Introduction and familiarity with different types of temperature controllers
What is the temperature?
Temperature is obtained by quantitative measurement of heat (amount of heat). Celsius and Fahrenheit are both scales and units of temperature.
Celsius and Fahrenheit are the scale and unit of temperature. They are relative values for freezing and boiling points, and it is important to note that the main criterion for freezing and boiling points is water. These scales are useful in many situations and are used all over the world.
The Celsius Scale takes its name from the Swedish astronomer Andres Celsius, who introduced his discoveries and observations about the two degrees on the thermometer to the world of science in 1742. It was originally called the Celsius scale, but due to some issues, it was officially renamed Celsius with the symbol ° C. Many countries use this scale as the main and standard scale for measuring temperature due to its ease of use.
The Fahrenheit scale was proposed by the German physicist Daniel Gabriel Fahrenheit in 1724. This scale was first used for climatic, industrial and medical purposes and mainly in the West in the 1960s. But little by little, the scale became common in most countries. Fahrenheit is still used in other countries, such as the United States. By adopting this system, in fact, negative temperature readings are minimized.
The difference between Celsius and Fahrenheit
The main difference between the temperature scales lies in the relative value of freezing and boiling points. In the Celsius scale the boiling point and freezing point of water C ° 100 C ° 0 ‘s. At the Fahrenheit scale, the boiling point of water is 212 degrees F and the freezing point is 32 degrees F. Although it seems easier to understand the Celsius scale than the Fahrenheit scale, the Celsius scale has fewer points between the freezing point and the boiling point, which means that the actual temperature changes can be greater. This makes decimals or fractions on the Celsius scale very important.
Another difference between the two scales is that the Fahrenheit scale is used in the imperial measurement system while the Celsius scale is measured in the metric system.
It does not really matter what scale is used to measure, because it can be converted to the equivalent of Fahrenheit or Celsius, and get the same temperature. So to convert Celsius to Fahrenheit, read the number Multiply 5.9 and add 32. To convert Fahrenheit to Celsius, subtract 32 from the reading and multiply the result by 9.5.
What is a temperature controller?
A temperature controller is a device used to control a heater or heating device. These devices compare the output of the temperature sensor with an adjustable value and control the temperature by performing calculations commensurate with the amount of deviation from each other. Equipment that works with sensors other than temperature, such as pressure sensors, humidity, flow, etc., are called controllers. Electronic controllers are specifically called digital controllers.
Temperature controllers control the temperature so that the value of the process is equal to the set value, but the results will often vary due to the properties of the controlled object as well as how it is controlled. Consider Figure 2 as an example, where the process value quickly reaches the set point without jumping, requiring a temperature controller. Note also in Figure 1, where the temperature rises very rapidly but causes a jump in temperature, or in Figure 3, where the temperature rises slowly.
1- The answer that the value of the process reaches the set value very quickly but has temperature fluctuations.
2- Appropriate answer
3- Answer in which the value of the late process reaches the set value.
The following example shows a simple combination of temperature control.
First, let us recall the phrases we need to control the temperature:
|Set point||SP||The temperature we need to reach|
|Set Value||SV||All the values that are suitable for us according to SP|
|Present Value||PV||Current temperature measured using sensors|
|Manipulated Value||MV||The amount of output that is intended as the output of the temperature controller to turn on or off the output equipment such as heater|
How the temperature controller works
There are generally two major types of controls.
- Sequential Control
In this control method, a series of predetermined processes are performed in sequence.
- Feedback Control
Processes that are constantly being re-examined and several things are being done over and over and over and over again. In this case, our final value is always being evaluated.
Principles of temperature control
The following figure shows an example of a feedback control system for temperature control. The main parts of the feedback control system are designed and built inside a temperature controller. A feedback control system can be built and the temperature can be controlled by combining a controller and a sensor that fits the subject to be controlled.
Combining a feedback control system
Control specifications in the temperature control process – Specifications of the controlled object
In order to perform a temperature control process well, we need to know the control specifications for temperature regulation
and use those specifications and features to purchase a temperature controller and a temperature sensor.
- heat capacity
Heat capacity varies for different objects.
The heat required to heat a food container is far less than the heat required to heat a food pot.
That is, to heat an object, the mass and type of that object are very important.
Heat capacity is by definition equal to the heat required to raise the temperature of an object by one degree Celsius or Fahrenheit (C / ° F °).
- Static specifications
When we put two dishes with the same contents on the fire or stove, the
dish that heats up with more heat and flame, heats up faster and the maximum temperature that will be in its steady
state (the case where the temperature of the dish does not change much anymore). , Far more than any other dish.
In other words, the final temperature will vary according to the heat output and the intensity of the flame.
This relationship between the amount of heat (heat capacity) and the final temperature is called the static characteristic.
- Dynamic specifications
The speed at which the food reaches the final temperature depends on the material of the dish.
In other words, with the same heat, a copper vessel reaches the final temperature faster than an iron vessel, and an iron vessel also reaches the final temperature faster than an earthenware vessel. The rate of temperature increase is called a dynamic characteristic.
- External disturbances
An activity that leads to disturbance of temperature stability and stagnation is called external disturbance.
For example, if we pour a pitcher of cold water on a container that contains food and has reached a constant temperature due to heat, the temperature of the container decreases rapidly and must return to its final state over time.
Temperature control methods
Temperature control methods using a temperature controller can be divided into the following two categories:
- Discrete temperature control (ON / OFF control)
In this method, the temperature is adjusted simply by turning the heater on and off.
In this method, the
heater is turned on when the temperature is lower than the specified temperature, and the heater is turned off when the temperature is higher than the specified temperature.
This type of control, in which the temperature is controlled by turning the heater on and off according to the relationship between the current
temperature and the desired temperature, is called ON / OFF control.
ON / OFF control may lead to some phenomena such as overshoot or hunting Hunting. To prevent these phenomena from occurring, it is better to control more carefully.
However, due to the ON / OFF control method of overheating and oscillating motion, this method is used for cases that do not require high precision, and it can be said that it is an old and outdated method.
The vertical axis is temperature. The horizontal axis is time.
When the temperature reaches the desired value, the heater turns off.
When the temperature drops below the desired value, the heater turns on.
This behavior is done continuously.
Overshoot (Overshooting): jumps higher than the desired temperature shot Orr said.
In some industrial applications where overheating leads to damage, we must control the system overflow.
Swing (Hunting): a phenomenon in which the temperature near the desired value and switch between higher and lower than the desired value change.
- Temperature control PID Control_ PID
PID controller or PID controller (PID controller) is one of the most common control algorithms that is used in many control processes such as:
- DC motor speed control
- Pressure control
- Temperature control
- And other cases
PID stands for Proportional, Integral, Derivative.
PID controller is a tool used for control applications in industry to regulate temperature, flow, pressure, speed and other process variables. PID controllers (Proportional-Integral-Derivative) controllers use a feedback loop mechanism to control process variables and are considered to be one of the most accurate and stable controllers.
PID control is a well-known way to steer the system to a target level or position. This is a practical solution that is always available for temperature control, which in addition to automation in scientific and chemical processes, is also extremely effective. A PID controller uses a loop-closed control feedback to keep the output value as close as possible to the preset output or target.
A PID temperature controller, as its name implies, is a tool that is mainly used to control the temperature without extensive intervention by the operator. A PID controller in a temperature control system uses a temperature sensor such as a thermocouple or RD as input and compares the actual temperature with the intended control temperature; It then sends a corresponding output to a control element.
What is a Digital PID Controller?
A digital PID controller receives the signal from a sensor, usually a thermocouple, or RTD, and displays it digitally after converting the scale to engineering units such as Fahrenheit or Celsius.
How does a PID controller work?
A proportional-integral-derivative controller (PID) can be used to control temperature, pressure, flow and other process variables. A PID controller, as its name implies, combines proportional control with integral and derivative corrections, thereby enabling automatic compensation of any changes in the system.
In this method, compared to the ON / OFF method, we reach the desired temperature with less speed, but it has less fluctuations and Orshot is minimized. In this method, it is possible for us to reach the desired temperature with high accuracy after reaching the desired temperature. Keep a constant value. So in this method the control characteristics are better than the previous method and we will not have overshot and hunting.
Also, if a disturbance enters the system through external disturbances, the system quickly returns to its desired state.
In the image below, you can see an overview of temperature control by PID method.
PID control is done by setting three coefficients.
Proportional Band (P), Integral Time (I), and Derivative Time (D)
In order to control the temperature by PID method, we have to set these three coefficients in the controller.
It should be noted that if these coefficients are not selected correctly, the result of PID control may be much weaker and worse than the ON / OFF method; for example, reaching the desired temperature may take a long time or the fluctuations may be large and may We never reach a constant value.
PID controller operating principles
The basic rule behind the operation of a PID controller is that proportional, integral, and derivative expressions must be set or “tuned” separately. A correction factor is calculated based on the difference between these values and applied to the input. For example, if the temperature of an oven is colder than required, the heat will rise. These three steps are explained here:
- Proportional adjustment
Involves correcting a goal in proportion to the extent of the difference; Therefore, the goal value is never reached because the closer the difference is to zero, the lower the proportion of the corrective action.
- Integral adjustment
By accumulating the error resulting from the “P” operator, it tries to improve this situation with the aim of increasing the correction factor. For example, if the oven temperature remains below the desired level, the “I” operator acts to increase the delivered heat. However, the “I” operator, after reaching the intended target, tries to reduce the accumulated error to zero, which will lead to an oversteer.
- Derivative adjustment
It tries to minimize this overshoot by reducing the correction factor.
The basis of the work of the PID controller
A proportional-integral-derivative controller (PID) can be used as a tool to control temperature, pressure, flow and other process variables. A PID controller, as its name implies, combines proportional control with integral and derivative corrections, thereby enabling automatic compensation of any changes in the system.
Types of PID controllers
There are three general types of PID controllers: On-Off, Proportional, and PID. Depending on the type of system in need of control, the user can choose one of these control processes.
- On / Off controller
An on / off PID controller is the simplest type of temperature control device. The output of this device is either off or on and there is no middle ground. An on / off controller changes the output status only when the temperature exceeds the preset value. A special type of on-off controller is a limit controller. These types of controllers use a holding relay. These relays are used to deactivate the process when it reaches a certain temperature, and they must be reset manually.
- Proportional controller
Proportional controllers are designed to eliminate the cycles associated with on / off controllers. A proportional control reduces the average power fed to the heater as the temperature approaches the set point. This reduces the heat generated by the heater to prevent it from overheating at the set point and keeping the temperature at a stable point near the set point. Achieving this proportional function is done by turning the output on and off at short intervals. This “temporal proportion” will vary depending on the ratio of “clear” time to “off” time.
- Standard PID controller
The standard PID controller combines proportional control with integral and derivative control, thus enabling automatic compensation of any changes in the system. These integral and derivative corrections are expressed in time-based units; They are also referred to using their equations, ie resets and rates, respectively. Proportional, integral, and derivative expressions must be set or “tuned” separately for a given system by trial and error. Of the three types of controllers mentioned, PID controllers provide the most accurate and stable control.
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