How a 15 V Stick-Slip Piezo Motor Works: The Whole Bench on One Page

How a line of firmware becomes a step

An Arduino Mega streams an 8-bit sawtooth table out of its parallel ports into a DAC0800 current-output DAC. A TDA2050 audio amplifier does two jobs at once, converting that current to voltage and then pushing a 0→~15 V sawtooth with a sharp flyback into a 1.8 µF piezoelectric stack [11, build guide]. The stack, an 18 mm CoreMorrow PSt150/5×5/20H, extends about 2 µm at that voltage (Derived: 20 µm × 15/150, from the datasheet-verified 20 µm at 150 V [9], scaled linearly, which is a rough assumption at a tenth of rated field). A printed foot rides the stack's nose and presses sideways into a polished alumina plate on the slider of a fully 3D-printed rail. The foot's tip is a 2 mm silicon-nitride ball (a bare resin dome, until August). The pressing force comes from a spring I can set in calibrated 0.099 N clicks. During the slow ramp, friction drags the slider along. During the fast flyback, the slider's inertia wins and the foot slips underneath. Each cycle nets a step on the order of a micrometer. When the interferometer is on the bench, a mirror on the slider's end face turns those steps into interference fringes, a BPW34 photodiode turns the fringes into a voltage, and the oscilloscope digitizes that voltage over USB for a Python script to count. One fringe per 260 nm of travel. That is the design. As of this writing the measurement mirror is still on the old rig, so the travel numbers in this write-up were read with a ruler, and the metrology section below says how that happened.

Spec box

Drive electronics

The signal starts as a lookup table in Arduino Mega firmware, clocked out over the Mega's parallel ports (Timer 4 pacing PORTA/C/L) so that all three DAC channels can update at the same instant. The board is a three-channel design because the endgame is a three-legged robot running 120°-offset waveforms (§M1, [8]). Each DAC0800 outputs a current. Each TDA2050, which is nominally an audio power amp, converts that current to voltage (15 kΩ feedback with 68 pF compensation) and then drives the stack, which is a very capacitive load [11, build guide]. The rails come from a NICEPOWER bench supply at ±15 V.

Two measured numbers define what this subsystem can and can't do. The first is amplitude. The output is ~15 V peak-to-peak, unipolar 0→+15 V, but for weeks the project's own documents said "30 V p-p." That figure turned out to be the supply span, not the signal, and the scope settled it (measured 2026-07-17, corrections logged). The second is bandwidth. Into the stack's 1.8 µF, the amplifier holds a clean full-amplitude sawtooth only up to a few tens of hertz, and by 200 Hz the amplitude has collapsed to about 3–5 V p-p (measured 2026-07-13). That is why the bench runs at 5 Hz, why the motion is invisible to the naked eye, and why the interferometer exists at all. The full story of this board, including the defective published Gerbers and their correction, is §M6. The sawtooth troubleshooting is §M7.

One item stays open, and it's flagged here rather than hidden. The stack is wired across two channel outputs ("15 volts across Ch1 and Ch2" is how my bench notes put it), and which channel plays which role has not been pinned down with the scope. What has been measured, on 2026-08-10, is the voltage at the stack's own wires, 14.7 V p-p, with a flyback of about 42 µs against a commanded 60 µs [run log 2026-08-10]. Step-size claims in §M11 use that measured voltage, not the commanded one.

The actuator and its foot

The stack is the one thing in the motion path I could never have made myself, hundreds of thin PZT layers co-fired into an 18 mm column (the exact layer count isn't in the datasheet, so it isn't here either). It converts charge into length by the inverse piezoelectric effect, ΔL = n·d₃₃·V in the ideal stack model [5]. The datasheet numbers (20 µm at 150 V, 1600 N blocking force, 50 kHz resonance [9]) make it both the strongest and the most fragile thing on the bench. It could hold a person's weight, though only across the 20 µm it can move, and it must never see tensile stress or a reversed overvoltage.

On its nose sits my part, an MSLA-printed resin foot. Through July the foot ended in a bare resin dome tip, which keeps the contact patch small and predictable, and the same foot geometry carried a conical socket for a 2 mm silicon-nitride ball. The ball was bought because resin was expected to gall against alumina within minutes of sliding (design_notes §16), and it stayed in the drawer for the first-motion hunt, on the reasoning that each cycle only slides the foot a micrometer or two. That reasoning was right per cycle and wrong in total. The foot slips back across the plate on every flyback, so the rubbing accumulates even when the net step does not, and a ten-minute run at 5 Hz is millimeters of sliding, not micrometers. In August the dome proved that note right. A fresh one ran twice, went dead within the same session, and two plate changes did not revive it (§M11). The ball went into its socket in mid-August and has been the working contact since, and the lever flexure went into service the same week (§M9's postscript carries that story). Photographing the worn resin tips is still on the data list, because surface-pairing effects are one of the things the reviews never report [5, 6, 7].

Mounting, since every builder asks. The stack sits in a hole printed into the flexure body and is clamped there by a screw driven in against it, with no adhesive, so the stack stays removable [builder, 2026-08-06].

Rail, preload, and the physics they serve

The rail is the newest part of the machine and the reason the July 30 result exists. Version one was a commercial MGN12H miniature linear guide, a reasonable choice with the wrong outcome. Its breakaway friction was bigger than the entire drive force my contact could produce, which I only established after building a force budget (Derived, §M9, §M10). Version two is fully printed. The slider carries bearing bolts, and the bearings ride printed V-grooves, one set in the rail bottom and one set in the side wall. The bottom grooves carry the slider's weight. The side grooves react the lateral preload that the actuator pushes into the slider. Two groove sets handle two force directions separately, with rolling contact where the old carriage had sliding seals. Figure M2-1's inset shows the cross-section.

For the record, that two-groove cross-section came from asking what the rail fundamentally had to resist. I sketched it before modeling anything, and on paper it simply made sense [builder, 2026-08-06].

Preload, the N in the friction force µ·N that drives every step, comes from a spring-and-screw mechanism calibrated to 0.099 N per ⅛-turn tick (design_notes §18), with a paper-strip touch-off defining zero. That turns the single most sensitive parameter in stick-slip physics from a guess into a dial. The July 30 motion showed up at the bottom of the dial, with the foot barely touching, and so did every August run. That is also a clue about how low this rail's drag really is. The breakaway measurement itself is still on the to-do list (§M9).

Metrology

At 5 Hz, a perfectly working motor here moves a few micrometers per second. No eye can see that, and for about a month I effectively ran blind experiments without knowing it (§M10 tells that story properly).

The fix is a Michelson interferometer built from hobby parts on the same baseplate: a 520 nm, 5 mW diode laser module, a 50:50 beamsplitter, a reference mirror, and a measurement mirror mounted on the slider itself so the beam reads travel-axis motion directly. The recombined beams land on a BPW34 photodiode, reverse-biased into a 50 kΩ load made of five 10 kΩ resistors in series, because that's what the parts bin had. Every 260 nm of slider travel walks the pattern through one full fringe (λ/2 at 520 nm). The DHO804 digitizes it, and two Python scripts (interferometer_capture.py, then fringe_to_displacement.py) turn scope samples into displacement versus time.

First fringes showed up on 2026-07-06 as a bull's-eye pattern (wavefront-curvature mismatch from the diverging diode beam, usable anyway with the photodiode at the pattern center). A fingertip tap on the measurement mirror visibly modulates the scope trace, which was the end-to-end proof that the sensing chain works. One status flag comes with all this. That alignment was done on the old rig, and as of this writing the measurement mirror has not yet moved onto the new slider. August's diagnosis took the bench days the realignment was supposed to get, so every travel number in §M11 is a ruler-and-masking-tape measurement. The interferometer goes back to being the project's ground truth once the mirror is on the slider (§M8).

Everything else on the table

The two printers are the factory. The Mars 5 Ultra (MSLA resin) handles anything precise: feet, coupons, and flexures. The Bambu A1 mini (FDM) handles anything structural: fixtures, mounts, the rail's larger prints. One firm internal rule keeps them straight. No FDM for flexure hinges, ever, because layer adhesion under cyclic load is a fatigue crack on a schedule (project CLAUDE.md §2). The DHO804 doubles as metrology digitizer and electronics debugger. The whole thing fits on a desk, and nothing in the motion path except the stack, the bearings, the alumina, and the fasteners is store-bought.

What the system leaves out, for now

Reading §M1's ambitions against this bench, the gaps are deliberate. The July 30 run was on the bare stack, with no flexure amplification. The ~3.5× lever leg that §M9 writes up has been the hardware since August 11, which means the August 20 run was on a different machine from the July one, and the characterization that matters gets done on the lever. There is one axis and one foot, not three legs. The bench runs at 5 Hz by choice, and full amplitude holds to a few tens of hertz before it starts to fall away, so speeds stay in µm/s for now. Why the amplitude collapses by 200 Hz is not yet pinned down, and §M7 has the test that would settle it. And the July 30 motion spent three weeks as "provisional," until the August 20 run with the sawtooth's ramp and flyback swapped closed out the artifact explanations that only work in one direction (§M11). Each limitation is a planned rung. The system as drawn is already enough for what Phase 1 exists to produce, a measured speed-versus-preload curve for a printed, 15-volt stick-slip motor.

Still missing from this chapter

An annotated bench photo (the drawn schematic stands in until then), the bench supply's model number, a close-up of the stack in its printed pocket, and the bearing-bolt count and type for the rail cross-section caption. They get added as they're recorded. Nothing here gets guessed.