AB Guru help needed

I installed a micro 1000 analog on a press about a year ago. The PLC controls only one aspect and as such the program is very short and the operation is very simple.

The PLC has faulted out do to a math error twice in a 12 month period. It appears this PLC has no way via program bit set for customer to reset fault. Therefore I have to go out and reset via RS Logix. I am not sure if I have a preset value entered wrong in one of the SCL blocks or the data coming in is beyond expectations.

The logic is; A 4-20ma comes into PLC via joystick. This value is scaled to output to a valve 4-20ma directly proportional to the input. I could not just move the raw input data to the output as the internal scale between the PLC input and output is different.

There is a second 4-20ma signal input that serves as a max output limit. If the joystick input is greater than the Max setting, the output will be limited to the Max setting.

There is a retract signal which gives a full output command when activated. I don't believe this to be any problem.

I added a "u" on s:5/0 on the last rung as a temporary fix recommended by local AB rep until I find the math problem.

What did I do wrong?

Robert,

You have used two SCL (Scale Data) instructions, in Rung 000 for I:0.6, analog input from the joystick, and in Rung 002 for I:0.7, analog input from a potentiometer. I would guess that one or the other of the physical devices is worn, and at times is sending an out-of-range input signal. Perhaps the wiper on the pot is not making good contact, or the joystick is "bouncing". I usually install a Limit comparison check for each analog input, and only pass it on to the Scale function if it is within the valid range of the analog input module. Here is a warning note from Rockwell about the SCL instruction:

Note If the result of the Source times the Rate, divided by 10,000 is greater than 32767, the SCL instruction overflows, causing error 0020 (minor error bit) and places 32767 in the Destination. This occurs regardless of the current offset. If this occurs reset bit S:5/0 with your ladder program before the end of the current scan or else a major error will be declared.
Rockwell Software 2000

Click to expand...

Some input limits (as below) on the SCL can do wonders to prevent those annoying faults!

Robert, I suggest you set the S:1/8 bit (Fault Override At Power Up) next time you connect to the PLC. This bit can be found in the processor status file under the Errors tab.

I also suggest you set the S:1/12 bit (Run Always) so that the controller automatically goes into run mode when power is cycled. The Run Always bit can be found in the processor status file under the Protection tab.

That way if a fault occurs in the future it can be cleared by cycling power to the processor, saving you the trip.

Thanks guys.


I will look into adding the limits as well as setting the bits to allow customer to reset via power cycle. I have to stop by Monday AM to reset. I better check the pots on input devices as well. My math on the SCL looks OK though?

Robert,

I can't really tell if your math is correct. It depends on what are the actual maximum possible input values for I:0.6 and I:0.7. In Rung 0, if 4-20 milamp Input I:0.6 has the normal max value of 32767, then your SCL instruction multiplys this by 10500/10000 (or 1.05), then adds 0 offset, which would give 34405, a number that is out-of-range and would cause a math overflow and a fault shutdown.

Was this scale multiplier of 10500 used because the actual input is low for some reason?