Noise problem?

I have a systwem that I set up that uses 2 paddle wheels npn devices and an encoder card. I have a digital meter hooked up that correctly counts the pulses. However the encoder card is another issue. It reads too many pulses. I have my shielding grounded at both ends, if I only use one end the noise is worse(the counts grow even larger). Any advice on how to eliminate noise as a problem? I still wonder if I'm having a shielding problem or something else. I would like to eliminate incorrect shielding as a problem.
The cable has 4 pair that are individually shielded along with a shielded jacket. Right now I have the jacket grounded at both ends and the other 2 pair grounded at one end. I've tried a couple different scenarios and its not working.

Thanks

1) Ground the sheild ONLY at the counter module end, however make sure you have a good chassis ground between the encoder body & your panel ground. (most encoders with pigtails do not have grounded sheilds (grounded to the encoder body)).

2) make sure you repair the sheild, or fully sheild any splices. Just connect the two sheilds and wrap the splice with foil, then plastic tape.

3) Run your encoder cable in a seperate duct (at least 6" away from power cunductors such as motor leads). Common error on apps with encoders mounted on the motor.

4) check the encoder power supply with a scope to see if you have noise on the power supply. Anybody playing with encoders & motors should have a O-scope in their toybox.

5) use a linier regulated (not switching) power supply for the encoder. Also use a dedicated power supply for the encoder.

If you've done all of these things, then you need to post more detaild info on the encoder, counter module, and application.

poisonfrog said:

I have a digital meter hooked up that correctly counts the pulses. However the encoder card is another issue. It reads too many pulses.

Click to expand...

This sounds strange to me. A few things come to mind.

• Is the card always counting 2x or 4x the digital meter? If so, the card is set up for duplex or quaduature counting and the meter is not. (Doesn't seem likely as you state that the sheilding affects the cound).
• Do you have the meter and card hooked up at the same time and they might be affecting each other?
• Does the card have a much higher frequency rating than the meter. Can you program a minimum pulse on time required in the card to filter out the noise as may be happening in the digital meter?
• Does the card have a lower minimum pulse on voltage level than the meter?

Of course, all the other grounding suggestions should be implemented but something is allowing the meter to work properly while the card is not. Find that difference and you are on your way to fixing the situation.

poisonfrog,

I have attached a document with some cabling advice which is an extract from a control systems EMC guide.

You may find Rule 8 interesting.....



The complete guide is available for download over at MrPLC...

http://forum.mrplc.com/index.php?&act=Downloads&CODE=02&id=502
.
.
.
.

Get a O-Scope

Why is it that I rarely see oscilloscopes mentioned on this forum?
Then you know for sure whether pr not the problem is noise. Perhaps it is the card. The encoder card should see each of the four phases occur in order. It is rare that noise can occur in such a way that it generates all four phases in the correct order. The encoder card should detect changes to illegal phases and generate an error. If not why not?

Why is it that I rarely see oscilloscopes mentioned on this forum?

Click to expand...

I am an Oscope fanatic. My Fluke grew legs but I still have the software and cable...the Fluke kind of sucked anyway, would not stay charged. I have a dual channel 40Mhz B&K that I prefer to use.

Any situation where I suspect "noise" etc is a problem I break out the scope.

I kind of think this may not be a noise issue though.

Ensure that the wiring goes directly from the encoder to the card. There should be no terminal blocks anywhere in between, as this is a primary point for noise to enter.

The problem may not be noise at all

Are you sure that the device is correctly connected to the Encoder Card? Most encoder cards are designed for a differential output device. If you are using a NPN output, you will need to connect a pull up resistor between the device output and the power supply positive. This resistor must be sized to give the correct voltage to the Encoder Card when the NPN output is off (open). If the pull up is not sized correctly, the Encoder Card count will be erratic. Check the Encoder Card Installation Instructions. Usually the manufacturer will detail how to size the resistors.


As Peter said, looking at the signal with a scope will quickly show what the problem is.

Thanks for the advice guys... I'm kinda leaning away from noise as the problem and am now looking at the wiring issues. I have a 2.2K resistor pulling up the signal wire. I have a 24 VDC power supply. It was mentioned to me that I should also try a resitor going from the 24VDC+ to the 24 VDC- to create a voltage divider?? I'm not an electronics fanatic so I have no idea if this is reasonable or not. I adjusted the voltage power supply from 20VDC to 30 VDC... As far as the noise is concerned it looks like the pulse value is very stable (although to high still). Any other wiring ideas?

Thanks

Give up the make / model of the encoder and card. Also post a drawing or sketch of your wiring scheme.

Quote: "I have a systwem that I set up that uses 2 paddle wheels npn devices and an encoder card"

I don't think he has an encoder. He has some device that produces pulses.
So, is this device solid state or mechanical.If it is just a switch(mechanical) it sounds like a debounce problem. Just a WAQ

I've got the impression that its a duel output quad encoder?? But I'm not sure.

Mike,

I think if he responds to your previous post about make and model we will all know

:nodi: Ok guys... I have 2 paddle wheel sensors that I'm collecting pulses off of. I am sending them to a AB VHSC card... As far as the wiring I'm using a 2k pull up resistor that goes from the 24 VDC+ to 24VDC-. I have a 2.2K resistor that goes from the 24VDC+ to the output of the paddle wheel sensor. I have the output going to channel A and the common going to A return. I have the other paddle wheel connected in the same manner. I should get roughly 134 pulses but I usually end up with 196-202 on the one sensor, the other one I don't care about right now... Let me know what other info I can get for you guys and I'll get it...
Thanks for the help. Oh, the paddle wheel is a hall effect sensor.