Harder than it sounds RS 500 SCP block

OK I'm really stuck on this one.. I have a 4-20mA signal from a micrologix 1200 going to a AB 1336 drive. The system is on a Army Truck assembly line that pulls the trucks down the line at a set speed. My input is a HMI were the operator enters how many trucks he wants to produce during his shift. This in turn will set the speed of the drive so that the right amount of trucks go down the line in a given amount of time. Now here is the tricky part. The drive speed (Hz) determins the assembly rate. At the max of 120HZ they will produce 4 trucks per hour. Now I have three shifts of 8 hours, 9 hours, and 10 hours. If the operator enters into the HMI that he wants to produce 28 trucks during his 8 hour shift then I need to program the ladder logic to generate a 13.3 mA signal (Binary 27225) to the drive. This card operates from 0 to 32,760 bit (4 to 20 mA).

So I have four parameters to deal with. I have the drive frequency (Speed) the length of the shift and the numbers of trucks per shift requested. I been trying to program this using scaling blocks (SCP). I really could use a fresh frame on mind on this ..

Thanks Guys and Gals.

Jim

strebej said:

At the max of 120HZ they will produce 4 trucks per hour. Now I have three shifts of 8 hours, 9 hours, and 10 hours. If the operator enters into the HMI that he wants to produce 28 trucks during his 8 hour shift then I need to program the ladder logic to generate a 13.3 mA signal (Binary 27225) to the drive. This card operates from 0 to 32,760 bit (4 to 20 mA).

Click to expand...

Number of trucks to produce on this shift / hours on this shift = trucks per hour.

trucks per hour / max trucks per hour (4) * max raw counts (32760) = raw counts to drive.

You could use an SCP for the second calculation if you like.

Your example is a little off. 28 trucks / 8 hours = 3.5 trucks/hour. 3.5 / 4 = 87.5% of maximum. 0.875 * 32760 = 28665 raw counts. In ma, that would be 0.875 * 16 + 4 = 18 ma.

Good luck,

I would suggest using floating point registers for the calculations due to the range of the numbers you are working with. For example, if your program determines the rate to be 3 trucks/hour, you are running at 75% capacity, and the current signal should be calculated from that. But if you do 3/4 on the PLC using integer registers you will get 0 (or 1... I don't remember which way it rounds).

Please look up a manual as my understanding of ML1200 is that the OF cards are only 12 bit..0 - 4095 and are scaled across 0 - 21 ma
which changes the values you are trying to use

Gil,

You are correct for the PID format, but there are two selectable formats. The Raw format has a range of 0 to 32,760, while the PID format has only 12-bit resolution. I assume that Jim is using the Raw format, with the module Input Word Bit 13 set = 0.

Here is an excerpt from the Micrologix 1200 1762-OF4 Analog Output module user manual.

Raw/Proportional Format

​

Word Bit Position​

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0​

​

0 0 Channel 0 Data 0 to 32,760 0 0 0​

1 0 Channel 1 Data 0 to 32,760 0 0 0​

2 0 Channel 2 Data 0 to 32,760 0 0 0​

3 0 Channel 3 Data 0 to 32,760 0 0 0​

​

Scaled-for-PID Format

​

Word Bit Position​

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0​

​

​

0 0 0 Channel 0 Data 0 to 16,380 0 0​

1 0 0 Channel 1 Data 0 to 16,380 0 0​

2 0 0 Channel 2 Data 0 to 16,380 0 0​

3 0 0 Channel 3 Data 0 to 16,380 0 0​

Click to expand...

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Thanks for the help guy's.. I can put this one to bed. Again thank you

Jim

i hate to bring a thread back from the dead but this has some info that i believe will help with my problem.

I am using the same 1762-OF4 and my output is erronoeous. I want a 4-20ma output and right now i am getting 0-21. In the configuration it is set to 4-20mA, scaled for pid. In the program it recieves a scaled value from 0-16383. Is this correct?

Apparently you module is not set to produce 4-20 miliamps, but is set for 0-21 miliamps. Double check your module configuration. I think these OF4 Output modules are configured by storing a 1 value to bits in the equivalent Input Word. Check your 1762-OF4 User Manual.

Your output cable should be wired to an I Out X terminal, where x is the current terminal output (from 0 to 3), and the other wire goes to COM.

i got it figured out this morning. The 1762-OF4 manual says the scaling is 0-16383 for pid format and 0-32764 for raw/proportional. Its a 4-20 or 0-10V card depending on how you wire it. I had is set 4-20mA and scaled for PID. The only time the book mentions 0-21 is when there is a fault condition. Well i was entering the new scale mentioned on here(in another thread), and i started glance through the manual for programming the 1100 when i saw the card config section. I decided to check it out and sure enough in the 1100 manual, not the card manual, it states that you use 0-16383 for 0-21mA and 3120-15600 for 4-20mA.

DOH

kurt.brinker said:

i got it figured out this morning. The 1762-OF4 manual says the scaling is 0-16383 for pid format and 0-32764 for raw/proportional. Its a 4-20 or 0-10V card depending on how you wire it. I had is set 4-20mA and scaled for PID. The only time the book mentions 0-21 is when there is a fault condition. Well i was entering the new scale mentioned on here(in another thread), and i started glance through the manual for programming the 1100 when i saw the card config section. I decided to check it out and sure enough in the 1100 manual, not the card manual, it states that you use 0-16383 for 0-21mA and 3120-15600 for 4-20mA.

DOH

Click to expand...

The scaling of the analog input cards and outputs cards is not very clear in any of the manuals.
Maybe I am being unreasonable to expect a section in the card manual "Scaling Your Inputs/Outputs"

I have run into the same issue many times on the Compact, Logix, and SLC platforms. Don't feel bad

Ian

HI,

Couldn't agree more "expect a section in the card manual "Scaling Your Inputs/Outputs".

Evidently whoever wrote these manuals hasn't ever installed one. And why the PID scaling? What the heck does that do for me? It is a conspiracy!!!!
My $0.02