University of Wisconsin Madison Biomedical Engineering Design Courses
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Hospital Bed-Back Angle Controller (RERC National Design Competition) Team Members Advisor Katy Reed Brenton Nelson Ibrahim Khansa Shikha Client Dr. Willis Tompkins Dr. John Enderle - University of Connecticut
Background: Current Hospital Beds • Bed-back can only be operated at one fixed speed • Disadvantages: • Buttons are hard to push, and inaccessible for some patients • Fixed speed less control and flexibility • No feedback to user
Background: Extended Physiological Proprioception (EPP) • D. C. Simpson, 1974 • Controllers are more intuitive when they act as a natural extension of the user’s body
RERC National Design Competition • RERC: Rehabilitation Engineering Research Center • Conducts projects in Accessible Medical Instrumentation (AMI) • Competition organized by Marquette University and the University of Connecticut • 10 projects funded every year • Client: Dr. John D. Enderle, Professor of Biomedical Engineering at the University of Connecticut.
Problem Statement An intuitive hospital bed control system, which gives the user better control over the velocity of bed-back elevation, is desired. The user would be able to operate a controller based on force-assist concepts, and the velocity would vary with the amount of force applied.
Requirements • Ability to control velocity • Accessible for patients with specific disabilities • Intuitive and ergonomically designed controller • Support a maximum load of 180 lbs on the bed-back • Bed-back brake system during power loss • Maximum operator force should not exceed 20 lbs on the controller • Budget less than $2, 000
Design Overview Feedback to user User Feedback to controller Sensor Bed-back angle Controller Controllable electric frequency Motor
Design Overview Feedback to user User Feedback to controller Sensor Bed-back angle Controller Controllable electric frequency Motor
Motor Control • Bed back is controlled by a single-phase AC induction motor • Motor speed cannot be controlled by magnitude of input current • To control motor speed, modulate input current frequency Motor speed (rad/sec) Input current frequency (Hz)
Design Overview Feedback to user User Feedback to controller Sensor Bed-back angle Controller Controllable electric frequency Motor
Feedback System Reference angle θref Sensor measures angle θactual Error: ξ = θactual – θref Differentiator Change in error dξ/dt Inputs to feedback loop
Feedback System (cont. ) • Alternative 1: PD Loop • PD = Proportional-Derivative (PDI loop without integral term) • Output = K 1ξ + K 2 dξ/dt • The output of the PD loop is used to minimized the error • K 1 determines how fast the error is reduced • K 2 determines how smooth the transitions are
Feedback System • Alternative 2: Fuzzy Logic • Used when the mathematical basis of the system is uncertain or not needed • Inputs and outputs are ranges of values, each given a discrete qualifier (“Slow”, “Medium”, “Fast”). Probability slow medium fast Velocity
Design Overview Feedback to user User Feedback to controller Sensor Bed-back angle Controller Controllable electric frequency Motor
User Interface • User has the choice of two controls: • Cruise control: user specifies angle and velocity • Controller for fine angle adjustments • User receives feedback on the velocity of motion • Vibrations in controller (similar to video game controllers) • Resistance on controller • Combination of visual and auditory cues.
Design Alternative 1 • Substitute a DC motor for the existing AC motor on the bed AC current (from wall) AC/DC converter Current amplitude controller • Advantage PD loop • Simple • Disadvantages • DC motors rotate when power is off. • Cost DC motor
Design Alternative 2 • Analog error compensation PD loop Analog output AC Motor • Advantages: • Accurate • Smooth transitions • Disadvantage: • PD loop requires a complete knowledge of the physics of the system
Chosen Design • Logical error compensation Fuzzy loop Discrete output AC Motor • Advantages • Does not require accurate knowledge of the system • Corresponds more intuitively to the human mind • Disadvantage • Less precise (but not a problem for our purposes)
Plan for Future Work • October-December: • Analyze the electrical and mechanical aspects of the bed • Build a preliminary prototype based on fuzzy logic • Conduct preliminary testing • January-May: • Build final prototype • Test ergonomics, ease of operation and safety on volunteers or patients
References Doubler, J. A. , Childress, D. S. An Analysis of Extended Physiological Proprioception as a Prosthesis-Control Technique. Journal of Rehabilitation Research and Development, (21), Issue 1, pp. 5 -18. Simpson, D. C. (1974). The choice of control system for the multimovement prothesis: extended physiological proprioception (EPP). The Control of Upper-Extremity Prostheses and Orthoses. (P. Herberts et al, ed) Springfiled, Illinois, C. C Thomas. pp. 146 -150. Simpson, D. C. (1973). The control and supply of a multimovement externally powered upper limb prosthesis. Proc. 4 th Int. Symp. External Control of Human Extremities, Belgrade, Yugoslav, pp 247 -254. Zadeh L. A. (1968). Fuzzy algorithms. Information and Control, 5, pp. 94 -102
Questions?
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