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Biomedical Instrumentation
Sensors I
Instrumentation
• Devices that can be used to make a
measurement and give quantitative (or
sometimes qualitative) results
Biomedical Instrumentation
• Devices that can be used to make
biological or medical measurement and
give quantitative (or sometimes qualitative)
results
Biomedical Instrumentation
Examples
Future Biomedical
Instrumentation
• Tricorder (Star Trek)
– Completely noninvasive
– Internal and external
measurements
– Imaging
– Intelligence to make
diagnosis and
suggest therapy
Basic Biomedical Instrument
Sensor
Processor
Display
Storage
Fundamental Rules of
Biomedical Instrumentation
• Minimum disturbance to the physiological
system.
• Maintain simplicity
Control
And
feedback
Power
source
Sensor
Measurand
Primary
Sensing
element
Calibration
signal
Variable
Conversion
element
Signal
processing
Output
display
Data
storage
Data
transmission
Perceptible
output
Radiation,
electric current,
or other applied
energy
Generalized instrumentation system The sensor converts energy or
information from the measurand to another form (usually electric). This signal is
the processed and displayed so that humans can perceive the information.
Elements and connections shown by dashed lines are optional for some
applications.
Important Instrumentation Terms
• Sensitivity—Change in output as a function of a
change in input.
• Stability—Consistency in output for a constant input.
• Specificity—Ability to distinguish desired variable
from other competing variables.
• Accuracy—Difference between true value and
measured value divided by the true value.
• Precision—Number of distinguishable alternatives
from which a given result is selected
• Resolution—Smallest increment that can be
measured with certainty.
• Reproducibility—Same output for the same input.
Electrical
Thermal
Mechanical
Chemical
Optical
Hydraulic
Acoustic
Sensors
Types of Sensors
• Physical
• Chemical
• Bioanalytical
Physical
•
•
•
•
Strain gauge
Accelerometer
Load cell (force)
Pressure sensor
Potentiometers
Linear and rotational position measurement
• For example a potentiometer
– voltage is a fraction of supply voltage is typically a few V
– small gain required
– may require buffering with an amplifier to prevent
sensitivity to load impedance and long cables
• i.e. to make into voltage source
Vs
Vs
Ro 
R2
R1
R1
Vo 
Vs
R2  R1
x
 Vs
l
R2 R1
R2  R1
l
+
x
Potentiometer
Vo ' 
Ri
Ro ' ~ 0
Ri
Rf
Rf
Vo
Measurement techniques
• Resistance varies in relation to a parameter
– thermisters (temperature)
– potentiometers (position)
• use an amplifier to buffer against lower load
impedances
VS

?
 R f  Rs

Vo  1 
VS

Ri  R1  RS

R1
Vo
+
sensor
Vo  
RS
Ri
Rf
Vo 
Rf
Rs
VS
Ri R1  RS
Rf
Ri
VS
X
?
X
?
Measurement of small fractional changes
• Many sensors vary their electrical characteristic by a small
fraction over the range over which they are used
– thermister - resistance varies by 4%/ºC
– temperature IC varies by 1mA/K (i.e. 290-300 K = 3% change in i)
• A differential amplifier can help to interface this to an ADC (or
other device) to give a large fractional change
Wheatstone bridge
R3
R3
R1
R1
+
-
Vo
+
Rs
R2
RS
R2
x1
Vo'  Vo 
R2
Vs
R1  R2
For example:an electronic medical thermometer
• 4% per ºC
• At 40 ºC RS=10k
x 35 ºC to 40 ºC
– Approximately
linear
1.045=12.166kW
– Voltage at 40 ºC:
Vmax
Vmin
12166

 0.549VS
12166  10000
 0.5
• Variation is 0.049/0.549=9%
– 9% of ADC dynamic range is used
– 23 counts for 5 ºC
=0.22 ºC/count
0.4
0.3
128
0.2
0.1
35
36
37
38
39
temperature
VS
0
Vmax=0.549V
10 k
Vo
Vref
ADC
sensor
40
RS
Vo 
Rs
VS
R1  RS
ADC output
< 0.1 ºC sensitivity required
8-bit ADC
Thermister RS=10kW at 35 ºC
Temperature coefficient
v0 vmax
–
–
–
–
255
0.5

• Employs a thermister to
measure temperature in range
35-40 ºC.
– only levels 0-232 are
unused:
• For this application
– voltage across thermister is
high
– ADC has high input
impedance
– ADC is close to thermister
• Thermister voltage can be
applied directly to ADC
without amplification
• But the poor use of the
ADC
dynamic range means that
the required resolution (0.1
250
v0 vmax
• Or in graphical format
245
240
0.22
0.5
35
36 37 38 39
temperature
VS
10 k
Vo
ADC
sensor R
S
Vo 
Rs
VS
R1  RS
235
40
ADC output
255

0.549
• A solution is to subtract the bias voltage with a differential
amplifier (slide 93)
• The voltage range is unchanged, but the voltage at the
ADC input at the minimum temperature of 35 ºC is now
zero volts
• Using a differential amplifier with a gain of 10 with a Vref of
0.5V is now 0.49V, making good use of the ADC dynamic
VS
range
10k
0.5VS
10k
Vo
VS/2
RS
10k
V’o
+
x10
-
Vref
ADC
 RS T 
1 

 10 4
 V
 10  R T  2  S
S


Vmin  0
Vo'
Vmax  0.49 VS
Strain Gage
• When external forces are applied to a
stationary object, stress and strain are the
result.
– Stress is defined as the object's internal
resisting forces, and
– strain is defined as the displacement and
deformation that occur.
Stress
• Stress can be calculated stress can be
calculated by dividing the force (F) applied
by the unit area (A):
F
Stress  
A
F
Stress  
A

F

F
• Strain is defined as the amount of
deformation per unit length of an object
when a load is applied.
– Strain is calculated by dividing the total
deformation of the original length by the
original length (L):
L
Strain  
L
L
Strain  
L

F

F
L
L
• The ideal strain gage is
–
–
–
–
small in size and mass,
low in cost,
easily attached, and
highly sensitive to strain
but insensitive to
ambient or process
temperature variations.
Typical bonded strain-gage units
Arrows above units show direction
of maximal sensitivity to strain.
Accelerometers
• Accelerometers are
devices that
produce voltage
signals in proportion
to the acceleration
experienced.
• The most general approach to
acceleration measurement is to take
advantage of Newton's law, which states
that any mass that undergoes an
acceleration is responding to a force given
by F = ma.
• Piezoelectric
accelerometers are on
the market, and are
primarily offered for
vibration measurement.
– For moderate signals
(milli-gs), fairly small
devices with simple
circuits are quite
sufficient, so these
devices can be in the
10-100 dollar range.
Pressure sensors
• Aside from some fairly
exotic approaches,
pressure sensors all
operate on the basis of
the same principle: the
detection of a physical
force which arises due
to pressure.
Flow
• There are three basic approaches to the
measurement of flow.
– The first of these categories involves the use
of thermal effects to measure fluid motion.
• In general, this approach uses a heat source to
deposit heat into the fluid, and a thermometer to
measure the temperature of the fluid.
• If the heat source is upstream of the sensor, flow
increases heat transport and causes the sensor
temperature to increase.
• Another possible arrangement is to heat a
thermistor with a fixed power, and measure its
temperature.
• In this case, fluid flow acts to cool the thermometer.
• A slightly more complicated approach
relies on Bernoulli's Equation, which is:
v
v
P1 
 gh1  P2 
 gh2
2
2
2
1
2
2
• This roughly states that
the Pressure + the
kinetic energy density +
the gravitational
potential energy density
is a constant
throughout a fluid.
• This principle is applied by measuring pressure
at a pair of points in a fluid.
– When water flows through a pipe with a varying
diameter, the total flow rate in each region is a
constant (since the fluid must all get through the
tube).
– Therefore, changes in tube diameter are
compensated for by changes in fluid velocity.
– By measuring the pressure in regions with different
diameter, it is possible to measure fluid velocity.
• The last technique for flow measurement
is based on measurement of Doppler
effects in sound transport.
• Since sound is carried by pressure waves in a
medium (the fluid), its transport laterally across a
channel is affected by the motion of the fluid.
– It is possible to measure the change in sound
frequency due to fluid motion (direct Doppler effect, or
listen for changes in the travel time from transmitter to
receiver.
• High sensitivity techniques generally measure
frequency shifts, since excellent accuracy may be
obtained by use of analog or digital signal processing
techniques to measure small frequency shifts.
Chemical
•
•
•
•
Oxygen electrode
Glass electrode (pH)
Ion-selective electrode
CO2 sensor
• The human nasal
sensing apparatus
contains a
remarkably flexible
and sensitive
detection capability.
• You are capable of detecting and
distinguishing thousands of different
smells with almost instantaneous
recognition.
– Odor detection is made very complicated
because of the lack of actual uniqueness in
the chemical basis of most smells.
• There is no "garlic molecule" that is distinct from
the "enchilada molecule"; yet you can easily
distinguish these smells.
• If a chemical sensing application requires
detection of a particular molecule, several
techniques are available.
– These techniques are based on the unique
properties of particular molecules.
• Molecules may be
recognized by their
mass (or mass
spectra...).
– A mass spectrometers
are used to detect and
distinguish molecules.
• Gas chromatography instruments are
available.
– In these instruments, the varying molecular
diffusivities are used to detect specific
molecules.
Bioanalytical
• Glucose sensor
• Lactate sensor