How to read time-period in SCL?

Hi,

I am using SIMATIC 400H with CPU 417-5 H PN/DP in my project. I want to implement a rate algorithm in SCL. Something like 

RATE = (input - previous input) / TIME

now, for 'previous input' variable, i can store the current input in the static variable part for later use, what I don't understand is how to implement the time variable. Algorithm says its "TIME = sampling time (loop time)".

It would be great if anyone could give me some idea on how do I extract this "loop time".

thanks in advance.

Sabeeh

Hi,

I am using SIMATIC 400H with CPU 417-5 H PN/DP in my project. I want to implement a rate algorithm in SCL. Something like 

RATE = (input - previous input) / TIME

now, for 'previous input' variable, i can store the current input in the static variable part for later use, what I don't understand is how to implement the time variable. Algorithm says its "TIME = sampling time (loop time)".

It would be great if anyone could give me some idea on how do I extract this "loop time".

thanks in advance.

 Dear Sabeeh,

Do you want to read the time of PLC for tracking the log if this is the case you can call the Read PLC time.

https://support.industry.siemens.com/tf/WW/en/posts/reading-date-and-time-in-scl/54322?page=0&pageSize=10

could you elaborate your requirement with an example to provide you the solution

Hi Sabeeh,

U can use OB30-38 (watch dog timer interrupts). U can modify sampling time for OBs in HW configuration.

Another simple solution is add trigger input(bool) to SCL function. U can connect eg. 1 seconds pulse when u call this function. Note that this pulse should be on for only one scan.

I have used similar logic for flow totalizer where i add calculated value on every sampling interval.

Dongerpure

 

Sabeeh

Hi,

I am using SIMATIC 400H with CPU 417-5 H PN/DP in my project. I want to implement a rate algorithm in SCL. Something like 

RATE = (input - previous input) / TIME

now, for 'previous input' variable, i can store the current input in the static variable part for later use, what I don't understand is how to implement the time variable. Algorithm says its "TIME = sampling time (loop time)".

It would be great if anyone could give me some idea on how do I extract this "loop time".

thanks in advance.

 Dear Sabeeh,

Do you want to read the time of PLC for tracking the log if this is the case you can call the Read PLC time.

https://support.industry.siemens.com/tf/WW/en/posts/reading-date-and-time-in-scl/54322?page=0&pageSize=10

could you elaborate your requirement with an example to provide you the solution

 What my understanding is the following,

if input is read at time1 and then again read at time2, then rate is calculated as
rate = (input at time2) - (input at time1) / (time2-time1).

so my confusion is how can i keep track of this time? should i use this function in the timer OBs or what?

Hi Sabeeh,

The smiffy solution is best suited to ur problem. I think initialization is through restart OBs[OB100]. U have integrate above function in ur RATE calculation function in SCL.

@Smiffy: I will definately use ur scheme in my logic whenever required.

smiffy

You could use some static variables and the system time thus:-

PostTime := TIME_TCK(); // Get current tick time now
IF PostTime < PreTime THEN // If time has rolled over then
ElapsedTime := TIME_TO_DINT((T#24D20H31M23S647MS - PreTime) + PostTime); // Get time elapsed since last time we ran (roll-over)
ELSE                       // Otherwise no roll-over, so perform normal calculation
ElapsedTime := TIME_TO_DINT((PostTime - PreTime)); // Get time elapsed since last time we ran (normal)
END_IF;
PreTime := PostTime; // Store 'old' time

You could need to initilase the PostTime and PreTime variables like this when the PLC starts up:-

PostTime := TIME_TCK(); // Initialise timers
PreTime := PostTime;

The ElapsedTime variable will be the cycle time in ms. Or you could remove the TIME_TO_DINT stuff and simply and make ElapsedTime a DT (DATE_AND_TIME) type and get the value as a DT type cycle time value.

 Beautiful, I was looking for something like this. Thanks.

I didn't know about TIME_TCK(). So, it is SFC64 :)