Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!mailrus!uwm.edu!wuarchive!zaphod.mps.ohio-state.edu!usc!rutgers!netnews.upenn.edu!grip.cis.upenn.edu!ulrich From: ulrich@grip.cis.upenn.edu (Nathan Ulrich) Newsgroups: comp.robotics Subject: Re: High Accuracy manipulators Keywords: accuracy, position control vs. force control Message-ID: <29067@netnews.upenn.edu> Date: 1 Sep 90 19:21:27 GMT References: <1990Aug28.234809.15660@portia.Stanford.EDU> <1990Aug30.014817.8794@portia.Stanford.EDU> <141582@sun.Eng.Sun.COM> <5829@hplabsb.HP.COM> Sender: news@netnews.upenn.edu Reply-To: ulrich@grip.cis.upenn.edu (Nathan Ulrich) Organization: University of Pennsylvania Lines: 68 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