The document discusses biomedical transducers with a focus on inertial sensors, including accelerometers, gyroscopes, and magnetometers. It covers the principles of inertial reference frames, evaluates static noise, and conducts a sit-to-stand task using IMUs. Conclusions highlight the improved performance of the Xsens IMU compared to the Opal IMU in terms of noise characteristics and measurement accuracy.

1. Biomedical Transducers a.a.
2011/12
Inertial Sensors
Daniele Antonioli
Luca Faggianelli
Jian Han
Mekki Mtimet
6/16/2012 1Biomedical Transducers - Inertial Sensors

2. Outline
 Introduction to Inertial Sensors;
 Static Evaluation of the Noise;
 Sit to Stand Task Evaluation;
 Conclusions.
6/16/2012 Biomedical Transducers - Inertial Sensors 2

3. Inertia and Inertial Frame
• Inertial Frame of Reference: is a frame in a
state of constant, rectilinear motion with
respect to one another: an accelerometer at
rest in one would detect zero acceleration;
• Newton’s First Law of Inertia: an observer in a
inertial frame of reference observes a body:
inertia is the natural tendency of that body to
remain immobile or in motion with constant
speed along a straight line;
6/16/2012 Biomedical Transducers - Inertial Sensors 3

4. Inertia and Inertial Frame
• Newton’s Second Law: A force will accelerate
a body, in the direction of the force at a rate
inversely proportional to the mass of the
body;
• Mass is the linear quantification of inertia;
• The laws of Classical Mechanics
(Biomechanics included) are valid and
maintain the same form in all inertial
reference systems.
6/16/2012 Biomedical Transducers - Inertial Sensors 4

5. What is a sensor?
• Instrument capable to transduce a physical
quantity to a measurable electric signal;
• Accuracy vs Precision;
• Inertial sensor: functioning principle based on
inertial phenomena.
6/16/2012 Biomedical Transducers - Inertial Sensors 5

6. Inertial Sensors
• Accelerometers: sense linear acceleration
[m/s^2] along a specific axis;
• Gyroscopes: sense angular velocity axis,
measured in [rad/s];
• Magnetometer: sense the strength of a
magnetic field, measured in [mGauss].
6/16/2012 Biomedical Transducers - Inertial Sensors 6

7. Inertial Sensor Benefits and
Applications
• Low cost;
• Small size, Portable;
• Ultra Low-power systems;
• Wireless.
• Ambulatory monitoring;
• Unsupervised monitoring;
• Fall & Gait;
• Activity detection.
6/16/2012 Biomedical Transducers - Inertial Sensors 7

8. 2.STATIC CALIBRATION
EXPERIMENT
6/16/2012 8Biomedical Transducers - Inertial Sensors

9. 2.1 Brief Hardware Description
6/16/2012 9Biomedical Transducers - Inertial Sensors

10. 2.2 Static Noise Evaluation
2.2.1 Description
6/16/2012 10Biomedical Transducers - Inertial Sensors

11. INERTIAL MEASUREMENT UNITS
XSENS SENSOR(with cables) OPAL SENSOR(wireless)
6/16/2012 11Biomedical Transducers - Inertial Sensors

12. 2.2.2 Evaluate and characterize the
noise in terms of mean and standard
deviation of the ouputs
• Mean() function
• Std() function
6/16/2012 12Biomedical Transducers - Inertial Sensors

13. The results for XSENS IMU are as follows:
6/16/2012 13Biomedical Transducers - Inertial Sensors

14. The results for OPAL IMU are as follows:
6/16/2012 14Biomedical Transducers - Inertial Sensors

15. 2.3 Evaluate the drift effect
• Detrend() function
• Polyfit() function, y=mx+b
6/16/2012 15Biomedical Transducers - Inertial Sensors

16. The results for XSENS IMU are as follows:
6/16/2012 16Biomedical Transducers - Inertial Sensors

17. The results for OPAL IMU are as follows:
6/16/2012 17Biomedical Transducers - Inertial Sensors

18. 2.4 What are the main difference
between the noises on each sensor?
6/16/2012 18Biomedical Transducers - Inertial Sensors

19. Ay vs Ay1
6/16/2012 19Biomedical Transducers - Inertial Sensors

20. From these plots we can conclude that:
• The Xsens IMU, has overall better
performance with respect to the Opal
IMU;
• The Xsens trend of noise drift is almost
parallel to the time axis and the signals
have lower offsets with respect to the
Opal signals.
6/16/2012 20Biomedical Transducers - Inertial Sensors

21. 2.5 Does the standard deviation of the noise
correspond to that reported in the data sheet?
• Xsens: As we can see in the tables above, the data reported in the
datasheet and our measured ones, differ from a factor of ±.001; So we
obtain very good measurements in terms of accuracy and precision;
• Opal: In this case we have to convert the data from [μg/»Hz] to [m/s2]
for the linear acceleration Noise and from [°/s/»Hz] to [rad/s] for the
angular velocity, using the bandwidth data B = 50[Hz]. Also in this case
we obtain good measurement in terms of accuracy and precision.
6/16/2012 21Biomedical Transducers - Inertial Sensors

22. 3. Sit to Stand
• Opal IMU1 placed on the Thigh, in lateral
position;
• Opal IMU2 placed on the Trunk, at L5 height;
• 4 trials with 5 repetitions at different speed;
• f_{sample} = 128[Hz];
6/16/2012 Biomedical Transducers - Inertial Sensors 22

23. Sit to Stand
6/16/2012 Biomedical Transducers - Inertial Sensors 23

24. Extracted Signals
6/16/2012 Biomedical Transducers - Inertial Sensors 24

25. Digital Filtering
2
sample
cut
n
f
f
W
6/16/2012 Biomedical Transducers - Inertial Sensors 25
Normalized CutOff Frequency
Because of Noisy signals: Lowpass Filtering needed
[b,a] = butter(order,Wn,type): extract the coefficients;
filtfilt(b,a,input): No Phase Shift, forward + backward
filtering.

26. Algorithm
6/16/2012 Biomedical Transducers - Inertial Sensors 26
Results
LPF
Pulses
Detection
Edges
Detection
Integration
Validation
Good/Bad
Knee
Angles
Timings
Acc(x,y)
Gyro(z)

27. Results: Plots
6/16/2012 Biomedical Transducers - Inertial Sensors 27
Thigh Accelerometer x and y
axis Thigh Gyroscope z axis

28. Results: Table
StS Time mean
[s]
TtS Time mean
[s]
StS Angle
mean [°]
TtS Angle
mean [°]
Trial 1 1.7984 1.4375 94.7484° - 90.1806°
Trial 2 1.391 1.1719 96.3518° - 92.9096°
Trial 3 1.4672 1.3531 75.5568° - 71-7260°
Trial 4 .9906 .09562 71.6656° - 69.1158°
6/16/2012 Biomedical Transducers - Inertial Sensors 28
4 Trials 5 Repetitions StS = Sit to Stand Task TtS = Time to Sit Task

29. Sit to Stand Conclusions
+ Results achievable with only 1 IMU (on the
thigh)
+ Robust algorithm
• Kalman fusion filter to improve the algorithm
6/16/2012 Biomedical Transducers - Inertial Sensors 29