Foreign engineers thought drone motors were too heavy, so they made a PCB motor[Copy link]
IEEE SPECTRUM recently reported an engineer's wonderful idea. Axial flux motor using PCB traces as electromagnetic coils Each layer of the motor printed circuit board has a group of coils, which are stacked together and connected to each other to form a continuous trace. I started out wanting to just make a very small drone. But I soon realized that one limiting factor in my design was the size and weight of the motors. Even small motors are still discrete devices that need to be connected to all the other electronics and structural elements. So I started wondering if there was a way to consolidate these components and shed some of the mass. I was inspired by how some radio systems use antennas made from copper traces on a printed circuit board (PCB). Could I use something similar to create a magnetic field strong enough to drive a motor? I decided to see if I could use electromagnetic coils made from PCB traces to make an axial flux motor. In an axial flux motor, the electromagnetic coils that form the motor's stator are mounted parallel to a disc-shaped rotor. Permanent magnets are embedded in the rotor's disc. Driving the stator coils with alternating current causes the rotor to spin. The first challenge was to make sure I could create enough magnetic flux to turn the rotor. It would be simple enough to design a flat spiral coil trace and run current through it, but I limited my motor to 16 mm in diameter to make the overall motor diameter comparable to that of the smallest off-the-shelf brushless motors. 16mm meant I could only fit a total of 6 coils on the underside of the rotor disc, with about 10 turns per spiral. Ten turns isn't enough to create a large enough magnetic field, but it's easy to make multi-layer PCBs these days. By printing the coils in stacks (with coils on each of the four layers), I could get 40 turns per coil, enough to turn a rotor. As the design moved forward, a larger problem emerged. 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 control circuitry that implements a feedback system. Photo credit: Carl Bugeja Left: The finished four-layer printed circuit board. Center: Pulsing these coils drives 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 previous brushless motor driver I built, I measured back EMF as feedback to control speed. Back EMF is generated because the spinning motor acts like a small generator, generating a voltage in the stator coils that opposes the voltage used to drive the motor. Sensing the back EMF provides feedback about how the rotor is spinning and allows the control circuit to synchronize the coils. But in my PCB motor, the back EMF was too weak to be used. 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 fed into the motor control circuitry. 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-on shaft as an integral part of the rotor. This simplified the physical components to just the rotor, four permanent magnets, a bearing, and the PCB that provided the coils and structural support. I soon had my first motor. Testing showed that it produced 0.9 g cm of static torque. This wasn't enough to meet my initial goal of making a motor that could be integrated into a drone, but I realized that the motor could still be used to propel a small, cheap robot on wheels along the ground, so I kept going (the motor is usually one of the most expensive parts of a robot). This printed motor can operate on voltages between 3.5 and 7 volts, although it will heat up significantly at higher voltages. At 5 V, it operates at 70°C, which is still manageable. It draws about 250 mA. Currently, I’ve been working on increasing the torque of the motor (you can follow my ongoing research progress on Hackaday https://hackaday.io/project/39494-pcb-motor). By adding ferrite sheets to the back of the stator coils to contain the coils’ magnetic field lines, I was able to nearly double the torque. I’m also working on designing other prototypes with different winding configurations and more stator coils. Additionally, I’ve been working on using the same techniques to build a PCB electric actuator that can drive a 3D-printed slider that slides over a row of 12 coils. And, I’m testing a flexible PCB prototype that uses the same printed coils to perform electromagnetic actuation. My goal is—even if I can’t build a little drone that can fly—to start building robots with smaller and simpler mechanical construction than existing robots. This article will appear in the September 2018 print edition of IEEE SPECTRUM, titled "The Printable Motor." [font=myFont,Source: IEEE SPECTRUM
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