Encoder with Micrologix 1400

I have a machine I am upgrading and would like to use an encoder to keep track of machine position. My problem is I had already ordered a A/C input model when I decided to add the encoder and I don't know if the pulses can be picked up with the D/C input module. I know the specs state a 8ms on delay.

This machine runs off an intermitter and it runs slowly on top of that. The shaft where I'll be taking the reading goes about 32 RPM. I've never done anything with an encoder and I was wondering if I have to have high speed inputs. I only need around 30ppr so thats roughly 15 pulses per second. I didn't know if pulse duration on an encoder is relative to speed. Also, if its relavent, the encoder doesn't need to be absolute.

I was looking at encoders from automation direct. If for some reason you don't care for this brand, please let me know.

Thanks in advance,
Roy

I thought because of the time and day I posted this, I got very limited views. So I thought I would move myself to the front of the line during normal living people hours. Thanks for looking.


Roy

At 15 pulses per second that is roughly 33.3 msec on and 33.3 msec off. The I/O has a listed 8.0 msec on delay and off delay so you lose that 16 msec and are left with 17.3 msec to find.

Now how long is the program scan time + communications overhead? That will depend on the program... If you have it up and running without the encoder then you can look online and see what it is. If you haven't converted the program to the 1400 yet then it is a bit of a wildcard.

The last question is how important is this information. If you miss a count will you kill the process? If so, then I would use a dedicated high speed counter. If not maybe you can test out using the standard inputs and see if it works for you...

I'm sure someone else will jump in with more details but if your micro has a build in high speed input, you should be all set.
Order a 360 degree encoder, 24 VDC (not a 5VDC) .
This will make your life easier as it is your first time with encoders.
Decide how to scale the 360 or even use it as degrees to show position on your screens.
You will have to use a "Function" file which is build in the micro (as long as you have the high speed inputs that is) and do a bit of reading on how to set up the "Function".

I think the yes/no answer to your question depends on the application as stated by nOrM.

Given you mentioned the encoder doesn't have to be absolute I should make the following comment:

If you used an absolute encoder you could poll your inputs at any time and get the current position. Whereas if it were an incremental encoder or something similar you would have to run a routine to sum the pulses to determine position (Think high overhead, large chance of failure based on scan times due to loss of pulse-count intercepts).

So I'd say you may be fine with an absolute encoder, and much less likely ok with an incremental.

Someone please correct me if my understanding of the different types of encoders is wrong!

Thanks for the replies.

Norm,
I don't have the program up and running yet. This machine is still on A/C relay logic. That was the reason for getting the A/C input model. Hence no high speed inputs. The program should be pretty small as this is a very simple machine. If a pulse was missed here and there it would not be the end of the world. I think i will try your idea and test it out with the DC input card. The encoder functions were mainly user friendly upgrades and are not critical to production.

Thanks again,
Roy

If you're going to use a quadrature encoder (the most common kind since it determines direction too), you'll need to differentiate the A and B outputs so you'll need to run twice that 33mS rate. And to satisfy the Nyquist criteria, you'll need to sample at least twice as fast again - at around 8mS. Without a high speed counter input, an 8mS scan rate is dicey.

I'd recommend you save yourself a lot of grief and buy a 1400 with the high speed (DC) inputs, then add some 1762-IA8 AC input modules to handle your AC inputs. If you get relay outputs, you can drive your AC outputs directly. Then you'll have the best of all worlds.