User guide

Chapter 26
PID Instruction
26–21
Feed Forward
Applications involving transport lags may require that a bias be added to the
CV output in anticipation of a disturbance. This bias can be accomplished in
the SLC 5/02 processor by writing a value to the Feed Forward Bias element,
the seventh element (word 6) in the control block file (see page 26–7). The
value you write will be added to the output, allowing a feed forward action to
take place. You may add a bias by writing a value between 0 and 16383 to
word 6 with your HHT or ladder program.
Time Proportioning Outputs
For heating or cooling applications, the Control Variable analog output is
typically converted to a time-proportioning output. While this cannot be
done directly in the SLC 5/02 processor, you can use the program on the
following page to convert the Control Variable to a time proportioning
output. In this program, cycle time is the preset of timer T4:0. Cycle time
relates to % on-time as follows:
T4:0.PRE is the cycle time
100% output on–time
% on–time