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Exam 1: Closed Notes
Terms in this set (11)
1. Describe what sensors are and give several examples of sensors including at least one related to biomedical engineering.
A device that responds to a physical stimulus in a useful way
Ex: Heart rate monitor, accelerometer, pressure sensor
4. Give an example of a transducer.
Loudspeakers - transforms electric signals into sound energy
With regard to sensors, describe what is meant by the terms stimulus and measurand.
Stimulus - The quantity that is sensed, the input to a sensor
Measurand - Another name for stimulus
Define what is meant by the phrase systems perspective.
Systems Perspective: a perspective that focuses on the behavior and interaction of the sub-systems that make up the system of interest
Express the ratio of power and field quantities in decibels (dB) and perform simple calculations involving decibels.
Power: (power, power density, energy)
P0 is the reference power value.
Field related: (voltage, current, pressure, force)
V0 is the reference value
Recognize and discuss a type of transfer function, for example linear or non-linear, monotonic or non-monotonic, or one or multi-dimensional.
Linear vs. Non-linear like math. Monotonic is one x value for every y value. Non-monotonic are like parabolas, multiple x values for y (x and y are interchangeable). One dimensional is one input and one output. Multi-dimensional are multiple inputs with a single output.
Name and briefly describe 3 broad categories of electrical temperature sensors.
Thermoelectric sensors: thermocouples, that are 2 dissimilar materials welded together and connected to a micro-voltmeter
Thermoresistive: Conductor based: RTD (resistive temperature detector), typically lower resistance (<1000 ohms). Traditionally metal wire, then thin metal film, now silicon
Semiconductor based: thermistors (thermal resistors), typically higher resistance (>1000 ohms). NTC (negative temperature coefficient) are more common, opposed to PTC (positive temperature coefficient)
Other: Based on secondary effects like speed of sound and phase of light, non-contact sensing, expansion of metals, phase changes
Describe the construction of thin film RTDs.
Produced by depositing a thin layer of the material on a stable, electrically non-conducting, thermally conducting ceramic. May be calibrated by laser trimming.
Describe the general features, including transfer function and operational temperature range, of silicon resistive sensors.
-Limited range of operation
-Max range is -55 to 150 degrees, sometimes 300.
-Typically 1 kilo-ohm at 25 degrees.
-Can be calibrated
-Made as small chip with two encapsulated electrodes
-Acts sort of like diodes(polarized)
-Commonly used in integrated circuits (IC's), like modern CPU's.
-Conductivity increases with pressure.
-45 deg C to +85 deg C or ) deg C to +80 deg C
Discussion the general features, benefits and potential problems with thermistor temperature sensors.
-Most are NTC
-Based on oxides of semiconductors
-High temp coefficients
-Small, simple, inexpensive
-High resistance (usually)
-Has self-heating errors, but lower because resistance is higher (lower current)
-Wide range of resistances up to mega-ohms
-Can intentionally be used in self-heating mode
-Good repeatability and accuracy
-Temp range: -50 to 600 degrees C
-Linear for narrow ranges
-Many sizes, shapes, probes
-Less expensive than RTDs, and less repeatability.
Describe the general behavior of thermoelectric emf production and thermocouple temperature measurement. You may use the various thermoelectric effect names, but will not be required to distinguish among them.
-Peltier effect: Heat is deposited or absorbed at the junction of two dissimilar materials when electric current flow through the junction- applications in cooling and heating
-Thomson effect: A current carrying wire will absorb or radiate heat depending on the direction of the current, and if the wire is unevenly heated.
-Seebeck effect: An emf is produced across the junction of two dissimilar conducting materials connected together, the sum of Peltier and Thomson effects
-If both ends are connected (in a loop) and there is a temperature difference, a thermoelectric current will flow through the closed circuit, and will be generated.
-If an opening appears in the circuit, an emf (electromagnetic field) will be created across the opening, and is what is usually measured in the sensor.
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