Encoder connections with cDAQ 9401

[JohnnyF]

I'm hoping some of the experts here can help me out with this one as I'm new to encoders. I want to make sure I connect this right but am finding conflicting info on it. I found this tutorial, which shed some light on some things for me.  If I am interpreting it correctly, the 9401 card I'm using (with 9172 chassis) can provide the 5 VDC excitation necessary for the encoder on any unused DO line. However, I saw a reply from an NI applications engineer that suggests an external supply is necessary in this thread.

So, my first question is whether or not I need an external supply.

Second, I have an encoder with 8 wires and am a bit unsure of whether I need to connect all of them for my angular position measurement, as I don't see any reference to A', B', Z' in the documentation I've found. Here is the data sheet for the encoder I am using, in line driver configuration (I believe it is x2).

I've also attached an image of the pin-outs and counter configuration for this encoder for anyone that doesn't want to search the entire data sheet.

Thanks very much in advance to anyone that can help!

Message Edited by JohnnyF on 03-31-2009 09:46 AM

[JohnnyF]

I hate to reply to my own post but it's not letting me edit it again. It seems from setting up a DO task in MAX and checking with a multimeter that I can't actually generate 5V with a logic high command so that takes care of my first question. I'd still be all ears if someone could enlighten me on the wiring, though.

Message Edited by JohnnyF on 03-31-2009 02:56 PM

[Chris_W.]

Hi Johnny,

You are right that this module may not be able to supply a solid sustainable +5V output by setting the DO line to high. In the specifications document for the 9401 on page 13, it specifies that the MAX output Voltage is 5.25V, but when the module is sourcing current that voltage is going to be lower. For example, in the same document, it specifies that if you are sourcing 2mA of current the max output voltage will be 4.3V. That is good that you found that out already, I just figured I'd point you to where that is specified. 

As far as connecting your quadrature encoder to your 9401, you must have that module in either slot 5 or 6 to be able to have external access to the counter on the backplane of the cDAQ chassis. Also, to figure out the right connections you should go to your 9401 in MAX and right click it and select "Device Pinouts." This is where you can see the connection pins to use for specific counters. One other way to see what pins you'll need to connect to when performing the angular encoder task, is to create a task in MAX and it will show you which pins need to be connected and how. I have attached an image of an angular encoder measurement task in MAX. This is basically the same thing that you'll want to use. 

The other thing I forgot to mention about your specific encoder, is that it has those other connections, A', B', and Z', because it is a differential quadrature encoder which is better suited to provide noise immunity. However, NI counters do not support differential encoder measurements, so you simply need to use A, B and Z signals only and as long as your signal quality is acceptable, your measurements will work the same. 

I would also take a look at the cDAQ 9172 manual for more information about counter operations. I hope this gets you started.

Chris W

[JohnnyF]

Chris,

Thanks very much for the reply and for pointing me to the specs on the current/voltage limitations. I'd missed that in the spec sheet and was wondering why I was actually able to get what seemed to be a pretty consistent 5 VDC, but the current draw explains that. The only thing I'm not getting there is that if my encoder input current is rated at 65 mA typical (from the spec sheet linked above), how can I decide whether the module output is acceptable? If I think of things simply in terms of Ohm's law, it suggests that the voltage output would increase with increasing current draw, but am I thinking of things incorrectly and oversimplifying?

Thanks also for the clarification regarding differential encoder compatibility with the 9401 and the pinout info. I've set up my task in MAX and am ready to go; I guess I just need to verify whether the module output voltage will suffice.

[Chris_W. · Accepted solution]

Hi Johnny,

In this case, like we said before, this module isn't going to be able to excite your encoder properly using the digital lines. It won't be able to provide the full 5V output on a given channel. Also, since the input voltage for your encoder is 4.75V minimum, you will need an external power supply. That will be providing the proper current to drive that encoder. As long as you can excite the encoder properly, this module should be fine for reading the signals and interpreting them for an angular position measurement.

Chris 

[JohnnyF]

Great, thanks for the quick reply. I've got a 24 VDC power source on a relay module as part of this test stand so I can just run another line from there. My wiring won't be as clean but since it will actually work that way I guess I'll suffer through it. 😄

Thanks again for taking the time to help with some newbie questions!