Add basic support for EIS

batdata/schemas/eis.py

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+"""Schemas associated with Electrochemical Impedance Spectroscopy"""
+
+from typing import List
+
+from pandas import DataFrame
+from pydantic import Field
+import numpy as np
+
+from .cycling import ColumnSchema
+
+
+class EISData(ColumnSchema):
+    """Measurements for a specific EIS test"""
+
+    test_time: List[float] = Field(None, description="Time from the beginning of the cycling test. Times must be "
+                                                     "nonnegative and monotonically increasing. Units: s",
+                                   monotonic=True)
+    time: List[float] = Field(None, description="Time as a UNIX timestamp. Assumed to be in UTC")
+    frequency: List[float] = Field(..., description="Applied frequency. Units: Hz")
+    z_real: List[float] = Field(..., description="Real component of impedance. Units: Ohm")
+    z_imag: List[float] = Field(..., description="Imaginary component of impedance. Units: Ohm")
+    z_mag: List[float] = Field(..., description="Magnitude of impedance. Units: Ohm")
+    z_phase: List[float] = Field(..., description="Phase angle of the impedance. Units: Degree")
+
+    @classmethod
+    def validate_dataframe(cls, data: DataFrame, allow_extra_columns: bool = True):
+        # Check that the schema is supported
+        super().validate_dataframe(data, allow_extra_columns)
+
+        # Ensure that the cartesian coordinates for the impedance agree with the magnitude
+        z_real_from_polar = np.multiply(data['z_mag'], np.cos(np.deg2rad(data['z_phase'])))
+        z_imag_from_polar = np.multiply(data['z_mag'], np.sin(np.deg2rad(data['z_phase'])))
+        if not np.isclose(z_real_from_polar, data['z_real'], rtol=0.01).all():
+            raise ValueError('Polar and cartesian forms of impedance disagree for real component')
+        if not np.isclose(z_imag_from_polar, data['z_imag'], rtol=0.01).all():
+            raise ValueError('Polar and cartesian forms of impedance disagree for imaginary component')

tests/schemas/test_eis.py

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+from pytest import fixture, raises
+import pandas as pd
+import numpy as np
+
+from batdata.schemas.eis import EISData
+
+
+@fixture()
+def example_df() -> pd.DataFrame:
+    output = pd.DataFrame({
+        'frequency': [5e5, 4e5],
+        'z_real': [0.241, 0.237],
+        'z_imag': [0.431, 0.327],
+    })
+    output['z_mag'] = np.linalg.norm(output.values[:, -2:], axis=1)
+    output['z_phase'] = np.rad2deg(np.arcsin(output['z_imag'] / output['z_mag']))
+    return output
+
+
+def test_pass(example_df):
+    EISData.validate_dataframe(example_df)
+
+
+def test_consistency(example_df):
+    example_df['z_imag'] *= 2
+    with raises(ValueError) as e:
+        EISData.validate_dataframe(example_df)
+    assert 'imaginary' in str(e.value)
+
+    example_df['z_real'] *= 2
+    with raises(ValueError) as e:
+        EISData.validate_dataframe(example_df)
+    assert 'real' in str(e.value)