1.2
MASTERTRACE Chapter 1 Overview
1.8 Theory of Operation
Controller functions are controlled by a microprocessor
that measures all analog signals and logic inputs, control
heater outputs and alarm contacts, and reads all user
input including communications and outputs to the
faceplate display and LEDs. The remainder of this chapter
describes the algorithms and operation of some of the
controller functions.
RTD Sensing
An RTD changes its resistance in a precision relationship
to temperature. This resistance is sensed by passing a
constant current through the RTD and measuring the
resulting voltage across the RTD (resistance = voltage/
current). The voltage appearing across RTD terminals also
includes the resistance of the inter-connecting wiring to
the RTD, which varies with wire length, size and ambient
temperature. By using a three-wire sensing scheme and a
lead resistance compensation circuit, the lead resistance
is cancelled out to give a voltage proportional to the true
RTD sensor temperature.
RTDs respond in a known but non-linear fashion to
temperature, which if uncorrected could lead to significant
errors over the temperature range of the controller.
Consequently, some means are needed to convert the
input voltage to a linear and useful range. The CPU
applies gain, offset and non-linearity corrections through
a linearization algorithm.
Current, Ground Fault and Voltage Sensing
Current transformers and high impedance voltage dividers
are used to scale-down the incoming heater current,
ground fault current and voltage. All three signals are
then passed through a full wave rectifier and filter to
obtain a DC signal. The DC signals are then converted to
digital values by a 10 bit A/D converter before finally
being passed on to the CPU for analysis.
Each of the three DC signals are sampled 300 times with
zero cross synchronization so that the sampling covers an
exact span of ten power cycles. This is to ensure that
heater current values are consistently measured when the
heater output cycle is modulated by the powerlimit and
proportional control functions.
Powerlimit
The powerlimit function allows the heater to operate
below its rated power by cycle modulation. Cycle
modulation is accomplished by controlling the integral
number power cycles into the heater over a periodic time
frame. The MasterTrace control uses a ten cycle time
frame. The integral number of power cycles per time frame
is called a duty cycle. With a ten cycle time frame, there
are ten duty cycles possible. For each duty cycle, there is
a fixed pattern that defines the number of power cycles in
which the heater is on and off. This is shown in figure 1.1.
Figure 1.1 Cycle Modulation - 10 Cycle Frame
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