This FPC-Based Stepper Motor "Oversteps" the Mark of What We Thought Was Possible with This Tech!

If there's one person who knows more than a thing or two about the construction of PCB-based motors, it's Carl Bugeja.

We've seen his flexible, FPC fabricated electromechanical actuators "making the rounds" on the front pages of many hacker news sites — though we're sure you'll have been getting your news fix from us, right?

Bugeja's latest lot of tinkering comes to us in the form of a super-flat, super-thin, FPC-based stepper motor looking to answer the burning — though hopefully not literally... — question of if such a device would be capable of microstepping.

Microstepping?

Microstepping is the mumble-rapping equivalent to breakdancing. Does that make sense? It covers a few generations in one analogy, so stick with me, I'll try and clear it up.

Rather than rely on the conventional electromotive force supplied by driving coils solely in antiphase, microstepping also augments those forces by driving complimentary and adjacent poles with a secondary current flow.

This combination of primary and secondary electromagnetic fields in conjunction, should enable finer grained control of the "positions" that the permanent magnets can be driven do, depending on the direction and sequencing of the currents flowing through the coils.

Microstepping is a really nice feature to have in a driver, and while it's not present in all of them, it's fairly common, even on commodity drivers such as the well known DRV8834 — which has until now, been the basis of many of Bugeja's builds.

It's a really neat feature that can allow a well designed stepper motor to achieve incredibly small incremental positional steps of angular resolution., and in this specific, flexible implementation, it may possibly be the key to opening the doors to all sorts of weird and wonderful, perhaps even deformable, planar stepper actuators.

Stepping on from microstepping

We took a quick look at the coil arrangement in our micro-module on micro stepping above, but let's take a look at the entire layout of this flexible electromagnetic FPC.

Designed as one single FPC circuit board, the motor coils and driver circuit itself are formed as one circuit board.

We recall certain experiments with hybrid-flex stackups — fusions of conventional, thicker FR4 with FPC layers fused into the laminate — but this is (usually) an excessively expensive option when it comes to pricing up PCB stackups!

Fitting everything onto the single-technology FPC production line can help keep costs down in that aspect.

With Bugeja somewhat bucking the trend and opting to design this deformable motor in Alitum Designer, we can imagine there must be a few handy features in that package to facilitate laying out so many tessellated tracks, to form those tiny coils of this two-phase, four-pole configuration motor.

We can see the bottom layer shown above in the board view of Altium Designer — keep in mind this effort is twice tracked out — once again on the top layer.

Indeed, this is one layout you wouldn't want to be left to have to make by manual means — having to manipulate all those meandering tracks into place. Yikes, no thanks.

Incredibly, all of these windings can be wound up into a circularly outlined layout, with a radius of only 15mm! That's right, this motor is some 30mm at its widest point in the windings!

While we can think of a few crafty ways to script this in KiCAD — it's clear that there are still some proprietary features in Altium Designer that must make this sort of layout exercise slightly less insanity inducing.

But — why is Bugeja so focused on FPC?

Well, for a start, these sorts of stackups are super thin.

With a dual-layer FPC stackup here, showing a total thickness for both copper layers and the mylar substrate coming in at a tiny 0.2mm, this reduced distance between the layers of coils should help with the efficiency when it comes to generating the required electromagnetic fields needed to effect the permanent magnet rotor.

With boards designed and received from PCBWay way back in September, we can tell Bugeja has been a busy man, having only just recently found the time to get these PCB's populated and programmed — even if he was faced with some predicament at certain points in both processes!

Diving into the driver!

We've focused on the flexible coil form, but let's not skip over taking a scan of the driver part of this FPC stepper motor.

Temporarily moving away from his previously proven workhorse of a driver IC, the DRV8834 from Texas Instruments, Bugeja is testing the waters with some new types of driver — having been thrown a tip from a Patreon subscriber suggestion that he take a look at the TMC2300, one of a nifty range of stepper motor drivers from Trinamic — now part of the Maxim IC family.

The TMC2300 is targeted at low-power applications, so makes perfect sense here for this super-thin project. We can see Bugeja's implementation of the IC in the stepper driver portion of his schematic below — and it looks like a nice way to get stepper functionality into a small space — this part is crammed into a tiny 3x3mm QFN-20, so it should fit into all but the smallest spaces.

As for the control of this phase switching peripheral IC, Bugeja turns to his handy family of well known PIC16F MCU, which we've seen put into practice in previous projects from this prolific PCB artist.

With one final block of schematic left to review, this deceivinglu simple duo of connectors are far far more problematic to a FPC than their simple schematic drawing suggetsts they ever could be...

Bugeja's cooking up some PCBs, fancy a slice?

With a fancy new hotplate, and possibly some previous work experience in a pizza joint showing through, Bugeja turns to this direct-heating method for reflowing his components in place.

It's simple and effective, and forgoes any risk of air circulation flowing in conventional ovens flipping the lightweight FPC out of place.

With paste application to be proud of, there's a perplexing bit of theatrical fireworks on the reflowing of one for the final boards — with placed components popping off the PCB like popcorn kernels.

It's very worth watching the video of the entire bring-up process of these FPC stepper motors, as the still from below doesn't do justice to the unscheduled, energetic disassembly of the pasted and placed components — but that hotplate was free from the scattering of parts that previously populated that PCB's pads!

FPC — the clue is in the name... flexible...

The thing about FPC circuits is that they are freakishly thin — but as far as mechanical rigidity is concerned, they are pretty welterweight.

One thing is for sure, they don't exactly support screw terminals. You can see even a mild application of torque quite literally tearing the terminal pads from the FPC substrate.

This is quite atypical, I'd like to make that point. Normally, the connector pads aren't meant to fall off.

This fiendish flexibility also might lead a line of enquiry as to the popcorn fireworks we saw previously during paste reflow. This inherent flexibility of the substrate leads to some unforeseen problems when making sure it's being reflowed properly — not aided by the almost instantaneous reflow profile provided by the hotplate used for heating.

So the major question — how well does this micro stepper move?

Well, as with all things, when it gets to the mechanical side, there's a bit of fine tuning involved. First up, you need to get the 3D printer out, usually.

First on that checklist of FDM components is a stator capable of holding some precisely fitted magnets and a bearing in the central hole.

Like we said, precision is the name of the game when it comes to mechanics.

But with a bearing bashed into place, and some magnets rather heavily manipulated into their required mountings, the end result is magnificent. We won't spoil the surprise, and really do suggest you take a look at some of the problems that this flexible FPC substrate has put forwards — and the ways in which Bugeja is addressing them...

One of the most interesting of which, is perhaps the one that has the very core property of the flexibility of the FPC motor as its as its main foible — with the deflection of the coils in repulsion to the stator magnets shown in the side-by-side screencap below.

Bugeja is always cranking out great YouTube content, so both his channel and his Twitter are worth following! If you'd like to go a little further and support his work, we encourage you to check out his Patreon page, where he really does make the effort to interact with his supporters — he's an all-round great guy!