Robotics/Sensors/Thermal Sensors

Robotics: Sensors: Thermal Sensors

Temperature can be an interesting parameter to measure. The temperature of the surroundings might not be very useful for a mobile robot, the temperature of a motor or battery back is.

What is temperature?
"Temperature: A measure proportional to the average translational kinetic energy associated with the disordered microscopic motion of atoms and molecules. The flow of heat is from a high temperature region toward a lower temperature region.

Units and their conversions
The most used units for temperature are Degrees Celsius, Kelvin, and Degrees Fahrenheit.

Degrees Celsius
This temperature scale is defined around water. It defines 0 °C as the temperature on which water freezes at standard atmospheric pressure. 100 °C is defined as the temperature at which water boils.

Conversion:
 * To Fahrenheit: Tc * 9/5 +32
 * To Kelvin: Tc + 273,15

Kelvin
Kelvin is a temperature scale with one defined temperature: 0 Kelvin is the lowest possible temperature, which is about -273,15 °C. An increase of 1 Kelvin has the same size as an increase of 1 °C. The Kelvin (note it's not degrees Kelvin) temperature scale is used in science and engineering, most formulas containing temperature require Kelvin, unless it involves temperature differences, then one can use both Celsius and Kelvin as the difference between 10 and 20 Kelvin and between 10 and 20°Celsius are equal.

Conversion:
 * To Fahrenheit: Tk *9/5 -459,67
 * To Celsius: Tk - 273,15

Fahrenheit
This scale uses the lowest temperate that could be measured at the time when this scale was developed and the temperature of the human body. The first temperature is defined as 0 °F the latter as 100 °F. This places 0 °C at 32 °F and 100 °C at 212 °F. A difference of 1 °F isn't the same size as a difference of 1 °C.

Conversion:
 * To Kelvin: (Tf + 459,67) * 5/9
 * To Celsius: (Tf - 32) * 5/9

Non-electric methods
Are any of these non-electric methods useful for robotics?

The old fashioned way to measure temperature uses the expansion of mercury in a glass capillary tube. Newer thermometers use other liquids like alcohol. Other methods use two thin metal strips with different thermal expansion rates fused together and are called bimetal or bimetallic.

Yet another way uses the expansion of gas or liquid or a gas-liquid combination in a closed metal capillary tube system, called gas thermometers and filled systems. Other ways use the change of phase of some materials, such as paints and pastes whereby their reflectivity or color changes permanently when a specific temperature is reached. These are often called temperature or thermal paints and labels.

Another method uses the variation in reflectivity of certain liquid crystals with temperature. One of the original thermometers, named after Galileo, uses several precision weights in labeled small glass globes, of known volume, all placed in a vertical cylinder of liquid like water, sufficient in size to allow all the globes to fit and with sufficient width to enable the globes to move freely past one another. As the temperature of the assembly changes, the density of the water changes and the globes assume differing heights in the cylinder according to their density with the globe in the highest position indicating the temperature to be at or below its labeled value.

Thermocouples
Thermocouples consist of a pair of wires of different metals connected together at both ends and insulated but kept thermally close to each other over their length. When they encounter a temperature gradient form one end to the other, an electrical current flows through the circuit due to the Seebeck Effect. If one of the wires is opened and the voltage between the open ends is measured with a suitably high impedance voltmeter, a voltage nearly equal to the average relative Seebeck coefficient times the temperature difference between the hot and cold ends will be measured.

Thermoelectric properties of many material combinations have been developed over the past hundred years or so and also have been standardized for several of them, more commonly known by their ISA letter designations, as standard. Their electrical outputs for cold end reference temperature at 32 Deg F and 0 deg C are widely published in ASTM Standard E230. Many sources on the web provide these tabular values including the National Institute for Science & Technology (NIST) in the USA.

Temperature sensitive resistors
Many electrically conductive materials have a resistance that varies with temperature—they are temperature sensitive resistors.

There are two generic types of temperature sensitive resistors: Positive Temperature Coefficient (PTC) devices and Negative Temperature Coefficient (NTC) devices.

Usually the PTC devices, if made of metal, are termed Resistance Temperature Detectors or RTDs. Ones made from other materials are usually called Thermistors, short for Thermal Resistors.

RTDs are a highly developed technology worldwide and several international standards exist for the most widely used type, those made of pure Platinum. They are usually called Platinum Resistance Thermometers, or simple PRTs. Very special PRTs are used as interpolating devices in the International Temperature Scale of 1990 (ITS-90), these and similar devices sold commercially are often known as Standard Platinum Resistance Thermometers, or SPRTs.

Tables for the most commonly used RTD materials, Platinum, Nickel and Copper, can be found in several International and national standards as well as many locations on the Web.

Thermistors are also very highly developed, but there are no uniform standards for their properties. Each supplier makes their own products and provide their own calibration tables. Some of these tables have been cataloged on the measurement database website.

Many popular thermistors are manufactured using material with a B of 3300 K or a material with a B of 5133 K or some other material with an intermediate value.

Independent of the material, most thermistors are manufactured such that the resistance at room temperature (298.15 K = 25 °C) is one of these preferred values: 1 kΩ, 2 kΩ, 5 kΩ, 10 kΩ, 20 kΩ, 50 kΩ, 100 kΩ, 200 kΩ, 500 kΩ, 1 MΩ.


 * PBASIC Programming/RCTIME and "RepRap: ExtruderIO" have more details on measuring the RC time constant of a capacitor + thermistor circuit; then from that time calculating the temperature.
 * Embedded Systems/Low-Voltage Circuits and "RepRap: Thermistor" have more details on measuring the output voltage of a "resistor divider" resistor + thermistor circuit; then from that voltage calculating the temperature.

Semiconductor
Semiconductors, such as diodes and transistors have properties that vary greatly with the temperature. This arises from the temperature sensitivity of the bandgap in such devices which is highly repeatable and very stable, especially in Silicon semiconductors.

Several Integrated Circuit (IC) manufacturers, such as Analog Devices, Dallas Semiconductors and National Semiconductors manufacture many variants of the basic diode temperature sensors with additional circuitry to scale the output for use with simple digital display meters.

Radiation Thermometer & Optical Pyrometer
The popular ear thermometer and low cost "laser" thermometer of today are variants of the radiation thermometer (pyrometer) that began its development in the 19th Century. The devices are all based on the thermal radiation properties of literally everything in nature, whether it be gas, liquid or solid. The physical processes at work inside all materials includes emission of electromagnetic radiation that is a function of the temperature and optical properties of an object.

The physical emission of thermal radiation was first described correctly by Max Planck in the last year of the 19th Century. His theory led to the quantum revolution in Physics. It has stood the test of time as an accurate and reliable theory of thermal emission of radiation and enables instrument designers to develop instruments that can measure temperature without contacting the object that is being measured with remarkable levels of accuracy and repeatability. There are very few standards for such devices.