DC Motors

Two types of DC motor were readily available in the lab for us to use: brushed and brushless. Though they have some similarities, their differences ultimately lead us to choose one over the other. (For our hands-on experiences working with the motors, check our mechanical design page.)

Where Brushed beats Brushless

H bridge layout. The switches can be closed independently of each other to reverse the current through the motor M.

Brushed motors have several advantages over their brushless counterparts. The main advantage is the direction of the motor can be reversed by reversing the current flow through the motor. This is achieved without switching the wires by using several switches or an H bridge chip. The brushless DC motor, on the other hand, has 3 control leads which must be manually re-wired to reverse the motor direction.

Controlling the speed of a brushed motor is very simple: to increase the speed, one needs only to increase the voltage through the motor. The brushless motor, however, is controlled  by pulse-width modulation; that is, the width of the rectangular input signal affects the motor’s speed.

Though it is easier to increase the speed and reverse the direction of the brushed motor, those strengths do not play to the project’s goals.  We never require the motor be able to go forwards and backwards – with lower or no airflow, gravity will do the work of pulling the ball towards the base station – so switching it electronically is not important.  Similarly, the Arduino Uno and Seeduino Mega have several PWM slots and ready software libraries, so generating an appropriate signal for the brushless motor can be easily programmed.  See this page for how to work with brushelss DC motors.

Advantages of the Brushless DC Motor

Compared to brushed motors, brushless motors have:

  • more torque per weight
  • more torque per watt (increased efficiency)
  • increased reliability
  • reduced noise
  • a longer lifetime (no brush and commutator erosion)
  • elimination of ionizing sparks from the commutator
  • an overall reduction of electromagnetic interference

Though some advantages, like lifetime concerns, are negligible benefits for us, the main strengths of brushless motors for us are in their increased efficiency and reduction of EMI. The former is crucial for the thrust required, and the latter is important in terms of signal interference.  This was an issue appearing in past students’ projects, which adversely affected the precision and operation of their radios and sonars.  Since the motor is the noisiest component in our system, this is a non-trivial issue for us.

Final Choice

Both motors were highly available and easy to use.  We originally started with brushed DC motors (CPU fans), but after multiple tests and attempts to use them, we ultimately shelved them as they did not give us nearly enough air flow.  With the brushless motor, we have a device that is easy to use, gives ample thrust, provides many inherent benefits in noise reduction and longevity, as well as giving us the opportunity to become familiar with a type of motor that can and has been used in a quadcopter.  Since becoming familiar with hardware components for a quadcopter is a desirable goal of this project, going with the brushless motor is the appropriate choice.