Temperature control instrument for heat treatment

Chengchi specializes in providing heat treatment equipment, electric furnaces, and temperature control instruments for industrial furnaces. Brands include Fuji FuJi, Yudian, and Eurofins, RKC, Omron, Shengang, Island Electric, Shanwu, WEST， Conduction, conductivity, physical and chemical properties, Fanda, Yangming, Taiyi, Delta Honywell， Hongrun, Yatai, Yuyao, etc.

Temperature control instruments, abbreviated as thermometers or thermostats, are a series of automatic control components that undergo physical deformation inside the switch according to changes in the working environment temperature, resulting in certain special effects and causing conduction or disconnection actions. Electronic components work on different principles at different temperatures to provide temperature data to the circuit for collecting temperature data.

The ideal temperature control is to faithfully track the controlled temperature when the target temperature changes (changing the set value, starting when the power is turned on). In reality, due to time delays in the control object, temperature detection part, operation part, etc., the control part takes corrective actions on the delayed return temperature. Therefore, there is a phenomenon of "upward rush" and "downward rush". If the gain (responsiveness) of the control action is reduced in order to obtain good control results, the time to reach the target temperature will become longer, or the oscillation will not decrease, or even increase.

Types of control actions

● Two position action (switch action)

The temperature control of electric heaters or irons is switch control, which means that if the actual temperature is higher than the set value, the power supply of the electric furnace wire is turned off (OFF), and if it is lower than the set value, the power supply of the electric furnace wire is turned on (ON). For setting the temperature, the OFF/ON control based on the measured temperature is called a two position action or switch (OFF/ON) action.

Simple control, the disadvantage is the generation of oscillations.

Proportional action (P action)

Output an operation quantity proportional to the deviation between the set value and the measured value for control.

Set the proportional band around the set value, and once the measured temperature enters the proportional band, gradually reduce the amount of operation.

The temperature tends to stabilize at the equilibrium point within the proportional band, but the measured values are rarely consistent with the set values.

The deviation between the set temperature and the stable temperature is called residual deviation.

● Integral action (I action)

Controlling with proportional actions will result in residual deviation. Use integral action (I action) to eliminate residual deviation.

The integral action is the area enclosed by the magnitude of the output deviation (the difference between the set value and the measured value) and the time at which the deviation occurred, which is proportional to the magnitude of the integral value.

In view of this, as long as there is a deviation, the integration action works to eliminate the deviation and thus eliminate residual deviation.

The strength of the integral action is represented by the integral time. The time elapsed when the output (operation amount) generated by the integral action is equal to the output (operation amount) of the proportional action product is called the integral time.

The shorter the integration time, the stronger the integration effect. The longer the integration time, the weaker the integration effect.

If the integration effect is too strong, it is prone to oscillation and instability.

Differential action (D action)

The action of controlling the operation quantity proportional to the speed at which the deviation (difference between the set value and the measured value) occurs to prevent the deviation from becoming greater than expected is called differential action (D action).

The strength of differential actions is represented by differential time. Differential time refers to the time elapsed when the output (operand) generated by a differential action is equal to the output (operand) generated by a proportional action.

The longer the differentiation time, the stronger the differentiation effect. The shorter the differentiation time, the weaker the differentiation effect.

If the differential effect is too strong, even if the deviation change is small, there will be large output changes, oscillations, and instability.

● PID action

PID action is a combination of proportional action, integral action, and derivative action.

Using proportional actions can obtain stable control results without oscillation, using integral actions to eliminate residual deviations, and using differential actions to improve the impact of external disturbances.

Suitable for situations where the ineffective time exceeds the adjustment (overshoot).

Types of control actions

● Forward and backward movements

Positive action is to increase the amount of operation when the actual temperature is higher than the set value.

Positive action is applied to cooling control.

Reverse action is to increase the amount of operation when the actual temperature is lower than the set value.

Reverse motion is used for heating control.

Heating and cooling control

Control is divided into heating and cooling control.

By using one temperature controller, two types of operation quantities can be output: heating and cooling.

● Position ratio control

In the control of using controllable motors, input the opening degree of the controllable motor (position of the resistance ruler) and output the control signal. There is also a corresponding "no need for resistance ruler" controllable motor temperature controller.

● Cascade control

Effective for objects with significant time delay between the temperature control area and the heat source.

Use the control output of the primary controller (master) as the remote setting input for the secondary controller (slave).

The secondary controller adjusts the temperature setting value while controlling the temperature of the heat source using the control output of the primary controller.

● Manual control

It is not automatically controlled through a controller, but manually controlled by changing the operation output.

Used for process control during start-up, trial operation, etc.

PV bias

Add the value set with PV bias to the measurement input to correct the measurement input.

Used to correct the uneven deviation of various sensors or the deviation from the measured values of other instruments.

Example: When measuring the temperature of the same point with two temperature controllers, the displayed measurement value is

Temperature controller A: 200 ℃

Temperature controller B: 198 ℃

If the PV bias is set to+2 ℃ in temperature controller B, the displayed value is:

Display value=measured value+PV offset value

=198 ℃+2 ℃=200 ℃.

● Digital filtering

Used to reduce input clutter interference. Equivalent to a CR low-pass filter with one delay.

The time constant of the filter is set according to the characteristics of the controlled object and the level of noise, which can suppress the influence of input noise.

If the time constant is too small, the filtering effect cannot be obtained. If the time constant is too large, the responsiveness will deteriorate.

Cold junction temperature compensation circuit

Thermocouples generate corresponding thermoelectric potentials based on the temperature difference between instrument terminals and measurement points.

The temperature at the terminal is the indoor temperature of the instrument, so only a thermoelectric potential equivalent to the temperature difference between the instrument terminal and the measuring point is generated.

The cold junction temperature compensation circuit detects room temperature and compensates for it by adding the thermoelectric potential of the room temperature portion, making it the thermoelectric potential corresponding to the temperature measurement point.

● Square root algorithm

When measuring flow, using a differential pressure flowmeter, the output signals Δ, P (differential pressure), and flow rate (Q) are generally related as follows:

Q=∝√ Δ P

Therefore, the output signals Δ and P from the flowmeter can be squared to obtain the flow rate (Q).

● Cut off PV low input

In the case of performing square root calculations, even if the differential pressure change amplitude is small when the input is small, it can lead to significant changes in the measured flow rate or instability caused by input noise. To avoid the above phenomenon, the function of counting the measured part below Δ P1 as zero.

● Set limiter

The function of limiting the setting range of the set value.

● Set the rate of change limiter

The function of setting the amount of change in the set value per unit time when the set value is changed.

Used in situations where the set value has been changed and there is no desire to output dramatic changes, or in situations where simple program control is required.

● Multi storage area function

The function of pre registering various parameter groups such as set values (SV), PID constants, alarm set values, proportional bands (P), integration time (I), differentiation time (D), etc. on several memories.

The number of parameter groups that can be logged in is called the number of memories, and the situation where 8 groups can be logged in is called 8 memories.

Retrieve the corresponding memory (area) as needed for control.

Can simplify the tedious setting changes.

● Remote setting

Set the set value (SV) using an external analog signal.

·RS magnification

For the function of multiplying remote set values by magnification.

·RS bias value

The value obtained by adding (subtracting) RS bias for remote setting is the set value.

Regarding control

Agile PID Control

PID control is widely used to achieve stable control results by setting constants for P (proportional band), integration time (I), and differentiation time (D). However, the drawbacks of this PID control are:

If PID constants are set in order to improve the response to the corresponding settings, the response to external disturbances will deteriorate; On the contrary, if PID constants are set to improve the response to external disturbances, the response to external disturbances will deteriorate.

Agile PID control, based on the PID parameters set to achieve good response to external disturbances, can choose from three shapes of "response to corresponding settings": Fast, Medium, and Slow.

These three response shapes are called control response parameters. If you focus on response speed, choose "Fast"; if you want to avoid overshoot, choose "Slow".

● Automatic Calculus (AT)

Automatic calculation (AT) is the function of automatically calculating and setting the optimal PID constant for a set temperature.

Automatic calculation can start from any state during temperature rise and stable control after power on.

● AT bias

AT bias is set in automatic calculations where the measured value (PV) does not exceed the set value (SV).

If the AT bias is set, the set value (SV) for automatic calculation, i.e. [AT point], can be changed.

● Control state judgment type self calculation

When the judgment control is disrupted, the self calculation function comes into play.

In normal control, self calculation is not implemented, and trust temperature and stability are considered.

RFB (Reset Feed Back) limiter

When the deviation between the measured value (PV) and the set value (SV) persists for a long time, the PID calculation result will exceed the effective range of the operating quantity (0-100%). Especially when the output value of the integral (I) exceeds the required value, even if the deviation decreases, the implementation of corrective actions is slow.

RFB limiter is a correction action taken when the PID calculation result exceeds the limit point (100%), in order to frequently feedback the excess part as an integral value within the effective range of the PID calculation result, and keep the calculation result at the limit point.

● arw（Anti Reset Windup）

In the case of PID control, if the integral (I) action is activated from the start of the controlled object, a significant overshoot will occur.

ARW is a function that suppresses overshoot by limiting the effective range of the integral action (I). The integral action only works when eliminating residual deviation, so reducing the range of the integral action within the proportional band can minimize the overshoot.

Regarding alarms

● Deviation alarm

When the deviation [measured value (PV) - set value (SV)] reaches the alarm setting, it is in an alarm state.

The movement of the alarm set value changes with the set value.

● Input value alarm

When the measured value (PV) reaches the alarm setting, it is in an alarm state.

● Set value alarm

When the set value (SV) reaches the alarm setting, it is in an alarm state.

● Alarm action gap

When the measured value (PV) is near the alarm set value, sometimes the drift of the input value can cause the alarm output to repeatedly turn on OFF。 Setting the alarm action gap can prevent repeated ON OFF。

● Alarm standby action

The so-called standby action refers to the action of disabling the alarm function when the power is turned on, or when the operation mode is switched from STOP to RUN, or when the set value is changed, even if the measured value (PV) is in the alarm area, until the measured value (PV) leaves the alarm area.

Please note that some instruments are called standby action, which refers to situations where the standby action includes changing the set value (SV). The standby action does not include the standby action of changing the set value (SV).

● Alarm delay timing

Alarm delay timing refers to the function where the measured value (PV), even if it enters the alarm area, must go through the set time of the alarm delay timing before it becomes an alarm state.

● Alarm lockout

Alarm locking refers to the function of maintaining an alarm state even if the measured value (PV) leaves the alarm area again once it enters the alarm area.

The alarm lock can be released using the front operation buttons or external contacts.

● Excitation/non excitation of alarms

·Excitation alarm: When in an alarm state, the relay contacts close.

·Non excitation alarm: When in an alarm state, the relay contacts are disconnected.

Regarding control

Heater wire breakage alarm (HBA)

The heater disconnection alarm is a function that uses a current detector (CT) to detect the current passing through the electric heater, compares the detected value with the set value of the electric heater disconnection alarm (HBA), and is in an alarm state in the following situations.

① When the control output is ON, the input value of CT is below the set value of the electric heater disconnection alarm

Reason: Disconnected electric heater, abnormal operator, etc.

② When the control output is OFF, the input value of CT is below the set value of the electric heater disconnection alarm

Reason: Welding of relay contacts, etc.

● Control circuit disconnection alarm (LBA)

The control circuit disconnection alarm starts when the control output becomes above 100% (or the upper limit of the output limiter) or below 0% (or the lower limit of the output limiter), and detects the change in measurement value (PV) every LBA set time unit, and determines whether there is an abnormality in the control circuit based on its change.

The following situations are in alarm state:

① When the control output is above 100% (or the upper limit of the output limiter)

In the case of positive action: within the set time of LBA, the decrease in measured value (PV) is less than the LBA judgment change (2 ℃).

In the case of reverse action: within the LBA set time, the increase in measured value (PV) is less than the LBA judgment change (2 ℃).

② When the control output is above 0% (or the lower limit of the output limiter)

In the case of positive action: within the LBA set time, the increase in measured value (PV) is less than the LBA judgment change (2 ℃).

In the case of reverse action: within the set time of LBA, the decrease in measured value (PV) is less than the LBA judgment change (2 ℃).

reason

Abnormal control objects: heater disconnection, failure to supply load power, wiring errors, etc.

Sensor abnormality: sensor detachment, short circuit, etc.

Operator abnormality: fusion of relay contacts, etc.

Internal abnormalities of the instrument: fusion of contacts of relays inside the instrument.

The control circuit disconnection alarm detects abnormalities within the control circuit, but cannot identify the location of the abnormality and requires confirmation of the control system.

·LBA not sensitive to tape

Due to external disturbances (such as the influence of other heat sources), even if the control system is not abnormal, it may still become an alarm state (control circuit disconnection alarm). In this situation, by setting the LBA non sensing band (LBD), it is possible to distinguish between not becoming an alarm state.

Regarding output

● Output limiter

The function of limiting (upper limit, lower limit) the range of output control. If controlling the output to 100% will have adverse effects on the device, set the output limiter.

● Output rate limiter

The function of setting the change in control output per unit time.

A device used to prevent drastic changes in output.

● Analog output (converted output)

Output the measured value (PV), set value (SV), control output value (MV), deviation value between measured value and set value (DEV), and opening input value in the form of DC voltage and current.

Can be used as input for recorders and other devices.

Regarding contact input

● Item input (external contact input)

External signals can be used to control stop/start, remote/local switching, storage area switching, ladder (SV1/SV2 switching), program model switching, etc.