Re: [CAD_CAM_EDM_DRO] Major encoder trouble, need advice.

Henrik Olsson wrote:

> Hi guys,
> I have a problem I've been working on for several weeks now and I need any
> help I can get. I appologise in advance long post but I'll try to cover what
> I've done and what I've tried. But first a bit of background....
>
> I'm aiming to use the new HP-UHU drive to run my motors and after quite a
> bit of struggle with various issues I now have three drives that seems to
> work pretty good. The problem is with the encoder signal.... The drive has a
> counter that increments for every invalid transition it sees (or think it
> sees) on the encoder input. Invalid transitions can come from overspeeding
> (the CPU can't keep up with the frequency) or from noice.
>
>
>
> Now, I've tested the drives and motors with a 3600 lines encoder, (14400
> counts / rev) and it works fine up to around 541rpm with that encoder then
> it starts to report invalid transitions.

Well, that's 14400 * 541 / 60 = 129840 counts per second. What is the
software sampling rate of the (PIC ?) CPU there? I'll bet it is about
130,000/second, or every 7.7 us.

> I knew the bandwidth was limited
> from the get-go so that's fine - now I know where the limit is and that the
> *drive* AND the differential interface (based on SN75115) on the drive
> works. (Initially it didn't work due to a problem with the drive but now it
> does.)
>
> Motors are rated at 2000rpm so I settled for USDigital E7P, 625 lines with
> differential output. With my mechanics this will give me 0.001mm theoretical
> resolution and still stay inside the envelope of both the drive and Mach3.
>
>
>
>
> OK, finally to the point. With the E7P in place I can not get it to run
> reliably at ANY speed. The setup is the same as what was working with the
> 3600 lines encoder, same powersupply, same motorcable, same
> control-computer, same motor just another encoder and the same grounding
> scheme. The motorcable is screened and screen connected to motor supply
> -ive. Encoder cable is screened twisted pair (only 300mm long for now) with
> screen connected to logic level GND.
>
> Each differential pair is terminated, where entering the drive, with a
> 120ohm resisitor in series with 4.7nF capacitor as per USDigital
> instructions. It was impossible to get it to work reliably so since the
> encoder was new I turned towards the drive which has had it's issues. I
> scoped around, swapped the differential reciever IC, swapped drives, and
> generally tried whatever trick I could come up with but nothing helped.
>
> I've tried with and without the encoder cable shield grounded, with and
> without termination, with only the 120ohm resistors as termination. I've
> tried bypassing the differential reciever circuit using the A and B channels
> only. I've tried running the A and B channels to homebrew differential
> transmitter and running THAT to drive. NOTHING seemed to help.
>
>

OK, the one constant is that the original encoder DID work, up until it
exceeded the UHU input rate. I think that is the most significant data
point! Something about this US Digital encoder is not working properly.

>
>
> Then I mounted an AMT encoder (you know those capacitive ones with
> adjustable resolution that Digi-key sells) my second motor and gave it a go.
> NO PROBLEMS, cycled thru my testprogram for 45 minutes without trouble so
> the drive IS INDEED OK.
>
> I contacted USDigital and they suggested that I change encoder to a new one,
> might have damaged the disc while installing it.... Since trying everything
> else I gave it go. I VERY carefully installed the new encoder (electronics
> AND disc) with the supplied mounting tools to get correct distance etc and
> checked runout of disc - it was 0.02mm, can't be much better than that.
>
> 100% sure of having fixed the problem I gave it go. Couldn't even jog for a
> couple of seconds without getting into trouble. I even lowered the motor
> supply voltage from 130V to 30V and run the motor 50revs at 20rpm and it
> STILL acted up. I can't imagine it being noice then....
>
> I'm getting desperate here. USDigital says it's not the encoder since I
> changed that. I say it's not the drive since it works with another encoder
> and it's not the motor since it used to work before mounting the E7P. But
> what the heck is it??
>
> I've checked the output of the encoder and it looks fine. Nice sharp square
> pulses, 50% duty cycle, 90deg shift. Each complementary pair is always the
> inverse of each other etc. I don't own a logic analyzer so it's REALLY hard
> to detect if a single pulse "dissapears".
>
>

Well, it IS the encoder, period. Have you tested the US Digital encoder
when it is CONNECTED to the UHU? That is what matters. Is there a mode
to measure position through the UHU with the drive not "driving"? My
guess is the switching transients of the power stage of the UHU are
upsetting the US Digital encoder. If it is OK when the power stage is
off, but goes haywire when the drive is active, you have a major piece
of information. One thing to try is a significant capacitor, like 4.7
uF across the DC input pins of the encoder, RIGHT at the encoder.
Is the 5 V power for the encoder coming from the UHU, or some other
power supply?

Jon