Micrologix 1100, 1762-IT4 and dynamically changing the input filter frequency?

Hello there,
Using the Micrologix 1100 with the 1762-IT4 Thermocouple/mV input module, is it possible to have the ladder logic program update the analog input filter frequency dynamically during execution?

I would like to have the ladder program be able to jump between 10Hz (slow but stable) and 60Hz (faster but less stable).

I know how to do the "initial setup" in the IO Configuration dilog where you can set the filter frequency....but what are the addresses for having the program change these 3 bits? The manual says that these 3 bits are in the "Configuration Data File" bits 0,1 and 2....but how do I access them from the user program?

BONUS QUESTION: What is the best instruction to write to the lowest 3 bits of a configuration word? Should I do it bit-by-bit or all 3 bits at once?

Thank you very much!!
frenchy

It doesn't look like there is a way to do that. At least I can not see it.
You might want to consider applying a filter in a ladder file. See PDF below for one I use. Just make the constant value a floationg point data file and write to it.

Mickey said:

It doesn't look like there is a way to do that. At least I can not see it.
You might want to consider applying a filter in a ladder file. See PDF below for one I use. Just make the constant value a floationg point data file and write to it.

Click to expand...

Thanks for the post. I have started out my week by learning something new.

Mickey said:

It doesn't look like there is a way to do that. At least I can not see it.
You might want to consider applying a filter in a ladder file. See PDF below for one I use. Just make the constant value a floationg point data file and write to it.

Click to expand...

After thinking about your solution a little bit more, I have the following thoughts:

Your formula is, in reality, a software implementation of an RC low pass filter.

Though this formula might me adequate for many applications, should it be included in a STI routine so that it will have a consistent effect?

Electrically_Bound said:

After thinking about your solution a little bit more, I have the following thoughts:

Your formula is, in reality, a software implementation of an RC low pass filter.

Though this formula might me adequate for many applications, should it be included in a STI routine so that it will have a consistent effect?

Click to expand...

Never gave that a thought. I use it for noise and unstable ( caused by the process) analog signals. Always on an unconditional rung.
Not sure what the OP is trying to filter so not sure if my solution is useful.

Thank you very much for your response Mickey!! I just played around with your filter suggestion and it will take me a while to get things tuned the way I need them, but I like the quickness of your idea compared to the built in filter.

Just for the record, I am filtering the analog input of a loadcell which is coming into the 1762-IT4 module set @ +/-50mV. I am only using the 0-30mV range, but the resolution seems fine (0.5lbs per count over 0-10,000lbs). The loadcell reading is in response to a hydraulic arm which is controlled using the analog output of a 1762-IF2OF2. I am using a PID loop which is being triggered by the STI every 20ms. I am in an industrial environment with noisy things going on, so the loadcell value jumps around when unfiltered. The 10Hz filter setting smooths things out very nicely but takes quite a while to update the loadcell value. I like keeping it on 10Hz most of the time to keep things nice and stable, but for certain PID setpoints I am finding that it is just too slow. Using your ladder filter, I would always leave the IT4 loadcell filter @ 60Hz (much faster), but dynamically change your "damping factor" depending on how much filtering I want....I gotta think about how this will work with respect to smoothing the loadcell response and the timing.... My initial thought is that with your solution the time it takes to filter never changes no matter what the filter factor is.....hmmmm. I like that.

Bonus question: What do you professionals usually use to "read in" a 0-30mv loadcell and optimize the resolution? The 1762-IT4 set for +/-50mv is only using 30mv out of a 100mv range or 30% of the usable range....seems like I could get much better use of resolution if I threw more money at some type of middleman?

thanks again,
frenchy

frenchy said:

Bonus question: What do you professionals usually use to "read in" a 0-30mv loadcell and optimize the resolution? The 1762-IT4 set for +/-50mv is only using 30mv out of a 100mv range or 30% of the usable range....seems like I could get much better use of resolution if I threw more money at some type of middleman?

thanks again,
frenchy

Click to expand...

First, Your Welcome.

Unless you are going to use all the inputs on your thermocouple card I prefer to use an analog input card i.e. 1762-IF4 ( current/voltage) which gives me more flexibility with different types of inputs. Then use a signal converter (transmitter) to convert the mv signal to 4 to 20ma. See link below for one option, there are many more brands.

http://www.miinet.com/products/sg_temperature.shtml