“Each layer of a motor printed circuit board has a set of coils that are stacked on top of each other and connected to each other to form a continuous trace.
Each layer of a motor printed circuit board has a set of coils that are stacked on top of each other and connected to each other to form a continuous trace.
I just wanted to make a very small drone at first. But I quickly realized that there was a constraint in my design, and that was the size and weight of the motor. Even small motors are still discrete devices that need to be connected to all other Electronic and structural elements. So I started wondering if there was a way to incorporate these elements and lighten some of the mass.
I was inspired by how some radio systems use antennas made from copper traces on a printed circuit board (PCB). Can I use something like this to make a magnetic field strong enough to drive a motor? I decided to see if I could make an axial flux motor using solenoid coils made from PCB traces. In an axial flux motor, the electromagnetic coils forming the stator of the motor are mounted parallel to the disc-shaped rotor. Permanent magnets are embedded in the discs of the rotor. The rotor is rotated by driving the stator coils with alternating current.
The first challenge was making sure I could create enough magnetic flux to turn the rotor. Designing a flat helical coil trace and letting current flow through it is trivial, but I limited the diameter of my motor to 16mm so that the entire motor is comparable in diameter to the smallest off-the-shelf brushless motors. 16mm means that I can only fit a total of 6 coils under the rotor disc, about 10 turns per helix. Ten turns are not enough to generate a large enough magnetic field, but it’s easy to make multi-layer PCBs these days. By printing as stacked coils (coils on each of the four layers), I was able to get 40 turns per coil, enough to turn a rotor.
As the design moved forward, a bigger problem arose. To keep the motor spinning, the dynamically changing magnetic fields between the rotor and stator must be synchronized. In a typical motor driven by alternating current, this synchronization occurs naturally due to the arrangement of brushes that bridge the stator and rotor. In a brushless motor, what is needed is a control circuit that implements a feedback system.
Left: The finished four-layer printed circuit board. Middle: These coils are pulsed to drive a 3D-printed rotor with embedded permanent magnets.
Right: While not as powerful as a traditional brushless motor, the PCB is cheaper and lighter.
In a brushless motor driver I made before, I measured the back EMF as feedback to control the speed. Back EMF is created because a spinning motor acts like a small generator, creating a voltage in the stator coils that is the opposite of the voltage used to drive the motor. Sensing the back EMF provides feedback on how the rotor is spinning and allows the control circuit to synchronize the coils. But in my PCB motor, the back EMF is too weak to use. To do this, I installed Hall Effect sensors, which directly measure changes in the magnetic field to measure how fast the rotor and its permanent magnets are spinning above the sensor. This information is then input into the motor control circuit.
To make the rotor itself, I turned to 3D printing. At first I made a rotor that I mounted on a separate metal shaft, but then I started printing the snap shaft as an integral part of the rotor. This reduces the physical assembly to just the rotor, four permanent magnets, a bearing, and a PCB that provides the coils and structural support.
I got my first electric motor very quickly. Tests have shown that it produces a static torque of 0.9 gcm. This wasn’t enough for my original goal of making a motor integrated into the drone, but I realized that the motor could still be used to propel a small, cheap robot on wheels along the ground, so I stuck with the research (the motor Usually one of the most expensive parts on a robot). This printed motor can operate from 3.5 to 7 volts, although it heats up significantly at higher voltages. At 5 V, its operating temperature is 70°C, which is still manageable. It draws about 250 mA.
Currently, I’ve been working on increasing the torque of the motor. By adding a ferrite sheet to the back of the stator coil to contain the coil’s field lines, I can almost double the torque. I’m also working on designing other prototypes with different winding configurations and more stator coils. Also, I’ve been working on using the same technique to build a PCB motorized actuator that drives a 3d printed slider that slides over a row of 12 coils. Also, I’m testing a flex PCB prototype that uses the same printed coils to perform the electromagnetic actuation. My goal – even if I can’t make small drones that can fly in the sky yet – is to start making robots with smaller and simpler mechanics than existing robots.
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