Motor manufactures’ data sheets provide excellent performance information about a motor’s optimum operating parameters such as voltage and current at the motor’s rated speed. Most manufacturers also provide tables and graphs that show how the motor responds to different voltages and the corresponding changes to RPM, torque, and power. However, very little information is available about how a particular motor responds to PWM signals.
We already know that decay mode and PWM duty cycle can control a motor’s speed, but to understand what happens when the mode is changed or frequency is varied, we need a way to measure the motor’s response to these parameters. If we measure the motor’s performance, picking the best-performing decay mode and PWM frequency is straightforward.
For example, the spin threshold frequency response spectrum of the Yellow-TT motor shows the motor voltage when spin threshold is reached (the blue line) as the PWM frequency is swept from 1Hz to 50kHz. At a PWM frequency of 20Hz, the motor begins to spin at 0.5 volts. As the frequency increases, the spin threshold changes eventually settling at approximately 0.6 volts. If low speed is your robot's thing, a PWM Frequency of 20Hz may be ideal -- as long as some motor chatter is tolerable. If smoother operation is needed, consider trying 400Hz. The chatter will decrease while maintaining a fairly low spin threshold. Selecting a lower frequency usually means that the motor's spin threshold torque will be relatively higher, as well.
The PWM frequency also determines the motor's speed at the spin threshold. The following chart compares a few PWM frequencies used to control a Yellow-TT motor. At 25Hz, the motor starts spinning at 200 RPM when the motor voltage is 0.5 volts. When the frequency is 500Hz, the motor spins at 1500 RPM when the spin threshold voltage of 1.5 volts is applied. Keep this in mind when choosing a frequency, especially if a low motor speeds is needed.
The motor speed curve noticeably changes when fast current decay mode is chosen instead of slow decay.
Test the decay mode and frequency you selected with the motor attached to your robot and operating with its normal power supply source. Remember that PWM frequencies of 60Hz or lower can cause mechanical vibration depending on the motor and the quality of its gearbox. Adjust the PWM frequency for the best balance of vibration and low-speed operation.
Motor performance charts for a variety of popular brushed DC motors can be found in the Motor Performance Charts section.