Kenneth Lerman wrote: > > Jon Elson wrote: > >>Ray Henry wrote: >> >>>Are there swivel joints between the two screws so that the motors can >>>move independent of each other? >> >>Excellent point, and no, there do not appear to be any joints >>between the two sides. (Dr. Lie has sent me photos of the >>machine.) That is going to make it MUCH harder to properly home >>the thing without bending the rails or ballscrews. >> >>I wonder if it would work better to home one side, with the >>other side's servo amp disabled, and then home the second side >>the last few encoder counts. Or, home the system with the servo >>P gain turned WAY down, just high enough to get movement, and >>then turn it up after the axes are in sync. > > > That solution assumes that one side can back drive the other. That isn't > necessarily the case. > Right. I've seen pictures of the machine, it looks quite rigid, a welded steel box-tube frame with round slider ways bolted every 100 - 200 mm. A modestly robust gantry, although it looks like 80-20 extrusion plus several round ways between the two ends. > It might be worth considering a home switch on just one side and a > squareness indicator on the other side. (Somehow, measure the offset or > flexing between the two screws.) Then drive one side to make the axis > square. Then lock the two axes to each other and home the whole thing. Well, the problem is when the thing fires up, the two servo drives will be fighting each other and distorting the frame. It needs some scheme to keep the two drives from fighting against each other, even BEFORE the axes are homed. It is not real clear how you do this, especially since it is relatively rigid. How do you determine the flex in the frame? Strain gauges? Wire-spool encoders on each side? Absolute encoders? Actually, matching up absolute encoders sounds like a possible solution.
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