Bi-directional incremental encoder (Understanding resolution and requirements)

I'm trying to solve a problem to where I'm monitoring a physical position with a bi-directional incremental encoder. The problem is that over several days, the accuracy is drifting further away with time. I did not design or do the selection on the type of encoder used to measure our required position but what I want to understand is if the specifications on the encoder qualify for my desired measurement. I've listed information below:


Required measurement: (0.001")
Encoder used: Dynapar 2200121000 (Bi-directional incremental encoder, 12 pulses per revolution, open collector output 5-26VDC)
Control system: AB Controllogix PLC with 1756-HSC/B High speed counter IO Module)


At the moment, the only thing I'm interested in understanding is how I can determine the amount of thousandths of change with one complete revolution in the encoder. 
So, 1 complete shaft turn on the encoder is how many thou?
I assume this question has to be answered by understanding how the high speed counter IO module is working with the encoder but its not so easy to see that thus far.
Any suggestions/experience with how I could find this information out?


Thanks,

A bit more information is needed. You have mentioned 0.001", this is a linear measurement, which an encoder cannot do directly, how is the linear motion being translated to a rotational one?Cheers,Richard

Richard is correct, to answer this question, you need to know how the linear motion is translated into a rotational motion. I'd expect there to be a gear train somewhere although it might be a worm drive.
I don't think a 12 pulse per revolution encoder with a 3/8" shaft is going to give you 0.001" resolution without a high gear ratio. The circumference of the shaft is about an inch so you'd need 1000:1 ratio or so...

Ok. That is currently what I'm trying to figure out. How the ratio is working and the specifications on it.
Thanks for the replies.

Are you saying that you have a  mechanical system in place already that theoretically gives a .001" resolution, if so, how has it been done.Normally pulleys or gears are used to do the translation from linear to rotational, and to ensure the resolution target was met, you need more counts than the basic spec. Since you have not specified what accuracy you need, I can't comment on that any further. As Elisabeth said, the ratio is greater than 1000:1 best case, and using gears or pulleys at that sort of ratio is to my mind not on, particularly if the system is bi-directional, backlash/hysteresis  will be terrible, and miscounts can accumulate rapidly.Additionally, if the system stops and starts, and uses a stepper motor drive for example, they can  jitter badly, which again may create miscounts. This may require more intelligence to sort out than is provided in your input module. In one job I had, I resorted to using a small FPGA at the front end to add additional logic to help resolve some of these issues.The range of encoders you are using includes models of up to over a 1000 counts/rev, which to my mind may be better suited to the task, but without knowing what your mechanical set up is, it is hard to say.Cheers,Richard