Second Design Phase

We consolidated feedback from our mentors after presenting to them our preliminary design idea. For our improved design, we will have to take into account the following design factors that were previously not well-addressed:

1. Kinematic Compatibility – The exoskeleton should have a mechanism that mimics the actual hand.
2. Increase in skin length – As our finger flexes, there will be an increase in the length across the top of the finger along the phalanges
3. Hyperextension – our designs have to prevent or avoid the hyperextension of fingers which is a safety issue
4. Wearability – This means that they should be easy and comfortable to wear for stroke patients

Based on these design considerations, we have spent the past week brainstorming for better ideas and have come up with 3 possible designs for our exoskeleton. We spent time evaluating these designs with our supervisor and mentors. The results of our evaluation are detailed below.

Design #1 – Overhang

ORIGINAL DESIGN (by our group)
Pros	Prevention of hyperextension with the overhang structure on the hinges
Cons	Many modular parts to attach → affects wearability and requires more specific measurements to fit the device onto different patients Hinges are not attached →  The hinges may move around or rotate during flexion/extension
MODIFIED DESIGN (edited under the guidance of our supervisor and mentors)
Improvements	Hinges are attached to prevent movement or rotation of the parts along the finger Fewer parts to reduce complexity and the need for specific measurements. Greater ability to cater to different finger lengths

DESIGN #2 – Caterpillar

Pros	Hinges are kinematically compatible with normal motion of our fingers, and accounts for the increase in length of our skin as we bend our fingers Easy to understand the motion
Cons	Does not prevent hyperextension More anchor points affect wearability. There has to be 3 anchor points (the brighter pink attachments as can be seen from the sketch) Bulkier exoskeleton, less appealing to patients

DESIGN #3 – Slider

For the end product, we will have to choose one of these joint designs to implement. In order to aid our decision making, we used a total of 6 criteria:

• Kinematic compatibility refers to the ability of the hinge mechanism to achieve the desired motion to mimic a hand.
• Adapt to Variable Finger Length: As the size of the hand varies from person to person, we want to create a device that can adapt to variable finger length so that there is no need for customization, thus bringing the cost of production down.
• Prevent Hyperextension: Our fingers have the natural mechanism to prevent or minimise hyperextension which we must mimic in our device for the safety of the patients.
• Degree of Freedom: We have to mimic is the degrees of freedom with regards to our finger. For extension and flexion of fingers, we only require 3 degrees of freedom. Any device with less than 3 degrees of freedom will not be able to work while any device abv 3 degrees of freedom might result in unwanted movement that may injure / apply stress to the patient’s finger.
• Wearability: In order to differentiate our project from other research projects that involve a glove which is difficult to put on for a spastic hand, we aim to make our robotic hand more wearable in the form of a clip on.
• Patient acceptability: The design of the exoskeleton must not be too bulky or odd looking as they may not be inclined to make use of such devices.

These criteria are summarised in our decision making table below.  Based on the criteria, we decided to go alone with design #3 which is also known as the slider design. The first four criteria (kinematic compatibility, adapt to variable finger length, prevent hyperextension and degrees of freedom = 3) are the basic criteria that have to be fulfilled for the safety of the patients. Although the slider design meets most of these 4 basic criteria, it does not meet the criteria of only having 3 degrees of freedom which could be dangerous for the patient. However, we think that fixing part 2 to part 3 may be able to solve the problem. The slider mechanism is also more wearable than the other two designs as it only requires 2 points of attachment. Even while satisfying all these criteria, the design remains more compact and will likely be more acceptable to patients as well.

Moving forward, we will have to work on these areas for the slider design:

• • Dimensions and sizing measurements
    • Calculation of the length of the parts that would be best to fit most finger sizes
    • Make 2 different sketches for 2 different finger sizes and print both prototypes
  • Improvement of wire routing
    • Transmission of forces currently do not look smooth
    • Subjecting wires to sharp angles may cause them to break
  • Possible simplification of the design
    • Translation of 1 joint with respect to another follows a trajectory
    • The slot in part 2 may not necessarily be the only solution