On Mon, 14 Jul 2008, John Kasunich wrote:
> Date: Mon, 14 Jul 2008 21:11:13 -0400
> From: John Kasunich <[EMAIL PROTECTED]>
> Reply-To: "Enhanced Machine Controller (EMC)"
> <emc-users@lists.sourceforge.net>
> To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net>
> Subject: Re: [Emc-users] Tandem Axes-YY
>
> Jon Elson wrote:
>> 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.
>>
>> Jon
>
> One approach is to consider the power-up state as "acceptable" meaning
> "since the motors were free-wheeling a minute ago, there can't be too
> much stress in the machine". Record the offset between the two encoder
> feedbacks before enabling the amps, and maintain that offset while
> moving toward home. The gantry may be a bit out of square, but it won't
> get any worse. When one axis hits home, record the position and keep
> moving. When the other hits home, record its position and stop (still
> keeping the same offset between motors). Then use the two recorded
> positions to determine the exact offset between the encoders that will
> result in a square gantry. Then move one (or both) motors to that offset.
>
> Regards,
>
> John Kasunich
>
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A similar scheme if the startup condition is acceptable (enough for one turn
of the motor), is to do the above scheme to move the motors one turn and
locate the encoder index points, then use the measured offset between indexes
to synchonize the axis (assuming the matched index means perfect mechanical
alignment), then only one limit switch is needed
Peter Wallace
Mesa Electronics
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