Hydrophone-based Localization of Snake Robot End-Effector

Last updated: 5/14/2013


This project revolves around using an optical hydrophone to determine the location and orientation of a robot's end-effector within a patient's anatomy by sensing the ultrasound intensity within a volume. Specific applications would include sensing the tip of the APL snake robot during a procedure to remove osteolysis caused by artificial hip joints.

  • Students: Stephen Van Kooten
  • Mentor(s): Emad Boctor, Mehran Armand

Background, Specific Aims, and Significance

The APL snake robot is to be used in removal of osteolysis caused by hip implants. The procedure involves drilling a hole into the pelvic bone or using an existing screw hole and inserting the manipulator into the osteolystic cavity. The manipulator then explores the cavity and extracts osteolytic bone.

Current control methods that track position solely through kinematics have limited accuracy (within 1.3mm for the simplest configuration), so it is necessary to develop a means of directly measuring the manipulator location and orientation to better track the robot within the osteolytic cavity.

The techniques used will make use of ultrasound and an optical hydrophone to triangulate the position of the manipulator tip.

Ultrasound is sound with frequencies above the threshold of human hearing (50kHz). Typical frequencies for medical imaging are in the range 1 MHz to 20MHz. Modern ultrasound transducers use an array of piezoelectric elements that can be activated in sequence or tandem to get a focused beam or scanning series of beams.

An optical hydrophone consists of a fibre-optic cable with a Fabry-Perot interferometer attached to one end. A laser beam of frequency matching the resonance of the interferometer’s unstressed state is transmitted down the optical fibre and a machine measures the strength of the reflected signal. When ultrasound hits the end of the fibre-optic cable, the distance between the two reflective layers of the interferometer changes and a corresponding change in the reflected laser light is sensed by the controlling machine.


  • Minimum: (Expected May 6)
    1. APL snake robot end effector fitted with optical hydrophone fibers.
    2. Software and hardware to determine manipulator tip position within 5mm.
  • Expected: (Expected May 11)
    1. APL snake robot end effector fitted with optical hydrophone fibers.
    2. Software and hardware to determine manipulator tip position within 1mm.
    3. Rudimentary visualization showing tip position.
  • Maximum: (Not expected to meet)
    1. APL snake robot end effector fitted with optical hydrophone fibers.
    2. Software and hardware to determine manipulator tip position within 1mm and angle with 10 degrees.
    3. Advanced visualization of tip position, shows tip relative to anatomy, tracks progress, indicates what material needs to be removed.
    4. Interface with robot control system to automatically calibrate system.

Technical Approach

For the minimum deliverable of determining tip position, a single hydrophone fibre-optic cable will be inserted into the manipulator of the snake robot. A linear array ultrasound probe with 128 transducers that can be separately controlled will be used to create signals to be picked up by the hydrophone. An EM tracker marker or set markers will be attached to the ultrasound probe so that its position and orientation can be 1tracked by an external EM tracker. Each of the 128 transducers will be pulsed in turn and the time between pulse initiation and pickup by the hydrophone will be measured. This time and the estimated speed of sound in tissue (1540 m/s) will be used to determine the distance between the transducer and the manipulator tip. The ultrasound probe will then be moved perpendicular to the linear array to get a second set of out of plane readings. Once two linear scans are complete, the position of the manipulator tip will be calculated by triangulation. Due to the high frequency of the ultrasound used (10 MHz), a specialized circuit will be used to rectify and integrate the analog waveform from the optical hydrophone. This will greatly simply sampling and allow greater time resolution when finding time of flight of ultrasound pulses. To calculate the angle of the ultrasound probe two methods can be. One method is using two optical hydrophones inserted on opposite sides of the snake robot manipulator and measuring the phase shift between the two analog waveforms they pick up. This would allow a very high time resolution between time of flight of the two points. Triangulation as before would then allow calculation of the vector between the two points. The constraint that this vector lies on the plane of the robot manipulator’s tip, combined with the modeled robot kinematics and calculated position would then allow a more accurate estimate of the manipulator tip’s orientation.


  1. Access to ultra-sound probe and imaging system, cannot test without (MET)
  2. Phantom of pelvis and hip joint with bone erosion. Currently have a thin cross-section for proof of concept testing. Will need 3D model with tissue phantom for later tests. (DEFERRED)
  3. End effector of APL snake robot (MET)
  4. Optical hydrophone system (light source, fibers, and sensors) (MET)
  5. Software for reading hydrophone data (MET)
  6. Arduino board (MET)
  7. EM tracker and API (MET)

Milestones and Status

  1. Milestone name: Complete circuit on Arduino board to time ultrasound time of flight.
    • Planned Date: March 4
    • Expected Date: March 4
    • Status: Circuit constructed, needs testing.
  2. Milestone name: Check feasibility of project.
    • Planned Date: March 8
    • Expected Date: March 8
    • Status: Need meet with sponsors, need obtain wet bone
  3. Milestone name: Complete program to interface with EM tracker, US machine, and Arduino.
    • Planned Date: March 11
    • Expected Date: March 11
    • Status: None
  4. Milestone name: Complete triangulation algorithm
    • Planned Date: March 18
    • Expected Date: March 18
    • Status: Basic functionality, need to account for noise and distortion
  5. Milestone name: Test all components together (circuit, interface, algorithm)
    • Planned Date: March 25
    • Expected Date: March 25
    • Status: Dependent on previous milestones
  6. Milestone name: Complete rudimentary vision system
    • Planned Date: April 1
    • Expected Date: April 1
    • Status: Skeleton program set up.
  7. Milestone name: Implement kinematic robot kinematic model to decrease error.
    • Planned Date: April 7
    • Expected Date: April 7
    • Status: None

Reports and presentations

Project Bibliography

[1] Cox, B. T., et al. “Fabry Perot polymer film fibre-optic hydrophones and arrays for ultrasound field characterisation.” Journal of Physics: Conference Series. Vol. 1. No. 1. IOP Publishing, 2004.http://iopscience.iop.org/1742-6596/1/1/009/pdf/1742-6596_1_1_009.pdf

[2] Liu, Wen P., et al. “Sensor and Sampling-based motion planning for minimally invasive robotic exploration of osteolytic lesions.” Intelligent Robots and Systems (IROS), 2011 IEEE/RSJ International Conference on. IEEE, 2011. https://jshare.johnshopkins.edu/wliu25/public_html/IROS2011.pdf

[3] Kutzer, Michael DM, et al. “Design of a new cable-driven manipulator with a large open lumen: Preliminary applications in the minimally-invasive removal of osteolysis.” Robotics and Automation (ICRA), 2011 IEEE International Conference on. IEEE, 2011. https://bigss.lcsr.jhu.edu/main/images/1/11/MorphableDesignICRAforMay2011.pdf

[4]Precision Acoustics (PAL) Fibre-Optic Hydrophone Documentation http://www.acoustics.co.uk/search/files/Fibre-Optic%20Hydrophone/

[5]Specifications for LM348N Quadruple Operational Amplifier Chip http://www.ti.com/lit/ds/symlink/lm348.pdf

[6]Specifications for 1N4001 Diode http://www.diodes.com/datasheets/ds28002.pdf

[7] Filonenko, Viacheslav, Charlie Cullen, and James D. Carswell. “Asynchronous ultrasonic trilateration for indoor positioning of mobile phones.” Web and Wireless Geographical Information Systems. Springer Berlin Heidelberg, 2012. 33-46.

Other Resources and Project Files

  • Simulation files (Not organized or necessarily useable as is but presented for reference)
  • Python wrapper for MEDsafe EM tracker
    • Contact me at svankoot1@gmail.com
courses/446/2013/446-2013-12/446-2013-12.txt · Last modified: 2013/05/14 21:58 by svankoo1@johnshopkins.edu