MS-2102
1.5
Chapter 1
Product Overview
Cycle modulating the current through the heater has the
effect of turning the heater on and off rapidly and
therefore, power output is reduced in the long run. Since
the switching is zero-cross controlled, the controller
knows exactly when power cycles start and finish. Zero-
cross switching also helps reduce power harmonics that
generate unnecessary interference.
The heater current (average current) measured by the
controller while cycle modulation is in effect may be
approximated as follows:
Heater Current at 100% x Duty Cycle = Average Current
When powerlimit is enabled, a powerlimit current is set
by the user. This is essentially the desired average current.
The powerlimit control algorithm ensures that the actual
current will not exceed the powerlimit setting while
optimizing the maximum duty cycle possible. When the
average current exceeds the powerlimit setting, the duty
cycle is reduce by 10%. When the average current is
below the powerlimit setting, the duty cycle is increased
by 10%. Before the algorithm increases or decreases the
duty cycle, the controller waits until the heater current has
reached steady-state at the current duty cycle setting. If
the heater is initially off and the controller calls for heat,
the duty cycle starts at zero and increases by 10%
increments until it reaches a steady-state value. This
ramping up effect provides a current-driven softstart
whenever the controller calls for heat.
Proportional Control
Unlike on/off control where the heater is fully on or off,
proportional control can partially turn on the heater. The
heater output is proportional to the difference between
actual temperature and heater setpoint. The relationship is
expressed as follows:
(actual temperature – heater setpoint) x k = heater output
where k is the proportional gain
To partially turn on the heater, the proportional control
function uses cycle modulation in the powerlimit function.
By incorporating cycle modulation into the proportional
control equation, the algorithm is expressed using the
Equation 1.
The deadband factor
DB(t)
is a time constant that
determines the slope of change of the proposed heater on
duty cycle with the temperature difference. It is adjusted
between 1 to 10 each hour to minimize the difference
between the measured temperature and the temperature
seconds
in
time
C)
(
C)
(
perature
heater tem
C)
(
emperature
setpoint t
heater
cycle)
C/duty
(in
factor
deadband
)
(
cycle
duty
)
(
Where
)
(
)
(
1
)
(
(1)
)
(
)
(
0
)
(
)
(
0
)
(
0
)
(
=
°
=
=
°
=
°
=
°
=
=
≥
=
<
<
=
≤
=
t
∆
T
Ts-T(t)
e(t)
T(t)
Ts
t
DB
t
d
t
DB
t
e
if
t
d
t
DB
t
e
if
DB(t)
t
e
t
d
t
e
if
t
d
setpoint. Every hour after power up, the controller
calculates the absolute values of the temperature
differences
e(t)
and sums them during the hour. Then the
total absolute temperature difference is divided by the
number of temperature readings taken during the hour.
The result is called the Average Absolute Temperature
Difference (AATD) for the hour. If current AATD is
smaller than the AATD in the previous hour, the
deadband factor will be increased or decreased in the
same direction. If current AATD is larger than the AATD
in the previous hour, the deadband factor will be
increased or decreased in the reversed direction. At
steady state, the deadband factor used will fluctuate
around a optimum value.
Figure 1.3 shows the relationship between the proposed
heater on duty cycle and the temperature difference for
different deadband factors used.
Figure 1.3
Proportional Control
Duty Cycle vs. Temperature Difference