Design and Control Challenges for a Surgical Radiographic Imaging System

Tim ElmoreOrthopaedic Biomechanics Laboratory

Department of Mechanical & Aerospace EngineeringUniversity of Florida

The Motivation

What surgeons can see… What surgeons want to see…

http://www.theploughhundon.co.uk/steak%20tuesday.jpghttp://www.flickr.com/photos/stanfordmedicine/3660598599/

The Motivation

The standard in industry …

Over 100 ft3 total volume!

Current O-arm technology remains in sterile field, giving surgeon limited access to patient

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The Motivation

Existing project development… Gator-Ray !

The Motivation

Existing project development… Gator-Ray !

Design Challenges for Surgical GatoRay

• Kinematic– DOF– Obstacles– Reach– Accuracy

• Kinetic– Payload mass– Velocity/Acceleration– Bearing life

OR table, surgeons, equipment

Under OR table to lateral view

+/- 1mm, +/-1 deg

30-60kg (65-130lbs)

0.25m/s, 2.5m/s^2

1,500,000 cycles (100/day, 40yr)

5 degrees of freedom

Mitsubishi PA10-6CE Specifications

Six Degree-of-Freedom Robot Arm

3-phase AC Brushless Servo Motors

Harmonic Drive Transmissions

Manufacturer Specified Positional Repeatability of

±0.1mm ANDJoint Resolver Accuracy

= ±0.44 arc min (0.0073º)

10kg payload limit

Mitsubishi PA10-6CE Specifications

This payload would result in a 700% overload!

0 20 40 60 80 100 120 140 160 1800

5

10

15

20

25

30

35

Torque Limit

Inertia Limit

Payload

Rbar, distance from EE to CoM

kg

lim

it

Alternative Manipulators

Panasonic VR-032 RobotAxes: 6Payload: 32 kgH-Reach: 1866 mmRepeatability: ±0.15 mmRobot Mass: 580 kgMounting: floor, ceiling

Disadvantages:- Size- Mass- EE rotations are translation

dependent- Cost ($20,000 used)

Table Height:32 inches/810mm

Alternative Manipulators

Parallel ManipulatorsDegrees of Freedom: 6Payload: high (>30kg)Repeatability: good (<1mm)Robot Mass: low (<200 kg)

Disadvantage:- Limited rotation angles- Limited translations

Alternative Manipulators

Ballscrew/Rotary tablesDegrees of Freedom: 1 per tablePayload: high (>100kg)Repeatability: good (<1mm)Robot Mass: low (<200 kg)

Advantages:- Each axis is customizable- Mass supported by frame, not

torque

The Proposed System (v0.1)

X-Ray Source

Undertable

Robot

The Proposed System (v1.0)

X-Ray Sensor

PA-10

X-Ray Source

Under table

Robot

The Proposed System (v1.0)

X-Ray Source

Undertable

Robot

X

YZ

The Proposed System (v1.0)

Design Goals for Surgical GatoRay

• Kinematic– 5 DOF (3 translation, 2 rotation)– Reach around table into lateral view– Displacement: 3 translation, 2 rotation– Accuracy 0.2mm/axis, 1mm total

• Kinetic– 30-60kg payload– 0.5m/sec velocity, 5m/sec2 acceleration– 1,500,000 cycles at 30kg load

Kinematics

• By inspection, task space dependencies:– Rx = f(xjs,Θzjs)

– Ry = f(yjs,Θzjs)

– Rz = f(zjs)

– Θx = f(Θxjs)

– Θz = f(Θxjs, xjs, yjs) X

YZ

Kinematics

Kinetics

• Motor choices– Servo

• Pros: torque, speed• Cons: size, price, control

– Stepper• Pros: size, price, control• Cons: torque, speed

Kinetics

Servo Control

Stepper Control

Input Output

Input Output

Stepper Motors

Stepper Motors

125kg horizontal payload 30kg vertical payload

time time