Re: Various bugs in March 15 EMC release

[Jon Elson <jmelson-at-artsci.wustl.edu>]

Fred Proctor wrote:

> The way homing works is that motion proceeds at the [AXIS_#] HOMING_VEL
> speed in the direction specified by [AXIS_#] HOMING_POLARITY until the
> home switch trips. The polarity of the home switch is set by [AXIS_#]
> HOME_SWITCH_POLARITY. Then, the axis position at the trip point is saved
> and motion is aborted. Once the move stops, a move back to the saved
> trip point plus any offset in [AXIS_#] HOME_OFFSET is done, and the axis
> position is set to the value specified by [AXIS_#] HOME.
>
> With HOME_OFFSET and HOME set to 0, the homing sequence would look like
> this:
>
> 1. slow jog toward home switch.
> 2. switch trips.
> 3. jog decelerates and move terminates shortly after home switch.
> 4. jog back to switch trip position.
> 5. position set to 0, and digits turn green.
>
> With HOME_OFFSET set to -0.1 (sign depends on value of HOMING_POLARITY),
> and HOME set to 3.0, the homing sequence would look like this:
>
> 1. slow jog toward home switch.
> 2. switch trips.
> 3. jog decelerates and move terminates shortly after home switch.
> 4. jog back to point 0.1 units past switch trip position (to get off
> switch if you need to)
> 5. position set to 3.0, and digits turn green (lets you specify 3.0 as
> home for this axis).
>
> This works on my servo system, but I haven't tried it on a stepper
> system with switches. It sounds like it's not working, but it's hard for
> me to tell.

So, this means you don't use the index channel of the encoder at all?
That seems like a big waste.  We should, of course, support both stepper
machines with no encoder, and encoders that don't have an index
channel, but for machines WITH encoders with index, we should find
a way to use them.  I know that the LS7166 chip has a VERY wierd
way of dealing with the index position latch, but it seems that it could
be used.  You have to run open loop during the time you are making the
final approach to the index position, but I think that could be done.
(The index signal latches the position value, so you can't read the
position reliably while the index controls the latch.)

Jon