Pid Control Parameters; Period; Proportional Band And Integral Time; Adjusting Pid Control Parameters - McQuay MicroTech II Operation Manual

Table 58: MicroTech II Modulating Output Devices
Modulating Device
Hot Water Valve Actuator
Steam Valve Actuator
Gas Valve Actuator
Chilled Water Valve Actuator
Heating F&BP Actuator
Cooling F&BP Actuator
Economizer Damper Actuator
Return/Exhaust Fan Inlet Vane
Actuator or VFD
Enthalpy Wheel VFD
Effective Discharge Cooling Set
Point
Effective Discharge Heating Set
Point

PID Control Parameters

Associated with each modulating output device, there is a set
of three adjustable PID control parameters. These are the
Period, Proportional Band and Integral Time.
Note: Since derivative control is not generally required in
HVAC control strategies, the Derivative Time
parameter of the PID object is set to 0 and is not
adjustable through the unit keypad/display. The
Derivative Time parameter can only be adjusted
using the MicroTech II Service Tool.
Since the factory settings for these parameters should pro-
vide the best control action, field changes are not recom-
mended. If problems arise, these parameters should first be
set back to the factory settings if they have been changed in
the field. If adjustment is then required, caution should be
used since parameters grossly out of adjustment can cause
erratic unit operation and possible equipment damage.
Changes should only be made in small increments and to
only one parameter at a time. After each change, enough
time for the system to stabilize should be allowed before fur-
ther changes are made. Refer to the following sections for
guidelines regarding how the specific parameters affect the
control action of the PID algorithm.

Period

Unless grossly misadjusted, changing the Period parameter
generally has a relatively small effect on the PID control
action. It should generally be set to a value of the same mag-
nitude and within 1/4 to 2/3 the time constant of the system
being controlled. If the Period is set too long, loss of system
control will likely occur. If the Period is set too short, dra-
100
Decrease Binary Output
MCB-BO9
MCB-BO9
MCB-BO9
MCB-BO7
MCB-BO9
MCB-BO7
MCB-BO5
MCB-BO15
ERC1-BO7
-
-
Increase Binary Output PID Method
MCB-BO10
MCB-BO10
MCB-BO10
MCB-BO8
MCB-BO10
MCB-BO8
MCB-BO6
MCB-BO16
ERC1-BO8
-
-
matic system "hunting" will likely occur. When in doubt,
this parameter should be set to the factory default setting.
Note: The Period parameter is not allowed to be set higher
than the Integral Time parameter. If this is
attempted the Period parameter reverts to its previ-
ous setting.

Proportional Band and Integral Time

The Proportional Band and the Integral Time parameters
have a similar effect on the PID control action. Increasing
either the Proportional Band or the Integral Time has a slow-
ing effect on the control action. Decreasing either the Pro-
portional Band or the Integral Time has a speeding effect on
the control action. Increasing or decreasing both parameters
at the same time has a more dramatic slowing or speeding
effect. Increasing one while decreasing the other tends to
have countering effects on the control action. Therefore, if it
becomes necessary to slow down or speed the control action,
these two parameter should generally not both be changed at
the same time. One parameter should remain unchanged
while the other is increased or decreased. Proper "tuning" of
the Proportional Band and the Integral Time parameters
involves finding an acceptable "balance" between the two.
Note: The Integral Time parameter is not allowed to be set
lower than the Period parameter. If this is attempted
the Integral Time parameter reverts to its previous
setting.

Adjusting PID Control Parameters

Correcting System Instability ("Hunting")

If the system control is unstable ("hunting"), this generally
means the control response is too "fast." To correct system
Controlled Variable
Direct Discharge Temperature
Direct Discharge Temperature
Direct Discharge Temperature
Direct Discharge Temperature
Direct Discharge Temperature
Direct Discharge Temperature
Cascaded Discharge Temperature
Cascaded Building Static Pressure
Enthalpy Wheel Exhaust Air
Direct
Temperature
Direct Control Temperature
Direct Control Temperature
OM 138