How to get conditioned Euler angles, angular velocity, and linear acceleration from MetaBase output?

dryheat122

Hi. I am trying to use MMS to evaluate bowling throws. For that I need Euler angles and angular velocity to describe arm movement, and linear acceleration to detect steps.

I am getting sensor data from the MetaBase app. but it will not record all three sets of values. Euler angles preclude everything else.

I know it should be possible to get avelo and accel from the angles, but the math is over my head. I've not had much luck finding python libraries to do the job. Can anyone point me in the right direction or share code?

dryheat122

I got this figured out with considerable help from ChatGPT. Here's the code for anyone trying to do the same thing. For reasons I'm not sure about, for my test data this produces angles in the range -8564 to 10,306. I'm only using the result in plots so it doesn't really matter to me. If you wanted it in the range -360 to 360 you'd need to scale the angles.

import csv
import numpy as np
import pandas as pd
from scipy.integrate import cumtrapz
import matplotlib.pyplot as plt
from scipy.signal import butter,filtfilt

def lowpass(data, cutoff, fs, order):
    normal_cutoff = cutoff / nyq
    # Get the filter coefficients 
    b, a = butter(order, normal_cutoff, btype='low', analog=False)
    y = filtfilt(b, a, data)
    return y

# Read the files and populate the acceleration and angular_velocity arrays
avelo = []
filename = "accel-gyro_MetaWear_2023-06-09T13.59.19.303_EABF46360571_Gyroscope_100.000Hz_1.7.3.csv"
with open(filename) as csvfile:
    csvreader = csv.reader(csvfile)
    next(csvreader) # skip header
    for row in csvreader:
        avelo.append([row[3],row[4],row[5]])
avelo = np.array(avelo,dtype=float)

accel = []
filename = "accel-gyro_MetaWear_2023-06-09T13.59.19.303_EABF46360571_Accelerometer_100.000Hz_1.7.3.csv"
with open(filename) as csvfile:
    csvreader = csv.reader(csvfile)
    next(csvreader) # skip header
    for row in csvreader:
        accel.append([row[3],row[4],row[5]])
accel = np.array(accel,dtype=float)

# ensure arrays are the same length and create index for plots
arows,foo = avelo.shape
brows,foo = accel.shape
if arows > brows: # then remove rows from avelo
    avelo = avelo[0:brows]
    rows = brows
elif brows > arows: # then remove rows from accel
    accel = accel[0:arows]
    rows = arows
else:
    rows = arows

idx = np.arange(0, rows, dtype=int) # index for plots

# Integrate angular velocity to get Euler angles
euler_angles = cumtrapz(avelo, axis=0, initial=0.0)

# combine the dataframes into one
dfs = {
    'accel': pd.DataFrame(accel, columns=list('xyz')),
    'angle': pd.DataFrame(euler_angles, columns=list('xyz')),
    'avelo': pd.DataFrame(avelo, columns=list('xyz'))
}
df = pd.concat(dfs, axis=1)

# configure and apply lowpass filter to remove noise
fs = 100        # sample rate, Hz
cutoff = 3      # desired cutoff frequency of the filter
order = 4       # desired order of the filter
nyq = 0.5 * fs  # Nyquist Frequency

parameters = ["accel","angle","avelo"]
for p in parameters:
    for var in ('x','y','z'):
        df[p,var] =  lowpass(df[p,var], cutoff, fs, order)

# Plot the three series
ax1 = plt.subplot(3, 1, 1)
ax1.plot(idx, df['angle']['x'], label='x')
ax1.plot(idx, df['angle']['y'], label='y')
ax1.plot(idx, df['angle']['z'], label='z')
ax1.set_title('angle')
ax1.legend(loc="best")
ax2 = plt.subplot(3, 1, 2)
ax2.plot(idx, df['avelo']['x'], label='x')
ax2.plot(idx, df['avelo']['y'], label='y')
ax2.plot(idx, df['avelo']['z'], label='z')
ax2.set_title('avelo')
ax2.legend(loc="best")
ax3 = plt.subplot(3, 1, 3)
ax3.plot(idx, df['accel']['x'], label='x')
ax3.plot(idx, df['accel']['y'], label='y')
ax3.plot(idx, df['accel']['z'], label='z')
ax3.legend(loc="best")
ax3.set_title('accel')
# plt.subplots_adjust(hspace=10)
plt.tight_layout()
plt.show()