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Instrumenting an abstract object
Group Design Project : Group 29
Team : Alexandros Nikolaou
Christopher Long Chung Pang
Hamish Dalrymple
Nikhil Banerjee
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Southampton Hand Assessment Procedure The Southampton Hand Assessment Procedure (SHAP) is a simple measure
of hand functionality.
Originally developed to assess the effectiveness of upper limb prostheses,
the SHAP has now been applied to assessment of musculoskeletal andneurological conditions.
Used in conjunction with other tools, it can observe the function of the
entire limb
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SHAP SHAP can be used to monitor progress and compare performance. This is
a clinically validated hand function test.
The SHAP is made up of 8 abstract objectsand 14 Activities of Daily Living (ADL). Eachtask is timed by the participant, so there isno interference or reliability on the reactiontimes of the observer or clinician.
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Project proposal The basic aim of this project is to aid SHAP in achieving more detailed
results, helping with its overall assessment
We aim to provide a wireless instrumentation module for one of the
abstract objects (sphere) in the SHAP tests
Information related to the motion of the sphere in 3D space subjected by
the user will be transmitted wirelessly to an external data logger
This data will ensure a more detailed analysis of the users limb
effectiveness and will further aid the assessment of the users recovery
USER
TASK
SENSOR
MODULE
WIRELESS
TRANSMISSION
DATA
LOGGER
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Understanding the engineering problem The object is under motion in three dimensions :
6 Degrees of Freedom which need to be measured
Inertial Measurement Unit Gyro Free : Due to large
inaccuracies in gyros in instrumenting small objects
accelerometers will be used in a predefined arrangement
Wireless adaptor for communication outside the device
Power: Maintaining enough power for sensors and wireless transmitter keeping
in mind space constraints Robustness : Keeping the components secure inside the object while it is
manipulated by the user
Size: Module must be small enough to fit in the given abstract object
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Visualization
Diameter = 72mm
Microprocessor
Wireless Transmitter
Data
Logger
Accelerometers
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GF-IMU : Gyro free - Inertial Measurement Unit The object under observation has 6 degrees of freedom which will be
measured using accelerometers alone (gyro free)
We aim to establish an optimal number and geometrical arrangement of the
accelerometers
The purpose is to achieve a good measure of accuracy and observability of
the linear and angular accelerations
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Accelerometer arrangement Mechanization equations: Differential equations describing the position,
velocity and attitude as functions of the sensor outputs
They depend on the physical configuration of the sensors relative to both
the object body and the reference frame
Strapdown Mechanization : Sensors are strapped onto the body frame
Advantage: Decrease in size and weight
Disadvantage: Computational complexity
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Geometrical Arrangement Tetrahedron
Reasoning:
No. of
Acceleromet
ers
2 4 6 8
No. of Wires
required
6 12 18 24
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2D analysis of the equations Assuming we are using two-axis accelerometers Let the readings be y1, y2, etc.
For a static object, the readings will be
y1 = y3= y5 = 0
y2 = y4 = y6 =g
2
1
6 4
5 3
g
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2D analysis of the equations The readings arey1 = y3
= y5 = g sin
y2 = y4 = y6 =g cos
2
1
6
5
4
3
g
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2D analysis of the equations While in linear motiony1 = y3
= y5 = g sin + a cos
y2 = y4 = y6 =g cos + a sin
This set of equations can be solved numerically
2
1
6
5
4
3
a
g
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2D analysis of the equations Radial acceleration (towards the centre of rotation) Tangential acceleration
= 2 =
2
1
6
5
4
3
g
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General equation From Hanson
Combining linear and angular accelerations
= +
1
21
2
+
1
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General equation
= +1
21
2
+1
y is the vector of the readings from the accelerometersy = (y1; y2; ...; yN)
is the vector of unknowns, i.e. the angular and linear accelerations in the
x, y, z axes.
H is the regressor matrix
=1 1
1
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The regressor matrix Used to solve the equation
Also provides a condition number
= 1
The condition number measures the worst case of how much accuracy
would be lost due to small changes in the data received
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HardwareParameters
System Physics behind
Scale
Ease of use Other parameters
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Wireless Data Transmission Requirements
12 channels without processor
6 channels with processor
Low power consumption
Small size
Short range
Ease of use
Cost
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Wireless Data Transmission Easyradio ER900TS transmitter
31x12x4 (mm)^3
3.3V
82.5mW
14.56 GBP low cost
Data Encryption 16-bit
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Wireless Data Transmission Easyradio ER900RS receiver
Pair with ER900TS transmitter
23.81 GBP
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Main PCB - Processor Requirements
6 inputs/outputs
Small size
Low power consumption
High processing speed not required
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Main PCB - Processor Arduino Pro mini 328
3.3V
3.3x1.8x0.8 (mm)^3
8 MHz processing speed
13.22 GBP
Low power consumption
USB connection using adaptor
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Power Supply Requirements
Small size
High endurance
Lightweight
Rechargeable
Low cost
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Hardware Accelerometers
Requirements
> 1kHz Bandwidth
Measuring range ~ +/- 6g
Low power consumption
Considerations
Type of chip mounting
Design a PCB?
Size of complete package
Synchronization
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Hardware Accelerometer
Suitable device found:
Triple Axis Accelerometer ADXL345
Measurement range: +/- 2, 4, 8 & 16 available
Output data as 16-bit twos
SPI (3 or 4 wire) digital interface
3.2 kHz Bandwidth
PCB dimensions: 15.5 x 22 mm
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CAD Design Rapid Prototyping Machine
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CAD Design Design Constraints
Cost of material
Size Required
Geometry of Accelerometers
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CAD Design Design Considerations
Accessibility of the components?
Access to sphere after construction?
Design so far:
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Accelerometers
Easy Radio
Microprocessor
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Battery
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Programming, Data extraction and DataProcessing
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Data Extraction : Communication modes Data Extraction from the ADXL
Two communication modes : SPI and I2C
I2C communication drawback :
X Hard coded I2C address, same for all accelerometers
X Cannot address accelerometers individually
SPI Communication chosen :
No addressing required on bus while communication
Communication is faster
No delays between inflow and outflow of data
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SPI communication
4 wire communication
For selecting accelerometer: SS slave select wire LOW
Digital output pins served this purpose on the master device i.e. Arduino Pro
Mini
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Accelerometer ConfigurationRequirements:
Continuous 3 axis readings for acceleration
Low power consumption
5 g range for acceleration
Register values in ADXL345 were set as per requirements
The decided rate was 1600 Hz with bandwidth 800 Hz (minimum 90A current),thus we use the rate code 1110 for this register.
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Program Structure Setup :
o SPI communication mode, rate, digital SS pins
o Serial communication rate
o Initialize velocity displacement variables
o Configure accelerometers g-range, data rate, etc
Loop
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Data flow
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Testing
Static Test
Use Horizontal and vertical rotary table
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Testing
Motion Test
Use gravity and standard equations
(v=u+at ect)
Use Robot arm
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Gantt Chart
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Future Work
Circuit design
Connections
Built of outer cell using rapid prototype machine
Data transmission
Software for computation
Testing