial) figures I remember from the 80's are approx. .001"
   repeatabilty for a _light duty_ electronic assembly robot (working
   envelope ~ 1 cubic foot).  

Michael Smith (mgsmith@hplabsb.UUCP) responds:
   One reason humans are capable of high accuracy tasks (putting a chip
   on a circuit board for example) using a low accuracy manipulator
   (the human arm) is because of the use of end point control.  One looks
   at what one is doing while doing it.  Instead of making a robot more accurate, 
   the loop needs to be closed at the end point using either vision or other
   sensors.  Then only the resolution of the robot is important which of course
   is much simpler than improving the accuracy of a manipulator.

High precision manufacturing can be handled in two distinct ways.  The current
automated approach is to use very massive and very stiff machines with linear
slides and precise position control.  There are grinding operations which
are toleranced in millionths of an inch (that's 0.000001") and the machines
that can handle this are vibration-isolated and temperature controlled.  

You will never get near this precision with a serial robot arm--the geometry of
cantilevered links connected by revolute joints just does not lend itself
to this kind of accuracy.  Some small SCARA arms claim repeatability under
controlled conditions of 0.001" inch, but this is mostly BS.  I challenge
anyone to show me a serial robot arm that can obtain accuracy (not
repeatability) of better than 0.01" inch outside of controlled testing and
calibrating environments, and throughout its claimed payload range.

But we've all heard about engravers that could put the Gettysburg Address on
the head of a pin by hand.  How is this possible, given that the human arm
has lousy position control in free space, even in comparison to robot arms?
You may claim that the engraver uses a magnifying device and his advanced 
vision system to perform "endpoint" control, and if robot vision systems were 
as good we could do the same thing with robot manipulators.  Ridiculous!

The engraver does the same thing that we do when we write:  he grounds his
high workspace, low accuracy arm to the workpiece and uses his small workspace
high accuracy hand to perform the work.  Have you ever tried to write anything
small and legible without resting your hand on the paper?  "Endpoint" control
will not overcome the accuracy limitations of a robot arm, which are related
to position sensing, stiction, and stiffness.  And any incidental vibration
can destabilize such as system--not a problem if the workpiece and the
manipulator are in contact.

But I'm not a pessimist.  I think high-accuracy operations can be realized
with current technology and with serial robot manipulators.  But not with
precise position control and not with "endpoint" control, but with force
control.  This same low-accuracy human manipulation system can locate its
two hands relative to each other in space with enough accuracy to put a
0.9995" peg in a 1.0000" hole.  How?  By using compliance and force control.
This is better than any robot manipulator can accomplish without huge chamfers
and the use of a RCC (which is passive compliance).

My opinion only.  Now tell me why I'm wrong.

Nathan Ulrich                           "If it was easy, someone would have
ulrich@grip.cis.upenn.edu                done it already..."
DoD #0080 - GT750 pilot