Download final report - Initial Set Up

Final Project Report
Cold Stimulus for Teeth
1
INTRODUCTION ............................................................................................................................................... 4
2
STATEMENT OF WORK ................................................................................................................................. 5
2.1
GENERAL ........................................................................................................................................................ 5
2.2
CURRENT METHODS....................................................................................................................................... 5
2.2.1
Carbon Dioxide (Dry Ice)..................................................................................................................... 5
2.2.2
Ice Stick.................................................................................................................................................. 5
2.2.3
Endo-Ice................................................................................................................................................. 6
2.3
PRODUCT PERFORMANCE .............................................................................................................................. 6
2.3.1
Inputs and Outputs ................................................................................................................................ 6
2.4
PHYSICAL DIMENSIONS .................................................................................................................................. 6
2.5
DELIVERABLES ............................................................................................................................................... 6
2.6
SAFETY ........................................................................................................................................................... 6
2.7
RELIABILITY AND MAINTAINABILITY............................................................................................................ 6
2.8
MANUFACTURED COST .................................................................................................................................. 7
3
REQUIREMENT SPECIFICATION .............................................................................................................. 8
3.1
PRODUCT PERFORMANCE .............................................................................................................................. 8
3.1.1
Inputs and Outputs ................................................................................................................................ 8
3.1.2
Special Requirements ............................................................................................................................ 9
3.1.3
Physical Properties ............................................................................................................................... 9
3.1.4
Operating Environment ........................................................................................................................ 9
3.1.5
Testing Philosophy and Testing System ............................................................................................. 10
3.2
RELIABILITY AND MAINTAINABILITY.......................................................................................................... 10
3.2.1
Reliability............................................................................................................................................. 10
3.2.2
Maintainability .................................................................................................................................... 11
3.3
DEVELOPMENT COSTS ................................................................................................................................. 11
3.4
MANUFACTURED COST ................................................................................................................................ 11
4
SYSTEM SPECIFICATION ........................................................................................................................... 12
............................................................................................................................................................................... 12
4.1
PROBE THERMAL MASS ............................................................................................................................... 12
4.1.1
Description of Block’s Operation....................................................................................................... 12
4.1.2
Physical Constraints ........................................................................................................................... 13
4.1.3
Inputs and Outputs .............................................................................................................................. 13
4.1.4
Operating Point................................................................................................................................... 13
4.1.5
Testing, Reliability and Acceptance ................................................................................................... 14
4.2
INTERNAL THERMAL MASS ......................................................................................................................... 14
4.2.1
Description of block’s operation ........................................................................................................ 14
4.2.2
Physical Constraints ........................................................................................................................... 15
4.2.3
Inputs and Outputs .............................................................................................................................. 15
4.2.4
Operating Point................................................................................................................................... 15
4.2.5
Testing, Reliability and Acceptance ................................................................................................... 15
4.3
THERMOELECTRIC COOLERS ....................................................................................................................... 16
4.3.1
Description of block’s operation ........................................................................................................ 16
4.3.2
Physical Constraints ........................................................................................................................... 16
4.3.3
Inputs and Outputs .............................................................................................................................. 16
4.3.4
Operating Point................................................................................................................................... 16
4.3.5
Testing, Reliability and Acceptance ................................................................................................... 17
4.4
HEAT REMOVAL SYSTEM............................................................................................................................. 17
4.4.1
Description of block’s operation ........................................................................................................ 17
4.4.2
Physical Constraints ........................................................................................................................... 17
4.4.3
Inputs and Outputs .............................................................................................................................. 18
4.4.4
Operating Point................................................................................................................................... 18
2
4.4.5
Testing, Reliability and Acceptance ................................................................................................... 18
4.5
COMPARATORS ............................................................................................................................................. 18
4.5.1
Description of block’s operation ........................................................................................................ 18
4.5.2
Physical Constraints ........................................................................................................................... 18
4.5.3
Inputs and Outputs .............................................................................................................................. 18
4.5.4
Operating Point................................................................................................................................... 19
4.5.5
Testing, Reliability and Acceptance ................................................................................................... 19
4.6
LED DISPLAY ............................................................................................................................................... 19
4.6.1
Description of block’s operation ........................................................................................................ 19
4.6.2
Inputs and Outputs .............................................................................................................................. 19
4.6.3
Operating Point................................................................................................................................... 19
4.6.4
Testing, Reliability and Acceptance ................................................................................................... 19
4.7
TRANSDUCERS .............................................................................................................................................. 19
4.7.1
Description of block’s operation ........................................................................................................ 19
4.7.2
Physical Constraints ........................................................................................................................... 20
4.7.3
Inputs and Outputs .............................................................................................................................. 20
4.7.4
Operating Point................................................................................................................................... 20
4.7.5
Testing, Reliability and Acceptance ................................................................................................... 20
4.8
POWER SUPPLY............................................................................................................................................. 20
4.8.1
Description of block’s operation ........................................................................................................ 20
4.8.2
Physical Constraints ........................................................................................................................... 20
4.8.3
Inputs and Outputs .............................................................................................................................. 21
4.8.4
Testing, Reliability and Acceptance ................................................................................................... 21
4.9
CASE AND PACKAGING ................................................................................................................................ 21
4.9.1
Description of block’s operation ........................................................................................................ 21
4.9.2
Physical Constraints ........................................................................................................................... 21
4.9.3
Inputs and Outputs .............................................................................................................................. 21
4.9.4
Testing, Reliability and Acceptance ................................................................................................... 21
5
CIRCUIT/MODULE DESIGN........................................................................................................................ 22
5.1
INTRODUCTION ............................................................................................................................................. 22
5.2
PROBE THERMAL MASS ............................................................................................................................... 22
5.2.1
Schematic Diagram............................................................................................................................. 23
5.2.2
Module Operation ............................................................................................................................... 24
5.3
INTERNAL THERMAL MASS AND HEAT SINK/FAN ...................................................................................... 24
5.3.1
Schematic Diagram............................................................................................................................. 25
5.3.2
Module Operation ............................................................................................................................... 26
5.4
THERMOELECTRIC COOLER / LED DISPLAY / COMPARATOR CIRCUIT ...................................................... 26
5.4.1
Schematic Diagram............................................................................................................................. 27
5.4.2
1.3.2 Circuit Operation ....................................................................................................................... 27
5.5
POWER SUPPLY............................................................................................................................................. 27
5.5.1
Connector Diagram ............................................................................................................................ 27
5.5.2
Circuit Operation ................................................................................................................................ 28
6
CONCLUSION................................................................................................................................................... 29
3
CHAPTER 1 Introduction
We were approached by Dr. Dean Kolbinson of the College of Dentistry, University of
Saskatchewan to undertake a design project which delivered a prototype of a new cold
stimulus pulp tester. Cold stimulus pulp testing is done to diagnose a tooth’s health, and is
used by Endodontists to determine the necessity of a root canal.
The pulp tester was to be used by two third year dentistry students, Brent Yaremko and
Roman Koutsil, who would undertake the testing and viability studies while using the
prototype for their table clinic. Controlled tests were completed on other students to
determine the effectiveness of the new tester. Brent and Roman were consulted
throughout the design process for their input on specific sizes, shapes, and other
specifications required in the design.
This pulp tester needed to generate a cold temperature that could be applied to a tooth in
a patient’s mouth. The vehicle used to transfer this temperature needed to provide a safe
and reliable means, with no mess or harm done to the patient. The cold must be
generated using a self contained system that requires no material input other than
standard wall power (110V @ 60 Hz). The target temperature goal was to reach -25ºC,
and colder temperatures would be considered a success.
The design process began in September 2004, and the working prototype was to be
delivered by February 1 / 2005. The design procedure laid out by the EE 495 design class
was to be followed, and modified somewhat to ensure the early prototype deadline would
be met.
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CHAPTER 2 Statement of Work
2.1
General
The development of a small, hand-held device to apply a cold stimulus to teeth was asked
to be devised for use in the dental clinic environment. The instrument would be used to
apply a cold stimulus to teeth to determine their status in regards to pulp vitality and/or if
the pulpal tissue is inflamed to an irreversible degree. This pulp vitality testing is done to
determine if a root canal needs to be performed by a root canal specialist or endodontist.
The reaction time of the patient’s response to the cold stimulus is also critical in
determining the health of the tooth.
We met with Dr. Dean Kolbinson from the College of Dentistry along with Roman Koutsil
and Brent Yaremko, two third year dentistry students. Mr. Yaremko and Mr. Koutsil
performed the testing and compared the results to that of the CO2 method. Dr. Kolbinson
acted as their supervisor and oversaw the initial stages of development.
2.2
Current Methods
Currently pulp vitality testing is performed using cold stimulus methods. Previously it was
performed using electronic means with a device known as an electronic pulp tester. We
were asked to utilize the cold stimulus method in our design. The three methods of cold
stimulus are described below.
2.2.1
Carbon Dioxide (Dry Ice)
Attaching a special device to a tank of compressed CO2 allows a thin stick of dry ice to
form. This dry ice stick is then applied directly to the tooth by means of a type of syringe.
This is the most widely used method of cold stimulus because it produces consistent
results with fast reaction time from the patient.
Regularly pieces of dry ice break off from the syringe and fall into the patient’s mouth.
This is not harmful to the patient but is inconvenient as the test must be stopped
momentarily.
2.2.2
Ice Stick
Used rarely to apply a cold stimulus, a stick of ice is created in a syringe and applied to the
tooth.
This method has many drawbacks because it does not produce consistent pain reactions,
sometimes false responses or no response at all. As well, the melting of the ice creates a
mess.
5
2.2.3
Endo-Ice
This method is known only as the product that is used in the test. Endo-Ice is a small can
of compressed refrigerant which is sprayed on a cotton swab. The frozen cotton is then
applied to the tooth.
This method is used when the CO2 is unavailable. The main drawback is that the
refrigerant has a terrible smell. Also the performance is not as good as the CO2 method
because the Endo-Ice does not get as cold. The reaction time of the patient is usually less
as well as the severity of the pain that is felt.
2.3
2.3.1
Product Performance
Inputs and Outputs
The primary output is the physical cold temperature which will be applied to the tooth. A
display was also used to indicate correct operation of the device.
Standard 110V, 60Hz AC power was the only input to the circuit besides the user
selection. Battery power was a consideration but was dropped after the requirements
were fully realized.
Dr. Kolbinson had initially discussed the possibility of implementing a function for heat
testing. The final construction of the Sledgehammer prevented this from becoming
realized.
2.4
Physical Dimensions
The Sledgehammer was designed to be easily integrated into a regular dentist’s station.
As well, the probe which is used in the patient’s mouth had to be hand-held. We decided
on a ball point pen shape for this.
2.5
Deliverables
The deliverable was a fully functioning prototype to be used in the clinical trials of the
dentistry students.
2.6
Safety
The major safety concern was within placing the probe into the patient’s mouth. We had
to protect the patient from the possibility of a ground fault or short circuit within the power
supply and having an immense amount of current flowing into the probe.
Cleanliness was also a factor because the device would be placed into a patient’s mouth.
2.7
Reliability and Maintainability
In the event of a major malfunction the Sledgehammer’s low cost will allow for the unit to
be replaced rather than repaired.
There are not many components that can be replaced easily without affecting the entire
system. The power supply is the only component that can be replaced easily but this will
6
only be possible on a fully produced unit as the Sledgehammer’s design is not fully
optimized for modularity.
2.8
Manufactured Cost
We hoped to develop the Sledgehammer at a low cost so that multiple units could be used
in a dentistry office. If the cost were to rise because of the need for higher cost
components such as a power supply then the Sledgehammer was also designed to be
portable.
7
CHAPTER 3 Requirement Specification
3.1
Product Performance
Our device is meant to replace current methods of pulp vitality testing within the dentistry
profession. These current methods rely on the patient’s response to a slight pain through
the sensitivity of their teeth. The most widely used, current method creates a response by
applying a cold stimulus to the tooth suspected of infection. The current methods are
cumbersome, unreliable, costly, and accident-prone. Our device will use the Peltier Effect
to produce a cold temperature at the end of the probe which is then applied to the patient’s
tooth.
We have decided that for the best operation we would like our probe to be at a
substantially low temperature (between -24 oC and -64 oC). After application to the tooth,
we want instant feedback from the patient if the tooth is healthy. It was found that a lower
temperature will cause a quicker response from the patient, as it will remove more heat
faster. A fast and reliable response is key to relying on the patient’s response to the
stimulus. A slower cooling device would lead to doubt that the device is cooling to the
appropriate temperature, and if the patient is even feeling it. In other words we do not think
that the entire tooth should be cooled down, rather a sensation must be felt by the patient
once the probe is applied.
3.1.1
Inputs and Outputs
Table 1: Physical and Electrical Inputs/Outputs
Inputs
Outputs
Power 120V
Heat ( from TEC / heat removal)
Heat - Tooth
Display – On
Temperature Select
Display – Probe Ready
On/Off
Display – Internal Ready
Heat - Ambient
“Cold” - Probe
Airflow In
Airflow Out
The early deadline of Feruary 1st, specific to our product will limit the type of output we
have to the user besides the actual temperature. We would like to install a display which
provides real-time outputs of the actual temperature of our probe however, because we
may be time-limited when we begin the more detailed part of the design, simple ON/OFF
and COLD/HOT indicators by using LEDs might be used. The control of this system will
most likely be designed around rocker, push-button or toggle switches. In addition to
power, some type of temperature selection will also be enabled in the system, a common
user interface will be provided for easy use.
8
The system will also need to input heat energy from the thermal masses and remove it
somehow. Most likely it will be through the environment via a heat sink attached to the
TEC. This will need to dissipate heat by using airflow. Alternately, we can also look at the
heat from the tooth as an input which will output to the internal mass and so on.
Power input to the system will be from a standard 120V wall plug. A power supply will
need to be purchased or constructed to regulate the power to usable levels for the TEC
and support electronics. Heat from the power supply will also need to be accounted for in
the heat removal, since it is guaranteed to get hot.
3.1.2
Special Requirements
There will be the choice between using a cold stimulus as well as a heated stimulus. The
heat stimulus was a special request by Dr. Kolbinson as occasionally a tooth’s sensitivity
is tested by heating methods. We initially thought this feature could be implemented but
after more research we are unsure if we can apply it to our design considering the time
constraint. This feature will have the same fate as the LCD display; it will be implemented
time and ease-of-install permitting.
With the need to have a working prototype available for the dentistry students to begin
their table clinic we will have an accelerated time table as compared to the other design
groups.
Additionally special requirements may include the dimension restraints on the probe
assembly since it must be usable by the dentist in the confines of the human mouth. This
may come into play in the final design since some features may need to be dropped to
meet this consideration.
3.1.3
Physical Properties
After meeting with our dentistry student counterparts we have now got a good picture in
our minds as to what the “wand” component of our device must look like. We have been
told that for the optimum dexterity the wand must be similar in shape and size to a
ballpoint pen. The probe shall have dimensions of approximately 5 mm radius with a
length of approximately 100 mm.
The housing for the cooling apparatus will be approximately the size of a toaster for
reference. Or more specifically it will be approximately 0.00002 Km wide x 0.00003 Km
deep x 0.000015 Km high. These dimensions should be small enough that the unit is
compact and easy to use.
3.1.4
Operating Environment
We have seen a typical work area for a dentist and have been shown that sanitation is a
very big concern. The sanitation practices we saw included the wrapping of all tools with
cellophane wrap. This is mainly for the ease of cleaning as the devices are used by many
people throughout the day. Our device must then meet the same criteria and be easily
cleaned after use. In fact the device will be wrapped in cellophane prior to use so that it
will be sanitary for the patient. Future models will possibly use a disposable tip or cover for
easier use.
The Device will be operated in indoor conditions. So we will design to expect an average
ambient temperature of 21 oC. We also are assuming an ambient relative humidity of
9
nearly 25%. As well 1 Atm can be assumed if that really matters at all. Basically there
are no anomalies preset in the dentist office that need to be accounted for.
The Design temperature is a minimum of -25 oC. Maximum performance temperature is
variable depending on ambient temperature an air flow to the unit, so it cannot be
specified at this time.
3.1.5
Testing Philosophy and Testing System
Our primary goal for testing the device will be its ability to cool to a desired temperature
and the speed at which it can do so. From research it can be assumed that the lower
temperature desired the more energy must be pumped out. Therefore a much lower
temperature should take more time to reach. So our aim is to determine the point which
we can reach quickly and is of a suitably low temperature for testing. It is our goal to later
develop a set of criteria to measure against. The criteria will most likely be developed from
the older established methods. As the older methods have been accepted by the dental
community, we can only improve on these.
The initial testing of our product will be done using mounted, human teeth, a thermocouple
and a stopwatch. We will have access to 2 different mounted teeth; one will have typical
anomalies such as fillings and the other will be a normal tooth. We have found out that the
thermal conductivity is quite low for a tooth’s enamel (similar to the conductivity of
porcelain) so we must be able to achieve quite a low temperature to get a quick response
from the patient. We will cool our device down to a necessarily cool temperature and
throughout a range of temperatures we will determine the time it takes for the tooth to
reach a necessarily cold temperature. This will then allow us to make a temperature
versus time graph to determine the optimum operating temperature.
Once the prototype is developed Mr. Yaremko and Mr. Koutsil would like to use it for their
table clinic project beginning in February, 2005 where they will be using human test
subjects to determine if our product will meet their needs.
At the time of submittal of the requirement spec it was unknown exactly the process that
would be used by the dentistry students for their tests however we can now elaborate a bit
on the process. Test will be conducted on fellow dentistry student classmates. The test
will evolve a direct comparison between CO2 and our method. The front tooth or incisor
will be tested, and then one week later the test will be reversed to ensure an accurate test.
After being tested the patient would be asked to provide a quick survey on the intensity of
the two methods to determine the intensity of our method over the CO2.
3.2
3.2.1
Reliability and Maintainability
Reliability
It is critical that the power supply, thermoelectric coolers, thermoelectric controller, and all
interconnects between devices are chosen to be high quality pieces, in order to maintain
operational status throughout an extended usage period, of approximately 5 years. The
overall design goal in this case is not to create a cheap device that will output a cold
temperature, but rather a reliable, trustworthy device that will last and provide worry free
operation for its users.
10
3.2.2
Maintainability
The device will need to be modular and make use of readily available components. This
will enable cheap, quick repairs, in the event that something goes wrong. As such, it will
be more financially appealing to have the unit repaired by a technician.
3.3
Development Costs
The main costs of this design will of course be the TEC but as well, the thermoelectric
cooler controller that will be used if that is implemented. Asside from components another
factor will be the engineering time that will go into designing the product. This can be
assumed to be negligible in our analysis since in all cases it will be the same, however
working less on the project is an attractive option to us. TEC controllers are available in
packages ranging from a basic circuit board that would be integrated into your system to a
fully packaged power supply contained system where you need to add your TEC and
some control circuits and it’s ready to go. Due to our accelerated time table for delivery of
the original prototype, a fully packaged controller may be needed to begin testing of the
TEC’s actual capabilities when employed as we need them. The price difference of the
basic board to the fully contained controllers is significant, but no final numbers have yet
been concluded, since we haven’t yet designed the thermoelectric cooler’s housing
(wand), so no specific thermoelectric cooler has been chosen. Also, removing the
controller aspect from the design, we will only need a power supply to actually power the
TEC. This would mean a cost saving as well as a time savings in implantation and for the
most part would accomplish the same thing.
The project time line as stated has been accelerated somewhat. Our counterparts in the
College of Dentistry have requested that a working prototype be developed for February.
This has heavily impacted our design in the sense that a simplified product that just
performs the basics will first be developed, and later a more complex design will be done.
3.4
Manufactured Cost
Manufacturing costs can only be drawn from the manufacturer’s or vendor’s website.
Additional pricing from stock components available from the techs will be taken from
vendor’s websites for an equivalent purchase. Realistically we are only planning for a
single prototype. However, in further analysis of manufacturing we will assume a price
break at 1000 units.
At this point in the design process we are on the brink of realizing what the actual cost
could be for a device of this nature. When the original starting goal of $130 USD was set
forward, we had assumed that one thermoelectric cooler would be capable of obtaining a
cold enough temperature. From our research we have discovered that the number of
thermoelectric coolers needed may actually be a multiple of anywhere from 2 up to 10.
This change has come about because of the extremely large difference in temperature
required across the hot and cold side.
11
CHAPTER 4 System Specification
Figure 1: System Block Diagram
4.1
4.1.1
Probe Thermal Mass
Description of Block’s Operation
The probe thermal mass will be the first contact between the tooth and the heat removal
procedure. It will be made of either copper or aluminium.
12
The connection to the tooth is one of the most vital components to this block. From
research the maximum heat that can be transferred from the tooth is proportional to the
surface area that is in contact with the metal. However the larger surface area also has a
downside. The larger the surface area is the more energy that will need to be removed
from the tooth to trigger a reaction from the patient. This is because the tooth’s enamel is
actually very thin somewhere in the range of 0.2 to 1.2 mm. Beyond this are the nerve
endings that should detect the cold stimulus. So there is not much material between the
tooth and the nerves and a larger surface area will only cool more enamel and not cause a
reaction. By some early work done we can make some guesses about the requirements.
Once the heat has been transferred from the tooth we need some sort of reservoir that
can hold it. This is the concept of the thermal mass. It is designed to draw heat from the
tooth and to store it as an increased temperature. We can think of it as a house cooling in
the winter the environment constantly removes heat from the house. And since the
environment has much more thermal mass the heat from just one house will not change
the temperature outside.
4.1.2
Physical Constraints
Based on the required energy that needs to be removed we can calculate the mass
required. We calculated the cooling potential that the probe would produce for a variety of
materials.
probe volume
dt
material
Volume
m^3
Copper
Aluminum
Stainless Steel
Silver
1.87E-06
1.87E-06
1.87E-06
1.87E-06
Density
g/m^3
8930000
2700000
7500000
10490000
=21-(-25)
Mass
g
0.000001868
46
Specific Heat
J/g oC
16.681
5.044
14.010
19.595
0.385
0.902
0.500
0.230
m^3
oC
Energy Req.
J
295.42
209.27
322.23
207.32
Our probe design incorporates a thermal mass that holds at least 10 times the energy
required to cool the tooth to the temperature of the probe. This will be sufficient to provide
quick and consistent cooling of the tooth.
4.1.3
Inputs and Outputs
Table 2: Heat Energy
4.1.3.1 Inputs
4.1.3.2 Outputs
4.1.3.3 Heat energy from the tooth
4.1.3.4 Heat transferred to the internal
thermal mass
4.1.3.5 Heat from the surrounding
environment
4.1.3.6 Heat energy transferred to the
transducer
4.1.4
Operating Point
It is essential that the unit operate at sub zero temperatures so material selection will be
affected by the temperatures in operation. We are optimistically designing for an extreme
low of -40ºC but realistically the design is focused on obtaining a temperature of -25ºC. In
13
either case we need to provide insulation to protect the user and to maintain the
temperature of the thermal mass.
4.1.5
Testing, Reliability and Acceptance
This is expected to be an extremely reliable block since upon completion, assuming that it
is designed correctly, it will never fail. However there are some considerations that need
to be dealt with such as condensation build up on the exposed metal. Testing to make
sure that the design meets the system’s requirements and will fit in with the other blocks
will be accomplished via the flow chart:
Define excess heat required
to remove
Material Selection
Too much time?
Size constraints?
Define thermal Mass
Required
Done
This block will be tested once the prototype is complete and the system tests are
underway. Tests will include the cooling and warming time of various materials used and
the different times associated with contact / no contact with a tooth for warming.
From prior analysis we know that we need to remove 23.19J of heat from the tooth to
achieve a response we have designed the probe to have sufficient capacity to store 250J
of heat energy giving approximately a 10 times buffer.
4.2
4.2.1
Internal Thermal Mass
Description of block’s operation
The operation of the internal thermal mass is similar to the probe thermal mass. We need
an energy deficient mass that will remove the heat from the probe’s thermal mass quickly.
This is dictated by the surface area between the two thermal blocks. The more surface
area in contact with the probe, the better the heat transfer capabilities. Another benefit to
using an additional thermal mass instead of directly cooling the probe is that during testing
the probe can be returned to the base unit to rapidly cool it back to a suitable temperature.
This is done by having the coolers constantly cooling the internal thermal mass which has
another 10 time buffer in heat capacity to 2500J of heat energy storage capability. This
large “buffer” will enable the probe to be cooled without relying on the coolers for a time
delay.
The actual heat removal from the thermal mass will be affected by the initial temperature
of the thermal mass, the initial temperature of the probe, the final temperature that we
14
need to achieve, the amount of insulation used, the surface area in contact with the probe
and the heat transfer capabilities of the coolers used. All of these factors were considered
when calculating the required energy to cool the internal block as well as the material, size
and shape of the block. The materials considered for the block were copper, aluminum,
silver and stainless steel. Stainless steel does not offer enough heat transfer capability
and silver is too expensive. The final decision rested with aluminum as it provided the
perfect cost, heat storage and heat transfer package for this.
4.2.2
Physical Constraints
Here we are mostly concerned with the surface areas involved. Basically three areas are
of concern. The contact area between the probe’s thermal mass and the internal thermal
mass must be as large as possible to provide quick cooling. The other surface areas not
in contact with anything will need to be sufficiently insulated to ensure that the active
cooler can operate optimally without giving off the cooling energy stored to the
environment.
To ensure maximum cooling from the active cooler it is important that we match the size of
the thermal mass to the dimensions of the thermoelectric cooler and ensure an optimum
contact between the two. This will include the use of a thermal grease to ensure a positive
contact patch. A very simple concept for this design is given in the appendix as well as
some calculations.
4.2.3
Inputs and Outputs
Inputs
4.2.4
Outputs
Heat energy from probe thermal mass
Heat energy to Thermo-Electric Cooler
Heat energy from environment
Heat energy to transducer
Operating Point
The system will need to operate at low temperatures so care should be taken in its design.
This especially makes the selection of the thermal mass’s shape to be important. We
need to provide the best contact with the probe and minimize the surface area not
touching the probe. An acceptable thermal mass component should be able to handle
temperature in the range of -50ºC to +22ºC.
4.2.5
Testing, Reliability and Acceptance
We have calculated so far that the internal thermal block will need to both act as storage of
heat energy created by the thermoelectric cooler (TEC) and as a means of heat transfer
between the TEC and the probe. The testing of the thermal block will be done along with
the TEC’s and heat removal system but will specifically focus on the insulation used and
it’s effectiveness at keeping heat from the environment and inside the case out of the
system. The block will have no reliability problems as it has no moving parts or possibility
of failure.
15
4.3
4.3.1
Thermoelectric Coolers
Description of block’s operation
The thermoelectric cooler(s) used (TEC’s) will use an input voltage and current to output a
temperature difference with heat transfer capability. The cooler works on the principal of
the Peltier effect. The Peltier effect states that if a current is passed across a junction of
two different conductors with different Peltier coefficients heat will be produced at a certain
rate. As the direction of this current is changed the heat transfer will be changed from
heating to cooling.
4.3.2
Physical Constraints
The coolers will be housed in a metal case of which the size will be determined by the
requirements of the internal components. The coolers physical size is only determined by
the capacity of the coolers of different dimensions. Smaller coolers have generally lower
heat transfer and temperature difference specifications and larger coolers have generally
greater capabilities. The cooler size we have chosen is a range from 20mm x 20mm x
2mm up to 50mm x 50mm x 5mm. This size allows for a fairly wide power range in the
coolers from 50 to 320W from some common manufacturer’s specification sheets.
The coolers will be sandwiched between the internal thermal mass and the heat sink.
4.3.3
Inputs and Outputs
Inputs
DC Power from the power supply (12V, 12A(max))
Outputs
Heat transfer from the internal thermal block
Heat energy from environment
4.3.4
to the heat transfer block
Operating Point
From the specifications we have researched on common TEC’s the operating point will be
determined by a graph similar to this one. The operating point on this specific TEC will be
the 7A curve as it reaches an acceptable temperature difference and offers our desired
thermal power output.
16
4.3.5
Testing, Reliability and Acceptance
While testing the TEC’s we will need to have the internal thermal block and the heat
removal system in place so we can apply a controlled current using a lab power supply
and other instruments to measure power consumption, heat transfer, and temperature
difference for different operating conditions (power supply settings). Once the cooler has
been tested in this manner we will have officially verified that the design will sufficiently
meet our requirements specification and our major design work has been completed.
The cooler will need to be tested for duration of time, a number of on/off cycles and
operated at both small temperature differences and large to ensure it will be reliable when
used in the system. The small temperature difference must be tested to simulate the
beginning of a cooling cycle and the large difference to simulate the cooler running at
maximum capacity.
4.4
4.4.1
Heat Removal System
Description of block’s operation
The heat removal system consists of all components necessary to remove the heat
generated on the hot side of the TEC’s and the power supply from the system. This will
include a heat sink and a combination of fans to move air within and into and out of the
system case. The heat sinks will be mounted strategically within the case to ensure
maximum heat removal from the system and perhaps even remove the necessity of fans.
4.4.2
Physical Constraints
The heat sink will be matched to the size of the TEC used to cover the entire surface area
of the hot side. The fan used to cool the heat sink will have roughly the same area as the
heat sink. The thickness of this fan will not be an issue so we’ll constrain the thickness to
25mm or less. The heat sink fan will have a minimum rating of 25 CFM.
17
4.4.3
4.4.4
Inputs and Outputs
Inputs
Outputs
Heat energy from the TEC and case electronics
Heat to the environment.
Power from power supply (12V, 12A max (DC))
Audible noise from the fan.
Operating Point
The fans included will operate on 12V DC power available from the power supply. They
must be able to operate in temperature conditions from 10ºC to 60ºC.
4.4.5
Testing, Reliability and Acceptance
The heat removal system will be tested using a source of heat and a thermal couple to see
if the ambient temperature is suitable. Essentially we need to ensure that heat is removed
from the surface of the heat sink and dissipated. A mock up of the case might be used if
the fans are available to check for the air temperature and to see if there is adequate
airflow.
4.5
Comparators
4.5.1
Description of block’s operation
A TL082 op amp chip will be used to compare voltage levels in two cases in our system.
The first will compare the user’s temperature selection to the voltage reading from the
temperature measurement of the internal block temp. If the measured signal represents a
warmer temperature than that of the user’s selection, the comparator will output to the
green LED to remain off and if the actual temperature is colder than user’s input, the
comparator will turn on the green LED. The second comparator will compare the user’s
selection to the voltage reading from the probe’s thermistor. If the temperature is colder
on the probe than the user’s selection the output state of the yellow LED will be on. The
yellow LED is also connected through an AND gate which takes input from the control
circuit’s green LED output and the proposed output to the yellow LED. Both of these must
be set to the on state for the yellow LED to illuminate to ensure that the probe will not
appear ready when the cooler has not yet reached the appropriate level.
4.5.2
Physical Constraints
There is not a real concern in choosing the chip on a size basis as the common chips are
significantly smaller than the other components included in the system. The chips must be
able to operate in the temperature range of 10ºC to 60ºC.
4.5.3
Inputs and Outputs
Inputs
Outputs
Voltage readings from the transducers and
+5 V DC, or 0 V DC depending on the state
user input setting.
of the comparison
18
+/-5VDC from Power Supply
4.5.4
Operating Point
The chip will not require a fast switching speed as a slower switching speed will eliminate
fluctuations of the on/off state when comparing close voltage readings. The chip is
powered at +/-5V DC from the power supply.
4.5.5
Testing, Reliability and Acceptance
The comparators can be tested using lab equipment to set up similar conditions as would
be found in the system, as shown by the data sheets of the connected components. This
testing will insure that the status LED’s will operate to our specifications. Variable
resistances or pots will be used to simulate the thermistors in action.
4.6
4.6.1
LED Display
Description of block’s operation
A three LED display will be used. A red one will be on when there is power to the system.
A green LED indicates that the internal thermal block has reached or exceeded the user’s
input setting. A yellow LED indicates that the probe’s has met or exceeded the user’s
input setting. There are no physical constraints on the LED’s, as they are very small and
standard sized and commonly work in any temperature condition we will be presenting
them.
4.6.2
Inputs and Outputs
Inputs
Outputs
Power supply
Red LED  Power on
Internal block ready signal (+5 / 0 V DC )
Green LED  Internal block ready
Probe ready signal (+5 / 0 V DC)
Blue LED  Probe ready
4.6.3
Operating Point
The LED’s are illuminated when a +5 V signal is received and are off when the voltage
level is 0V.
4.6.4
Testing, Reliability and Acceptance
The LED’s testing is included in the comparators testing.
4.7
4.7.1
Transducers
Description of block’s operation
The two transducers in the system take heat energy from the thermal blocks, and produce
an electrical response depending on the temperature of the blocks. This electrical
19
response is used to obtain a temperature reading using the comparators and the known
response of the components to specific temperatures. These transducers will be the
NCP15XQ102J03RC thermistor from Murata Electronics ordered through Dig key.
4.7.2
Physical Constraints
The transducers must be able to read temperatures down to -40oC and convert these
temperatures into a varied voltage level. The thermistors must be sufficiently small to fit in
a 3mm diameter hole which is bored to a depth of 5mm.
4.7.3
Inputs and Outputs
Inputs
Outputs
Power supply (5V DC)
Voltage levels to the control circuit
Heat energy from thermal blocks
4.7.4
Operating Point
The power supply will input +5V DC and the transducer will output a temperature
dependant range from 0 to 5 V DC.
4.7.5
Testing, Reliability and Acceptance
The transducers will be tested in the lab using a cold spray or ice to provide a cold
temperature simulation. The output of the transducer circuit will be monitored as the
temperature is also recorded and a table will be generated showing the output from the
transducer at different temperatures. This table will enable us to calibrate the user input
for different desired temperatures. The transducers will need to be tested to ensure they
will handle multiple freezing, thawing, freezing cycles so it can be trusted when integrated
into the system.
4.8
4.8.1
Power Supply
Description of block’s operation
The power supply will need to provide power to all other elements of the system. The
main area of concern with the power supply is supplying a large current to the TEC’s. We
are setting the TEC’s voltage at 12 V DC. The coolers we are currently considering
require a range of 3 to 7 Amps to reach a temperature difference of roughly 50oC. The
fans in the heat removal system will also run on the 12 V DC source. The power supply
also provides a +/- 5 V DC output, which powers the LED display, comparator circuits, and
transducers. These components do not have a large power consumption, so we have set
the current rating of the 5 V supply to 200mA max.
4.8.2
Physical Constraints
The power supply must be housed inside the system case, so its physical size will be
limited to 100 mm X 150 mm X 150 mm. This will ensure that the case doesn’t get overly
large and use up a lot of room. The power supply will definitely produce heat, which will
be removed from the system by a dedicated heat sink and a fan for the power supply.
20
4.8.3
Inputs and Outputs
Inputs
Outputs
110V AC Power from wall
Power to comparators, TEC’s, heat removal system, LED’s and
On / Off switch
transducers. (12 A max on the 12 V line)
4.8.4
Testing, Reliability and Acceptance
The unit can be tested, by operating the system through all possible input settings, and
cooling situations, to ensure that it can handle the power consumption of the system. The
test will be run for the duration of 2 hours with all current and voltage settings monitored.
4.9
4.9.1
Case and Packaging
Description of block’s operation
The case must be able to hold all the components of the system, except for the probe. Its
primary function is to house the system, but also provide thermal insulation from the
surrounding environment.
4.9.2
Physical Constraints
There must be adequate space allocated for all the components to be fixed within, as well
as sufficient extra space for proper airflow for the cooling systems. The insulation on the
case will need to provide protection for the circuitry from condensation due to the cold
temperatures created within. The insulation will need to keep its thermal properties to at
least -40oC. The connection to the probe, leaving the case, will need to be at least 500mm
long and contains the wiring to the temperature sensor contained within the probe. The
control knob for the user temperature input will be a varied resistance which changes with
the position.
4.9.3
Inputs and Outputs
Inputs
4.9.4
Outputs
Power Cord (110V @ 60Hz)
User Display (LED’s)
User Input
Heat to the Environment
Testing, Reliability and Acceptance
The case will not have any reliability issues, as it has no actual function or moving parts.
Its testing will be mainly in the area of heat removal, as we will need to mount the system
into the case once it has been tested, and ensure that there isn’t an excessive heat build
up once enclosed. The case should not be mishandled or dropped, in which case system
failure is expected.
21
CHAPTER 5 Circuit/Module Design
5.1
Introduction
The Sledgehammer is comprised of four main system modules. The power supply
accepts 110V, 60Hz AC power from the wall and outputs the necessary DC voltage and
high current to power the thermoelectric coolers and TTL voltage levels for the control
circuit and cooling fan.
The Sledgehammer’s control circuit displays the operation of the Sledgehammer by
means of a comparator circuit which is biased by thermistors mounted on each thermal
mass. Three differently coloured light emitting diodes indicate each state of operation.
The probe is comprised of a thermal mass attached to a plastic handle. A thermistor is
placed within a cavity bored out of the thermal mass. The thermistor is connected to the
Sledgehammer through a disconnect-able wire. This ensures patient safety.
A copper and aluminum heat sink/fan combination mounted with a compression assembly
including foam insulation remove the heat generated by the TEC’s which allows for a
greater temperature differential created by the TEC’s. The internal thermal mass is
machined from aluminum and attached to the cold side of the TEC’s.
5.2
Probe Thermal Mass
The probe thermal mass had two different designs. The need for a second design arose
for experimentation purposes where a smaller mass was thought to perform better than a
large one. The small design for the probe thermal mass was constructed three times,
each from a different material. We machined the mass out of steel, aluminum and copper.
The original, large design was made from copper.
The thermal mass is threaded to a cylindrical handle made from Delron plastic with a hole
drilled down the middle for the entire length of the handle.
22
5.2.1
Schematic Diagram
Figure 2: Original Thermal Mass (all measurements in millimetres)
B
a
c
k
F
r
o
n
t
S
i
d
e
Note: Threads on thermal mass not shown
Figure 3: Delron Probe Handle
Note: Threads on thermal mass not shown
23
Figure 4: Probe Tip Thermal Mass Second Revision
Note: Threads on thermal mass not shown
5.2.2
Module Operation
The temperature of the copper tip of the probe is measured by a thermistor installed within
a bored-out hole in the center of the copper tip. The thermistor is connected to two wires
which run through the middle of the probe handle then into the central unit. As the
temperature of the copper tip decreases the resistance increases. The chosen thermistor
has a maximum resistance of 1MΩ at about -100oC with a nonlinear temperature
response. Our response does not need to be linearized since we are only concentrating
on the -25oC threshold crossing. The -25oC reading of the thermistor is roughly 100KΩ.
5.3
Internal Thermal Mass and Heat sink/Fan
WKRP in Saskatoon purchased a Zalman CNPS7700-AlCu heat sink with a 120mm fan to
cool our TEC’s. This device is intended to be used as a cooling device for current PC
CPU’s. We decided upon this heat sink because of its good thermal conductivity and
large cooling fan.
The internal thermal mass was designed to fit the surface of the TEC’s we were going to
be using. A conical hole that matches that of the tip of the thermal mass was bored out of
the internal thermal mass to allow the probe to be placed inside.
24
5.3.1
Schematic Diagram
Figure 5: Heat sink, fan, TEC, compression and insulation assembly
DC Fan
Heat sink
TEC
Mounting Bracket
Internal Thermal Mass
Insulation Box
Figure 6: Internal Thermal Mass (All measurements in millimetres)
25
Figure 7: Zalman Heat Sink
Zalman CNPS7700-AlCu
- Dimensions
- Weight
- Base Material
- Dissipation Area
- Bearing Type
- Speed
- Thermal
Resistance
- Noise Level
5.3.2
: 136(L) x 136(W) x 67(H)mm
: 600g
: Pure Aluminum & Pure Copper
: 3,268 cm2
: 2-Ball
: 1,000 ~ 2,000rpm ± 10%
: 0.21 ~ 0.28°C/W
: 20 ~ 32dB ± 10%
Module Operation
A similar thermistor to the one installed within the probe tip is attached to the internal
aluminum thermal mass. The thermal mass is cooled until the -25oC threshold is
surpassed triggering the control circuit to illuminate the green LED. The thermal mass is
encased within a metal box which is filled with insulation. The box has a hole cut out the
same diameter as the bored out cone in the thermal mass to allow the probe to be
inserted.
The TEC’s are sandwiched between the heat sink and thermal mass using a
compression-type assembly. We have found that a TEC can withstand anywhere from
150 to 300 lbs/square inch of pressure in a compression assembly. The greater the
pressure on the TEC and its attached masses the more optimal the cooling effect will be.
The higher pressure will ensure that the heat transfer between the contacting surfaces will
be occurring optimally.
The Zalman heat sink shown in Figure 5 replaces the heat sink and fan assembly shown
in Figure 3 above.
5.4
Thermoelectric Cooler / LED Display / Comparator Circuit
The three components described in this section all run off of the DC voltages created by
the power supply. The LED’s are controlled by the comparator circuit which runs off the
5V rail while the TEC’s are powered by the 12V rail.
26
5.4.1
Schematic Diagram
Figure 8: Comparator Circuit
5.4.2
1.3.2 Circuit Operation
The thermistor installed within the probe tip is labelled Therm. A -25 in Figure 6 and is
compared to a variable resistance R4. Once the thermistor reaches -25oC the green LED
(D2) will turn on.
The second comparator circuit is biased using the thermistor Therm. B -25 and the
thermistor bias from the first comparator circuit. Once the thermistor on the probe reaches
-25oC the value of Therm. B should be very close to the resistance of Therm. A which will
engage the blue LED (D3). Power to the circuit is indicated by the red LED (D1). We
included a logic IC so that the yellow LED will not turn on unless the green LED is lit as
this is the only way the circuit should operate.
5.5
5.5.1
Power Supply
Connector Diagram
Figure 9: ATX Motherboard Connector
27
5.5.2
Circuit Operation
Using an ATX standard computer power supply outside of a PC environment is only
possible if the PS-ON pin (pin 14 in Figure 7) is grounded. When installed inside a PC
and connected to a motherboard this pin is connected to ground to signal that it is indeed
connected to the motherboard. We are using the +12 VDC (pin 10 in Figure 7) to run the
TEC as well as the cooling fan. The +5 VDC (pins 19, 20, 4, 6, 9 in Figure 7) are used to
power our control circuitry. The notion that some ATX power supplies will not start unless
there is sufficient load on certain voltage rails did not seem to affect the particular power
supply we have been using.
28
CHAPTER 6 Conclusion
We feel that the cold stimulus was a success as far as our design was concerned. The
design met our goals and as such was fully functional and ready for testing by our
customers. Unfortunately due to some unforeseen events the device did break down a
number of times during the two month testing period and it required our intervention. We
will briefly recap some of our successes and failures.
Above all the fact it even got cold was a plus. After some preliminary design calculations,
it seemed next to impossible to achieve even a quarter of our goal temperature.
Persistence paid off and with some guidance of professors and various manufactures
websites we where able to not only achieve our goal temperature of -25oC, but surpass it.
At maximum performance under ideal conditions the unit was able to get as low as -34oC.
Cost savings and simplicity were always on our minds in the design. We achieved this by
making use as much as possible of off the shelf components for the prototype. Due to the
nature of the project, highly specified components would cost an arm and a leg to acquire
so we had to make due with what was available to us. Because of ever tightening time
constraints, caused by both the reporting procedure of the class and breakdowns and
misunderstandings in the supply chain of components, our design seemed to get
simplified every chance we could while maintaining the same level of usability for the end
user.
It was stated that we had reached our goal temperature, but it was always known that
lower temperature would give a quicker and more intense reaction from the patient. So
anything extra would help with accurate testing. Since CO2 reaches roughly -70ºC, our
prototype did not cause the same intensity in reactions of patients. The dentistry students
had noted that approximately 10oC colder would probably have been sufficient to
reproduce the results from CO2. Unfortunately, only a complete redesign would suffice in
achieving those kinds of temperatures. Our device’s self contained design does present
an advantage over CO2 testing that cannot be overlooked.
Another design flaw was the power supply choice. In retrospect a constant current supply
should have been used. However, it would have been extremely costly to purchase or
build one that could output this large DC current, so it was not a viable option. In the end
we had to make due with the available lab power supplies to provide a reliable constant
current for testing.
29