Servo-Controlled Blood Vessel Occluder
description
Transcript of Servo-Controlled Blood Vessel Occluder
Servo-Controlled Blood Vessel Occluder
Ahmed El-Gawish, Alan Chen,Hugo Loo, & Imad Mohammad
Advisor: Ki Chon
Background
Goal: measure effect of drugs and hypertension on MYO and TGF in mediation of renal autoregulation
Needed: a device that will automatically occlude and release blood vessels based on the user’s settings
Requirements
Pressure control intervals: 20 mmHg - simulating rapid step change in renal arterial pressure (RAP)
Control over period of 200 secs – stabilizes pressure effects
Mean controlled pressure range 80 – 120 mmHg – range over which the MYO and TGF mechanisms hypothetically take effect in autoregulation
Components
Occluder
Sensor
Pump
Motor
Software
Occluder
Made from 100% silicon material Proven to be physiology compatibility
Size used is depends on lumen diameter Range is between 2mm to 24 mm
Inject air or liquid into occluder to adjust the diaphragm/lumen diameter
Air
Advantages Simplicity Availability Ease of pressure control Doesn’t damage occluder
Disadvantage Dangerous!
Used for either short or long-term occlusion
Water and saline solution
Advantages Simplicity Availability Ease of pressure control Biocompatibile
Disadvantage Transpires/Evaporates through silicone rubber Causes damage of silicone rubber
Used for short-term occlusion (up to one hour)
Glycerin
Advantages No transpiration No evaporation Doesn’t cause damage occluder Biocompatible
Use for long-term occlusion (excess of one hour)
Sensor
Blood pressure sensor is an invasive or non-invasive sensor Invasive - used or implanted directly at the
measurement site (e.g., intra cardiac, blood vessel) Non-Invasive - measure systolic and diastolic blood
pressure utilizing the oscillometric technique
Designed to measure human blood pressure
Invasive Sensors
Internally placed catheter-tip sensor
Catheter fluid is coupled directly to an external transducer
Blood pressure is measured by observing the cavity’s changes in length with an optical signal conditioner
Measuring scheme is based on white light interferometry
Can be used on rats as well as humans
Fiber Optic Pressure Sensors (Invasive)
Non-Invasive Sensors
For humans: the sensor is attached to the cuff on the wrist.
Oscillometric technique is used to measure the blood pressure
For rats: high cost
Photoplethysmography
Relatively inaccurate
Imprecisely measures systolic blood pressure
Over-saturation of the blood pressure signal by ambient light
Difficulty in obtaining adequate blood pressure signals in dark skinned rodents
Correlate poorly with direct blood pressure measurements
Piezoplethysmography
Similar clinical limitations to Photoplethysmography
Utilizes piezoelectric ceramic crystals to record blood pressure readings
More accurate than light-based/LED sensors
Correlate poorly with direct blood pressure measurements.
Volume Pressure Recording-VPR
Independent clinical validation study in 2003 conducted at Yale University VPR correlated almost 100% with direct blood pressure
measurement
Utilizes a specially designed differential pressure transducer
Measure six (6) blood pressure parameters simultaneously: systolic blood pressure, diastolic blood pressure, mean blood pressure, heart pulse rate, tail blood volume, and tail blood flow
Non-Invasive Sensors
Motor
Drives the medium based on control signals
Two Types Stepper Motor Servo Controlled Motor
Stepper Motors
Resolution is set at steps per revolution
Inexpensive
No need for feedback Remembers current position and knows number of
steps to reach another position
Uses current even when stationary Generates heat (50C - 90C)
Stepper Motors (cont’d)
Digitally controlled Signals cause it to settle in positions based on
coil states
Speed determined by controller
Maintains position without signal changes Higher holding torque
Stepper Motors in Action
Animation source: http://www.interq.or.jp/japan/se-inoue/e_step1.htm
Servo-Controlled Motors
Higher resolution
Smoother motion
Less heat generated
More expensive than stepper motors
Lower Holding Torque
Servo-Controlled Motors (cont’d)
Faster than stepper motor
Feedback determines correct positioning More complex than stepper motor
Oscillates when close to the desired position due to feedback
Servo-Controlled Motors (cont’d)
Diagram Source: http://www.machinedesign.com/ASP/viewSelectedArticle.asp?strArticleId=58153&strSite=MDSite&Screen=CURRENTISSUE&CatID=3
On-Off Controller
Logic control with only two states e.g. temperature control with a boiler that can
only be turned on or off
Determines whether the measurement is below a threshold If below threshold, take action Otherwise, no action is required
PID Controller
Proportional Integral Differential Controller
Alternative to on-off control
(error) = (set point) – (measurement) Set point is what you would like the value to Error would be the difference between the set
point and actual value
PID Controller (cont’d)
Image Source: http://www.netrino.com/Publications/Glossary/PID.html
P (proportional)
If the proportional gain is well chosen, the time it takes to reach a new set point will be as short as possible, with overshoot (or undershoot) and oscillation minimized.
Large proportional gain needed for quick response
Small proportional gain needed to minimize overshoot and oscillation
Achieving both at the same time may not be possible in all systems.
D (differential)
If the output is changing rapidly, there is a high chance of overshoot or undershoot
The derivative is the change in value from the previous sample to the current sample
Adding this to the proportional controller slows down the response time, but also decreases overshoot and ripple effects
I (integral)
If the is no change in value over time, the output may settle at the wrong value.
Integral value is small in general
Persistent error will cause sum to become large enough to cause a change
PID Controller Summary
Derivative and/or integral terms are added to proportional controllers to improve qualitative properties of a particular response.
When all three terms are used together, the acronym used to describe the controller is PID.
Schematic Presentation of PID
Software
Used to configure the PID controller
Manages data acquisition from sensor
DataLab 2000
References
Wang, H. Siu, K., Ju, K., Moore L. C., and Chon, K. H., Identification of Transient Renal Autoregulatory Mechanism Using Time-Frequency Spectral Techniques. IEEE Transactions on Biomedical Engineering, June 2005 (52) 6:1033-1039
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Brainstorming . . .