icebi ’07 icebi ’07 august 29th – september 2nd 2007 in graz, austria an energy efficient...
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ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA
An energy efficient wearable tissue monitor
ANNUS, Paul; MIN, Mart; PARVE, Toomas; LAND, Raul; HALDRE, Eero; KUUSIK, Alar; POOLA, Gustav
Why, what and how
ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA
Need for monitoring of the tissue parameters after transplantation
Two primary characteristics of tissue are considered for measurement:
Bioimpedance Temperature
Healing process is relatively slow Multiple simultaneous measurement points are needed Shortened pulse wave measurement of impedance and
temperature is used Primary circuitry is preferably close to tissue sample RF link to data collecting and processing equipment - PC
In July of 1886 Oliver Heaviside introduced term
"impedance"
In 1893, Arthur Edwin Kennelly presented a paper
on 'Impedance" to the American Institute of
Electrical Engineers in which he discussed the first use of complex numbers as applied to Ohm's Law in
alternating current circuit theory.
Electrical impedance is a measure of opposition to a sinusoidal electric current.
How to measure tissue parameters? Impedance
Temperature
ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA
Bioimpedance Essentials of the Electrical
Bioimpedance EBI
3-element equivalent of the static EBI
phasor diagram of the static EBI for two frequencies, low ωl and high ωh.
rint C
rext
–Im Ż X Imaginary part
R Real part Re Ż
Ż(ωl) = R1 (ωl) + j X1 (ωl)
Re Ż + j Im Ż R* C*
true complex frequency response
two-element approximation
Ż(ωh)
ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA
Design choices
ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA
Excitation waveform Analog versus Digital Current sources Multisensor or small single device Electrodes Current consumption Frequency range and Impedance range Speed User interface and representation of results
Sinusoidal signal(s) and square wave
ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA
4.0
-1.0
0.0
1.0
2.0
3.0
100001 10 100 1000
0.01
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0.00
100001 10 100 1000
Frequency responses (kHz) of relative magnitude errors (a) and phase errors (b) caused by odd harmonics of the order of 3, 5, 7, 9, 11, and 13 in the case of using regular rectangular waveforms for both, excitation and
reference signals when measuring 3-element bioimpedance equivalent with a pole at f1 = 160 kHz, and a zero at f2 = 2.5•f1 in its frequency response.
(Multi) Sinusoidal signal: accurate measurements according to definition not very easy to generate
Square wave signal: extremely easy to generate with almost any digital circuitry large measurement errors due to higher harmonics
Shortened rectangular pulses
...5sin
5
5cos3sin
3
3cossin
1
cos4)( tttatf
1.0
-1.0
-0.5
0.0
0.5
1.00.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
The 1st harmonic
1.0
0.0
0.2
0.4
0.6
0.8
250 1 3 5 7 9 11 13 15 17 19 21 23
All the harmonics are
coinciding
Relative magnitude of harmonic
1.0
-1.0
-0.5
0.0
0.5
1.00.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 t/T
18º 30º The 1st harmonic
1.0
0.0
0.2
0.4
0.6
0.8
250 1 3 5 7 9 11 13 15 17 19 21 23
Relative magnitude of harmonic
The coinciding harmonics
1
sincos4
i
tii
ia
a is the constant amplitude value of the pulse signal, and
β characterizes the shortening of pulses and is equal to the signal’s zero value interval within one half period β = 0 … pi / 2
a) square wave excitation and demodulation b) same with shortened pulses
ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA
From almost analog…
ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA
VZ
Vref,Re
Iexc
Vref,Im
VRe
VImt;
Texc
18
30
90
Analog synchronous detection waveforms for synchronous detection
… to mostly digital -
ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA
VZ
t;
Texc 2+S
sampling(b)
(a)
Vref,ReIexc
VZ
DRe
t
Sampling instants
sampling and processing of the signal:
(a) uniform sampling with 6°distance between samples
(b) 366° undersamplingDigital multichannel solution
Device
ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA
Block diagram of the measurement unit
Electrodes
ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA
Needles from ordinary disposable hypodermic syringe
ETHICON Surgical Stainless Steel Suture - Temporary Pacing Wire
Current consumption
ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA
Design errors (LFO, SWPS) – 10 mA
AD8251 – 4x5mA
Oscillator (12 MHz) – 2 mA
Microcontroller (AVR) – 10 mA
Bluetooth – 15 mA
Power supplies – 10 mA
Average (Agilent 34410A) – 100 mA
Device start | communication start | measure
User interface and some results
ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA
270 ohms, all 4 channels
270 ohms parallel with 0,25 uF (2358 Hz)
Where to go from here
ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA
Final sensor unit to build, with: Current source Preamplifier Integrated electrode temperature probe
Search for new waveforms for measurement
Compact and meaningful representation of measured parameters
Search for even lower power solutions
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