Step 7 ET200s - PID controller - dt problem

[naturephoenix]

How can I get the curret "t" in SCL
dt=tnow-tprevious

I dont know how to get "tnow". I am not sure should it be in s(seconds) or miliseconds. I know that a cycle of my OB1 is somthing like 5ms.
So any help out there?

 

[mk42]

I would recommend running your PID code out of a cyclic OB, like OB35, instead of OB1. They have a much more defined interval than OB1.

 

[wimpiesplc]

You might use the TEMP var OB1_PREV_CYCLE in OB1,
or running from OB35 like mk42 suggests use OB35_EXC_FREQ var

 

[BryanG]

I use this to calculate the time between PID calls:

[COLOR=SeaGreen]// Calculate the sample time[/COLOR]
[COLOR=SeaGreen]// Transfer previous time from last Current[/COLOR]
#Previous_Time:= #Current_Time;
[COLOR=SeaGreen]// Transfer Current time to previous time[/COLOR]
#"Cycle_Time_n-1":= #Cycle_Time;
[COLOR=SeaGreen]// Get the new Current Time[/COLOR]
#"RD-Sys_T_return":= RD_SYS_T(#Current_Time);
[COLOR=SeaGreen]// Subtract Current from Previous to get Time in ms[/COLOR]
#Cycle_Time:= T_DIFF(IN1:=#Current_Time, IN2:=#Previous_Time);
[COLOR=SeaGreen]// Convert to Real and divide by 1000 to get time in seconds[/COLOR]
#Time_in_Secs:= DINT_TO_REAL(TIME_TO_DINT(#Cycle_Time))/1000;

I am working with S7-1200 so I don't know if there is anything similar for the ET200s. I can't use a Cyclic Interrupt because the data isn't always available in a fixed period.

 

[KalleOlsen]

For 300/400 there is a SFC called (...twisting what's left of my brain, don't have S7 here...) I think it is TIME_TCK, that can be used in Bryan's code.

Kalle