Pulse Width Modulation and application in automobiles

Pulse width modulation also known as pulse time or pulse duration modulation reduces the average power delivered to a load by chopping electrical signals into discrete parts.

The average voltage and current delivered to the load is controlled by ON and OFF switching at a particular frequency, mostly at a fast rate.

The longer the switch is in the ON position relative to OFF position, the higher the total power supplied to the load.

The literal meaning of pulse width modulation is varying the width of pulse. PWM is used for varying analog signals from digital signals.

Advantage of using pulse width modulation in automobiles

1. In the automotive world, there is emphasis on efficiency and fuel economy. Manufacturers are improving every component to get better efficiency, less fuel consumption and less power consumption. PWM is used to minimize power consumption since the power to the load is not constantly ON. The switching frequency also known as duty cycle has to be high enough in order not to affect the load.

2. Another advantage which is linked to reduced power consumption is that power loss in switching devices is very low.

3. Because of the ON and OFF nature of a pulse width modulated signal or power, its duty cycle can be easily set.

What is duty cycle?

It is the proportion of the ON time to the regular interval of time. A high ON time translates to a high duty cycle. Duty cycle is expressed in percentage and a 100% duty cycle signifies fully ON. If it is ON half of the time, it has a duty cycle of 50% and the waveform will resemble a square wave.

Where PWM is used in automobiles

1. PWM is employed in Antilock braking system (ABS) of automobiles. It switches the direction of DC motor during braking and it gives about 36% improvement in braking. An ABS does not apply brake pad to the brake disc continuously but applies it in a remove and apply manner continuously until the driver stops braking.

2. It is used in efficient voltage regulators. It does this by switching voltage to the load (for example battery) at an appropriate duty cycle. Inductor and capacitor filters the switching noise.

3. PWM is used in cooling fan operation and speed control. The engine control unit (ECU or ECM) is the source of the PWM signal which controls the output of the cooling fan module that is connected to the fan motor.

4. PWM is used to control solenoid valves and actuators. The power fed to these components are not constant power but pulsed. This is the reason why on a bench test, it is not appropriate to continuously feed them with battery voltage to test whether they are working or not. By doing that, they can get damaged.

5. PWM is used to convert DC to AC by using a smaller signal to modulate the power. This application is in inveters. Inveters are more common in hybrid and electric cars.

6. Ignition coils in automobiles won't function as step up transformers without break in current provided by PWM.

7. Lights like LED (light emitting diode) brake lights or direction indicators can be dimmed or pulsed at a rate that the himan eye cannot detect using PWM.

8. Radio volume control and sound effects.

9. In modern cars, fuel pumps are not driven directly by battery voltage but driven by a pulse modulated voltage. This is why we see fuel pump duty cycle as one parameter in diagnostic tool.

Apart from these, PWM has found wide application in other systems of automobiles and other electrical systems.