QPolargraph

A Qt-based instrument interface for a two-dimensional polargraph scanner. Built on QInstrument.

A polargraph translates a payload to any position within its scan area using two stepper motors and a timing belt. Originally developed by Jürg Lehni and Uli Franke as a drawing machine, the design scales easily to large areas, delivers millimeter-scale positioning accuracy, and is very cost-effective to build.

Hardware

The belt forms a V-shape between the two motors. Each motor changes the length of one side of the V, moving the payload in two dimensions.

Firmware

The Arduino must be flashed with hardware/arduino/acam3/acam3.ino before first use. The easiest way is to use the built-in installer:

qpolargraph-flash

This opens a dialog that detects the attached Arduino, installs any missing Arduino libraries (Adafruit Motor Shield V2 Library, AccelStepper), compiles, and uploads the firmware — all without opening the Arduino IDE. It requires arduino-cli to be installed and on PATH.

Alternatively, open hardware/arduino/acam3/acam3.ino in the Arduino IDE and upload manually.

The firmware and package versions are coupled: Motors.identify() checks that the connected Arduino reports the expected firmware version and that the Adafruit Motor Shield is detected at I2C address 0x60, refusing to open the port if either check fails.

Installation

Clone the repository and install in editable mode:

git clone https://github.com/davidgrier/QPolargraph
cd QPolargraph
python -m venv .qp
source .qp/bin/activate          # Windows: .qp\Scripts\activate
pip install -e ".[dev]"
pip install PyQt6                # or PyQt5, PySide2, PySide6

QInstrument is installed automatically as a dependency.

Calibration

Five geometric parameters describe the polargraph. Set them in the QPolargraphWidget control panel or pass them directly to Polargraph():

Measure ell and y0 with a ruler after mounting the motors. pitch, circumference, and steps are determined by the belt and gear specifications and normally do not need to change.

Settings are saved to ~/.QScanner/ automatically when the application closes, so calibration only needs to be done once.

Quick start

Scanner application

python -m QPolargraph
# or, after installation:
qpolargraph

The application opens a live plot showing the belt geometry and the scan trajectory. Use the Scan button to start a polar-arc sweep, or Home / Center to move the payload to the home or centre position.

Embedding in your own application

Subclass QScanner to add experiment-specific data acquisition:

from qtpy.QtWidgets import QApplication
from qtpy import QtCore
from QPolargraph.QScanner import QScanner
import sys


class MyScanner(QScanner):

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.scanner.dataReady.connect(self.acquire)

    @QtCore.Slot(object)
    def acquire(self, position):
        x, y = position
        # measure something at (x, y) and call self.plotData(x, y, hue)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    scanner = MyScanner()
    scanner.show()
    sys.exit(app.exec())

Controlling the hardware directly

from qtpy.QtCore import QCoreApplication
from QPolargraph.Polargraph import Polargraph
import sys

app = QCoreApplication(sys.argv)
pg = Polargraph(ell=0.8, y0=0.15).find()   # auto-detects USB port
print(pg.position)                           # (x, y, running) in metres
pg.moveTo(0.0, 0.3)
while pg.running():
    pass
pg.release()

Architecture

QInstrument.QSerialInstrument
└── Motors               # acam3 serial protocol: goto, speed, position, …
    └── Polargraph       # geometry: step indexes ↔ Cartesian coordinates

QInstrument.QInstrumentWidget
└── QRasterScanWidget    # scan-pattern parameter controls

QInstrument.QInstrumentWidget
└── QPolargraphWidget    # polargraph hardware controls

QtCore.QObject
└── QScanPattern         # base: rectangular perimeter trajectory
    ├── RasterScan       # row-by-row zigzag raster
    └── PolarScan        # arc-by-arc sweep centred on the left pulley

QtWidgets.QMainWindow
└── QScanner             # full application: live plot + scan controls

Development

Run the test suite:

source .qp/bin/activate
pytest tests/

Tests run automatically before every git push. To install the pre-push hook in a fresh clone:

git config core.hooksPath .githooks

Acknowledgements

Work on this project at New York University is supported by the National Science Foundation of the United States under award number DMR-2438983.

References

H. W. Gao, K. I. Mishra, A. Winters, S. Wolin, and D. G. Grier, "Flexible wide-field high-resolution scanning camera for continuous-wave acoustic holography," Review of Scientific Instruments 89, 114901 (2018). https://doi.org/10.1063/1.5053666