How does a LPF work in RS 5000?

[GHaulk]

Can anyone tell me excatly how a LPF instruction works in RS 5000? I have an application where we are taking a signal from a load cell and passing it through a LPF in the 5000 program. The application is a loss in weight feed system. The problem I am having is that whenever we refill the hopper, (add weight to the load cell)it takes about 45 seconds for the LPF to update the correct weight. I was wondering if someone knew of a way around this.

 

[dchartier]

Hello GHaulk;
I am missing something in your process description, I think.
An LPF instruction in RSLogix5000 is a "Low Pass Filter" instruction:

"The LPF instruction attenuates signals that occur above the configured cutoff frequency. This instruction is typically used to filter out high frequency "noise" or disturbances that originate from either electrical or mechanical sources. You can select a specific order of the filter to achieve various degrees of attenuation. Note that higher orders increase the execution time for the instruction." (From the Online Help).

If this instruction is used to remove unwanted noise from the input signal of your load cell, and your resulting execution time is very long, check whether the ORDER input parameter is set at 3 (valid entries are 1 to 3). If it is, try lowering it to 1 or 2 and see if updates are faster. To access this parameter, click on the button that appears inside the instruction box. However, the signal will not be as clean, of course, and could have adverse impact on your process, so be careful.

Hope this helps,
Daniel Chartier