Pulse Width Modulation (PWM) Signals

Pulse Width Modulation Signals:

Background
On modern cars it is often necessary to send information between different systems, and some times between systems of different makes. Analogue transfer of information has proved to be unreliable. Voltage drops in cables and connectors, as well as tolerances in the various control modules and their sensors, are some of the causes of this unreliability.

PWM (Pulse Width Modulation)
This problem is solved by using PWM signals. PWM stands for Pulse Width Modulation, which means that it is the time the cable is energized that is important, not the voltage level.

The PWM signal can also be described as a square wave, where the width of the voltage pulses corresponds to the information. Since the voltage has only two states, on or off, the signal is said to be digital.

Example:
A throttle position sensor applies a voltage to a Motronic control module. The voltage level depends on the position of the throttle valve, so that the voltage at the closed throttle position may be around 0.2 V and at wide open throttle about 4.5 V. The voltage is thus proportional to the angle of the throttle butterfly valve and the value is said to be analogue.

Information about the throttle position can then be conveyed to other systems, such as the TCS, automatic transmission, etc., but as a PWM signal rather that an analogue voltage.

PWM signals in this case always have a pulse ratio of 9% in the closed throttle position (higher when the accelerator is depressed), regardless of any voltage drop or tolerances.

Pulse Width Modulation Signal Frequency:

PWM Signal Frequency
The information that is sent from one control module to another often changes very quickly and it must therefore also be possible to change the pulse width very quickly.

This also means that the pulses have to occur very often, i.e. the signal has to have a high frequency.

The PWM signal generally has a fixed frequency of between 75 and 225 Hz, which means that the pulses occur 75 to 225 times a second. *)

Common frequencies in cars are 100, 122 and 200 Hz for transmitting control modules while receiving control modules generally accept relatively wide deviations from these values. In this way we obtain very reliable communication.

*) In exceptional cases the frequency is not fixed, such as the engine load (Tq) signal from the Trionic system, the frequency of which depends on engine speed but the pulse width reflects the engine load.

Characteristics Of The PWM Signal:

Characteristics Of The PWM Signal
Frequency
The actual frequency does not tell us anything about the value of the information carried by the cable but it can confirm that contact has been established with transmitting control modules and we can check whether the frequency is within reasonable limits.

Pulse Length
At 100 Hz there are 100 pulses a second, which means that the maximum pulse length is 1/100th of a second, i.e. 10 milliseconds. The pulse length at 100 Hz can therefore vary between 0 and 10 milliseconds.

Pulse Ratio
If we have a pulse length of 0.9 ms at 100 Hz, it is said that the pulse ratio is 9% (the duration of the pulse is 0.9 ms out of a possible 10 ms). When information is being transmitted, the pulse ratio is between 8% and 92%.

Pulse Width Modulation Signal:

When The PWM Signal Is Not To Be Regarded As A Signal
The PWM technique can also be used for purposes other than transferring information between control modules. PWM can be applied directly to a consumer (item of electrical equipment) in the system, in which case we do not refer to it as a PWM signal because it is not information as such that is transmitted.

One example is the instrument lighting which is supplied with PWM voltage, where a low pulse ratio causes the lamps to shine dimly while a higher pulse ratio increases their brightness. Since the frequency is high, the eye cannot perceive that the lamps are "flashing".

Other examples are the throttle actuator motor in the traction control system, the IAC valve, the boost pressure control valve for the APC and the EVAP canister purge valve.

It can also be said that the power supplied to the injectors is pulse width modulated, as the pulse width corresponds to the time the injectors are open.

In the above examples we refer to PWM outputs (the transmitter) and PWM inputs (the receiver). The system manufacturer has a free choice of frequencies and pulse ratios with regard to these inputs and outputs. Some common frequencies are 70-90 Hz for the boost pressure control valve in the Trionic engine management system, 56-90 Hz for the instrument lighting and 500 Hz for the throttle actuator motor of the TCS system.