2013 Seventh International Conference on Sensing Technology
Thick Film Flow Sensor for Respirator Applications
Steven Frenzer, Will English IV, Ryan Rodebush, and
Michael J. Haji-Sheikh
Department of Electrical Engineering
Northern Illinois University
DeKalb, Illinois 60115
mhsheikh@niu.edu
Abstract—A new approach to monitoring respiration activity for
first responders is proposed. This approach emphasizes low cost
and portability. Current commercial methods cost upwards of
$100 and aren’t reusable or portable. The design goal was to
achieve a portable stand alone unit that measures and displays
both breaths per minute as well as inhalation/exhalation using
simple low cost commercial components. The prototype device is
portable and battery operated as well as accurate. This approach
has benefits in the field for emergency medical technicians as well
as other medical professionals.
Keywords; Respirator; Arduino; Thick-Film; air flow; portable;
INTRODUCTION
Recently, several approaches to monitor respiration
have been demonstrated [1,2]. These approaches rely on
humidity sensor arrays and wearable textile sensors. The
optimal sensor for this type of situation is a thermal mass flow
sensor. These types of sensors can detect small flow rates and
the direction of flow. The approach used in this project was to
adapt a thermal mass flow sensor into a common respiration
unit, used by paramedics to monitor an injured person's
respiration. The sensor is thick film sensor designed at the
Microelectronic Research and Development Laboratory and
manufactured in the Northern Illinois University clean room.
This sensor functions in a method similar to the
microelectronic machines and systems (MEMS) device
invented and manufactured by Honeywell's Sensing an
Control division [3]. The sensor uses a Wheatstone bridge
design excited by 4-9 V DC and 4 thermally sensitive
resistors. Two of the resistors are on one side of a heating
element and the other two across in the direction of flow. As
air passes over the first set of resistors it is cooled by the
heating element and the second set is warmed by the flow. The
change in air temperature creates a change in voltage across
the bridge. As flow is reversed, like the difference of exhaling
compared to inhaling, the temperature changes and then
voltage difference is reversed. The sensor also has an acrylic
coating on the surface to prevent moisture from affecting the
results. Currently the main methods of measuring respiration
(spirometers) are bulky or expensive. In the research we did,
none of the products were marketed as portable. A wireless
version of this system could become a vital tool in monitoring
patients, as well as other markets, due to its portable
inexpensive design.
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Fig. 1. Printed Mass flow Sensor showing the heater and thermistor bridge.
DESIGN DESCRIPTION
The finished flow sensor is shown in Fig. 1. The
fabrication process for this sensor uses a four layer screen
printing process. The screen pattern is made using a
photoimageable mylar film with a pattern. This film is then
placed on top of the screen (the screen is coated with a
negative acting photoimageable polymer) which is then
exposed to ultra-violet light. The photo-material then hardens
resulting in a negative imprint of the pattern. The unexposed
regions are then washed off with water. These screens are
then used with an AMI/HMI Presco 485 printer. The 485
printer is an industrial printer which has the ability to set
various parameters that range from squeegee sweep rat to
screen substrate and planerity. Beginning with an alumina
(Al2O3) ceramic substrate for the sensor, the first layer of gold
(used for the long interconnects) is screen printed. After each
layer is printed, the sensor is fired to 850°C. The second layer
is an oxide which is meant to insulate so leads can be placed
over the interconnects. This is then dried 150ºC and fired at
850ºC and forms a hard glassy coating. Thermistor paste
material is added, dried and fired, and the last two layers are
gold. Each layer use a screen with a different pattern and the
totality of the process produces the final device. The design
parameter that is the most crucial is that the closer the
thermistors are to the heater the more sensitive the thermistors
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become. This close proximity is desired since this allows the
device power to be reduced. However, due to the inaccurate
nature of screen printing, there is a chance of shorting out the
sensor, which is a drawback to the close proximity. To lessen
the chance of the sensor shorting, the squeegee pressure has to
be turned up high (called a dry squeegee). This additional
pressure causes the material to thin out more making the
material resistivity to increase to about 20K ohms per square
instead of 5K ohms per square. Unfortunately the high
resistance of the sensor causes a high susceptibility to noise.
Leads must be kept short or be shielded. Fig. 2 shows the
flow sensor assembly.
The sensor itself has a high change in voltage for
relatively low temperature change due to the high TCR
(Temperature Coefficient of Resistance) of the sensing
material. The equation for the TCR is as follows,
input level of 0-5 volts. Meaning every bit corresponds to
around 4 mV, which corresponds to around ± 0.01 V of
absolute accuracy. The initial measurements from the sensor
showed a 40 mv peak to peak swing between sides when
breathing normally. This means the signal from the sensor
should be amplified if shallower breaths are to be detected.
The first stage of amplification was a difference amplifier
which is shown in the schematic as shown in Fig. 3. The initial
amplifier design was proposed to use a LM series instrument
amp but it was decided to change op-amps from the suggested
LM to an OP07 due to availability and better noise/offset
characteristics. The amplifier (schematic capture shown in
Fig. 4) was modeled in PSpice. A second low gain amplifier
stage was then incorporated which amplified the signal to a
usable level and further suppressed high frequency noise.
After each step preliminary measurements were taken and
further calculations were based off these numbers.
2
R=R 0 (1+α∗T +β∗T ) (1)
Where R0 is the resistance at 0ºC, and α is the first order
coefficient and β is the second order coefficient and
temperature in degrees Celsius. The thermistor material used
in this design has a high β (1600 ppm/C2) term which allows
for the sensor to have a reasonably high sensitivity.
Fig. 3. Simple difference amplifier used to amplify bridge signal. Signal
conditioning is done on board the Arduino Nano.
Fig. 2. Mass flow sensor assembly. The construction is thermistors on
alumina with printed gold leads.
The voltage change from both sides of the bridge is
then compared using a unity gain differential amplifier. There
is a second stage of amplification prior to the analog to digital
(A/D) converter on the Arduino. Both stages of amplification
uses a OP07 amplifier (low noise low drift low offset) [4].
from there the Arduino is coded to measure period of a breath
and display the result. The entire design was modeled in
Solidworks and Altium Designer 10 to verify everything
physically. All the circuit elements can eventually fit on the
same ceramic as the sensor is built on.
The Arduino development board used includes an
Atmel ATmega 328 microcontroller [5]. The ATmega 328
features include an Analog to Digital (A/D) converter which is
used to convert the analog signal from the op-amp stages to
digital values for further processing in the microcontroller.
This A/D converter has ± 2 LSB (least significant bit) absolute
accuracy [6]. The chip offers 1023 steps of resolution with an
978-1-4673-5221-5/13/$31.00 ©2013 IEEE
Fig 4. PSpice Model used to design the difference amplifier used to amplify
bridge signal.
For this design, it was necessary to control 16 light emitting
diodes (LED). These 16 LEDs were 7 segment displays plus
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the two decimal points adjoining each of the 7 segments. The
Arduino development board only has 14 pins of digital I/O so
our design needed to incorporate multiplexing to control the
16 LEDs. The multiplexing software switches which LED
segment is illuminated by selecting which common cathode to
drive via a NPN transistor. The NPN transistors are necessary
because the Arduino can only sync/source 40 mA per pin.
With multiple segments illuminated the design would exceed
40 mA. A 2N4401 switching transistor was readily available
and met the specifications necessary. The base resistor was
chosen to be 1.2K ohm to supply the current [7]. The resistors
chosen for the display were chosen based on the data sheet of
the display. The LED’s were designed to be driven with less
than 35 mA at 50% duty cycle [8]. The voltage drop across
the display meant that choosing a 270 ohm resistor would
achieve ~10 mA which is well within the specifications for
adequate brightness. The full schematic design was done in
Altium Designer 10 shown in Fig. 5.
voltage change can be calculated by knowing the velocity of
the air across the sensor as well as the properties of the resistor
material, substrate and heating element. Physically, the sensor
was measured using a digital multimeter (DMM) and an
oscilloscope. The results of the Pspice simulation is shown in
Fig. 6 and the un-amplified results of the initial sensor test are
shown in Fig. 7. The final amplified results are shown in Fig.
8. The results from physical testing gave the voltages
necessary for the amplifier calculations. We were able to
experimentally confirm our accuracy and results. By video
recording operation and comparing the time coding of the
video to the displayed number the results were verified Fig. 4.
The video was recorded in 1080p 23.976 (NTSC format)
Frames per second on a Nikon D5100. By counting frames
and watching the video it is verified that the display goes to
“ - - “ after 30 seconds of inactivity as an alert. The same
technique was used to verify accuracy of breaths per minute.
The inhale LED and exhale LED were verified via audio track.
Fig 5. Altium Designer 10 model of the entire structure.
Fig 6. PSpice output of the first stage amplifier. Gain of 6.
The completed design was then soldered to the board and then
programmed. Programming on the Nano then takes place
using Arduino’s software development environment and was
written in “Arduino C”. The program included debouncing,
sampling the digital measurement values from the output of
the integrated A/D converter, as well as code for multiplexing
the display. The program works by using the microcontroller
to measure the period of a breath. The internal RC oscillator
on the ATmega328 only has an accuracy of ±10% which
significantly impacts the accuracy of measurements. As
suggested by Atmel, a more accurate external oscillator was
used to drive the microcontroller. The 16 MHz oscillator
which is available on the Arduino board was chosen. The
program also illuminates the decimal places on the segments
to visually display inhaling vs. exhaling.
MEASUREMENT METHODS
Measurement of the sensor was done both
mathematically as well as in normal operation. Because the
sensor was being built in house there was no data sheet for it,
but the sensitivity can be measured directly. The expected
978-1-4673-5221-5/13/$31.00 ©2013 IEEE
Fig 7. Measurement results of the un-amplified sensor. The signal range is 41
mV peak to peak.
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conclusion, this novel design allowed the measure breaths per
minute accurately and inexpensively. The size and cost would
be reduced if a microcontroller was used instead of using a
development board like the Arduino. The Arduino was by far
the largest and most expensive part in the design. In the future,
it would be nice to see the device integrated with a better
display as well as variable set points to trigger alarms. Further
improvements can be made by using a application specific
integrated circuit (ASIC). The original idea was to use an
instrumentation style differential amplifier with a higher gain.
After finding out the sensor was unbalanced, the only way to
balance the offset was not generally not practical for this
prototype. Design modifications are needed to the sensor to
allow the offset be trimmed to near zero. The sensor had issues
with high frequency which will require that the impedance be
lowered. This noise was caused by the high resistance in the
bridge legs. The noise was overcome in the design phase by
using shorter wires and adding capacitors. The capacitors
however added a time delay. This time delay was acceptable
because the frequency of human breathing in normally less
than 1 Hz. Additionally, a voltage regulator will be added to
help control the signal to battery life.
Fig 8. Signal after first stage amplification and filtering. Actual gain was only
3. This is due to the lack of a PSpice model of the OP07 op-amp. For this
application this design error was acceptable.
ACKNOWLEDGMENT
The Authors would like to thank their families and Mr.
Gregg Westberg for their support and help. I would additionally
like to thank Misty Haji-Sheikh for her editing help over the
years.
REFERENCES
[1] Andre, N.; Druart, S.; Dupuis, P.; Rue, B.; Gerard, P.; Flandre, D.; Raskin,
J.-P.; Francis, L.A., "Dew-Based Wireless Mini Module for Respiratory
Rate Monitoring," Sensors Journal, IEEE , vol.12, no.3, pp.699,706,
March 2012 doi: 10.1109/JSEN.2011.2161668
[2] Merritt, C.R.; Nagle, H.T.; Grant, E., "Textile-Based Capacitive Sensors
for Respiration Monitoring," Sensors Journal, IEEE , vol.9, no.1,
pp.71,78, Jan. 2009 doi: 10.1109/JSEN.2008.2010356
[3] Bob Higashi, Semiconductor device microstructure, US patent 4,696,188
[4] Analog Devices. (2011). 150 μV Maximum Offset Voltage Op Amp.
Norwood: Analog Devices, Inc.
[5] Arduino. (2009). Arduino; Schematic. Retrieved January 5, 2013, from
http://arduino.cc/en/uploads/Main/ArduinoNano30Schematic.pdf
Arduino. (2012, November 23). Arduino Board Nano. Retrieved February
1, 2013, from http://arduino.cc/en/Main/ArduinoBoardNano
[6] Atmel. (2009, November). ATmega48PA/88PA/186PA/328P. Retrieved
Febuary 1, 2013, from http://www.atmel.com/Images/doc8161.pdf
[7] Storr, W. (2013, April 28). Transistor as a Switch: Electonics-Tutorial.
Retrieved 2013, from http://www.electronicstutorials.ws/transistor/tran_4.html
[8] LITE-ON ELECTRONICS, INC. (n.d.). LITE ON. Retrieved from
P_100D5623AG.
Fig 9. Image capture of the functioning prototype. This device can be easily
miniaturized.
DISCUSSION AND CONCLUSIONS.
The simulated results were more generous than the
measured results for both the sensor and amplifier operation.
The amplifier simulation in PSpice did not use an OP07
directly so this could be the reason for the discrepancy. In
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