Microtech Ii Ddc Features; Direct Pid Method; Cascaded Pid Method - McQuay MicroTech II Operation Manual

MicroTech II DDC Features

The MicroTech II unit controller uses PID control algorithms
(referred to as "PID objects") to control modulating output
devices in order to keep a controlled variable at or near the
desired set point. These devices are generally either floating-
point actuators or variable frequency motor drives (VFD).
There is a pair of outputs defined for each modulating device
to be controlled. One output energizes to increase the capac-
ity and the other energizes to decrease the capacity of the
modulating device. There are two different PID controls
schemes used to control the modulating output devices.
These are the "Direct PID Method" and the "Cascaded PID
Method." The "Direct PID Method" uses one Velocity PID
object to directly control the modulating device outputs. The
"Cascaded PID Method" uses one Position PID and one
Velocity PID object in a "cascaded" manner to control the
modulating device outputs.

Direct PID Method

In some cases, the controller uses a Velocity PID object
directly to turn actuator control outputs on and off to main-
tain a controlled variable at a set point. In these cases the set
point and current value of the controlled variable are input to
the Velocity PID object. The Velocity PID object determines
a "time on" for either the increase or decrease output as nec-
essary to reduce the "error" between the set point and the
controlled variable. This information is input into a position
adjust output object (PAO) which in turn commands the out-
put device increase or decrease output on and off, reposition-
ing the actuator.
Figure 10: Direct PID Method Control Scheme
Set Point
Figure 11: Cascaded PID Method Control Scheme
+
Set Point
-
Controlled
Variable
OM 138
+
Velocity PID
-
Controlled
Variable
+
Position PID
-
Note: In this method, position feedback from the control
actuators is not used for control purposes.
Table 58 on page 100 lists the modulating devices controlled
using this method. Figure 10 is a schematic representation of
this control scheme.

Cascaded PID Method

In some cases, the controller uses actuator position (or VFD
speed) feedback and a combination of a Position PID object
and a Velocity PID object to control actuator (or VFD) con-
trol outputs to maintain a controlled variable at a set point. In
this "cascaded" control scheme, the set point and the current
value of the controlled variable are input to the Position PID
object. The Position PID object then calculates a new actua-
tor position (or VFD speed) set point and inputs this into the
Velocity PID object. The Velocity PID object then compares
the current actuator position (or VFD speed) feedback signal
to the new position set point and determines a "time on" for
either the increase or decrease output as necessary to main-
tain the feedback signal at the current position (or speed) set
point. This information is input into a position adjust output
object (PAO) which in turn commands the output device
increase or decrease output on and off, repositioning the
actuator (or changing VFD speed).
Table 58 on page 100 lists the modulating devices controlled
using this method. Figure 11 is a schematic representation of
this control scheme.
PAO
Velocity PID
Actuator Position (or VFD Speed) Feedback
Actuator
Position
Actuator or
PAO
VFD Position
99