🚀 go-pugleaf

In article <5829@hplabsb.HP.COM> mgsmith@hplabsb.HP.COM (Michael Smith) writes:

   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.

   End point control is also advantageous because it can eliminate the 
   need for special linear slide joints, a temperature controlled environment, 
   an isolating granite slab, and frequent recalibration.


True to a point, especially with more conventional accuracy requirements-
but the discussion was about manipulators with ~ .0001" accuracy.  This
would imply some very accurate sensors indeed.  Unless the sensor system
directly measures the positioning error (eg end-effector to workpiece) as a
differential error , one would still need to calibrate the sensor to the
end-effector. Most vision-based sensor systems would have considerable
trouble at this resolution level, and would have a very small field of view.
A contact-based sensing systems is generally much more accurate, but also is
more delicate, has more "observability" problems and recalibration is more
difficult.

In addition, for most assembly tasks the sensor-based system would be
considerably slower, since the robot would need to come to a stop (with time
to damp out all vibration to well below .0001") at least twice before
attaining the final position.

The original poster was interested in moving this sort of technology out of
the lab...that isolated granite table is still probably required, since the
forklifts running down the corridor outside tend to make the robot shake a
thou or two....  And I wonder how stable the camera and lens are over the 20
degree temp variation we get between the night and day shifts :-).

I'm still not sure what product really requires this level of accuracy in
assembly - can anyone clue me in?  It would be interesting to examine the
error budget for the complete system.

- Bart

barts@Eng.Sun.Com

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