feeling whether it is too warm or too cold as in the case of bathing we mix cold water or hot water to have the temperature of the bath being comfortable. In this case it is purely by sense of feeling.

In the case of refrigerator the desired chamber temperature is preset with the help of a thermostat. The actual temperature in the refrigerator cabinet is monitored and compared with the desired value. A switch closes to cause the refrigerant to flow into the coils when the actual temperature increases above the preset value. Heat is taken from the cabinet causing its temperature to fall. The reverse of this process takeplace when the temperature falls below the preset or desired value. That is the thermostat switch opens and cutoff the refrigerant flow into the coils. Only when there is deference in temperature between the preset and the desired value i.e., error the system will operate. That is the system is discontinuous. Opening of the refrigerator door, placing warm food etc., are external disturbances to the system.
Sequential controllers
In many situations necessity arises to control a cycle of operations in sequence i.e., to carry out the different operations in sequence. The sequences of steps may be time based or event driven. One such cycle of operations is washing of clothes, the sequences being, soaking, washing, rinsing and drying. These operations are carried out using washing machines incorporated with relays and cam operated switches to carryout the sequence of operation of the washing cycle. Such type of control system are now being replaced by microprocessor based control system; the sequence being controlled by a software embedded in it. Human operator has to only press a button to activate the system after the clothes have been put into the drum, of course with a provision for continuous supply of water.
Note: Without our knowledge we come across many open and closed loop control systems. But it is for the reader to identify them.
Advantages of Mechatronics
a. A Mechatronics product can be better than just sum of its parts.
b. Mechatronics products exhibit performance characteristics that were previously difficultto achieve without the synergistic combination.
c. The mechatronic approach results in greater productivity, higher quantity and productionreliability by the incorporation of intelligent, self correcting sensory and feed back systems.
d. The integration of sensors and control systems in a complex system reduces capital expenses.
e. Maintains high degree of flexibility
f. Higher percentage of machine utilization
g. Products produced are extremely attractive to the consumer in quality and cost effectiveness.
Disadvantages of Mechatronics
a. Expensive to incorporate to an existing (old) system.
b. Initial cost of the system will be high.
c. Knowledge of different engineering fields is necessary for design and implementation.
d. Each system will have its own specific problem and depends on the system used for theapplication and hence to be properly addressed.

Microprocessor based controllers
Simple control problems can be solved by electronic control system involving combination of sequential logic 1C circuits using hardwired connections. In the past most control applications were relay based with nearly a one -to-one input-output ratio. Control systems, controlling a of group of relays, were capable of controlling machine tools, conveyor systems and other systems such as motors, industrial vehicles etc, followed by Programmable Logic Controllers (PLC) based applications. With the advent of semiconductor technology a more complex situation with more variable to control in a more complex control sequence using a microprocessor and software to make the 'interconnections'.Microprocessor and micro controller-based control systems have become increasingly popular not only in mechatronics systems but in many industrial applications because of their versatility, functionality and high integration level. The number of discrete components get reduced, resulting