Danu Hankins, Jay Hartshorn, Justin Postma
1430 W A St.
APT 201
Moscow, ID 83843
03DEC2012
PJ Henscheid, P.E.
Mechanical Engineer
Avista Utilities
1411 E. Mission
Spokane, WA 99220
Phone 509-495-4323
Fax 509-777-9569
Dear PJ:
Here is an interim report for the progress made to date on the “Rotor Temperature Sensing
Network” design. The report includes:
1.
2.
3.
4.
The problem definition used in the design process.
Concepts considered as solutions to the problem definition.
Which concepts we have decided to pursue further next semester.
Our plan for testing and implementing the solutions.
To this end the report also includes any worked calculations, figures, and data sheets associated
with the concepts aforementioned. We have also included a budgeted item list for your
approval.
Sincerely,
Danu Hankins
Encl.
Jay Hartshorn
Justin Postma
Contents
Executive Summary....................................................................................................................................... 3
Background ................................................................................................................................................... 4
Problem Definition ........................................................................................................................................ 4
Project Plan ................................................................................................................................................... 5
Concepts Considered .................................................................................................................................... 6
Sensor Options .......................................................................................................................................... 6
RTD ........................................................................................................................................................ 6
Thermocouples ..................................................................................................................................... 9
Communication Options ......................................................................................................................... 10
Power Line Carrier Communication (PLCC)......................................................................................... 10
Radio Frequency Integrated Circuit (RFIC) .......................................................................................... 12
Optical ................................................................................................................................................. 13
Magnetic Coupling .............................................................................................................................. 13
Power Options ............................................................................................................................................ 14
Magnetic Field......................................................................................................................................... 14
Solar Power ............................................................................................................................................. 14
Piezoelectricity ........................................................................................................................................ 15
Concept Selection ....................................................................................................................................... 15
System Architecture.................................................................................................................................... 16
Rotor Circuit ............................................................................................................................................ 16
External Circuit ........................................................................................................................................ 23
Architecture References ......................................................................................................................... 23
APPENDIX A: MATHCAD DOCUMENTS ...................................................................................................... 25
APPENDIX B: COST BREAKDOWN ............................................................................................................... 26
Executive Summary
We are designing a temperature sensor network to monitor temperatures on the rotor of the
synchronous generators at the Noxon hydroelectric dam. There is no temperature
measurement on these poles currently. The lack of damper windings on these poles causes the
poles themselves to act as damper windings during any transients. With the increasing number
of transients that Noxon experiences as the load leveling station for Avista there is concern that
the temperatures are close to exceeding the rating of the insulation. The solution will be
scalable to allow for monitoring of temperature hotspots on poles and will interface with the
monitoring PLCs already at the dam. Implementing this solution will provide important metrics
for the safe and efficient operation of the generators at Noxon.
Background
The Noxon hydroelectric dam has no damper windings on its rotor poles. As one of the
load leveling generators for the power grid it goes through transients more often than other
generating stations. When it goes through transients the rotor poles themselves act like the
damper windings and have eddy currents run through them. This heats up the poles which are
not measured by any sensor currently. The temperatures potentially approaching or exceeding
the insulation’s temperature rating is a cause for concern for safe and efficient operation of the
generators. By providing metrics on the rotor pole temperatures Avista will be able to operate
the generators with the greatest efficiency while maintaining safe operation.
Problem Definition
Our project is to design a proof-of-concept for a temperature sensing network that will
take temperatures from the surface of the rotor and send that data to the Avista PLC. The
temperature sensor network had to meet a few specifications from the client including:
1. +/- 5 degree accuracy.
2. Being able to provide two temperature readings per minute to the PLC.
3. Being able to interface with the existing Avista PLC.
4. Be able to take temperature readings up to 250C.
a. The insulation on the rotor is rated to 180C; however, we want to be able to
continue monitoring temperatures beyond the rated.
Without a mechanical engineer on the team we constrained ourselves to assuming that we
could mount the sensor onto the rotor and the circuitry on the spider arms of the generator.
This means we could focus on the functionality of the system rather than trying to find a way to
mount it safely within the generator housing.
In addition to these constraints and specifications we added another specification,
scalability. We want our concept to be able to take in multiple temperature sensor readings so
Avista can monitor multiple potential hotspots on the rotor.
Project Plan
The first semester was spent on concept design. At this point we have two slightly
different approaches to implementation which will have to be tested in the next semester
(Spring 2013). Next semester will approximately follow the schedule below.
Figure 1: Task Schedule for Spring 2013
We hope to do the two loop coil test on the DC machine before winter break; however,
we should have the data within the first week of returning regardless. We will be testing each
component’s functionality individually before trying to integrate them all into one system.
The testing schedule has approximately one month of extra time in case any one of the
systems takes longer than anticipated to get online. We are currently particularly concerned
with the wireless communication system since that will require learning a protocol which we
are not familiar with. Depending on how steep the learning curve is that could easily add a
week to testing the communication system.
After the testing is complete we will compile another report detailing the results. In
addition, we will add recommendations for further additions to the project in coming years.
Concepts Considered
Sensor Options
At the start of the project we had three different options for temperature sensing:
Resistance Temperature Detectors (RTDs), Thermocouples, and InfraRed (IR) camera. IR was
quickly ruled out after speaking with our client the first time. The space requirements for an IR
sensor and where it would have to be placed made it a far worse option than the other two.
The other two options are still on the table.
RTD
The RTD bases its temperature measurements on the principle that a metal’s resistance
changes based on its temperature. An electrical current is passed through a resistor and the
resistance is compared to the known characteristics of the metal. An RTD comprises of a few
elements1:
1.
RTD platinum resistance element: This is the actual temperature sensing portion of the RTD. The standard
resistance is 100 Ω at 0° C.
2. RTD Outside diameter: The most common outside diameter is ¼" in the US or 6mm (.236") for non-US
applications. However, outside diameters range from .063" to .500"
RTD Tubing Material: 316 Stainless steel is commonly used for assemblies up to 500° F. Above 500° F it is
advisable to use Inconel 600.
1
JMS Southeast. http://www.jms-se.com/rtd.php
3. RTD Process Connection: Process connection fittings include all standard fittings used with
thermocouples (i.e. compression, welded, spring-loaded, etc.).
4. RTD Wire Configuration: RTDs are available in 2, 3 and 4 wire configuration. 3 wire configurations are
the most common for industrial applications. Teflon and fiberglass are the standard wire insulation
materials. Teflon is moisture resistant and can be used up to 400° F. Fiberglass can be used up to 1000° F.
5. RTD cold end termination: RTDs can terminate on the cold end with plugs, bare wires, terminal heads
and any of the reference junctions common to thermocouples.
Figure 2: General RTD Composition
The RTD has a few common configurations called “Wiring Configurations”2.
Two-wire configuration
Figure 3: Two-Wire Schematic
The simplest resistance thermometer configuration uses two wires. It is only used when
high accuracy is not required, as the resistance of the connecting wires is added to that of the
2
Wikipedia RTD Article. http://en.wikipedia.org/wiki/Resistance_thermometer
sensor, leading to errors of measurement. This configuration allows use of 100 meters of cable.
This applies equally to balanced bridge and fixed bridge system.
Three-wire configuration
Figure 4: Three-Wire Schematic
In order to minimize the effects of the lead resistances, a three-wire configuration can
be used. Using this method the two leads to the sensor are on adjoining arms. There is a lead
resistance in each arm of the bridge so that the resistance is cancelled out, so long as the two
lead resistances are accurately the same. This configuration allows up to 600 meters of cable.
Four-wire configuration
Figure 5: Four-Wire Schematic
The four-wire resistance thermometer configuration increases the accuracy and
reliability of the resistance being measured: the resistance error due to lead wire resistance is
zero. In the diagram above a standard two-terminal RTD is used with another pair of wires to
form an additional loop that cancels out the lead resistance. The above Wheatstone bridge
method uses a little more copper wire and is not a perfect solution. Below is a better
configuration, the four-wire Kelvin connection. It provides full cancellation of spurious effects;
and cable resistances of up to 15 Ω can be handled in this configuration.
Figure 6: Four-Wire Kelvin Schematic
RTDs have been replacing thermocouples for systems that are under 600C since the
RTDs provide better accuracy.
Thermocouples
Thermocouples operate off the Seebeck effect. As a conductor is exposed to a thermal
gradient, i.e. a temperature differential, it generates a voltage. Two dissimilar metals are
exposed to a heat source and the resultant voltage difference is measured and a temperature
extrapolated.
Figure 7: Thermocouple Circuit3
Communication Options
One of the challenges faces by the team is getting the temperature readings of the rotor
to the programmable logic controller. Originally, the team opted to use infrared technology
and place a sensor on the stator. The pole temperature would be taken remotely as each pole
passed by the sensor. Unfortunately, this technology already exists and Avista has a beta
version installed on one unit at Noxon. The Beta version does not work well so Avista opted for
a solution with the sensor mounted on the rotor. Therefore, the design must implement a
communication channel. Four different technologies are under consideration.
Power Line Carrier Communication (PLCC)
PLCC is based on the principles of superposition, allowing one conducting member to
carry multiple signals of different frequency. PLCC is used today to send data over power lines
3
http://en.wikipedia.org/wiki/Thermocouple
since they span great distance rather than creating a separate communication channel. The
signal is coupled onto the conductor using inductors and capacitors. The passive filters isolate
the transceivers from harmful voltages or currents. Additionally, passive filters are used to
steer the signal and block it from low impedance loads. A simplified diagram1 shows how the
passive components are used to couple a signal onto one of the lines of a three phase system
and direct the signal between transmitter and receiver.
Figure 8: PLCC General Schematic
This technology is of interest because the rotor field windings of a synchronous machine
are electrically connected to a DC source through slip rings. Some of the obstacles faced by this
communication method are electrical noise and directing the signal. Noise is introduced by
system transients and the mechanical contact of the slip rings. The effect of noise can be
mitigated by determining the frequency content of the noise and using a carrier frequency for
our signal that is well outside this range. If the noise is evenly spread over the usable
bandwidth of the channel then the signal to noise ratio (SNR) must be made sufficiently large to
distinguish between signal and noise. The expense of increasing the SNR will be larger power
consumption since the signal magnitude must be increased. Presently, no data has been
determined on the noise frequency content of the generators at Noxon. It is difficult to obtain
real-time data of the voltage on the rotor field.
Another obstacle of using PLCC is creating the filters necessary to steer the signal from
the rotor toward the slip rings rather than the rotor field. Adding passive filters to the rotor
field is not an option because this will require major modification of the field. The DC rating of
the field is 500V @ 1200A which means that the DC resistance is less than half an ohm.
Without the passive filters communication will be inefficient because signal energy will be
absorbed by the rotor field winding resistance. One way to increase the efficiency is to find a
frequency for which the rotor ‘acts’ like a high impedance load due to winding inductance and
capacitance between windings. Determining this frequency may not be possible without
testing on the unit.4
Radio Frequency Integrated Circuit (RFIC)
RFIC is a wireless communication solution that is designed as an off the shelf
component. These devices typically operate in the license free frequency bands that include
2.4GHz and 915MHz for North America. An RFIC is an all in one communication solution
because the chip takes care of all of the low level detail of wireless communication. The
wireless modulation techniques vary between devices as well as the usable range. Most of the
chips on the market send data packets and perform error detection so the user can view the
interface as a reliable ‘wire’.
4
All About Circuits filter tutorial. http://www.allaboutcircuits.com/worksheets/filter.html
Zigbee:
This chip sends packets up to 128 bytes and the range is between 90 meters and 1 mile
line of sight depending on the power used. Zigbee utilizes addresses for each device which
allows for up to 65000 nodes within a network. Communication to the Zigbee chip is
performed using SPI.
Optical
Optical communication typically uses a light emitting diode to transmit signal and a
special transistor to receive the signal wirelessly over short distances. The transistor is special
because the base is exposed to the light which modulates the drain current and amplifies the
signal simultaneously. The communication channel typically consisted of a low data rate but
the implementation is relatively cheap. An example of optical communication is a TV remote
which modulates the signal with a carrier of 38kHz over an infrared light wave.
Magnetic Coupling
Communication through the magnetic field can be performed using to inductors that are
magnetically coupled. The time varying magnetic field created by one inductor (the
transmitter) induces a voltage in the other inductor (the receiver). Essentially, they pair of
inductors can be viewed as a transformer when magnetically coupled. The limitation of this
technology is the orientation and distance between the two coils. The coefficient of coupling
decreases quickly as the distance between the coils increases.
Magnetic coupling will require one coil to be located on the rotor and the other on the
stator. Communication must take place while the two are aligned which causes some issues.
The rotor only rotates at 100rpm but the diameter of the rotor is on the order of 30 feet.
Therefore, the communication window is small and the rotor position must be monitored so
the circuit knows when it can transmit.
Power Options
There are many options that have been considered for powering the sensor and
communication devices on the rotor. Some of the concepts that were considered were the
rotor's DC windings, the vibrations of the machine, the magnetic field of the rotor, and solar
power. Although the rotor's windings were ruled out due to the imbalance it could cause on the
rotor's magnetic field. The remaining options were still under consideration.
Magnetic Field
Our primary consideration is to use the magnetic field from the rotor to provide power
to the circuit. This will be done by placing a loop of wire on the face of a rotor pole and utilizing
Faraday's law to create an emf in the loop. The major issue in this method is that the rotor's
magnetic field is primarily a DC field while Faraday's law states that a changing magnetic flux is
required to generate an emf. Thus we are planning on using the non ideal effects caused by the
teeth of the stator to obtain the changing magnetic flux and provide a voltage for the circuit.
Solar Power
The idea behind solar power is to place solar panels onto the rotor to generate the
power for the circuit. Solar panels operate off of the photovoltaic effect which is the
phenomena when light is incident onto a material the valence electrons present in the material
will become excited and break free. These electrons are what create the electrical generation
provided by the solar panels. The major downsides to using solar power would be the
extremely low efficiency of solar panels as well as the weak power generation from
incandescent lights and the relative expensiveness of the panels themselves.
Piezoelectricity
Another concept that was considered was to use the vibrations of the machine to power
the circuit. In order to do so we would utilize piezoelectricity which is the accumulation of
charges in certain solid materials when under mechanical stress. Thus by using the vibrations of
the machine we could theoretically generate charge. However this method did not have very
much information and was inconclusive on the amount of power that could be generated.
Concept Selection
For power we decided on using the slot harmonics of the magnetic field. We decided on this
after being reassured that it could provide enough power for the circuit as well as being easy
and cheap to implement. It is also particularly easy to modify the amount of voltage obtained
by linking more or less magnetic flux and by changing the area of the loop. Meanwhile the solar
would have been both difficult and relatively expensive to implement, while also not providing
as much power. The piezoelectric was lacking in too much information for a serious
consideration over the other options.
For the communication of the rotor circuit to the plc we decided on using radio waves.
In particular we decided on using Miwi over Zygbee due to the free open source stack and ease
of implementation that the Miwi provided. Optical would have proven to be more difficult to
implement with the timing constraints caused by the mechanical rotation without providing any
major benefit. The power line carrier would have also been more difficult to implement as well
as containing a loss of accuracy from the slip rings.
For the sensor the choice was primarily between an RTD and the thermocouple. The
RTD was chosen over the thermocouple as it provided a higher degree of accuracy across a
much larger temperature range, particularly at higher temperatures, while not costing a
significant amount more than the thermocouple alternative.
Table 1: Morphological Chart
Option 1
Option 2
Option 3
Option 4
DC windings
Magnetic Field
Solar
Piezoelectric
Wireless RFIC
Optical
PLCC
Sensor
Thermocouple
Infrared Camera
Power
RTD
System Architecture
Rotor Circuit
After Design one was chosen we began to brainstorm different circuit topologies in
order to analyze the sensitivity of the circuit. This semester the focus of the design has been on
the rotor circuit since it is the more difficult part of the system. As such, we chose to design the
rotor circuit first. Taking ideas from past course work and labs we designed the analog portion
of the circuit to measure the RTD’s resistance. The design is based on the Wheatstone bridge
which decreases sensitivity to common mode noise of the power supply. The choice of RTD
dictates the nominal resistance so we started designing the circuit from with this component.
After numerous searches we decided to use an RTD manufactured by Omega due to its size and
ease of mounting (i.e. #4 screw hole).
The next decision is choosing the Wheatstone bridge resistor values. In order to
maximize the voltage swing of the bridge, the resistor in series with RTD should be chosen as
the geometric mean of the RTD’s resistance extremes (𝑅𝑠𝑒𝑟𝑖𝑒𝑠 = √𝑅𝑇𝐷𝑚𝑎𝑥 ∗ 𝑅𝑇𝐷𝑚𝑖𝑛 ). The
problem with this design is the current draw through the Wheatstone bridge would cause
major error due to self-heating of the RTD. A standard 5V rail and a series resistor that
maximizes voltage swing for the sensor is on the order of 20C which is unacceptable. Other
methods, such as a current source, were examined to determine how to fix this problem. We
determined that the best fix is to increase the series resistance for the RTD until the selfheating is acceptable. The self-heating error was decreased to below 1C but the trade-off is
that the output voltage swing is decreased. The positive side to this is the Wheatstone bridges
output is more linear. We plan to use linear approximations in the microcontrollers to keep
calculations simpler and larger valued resistors will decrease these errors.
The next component we decided to use is an instrumentation amplifier to gain the
Wheatstone output voltage to the full scale range of the analog to digital converter (ADC).
Utilizing the full range of the ADC ensures all of the truncation and offset errors are minimal;
plus the amplifier eliminates loading of the Wheatstone bridge during measurements. After
amplifying the signal we will filter it with a simple RC filter to knock out high frequency noise.
We are anticipating some noise from the amplifier so it is better to design for this now rather
than trying to implement a digital filtering scheme later. The components mentioned are the
foundation for the analog circuit that is responsible for converting the RTD’s resistance to
voltage. After stepping back and looking at the design we determined a more useful design can
be created with the addition of one more component. Placing an analog multiplexer between
the instrumentation amplifier and the Wheatstone bridge allows for multiple sensors to be
measured by one common circuit (figure 1). This is a useful feature since the hot spots of the
machine are not yet known. Placing multiple sensors on a few rotor poles will give a thermal
mapping of the rotor pole which can be used to determine the final sensor position.
Figure 9: Analog Resistance to Voltage Circuit
The digital design for the sampling portion of the rotor circuit consists of an ADC, Zigbee
transmitter and a microcontroller (uC). Initially, we thought that the Zigbee could be used
without an external uC since its only purpose is to sample and send data. Programming a
microcontroller built into the Zigbee transceiver is not for the faint at heart because there is not
a lot of program space or resources left. In order to avoid future roadblocks we decided that an
external uC is a better solution; at least for the first design. The next step in the rotor circuit
design is to find part numbers for devices that will implement this circuit. In order to determine
which parts best fit our application a sensitivity analysis was performed.
Figure 10: Sensitivity Analysis Circuit
The first step in the sensitivity analysis is to determine a good model for the non-ideal
effects (i.e. tolerances from process, voltage and temperature, also bias currents and offset
voltages) of the components that correlate to information available in datasheets (figure 2).
The part search is a very iterative process because one component may change others in the
design but we had to start somewhere. Digikey has a superb search engine due to the
parametric filters. It is not uncommon to search for a part and have thousands of matches so
we applied different filters to get more ideal features for our design. Some of the filters are
easy to choose such as input voltage and the like but others were not so. After narrowing a
component down to about 200 parts we sorted by price. Starting with the cheapest part for all
components the sensitivity of the circuit was determined.
The results were not as pleasant as we would have hoped but this can be fixed by
choosing parts that cost a little more and have tighter tolerances. A Mathcad sheet was
created to perform all calculations and show what portions of the circuit were the sources of
error so better components could be chosen. Another factor that affects the sensitivity analysis
is the range of temperatures we expect to measure with the RTD. The Insulation rating of the
poles and field windings are both less than 200C. The circuit should be able to measure beyond
the insulation limits of the rotor since the purpose of the circuit is to measure temperatures
that may exceed ratings. We decided that a temperature range from 0C to 250C will be
sufficient.
One of the more important attributes of the rotor circuit is that it will be repeated for
every pole in order to increase redundancy and decrease wiring between poles. Each machine
at Noxon has 72 poles per rotor and 5 units which equates to a LOT of sensors. Cutting down
the cost of the design by so much as a dollar per rotor circuit is roughly a $350 savings.
Needless to say cost is a major factor in choosing components which is why we decided to use
an alternative to Zigbee for wireless communication. Microchip manufactures a 2.4GHz
transceiver that is ten dollars cheaper than the Zigbee chips we offered by Digilent. The tradeoff is that the protocol for the transceiver is no longer Zigbee and the rail voltage is 3.3V (we
anticipated using a 5V rail). Microchip has a similar protocol to Zigbee called MiWi that differs
in the number of nodes available in the communication network. MiWi Pro is limited to 8,000
nodes which poses no problems. Prior to finding this transceiver, the circuit was designed for a
5V rail but we decided to change the necessary components (eg. uC) in order to have a
common rail voltage for all the circuitry. A common rail voltage reduces complexity by
eliminating an additional voltage regulator as well as the hassle of changing logic levels
between ICs. The next step in designing the rotor circuit is to design a robust power supply.
Figure 11: Power Supply Circuit (Buck converter topology)
The input voltage to the power supply one of the big unknowns for this project so we decided
to treat the input voltage as a very poor quality alternating current (AC) voltage source and
design the power supply. Our model for the slot harmonics suggests an output voltage in the
teens is a very reasonable assumption. That said, we chose to base the power supply on a
direct current (DC) to DC buck converter. The slot harmonics will be harnessed using a
magnetic loop coil with a width that is half that of a slot (on the stator) in order to maximize the
change in flux. The AC voltage of the coil will be rectified and filtered into a rough DC voltage
using a simple full-wave rectifier and filter capacitor. A metal-oxide-varistor (MOV) is added to
the input of the converter for over-voltage protection (figure 3). In steady state operation the
field current is DC so the voltage induced in our power supply coil is only caused by slot
harmonics which should be relatively constant because the machine is synchronous. On the
other hand, the change in flux will be very large when the field winding is either powered up or
down. The MOV will short out any of the potentially harmful voltage levels for short transients
and protect the input of the power supply.
In order to determine which DC to DC converter is the best for our application we first
determined the minimum and maximum current draw of the rotor circuit from datasheet specs.
The minimum current is useful in determining the type of regulator (eg. PWM, burst) within the
converter as well as the inductor size. The maximum current is useful in determining the
saturation ratings for the inductor and the rating of the switch to ensure reliability. Digikey was
used to narrow down the number of parts to roughly 200 before using cost to begin analyzing
different buck converters. We decided that a fixed output is best for our application because it
reduces the external component count for the feedback loop by two resistors and reduces the
sensitivity. It turns out that the majority of the regulators that are available for 3.3V are rated
for currents on the order of 0.5A but we only need up to 100mA. The DC to DC converter we
chose has a high frequency oscillator (adjusted to roughly 500kHz with a resistor) to reduce the
size of external components and it has a low power mode. The low power mode is especially
useful because the minimum load is well below our specification for the rotor circuit.
A few more details for the power supplies design were considered prior to fabrication in
order to eliminate future issues. Part of the circuit for the rotor is analog and the rest is digital.
These two do not always play together well because the noise of the digital signals is coupled
into the analog signals. In order to reduce these effects the layout of our board will consist of
two separate rail voltages. A 3.3V rail for the analog portion will be created through a filter
from the digital 3.V rail. The analog circuitry will also have its own ground plane. The
connection of the grounds will take place at one point on the board to reduce current paths
between analog and digital signals. Coupling capacitors will be used for each IC in order to
reduce the effects of transients between IC’s. In addition to separating the circuit into two
sections the high frequency digital components (i.e. transceiver and uC oscillator) will be placed
on the opposite side of the board as the analog components. One last consideration for board
layout is to keep the reference voltage of the ADC physically close to the Wheatstone bridge in
order to reduce common mode noise.
External Circuit
The external circuit will consist of a transceiver and uC in order to receive the digital
signal from all of the transmitters for a generator. Once the digital signal is received it will be
converted into a temperature using linear approximations for both the Wheatstone bridge
(resistance to voltage output) and the RTD (temperature to resistance characteristics) using 32
bit floating point arithmetic. After the temperatures are determined the software will take the
maximum of all of the values to output to the programmable logic controller (PLC) and store
other values in a buffer. The buffer is useful if one of the temperatures exceeds a rating of the
machine because the temperature history can be extracted and analyzed.
The remainder of the design has been halted until the rotor circuit can be built and
more is known about the interface to the PLC. We are unsure of the wireless signal range for
the transceivers. If the signal can penetrate the concrete and rebar between the rotor and PLC
then the external circuit can be mounted next to the PLC. Otherwise, the signal must be routed
from within the rotor housing. Also, the interface to the PLC is known to be a 4-20mA signal.
The analog input to the PLC requires the temperature measurement to be converted back to
analog but this introduces more error. We have been recently informed that the PLC at Noxon
may have a digital interface according to a gentleman at the last snapshot. If this is true then
there is no need to introduce error by converting back to an analog current.
Architecture References
Refer to Appendix X for the circuit sensitivity analysis performed in Mathcad.
Refer to Appendix Y for the cost breakdown of the circuit components
APPENDIX A: MATHCAD DOCUMENTS
APPENDIX B: COST BREAKDOWN