Heated Clothing for Safe Body Temperature Regulation
for Quadriplegic Wearer
Proposal
February 20, 2009
Executive Summary
Spinal injuries can cause a disorder in which the body cannot properly regulate its
own temperature. Hot or cold weather causes large swings in core temperature, severely
impairing basic body functions. This disorder typically accompanies paralysis of the legs
or arms, leaving a person wheelchair-bound. In an effort to provide greater independence
and increased safety for people with this disability, we are designing a wheelchair
accessory which regulates body temperature. A network of tubes circulates temperaturecontrolled liquid around an all-weather garment, keeping the wearer’s core temperature
within a comfortable range. This keeps the wearer safe and allows them a greater degree
of mobility and freedom in a variety of environments.
Design Team One:
Albert Alexander
Steven Shane
Melissa Stroud
Stephen Zajac
Facilitator: Bob McGough
Table of Contents
Introduction......................................................................................................................... 3
Background: ........................................................................................................................ 4
Design Specification ........................................................................................................... 5
Conceptual Designs ............................................................................................................ 8
Risk Analysis ..................................................................................................................... 12
Process Organization........................................................................................................ 14
Distribution of Technical Tasks ........................................................................................ 15
Necessary Hardware and Software .................................................................................. 16
Cost ................................................................................................................................... 18
Project Timeline ................................................................................................................ 19
References ......................................................................................................................... 19
US Patent 6550471 - Heated clothing for use in cold weather and cold climate
regions. ...................................................................................................................... 19
April 22, 2003 <http://www.patentstorm.us/patents/6550471/description.html> .... 19
US Patent Application 20050047091 - Liquid cooling system and electronic
apparatus using the same .......................................................................................... 19
March 4, 2004
<http://www.patentstorm.us/applications/20050047091/fulltext.html> ................... 19
2
Introduction
The focus of this project is to design a non-medical device that will help the
customer compensate for their disability. As a result of a spinal cord injury, the customer
was diagnosed with a condition called quadriplegic poikilothermia. This disorder affects
the ability of the body to regulate its own core temperature, leading the body to adopt the
temperature to the local surrounding environment similar to cold-blooded animals, for
which the condition is named. Normal healthy bodies maintain a constant internal
temperature of 37°C (98.6°F), even in relatively extreme environments. Someone with
poikilothermia, however, might experience swings in body temperature, even in milder
environments, before natural thermoregulation kicks in. The customer’s body
temperature fluctuates during his daily activities that can cause headache, fatigue, and
reduced concentration.
The goal of the project is to provide a heating/cooling suit that will keep the
user’s body temperature within a safe and comfortable range. The top priority for the
project is safety, and that is where this system provides key innovations over prior work
in the market. Most products available today use resistive heating, which can cause burns
and requires expensive feedback systems to regulate with accuracy. In fact, these
products are usually marked with a warning, “not intended for use by quadriplegics,”
since they rely on user discomfort to indicate that the product is getting dangerously hot.
This project circulates temperature-controlled liquid instead, providing the ability to both
cool and heat the user depending on the environment. A sophisticated control system
tracks the user’s core temperature with an armpit sensor and adjusts the temperature of
the liquid accordingly to compensate for swings in body temperature. The control system
3
is completely automated and has its own diagnostic system, maintaining the user’s core
temperature without requiring the user’s attention. The liquid will be heated and cooled
by a completely solid state thermoelectric system, and diaphragm pump will circulate the
fluid around the body. The casing is designed to fit on a modern electric wheelchair, and
all interconnections are standard for electric wheelchair accessories.
Background:
There have been numerous inventions to create such a body suit that will
heat or cool an individual. These suits are designed for various activities including
working, medical treatment, or sport activities in adverse weather conditions. The
inventions use a heat source connected to stationary equipment from a heating system
attached to ones clothing. The heating garment must be connected to an energy source.
Due to the high power requirements, the battery power needed for this type of system is
either very bulky (lead acid) or prohibitively expensive (lithium-ion), resulting in very
limited mobility because of the electrical connections.
One other solution for heated clothing that does not involve electrical heating is a
garment disclosed in Japanese Patent No. 11050314(Desanto). For this design the
individual carries self-contained heating units that have hot gas passages throughout the
warming jacket. With this apparatus only a limited portion of the body would receive
heat, and there is a high mechanical difficulty in the implementation. Another
disadvantage of this system is the condensation of water vapors from the hydrogen
combustion process inside the warming jacket. Absorption pads inside the suit could be
used to keep a person dry and comfortable during physical activity.
4
Preliminary research indicated that the only prior art using temperature-controlled
liquid circulation is a medical device used for surgery patients. A silicone pad adheres to
the skin, and liquid is circulated through the pad to keep the patient’s core temperature
up. This implementation is upkeep-intensive and requires the user to be immobile.
This project improves on the schemes presented above by retaining user mobility
with a precise, autonomous, low-upkeep system.
Design Specification
The following parameters are of particular importance to the project. Their areas
of relevance are described below.
Safety: The parts of the system exposed to the user must not be hazardous to touch or
operate. Safe temperature limits will be established to prevent burns, the heat exchange
will be located underneath the wheel chair in a protective cage to avoid accidental
contact, and a sensor-driven diagnostic system will monitor the temperature of the user
and keep their core temperature within a safe range. For the benefit of the customer,
safety is the number one concern.
Comfort: The system is not intended to be a life-saving device. Rather, it is intended to
increase the comfort and freedom of the user in adverse environments. Therefore,
comfort is a necessary consideration for a successful product.
Of particular note is the garment to be worn by the user. The heating system will
be as unobtrusive as possible, and must be able to fit under a coat. The materials used
must cover the whole body, be light and easy to clean, permit mobility without stretching,
and must allow maximum bidirectional heat transfer. Where possible a neutral color such
5
as white should be used to prevent the garment from absorbing heat during the summer.
The main unit should also be as thin as possible for use during the summer, with extra
layers added for more warmth in the winter.
Robustness: The final system must be able to withstand extreme heat and cold, along
with the vibration and movement encountered when operating a wheelchair in an outdoor
environment. The heating and cooling system must be able to operate in conditions
where the device is not exactly level. The controller electronics also need to be able to
tolerate temperatures below zero degrees Fahrenheit. Operating the wheelchair outside
while the heating/cooling unit is not operational is also a concern, such that an antifreeze
solution will be circulated in the tubing to prevent damage to individual components such
as the pump and the tubing in this case.
Cost: While keeping the cost down is a significant concern, it is also not as important as
other design parameters. Like many devices catering to individuals with a specific
disorder, this product fills a unique need. Therefore a cost is unlikely to dissuade people
from using it. This system may even be covered by insurance in some cases. However,
for comfort devices of this type, components should be selected with a large margin of
overdesign.
Ease of Operation: Poikilotherms generally have limited ability to manipulate fine
objects, and their bodies are less sensitive to changes in temperature than a normal human
body. Any feasible design solution must be extremely simple to operate and require a
minimum of user feedback to perform properly. The end product will control the
heating/cooling performance based on sensor inputs, and will not require active user
input, save turning the system on and off.
6
Scalability: The primary goal of this project is to provide a working prototype for one
specific individual. It appears likely that functionality could be easily scaled to handle
more extreme conditions and a variety of body shapes, but having a single, operable,
user-specific prototype is a much higher priority. The design that we will produce will be
easily scalable to additional clothing such as temperature-controlled pants, while still
functioning properly.
Diagnostics: A well-defined set of fault states is of critical importance to the project.
Basic functional limits must be soundly defined in the control system, since without
proper controls, any unforeseen condition could jeopardize the functionality of the
product, possibly endangering the user. The goal is to have these issues addressed during
the design phase, such that all possible problems can be addressed. In the case of the
final product, it would be practical to have the unit shut down, rather than risk dangerous
thermal runaway. For this reason, a series of failsafes and shutoff conditions will be
incorporated into the final design.
Manufacturability: As with scalability, large-scale, generalized production is not a
priority for this project. The particular nature of the design (tubes attached to a garment)
could be modified to facilitate more efficient manufacturing on a wider scale, but the
current design is one that requires a degree of customization.
Design and Form: It is important that the garments are easy to put on and remove, since
customers using this product have reduced mobility. Therefore a pullover design is
preferred to a zip-up jacket. Where possible, aesthetic concerns must also be a part of the
design; the presence of the tubes must be as unobtrusive as possible, since the garment
may be the outermost layer of clothing worn, depending on circumstances. The final
7
product will use the smallest number of components necessary, not only to increase the
ease of manufacturability, but also to decrease the vulnerability of the system to
individual failures. For example, using a thermoelectric system replaces the need for
both a compressor-driven cooling system, along with a resistive heater.
Installation: The main unit consisting of the pump, heat exchanger, cooling device, and
controller electronic will remain on the wheelchair at all times. It would be ideal to locate
this assembly on the bottom of the wheelchair, such that it is as unobtrusive as possible.
This would also prevent burns/frostbite due to accidental contact. The only required
maintenance would be adding water to the system to ensure a constant flow. Quickconnects will be used to prevent fluid loss as the garment is connected and disconnected
from the rest of the system.
Conceptual Designs
The design criteria for the project as far as requirements and specifications have
been given above, but in this section we make their impact on the design process more
plain. In Table 4-1 below, these criteria are defined and then ranked in order of relative
importance, and assigned a weight which forms the basis of the design matrix below.
8
Table 4-1: Design Factor Matrix
Factor
Relative Order of
Significance
Safety
Comfort
Robustness
Ease of Operation
Cost
Design and Form
Diagnostics
Scalability
Installation
Manufacturability
Relative
Weight
1
2
3
4
5
6
7
8
9
10
10
9
8
8
6
8
4
4
4
4
Because the project is composed of many quite disparate systems acting together,
it makes sense to consider the design options from a more compartmentalized vantage. In
the course of choosing the proposed design, alternatives were considered and evaluated
for feasibility, according to Table 4-2 below.
Circulation System: The method by which heat will be distributed to the
wearer’s body. The choices identified are Tygon medical-grade tubing and a silicone-pad
technology.
Thermoelectric: Heat is pumped into or away from the system by the
thermoelectric device. The Peltier junction is capable of both heating and cooling without
significant wiring complexity, while standard inductive heating (common in electric
blankets) is incapable of cooling.
Control: Power to the thermoelectric is regulated by the control system. For this
purpose, a solid-state feedback system supplies a simple, economical solution, while a
9
microprocessor-controlled system allows for greater finesse in incorporating feedback
data over time.
Fluid: The medium that carries heat through the circulation system. Methyl
Alcohol has low viscosity at high and low temperatures and is largely immune to low
temperatures, but an water/antifreeze solution would be similarly flexible in sub-freezing
temperatures.
Thermometer Location: The location on the wearer’s body of an electric
thermometer to reliably report skin temperature. The eardrum is a medically-preferred
location for non-invasive measurements that provides core temperature readings
comparable to a rectally-placed thermometer. A thermometer located in the armpit is
more comfortable for the wearer, and provides a good measurement of skin temperature.
Power Source: Power is consumed by the thermoelectric element and drives the
control system and associated sensors. The primary concern is whether the power
consumption will be an acceptable drain on the battery in the customer’s wheelchair, or
whether a dedicated battery is required.
Garment: The type of garment that forms the framework of the circulation
system is important to consider. Two distinct styles, jacket or pullover, have been
identified, as well as four different materials.
10
Table 4-2: Component-Based Design Matrix
Design
Circulation
Tygon
Silicone
Factor
Thermoelectric
Peltier Inductive
Control
MCP Feedback
Fluid
Water/Antifreeze
Alcohol
Weight
Safety
Comfort
Robustness
Ease of
Operation
Cost
Design and Form
Diagnostics
Scalability
Installation
Manufacturability
Total
Thermometer Location
Eardrum
Armpit
10
9
8
5
6
7
5
7
5
10
8
10
3
5
5
9
5
10
7
5
6
8
5
7
7
5
6
8
6
8
4
4
4
4
5
8
8
6
5
5
4
5
4
6
5
5
7
8
5
6
5
5
7
5
7
7
5
5
5
7
5
6
5
6
5
8
8
4
7
5
8
5
5
5
6
5
5
10
5
5
5
7
7
5
6
5
5
5
5
5
392
365
464
333
439
375
417
359
Power Source
Wheelchair
Devoted
battery
battery
Garment Style
Material
Pullover
Jacket
Leather
Down
Cotton
Synthetics
5
7
8
4
3
6
7
5
7
5
5
6
6
8
5
6
5
5
5
5
8
5
5
5
10
5
5
5
8
5
7
5
5
5
6
5
5
7
4
3
5
5
5
9
5
6
5
5
5
5
5
5
5
5
5
4
5
5
5
5
6
6
5
4
4
7
5
5
4
5
7
4
5
6
5
4
5
5
6
5
5
7
5
6
6
5
5
5
5
5
7
7
5
7
6
6
5
5
5
5
380
374
397
347
365
317
342
340
357
393
Proposed Design
We will design, build, test and deliver a synthetic-fiber pullover integrated with a
path of Tygon medical-grade eighth-inch tubing which will carry a water/antifreeze
solution heated or cooled to provide core temperature regulation to the wearer. The fluid
11
will be temperature-regulated by a control system built around a Microchip PIC18F420
microprocessor to range within 100-120 degrees Fahrenheit when in heating mode, and
between 45-60 degrees Fahrenheit in cooling mode. The fluid will be circulated through
the Tygon tubing by a 24V 200L/hr electric diaphragm pump. The temperature of the
water will be raised or lowered by a 250W Peltier-junction thermoelectric device linked
to the tubing via a heat exchange. The thermoelectric will be coupled with an aluminum
heat sink.
The system will be powered by a 24-volt lead-acid battery already present on the
customer’s electric wheelchair. The pump, control system, and thermoelectric system will
be housed near the chair’s electric controller, in a harness beneath the user’s chair. Heat
transfer will be controlled by adjusting the duty cycle of a PWM signal controlling a
high-amplitude transistor, directly regulating current to the thermoelectric device.
Specific quantities in the operation of the control system will be indentified
through testing with a functional prototype, as well as safety and comfort trials with the
customer. Experiments performed will measure the heat delivered to the wearer of the
garment as a function of both water temperature and ambient temperature. Additional
experiments will determine the reaction of core body temperature to fluid temperatures
over time, providing the data necessary to establish heating and cooling gradients and
threshold points.
Risk Analysis
The health of the user is of primary importance. Since poikilotherms are less
sensitive to changes in temperature, any possible danger of under- or overheating the
system must be detected and prevented by the control system. First and foremost, the
12
system must not burn the user. A heating device which is comfortable at first can cause
tissue damage if left in the same place for a long time, oftentimes without the user
noticing, and the liquid in the tubes must not scald the user in case of a malfunction such
as a leak. This must be self-regulated by the system, especially since poikilotherms have
a reduced sensitivity to temperature and may not notice if they are being burned.
Secondly, the heating/cooling control must be both automatic and effective. If the
user is able to control or interfere with standard operation, undesirable system states may
occur—for example, cooling the user when it is already cold out—in which case the
combined effect of the system and the environment may place the user in an unhealthy or
even life-threatening state. The system must have a reasonable operable temperature
range, with high and low limits defined by the points at which it can no longer properly
regulate the user’s core temperature. The circulating liquid must neither freeze nor boil in
the normal operating range of environments.
Much less significant risks include proper power handling and energy
conservation. In order to be powered by an electric wheelchair battery, the system will
draw up to 10 amps of current, requiring a robust grounding scheme and insulated,
weatherproof interconnects to prevent any possible electrocution hazard. Additionally,
the power consumption must be low enough that a user will not find himself immobile
and stranded in a hostile environment because of a dead battery. Power requirements will
be well-defined and ensure at least a full day’s worth of operation without draining the
battery. An automatic shutoff may be implemented to allow all possible power to be
directed towards mobility if a battery is dying.
13
Process Organization
The following image indicates how the primary processes involved in the product
relate to key components and to each other. Each member of the group is responsible for
several individual processes as well as one higher level of system functionality: Albert is
doing circuit layout and analog integration, Steve Shane is in charge of the control
system’s functionality, Melissa is in charge of the design and mechical integrity of the
garment, and Steve Zajac is in charge of thermoelectrics and heat distribution.
Wheelchair
Battery
Thermoelectric
Device Power
(Zajac)
Microcontroller
Power
(Zajac)
ANALOG INTEGRATION
(Alexander)
Microcontroller
Digital-Analog
Interfacing
(Alexander)
Programming
(Shane)
Diagnostics
(Stroud)
Algorithm
Design
(Shane)
DIGITAL CONTROL SYSTEM
(Shane)
Hot/Cold
Control
(Alexander)
Thermoelectric
Device
THERMODYNAMICS
(Zajac)
Water Pump
Heat Exchange
(Zajac)
Sensors
USER INTERFACE
(Stroud)
Water Circulation
(Stroud)
Garment Design
(Stroud)
14
Distribution of Technical Tasks
The power system design is headed by Steve Zajac. This centers around adapting
an existing 24v wheelchair battery (the industry standard in electric wheelchairs) to
power a microcontroller and the thermoelectric device. Steve is well-experienced with lab
circuitry and prototyping, key skills for developing the power structure and understanding
trade-offs for efficiency and cost. Steve Zajac will also be in charge of design decisions
involving the thermoelectric device and heat exchange, due to his background in
thermodynamics.
Albert Alexander is the analog design lead. He will be in charge of interfacing the
microcontroller with the sensors and the thermoelectric. Albert will be working with
Steve Zajac to develop a control scheme that is optimal for the thermoelectric device.
Albert has experience designing and modifying high-amperage automotive electronics, so
he will also design the wiring harness and layout makes him secondary contact for power,
particularly with regard to high-amperage concerns and harness design.
Melissa Stroud is heading up the design of the garment and tubing network. She
will develop a scheme for body heat distribution based on communication with medical
professionals in the field. She will work with Albert and Steve on system diagnostics and
the control algorithm, applying her research to ensure the safety of the end user. Melissa
will also collaborate with Steve Zajac to optimize the exchange of heat by balancing flow
rate with thermocouple limitations. Melissa will also design the user interface, ensuring
that the entire system accommodates the unique needs of tetraplegic poikilotherms.
Steve Shane is the programming lead, due to his experience with microcontrollers
and control systems. He will be the primary designer of the control system algorithm and
15
implementation. Albert will collaborate with Steve Shane to develop system diagnostics
based on sensor feedback.
Additional system-level concerns will be more fully developed for the final
proposal, including a possible EMC analysis.
Necessary Hardware and Software
The primary components of the system include a pump, a microcontroller, a
thermoelectric device with a heat exchange, a base garment to build on, tubing and liquid
to fill it, and interconnects and casing. The project is being designed as an accessory for
electric wheelchairs, and will be powered by a 24v lead-acid battery, standard for electric
wheelchairs.
The thermoelectric device has already been specified and obtained. A Peltier
junction will be used due to its flexibility (can both heat and cool) and small form factor.
While there is a comfortable buffer for necessary output and power consumption, the
efficacy of the heat exchange still needs to be considered. However, preliminary research
indicates that an appropriate heat exchange will be obtainable for a reasonable cost, and
there is plenty of room on the wheelchair itself for the heat exchange as well as the pump.
The pump still needs to be specified. Small form factor, low noise, and low power
consumption are all desirable traits. A small, centrifugal pump, similar to those used for
computer liquid coolant systems, should provide adequate flow rate without being too
loud or too big.
Tubing has already been obtained. 2mm Tigon tubing will be used due to its low
price, high heat tolerance, flexibility, and widespread availability.
16
The microcontroller or PIC is still being researched. Many chipsets can provide
the functionality this project requires, but due to the simplicity of the control system,
cutting-edge hardware is unnecessary. In the interest of reducing cost and maintaining a
high level of manufacturability, a fairly basic, widely available controller will be
selected.
The garment needs to be lightweight, not too stretchy, simple, and breathable.
Lightweight, long-sleeve, athletic shirts can fit under a coat, and they are durable enough
to support a tube network but are not too warm for the summer. Both polyester and cotton
are being considered: cotton is more comfortable and more durable, but sweat-wicking
polyester is easier to keep clean and breathes better. Both fabrics are inexpensive and
easy to obtain.
Power interconnects are standardized for electric wheelchair accessories. Tubes
will be connected with Quick-Connect interconnects for fast, drip-free operation. Casing
will be manufactured in the engineering shop from folded aluminum.
Most prototyping and lab testing tools are readily available in the ECE 480
laboratory, but testing Peltier junction control schemes requires the use of a high
amperage power supply. This can be checked out from the 402 lab, or a wheelchair
battery can be used instead.
17
Cost
Total Budget: $500
Item
Cost Range
Selected Device Cost
Thermoelectric Cooler
$23-$65
$65
Heat Exchanger
$27-$145
$27
Heatsink
$5-$45
$20
Water Pump
$54-$64
$63.82
Tygon Tubing
$25
$25
Quick Connectors
$15-$30
$15
Pants/Jacket
$40-$60
$25
Temperature Sensors
$20-$100
$50
Circuit Components
$5-$50
$5
PCB Service
$0-$51
$17
Total Spent: $313
Almost all of the parts listed in the cost breakdown are critical to the functioning
of the final product. The only exception is the PCB service, which is meant to improve
the aesthetics of the electrical portion of the design. The TE module was chosen to
provide maximum cooling performance (heating performance is resistive and more than
enough for this project), while the heat sink was chosen to dissipate the maximum
amount of heat this module can generate. The heat exchanger was chosen for its large
copper surface, along with a flow rate that is compatible with our pump. The pump that
was purchased has a flow rate that is sufficient for 2 mm inner diameter Tygon tubing,
18
which was found to produce good experimental results. The jacket and quick-connectors
were chosen for comfort and ease of use, respectively. Temperature sensors need to be as
unobtrusive as possible. For this application, lightweight thermocouples were chosen for
their high accuracy and ability to be placed under the armpit. The final two items, the
circuit components along with the PCB service, are absolutely required for the operation
of the control system. A PIC was chosen as the microcontroller due to the fact that it is
well documented, and is available in an easy-to-prototype DIP package. Other processors
would require the use of an expensive and delicate surface mount adapter.
Project Timeline
Gant chart is attached.
References
FAQ & Technical Information. TE Technology, INC. 6 Feb. 2009
<http://www.tetech.com/FAQ-Technical-Information.html>.
"Introduction to Thermoelectrics, Peltier, Seebeck." Introduction to Thermoelectrics.
Dec. 2001. Tellurex. 06 Feb. 2009 <http://www.tellurex.com/cthermo.html>.
US Patent 6550471 - Heated clothing for use in cold weather and cold climate regions.
April 22, 2003 <http://www.patentstorm.us/patents/6550471/description.html>
US Patent Application 20050047091 - Liquid cooling system and electronic apparatus
using the same
March 4, 2004 <http://www.patentstorm.us/applications/20050047091/fulltext.html>
19