Biomedical Electrodes, Sensors, and Transducers. Definition of Biomedical Electrodes, Sensors, and Transducers. Electrode: Sensor: Transducer:

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Biomedical Electrodes, Sensors, and Transducers from: Chaterjee, Biomedical Instrumentation, chapter 6 Key Points Electrodes, Sensors, and Transducers: - types of electrodes - voltaic - electrolytic

1 Biomedical Electrodes, Sensors, and Transducers from: Chaterjee, Biomedical Instrumentation, chapter 6 Key Points Electrodes, Sensors, and Transducers: - types of electrodes - voltaic - electrolytic - Ag/AgCl electrodes - know the characteristics of sensors: - sensitivity - static & dynamic error - accuracy - precision - hysteresis - frequency response - resolution - pressure transducers - temperature transducers (esp thermistor vs thermocouple) - advantages of the instrumentation amplifier D. J. McMahon rev cewood Definition of Biomedical Electrodes, Sensors, and Transducers Electrode: Solid electric conductor Detects electrical activity Sensor: Detects change in physical stimulus Turns it into a signal Transducer: Converts energy types

2 block diagram of human-machine interface Biomedical Electrodes Electrode: Conductor Makes contact with a nonmetal Electrochemical cell Electrode is anode or cathode 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 voltaic cell: generates electrical current electrolytic cell: Uses the electrical current

3 Silver/Silver Chloride Reference Electrode Stable reference electrode Silver/silver chloride electrode silver/silver chloride button electrodes A view of a silver/silver chloride electrode

Types of Electrodes Include: Polarizable or nonpolarizable

monopolar or bipolar Provides a low-offset, low impedance

skin. Needle electrodes (typical for EMG) Cup electrodes (typical

electrodes cup electrodes (typ. EEG) needle electrode (typ. EMG).

4 Types of Electrodes Include: Polarizable or nonpolarizable Electrode shapes: button or bar types Electrode configurations: monopolar or bipolar Provides a low-offset, low impedance interface with the monitor with minimal effect on the patient s skin. Needle electrodes (typical for EMG) Cup electrodes (typical for EEG) Skin-surface electrodes (everything else) Skin-surface electrodes cup electrodes (typ. EEG) needle electrode (typ. EMG).

5 Classification of Sensors and Transducers If based upon stimulus: Mechanical Acoustic Electric Magnetic Optical Thermal Radiologic 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 Hysteresis Unable to repeat data in opposite direction Performance Characteristics (more) Accuracy How close output is to true value Reproducibility (precision) Ability to produce identical output values At different times under the same conditions Frequency response Range of frequencies Bandwidth 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

Linear Variable Differential Transformer A simple LVDT transducer: movement or displacement A balanced bridge circuit: Both gauges affected by what we don t want to measure.

6 Linear Variable Differential Transformer A simple LVDT transducer: movement or displacement A balanced bridge circuit: Both gauges affected by what we don t want to measure. Only active gauge is affected by what we do want to measure. Output voltage in a LVDT transducer LVDT in an amplifier circuit axis of s-t-r-e-t-c-h Flow Transducers Measure flow of gases, liquids, or solids Given as: Q = (area) (velocity) don t confuse that with: Blood flow detection by ultrasound & Doppler shift: A simple flow transducer RBC is also called formed element, larger than other blood components

7 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 simple thermocouple transducer 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

8 Summary (more) 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

1. A transducer converts

1. A transducer converts a. temperature to resistance b. force into current c. position into voltage d. one form of energy to another 2. Whose of the following transducers the output is a change in resistance?