Converting The Loop Output To A Scaled Integer Value; Forward- Or Reverse-Acting Loops - Siemens Simatic S7 Series System Manual

7.9.4

Converting the loop output to a scaled integer value

The loop output is the control variable, such as the throttle setting of the cruise control on an
automobile. The loop output is a normalized, real number value between 0.0 and 1.0. Before
the loop output can be used to drive an analog output, the loop output must be converted to
a 16-bit, scaled integer value. This process is the reverse of converting the PV and SP to a
normalized value. The first step is to convert the loop output to a scaled, real number value
using the formula given below:
R
Scal
M
n
Offset
Span
The following instruction sequence shows how to scale the loop output:
MOVR VD108, AC0 //Moves the loop output to the accumulator
-R
*R
Next, the scaled, real number value representing the loop output must be converted to a 16-
bit integer. The following instruction sequence shows how to do this conversion:
ROUND AC0, AC0
DTI
MOVW
7.9.5

Forward- or reverse-acting loops

The loop is forward-acting if the gain is positive and reverse-acting if the gain is negative.
(For I or ID control, where the gain value is 0.0, specifying positive values for integral and
derivative time will result in a forward-acting loop, and specifying negative values will result
in a reverse-acting loop.)
Variables and ranges
The process variable and setpoint are inputs to the PID calculation. Therefore the loop table
fields for these variables are read but not altered by the PID instruction.
The output value is generated by the PID calculation, so the output value field in the loop
table is updated at the completion of each PID calculation. The output value is clamped
between 0.0 and 1.0. The output value field can be used as an input by the user to specify
an initial output value when making the transition from manual control to PID instruction
(auto) control of the output. (See the discussion in the "Modes" section below.)
S7-200 SMART
System Manual, 09/2015, A5E03822230-AC
is the scaled, real number value of the loop output
is the normalized, real number value of the loop output
is 0.0 for unipolar values
is 0.5 for bipolar values
is the maximum possible value minus the minimum possible value
= 27,648 for unipolar values (typical)
= 55,296 for bipolar values (typical)
0.5, AC0 //Include this statement only if the value is bipolar
55296.0, AC0 //Scales the value in the accumulator
//Converts the real number to a 32-bit integer
AC0, LW0 //Converts the value to a 16-bit integer
LW0, AQW0 //Writes the value to the analog output
Program instructions
7.9 PID
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