Analog Lagging Techniques

Louisiana159

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Join Date
May 2017
Location
Louisian
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I am programming a system that will have a 4-20mA linear position sensor that will go from 4-20mA then 20-4mA in one full cycle. This cycle will continuously repeat itself at a set speed. Next, I have an identical hardware setup that I need to lag behind or follow the first cycle at roughly 30% of the cycle. It will be controlled by an analog output signal. I have had several ideas of how to do this that would work as the leading value is increasing from 4-20mA but I am running into a roadblock of how to get the second value to continue to increase to 20mA and then follow the leading value back to 4mA. I am programming in RSLogix 5000. Any help or ideas would be greatly appreciated!
 
Welcome to the forum.

it would help if you would explain what is happening, why does this setup need to lag behind the first?

what is the process?
this will help us understand what is going on so we can assist you.

james
 
The way the system will work is very similar to how the pistons of a triplex pump will work. Except that there is no rotating shaft controlling the sequence of strokes, each piston is being moved by its’ own independent hydraulic power unit. The speed of each piston is controlled by the speed of its’ own hydraulic pump.
 
I seem to recall there being a "Sine Wave" function or similar that essentially ramps a value in the form of a sine wave.

I've never messed with it, but I'm assuming you can change the frequency and maybe the amplitude? You may also be able to set a starting point for the cycle.

If you don't need a sine wave, just manually ramp the first one linearly by adding 1 unit to the output every 10ms or whatever, and then subtracting the same way. Then start the 2nd one after a time delay.
 
Last edited:
It sounds to me what you are really trying to do is have it behave like a cam function. Maybe motion instructions would be more suitable.


Bubba.
 
I do plan on looking into the motion PlantPAX instructions as well. I am just beginning to use the PAX instructions because I have been set in my ways and using custom add-ons that I have been using for years and finally finding the time to learn their full capabilities.
 
Motion control sounds good to me, as does a sinus or cosinus with a phase shift for the second output.

An alternative and simple approach could be to save the output that goes to the first linear actuator in a queue ('FiFo'). Every scan cycle you move one position in the queue. Send the output of the queue to the second actuator. The second output will lag behind the first by the PLC scan time multiplied by the length of the queue. Wether or not this is feasible also depends on the required lag time: creating minutes of lag (or more) is not practical as it would require a very long array of values for the queue.
 
Another way to do this would be to use your 4-20mA in an input and then use LIM instructions along with other conditionals to perform various parts of your process. I'm not sure how applicable that would be to your process, but I've seen it done on machines and it works quite well.
 
Thank you all for the feedback! I am exploring all of the ideas given and will update the thread in a week or two when I am in the testing stage of this project.
 
20ma - input_value +/- 4.8

a variation on that should work, I wouldnt use the sinefunction as phase shift and jitter will screw you up.
 

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