Controllability of electric heating
The basic function of the controller is to compare the actual temperature with the set value and generate an output for maintaining the set value.
Controller is a part of the whole control system. When choosing the right controller, the whole system should be analyzed. The following should be considered when selecting the controller:
Type of input sensor (thermocouple, RTD, card and temperature range)
Arrangement of sensors
Required control algorithm (on / off, proportional, PID, auto tuning PID)
Type of output hardware required (electromechanical relay, SSR, analog output signal)
Additional output or system requirements (required temperature and / or set point display, cooling output, alarm, limit, computer communication, etc.)
Input type
The type of input sensor depends on the required temperature range, the required measurement resolution and accuracy, the installation method and location of the sensor.
Arrangement of sensors
The correct arrangement of sensing elements relative to working position and heat source is the most important for good control. If the three can be arranged in close range, it will be easy to get very high precision, and even reach the limit precision of the controller. However, if the heat source is far away from the working position, and the sensor is located in a different location between the heater and the working position, the accuracy achieved will be very different.
Before selecting the position of the sensing element, it is necessary to determine whether the heat demand is basically stable or has changed. If the heat demand is relatively stable, the temperature change at the working position can be kept to a minimum by arranging the sensor near the heat source.
When the heat demand changes, the sensor will be placed near the working position so that it can perceive the change of heat demand more quickly. However, due to the increase of the thermal lag between the heater and the sensing element, larger overshoot and undershoot will occur, resulting in greater dispersion between the highest and lowest temperature. This dispersion can be reduced by selecting PID controller.
Control algorithm (mode)
The method adopted by the controller to try to restore the system temperature to the required level. The two most common methods are on-off control and proportional control.
On-off control
The on-off control has the simplest control mode. It has a deadband (difference) expressed as a percentage of the input span. The setting is usually in the center of the deadband. Therefore, if the input is 0-1000 ˚ F, the deadband is 1% and the set value is 500 ˚ F, when the temperature is 495 ˚ f or lower, the output will be full on until the temperature reaches 505 ˚ F, and then the output will be full off. It will remain fully open until the temperature drops to 495 ˚ F.
If the response rate of the process is very fast, the cycle between 495 ˚ F and 505 ˚ f will be very fast. The faster the response rate of the process, the greater the overshoot and undershoot, and the faster the contactor cycles when used as the final control element.
On-off control is usually used in situations where precise control is not needed, such as in systems where energy cannot be switched on and off frequently, where the temperature changes very slowly due to the high quality of the system, or as a temperature alarm.
A special type of on-off control used as an alarm is a limit controller. This controller uses a lock-in relay that must be manually reset to close the process when a specific temperature is reached.
Proportion
The proportional control is designed to eliminate the cycles associated with on-off control. The proportional controller reduces the average power supplied to the heater as the temperature approaches the set point. This slows down heater heating so that the temperature does not exceed the set point, but will approach the set point and maintain a stable temperature. This proportional effect can be achieved by switching on and off the output at short intervals. This time proportional control controls the temperature by changing the ratio of the on time to the off time.
The time between two consecutive "connections" is called "cycle time" or "duty cycle". The proportional action occurs in a "proportional band" near the set temperature. Beyond this proportional band, the controller functions in the same way as the on-off controller, and the output is full on (below the proportional band) or full off (above the proportional band). However, in the proportional band, the on and off of the output are proportional to the difference between the measured value and the set value. At the set value (the midpoint of the proportional band), the output on-off ratio is 1:1, that is, the on time and the off time are equal. If the temperature is away from the set value, the on time and off time will change in proportion to the temperature difference. If the temperature is lower than the set value, the output on time is longer. If the temperature is higher than the set value, the output cut-off time is longer.
The scale band is usually expressed as a percentage or degree of full scale input. It can also be called gain, which is the reciprocal of the proportional band. In many devices, cycle time and / or proportional band width can be adjusted so that the controller can better match a specific process.
The proportional controller has a manual reset (fine adjustment) adjustment device that can be used to adjust the offset between the steady-state temperature and the set value.
In addition to electromechanical and solid-state relay output, the proportional controller can also have proportional analog signal output, such as 4-20 mA or 0-5 VDC. In these outputs, the actual output level amplitude is changed, not the proportion of on-off time.
Proportional plus integral plus differential control mode (PID):
This controller works in the same way as the proportional controller, except that the fine adjustment function is automatically performed by the integral function (automatic reset), so as to automatically compensate the load change, so that the temperature is consistent with the set value under all working conditions, and the offset is eliminated.
Differential function (rate action) compensates for rapid load changes. One example is the conveyor belt drying path for intermittently conveying products. When the product enters the drying channel, the heat demand rises rapidly. When the conveyor belt stops, there is excess heat. Differential action can reduce the under adjustment and over adjustment of temperature in this case, so as to prevent unqualified products due to over or under baking.
Compared with on-off or proportional controller, PID controller can provide more accurate and stable control. It is most suitable for systems with relatively small mass and rapid response to changes in the energy added to the process. This controller is recommended for systems with frequent load changes. The controller can automatically compensate the available energy and the quality to be controlled as the set value changes frequently
Proportion, integral and differential terms must be "set", i.e. adjusted for a specific process. This operation is completed by trial and error method. Some controllers are called self-tuning controllers, and they try to adjust PID parameters automatically.
The time proportional output power to the load is a percentage of the fixed cycle time. For example, for a 10 second cycle time, if the controller output is set to 60%, the relay will be energized (closed, powered) for 6 seconds, then de energized (open, non powered) for 4 seconds.
Electromechanical relay is usually the most economical one. It is generally used in systems with cycle time greater than 10 seconds and relatively small load.
Selecting AC solid-state relays or DC voltage pulses can reliably drive external SSRs because they do not contain any moving parts. They are also recommended for processes requiring short cycle times. External solid state relays may require AC or DC control signals.
The amplitude proportional output is usually analog voltage (0-5 VDC) or current (4-20 mA). The output level of this output is also set by the controller. If the output is set to 60%, the output level will be 60% of 5V, that is 3V. For 4 to 20 mA output (16 Ma range), 60% is equal to (0.6 x 16) + 4, i.e. 13.6 ma. This kind of controller is often used with SCR power controller or proportional valve.
The mechanical contactor is an external relay, which can be used in case of large current that cannot be handled by the relay in the controller, or in some three-phase systems. They are not recommended for cycle times shorter than 15 seconds.
Compared with mechanical contactors, solid-state relays have the advantage that they have no moving parts, so they can be used in short cycle time. The shorter the cycle time, the smaller the dead time and the better the control. "Switching" occurs at the zero voltage crossing point of the AC cycle, so that no perceptible electrical noise is generated. The AC controlled solid-state relay is used with the controller's mechanical relay or three terminal bi-directional thyristor switch output for a current of up to 90 a at 480 vac. The DC solid state relay is used with the DC fixed driver (pulse) output. The "on" signal can be 3-32 VDC, with various models, and can control the current of 90a at 480 VAC at the maximum.
Zero crossing SCR power controller is used to control single-phase or three-phase power supply with larger load. They can be used for currents up to 200 a at 480 v. The controller usually needs to output a 4-20 mA DC control signal. The zero crossing SCR power controller converts the analog output signal into a time proportional signal with a cycle time of about 2 seconds or less, and switches at the zero crossing point to avoid generating electrical noise.
The phase angle SCR power controller is also operated by 4-20 mA DC controller output. The power supply of the load is controlled by controlling the on (on) point of a complete AC sine wave every half cycle. This has the effect of changing the voltage in a single period of 0.0167 seconds. By comparison, the time proportional controller changes the average power over the cycle time (usually greater than 1 second, often greater than 15 seconds). Phase angle SCR power controller is only recommended for low thermal inertia heating elements such as infrared lamp or electric heater
