If PctO (Secondary Menu) is turned On, the lower display changes to show the
active percentage of output as required to maintain SP1. The display is similar to the
Auto/Manual display above, except that the letter indicators do not flash, and the
output is displayed in whole percentages of output, not in tenths of a percent. If the
control has both SP1 and SP2, the lower display will alternate between the SP1 per-
cent output and the SP2 percent output.
Error messages are listed on Page 34 and 35.
PROGRAMMING AND OPERATION FOR RAMP / SOAK FEATURE
The ramp / soak feature offers a great deal of flexibility by allowing changes in the set
point to be made over a predetermined period of time. Soak (dwell) times can be
programmed, and the alarm output relay can be programmed to open or close dur-
ing any of the segments.
Theory of Operation
The CN74000 Series controls offer a very simple approach to programming a ramp.
Rather than requiring the operator to calculate an approach rate (usually in degrees
per minute), the CN74000 does the calculation internally. Thus, the operator only
needs to program the target set point and the time desired to reach that point. When
the ramp segment is executed by the control, it calculates the ramp required to move
the process from the starting value (current PV) to the desired value (programmed
SP) in the time allowed.
Soaks (or dwells) are ramp segments where the target set point is the same as the
beginning process value. This allows for multistage ramps without wasting interme-
diate soak steps. Care must be taken, however, that the process does actually reach
the soak value before the soak time starts. If not, the next segment will calculate a
slope from the starting PV to the target SP. Depending on your process require-
ments, this difference may be important. Make sure to test any program for desired
results before running production material.
Do not operate Self Tune while a ramp function is operating. The ramp func-
tion will prevent the Self Tune from operating properly. Make sure that all
tuning is set up before operating Ramp / Soak.