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The operation of the temperature controller
   There are three basic types of controllers: the switch controller, the proportional controller and the PID controller. According to the system to be controlled, the operator will be able to use a type of type or other type of controller to control the process.

Switch controller
  The switch controller is the simplest kind of temperature control device. The output of the device is only "open" and "shut" two states, without any intermediate state. Only when the temperature exceeds the set value, the switch controller will switch the output. For heating control, the output is "open" when the temperature is lower than the set value, and the output is "off" when it is higher than the set value.
  Because the temperature will exceed the set value, the output state will be changed, so the process temperature will continue to cycle, from the set point to the set value, and then back to the set value. If this cycle occurs quickly, in order to prevent the damage of the contactor and valve, the switch difference or "lag" is added during the operation of the controller. This difference requires that the temperature will be closed or open again after a certain degree of temperature is exceeded. If it circulate very fast in the set value, the switch difference can prevent the output "repeatedly" switching or fast switching.
  Switch control is usually used without the need for precise control. It can be used in systems that fail to handle frequent open and closed energy sources, and is used when the system is very large and extremely slow in temperature change, or is used for temperature alarm.
  A special type of switch control for alerts is the restriction of the controller. The controller uses a closed - lock relay that must be manually reset and is used to end a certain process after a specific temperature is reached.

Proportional controller
  The proportional control is specially designed to eliminate the cycle associated with the switch control. The proportional controller can reduce the supply of the average electric quantity to the heater when the temperature is close to the set value. This can slow the heating of the heater so that the temperature will not exceed the set value, but it will be close to the set value and maintain at a stable temperature. This proportional control operation can be realized by opening and closing the output in a short time interval. This "time scale control" controls the temperature by changing the ratio of "open" time and "close" time. The proportional control operation occurs within the "proportional band range" near the set temperature. Beyond this ratio, the controller used for the switch controller output state is fully open or fully closed (ratio below) (ratio above). However, when the ratio is within the range, the output state is open or closed according to the ratio of the measured deviation from the set value. At the set value (the middle point of the proportional band), the output is open: the rate is 1:1; that is, the opening time is equal to the closing time. If the temperature is far from the set value, the opening time and the closing time will vary depending on the temperature difference ratio. If the temperature is lower than the set value, the opening time is longer; if the temperature is too high, the closing time is longer.
  A proportional band is usually expressed as a full percentage or a degree. It can also be called the gain, the gain is the reciprocal of the proportional band. Please note that during the time proportional control, the heater applies full power, but it circulate between opening and closing,
  So the average time is different. In most of the proportional controllers, the cycle time and / or the proportional band are adjustable so that the controller can match the specific process better.
In addition to the output of electromechanical and solid state relays, the proportional controller can also be used for proportional analog output, such as 4 ~ 20 mA or 0 ~ 5 Vdc. Through these outputs, the actual output level is different, not just the opening and closing time, as with the use of the relay output controller.
  One of the advantages of proportional control is that it is easy to operate. It may require a small amount of adjustment (manual reset) for the operator to set the set point temperature at initial start-up time, or to adjust when the process condition changes significantly.
  A system that is prone to a wide range of temperature cycles is also required to use a proportional controller. It is necessary to determine the need for a simple proportional control based on the required process and accuracy, or the need for a proportional control with PID.
  The process (such as heat exchanger) with long lag time and maximum rising rate (such as heat exchanger) requires a large scale band to eliminate the oscillation. Large scale bands may lead to a large offset with the changes in the load. To eliminate these offsets, automatic reset (integral) can be used. The differential (rate) operation can be used in the process of long time delay to speed up the recovery rate after the process interference.

PID controller
  Third kinds of controller (PID controller) can provide integral and differential control for the proportional controller. The controller combines the proportion control with the other two adjustments to help the device to automatically compensate for the change in the system. These adjustments (integral and differential) are expressed in units based on time; they can also be represented by its reciprocal (RESET and RATE, respectively).
  The proportions, integral and differential conditions must be adjusted or "adjusted" for a particular system using a trial error method. In the three types of controllers, the PID controller can provide the most accurate and stable control, and is most suitable for systems with relatively small quality. These systems can react quickly to the energy changes added to the process. PID controller is recommended for systems where load changes frequently and expect controllers to automatically compensate for frequent changes in the set value, the energy provided or the quality to control.
  Other features should be considered when selecting the controller. These features include automatic setting or self setting,
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