snap ethernet-based i/o units protocols and programming guide

Equations Common to All Algorithms
Err = SP - PV
Span = (OutHi - OutLo) / (InHi - InLo)
Output = Output + FeedForward * TuneFF
Velocity Algorithm
CHAPTER 2: OVERVIEW OF PROGRAMMING
The velocity algorithm is similar to the algorithm used in OptoControl for Mistic I/O except that
the derivative does not act on setpoint changes.
TermP = (Err - Err_1)
TermI = TuneI * ScanTime * Err
TermD = TuneD / ScanTime * (PvIn - 2 * PvIn_1 + PvIn_2)
Output = Output + Gain * Span * (TermP + TermI + TermD)
Non-velocity Algorithms
These equations were derived from the article “A Comparison of PID Control Algorithms” by John
P. Gerry in Control Engineering (March 1987). These three equations are the same except for the
tuning coefficients; converting from one equation to another is merely a matter of converting the
tuning coefficients.
Equations common to all but the velocity algorithm:
Integral += Err
TermP = Err
TermI = TuneI * ScanTime * Integral
TermD = TuneD / ScanTime * ( PvIn - PvIn_1 )
“Ideal” or ISA Algorithm:
Output = Span * Gain * ( TermP + TermI + TermD )
“Parallel” Algorithm:
Output = Span * ( Gain * TermP + TermI + TermD )
“Interacting” Algorithm:
Output = Span * Gain * ( TermP + TermI ) * ( 1 + TermD )
Formatting and Interpreting Data
Data is formatted differently for different addresses in the memory map. The memory map tables
in Appendix A (page 103) show whether the data in each address is a mask, a signed or unsigned
integer, a float, and so on. This section explains some of these formats.
Mask Data
Some data is in the form of a 32-bit or 64-bit mask—four or eight addresses, each holding eight
bits. Each bit in the mask contains the data for one thing in a group: one point, one module, one
Scratch Pad bit, etc.
SNAP Ethernet-Based I/O Units Protocols and Programming Guide 41