pulse-echo-azimuthal-scanner

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Pulse-Echo Azimuthal Scanner

A single-transducer ultrasonic scanner that measures the crystal orientation fabric (COF) of an ice core disk. The machine rotates a 100 mm ice disk through a full turn while a spring-free vertical carriage repeatedly seats an ultrasonic probe against the rim under a controlled contact force. At each azimuth the probe fires a pulse across the diameter and records the back-wall echoes. Because sound travels faster along the crystal c-axis than across it, the velocity varies with azimuth, and the pattern v(theta) = v0 + A cos 2(theta - phi) reveals the predominant crystal orientation without cutting or melting the sample.

The measurement leans on two design decisions:

E2 minus E1 timing. The interval between the first and second back-wall echoes spans exactly one extra two-way transit of the diameter, so the couplant film and wear-plate delays cancel. Timing accuracy no longer depends on how much oil is under the probe.
Re-seat and revisit. The probe lifts and re-docks between repeat traces to average the couplant wedge, and the scan revisits a reference azimuth at intervals so slow drift (freezer cycling, rim shape) can be removed in analysis.

How a scan works

For every azimuth in the sweep the machine, fully unattended:

lifts the probe 2 mm clear of the rim,
rotates the disk to the target azimuth (witnessed by an absolute encoder on the rotation axis),
tares the load cell and docks: a fast descent to just above the remembered sample height, then a slow creep until the load cell registers the contact force (5 g nominal),
captures an averaged waveform from the oscilloscope, checking the echo amplitude and re-capturing if the signal is weak,
repeats the dock/capture cycle for the configured number of traces,
logs everything to disk row by row, so a crash or abort loses nothing and the run can be resumed.

When the sweep finishes the machine parks itself: the probe homes to the top stop and the stage unwinds back to azimuth 0.

Repository layout

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Pulse-Echo Azimuthal Scanner

The measurement leans on two design decisions:

E2 minus E1 timing. The interval between the first and second back-wall echoes spans exactly one extra two-way transit of the diameter, so the couplant film and wear-plate delays cancel. Timing accuracy no longer depends on how much oil is under the probe.

Re-seat and revisit. The probe lifts and re-docks between repeat traces to average the couplant wedge, and the scan revisits a reference azimuth at intervals so slow drift (freezer cycling, rim shape) can be removed in analysis.

How a scan works

For every azimuth in the sweep the machine, fully unattended:

lifts the probe 2 mm clear of the rim,

rotates the disk to the target azimuth (witnessed by an absolute encoder on the rotation axis),

tares the load cell and docks: a fast descent to just above the remembered sample height, then a slow creep until the load cell registers the contact force (5 g nominal),

captures an averaged waveform from the oscilloscope, checking the echo amplitude and re-capturing if the signal is weak,

repeats the dock/capture cycle for the configured number of traces,

logs everything to disk row by row, so a crash or abort loses nothing and the run can be resumed.

When the sweep finishes the machine parks itself: the probe homes to the top stop and the stage unwinds back to azimuth 0.

Repository layout

`hardware/pcb/`	KiCad schematic and layout for the controller board (RP2040 Pico, 2x TMC2209, HX711, DS18B20, AS5600 inputs), the bare-board STEP export, and the component 3D models
`hardware/3d/`	STEP and GLB exports of the assembled controller board (board plus fitted modules)
`hardware/Gcode/`	CNC milling toolpaths used to machine the prototype board, plus the Makera CAM source project (.mkc)
`software/firmware/`	Pico C++ firmware. Serial command reference in PROTOCOL.md
`software/GUI/`	Streamlit control application: jog, dock, sweep, run browser, analysis. See its README
`docs/`	LaTeX machine manual
`.vscode/`	editor tasks for one-click firmware build, flash and GUI launch
`Run Scanner GUI.bat`	double-click launcher; installs a private Python runtime on first use

Quantity	Value	Notes
Rotation drive	71.188 steps/degree	1.8 degree motor, 32 microsteps, belt reduction; calibrated against the absolute encoder
Azimuth truth	0.34 degrees RMS	AS5600 absolute encoder after a 2-harmonic correction for magnet eccentricity and tilt (2.14 degrees RMS raw)
Rotation backlash	~0.1 degree	approach direction is constant during a sweep
Vertical drive	1600 steps/mm	two-start T8 leadscrew, 4 mm lead; no encoder on this axis
Vertical homing	0.03 to 0.06 mm	sensorless StallGuard to the top mechanical stop
Dock repeatability	~5 um within-angle (best case)	position at 5 g contact, rigid target
Contact force	5 g nominal	load-cell noise 0.65 g rms idle, 2.6 g peak-to-peak; median filter plus 2-sample debounce; under 20 um over-travel at the 0.06 mm/s creep
Contact calibration	repeatable, not absolute	no reference mass on site; the force units are consistent counts, which is what the physics needs
Thermal drift	dv/dT about -2.3 m/s per K	DS18B20 in the tailstock logs temperature per trace; reference revisits detrend the residual
Cycle time	11 to 17 s per trace	dominated by the final slow creep; a 360 degree sweep at 1 degree steps with 3 traces per angle takes 3.5 to 6 hours

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