Quadrature Encoder Counting with 6024E

[amrish]

Hi Wise ones,
I am trying to measure the rotary position of a shaft with an Allen Bradley incremental encoder (1024 ppr) and 6024E DAQ card. I tried using the Quad._Encoder_with_E-series_(STC).VI available on the NI site.
The resulting output is a damped sinusoid (which was expected), but also includes a drift. Could anyone please explain the reason/s ?

Thanks !
Amrish

[Doug_Norman]

Hello Amrish,

If you go to ni.com and search on "Using Quadrature Encoders with E Series DAQ Boards" you'll find an entry of the same name. This paper explains all about using quadrature encoders with E Series boards. The typical problem is that the counter on this board was never designed for quadrature encoders. The trouble is that the counter cannot be set up to count both low-to-high and high-to-low transitions. It only counts one or the other. This works for most counting applications. But if you get into a case where you are rocking back and forth over a transition, what you want is for the counter to count up, down, up, down. But what you get is say up, up, and then your count is off.

The solution is to use an LS7084 clock converter chip
between your encoder and the E Series board. I've done this a lot, and it works flawlessly. This paper explains all the details.

Best Regards,
Doug Norman

[amrish]

Hello Doug,

I procured the LS7084 and after using it I see a different output than what I was expecting. I am using the Quad._Encoder_with_E-series_(STC).VI from the NI website.

The difference in the outputs is as follows:

1) When NOT using LS7084: There is a damped sinusoid present (frequency ~13 Hz), however there is a continuously increasing DC offset.

2)Upon using LS7084: The output is what "looks" like a damped sinusoid with a low frequency (~2 Hz).

I measured the frequency from the torque profile obtained from a torque transducer in the system and it is close to 13 Hz.

Are you using the same VI that I mentioned earlier or do you use some other VI ?

I am attaching the two data files in tab delimited text format. Could
you please look at them and comment ?

Thanks !
Amrish