My CHNC uses a 7I29 and a single end encoder on the X axis, works
fine. All the wiring goes through the same flex guide. Make sure your
motor leads are twisted to cut the noise.
The motor leads are screened and grounded to the drive. Adding the
chokes drastically dropped the radiated noise that I could detect with
my near-field probe but had no detectable effect on the noise picked up
by the 7i29.
Are the shields of the encoder cables grounded at the encoder end? Are
the encoders themselves grounded to the machine frame? Possibly
different grounding of the machine frame, or isolating the encoders
with plastic sheets and washers might help. (Don't forget the encoder
shafts.)
The encoder cables screens are grounded at the drive end. They are not
connected at the encoder end. I don't think the problem is noise pickup
on the encoder cables because connecting one of the encoders to a 7i33
fixed the noise on that encoder completely, even with filtering disabled.
Also, common mode chokes on the DC INPUT to the 7i29 might be of help.
The 7i29 doesn't have any big reservoir capacitors on board. I was
always taught that H bridges need a nice low impedance supply. Adding
inductance doesn't strike me as being a good idea. I could of course be
wrong.
Is there possibly more than one ground on the 7i29? Possibly a shield that
is connected at both ends or some thing else creating a group loop.
I don't think so. The power section of the board is electrically
isolated from the logic. Apart from the ribbon the only other connection
is to the encoders. The encoders work fine when connected to the 7i33 so
I doubt if it could be a grounding issue through the encoders.
If not grounding differences then there must be a difference in the input
networks of the two cards. Do you have schematics?
No, I don't have schematics.
A few other data points:
The power supply in my inverter sticks out a crap ton of fairly wide
band noise but I don't see a noticeable difference in the noise that the
encoders see with the inverter on or off.
I tried using the 5i20 with a standard PC grounded to mains earth and
with one running from an isolated power supply. I also tried a 7i80
powered through a voltage regulator from the machine 24V.
I have now moved both axes to the 7i33 and now see no noise on either
axis, even with filtering disabled. The machine ran for about 4 hours
repeatedly running a simple test program then moving back to two dial
gauges to verify position. After that time the gauges were still reading 0.
A couple of weeks ago the 7i29 stopped working. On further investigation
the two drive transistors on the dc-dc converter had failed. Going by
the soot stains and accumulated dust I think one failed several years
ago, putting more strain on the other which finally failed. Several
years ago the machine stopped and I noticed that burning electronics
smell. At the time I couldn't find the source and cycling power appeared
to fix it. That was probably the first transistor failing. Apart from
the transistors being difficult to access it was an easy enough repair.
I don't think this bears any relation to the noise issue as I have had
noise problems pretty much since I first wired it up.
The spindle is now on one of the 7i29 inputs. Without filtering I can
see noise on the spindle but filtering seems to be able to handle it.
Noise on the spindle is less harmful than on the axes so I now have a
machine that is usable but I would really like to figure out what is
going on. I have another job coming up that could use two of these
drives but if I am likely to have similar issues I'll go with something
else.
Les
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