Inclusive & Assistive Products
ISDN2001/2002: Second Year Design Project
Resistance Implemetation
Dorsiflexion and Plantar-flexion
Objective
Achieve safe and effective resistance exercise for Dorsiflexion and Plantar flexion.
Method
1. Research on current ankle strengthen exercise
2. Evaluate various source of resistance
3. Desgn resistance implementation mechanism
Resistance Source: Approach 1
Linear Actuator
The motor inside the linear actuator creates rotational motion which is converted into linear motion through gears, nuts and lead screw. The idea was to utilizes the linear motion of the linear actuator to opposes the motion of the patient’s foot, creating resistance.
Incorporate load cells to obtain the force exerted by the patient. The linear actuator moves opposite the patient’s movement with force proportional to the force exerted by the patient.
Adjustable resistance
The linear actuator includes encoder to precisely control the position, speed, and force output.
Force measurement
The load cell continuously measures the force exerted by the patient. This allows real-time monitoring and resistance adjustment.
Safety protocols
If too much force is detected by the load cell, the linear actuator reduce resistance to avoid injuries.
Heavy weight
~5kg weight. As it would be installed on the rotating plate and rotates during inversion and eversion, it burdens the motor.
Speed limitation
may not provide the high speeds required for the exercise.
Power dependency
require electricity force which limits portability.
Resistance Source: Approach 2
Springs
The deformation of springs create tension that can replicate the resistance of resistance bands.
As F = kx, we can control the tension of the spring by changing the spring constant k, or the deformation distance x.
Adjustable Resistance
manipulating the spring stiffness or the length of the spring
Small and lightweight
the small size of spring makes it easy to fit in mechanism design
Simple mechanism
a compact design that exert force from elongation or compression
Interview with physiotherapist
Why resistance is needed?
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to improving the ankle muscle strength of patients
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strength of patient's maximum strength is not an important information
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physiotherapists mainly want to know how patients feel when doing a certain level of resistance and if adjustment is needed.
Final approach: Spring
As the force exerted by the patient is not an important information, we decide not to use linear actuator with load cell to unnecessarily complicate the device's design. The lightweight and simple design of spring also benefits the portability of the device.
Iteration 1: Spring inside cylinder
Problems
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The length of the spring, add on with the length of the rod in the cylinder caused the footplate to be too high, patient need to lift their thigh or find a higher chair to do the exercise
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The spring is inside the cylinder, making it hard to swap.
Iteration 2: Lead screw and nut mechanism
Idea
use double nut screw to deform the spring at the two ends. The footplate stays at the same place as the magnitude of the upper and lower screw force is same
a. Resistance bands Force to Length graph (1)
b. Spring Force to Position graph (2)
Problems
Changing the length of the spring only changes its force at that point but not its stiffness (a). However, the resistance bands varies with stiffness (b).
Iteration 3: Swap and insert spring with slider assistance
(Final Approach)
Idea
The patient swap the 'spring box' for different resistance level. The sliders at the side to restrict and guild the movement of patient's foot.
Slide-to-insert design for user to easily change the resistance.