Washing Machine: An example of mechatronics system




The washing machine is an example of a sequential control system. A system operating with sequential control is one where a number of preset operations are performed in sequence. 

  • The control may be implemented as event-based, where the next action cannot be performed until the previous action is completed. 
  • Another mode of sequential control is termed "time-based", where the operations are sequenced with respect to time. Example: An example of a time-based sequential control system is shown in the following Figure 1. The pump (fill valve) is switched ON for a particular time to fill enough water into the tank to fill it to approximately the correct level. Following this, the pump is switched OFF.  


Figure 1: Time-based sequential control

  • Among these two modes, event-based control is intrinsically more reliable than time-based control.  An example of an event-based sequential control system is a washing machine. In this system, a number of operations or events have to be carried out in the correct sequence. The various events of the washing machine are given in Figure 2. Each event of the washing machine may consist of a number of sub-events or steps. For example, a pre-wash cycle involves various steps such as opening an inlet valve to fill water into the drum to the required level, closing the valve, switching ON the motor to rotate the drum to a number of revolutions or a particular time and operating the pump to empty the water from the drum.


Figure 2: Event-based sequential control in the washing machine

Washing machine

Working principle: A washing machine uses detergent to separate the dirt from the clothes. It sloshes your clothes in detergent suds for a while and the spin it fast to remove the water
Types of the washing machine: 

Figure 3: Types of washing machine

Top Load washing machine: 
A typical top load washing machine is as shown in Figure 4. The main components of the top load washing machine are:
Mechanical and electro-mechanical components:
  1. Lid: The lid on the top is used to drop the clothes into the drum.
  2. Drum (or tub): The drum is mounted on to the vertical axis and does not move. There are two drums in a top load washing machine. The inner drum has lots of holes that let the water in and out. The clothes are dropped into this inner drum. The outer drum contrarily is completely watertight.  
  3. Water inlet valves: The water required for various washing cycles in a washing machine is provided by the water inlet valves. This valve receives water from the water inlet connection which is connected to a water tank. When the clothes are loaded in the washing machine, this valve opens automatically and it closes automatically depending on the total quantity of the water required. The water control valve is actually the solenoid valve. 
  4. Water pump and/or drain pump: The water pump circulates water through the washing machine during various washing cycles. It works in two directions, circulating the water during the wash cycle and draining the water during the spin cycle. A separate drain pump is sometimes equipped to drain the water. The drain pipe enables removing the dirty water from the washing that has been used for the washing purpose.
  5. Agitator (Paddles or rotating disc): The agitator is located inside the drum of the washing machine. It is the most important component of the washing machine that actually performs the cleaning operation on the clothes. During the wash cycle, the agitator rotates continuously and produces strong rotating currents within the water due to which the clothes also rotate inside the drum. The rotation of the clothes within water containing the detergent enables the removal of the dirt particles from the fabric of the clothes. Thus, the agitator produces the most important function of rubbing the clothes with each other as well as with water. In some washing machines, instead of the long agitator, there is a disc that contains blades on its upper side. The rotation of the disc and the blades produce strong currents within the water and the rubbing of clothes that helps in removing the dirt from clothes.
  6. Motor: Single-phase motor or brushless DC motors are used in the design of washing machines. Controlling the motor is a very crucial part of the design. The pulse width modulation (PWM) feature microcontroller controls the motor speed. PWM output is fed to the driver circuit and then to the motor. To rotate the motor in two different directions, forward and reverse directional control blocks are used. A motor speed sensor is interfaced to a microcontroller. The microcontroller reads the speed of the and appropriately controls the speed of the motor in different phases of washing using PWM output. The agitator or rotating disc is coupled to this motor. 
  7. Sensors:  Several sensors such as door sensor, pressure sensor, temperature sensor, water level sensor is used to measure and monitor various parameters.  
Information systems and real-time interfacing systems
  1. Front panel: The front panel has buttons/switches to select various functions. Modern washing machine designs use a microcontroller which is programmed to perform a unique set of operations and controls the overall operation of the washing machine. 
  2. The real-time interfacing systems consist of the PWM (Pulse Width Modulation) control unit that controls the speed of the motor. 
Electronics systems: The electronics circuits consist of hardware circuits or interfacing circuits (usually signal conditioners) required to interface various sensors and actuators. The central processing unit is usually a microprocessor which is the main control unit for the overall effective functioning of the washing machine. The A/D converters and timers are also components of the electronics system. 


Figure 4: Components of a top load washing machine

The block diagram representation of a micro-controller based washing machine is shown in Figure 5. 


The various sequences of operation of the microprocessor-controlled washing machine system are explained next. 
  • During a pre-wash operation, the inlet valve is opened when the machine is switched ON and the valve is closed when the required level of water is filled in the drum. A water-level indicator sensor is used to give feedback to the microprocessor unit regarding the water level in the drum. After completing this operation, the microprocessor operates the motor to rotate the drum. The outlet valve is opened and a pump is operated to drain the water from the drum after completing the pre-wash operation.
  • The main wash cycle is then started by the microprocessor by operating the inlet valve to allow the water into the drum. The water-level sensor senses the water level in the drum and it closes the inlet valve after reaching a certain level. Now, the microprocessor switches ON the heating coil in the drum to heat the water. The temperature sensor sends the feedback of the temperature of water in the drum and switches OFF the heating coil when the water temperature reaches the preset value. Then the microprocessor operates the motor to rotate the drum. The motor rotates for a preset time or a number of revolutions and the microprocessor switch OFF the motor. The microprocessor operates the pump to drain the water from the drum.
  • The rinse cycle is started now, which has similar operations as explained above. This cycle is repeated two to three times. 
  • The spinning cycle follows the rinse cycle. In this cycle, the microprocessor switches ON just the motor, which rotates at a higher speed than the other cycles to drain the water from the clothes.

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