Type conversion and scaling a 4-20 mA signal

Why conversion is needed

An analogue input card does not hand you "73.4 percent". It hands you a raw count: 0…27648 on Siemens, 0…32767 on other cards. Turning that into something an operator understands is your job.

Explicit conversion

The standard wants conversions spelled out:

Scaled := INT_TO_REAL(Raw);
Steps  := REAL_TO_INT(Position);   (* rounds to nearest *)
Whole  := TRUNC(Position);         (* cuts the fraction off *)

Note that TRUNC(3.9) is 3 while REAL_TO_INT(3.9) is 4. That difference is a common source of one-step position errors.

The classic trap: integer division

Percent := Raw / 27648 * 100;      (* wrong: the result is zero *)

Raw and 27648 are both integers, so the division is integer and gives 0 before the multiplication. Convert first, or write the constants as reals:

Percent := INT_TO_REAL(Raw) / 27648.0 * 100.0;   (* correct *)

The scaling formula

For any raw range [RawLo…RawHi] and engineering range [EngLo…EngHi]:

Eng := (INT_TO_REAL(Raw) - RawLo) / (RawHi - RawLo) * (EngHi - EngLo) + EngLo;

Example: a 4-20 mA level transmitter spanning 0-5 m, on a card that gives 0…27648 for 0…20 mA. At 4 mA the raw count is about 5530, so:

Meters := (INT_TO_REAL(Raw) - 5530.0) / (27648.0 - 5530.0) * 5.0;

Always clamp

Sensors go out of range. Clamp the result so the operator never sees "-3 metres":

Meters := LIMIT(0.0, Meters, 5.0);

Better still, know why it went out: below 3.6 mA or above 21 mA means a sensor fault under NAMUR NE43, which is lesson 27.

A ready-made tool

The workshop on this site has a 4-20 mA and loop calculator: enter the range and the current and it gives you the engineering value and the loop voltage budget.