How to store Variable value in each PLC cycle in ST

Akgaurihar · Apr 22, 2020

Hello,

I have a variable that contains the velocity value. I have to store the velocity value in each PLC cycle and have to do further calculations. Is it possible to save 20 entries in an array and rotate it, always new entries will be added and the older ones will be deleted? Somebody can help with the code example in Structured Text?

CommissioningMan · Apr 22, 2020

Code:

FOR i:= 19 TO 1 BY -1 DO
  arVelocity[i] :=  arVelocity[i-1];
END_FOR
arVelocity[0] := rVelocity;

This will push newest entry to index[0] and shift towards index 19.

drbitboy · Apr 22, 2020

An alternate but equivalent approach, using a circular array.

(initialize both next_pos and N to zero)

Code:

IF next_pos < N THEN
  <remove value at arVelocity[next_pos] from calculation result>
END_IF

arVelocity[next_pos] := rVelocity;

next_pos := next_pos + 1;

IF N < next_pos THEN
  N := next_pos;
END_IF

<add value in rVelocity to calculation result; N is array size>

IF next_pos > 19 THEN
  next_pos := 0;
END_IF

This is more complicated than the shift algorithm described in post #2, but it moves less data around, because it leaves the data in place and moves the index over the data, where post #2 leaves the index in place (=0) and moves the data over the index.

CommissioningMan · Apr 22, 2020

True, but a measly 20 indices in a for loop shouldn't have any impact on performance, and I have always preferred having the latest entry in a fixed position for online viewing

drbitboy · Apr 22, 2020

CommissioningMan said:
True, but a measly 20 indices in a for loop shouldn't have any impact on performance, and I have always preferred having the latest entry in a fixed position for online viewing

agreed, just showing an alternative.

Akgaurihar · Apr 23, 2020

CommissioningMan said:
Code:

FOR i:= 19 TO 1 BY -1 DO arVelocity[i] := arVelocity[i-1]; END_FOR arVelocity[0] := rVelocity;

This will push newest entry to index[0] and shift towards index 19.

HI! Thank you for quick help. Being a beginner in programming, need little more help. Could you please check the following code.

Code:

VAR
 Vel_L_Array : ARRAY[1..10] OF INT;
 i : int;
END_VAR


FOR i:= 10 TO 1 BY -1 DO
  Vel_L_Array[i] :=  Vel_L_Array[1];
END_FOR;
Vel_L_Array[1] := Velocity_L;

Velocity_L is the variable giving me the value from the drive.
And now I want to access 10 values from the array for some arithmetic calculation. I hope I'm on right track.

parky · Apr 23, 2020

To access the "10" arrays you could do the same loop for example if you wanted to find an average over the ten readings then in the loop add the 10 and after the loop divide it by 10
AverageVal := 0; //Clear the average register
for I = 1 to 10
AverageVal := Myarray; //Add the 10 readings
End_For
AvarageVal := AverageVal / 10; //Divide to find the average

forgive me if my ST may be incorrect, but feel lucky as I went back to my pascal days & put begin & end in first time

drbitboy · Apr 23, 2020

Akgaurihar said:
HI! Thank you for quick help. Being a beginner in programming, need little more help. Could you please check the following code.

Code:

VAR Vel_L_Array : ARRAY[1..10] OF INT; i : int; END_VAR FOR i:= 10 TO 1 BY -1 DO <<<<<FIX THIS A Vel_L_Array[i] := Vel_L_Array[1]; <<<<<FIX THIS B END_FOR; Vel_L_Array[1] := Velocity_L;

Velocity_L is the variable giving me the value from the drive.
And now I want to access 10 values from the array for some arithmetic calculation. I hope I'm on right track.

You are close:

Where it says [<<<<<FIX THIS A] change [TO 1] to [TO 2]
Where it says [<<<<<FIX THIS B] change [1] to [i-1]

parky · Apr 23, 2020

Sorry I missed a crucial bit
AverageVal := 0; //Clear the average register
for I = 0 to 9
AverageVal := AverageVal + Myarray; //Add the 10 readings forgot the accumulated value doh.
End_For
AvarageVal := AverageVal / 10; //Divide to find the average

kamenges · Apr 23, 2020

If you are going to perform a true average be careful of data type and result overflow. The result of adding ten 16-bit integers that presumably span the range of a 16-bit integer will NOT fit back into a 16-bit integer.

Keith

Peter Nachtwey · Apr 23, 2020

I wish people would state what they really trying to achieve. This thread talks about tactics but the strategy may be wrong or not optimal.

Peter Nachtwey · Apr 23, 2020

Grumpy old engineer asks....

Why bother adding 10 velocities?
Why bother adding 20 velocities?

kamenges · Apr 23, 2020

I thought that as well. But they seemed to be having so much fun. I figured who am I to get in the way of their joy.

Keith

drbitboy · Apr 23, 2020

Peter Nachtwey said:
Why bother adding 10 velocities?
Why bother adding 20 velocities?

OP never said they were (numerically) adding velocities.
and these are speeds anyway.

janner_10 · Apr 23, 2020

kamenges said:
I thought that as well. But they seemed to be having so much fun. I figured who am I to get in the way of their joy.

Keith

It's not much help though and not what the OP asked.

He had the answer at #2. He may as well watched kitten videos on you tube after that.

How to store Variable value in each PLC cycle in ST

Akgaurihar

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