Automatic Temperature Control of Furnace Block Diagram
Heater driver
Heater
Sensor
Amplifier
(Power)
Error Amplifier
PID/ Temp.
Set temperature
Heater driver: The heater is a coil which gets heated up by receiving power from heater driver. It is a voltage supply of the heater. This heater driver voltage can be controlled by PID controller. Heater: It is a coil of wire which is powered by a voltage source. Sensor: A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. The seven basic types of temperature sensors.
Thermocouples Thermocouples are voltage devices that indicate temperature by measuring a change in voltage. As temperature goes up, the o utput voltage of the thermocouple rises - not necessarily linearly. Often the thermocouple is located inside a metal or ceramic shield that protects it from exposure to a variety of environments. Metal-sheathed t hermocoup hermocouples les also are available with many types of outer coatings, such as Te flon, flon, for trouble-free t rouble-free use in acids and strong caustic so lutions. lutions. Resistive Temperature Devices Resistive temperature devices also are electrical. Rather than using a vo ltage as the thermocouple does, they take advantage of another characteristic of matter which changes with temperature its resistance. The two types of resistive devices we dea l with at OMEGA Engineering, Inc., in Stamford, Conn., are metallic, resistive temperature devices (RTDs) and t hermistors. In general, RTDs are more linear than are thermocouples. They increase in a positive direction, with resistance going up as temperature t emperature rises. On the other hand, the thermistor has an entirely different type of construction. It is an extre mely nonlinear semi conductive device that will decrease in resistance as temperature rises.
Infrared Sensors Infrared sensors are non-contacting sensors. As an example, if you hold up a typical infrared sensor to the front of your desk without contact, the sensor will tell you t he temperature of the desk by virtue of its radiation - probably 68°F at normal room temperature. In a non-contacting measurement of ice water, it will measure slightly under 0°C because of evaporation, which slightly lowers the expected temperature reading. Bimetallic Devices Bimetallic devices take advantage o f the expansion of metals when they are heated. In these devices, two metals are bonded together and mechanically linked to a pointer. When heated, one side of the bimetallic strip will expand more than the other. And when geared properly to a pointer, the temperature is indicated. Advantages of bimetallic devices are po rtability and independence from a po wer supply. However, they are not usually quite as accurate as are electrical devices, and you cannot easily record the temperature value as with e lectrical devices like thermocouples or RTDs; but portability is a definite advantage for the r ight application. Thermometers Thermometers are well-known liquid expansion devices. Generally speaking, they come in two main classifications: the mercury type and the o rganic, usually red, liquid type. The distinction between the two is notable, because mercury devices have certain limitations when it comes to how they can be safely t ransported or shipped. For example, mercury is considered an environmental contaminant, so breakage can be hazardous. Be sure to check the current restrictions for air transportation of mercury products before shipping. Change-of-state Sensors Change-of-state temperature sensors measure just that - a change in the state of a material brought about by a change in temperature, as in a change from ice to water and then to steam. Commercially available devices of this type are in the form of labels, pellets, crayons, or lacquers. For example, labels may be used on steam traps. When the trap needs adjustment, it becomes hot; then, the white dot o n the label will indicate the temperature rise by turning black. The dot remains black, even if the temperature returns to normal. Change-of-state labels indicate temperature in °F and °C. With these types of devices, the white dot turns black when exceeding the temperature shown; and it is a nonreversible sensor which remains black once it changes color. Temperature labels are useful when you need confirmation
that temperature did not exceed a certain level, perhaps for engineering or legal reasons during shipment. Because change-of-state devices are non electrical like the bimetallic strip, they have an advantage in certain applications. Some forms of this family of sensors (lacquer, crayons) do not change color; the marks made by them simply disappear. The pe llet version becomes visually deformed or melts away completely. Limitations include a relatively slow response time. T herefore, if you have a temperature spike going up and then down very quickly, there may be no visible response. Accuracy also is not as high as with most of the other devices more commonly used in industry. Ho wever, within their realm of application where you need a non-reversing indication that does not require electrical power, they are very practical. Other labels which are reversible operate o n quite a different principle using a liquid crystal display. The display changes from black color to a tint of brown or blue or green, depending on the temperature achieved. For example, a typical label is all black when below the t emperatures that are sensed. As the temperature rises, a color will appear at, say, the 33°F spot - first as blue, then green, and finally brown as it passes through the designated temperature. In any particular liquid crystal device, you usually will see two co lor spots adjacent to each other - the blue one slightly below the temperature indicator, and the brown one slightly above. This lets you estimate the temperature as being, say, between 85° and 90°F. Although it is not perfectly precise, it does have the advantages of being a small, rugged, nonelectrical indicator that continuously updates temperature. Silicon Diode The silicon diode sensor is a device that has been developed specifically for the cryogenic temperature range. Essentially, they are linear devices where the conductivity of the d iode increases linearly in the low cryogenic regions. Whatever sensor you select, it will not likely be operating by itself. Since most sensor choices overlap in temperature range and accuracy, selection of the sensor will depend o n how it will be integrated into a system. Amplifier: Amplifier of required gain can be designed. S ince the voltage across the thermocouple is in microvolts it needs amplification. Error Amplifier: It has two inputs, one from the sensor, other is set t emperature signal. It amplifies the difference between the two signals and gives output accordingly. This out put is given to the PID which ultimately controls the voltage input of heater. PID
A proportional±integral±derivative controller (PID controller) is a generic control loop feedback mechanism (controller) widely used in industrial control systems ± a PID is the most commonly used feedback controller. A PID controller calculates an "error" value as the difference between a measured process variable and a desired set po int. The controller attempts to minimize the error by adjusting the process control inputs.
ON/OFF Control Action (For heating): The relay is µON¶ up to the set temperature and cuts µOFF¶ above the set temperature. As the temperature of the system drops, the relay is switched µON¶ at a temperature slightly lower than the point.
