PWM in effect creates an average voltage over a given period of time. This period is generally short, on the order of milliseconds or even microseconds, so that the dwell time between each state does not impede the intended operation of the device. Pulse width modulation signals are expressed as a rate called the duty cycle. The duty cycle represents the percentage of time over a given period that the device is in the "on" state. Figure 1 below represents graphs of a basic PWM signal at various duty cycles. Because the state is either "on" or "off", PWM signals produce a square wave pattern.
It is fundamental to understand that PWM is an example of digital logic, for which there are only ever two states - "off" (also "0" or "no") and "on" (also "1" or "yes"). Furthermore, only one state can exist at a time; the device is either receiving power from the source, or it is not. The source voltage remains consistent through the process, only the state of the voltage changes.
A simple example is using pulse width modulation to control the speed of an electric motor. For this example, consider an electric motor specified to operate at a maximum speed of 3,000 rpm at 12 volts DC. The chart below represents the angular velocity and average voltage as a function of PWM duty cycle:
| Duty Cycle | Angular Velocity (RPM) | Average Voltage |
|---|---|---|
| 10% | 300 | 1.2 |
| 25% | 750 | 3 |
| 50% | 1,500 | 6 |
| 75% | 2,250 | 9 |
| 100% | 3,000 | 12 [1] |
[1] - At 100% duty cycle, the average voltage is equal to the source voltage because the state is continuously "on"
Let us assume that the period of this signal is 10 milliseconds. At 10% duty cycle the motor will receive power for 1 millisecond followed by 9 milliseconds of no power, and this will repeat itself until the signal is modified by the controller. At a 50% duty cycle the motor will receive power for 5 milliseconds followed by 5 milliseconds of no power, and so on and so forth. The output of this system is plotted in figure 2 below.
Pulse width modulation is intended to be a rapidly executed process, i.e. the state of the signal must change at a very high rate to ensure intended results and smooth operation of the electromechanical device. If the signals are spaced too far apart (excessive time between the on and off states), the fluctuations in delivered power are going to be obvious and the device will behave erratically. If the signals are spaced close enough together, the fluctuations are undetectable and the device behaves smoothly, as intended.
Diagnosing PWM Devices
Pulse width modulation is used in a wide variety of car and truck components. Some of the more common parts that operate using PWM include EGR valves, turbocharger vane position actuators, engine cooling fan clutches, throttle bodies, camshaft actuators (variable timing camshaft systems), and even variable displacement fluid pumps.
When it comes to vehicle diagnostics and troubleshooting, PWM devices generally offer some form of self-diagnosis by comparison of actual to commanded values. A commanded value would be the duty cycle that the PWM controller (PCM, ECM, ECU, etc) is providing to the device. The actual value would therefore be the current position or state of the device. While this may not always be true, it is very common for both these values to be obtained using an appropriate diagnostic tool. Note that basic over-the-counter scantools generally lack this type of functionality and more sophisticated equipment is required.
Take for example an EGR valve. We may have access to parameter IDs (PIDs) for the actual and commanded positions of the valve and the ability to command the valve to any duty cycle we choose. You would first compare the actual and commanded values to ensure they were the same. If a PCM/ECM/ECU were calling for the valve to open at a 50% duty cycle but the actual reported position were only 10%, this would indicate that the EGR valve is not responding or has another condition that is impeding its function. If the actual and commanded positions were equivalent, the next step would be to manually cycle the valve open and closed by increasing/decreasing the duty cycle and monitoring the activity (assuming this functionality is available).
The same principles can be applied to a PWM controlled fan clutch where PIDs are available for fan speed and commanded duty cycle, as they often are. If the fan speed is commanded at 0% but monitored at 50%, there is an obvious defect in this circuit and/or this equipment. These same principles are widely applicable to all PWM devices. The process can become increasingly difficult when there is no such PIDs available for real-time viewing, but there is always a way to compare what a device is doing versus what it is supposed to be doing.