Joint Position Sensor

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Joint Position Sensor
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Joint Position Sensor

• Purpose
• Safely perform real-time measurement of the
  motion of an injured arm while presenting minimal
  risk of further injury to the user.
• Requirements
• Attachable to medical brace
• Does not impede rehabilitation or promote
  further injury
• Physically robust
• Accurate to 0.5 across 160 range of motion
• Low current (lt 10 mA)
• Compatible with microcontroller (output
  impedance lt 10 kO)

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Joint Position Sensor

• Proposed Solutions
• Exoskeletal (2-D)
• Radial Potentiometer/Transducer
• Attach a radial pot or radial position transducer
  to a rigid frame and measure single-plane angle
  of motion.
• Linear Displacement Potentiometer/Transducer
• construct a small pulley at the joint and convert
  linear displacement measurement to single-plane
  angle of motion.
• Flexible (3-D)
• Triple-Axis Accelerometer Network
• Use triple-axis accelerometers to measure
  orientation vectors across sections of the arm.

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Joint Position Sensor
Proposed Solution 1 - Radial Potentiometer Pr
os Simple to design and construct Linear
response simplifies measurement Physically
robust Controllable power level Reasonable Cost
( 30 for precision pot) Cons Exoskeleton may
be uncomfortable or obtrusive
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Joint Position Sensor
Proposed Solution 2 Linear Position
Transducer Pros High accuracy along a
single plane of motion Ultra-low current for
maximum battery life ( 1 µA) Cons Complex and
fragile structure Difficult to design for comfort
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Joint Position Sensor
Proposed Solution 3 Triple-Axis
Accelerometers Pros Allows 3-D motion
capture using relative orientation Low current (lt
1 mA per sensor) Comfortable and
unobtrusive Reasonable Cost ( 10 per
sensor) Cons Low accuracy for this
application Significantly complicates 3-D arm
rendering 5 connections per analog sensor
(digital unavailable)
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Joint Position Sensor
PASCO Scientific PS-2138PASPORT Goniometer Probe

• Attachable to medical brace
• Measurement Range -170 to 170
• Accuracy to 1 with calibration, resolution to
  0.04
• Low current (500 µA)
• Compatible with microcontroller A/D Input
  Impedance
• 10 kO multi-turn internal precision potentiometer

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Battery and Power
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Power Electronics

• Purpose
• Convert battery supply voltage to appropriate
  levels for electronic components and facilitate
  safe charging of battery using AC adapter.
• Goals
• Stable output voltage
• Minimize component size
• Requirements
• 24 hour battery life during normal continuous
  operation
• Rechargeable battery
• Regulated 5 V DC supply

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Battery Selection

• Three types of rechargeable batteries are
  considered
• Lithium Ion (Li-Ion)
• Nickel-Metal Hydride (NiMH)
• Nickel-Cadmium (Ni-Cad)

Prototype uses single 9V Lithium Ion
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MAX1555USB Compatible Li-Ion Charger

• Charges single-cell Li-Ion from AC adapter or USB
  port
• AC input charge current limit of 300 mA
• USB trickle-charge limited to 100 mA
• Thermistor input controls charge current for
  over-temperature protection
• Small Outline (SOIC) package
• AC auto-detect

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Battery Selection Guidelines
Based on measured current nominal draws 40 mA in
data logging mode 14 mA in real-time mode
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Regulated DC Voltage Supply

• Three types of supplies are considered
• Switched Power Converter (SPC)
• Low Dropout Regulator (LDO)
• Charge Pump
• According to research
• SPC provides regulated output but produces EMI
  during switching
• LDO provides regulated output but is less
  efficient than SPC
• Charge pump provides unregulated output but is
  more efficient across specific current draws than
  LDO or SPC.

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MAX719 Selectable Output SMPS

• Pros
• 150 mA supply suitable for application
• Dual selectable voltage outputs enables isolation
  of sensor
• Enables use of backup battery for uninterrupted
  operation
• Enables single-cell Li-Ion usage for
  compatibility with charger
• AC power detect bypasses battery
• Cons
• Extensive external circuitry even for single
  source
• Surface mount package complicates prototyping
• High cost at low quantities
• EMI due to switching may interfere with sensor
  readings

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Power Supply with Zener Reference
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Printed Circuit Board Layout