It doesn't matter what type of industry we are taking about; Temperature always play an important role. Every process or product has some specific temperature at which processing should be done otherwise product quality can alter. That's why temperature controllers are connected with temperature driven processes, It not only shows the temperature but it holds the temperature at desired level by connecting or disconnecting the heating or cooling element.
In this post, We will discuss:
- How these Temperature controllers works
- Control Methods of Temperature controllers
- Types of Temperature controllers
- Types of Input sensors
- Output controlling methods
WORKING OF TEMPERATURE CONTROLLERS
Temperature controller with the help of input temperature sensor continuously monitors present temperature of the system than controller compares the present value with the set/target value.
If there is error in between present value & set value than by connecting or disconnecting heating or cooling element, it maintains the temperature.
 |
| Temperature controller TC-513 by Selec |
 |
| Terminal connections temperature controller TC-513 by Selec Terminals 6,7,8- For input sensors connections. Terminals 5,4,3 Changeover relay- For load Output. Terminals 10,12 SSR output- For Load Output through SSR. Terminals 1,2- Supply terminals. |
 |
| Load(Heater) connections through SSR Temperature controller SELEC TC-513 |
"Methods of Connecting or disconnecting the heating OR cooling element at which time; for how much time is known as Control methods of Temperature Controllers.
Lets understand this with an Example
A system which is having the set temperature value is 50°C.
& Present value is 30°C.
Case1
Temperature controller connects the heaters until it reaches at 50°C & than heaters get disconnected but heating still raising & final temperature reaches to 56°C.
Now temperature starts decreasing & falls to 45°C.
Again heaters get connected & final temperature reaches to 56°C & cycle repeat it self.
Case2
Present value is 30°C.
Controller makes the heater complete ON until 45°C after that
Controller connects heater in pulse intervals such that desired temperature get achieved precisely & steadily which results temperature remains maintained at 50°C.
"
CONTROL METHODS OF TEMPERATURE CONTROLLERS
There are two controlling methods.
- ON OFF CONTROL
- PID CONTROL
1. ON OFF CONTROL
On Off control method is the simplest one in which controller connect the element ON Or OFF at fixed intervals around the set temperature. This controlling method oscillates around the set temperature but not able to get the stable set temperature. clearly explained in the graph below.
Let suppose target temperature is 50°C & Heating element is connecting through temperature controller.
At the time of starting, System's Ambient temperature is 20°C. Heating element get connected at starting which results temperature starts raising reaches to 30°C, 40°C & reaches to 45°C At which Heating element get disconnected.
But temperature still increasing gradually reaches to 50°C & than at last 60°C. At peak 60°C, Temperature starts decreasing reaches to 40°C at which heating element get connected again, Temperature starts increasing & than again heating element get disconnected at 45°C. This cycles remains continue but system is not the stable at set temperature of 50°C. This all control process is known as ON OFF CONTROL.
This type of control is useful where we do not need a stable set temperature.
 |
| Graph shows ON-OFF Temperature Control |
2. PID CONTROL
In ON-OFF Control, System temperature is not stable. Temperature fluctuates around the target temperature always.
System which requires an accuracy with Temperature; cannot survive on ON-OFF Control. PID Control is the best solution if we need a system with stable set temperature which remains stand still at target temperature.
PID control is basically a combination of three controls Proportional, Integral & Derivative. For PID Control explanation, we are again taking an example of heating element controlling through temperature controller. Let suppose target temperature is 50°C & ambient temperature is 20°C.
 |
| Graph Shows PID Temperature Control |
Proportional Control
In Proportional control, system's temperature control works directly proportional to the error. This means heater control whether it is ON or OFF entirely depend upon the error between the target value & present value.
At the time of starting As shown in a PID graph above, ambient temperature is 20°C & the difference between target temperature & present temperature is 30°C Which means heater will get directly ON for 30°C at the time of starting but as present temperature reaches towards the target temperature, Error value geos on decreasing which further get difficult to achieve the target temperature As system works proportional to the error value. Which results present temperature will remain lower than the target value in proportional control.
 |
| Manual of temperature Controller TC513 by SELEC, Showing Proportional, Integral & Derivative Value, Proportional Band having degree Celsius value while Integral & derivatives are time values| |
In an image of manual above, Proportional band having a value in degree Celsius which is 10°C by default While System target value is 50°C than range becomes 40°C to 60°C As ±10°C of 50°C.
Proportional control works directly upto 40°C if raising from low to high White it works directly upto 60°C if falling from high to low. After Proportional control, Integral Control Starts working.
Integral Control
After Proportional band in which present value left behind the target value, Integral control come into the action. Integral control works cumulatively to raise the temperature. While doing so integral control results in overshoot & present temperature crosses the target temperature.
In controller manual image above, Integral control has a value in time(minutes)(default Integral time is 2minutes in manual). if long time been selected than system overshoot less while if less time been selected than system overshoot more. This value of integral control all depend upon the type of process.
 |
| Graph showing Integral temperature control which is raising cumulatively ! |
Derivative Control
Derivative control is exact opposite of integral control which attempts to minimize the overshoot by slowing the correction factor applied As the target is approached. (Default derivative time is 30sec. in manual)
 |
| Graph representing Derivative Temperature Control |
TYPES OF TEMPERATURE CONTROLLER
Temperature controller can be classified on the basis of two Parameters.
- BASIS OF CONTROL
- BASIS OF PROCESS
 |
| Types of Temperature Controller |
BASIS OF CONTROL
- ON OFF control
- PID control
BASIS OF PROCESS
- Heating Temperature controllers
These temperature controllers are specially designed for heating processes. Electrical heaters are connected as a load in heating temperature controllers.
 |
| Heating Temperature Controller E5CWL by OMRON |
- Cooling Temperature controllers
These temperature controllers are specially designed for cooling processes. Compressors/Chiller units are connected as a load in cooling temperature controllers.
- Process controllers (Heating + Cooling)
Process controllers are the combinations of heating & cooling processes which manages the temperature at particular level by connecting or disconnecting heating or cooling element.
 |
| PID Temperature controller P104 by EUROTHERM control both heating & cooling process |
TYPES OF INPUT TEMPERATURE SENSORS
Digital temperature devices can't measure the temperature directly instead these devices observe electrical signals provided by temperature sensors.
For digital Temperature meter, there are two types of temperature sensors 'THERMOCOUPLES' & 'RTD'. Both the sensors work on different working principles. These sensors provide signal to temperature controller as per change in temperature & in return controller shows temperature As per stored information.
'THERMOCOUPLES' are junction of two dissimilar metals which produces milli-volt as per change in temperature. Thermocouples are two wired sensors with wide temperature range from -199°C to +1700°C. Thermocouples further have many types J,K,T,R&S & these types have different temperature ranges.
 |
| K- Type Thermometer with K-Type Thermocouple |
'RTD' stands for Resistance Temperature Detector. RTD contains a metal whose resistance value changes as per change in temperature. This resistance value of RTD continuously monitored by temperature controller. RTD are available in 2 wires, 3wires & 4 wires.
 |
| 3 Wired RTD PT-100, Dia 6mm/length 150mm |
OUTPUT CONROLLING METHODS
ELECTROMAGNETIC RELAY
Electromagnetic relay is a type of electromechanical relay which contains an electromagnet, electrical contacts & mechanical spring mechanism. When electromagnet get energized common contact get connected with NO contact. When electromagnet get deenergized common contact get connected with NC contact again due to the spring tension.
 |
| Diagram of Electromagnetic Relay changeover type |
 |
| 4 Electromechanical changeover relays fitted on UPS control card by MICROTEK |
SSR
Electromechanical relays contains mechanical moving contacts while SSR contains electronic components without any moving contact. SSR mainly contains three terminals electronic component in which 2 terminals are input & output while 3rd terminal is signal terminal. When supply get applied on 3rd terminal than input & output get connected.
|
| Selec TC 513 having both output controls changeover relay & SSR |