**Figure 5.**

*PTC (left) and NTC (right) thermistor electrical symbols [19].*

• They are easy to use

They are small and can fit into any smallest space [19].

A bi-metallic strip is created when two distinct metals, such as nickel, copper, tungsten, or aluminium, are bonded together to create the thermostat, an electro-mechanical

**Figure 7.** *Bi-metallic strip.*

contact type temperature sensor. When the strip is heated, the differing linear expansion rates of the two dissimilar metals cause a mechanical bending action.

The bi-metallic strip is frequently used to control hot water heating elements in boilers, furnaces, hot water storage tanks and vehicle radiator cooling systems. In addition, it can be used as an electrical switch on its own or as a mechanical method of operating an electrical switch in thermostatic controls [16].

**Figure 7** shows two metals with distinct thermal properties bonded back-to-back to form the thermostat. The connections are closed when it is cold, allowing current to flow through the thermostat. However, the bonded bi-metallic strip bends up (or down) and opens the contacts when it gets hot because one metal expands more than the other, blocking the current flow [16].

## **2.4 Thermostat**

A thermostat is a temperature-sensing tool that gauges engine coolant temperature. In order for internal combustion engines to operate at an efficient temperature, the component is intended to know when to open and close.

**Figure 8.** *On/off thermostat [16].*

If the coolant is not hot enough, the thermostats stay closed. However, when the coolant reaches a certain temperature, a valve opens, letting hot coolant flow into the radiator. The thermostat therefore functions similarly to a gate by allowing or preventing the passage of coolant from the engine to the radiator.

Modern automobile engines operate within a specific temperature range; typically, they operate between 194 degrees Fahrenheit, or 90 degrees Celsius, and 221 degrees Fahrenheit. The thermostat determines when to open and close based on the coolant temperature [20].

**Figure 8** shows the on/off the thermostat; there are two main types of bi-metallic strips with respect to their movement when subjected to temperature changes. They are:

1. snap-action

2.creeper types

Both the faster "creep-action" types gradually adjust their position as the temperature changes, and the snap-action types generate an instantaneous "ON/OFF" or "OFF/ON" type action on the electrical connections.

Snap-action type thermostats are frequently used in our houses to regulate the temperature set point of ovens, irons, immersion hot water tanks, as well as the domestic heating system. They can also be found mounted on walls [16].

In most creeper varieties, a bi-metallic coil or spiral slowly unwinds or coils up in response to temperature changes. Since the creeper-type bi-metallic strips are longer and thinner than the conventional snap ON/OFF varieties, they are typically more sensitive to temperature changes, making them perfect for use in temperature gauges, dials, and other similar devices [16].

Standard snap-action-type thermostats have a significant hysteresis range between the time the electrical contacts open and the time they close again, which is a drawback despite their low-cost and wide operating range when used as temperature sensors. It might be set to 20°C, for instance, but not open until 22°C or close again until 18°C [16].

Therefore, the temperature swing range might be rather wide. Bi-metallic thermostats that are sold for residential usage contain temperature adjustment screws that enable more exact pre-setting of the appropriate temperature set point and hysteresis level [16].

Contact sensors are employed in industries to control various automation temperature processes; hence, it is advantageous to use sensors in the industry, offices and home to regulate the environment's temperature.

### **2.5 What is a temperature controller?**

Temperature controls make sure a process gets the desired temperature and keeps it there. These are typically employed for closed-loop control, in which the temperature controller compares the actual temperature with the set point established by the programmer using data from a temperature probe (thermocouple, resistance thermometer or temperature transmitter). It then modifies its output signal to the appropriate control element as necessary (electrical heater, cooling circuit, steam control valve, etc.). A variable output, where the output signal to the process is between 0 and 100%, and a straightforward ON/OFF control, working like a thermostat, are possible. The latter is also called a 2-point, binary, or bang-bang control [21].

### **2.6 How does a temperature controller work**

The heating circuit is turned on for ON/OFF control when the temperature is below the set point and off when it is above. Additionally, a cooling circuit may be activated above and deactivated below the specified point. A proportional–integral– derivative (PID) controller frequently performs variable control (three-term controller). In order to attain and keep the set point with the least amount of overshoot and to retain it as steadily as possible, this controller applies a revised algorithm on the error (the difference between the set point and the measured value) [21].

## **2.7 What is a PID controller**

Depending on the needs of the process, three-term or PID controllers (proportional–integral–derivative) can be employed for proportional alone (P), PI or PID control. In proportion to the departure from the set point, proportional control modifies the output. A defined proportionate band is below and/or above the set point. The output for cooling (above) or heating (below) is 100% outside of this band. It decreases linearly within the band, reaching 0% at the set point. The integral term can then further alter the output based on the rate-of-change of the mistake because this can result in a sluggish approach to the set point (achieving the set point quicker). Due to the possibility of overshooting the fixed point, the derivative term predicts future errors and modifies the output [21].

#### **2.8 Advantages of temperature sensors**

Temperature sensors are possible when an object needs to be heated, cooled, or both, and it must maintain the desired temperature (setpoint) despite changes in its surroundings.

Open-loop and closed-loop controls are the two fundamental methods of temperature control.

Open-loop systems apply continuous heating and cooling without considering the actual temperature output. It is comparable to a car's interior heating system. You might have to set the heat all the way up on a chilly day to get the car up to 75 degrees. However, during warmer weather, the same setting would leave the inside of the car much warmer than the desired 75° [22].

Temperature sensors can control a given situation using the open and closed loops, as shown in **Figures 9** and **10**, respectively.

#### **Figure 9.**

*Open-loop temperature control diagram [23].*

#### **Figure 10.**

*Close loop temperature controller block diagram [13, 23, 24].*

Regardless of sophistication, all temperature sensors and controllers operate in essentially the same way. A controller keeps a variable or parameter constant at a predetermined value. The actual input signal and the desired setpoint value are the two variables that the controller needs. The input signal is also known as the process value. The controller determines how frequently the input is sampled [25].

The input or process value is then compared to the setpoint value. If the process value deviates from the setpoint, the controller changes the output signal based on the difference between the process value and the setpoint and whether the process value is getting closer to the setpoint or moving further away from it. The actual value is then changed in response to the output signal in order to bring it into compliance with the setpoint. Typically, the control algorithm updates the output power value before applying it to the output [25].

The control action is based on the type of controller being used. The controller decides whether the output should be turned on, off or left in its current state, for example, if it is an ON/OFF control [25].

One of the easiest control kinds to use is the ON/OFF control. By establishing a hysteresis band, it operates. To regulate the temperature inside a room, for instance, a temperature controller might be used. An error signal would display a 1° difference if the setpoint temperature was 68° and the actual temperature was 67°. The temperature would then be raised back to the setpoint of 68° by the controller sending a signal to increase the applied heat. The heater turns off when the room reaches 68 degrees. The controller does nothing, and the heater stays off for a temperature between 68° and 67°. The heater will, however, start up once the temperature hits 67° [25].

Unlike ON/OFF control, PID control determines the precise output value required to maintain the desired temperature. Power output ranges from 0–100%. When an analogue output type is used, the output drive is proportional to the output power value. If the output is a binary output type, such as a relay, Solid State Relay driver or triac, it must be time-proportional in order to provide an analogue representation [25].

A system that uses cycle time to proportion output values is called timeproportional. A system requiring 50% power will have its output on for 4 seconds and off for 4 seconds if the cycle time is set to 8 seconds. The time values would not change as long as the power value remained constant. The power is gradually averaged to the requested 50% amount, which is evenly split between on and off. The output would be on for two seconds and off for six seconds over an eight-second cycle if the output power needed to be 25% [25] as shown in **Figure 11**.

A shorter cycle time is desired, barring any other factors, because the controller can react to changes in the process and the output's condition more quickly. Due to the way relays operate, which may shorten their longevity, a cycle duration of less than 8 seconds is not recommended. For solid state switching components like an SSR driver or triac, quicker switching times are preferred. Longer switching times allow for higher process value variation regardless of the output type. A longer cycle time is typically desirable when employing a relay output, but only if the process allows it [25], as shown in **Figure 12**.

**Table 1** shows the comparison between NTC thermistor and thermocouple.

#### **2.9 Non-contact sensors**

Non-contact sensors are not in contact with the object that it measures; however, they measure the temperature by utilising the radiation of the heat source. An example of a non-contact sensor is the infrared (IR) sensor. IRs detect the energy of an object remotely and emit a sign to an electronic circuit that senses the object's temperature by a specific calibration.

Non-contact temperature sensors generally rely on technologies that are based on electrical, magnetic, optical, sonic or other principles rather than depending on physical contact or mechanical movement to obtain the measurements. The sensor often

**Figure 11.** *Output time proportioning [25].*

#### **Figure 12.**

*Overview of contact temperature sensor controller [1].*


**Table 1.**

*A brief comparison of thermistor and thermocouple [19].*

emits a form of energy such as radiation that can be used to detect a condition without physical contact.
