Servo - November 2015

28 SERVO 11.2015

Brushless motors are quickly becoming the standard motor for weapon drive in combat robotics.

In addition to removing the shock and current-sensitive brushes that wear over time, the brushless motor packs more copper windings and magnets into a given space. Put simply, there’s more power per given size. For driving the wheels of a robot, brushless drive seems to be a dream just out of reach ... but no longer as two solutions have come together to provide a reliable and controllable drive platform in the smallest of space.

As brushed motors are mechanically commutated to generate rotation given a voltage, brushless motors require a drive or ESC (electronic speed controller) that can start and run the brushless motor by controlling power through the motor’s three copper coils. Many small or inexpensive brushless motors lack feedback Hall-effect sensors that provide information on the direction and speed of the spinning rotor. To control these motors, ESCs must sense the changes in field voltage made by the rotation of the magnetic rotor inside the coils.

Most ESCs use firmware (hardware-specific software) to start the motor at a relatively moderate speed to get the rotor spinning, and adjust direction and speed to “lock in” the rotor synchronously (at the same time) with the change in coil magnetic fields. This type of control yields poor low speed performance, but is the ESCs. For aircraft, low speed is not necessary as the propellers used do not generate much lift at low speeds.

The demand for aerobatic flight controls has changed the nature of the startup procedure of brushless ESCs. Several software developers have created very sophisticated sensing and low speed timing code for several microcontroller platforms. For Atmel-based microcontrollers, one of the more popular firmware sets is Simon Kirby. Another popular firmware set is called BL Heli and was created by Steffen Skaug for Silicon Lab based microcontrollers.

Both platforms are open source (source code is documented and freely available on the Web) and can be reprogrammed through a bootloader (code in the chip that directs the initialization to a programmable section in memory). There are several graphical programmers that can load/reload code to a brushless motor drive. This is important to wheeled drive motors because we require most aircraft drives to be reprogrammed to work with either RC controller-based center stick or car style forward and reverse calibration. Most importantly, the low speed control characteristics provide the same fine control capable with brushed motors.

Figure 1 is an example of three USB-based programmers that can load the firmware into compatible ESCs through the receiver cable or special socket programmers.

Now that there are ESCs with the firmware necessary to operate a brushless motor, we need to determine how to get the high speed and low torque (rotation force) of the motor to the necessary low speed and high torque to the wheels. For this, we will need a gearbox. Once again, we can leverage aircraft motors as they are often inexpensive and designed to work with the drives noted above.