6.1.4 Controlling the machine

The major task of a washing machine microcomputer is to control the actions of the machine in accordance with the wash programme selected. To do this, the computer is electrically attached to a variety of:

• actuators that cause mechanical parts of the system to work;
  


• sensors that sense the state of some aspect of the machine, such as water temperature.
  

There is an actuator to open or close the water input valve. Another controls the turning of the drum, and another the pumping of water through the machine. There is also one for pumping the water to the drain and one for controlling the water that washes through the tray holding washing powder and fabric softener. Lastly, there is an actuator that turns on a heating coil if the water temperature is below the desired temperature for the wash.
As regards sensors, my machine has one to check water level, and another to check water temperature. It also has a sensor that weighs the dry laundry at the start of the cycle, providing data that the microcomputer program uses to determine how much water to use for each wash. This enables my machine to optimise water use, and hence conserve resources (water and the electricity to heat the water).

Exercise 16

The general wash programme cycle on my machine goes through the following steps.

1. It weighs the dry laundry once the door is shut and the ‘Start’ button is pressed.
   


2. It determines how much water should be used and opens the valve.
   


3. It pumps water through the washing powder tray for a period of time to flush the powder into the drum, and then pumps the water directly into the drum.
   


4. It locks the main door when the drum is in motion, or when the water level is higher than the bottom of the door (a safety feature).
   


5. It heats the water to the correct temperature for the programme and point in the washing cycle.
   


6. It turns the drum and recycles the water onto the laundry for the duration of the washing cycle.
   


7. It drains the water from the drum.
   


8. It spins the laundry at the correct speed for a certain time.
   

The programme stops.
For each of the eight steps listed above, list the actuator or sensor involved.

Discussion

1. The weight sensor.
   


2. The valve actuator.
   


3. The actuators for the water pump and for the valves channelling water in different ways through the machine.
   


4. The actuator for the door lock and the water level sensor.
   


5. The water temperature sensor, and the actuator for the heating coil.
   


6. The actuator for the motor spinning the drum, the actuator for the water pump and the actuators for the valves leading into and out of the drum.
   


7. The actuator for the pump and the actuator for the drain valve (or the actuator for water channelling, depending on the design or type of the machine).
   


8. The actuator for the motor spinning the drum and the actuators that drain water from the drum (as in point 7).
   

SAQ 8

What role do computer control systems play in many mechanical devices?

Answer

They manage sensors and actuators to control the actions of mechanical devices such as cars, microwaves and washing machines.
They provide interfaces that enable the user to control the workings of the machine.
They control the machine's actions in response to the user's choices and the state of the machine.