Pulse Width Modulation (PWM) is a technique that uses digitally controlled pulses of varying width. These pulses, when applied to a suitable load such as a motor winding, are “smoothed” and allow the current in the motor winding to be controlled.
Originally the PWM switching technique was used as a method of making a very efficient amplifier. In a Direct PWM system, as employed by Delta Tau, the output devices are controlled directly from the motion controller allowing much faster control of the current in the motor which in turn increases the overall performance.
In servo motion digital control has many clearly identified advantages. Setting the gains can be much easier, a matter of changing numbers in registers instead of tweaking potentiometers. Gain settings can very easily be copied from system to system. The data logging capabilities of a digital system make it easy to store and plot key data for tuning and diagnostics.
Above is a simplified example of pulse-width modulated (PWM) voltage supply to a magnetic circuit. The voltage (blue waveform) in a magnetic circuit is proportional to the rate of change of the flux density (red waveform). Therefore, by using PWM supply the resultant flux density can be controlled with relative ease. This method is commonly used for supplying electric motors, in which the torque is proportional to the flux density. As can be seen, with a series of appropriately modulated voltage impulses the resultant flux density can be modulated to be close to desired sinusoidal waveform. In the example shown (for 50 Hz), for clarity the switching frequency is 600 Hz, in real devices the switching frequency is much higher – even up to 10 kHz or higher. Of course, for higher fundamental frequency the switching frequencies would be even higher.