1746 HSC with compact logix....

darrenj

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Feb 2005
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I have a 1746 HSC i am setting up on a machine, all is good, i am seeing counts, controlling the loads fine, however after a couple of min's i am eather losing counts or gaining.
Full scope
1 ab frequency drive turning 2 cams, each cam has an eletrical (24vdc) clutch
2 Automation direct quad encoders, 360 counts
seperate 5vdc power supply for encoders
lead length of encoders is about 2-3' (using the cable that came with the encoder)
High speed counter is set to ring type

i home the system running the drive at low low rpm (2hrz)
after home is found i can run the machine with clutches disengaged and see no extra counts coming on the encoders, so i have effectivly ruled out noise. (???)

once i engage the cam clutch the machine works great, if i stop within a min or so and check home i am bang on, if however i let the machine run for 5 mins or so then check home i am way off..(i set the cam back to home and my encoder is reading 200 or sometimes 300)
In my config i have left it as x1 seeing as the 360 count suits my purpose well.

i see this happenening on both encoders...

should i regret the AD encoders?

I am sitting here thinking that i might have some noise on my 5vdc power supply line....

any suggestions??

thanks

Darren
 
Encoder noise is one of those things that just begs for oscilloscope analysis. It won't even have to be a fast one for this application.

I don't think that running the motors with the clutches disengaged totally eliminates noise as a possible issue because when the motors aren't running their loads, the current flowing through them won't be as high and likewise the EM field they create will be weaker and less likely to cause interference.

I generally refer to A-B's motion control grounding and shielding reference for things like braided vs. stranded ground conductors and cable routing. I'll look up the publication number.
 
Just as an update...

went back today and tried a couple of things,
First i used a bench top DC power supply to power the encoders, hooked up my power quality analizer, and saw a little iregualarity in the dc line.
ran the machine, it seems the encoder is giving me a max of +3.4v however on the transition it does not drop to zero, i am seeing a lot of +1.2 but also some +1.9 to +2.2.

next i hooked up the 5Vdc power supply i got from AD...And after putting that on a scope its the last power supply i get from them. The plus line almost looked like a sine wave, while it didnt come close to the zero mark it still drifted from +5 to +4.5 in a steady wave.

anyway, while running i didnt see much of a change on the meter, the irregularity didnt get any worse, the only thing i could see was the voltage MAY be above what the encoder takes as a pulse...could be i am way off...

i mesured between A0- and a0+

any one know what an AB 845H encoder puts out?


Thanks Ken
 
I'll have to look up the specs on the HSC module later but that's a differential output encoder. Does the HSC have differential inputs? If not you may need a different encoder (like the totem-pole output RZWD).
 
Now that I have been able to look at typical module specs (though I didn't find the number you stated - you may wish to check it) the module for CompactLogix does have differential inputs.
 
Is the Plc input not loading the line driver ic enough?
Suggestion!!!
Quick test is a 10k - 1k resistor in parallel with the input to 0vdc. Hopefully this will clamp the 1.2V to 2.2V signal experienced. On the encoder specs the current of the line output signals should be given this will determine the correct resistor value. I would make the output drive at least half its current. Remember the PLC only wants to make an Led and an optocoupler work about 7mA.
 
Originally posted by darrenj:

In my config i have left it as x1 seeing as the 360 count suits my purpose well.

Even if you don't need the resolution you should go with the X4 decoding. If you go with X1 the module is just looking for transitions on ChA and counting those. Its what allow you to use single channel encoders with this module. If you select X4 the edges on a given channel are evaluated relative to the level of the other channel and the count increments or decrements as appropriate.

So if you get a noise pulse on a channel and it triggers a count on the rising edge of the noise pulse the counter will take away a count on the falling edge of the same noiuse pulse. You effectively get no additional counts. This is the "other" benefit of A-quad-B encoders.

Keith
 
And the winner is Kamenges!!

That looks like what was going on, i set the encoder to x2 and away it went, same counts every time i checked...

Now maybe i missed that in the manual but i dont think so....where and how did you come up with that??
 
Originally posted by darrenj:

Now maybe i missed that in the manual but i dont think so....where and how did you come up with that??

Given your available configuration options it is just the way of things. The HSC doesn't have separate configuration selections for the decode count (X1, X2, X4) and the physical input type (A alone or A-quad-B). So the counter needs to infer the physical input configuration from the decode selection. If you select X1 the module needs to assume you only have one channel available. So it can't perform quadrature decoding. The X4 option absolutely requires two channels so that has to use quadrature decoding. The X2 option is a bit of a wildcard because technically it wouldn't require two channels. The counter could just count both edges of a single channel. But it appears to do a quadrature decode in that case.

Long story short it is just something I kind of knew from using encoders.

Keith
 

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