Biomedical Instruments Design
475 BMIS
Sensors (2)
Sensors
Sensor Types
Resistive
Inductive
Capacitive
Piezoelectric
Sensors
Sensor Types
Resistive
Inductive
Capacitive
Piezoelectric
Resistive Sensors - Potentiometer
 Potentiometer
 A displacement sensor
Displacement
Translation / Rotation
Resistance
Electrical Quantity
Voltage
Potentiometer
 Potentiometers produce output potential (voltage) change in response to input (e.g.,
displacement) changes
 typically formed with resistive elements e.g. carbon/metal film
 V = I R
 produce linear output in response to displacement
 Example potentiometric displacement sensors
 Translational: small (~mm) linear displacements
 Vo increases as xi increases
 Single-Turn: small (10-50º) rotational displacements
 vo increases as i increases
Resistive Sensors – Strain gages
 Strain gages
‫التواء‬
 A strain sensor
Strain
(Stretch)
Resistance
Electrical Quantity
Voltage
Resistive Sensors – Strain gages
 Consider: strain (stretch) a thin wire (~25µm)
 its length increases and its diameter decreases
 results in increasing resistance of the wire
 Can be used to measure extremely small displacements, on the order of nanometers
 For a rectangular wire
 Rline =  L / A
A = wt (area)
 : resistivity
Resistive Sensors – Strain gages
 Example of strain gage
50 kOhm
Resistive Sensors – Strain gages
 Example of strain gage
60 kOhm
Resistive Sensors – Strain gages
 Example of strain gage
90 kOhm
Exercise
We want to monitor the respiration of a patient using a biomedical instrument.
1. Identify the measurand in this context.
2. Which type of sensors is suitable for this application?
3. Propose a simple design that shows how the sensor is installed.
Vout (V)
4. The sensor converts the changing measurand into changing voltage. Draw
the waveform generated by the sensor.
Time (s)
Exercise
5. Choose the modes in which the device operates (continuous vs sampling)
6. Calculate the minimum sampling frequency suitable for this application.
7. Propose the bloc diagram of a circuit that measures the respiration rate.