Week 1: Thermal Sensors Flashcards
List some different types of sensors and potential applications for them.
Process sensors: petrochemical, pharmaceutical, semiconductor industries to measure and control chemical reactions
Position sensors: aircraft, vehicles
Motion sensors: smartphones, security systems
Ergonomic sensors: tablets, PCs, phones
Chemical sensors: fire, toxic vapors
Electrical sensors: lab instruments, security sensors
Temperature sensors
Hydraulic sensors
Pneumatic sensors
Flow sensors: metering, mixture sensing
Capacitive sensors: touchscreens
Light sensors: adaptive lighting
Compare and contrast electrical motors with mechanical actuators.
Electrical motors convert electrical energy to rotary motion.
Mechanical actuators (gears, belts, lead screws, etc.) either boost up the torque or they convert the rotary motion to linear motion.
What are the three flavors of electrical actuators?
Electrical actuators come in three flavors: stepper motor, brushed DC motor, and brushless DC motors.
How does a stepper motor work?
A stepper motor has multiple coils organized in phases that cause the motor to rotate in very precise steps.
Energizing the coils sequentially is what allows the stepper motor to rotate in very precise steps.
Describe the way a brushed DC motor works.
Specifically, how does it get continuous rotation?
What is the downside?
A brushed DC motor has a coil that generates a magnetic field around an armature (the component of an electric machine that carries alternating current). One side of the armature is repelled by a north pole magnet and the other side is attracted to the south pole, causing rotation.
The rotation is caused by the opposing forces which cause the armature to rotate in one direction. As the armature rotates in that direction, the torque eventually goes to zero and the magnetic field of the armature attracts to the north and south poles. Reversing current to the armature causes that side to be repelled by the south pole, but attracted to the north pole. Reversing the current exactly on that half cycle creates your continuous rotation.
The down side is that the brushes in the motor wear rapidly and have to be replaced at frequent intervals.
Describe how a brushless DC motor works.
Brushless DC motors have permanent magnets rotating around a fixed armature. An electronic controller switches the phase to the windings allowing rotation within the motor.
What is an actuator? What four classes of actuators are used in automobiles, production equipment, and home appliances?
An actuator is a component of a machine responsible for moving and controlling a part of the system. It requires a control signal and a source of energy.
The four classes are hydraulic, pneumatic, electrical, and mechanical.
What is excitation in terms of a sensor?
Excitation is when power is applied to a sensor so that it produces a usable output.
What functions does analog circuitry provide to accomplish signal conditioning?
Analog circuitry excites, filters, and amplifies the raw sensor signal. This process is called signal conditioning.
After signal conditioning with the analog circuitry, where does the signal get sent next?
After signal conditioning in the analog circuitry, the signal is sent to an analog to digital converter, or ADC.
What happens to the data outputted from the ADC?
The data from the ADC is usually converted to a serial format such as I2C, SPI, or RS 232.
Why is signal conditioning circuitry usually located very close to the sensor?
Raw outputs of most sensors are easily corrupted. By moving the signal conditioning circuitry closer to the sensor, one can help shield against external interference.
How does the nScope compare to a typical bench top digital oscilloscope?
The nScope is not nearly as fast or accurate. It’s power supply can only provide a modest amount of current and the signal generator goes only to 10 KHz.
Describe a thermocouple.
A thermocouple consists of two wires made form dissimilar metals joined at the heated end or junction, and connected to circuitry at the other end. The voltage read by the circuitry is proportional to the temperature at the heated junction.
What are some pros of using a thermocouple?
Thermocouples can measure up to 1800 degrees Celsius. They’re versatile due to the high temperature span for a single device. They are popular in metallurgy and semiconductor processes.
Describe the basic principle of an RTD.
RTD stands for resistance temperature detector. An RTD works on the principle that the resistance of a metal increases as the temperature increases. The most commonly used metal for this is Platinum whose resistance increases nearly linearly with temperature.
Compare and contrast an RTD to a thermocouple. What applications might use an RTD?
While thermocouples can measure up to 1800 degrees Celsius, RTDs can only measure temperatures up to 800 degrees Celsius. However, RTDs are much more stable and more precise than thermocouples. This makes them a good choice for applications where accuracy and repeatability are important. For example, pharmaceutical and biotechnology industries.
Describe the principle of a thermistor. What are the pros and cons?
Thermistors are made of centered semi-conductor or metal-oxide particles. NTC thermistors exhibit a large decrease in electrical resistance for a relatively small increase in temperature, meaning they are severely non-linear.
Thermistors are very accurate and the small package size makes them ideal for measuring temperature in phones and computers.
What’s the only non-contact temperature sensor? What is it used for?
Infrared sensors are the only non-contact temperature sensors. They’re excellent for measuring very high temperatures safely from afar and as such are commonly used to measure temperatures in hostile environments.
An infrared sensor calculates the surface temperature of an object from that object’s emitted infrared energy.
Which sensors have the highest measurement range of all the thermal sensors?
Thermocouples have the highest measurement range of all the sensors.
What is the downside to thermocouples?
Thermocouples are subject to drifting over time because their metallurgical properties change with large numbers of thermal cycles. This means they need to be re-calibrated yearly.
How does the stability of RTDs compare to thermocouples?
Thermocouples must be re-calibrated due to drift, but RTDs are stabled over many years and don’t need to be re-calibrated yearly.
How do you null out the effect of lead length on the resistance measurement of an RTD circuit?
You have to use either a three or four wire RTD circuit in order to null out the effects of lead length.
What are some strengths and limitations of thermistors?
Thermistors have attributes similar to RTDs, namely they’re accurate, stable, and easy to package. However, they measure only up to 200 degrees Celsius and because of their highly non-linear behavior, they need a lot of programming to handle them in your embedded system.
What are some challenges for getting accurate temperature measurements with infrared thermometers?
To get accurate surface temperatures of objects with an infrared thermometer, you have to know the emissivity of the surface. You also have to make sure that the surface you want to measure is fully contained within the field of view of the infrared lens. This can be very challenging when you’re very far away from the object.
