Biomedical Electrodes,
Sensors, and Transducers
from: Chaterjee, Biomedical Instrumentation, chapter 6
© D. J. McMahon 2014
rev 140928
Definition of Biomedical Electrodes,
Sensors, and Transducers
• Electrode:
– Solid electric conductor
– Detects electrical activity
• Transducer:
– Converts energy types
• Sensor:
– Detects change in physical stimulus
• Turns it into a signal
FIGURE 6-1 Various sensors
FIGURE 6-3 A block diagram of human-machine interface
Biomedical Electrodes
• Electrode:
– Conductor
– Makes contact with
a nonmetal
• Electrochemical cell
– Electrode is anode
or cathode
FIGURE 6-4 Charge density in the
muscle-electrode interface
Theory of Electrodes
• Electrochemical cell is divided into:
– Voltaic cell
• Generates electrical current
• Chemical reaction: two half reactions
– Electrolytic cell
• Uses the electrical current
FIGURE 6-6 A voltaic cell
FIGURE 6-7 An electrolytic cell
FIGURE 6-8 A table
of half-cell potentials
Silver/Silver Chloride Reference
Electrode
• Stable reference electrode
FIGURE 6-12a Silver/silver chloride electrode
FIGURE 6-12c A photo of silver/silver chloride
button electrodes
FIGURE 6-12b A view of a
silver/silver chloride electrode
http://solutions.3m.com/wps/portal/3M/en_US/3M-Patient-Monitoring-NA/electrodes-cablesleadwires/products/product-catalog/~/Patient-Monitoring-roducts?N=6271+5005428&rt=c3&Nao=30
Provides a low-offset, low impedance interface with
the monitor with minimal effect on the patient’s skin.
Types of Electrodes
• Include:
–
–
–
–
–
–
Polarizable or nonpolarizable
Electrode shapes: button or bar types
Electrode configurations: monopolar or bipolar
Skin-surface electrodes
Needle electrodes
Cup electrodes
FIGURE 6-13a A surface electrode
FIGURE 6-13b A needle electrode.
FIGURE 6-13c A cup electrode
Classification of Sensors and
Transducers
• If based upon stimulus:
–
–
–
–
–
–
Mechanical
Electric
Magnetic
Optical
Thermal
Acoustic
Performance Characteristics of
Electrodes, Sensors, and Transducers
• Sensitivity
– Output change divided by input change
• Static and dynamic error
– Difference between measured and actual value
• Static error: measured value doesn’t change
• Dynamic error: measured value changes
Performance Characteristics
(cont’d.)
• Accuracy
– How close output is to true value
• Hysteresis
– Unable to repeat data in opposite direction
• Frequency response
– Range of frequencies
• Bandwidth
Performance Characteristics
(cont’d.)
• Reproducibility
– Ability to produce identical output values
• At different times under the same conditions
• Resolution
– Smallest step of meaningful output
• Measurable over range
Pressure Transducers
• Pressure
– Force per unit area
• Types of pressure transducers:
– Metal strain gauge
– Linear variable differential transformer (LVDT)
FIGURE 6-15a A simple metal gauge
FIGURE 6-15b Backing
material of a metal gauge
FIGURE 6-16a A simple LVDT transducer
FIGURE 6-15c A balanced bridge
circuit with a metal gauge
FIGURE 6-16c LVDT in an amplifier circuit
FIGURE 6-16b Output voltage in a
LVDT transducer
Flow Transducers
• Measure flow of gases, liquids, or solids
– Given as:
• Q = (area) (velocity)
FIGURE 6-17
A simple flow
transducer
Blood flow detection by ultrasound & Doppler shift:
Temperature Transducers
• Types include:
– Resistance temperature detector (RTD)
• Metal wire resistance increases with temperature
– Thermistor
• Semiconductor device
• Negative or positive temperature coefficient
– Thermocouple
• Converts temperature reading into voltage reading
• Uses two dissimilar metals: Seebeck effect
FIGURE 6-19a A simple
thermocouple transducer
FIGURE 6-19b A thermocouple with ice bath
Optical Transducers
• Don’t require direct physical contact
– Avoid disturbances
• Photoconductive cell
– Common type
– Uses reflection of light
• Detects distance or level
Summary
• Acquiring biomedical signals
– Electrode or transducer is essential
• Electrode
– Solid electric conductor
• Sensor senses stimulus
• Strain gauge and LVDT
– Pressure sensors
• RTD, thermistor, and thermocouple
– Temperature sensors
Summary (cont’d.)
• Strain gauge
– Determines amount of strain
• Most common: metal foil gauges
• LVDT
– Provides output voltage proportional to displacement
• RTDs or thermistors
– Resistance varies with temperature
• Thermocouples
– Voltage varies with temperature