![]() Generating PWM signals is beyond the scope of this bulletin, instead, we will focus on how it can be used to drive the motor and assume that a suitable PWM source is readily available in the circuit. Many microcontrollers have PWM generators built in, alternatively, the antique technique is to generate it using an analogue signal, a sawtooth signal, and a comparator. This enables analogue control of the motor using digital signalling, making it very useful in electronics systems that use microcontrollers to control the motor. Changing the pulse widths results in a change of the average voltage after filtering, allowing any value between zero and the maximum voltage to be represented by increasing or decreasing the pulse width. The width of the on burst can be adjusted by whatever is producing the PWM signal (typically a microcontroller), which is complemented by the inverse change in the off pulse width, thereby maintaining the same frequency. Example of motor driven by a PWM signal The modulation doesn’t work if the load is a resistor With the motor as a load, the PWM signal is clearly averaged (though still a little spiky/noisy, but we can improve on that later). The example circuits below demonstrate the difference in the resulting voltage between an inductive load (with the motor) and a resistive load (without the motor). If the load was purely resistive then the PWM waveform would still be visible. Luckily, because of the inductive and resistive nature of a DC motor’s windings, it effectively has its own low pass analogue filter built in. Instead, we want the PWM signal to appear as a smooth averaged signal, which is proportional to the duty cycle of the digital signal. The specified frequency of the signal needs to be sufficiently high so that the load, in our case a vibration motor, does not see ‘bursts’ of high and low (the switching digital signal). Since a vibration motor’s speed and frequency of vibration is directly proportional to the voltage applied to the motor, we can use PWM to control precisely how the motor runs. This is useful because when the PWM signal is averaged with a simple analogue filter, a DC voltage is produced that is proportional to the duty cycle (which is the percentage of time that the PWM signal is high). ![]() square waves) because the time that the signal is high and low can be varied. ![]() The PWM signal differs from other digital signals (e.g. It alternates between bursts of ‘On’ and ‘Off’, also known as high and low respectively, at a fixed frequency. A Pulse Width Modulated signal is a type of digital waveform.
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