Download thermo grill illusion of pain

Probing The
Of
Experience
Pain
Team Members
Jeff Swift
Anna Karas
Stacey Hoebel
Advisor
Prof. Willis Tompkins
Client
Dr. Christopher Coe
UW-Madison Dept. of Psychology
Abstract
Our client, a psychologist at UW, wanted a
method to study pain disorders without actually
inflicting pain on patients.
The “thermal grill illusion” accomplishes this by
tricking the brain into feeling pain due to
alternating hot and cold components.
Our device employs solid copper rods as the
cold component (20°C) and strips of stainless
steel foil heated electrically as the warm
component (40°C).
Problem Statement
Design a device to test pain sensitivity in
patients with generalized pain disorders.
The alternating warm and cool
components of the device activate sensory
receptors and neurological thermal
pathways to create the illusion of pain.
Background
FIBROMYALGIA
Chronic musculoskeletal pain and fatigue
disorder
Cause of the disease is still unknown
Associated sleep disorder called the alpha-EEG
anomaly
Treatments
THERMO GRILL ILLUSION OF PAIN
Pain and temperature have intersecting neural
pathways
Previous Design
Created by A.D. Craig (Univ. of Arizona)
Alternating warm (40°C) and cool rods (20°C)
About 8” x 8” with 0.5”
diameter thermal rods
Presumed to be electric
Ominous Looking
Design Constraints
Consistent temperature components
– Warm: 40°C
Cool: 20°C
Reach temperatures in 1 sec.
Safe
Reusable
Non threatening appearance
Easy to operate
Materials and Cost
Material
Use
Cost
St. Steel Foil
Copper Bars
Wood (scrap)
Heater
Cool Component
Insulator for
heater, Platform
$24.00
$65.00
----------
Resistors & Wires
Breadboard
Wood Screws
Circuitry
Circuitry
Secure Copper
Bars
Circuitry
$1.00
---------$2.00
Power Amplifier
$40.00
TOTAL $132.00
Temperature Controlling Circuit
•Temperature remains constant 40° C
•Rapid heating
•No temperature overshoot
Warm Component
Stainless Steel foil strips heat when
voltage is applied by power supply
As temperature changes, the resistance of
the stainless steel feeds back to the circuit
If the resistance of the foil, when heated,
balances the wheatstone bridge, the
voltage supply is stopped and temperature
is held constant
Warm Component Material
Selection
Platinum
– Ideal resistivity and temperature coefficient of
resistivity
– Very high cost
Stainless Steel
–
–
–
–
Large temperature coefficient of resistivity
Large resistivity
Inexpensive
Available in appropriate thickness
Material Selection
Resistivity (micro ohm*cm)
Electrical resistivity vs Temperature for Stainless
Steel 321
140
120
100
80
y = -3E-10x4 + 4E-07x3 - 0.0003x2 + 0.1474x + 69.166
R2 = 1
60
40
20
0
0
200
400
600
Temperature (C)
800
1000
Material Selection
Calculations for foil dimensions:
– Want total resistance of heaters Rs = 2.5Ω
– Heaters to be connected in series so
resistance of each of 6 heaters = 0.42Ω
– Thickness of foil = 5.08 x 10-5 m
Length of foil strip = 0.2032 m
Resistivity of steel at 40°C = 7.46 x 10-7 Ω-m
– R=ρ*L/A  width of foil strip = 0.7 cm
Cool Component
Copper bars
– Heat transfer equation:
Q=A * k * ΔT
T=(m * Cp * ΔT) / Q
– Time to change 1ºK = 6.4 sec
Heat sink
– Adds thermal mass (absorbs heat)
– Adds surface area (dissipates heat)
Prototype Construction
Cool Component
– Cut copper bar into six
smaller bars (8” long)
– Drilled holes in ends of
each copper bar
– Used a countersink to
accommodate screw
heads
– Hammered wood
screw through copper
hole and into plywood
platform
Prototype Construction
Warm Component
– Cut wood blocks to
same size as copper
bars (8” x 1” x ½”)
– Cut stainless steel foil
strips to size for
optimal heating control
(8” x ¼”)
– Tacked foil and wire to
wood at ends
Prototype Testing
Verified total resistance of foil heaters
Tested prototype with feedback circuit:
– Power Source: +/- 20V and 1 amp
– Resulted in overload of power source
– Failure Analysis
Tested prototype without feedback circuit:
– Applied direct voltage of 3V to foil
– Resulted in successful heat generation
Assessment of Cool Component
– Advantages
Simple operator interface
Easy to manufacture
Constant temperature
Reusable
Aesthetically pleasing– conductive paint
– Disadvantages
Remains at the room temperature
Assessment of Warm Component
– Advantages:
Quickly heats up to 40°C (1-2 sec)
Remains at constant temperature
No temperature overshoot
No risk of burning patient
Easy to manufacture
– Disadvantages:
Somewhat primitive in appearance
Improvement of control circuitry necessary
Ethics
Inducing pain in human subjects
Adequate testing to assure engineering
temperature controls are reliable and safe
Future Work
Improve the aesthetics of device
Develop and test circuitry to control temperature
– Order 3W resistors for power amp
Look into possibilities of creating a switch
operated device
Use an aluminum sheet to serve as a heat sink
for the cold copper bars
Replace wood blocks with ceramic as the
thermal insulator for stainless steel heater
Special Thanks
Prof. Willis Tompkins Prof. John Webster
Prof. Jay Martin