54
SumZ Operation
— The SumZ algorithm is an adaptive PID
style of control. The PID (proportional, integral, derivative) is
programmed within the control and the relative speed of stag-
ing can only be influenced by the user through the adjustment
of the Z.GN configuration, described in the reference section.
The capacity control algorithm uses a modified PID algorithm,
with a self adjusting gain which compensates for varying con-
ditions, including changing flow rates across the evaporator
coil.
Previous implementations of SumZ made static assump-
tions about the actual size of the next capacity jump up or
down. This control uses a “rise per percent capacity” technique
in the calculation of SumZ, instead of the previous “rise per
stage” method. For each jump, up or down in capacity, the
control will know beforehand the exact capacity change
brought on. Better overall staging control can be realized with
this technique.
SUM Calculation — The PID calculation of the “SUM” is
evaluated once every 80 seconds.
SUM = Error + “SUM last time through” + (3 * Error Rate)
Where:
SUM = the PID calculation
Error = EDT – Cooling Control Point
Error Rate = Error – “Error last time through”
NOTE: “Error” is clamped between –10 and +50 and “Error
rate” is clamped between –5 and +5.
This “SUM” will be compared against the “Z” calculations
in determining whether cooling stages should be added or
subtracted.
Z Calculation — For the “Z” calculation, the control attempts
to determine the entering and the leaving-air temperature of the
evaporator coil and based upon the difference between the two
during mechanical cooling, determines whether to add or
subtract a stage of cooling. This is the adaptive element.
The entering-air temperature is referred to as MAT
(mixed-air temperature) and the leaving-air temperature of the
evaporator coil is referred to as EDT (evaporator discharge
temperature). They are found at the local display under the
Temperatures
CTRL sub-menu.
The main elements to be calculated and used in the calcula-
tion of SumZ are:
1) the rise per percent capacity (R.PCT)
2) the amount of expected rise for the next cooling stage
addition
3) the amount of expected rise for the next cooling stage
subtraction
The calculation of “Z” requires two variables, Z.PLU used
when adding a stage and Z.MIN used when subtracting a stage.
They are calculated with the following formulas:
Z.PLU = Z.GN * (10 + (4*(–ADD.R))) * 0.6
Z.MIN = Z.GN * (–10 + (4*(–SUB.R))) * 0.6
Where:
Z.GN = configuration used to modify the threshold levels used
for staging (Configuration
COOL
Z.GN)
ADD.R = R.PCT * (C.CAP – capacity after adding a cooling
stage)
SUB.R = R.PCT * (C.CAP – capacity after subtracting a cool-
ing stage)
Both of these terms, Z.PLU and Z.MIN, represent a thresh-
old both positive and negative upon which the “SUM” calcula-
tion must build up to in order to cause the compressor to stage
up or down.
Comparing SUM and Z — The “SUM” calculation is com-
pared against Z.PLU and Z.MIN.
• If “SUM” rises above Z.PLU, a cooling stage is added.
• If “SUM” falls below Z.MIN, a cooling stage is subtracted.
There is a variable called SMZ which is described in the
reference section and which can simplify the task of watching
the demand build up or down over time. It is calculated as
follows:
If SUM is positive: SMZ = 100*(SUM/Z.PLU)
If SUM is negative: SMZ = 100*(SUM/Z.MIN)
Mixed Air Temperature Calculation (MAT)
— The mixed-
air temperature is calculated and is a function of the economiz-
er position. Additionally there are some calculations in the con-
trol which can zero in over time on the relationship of return
and outside air as a function of economizer position. There are
two configurations which relate to the calculation of “MAT.”
These configurations can be located at the local display under
Configuration
UNIT.
MAT Calc Config (MAT.S) — This configuration gives the
user three options in the processing of the mixed-air tempera-
ture (MAT) calculation:
• MAT.S = 0
There will be no MAT calculation.
• MAT.S = 1
The control will attempt to learn MAT over time. Any time
the system is in a vent mode and the economizer stays at a
particular position for long enough, MAT = EDT. Using
this, the control has an internal table whereby it can more
closely determine the true MAT value.
• MAT.S = 2
The control will stop learning and use whatever the control
has already learned. Using this setting infers that the control
has spent some time set to MAT.S = 1.
First set MAT.S = 1. Then go into the Service Test mode,
turn on the fan and open the economizer to a static position for
5 minutes. Move to several positions (20%,40%,60%,80%). It
is important that the difference between return and outside
temperature be greater than 5 degrees. (The greater the delta,
the better). When done, set MAT.S = 2 and the system has been
commissioned.
Reset MAT Table Entries? (MAT.R) — This configuration
allows the user to reset the internally stored MAT learned
configuration data back to the default values. The defaults are
set to a linear relationship between the economizer damper
position and OAT and RAT in the calculation of MAT.
SumZ Overrides
— There are a number of overrides to the
SumZ algorithm which may add or subtract stages of cooling.
• High Temp Cap Override (H.TMP)
• Low Temp Cap Override (L.TMP)
• Pull Down Cap Override (PULL)
• Slow Change Cap Override (SLOW)
Economizer Trim Override
— The unit may drop stages of
cooling when the economizer is performing free cooling and
the configuration Configuration
ECON
E.TRM is set to
Yes. The economizer controls to the same supply air set point
as mechanical cooling does for SumZ when E.TRM = Yes.
This allows for much tighter temperature control as well as cut-
ting down on the cycling of compressors.
For a long cooling session where the outside-air tempera-
ture may drop over time, there may be a point at which the
economizer has closed down far enough were the unit could
ITEM EXPANSION RANGE
CCN
POINT
DEFAULTS
UNIT UNIT CONFIGURATION
MAT.S MAT Calc Config 0 - 2 MAT_SEL 1
MAT.R Reset MAT Table
Entries?
Yes/No MATRESET No
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