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...... 19 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...... 20 4.7.1 Description of block’s operation...... 20 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...... 21 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...... 23

5.1 INTRODUCTION...... 23 5.2 PROBE THERMAL MASS...... 23 5.2.1 Schematic Diagram...... 24 5.2.2 Module Operation...... 25 5.3 INTERNAL THERMAL MASS AND HEAT SINK/FAN...... 25 5.3.1 Schematic Diagram...... 26 5.3.2 Module Operation...... 27 5.4 THERMOELECTRIC COOLER / LED DISPLAY / COMPARATOR CIRCUIT...... 27 5.4.1 Schematic Diagram...... 28 5.4.2 1.3.2 Circuit Operation...... 28 5.5 POWER SUPPLY...... 28 5.5.1 Connector Diagram...... 28 5.5.2 Circuit Operation...... 29 6 CONCLUSION...... 30

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.

4 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 CO 2 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 Product Performance

2.3.1 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.

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.

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.

8 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 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.

9 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 Reliability and Maintainability

3.2.1 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.

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.

10 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 Probe Thermal Mass

4.1.1 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 0.000001868 m^3 dt =21-(-25) 46 oC material Volume Density Mass Specific Heat Energy Req. m^3 g/m^3 g J/g oC J Copper 1.87E-06 8930000 16.681 0.385 295.42 Aluminum 1.87E-06 2700000 5.044 0.902 209.27 Stainless Steel 1.87E-06 7500000 14.010 0.500 322.23 Silver 1.87E-06 10490000 19.595 0.230 207.32 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.

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 4.1.3.6 Heat energy transferred to the environment 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 Size constraints?

Define thermal Mass Too much time? Required

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 Internal Thermal Mass

4.2.1 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.

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.

Inputs Outputs Heat energy from probe thermal mass Heat energy to Thermo-Electric Cooler Heat energy from environment Heat energy to transducer 4.2.4 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.

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 Thermoelectric Coolers

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.

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.

Inputs Outputs DC Power from the power supply (12V, 12A(max)) Heat transfer from the internal thermal block Heat energy from environment to the heat transfer block 4.3.4 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 Heat Removal System

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.

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 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. 4.4.4 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.

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

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.

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.

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 +/-5VDC from Power Supply

18 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.

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 LED Display

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.

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

The LED’s are illuminated when a +5 V signal is received and are off when the voltage level is 0V.

The LED’s testing is included in the comparators testing.

4.7 Transducers

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 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.

19 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.

Inputs Outputs Power supply (5V DC) Voltage levels to the control circuit Heat energy from thermal blocks

The power supply will input +5V DC and the transducer will output a temperature dependant range from 0 to 5 V DC.

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 Power Supply

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.

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 Case and Packaging

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.

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.

Inputs Outputs Power Cord (110V @ 60Hz) User Display (LED’s) User Input Heat to the Environment

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 Fr ac on k t

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. DC Fan

Heat sink

TEC 5.3.1 Schematic Diagram Mounting Bracket

Figure 5: Heat sink, fan, TEC, compression and insulation assembly Internal Thermal Mass

24 Insulation Box Figure 6: Internal Thermal Mass (All measurements in millimetres)

25 Figure 7: Zalman Heat Sink

- Dimensions : 136(L) x 136(W) x 67(H)mm 5.3.2 Module Operation - Weight : 600g - Base Material : Pure Aluminum & Pure Copper A similar thermistor - Dissipation to the Areaone installed: 3,268 cm2 within the probe tip is attached to the internal o aluminum thermal - Bearing mass. Type The : thermal 2-Ball mass is cooled until the -25 C threshold is surpassed triggering the control circuit to illuminate the green LED. The thermal mass is - Speed : 1,000 ~ 2,000rpm ± 10% encased within a metal box which is filled with insulation. The box has a hole cut out the - Thermal same diameter as the bored out: 0.21cone ~ in0.28°C/W the thermal mass to allow the probe to be Resistance inserted. - Noise Level : 20 ~ 32dB ± 10% 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.

Zalman CNPS7700-AlCu

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 Power Supply

5.5.1 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.