Siemens Simatic S7-200 System Manual page 248

Instruction Set
Converting the Loop Output to a Scaled Integer Value
The loop output is the control variable, such as the throttle setting in the example of the
cruise control on the 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 = (M
Scal
where:
R
Scal
M
n
Offset
Span
The following instruction sequence shows how to scale the loop output:
MOVR
-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:
TRUNC
MOVW
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 discussion in the Modes section below).
10-60
- Offset) * Span
n
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
= 32,000 for unipolar values (typical)
= 64,000 for bipolar values (typical)
VD108, AC0 // Move the loop output to the accumulato.r
0.5, AC0 // Include this statement only if the value is bipolar.
64000.0, AC0 // Scale the value in the accumulator.
AC0, AC0 // Convert the real number value to a 32-bit integer.
AC0, AQW0 // Write the 16-bit integer value to the analog output.
S7-200 Programmable Controller System Manual
C79000-G7076-C230-02