Nextron MASTERTRACE MS-2102, Руководство оператора

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1.4
MS-2102 Chapter 1 Product Overview
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. Consult the hardware block
diagram in figure 1.5 for details. 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 RTD1 terminals 6-8
(designated to heater 1) or RTD2 (designated to heater 2)
terminals 10-12 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 signifi-
cant 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 MS-2102 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 1.2:
Figure 1.2 Cycle Modulation - 10 Cycle Frame