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Calculate $x$-position according to:
__ | C |
╲╱ a ⋅ | - - c_f |
| a |
x = ──────────────
c_d - c_f
where:
- $C$ is the measured capacitance.
- $c_f$ is the capacitance of the filler medium per unit area
_(e.g., air)_.
- $c_d$ is the capacitance of an electrode completely covered in
liquid per unit area.
- $a$ is the area of the actuated electrode(s).
Note that this equation for $x$ assumes a single drop moving across an
electrode with a length along the x-axis of Lx. If no value is provided
for Lx, the electrode is assumed to be square, i.e.,
Lx=Ly=sqrt(area)
'''
if self.calibration._c_drop:
c_drop = self.calibration.c_drop(self.frequency)
else:
c_drop = self.capacitance()[-1] / self.area
if self.calibration._c_filler:
c_filler = self.calibration.c_filler(self.frequency)
else:
c_filler = 0
if Lx is None:
Lx = np.sqrt(self.area)
return (self.capacitance(filter_order=filter_order,
window_size=window_size, tol=tol) / self.area \
- c_filler) / (c_drop - c_filler) * Lx"
952,"def mean_velocity(self, tol=0.05, Lx=None):
'''
Calculate the mean velocity for a step (mm/ms which is equivalent to
m/s). Fit a line to the capacitance data and get the slope.
'''
dx = None
dt = None
p = None
ind = None
t_end = None
if self.area == 0:
return dict(dx=dx, dt=dt, p=p, ind=ind, t_end=t_end)
x = self.x_position(Lx=Lx)
# find the first and last valid indices
ind_start = mlab.find(x.mask==False)[0]
ind_last = mlab.find(x.mask==False)[-1]
# if the original x value is within tol % of the final x value, include
# all samples
if x[ind_start] > (1 - tol) * x[ind_last] or x[ind_last] < 0:
ind_stop = ind_last
else: # otherwise, stop when x reaches (1 - tol) % of it's final value
ind_stop = mlab.find(x > (1 - tol) * x[ind_last])[0]
ind = [ind_start, ind_stop]
# if we have at least 2 valid samples
if len(ind) >=2:
dx = np.diff(x[ind])[0]
dt = np.diff(self.time[ind])[0] # ms
# suppress polyfit warnings
with warnings.catch_warnings():
warnings.simplefilter(""ignore"")
# fit a line to the data
p = np.polyfit(self.time[ind[0]:ind[1]], x[ind[0]:ind[1]], 1)
# find time when the the line intercepts x[ind_last]
ind_stop = mlab.find(self.time > \
(x[ind_last] - p[1]) / p[0])
if len(ind_stop):
t_end = self.time[ind_stop[0]]
else:
t_end = self.time[-1]
return dict(dx=dx, dt=dt, p=p, ind=ind, t_end=t_end)"
953,"def to_frame(self, filter_order=3):
""""""
Convert data to a `pandas.DataFrame`.
Parameters
----------
filter_order : int
Filter order to use when filtering Z_device, capacitance, x_position, and dxdt.
Data is filtered using a Savitzky-Golay filter with a window size that is adjusted
based on the mean velocity of the drop (see _get_window_size).
Returns
-------
pandas.DataFrame
This DataFrame is indexed by a utc_timestamp and contains the following columns:
frequency: actuation frequency (Hz)
target_voltage: target voltage (V)