A MICRO OPTICAL FORCE SENSOR FOR FORCE MEASUREMENT

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A MICRO OPTICAL FORCE SENSOR FOR FORCE
MEASUREMENT
J. Peirs, J. Clijnen, D. Reynaerts, H. Van
Brussel, P. Herijgers, B. Corteville, S.
Boone Katholieke Universities Leuven, Dept. of
Mechanical Engineering, Belgium
- Vivek Kumbala (K00217072)
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Overview

• Introduction
• Force Range and Sensor Design
• Principle of Measurement
• Flexible Structure
• Calibration of the Sensor
• Conclusion
• Summary

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Force Range

• A test set-up was built to measure the forces
  occurring during suturing (stitching). The figure
  shows a needle driver equipped with a 2-component
  force sensor based on strain gauges.
• The 4 strain gauges are glued on the instrument
  shaft with 90 interval. Two opposing strain
  gauges form a half bridge measuring the X or Y
  component of the force at the tip.
• The Z component is assumed to be of the same
  order of magnitude. The strain gauges are
  connected to AC bridge amplifiers and sampled at
  250 Hz.

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Needle driver with 2-component force sensor based
on strain gauges (enlarged).
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Sensor Design

• Specifications
• The sensor will be mounted at the tip of a 5 mm
  diameter
• instrument driver with two bending degrees of
  freedom.
• This instrument driver has an internal channel of
  2 mm
• diameter through which surgical instruments
  can be inserted.
• The sensor should have the same internal and
  external
• Diameters.

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• The sensor should be as short as possible because
  it will be
• mounted in front of the local degrees of
  freedom
• Three components (FX, FY, FZ) should be measured
  with a
• range of 2.5 N and a resolution of 0.01 N
  (0.2 of range).
• The radial forces act on the instrument tip, at a
  distance of
• 15 mm from the sensor front. The sensor should
  be
• biocompatible, sterilisable and robust.

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Principle of Measurement

• The working principle of the sensor Three
  optical
• fibres measure the deformation of the
  flexible structure
• through the intensity of the reflected
  light.
• The basic layout of the sensor consists of
  consists of two
• parts connected by a flexible connection.
• The upper part is connected to the tool
• The lower part is connected to the instrument
  shaft.

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Basic Layout of the Sensor
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• Three optical fibres, arranged at 120 intervals
  in the lower part, measure the relative
  displacement between upper and lower part through
  the intensity of the reflected signal.
• The fibres are placed axially because bending
  them inside the sensor over 90 would violate the
  minimum bending radius.
• Three sensing fibres are used here each of which
  have two possible configurations.

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• The first configuration uses separate emitter and
  receiver fibres.
• Here coupling losses do not occur, but high
  losses occur at the mirror.
• As the receiver fibre is located next to the
  emitter fibre, it can pick up only a small
  portion of the reflected light, which is far less
  than 25 .
• An additional drawback is that this configuration
  requires twice as much fibres to be integrated in
  the sensor. Therefore, the second configuration
  is chosen.

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• In the second configuration the light is
  reflected back into the same fibre.
• An optocoupler has to be used to couple the
  emittor (LED)
• and receiver (photodiode) to the same
  measurement fibre.
• Maximum sensitivity is obtained when a 50/50
  optocoupler
• is used. This means that 50 of the emitted
  signal is
• coupled into the measurement fibre while 50
  goes to the
• reference photodiode.

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• Similarly, the reflected signal is split equally
  into the emitter
• and receiver fibres.
• This means that maximally 25 of the originally
  emitted
• light is sent back to the receiver (when the
  sensor reflects all
• incoming light).
• The sensing fibre can measure the perpendicular
  distance
• from the surface or the lateral distance from
  an edge of this
• surface, depending on the sensor design.

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Flexible Structure

• The design tries to decouple the deformations
  caused by axial and radial forces by using
  parallelograms.
• An axial force causes the thin horizontal beams
  to bend, while the thick vertical beams deform
  negligibly.
• For a radial force the vertical beams deform
  while the horizontal beams are mainly stressed
  longitudinally, a direction in which they have a
  high stiffness.
• The flexible structure consists of 4 identical
  parallelograms
• placed in an axisymmetric arrangement.

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• The sensor is made of a strong titanium alloy
  (Ti6Al4V) because this material has a good
  corrosion resistance, superior biocompatibility,
  low Youngs modulus and high strength (both
  maximizing sensor displacement), high fatigue
  resistance, and good shock resistance.

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Flexible structure of the sensor (scale in
millimeters).
Dimensions of the flexible structure.
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• The circular holes are starting holes for the
  wire-EDM (Electro- Discharge Machine) process
  used to machine the slits.
• The EDM wire has a diameter of 30 µm.
• The square features located just above the upper
  holes are end stops protecting the sensor from
  axial overload.
• The vertical gap is 50 µm corresponding to an
  axial load of 2.5 N
• The end stop in radial direction is the core the
  gap between the core and the flexible tube is 85
  µm and closes at a radial force of 1.7N

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Calibration

• To calibrate the sensor, a short rod is fixed to
  its tip to act as a lever for applying torque.
• The length of the lever, 15 mm, corresponds to
  the distance from the instrument tip to the
  sensor front.
• The compliance matrix A between the applied
  forces Fi and the displacements di measured by
  the fibres is defined as follows

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Reflected signal as a function of perpendicular
and lateral distance between fibre and Si surface
edge.
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• The design and calibration values for the
  compliance matrix are as shown (in µm/N).
• These values can be converted to mV/N by
  multiplying them with the sensitivity of the
  optical system, 10.3 mV/µm
• The design sensitivity matrix is not symmetric
  due to the 120 spacing between the fibres and
  the chosen coordinate frame.
• The calibrated compliance matrix shows large
  differences with the design values because of
  production errors. But, the resolution of the
  optical measurement system is 0.3 µm,
  corresponding to about 0.04 N.

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Conclusion

• A 5 mm diameter tri-axial force sensor has been
  developed
• for minimally invasive robotic surgery.
• To define the required force range and
  resolution, a needle driver has been equipped
  with strain gauges. The vivo-tests with different
  types of needles and tissue showed that the
  required force range and resolution are
  respectively 2.5 N and 0.01 N.
• The new sensor is based on a flexible titanium
  structure of which the deformations are measured
  through reflective measurements with 3 optical
  fibres. It has a range of 2.5 N in axial
  direction and 1.7 N in radial direction

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References

• http//www.mech.kuleuven.be/micro/pub/medic/Pape
  r_MME2003_MIS_ sensor.pdf
• J. Rosen et al, Surgeon-Tool Force/Torque
  Signatures - Evaluation of Surgical Skills in
  Minimally Invasive Surgery, Proc. of medicine
  meets virtual reality, SanFrancisco, CA
• I. Brouwer et al, Measuring in vivo animal soft
  tissue properties for haptic modeling in surgical
  simulation. Medicine meets virtual reality
• J. Peirs, H. Van Brussel, D. Reynaerts, G. De
  Gersem, A Flexible Distal Tip With Two Degrees of
  Freedom for Enhanced Dexterity in Endoscopic
  Robot Surgery, Proc. Micromechanics Europe
  Workshop, Sinaia, Romania,

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Summary

• The design of a prototype gastro intestinal
  intervention system based on an inch-worm type of
  mobile robot was discussed in which the modular
  actuator for realizing the bending motion was
  presented and many technologies were discussed.
  
• It has been, hence, concluded that the device
  can be used as a kind of vehicle for inspection
  in the intestinal system.
• A 5mm diameter optical force sensor for
  minimally invasive robotic surgery has been
  developed, based on the specifications from
  in-vivo tests. And it has been found that it has
  a range of 2.5 N in axial direction and 1.7 N in
  radial direction.

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