Pulse Density Modulation

[esot.eric] was trying to drive a motor and naturally thought of using pulse width modulation (PWM) to control the motor speed. However, he found that even with a large capacitor, his underpowered power supply would droop before the PWM cycles were complete. So instead of PWM he decided to experiment with pulse density modulation.

The idea is to use smaller pulses over a longer period of time and make the average power equal to the percentage motor speed desired. With a PWM system, for example, if the time period is T, a 50% PWM drive would have the  drive high for T/2 and low for the other half of the cycle. With pulse density, each pulse might be T/10 (as an example) and then the output would be on for 1/10, off for 1/10, on for 1/10 and so on, until by time T you’d still get to 50%. The advantage is the output capacitor gets a kick more often and has less opportunity to droop.

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NXP’s ARM Micros With Motor Controllers


It’s still relitavely early in the year, and all those silicon manufacturers are coming out with new toys to satiate the engineer and hobbyist for years to come. NXP’s offering is the LPC1500, a series of ARM microcontrollers optimized for motor and motion-control applications.

The specs for the new chips include an ARM Cortex-M3 running at 72MHz, up to 256kB Flash, 36kB SRAM, USB, CAN, 28 PWM outputs, an a real-time clock. There are options for controlling brushless, permanent magnet, or AC induction motors on the LPC1500, with dev boards for each type of motor. Each chip has support for two Despite NXP’s amazing commitment to DIP-packaged ARM chips, the LPC1500 chips are only available in QFP packages with 48, 64, and 100 pins.

Don’t think the LPC1500 would be a perfect chip for a CNC controller – the chips only support control of two motors. However, this would be a fantastic platform for building a few robots, an electric car, or a lot of the other really cool projects we see around here.