Kevin Lynagh's Custom PCB Turns Magnets Into Tiny Robots via a PCB Stepper Motor System

Building on concepts from the 1980s, Lynagh's system flips a tiny magnet around a PCB in tiny programmatically-controlled steps.

Gareth Halfacree
2 years agoHW101 / 3D Printing

Designer Kevin Lynagh has taken a technique for turning printed circuit boards into micro-robotic "stepper motors," originally developed as part of DARPA's Open Manufacturing Program, and successfully replicated it — creating tiny "robots" that can walk around a surface, under manual or programmatic control.

"I saw this sweet video of tiny magnets moving around a PCB," Lynagh writes, referring to a demonstration published by SRI International back in 2014, "and decided to replicate. The SRI system consists of two main ideas: Generation of magnetic fields via PCB traces — think 'unrolled bipolar stepper motor'; [and] diamagnetic (passive) levitation of magnets via a graphite layer between the PCB and magnets."

A PCB, or a stepper motor? This build flings a magnet around, under programmatic control. (📹: Kevin Lynagh)

In other words: It's a system for converting low-cost PCBs into just-as-low-cost motors, offering a very small size yet with high precision. Intrigued, Lynagh set about researching the topic — going back to a 1988 PhD thesis and finding various makers, including Peter "bobricius" Misenko, who have created their own variants.

"The basic idea is to have two sets of serpentine coils and selectively run current through them to generate a moving magnetic field that drags along the permanent magnet 'robot,'" Lynagh explains. "Before designing a custom PCB I wanted to quickly establish viability, so I 3D-printed a template and hand-wound two serpentine coils of 30 AWG enameled magnet wire, covered with polyimide tape (for a smoother surface and to keep the wires from popping out), and drove with a TMC2209 stepper driver I had laying around at a 1.6A current limit."

Initially, Lynagh drove the steps manually before switching to programmatic control. Concept proven, a PCB was designed offering two zones for different step sizes, support for a controlling STMicro STM32F01 microcontroller, and current control for micro-stepping and power requirement experimentation.

Lynagh started with a 3D-printed prototype, just to prove his understanding of the core concept. (📹: Kevin Lynagh)

"I got the PCB working," Lynagh writes, "which I always find to be a pleasant surprise. For the 'robots' I tested both single 1mm diameter, 1mm thickness N52 disk magnets and arrays of five such magnets press-fit into a resin-printed carrier, with the center magnet's polarity opposite the outside four. This array could be reliably actuated at lower currents than the single magnet; the minimum was about 170mA compared to 200mA for the single free 1mm disk magnet."

Lynagh's full write-up is available on his website; he has also proposed a series of next steps, including integrating pyrolytic graphite to levitate the tiny robots and improving sliding performance with dry lubricant or pressurized air; at the bottom of the to-do list is finding a practical application, including a machine to "sort grains of sand" or a "self-moving tiny chess set."

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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