Well, I just figured out how to add an optical sensor without adding any
moving mass. This is the Y axis I'm working on, which is carried by the X
axis. If I home the X first, then I can use a fixed optical sensor to home
the Y axis. It would not be possible to home Y unless X was already at its
home position. I think that should be OK.

Neil

On Sun, Jun 6, 2010 at 1:51 PM, Neil Baylis <neil.bay...@gmail.com> wrote:

> Just wanted to point out that there are millions of inkjet printers and
> plotters in operation right now that use this exact technique. It's
> certainly practical. I think the reason they do it is to lower the cost. I
> imagine it also improves reliability by eliminating failure-prone parts
> (switches, connectors, etc).
>
> The reason I would do it is to lower the moving mass, as I'm attempting to
> build a very agile machine, and every ounce counts. If it's too complicated
> though, I'll rig up some optical sensor that can have low mass. On the other
> axis, I already have a reflective optical sensor that adds no moving mass
> and works well, but that's more difficult on this axis.
>
> Sam's point about the Integral term is important, and raises another
> question: is it possible to alter the PID tuning parameters after the .ini
> file has been read. I think the answer must be yes, because the calibration
> tool in the GUI does this, but not having read the code yet, I don't know
> how that works. But even assuming that's possible, I don't have a clear idea
> of where I would need to hook into EMC to do what I want.
>
> Neil
>
>
> On Sun, Jun 6, 2010 at 1:25 PM, Viesturs Lācis 
> <viesturs.la...@gmail.com>wrote:
>
>> Can You explain the reason, why would You want to do it this way? I
>> think that such approach is breaking machine, it creates unnecesary
>> loads and stresses to the construction of machine and simple switches
>> are cheap and wiring them is easy. I think that it is trying to
>> reinvent the wheel in some painful way.
>> No offense, just my personal opinion :))
>>
>> Viesturs
>>
>> 2010/6/6 sam sokolik <sa...@empirescreen.com>:
>> > I could maybe see monitoring following error...  When the servo hit the
>> > limit - the error would increase.  You could then use some logic that
>> > says when the following error reaches a certain amount - trip the
>> > 'virtual' limit switch.  Maybe..   I could see lots of issues and as
>> > gene says - you would want to limit the output to the servos.  If you
>> > have any I (in the pid) the pid loop will 'wind up' pretty quick sending
>> > the servos to maximum.
>> >
>> > Big picture it seems possible...  :)  (but I am just thinking out loud)
>> >
>> > sam
>> >
>> > On 6/6/2010 11:09 AM, Neil Baylis wrote:
>> >> Many printers&  plotters do not use limit switches. Instead, they move
>> the
>> >> print head slowly towards the end stop until the motor stalls, and then
>> back
>> >> off from that point a certain distance and that's the home position or
>> soft
>> >> limit.
>> >>
>> >> What, roughly, do I need to do with EMC to get this behavior?
>> >>
>> >> Neil
>> >>
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