On Fri, 10 Jul 2009, Leslie Newell wrote:
> Date: Fri, 10 Jul 2009 17:39:19 +0100
> From: Leslie Newell <les.new...@fastmail.co.uk>
> 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] Hostmot2 homing
>
> Hi Peter,
>
> If it is edge sensitive then as you say, inverting the signal has little
> effect. I was going by your previous statement where you said it was
> dependent on the state. After doing more tests, noise looks to be a
> possible culprit. It seems odd that I have a relatively bad noise
> problem on the index while A and B are fine. They all run down the same
> screened cable. I ran some tests and the other axis also very
> occasionally misses. Neither axis loses position while running so A and
> B don't seem to be affected.
>
> From your docs, A and B are filtered. Does Z have the same filtering?
> The 7i29 has a relatively long ribbon (about 4') so it may be that it is
> picking up noise. I will first add some common mode chokes to the 7i29
> motor outputs (works well for Geckos and Rutex drives). If that has no
> effect I'll try moving the encoders to the 7i33. That would entail a lot
> of rerouting and remaking cables so it would be a last resort.
>
> By the way, when the encoder inputs are set up for complementary
> signals, what happens if one wire becomes disconnected? Is there a fault
> signal?
>
> Les
Z hardware filtering is identical to A and B. With your current filtering
count rates up to 4 MHZ are possible and noise pulses shorter that ~.5 usec
are rejected. This may still be too fast for your noise environment.
Unfortunately, the HM2 driver does not support lowering the encoder sample
rate sample rate yet, but this could be tried by using the raw-write command
in your HAL init file. With a 5I20, the encoder sample rate is 33 MHz, when
the filter is on, 15 clocks are required to recognize an input (the ~.5 uSec
above) If for example the sample rate were lowered to 3.3 MHz, those 15 clocks
would now be ~5 usec, much better at eliminating PWM interference, and stil
capable are reading encoder inputs up to ~400 KHz
If you disconnect one side of the differential inputs, that input will likely
not work as differential inputs have a 130 Ohm termination resistor and enough
hyteresis that they will not switch when only one input is driven. There is no
encoder fault detection, that is better left to FE and PID saturation detect.
Peter Wallace
Mesa Electronics
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