dls_motor_scanning

Tools for characterising the motion performance of EPICS motors.

Two commands are provided:

• scan drives a motor through a range in fixed steps, recording the position actually reached and the time each move took, then reports the positioning error and move time as statistics, a data file and a plot.
• calibrate fits a 5th order polynomial that converts an unscaled feedback device, such as a potentiometer read as raw ADC counts, into engineering units, and emits it as an EPICS calc record and an Excel formula.

Scanning a motor

dls-motor-scanning scan MOTOR START STOP STEP DELAY [options]

START, STOP and STEP are in the motor's engineering units, and DELAY is the settling time in seconds to wait after each move before reading back. Scan PS-MO-WIRE-01:Y from 40 to 45 in half-EGU steps, with no settling time:

dls-motor-scanning scan PS-MO-WIRE-01:Y 40 45 0.5 0

The scan direction follows START and STOP, so a reverse scan needs no sign on the step:

dls-motor-scanning scan PS-MO-WIRE-01:Y 45 40 0.5 0

Each run writes Scan_<motor>_<date>_<start>_<stop>_<step>.txt and .png into the current directory, and prints a summary of the move times and position errors.

The output file

The data file is space separated, with a header row naming the columns:

Desired Actual MoveTime Timestamp(UTC)
40.5 40.499 1.1394305229187012 2026-08-14T09:15:56.375000Z
41.0 40.9992 1.1742472648620605 2026-08-14T09:15:57.625000Z

Timestamp(UTC) is the timestamp the IOC put on the readback record, not the time the client received it, which makes it directly comparable with archiver data. It appears only when --timestamp is given, as does the --extra-pv column.

Plotting on a remote machine

--plot needs a working display. Over a slow link, or on a headless machine, prefer:

dls-motor-scanning scan PS-MO-WIRE-01:Y 40 45 0.5 0 --no-plot

which skips the interactive window entirely and just writes the png.

A note on backends. cothread integrates with Qt and not with Tk, and the two segfault if combined. PyQt5 is therefore a dependency, and the tool selects qtagg explicitly rather than letting matplotlib work through its own candidate list, which would fall back to tkagg. If Qt cannot be imported, or DISPLAY is unset, the interactive plot is skipped with a message instead of taking the process down with it.

The png is always rendered under Agg and written to disk before any GUI toolkit is touched, so a display problem can never cost you the saved plot.

Calibrating a feedback device

dls-motor-scanning calibrate CSV_PATH [options]

The CSV needs one integer column, taken as the raw feedback, and one float column, taken as the scaled position to calibrate against:

raw,encoder
0,2.0
200,2.5
400,3.0

dls-motor-scanning calibrate pot.csv --raw-input-pv BL01I-MO-POT-01:ADC --excel-cell C2

This prints the six coefficients, a Builder records.calc entry with them loaded into fields B to G, and an equivalent Excel formula referencing --excel-cell.

Development

uv sync
uv run pytest

The scan code reaches EPICS through cothread.catools, which is imported lazily so the tests, --help and the calibrate command all work without an IOC. The test suite substitutes a fake Channel Access layer, so a full scan can be exercised without hardware.