These Controllers receive sensor signals and
control heaters or other devices to maintain a preset temperature. They
can also be used for humidity, pressure, and flowrate control. OMRON also
provides temperature and humidity sensors.
Temperature Control Configuration Example
The following example describes the basic configuration for temperature
control.
Temperature Controller
An Temperature Controller receives electrical signals input
from the Temperature Sensor, compares the electrical signal input to the
set point, and outputs adjustment signals to the Controller.
Controller
A Controller is used to heat or cool furnaces and tanks using devices such
as a solenoid that cuts off electric current to a heater or a fuel valve
that shuts off the fuel supply.
Temperature Sensor
The Temperature Sensor consists of an element protected by a pipe. Locate
the element, which converts temperatures into electrical signals, in
places where temperature control is required.
Temperature Control
The set point is input to operate the Temperature Controller. The time
required for stable temperature control varies with the controlled
object. Attempting to shorten the response time will usually result in
overshooting or hunting the temperature. The response time must not be
shortened to reduce overshooting or hunting the temperature. There are
applications that require prompt, stable control in the waveform shown in
(1) despite overshooting. There are other applications that require the
suppression of overshooting in the waveform shown in (3) despite the long
time required to stabilize the temperature. In other words, the type of
temperature control varies with the application and purpose. The waveform
shown in (2) is usually considered to be a proper one for standard
applications.
1. The temperature stabilizes after overshooting several times.
2. Proper response
3. The response is slow in reaching the set point.
Characteristics of the Controlled Object
Before selecting a Temperature Controller or Temperature Sensor, it is
necessary to understand the thermal characteristics of the controlled
object for proper temperature control.
Heat capacity, which indicates the ease of heating, varies with the
capacity of the furnace.
Static characteristics, which indicate heating capability, vary with the
capacity of the heater.
Dynamic characteristics, which indicate the startup characteristics (i.e.
excessive response) of heating, vary with heater and furnace capacity that
can affect each other in a complex way.
External disturbances cause temperature changes. For example, the opening
or closing of a door on a constant temperature tank can cause external
disturbances that generate temperature changes.
ON/OFF Control Action
As shown in the graph below, if the process value is lower than the set
point, the output will be turned ON and power will be supplied to the
heater. If the process value is higher than the set point, the output will
be turned OFF and power to the heater will be shut off. This control
method, in which the output is turned ON and OFF based on the set point in
order to keep the temperature constant, is called ON/OFF control action.
With this action, the temperature is controlled using two values (i.e., 0%
and 100% of the set point). Therefore, the operation is also called
two-position control action.
P Action
P action (or proportional control action) is used to obtain an output in
proportion to the input. The Temperature Controller in P action has a
proportional band with the set point in the proportional band. The control
output varies in proportion to deviation in the proportional band. In
normal operation, a 100% control output will be ON if the process value is
lower than the proportional band. The control output will be decreased
gradually in proportion to the deviation if the process value is within
the proportional band, and a 50% control output will be ON if the set
point coincides with the process value. (i.e., there is no deviation).
This means P action ensures smoother control with minimal hunting compared
with the ON/OFF control action.
Proportional control action
Example:
If a Temperature Controller with a temperature range of 0 °C to 400 °C has
a 5% proportional band, the width of the proportional band will be
converted into a temperature range of 20 °C. In this case, a full output
is kept turned ON until the process value reaches 90 °C, and the output is
OFF periodically when the process value exceeds 90 °C, provided the set
point is 100 °C. When the process value is 100 °C, there will be no
difference in time between the ON period and the OFF period (i.e. the
output is turned ON and OFF 50% of the time.)
I Action
I action (integral control action) is used to obtain an output in
proportion to the time integral value of the input.
P action causes an offset. Therefore if proportional control action and
integral control action are used in combination, the offset will be
reduced over time until the control temperature eventually will coincide
with the set point and the offset will cease to exist.
D Action
D action (derivative control action) is used to obtain an output in
proportion to the time derivative value of the input.
Proportional control action corrects the control results as does integral
control action. Therefore, proportional control action and integral
control action respond slowly to temperature change. This is why
derivative control is required. Derivative control action corrects control
results by adding the control output in proportion to the slope of
temperature change. A large control output is applied to radical external
disturbances to get the temperature quickly back under control.
PID Control
PID control is a combination of proportional, integral, and derivative
control actions. The temperature is controlled smoothly here by
proportional control action without hunting, automatic offset adjustment
is made by integral control action, and quick response to an external
disturbance is made possible by derivative control action.
Two PID Control
Conventional PID control uses a single control block to control the
responses of the Temperature Controller to a target value and to external
disturbances. Therefore, the response to the target value will oscillate
due to overshooting if importance is placed on responding to external
disturbances with the P and I parameters set to small values and the D
parameter set to a large value in the control block. On the other hand,
the Temperature Controller will not be able to respond to external
disturbances quickly if importance is placed on responding to the target
value (i.e., the P and I parameters are set to large values). This makes
it impossible to satisfy both the types of response in this case.
Two PID control eliminates this drawback while maintaining the strengths
of PID control. This makes it possible to improve both types of responses.
PID Control
(1) Response to the target value will be slow if response to the external
disturbance is improved.
(2) Response to the external disturbance will be slow if response to the
target value is improved.
Two PID Control
(3) Controls both the target value and the external disturbance response.
Temperature Measurement Categories
There are two categories of temperature measurement, as described below.
Thermocouple
Principle
A thermocouple uses the thermoelectric force (i.e., the Seebeck effect)
that is generated between different types of metal to measure temperature.
This type of combined metal wires is called a thermocouple.
The Law of Intermediate Temperatures and the Law of
Intermediate Metals
The size of the potential difference is determined by the two different
materials of the metal wires and by the difference in temperature between
the thermocouple junction (i.e., hot junction) and standard junction
(i.e., cold junction). Any difference in temperature in between has no
effect (Law of Intermediate Temperatures). There is also no effect if
there are different types of metals in between as long as there is no
difference in temperature (Law of Intermediate Metals).
Thermocouple Types
Among thermocouples, types K, E, J, and T use base metals, and types B, R,
and S use noble metals.
The type of thermocouple is chosen based on the measurement temperature,
environment, and accuracy. In general, however, types K, J, and R are
commonly used.
Characteristics of Thermocouple Potential Difference
Compensating Lead Wire
These are special extension wires (cable) used when the temperature
measurement point and the Temperature Controller are far apart or when a
thermocouple temperature sensor with a terminal block is used.
For the thermocouple wires, use compensating lead wires in an extension
cable considering the construction and resistive value. The compensating
lead wires consist of materials that match the potential difference of the
thermocouple to be used.
Two types are available depending on the temperature used: general purpose
(−20 to 90°C) and heat-resistant (0 to 150°C). The characteristics are
determined according to JIS.
Platinum Resistance Thermometer
Resistance Thermometer
This device exploits the constant relation of metal resistance to
temperature.
Conditions requited for metal wire material:
1. High temperature coefficient of electrical resistance and good
linearity
2. Stability
3. Ability to be used with a broad temperature range
The material which best meets these conditions is platinum. Only the
platinum resistance thermometer is prescribed by JIS.
Platinum Resistance
Thermometer
This device uses the characteristic of platinum (Pt) that causes its
electrical resistance to increase in proportion to temperature.
Pt (new JIS) is prescribed at present by JIS (C1604-1997). JPt (former JIS)
is often used in Japan but has been discontinued. Both types, however, are
still widely used. Pt and JPt have different characteristics, so the type
must conform to the input specifications of the Temperature Controller.
Compensating Lead Wire Types
The resistance of a platinum resistance thermometer is 100 Ω at 0°C and
the standard resistance ratio (R100/R0 value) is 1.3851 (Pt100).These
values are low and so they will be greatly influenced by the compensating
lead wire resistance.
Generally, wiring with a three-wire resistance thermometer is used to
eliminate the influence of the compensating wire resistance.
Three-wire Resistance
Thermometer
One resistance conductor is connected to two
wires and the other is connected to another wire to eliminate the
influence of resistance when lead wires are extended. All of OMRON's
three-wire platinum resistance thermometers are configured this way.
Connection of Three-wire Platinum Resistance
Thermometers
Temperature
Controller / Temperature Sensor Glossary >>
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