Arrays in Structured Text

When separate variables become the problem

You want the average of the last ten level readings. With separate variables:

S10 := S9; S9 := S8; S8 := S7;   (* ... nine more lines *)
Avg := (S1 + S2 + S3 + S4 + S5 + S6 + S7 + S8 + S9 + S10) / 10.0;

Twenty lines, and changing ten to twenty means rewriting all of it. An array fixes that.

Declaring one

VAR
  Samples : ARRAY [1..10] OF REAL;
  Motors  : ARRAY [1..32] OF BOOL;
  Grid    : ARRAY [1..4, 1..8] OF INT;   (* two dimensions *)
END_VAR

The range [1..10] sets the first and last index. The standard lets you choose, but plant convention is to start at 1, because the first motor is M1 and not M0, and that removes a class of off-by-one mistakes.

Indexing

Samples[1]    := Level;
Last          := Samples[10];
Samples[i]    := Samples[i - 1];
Motors[Index] := TRUE;

The index is an expression, so it can be a variable or a calculation, and that is where the trouble starts.

FOR over an array

Total := 0.0;
FOR i := 1 TO 10 DO
  Total := Total + Samples[i];
END_FOR;
Avg := Total / 10.0;

Four lines that work for 10 elements and for 200. To shift:

FOR i := 10 TO 2 BY -1 DO
  Samples[i] := Samples[i - 1];
END_FOR;
Samples[1] := Level;

Note the direction: top down, or you copy element one over the whole array.

The filter: a moving average

A level reading wobbles because the liquid moves and because electrical noise rides on the cable. A moving average calms it:

Tick(IN := NOT Tick.Q, PT := T#100ms);

IF Tick.Q THEN
  FOR i := 10 TO 2 BY -1 DO
    Samples[i] := Samples[i - 1];
  END_FOR;
  Samples[1] := Level;
END_IF;

Total := 0.0;
FOR i := 1 TO 10 DO
  Total := Total + Samples[i];
END_FOR;
AvgLevel := Total / 10.0;

Do not sample every scan. Ten samples at a 10 ms scan is a 100 ms window, which filters nothing. Sampling every 100 ms gives you a full second. Filter window = sample count x sample interval, and that is the number to work out before you type anything.

The out-of-range index

This genuinely stops production lines:

Samples[i] := Level;   (* what if i is 0? or 11? or 300? *)

What happens depends on the platform: some raise an exception and stop the controller, others quietly write over another variable's memory. The second is worse, because the program keeps running and keeps being wrong. The dangerous index is the one that came from outside: an HMI field, a comms message, a calculation.

Always guard it:

IF i >= 1 AND i <= 10 THEN
  Samples[i] := Level;
ELSE
  IdxFault := TRUE;
END_IF;

Or clamp it:

i := LIMIT(1, RawIndex, 10);

Clamping beats ignoring, and raising an alarm beats clamping silently.

Not runnable in the lab

The lab does not support ARRAY. The lab program below gets the same moving average out of four separate variables, which is exactly the style an array exists to replace.