CMC TECHNICAL REFERENCE MANUAL
1X36003 Version 2.52
1996-1999 Ingersoll-Rand Company
Date of Issue: 18-Oct-1999
7
Proportional Band
Proportional control varies the signal sent to the valves as a linear response to the
difference between the actual system pressure and the system pressure set point. Valve
responsiveness can be adjusted through the CMC with the proportional band,
Pb
, set point.
This scaling factor, graphically depicted in Figure 5, is the amount of change in the input
variable (actual minus set point pressures) to cause a full scale change in the output
variable (valve position).
In other words, if the pressure in the air system fluctuates frequently, it would be prudent to
set
Pb
to a low value to keep up with those system changes. Otherwise, if the system is
very stable, a larger value can be used.
Pb
is directly related to valve life and indirectly
related to valve cycling; so, as
Pb
decreases, valve life decreases and cycling increases.
As stated earlier, the CMC uses a proportional plus integral control algorithm. The result of
proportional only control is offset from the controlled variable, discharge pressure. This
means that if the set point pressure is 100, the actual pressure may only be 95. The value of
this offset depends upon the proportional band
value.
What is the valve response when the difference
between actual and set point pressures is zero?
There is no response. Proportional control only
functions when a difference or error exists. Design
discharge pressure could not be attained in a
proportional only control system. Therefore, an
integral control algorithm is added to achieve the
desired discharge pressure.
Integral Time
The offset produced by the proportional control
algorithm could be eliminated by manually
readjusting the system pressure set point. Using
the example above, the set point could be reset to
105 to obtain the 100 desired. Manually resetting
the set point would be required as the system
demand fluctuated. Integral control, also known as
reset control, automatically resets the desired
system pressure set point. For the CMC, the rate
at which the controller resets the system pressure
setting is known as Integral Time,
It
, and is
expressed in units of repeats per second.
If precise control of the specified discharge
pressure is required, the
It
set point should be set
for a fast value.
It
is inversely related to valve life
and directly related to valve cycling, therefore, as
It
decreases, valve life increases and cycling
decreases. For the CMC controlling Centac and X-FLO compressors,
It
values are typically
less than 1.00.
Output
Variable
(Valve Position)
Pb
high
Full Scale
0
Large Change
Output
Variable
(Valve Position)
Input Variable
(Actual - Set Point Pressures)
Pb
low
Full Scale
0
Small
Change
Slow
Fast
Response
Response
Figure 5: Proportional Band, Pb
