19.2 -> Design Process

This page outlines our design process from initial concept to final implementation. The first half focuses on the prototype phase, where we developed and tested early versions of the mechanism to validate core functionality. The second half highlights the final product, detailing the design iterations and refinements made after the prototype deliverable to improve performance, robustness, and overall behavior.

1. Prototype Phase

We use physical prototyping as a part of our design process to validate and refine the compliant finger mechanism. By building and testing, we aim to identify problems that are not always found in simulation. This may include assembly, material behavior, and the actuation performance. This process is iterative in which each new prototype will inform improvements for the next design. Our prototyping documentation is outlined below.

MotionGen Model

To begin our prototyping efforts, we developed a digital model using MotionGen to simulate the finger mechanism. This allowed us to approximate the desired motion profile and scale for the system. The preliminary model is shown below.

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Figure 1. Preliminary MotionGen Model to Guide Link Lengths and Scale

From this model, we retrieved approximate link lengths to guide our first physical prototype. The links are defined as follows:

The joint between Link 3 and Link 4, denoted by the plus sign in MotionGen, is where the torsional spring is located.

Note that, while the MotionGen model visually depicts the links with extra joints to create their unique geometries, all links are treated as binary links for the purposes of our project. The sketch below shows the links using their defined link numbers and without their geometric forms.

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Figure 2. Simplified Kinematic Diagram to Show Binary Links, Not to Scale

Initial CAD from MotionGen Model

From the MotionGen model, we created our initial CAD model using the approximated link lengths. The screenshots below depict the first version of our compliant finger mechanism.

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Figure 3. Isometric View of CAD Finger Assembly

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Figure 4. Top View of CAD Finger Assembly

However, during the assembly simulation, links 3 and 5 unexpectedly collided, due to the geometry of link 5. This restricted the finger from closing properly, so the geometry of links 4 and 5 were altered. The length of link 4 was also changed to 15 mm. These changes are reflected in the images below.

Updated Model to Avoid Collision and add Torsional Spring

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Figure 5. Isometric View of CAD Finger Assembly

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Figure 6. Top View of CAD Finger Assembly

In these CAD models, slots were also added to links 3 and 4 to pre-emptively account for the torsional spring’s integration. While at this time, the springs have not been ordered or received, we wanted to implement this design update early such that we can begin iteration as soon as the components arrive. The slot for the torsional spring can be seen in the CAD screenshot below.

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Figure 7. Torsional Spring Slot in Link 3

Updated Model Through Preliminary 3D-Prints

Through 3D-printing the updated CAD models, we identified a new issue. Although the finger appeared to bend as intended in simulation, the physical assembly (Figure 8) revealed that the distal segment bent excessively even when the proximal segment was only roughly perpendicular to the base.

To address this, we increased the length of link 3 from approximately 7 mm to 15 mm to limit distal over-flexion in the first stage of motion (Before Object-Contact/Unflexed Torsional Spring). With this modification, the updated assembly (Figure 9) shows that the distal segment remains appropriately aligned while the proximal segment maintains a similar position.

Open Figure 8. Short Link 3 Creates Unwanted, Early Distal Bend	Open Figure 9. Longer Link 3 Prevents Early Distal Bending

Open Figure 8. Short Link 3 Creates Unwanted, Early Distal Bend	Open Figure 9. Longer Link 3 Prevents Early Distal Bending

Furthermore, as seen in the above images, we chose to increase the base of our finger mechanism. This allows the base to provide another area of contact with the object being grasped, assisting the finger mechanism naturally as a palm would.

Updated Finger Geometry and Finalized Link Lengths

To evaluate the grasping capability of our compliant finger mechanism across objects of varying sizes, we selected a “watch cushion” as our test object. This cushion is a small rectangular block with rounded edges, measuring approximately 3” × 1.5” × 1.25”. Its geometry allows it to be oriented along its longer or shorter dimension, enabling us to test the mechanism under different grasping conditions. The test object is shown in Figure 10.

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Figure 10. Watch Cushion as our Test Object

Initial testing showed that the mechanism successfully grasped the object when oriented along its longer dimension. In this case, the system remained in its stage one four-bar configuration, and both proximal and distal segments made contact with the object effectively. However, we encountered an issue when testing the shorter orientation (approximately 1.5 inches). Here, the mechanism transitioned into the stage two four-bar behavior, where the torsional spring is expected to flex and allow the distal segment to wrap around the object.

Despite this intended behavior, links 2 and 3 came into contact with the object, preventing the distal segment from achieving a secure grasp. This limited the effectiveness of the mechanism when grasping smaller objects.

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Figure 11. Object is not tightly grasped, due to link geometry

To address this issue, we experimented with MotionGen to simulate different changes. Based on these iterations, we reduced the length of link 4 from 15 mm to 10 mm. Additionally, we increased its thickness, allowing the distal segment to engage the object earlier in the grasping motion. A comparison between the original (black) and updated (purple) link 4 designs is shown below.

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Figure 12. Link 4 Update (Old in black, new in purple)

This resulted in an improved performance, enabling a more reliable and tighter grasp as seen in Figure 13.

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Figure 13. Better, tighter grasp with updated link geometry

Prototype Results

Through iterative design and testing, we updated the mechanism to improve its functionality. The final link dimensions were selected based on these improvements and are summarized below.

Although the final prototype does not yet include torsional springs, the mechanism’s intended behavior is still demonstrated. When grasping larger objects, the finger operates as a collective system in its stage one configuration, with both proximal and distal segments making contact simultaneously (Figure 14). For smaller objects, the proximal segment contacts the object first, effectively grounding that link. This triggers the transition to stage two behavior, where the torsional spring would allow continued motion of the distal segment, enabling it to wrap around and securely grasp the object (Figure 15).

Open Figure 14. Finger grasps a larger object (Spring is unflexed)	Open Figure 15. Finger grasps a smaller object (Spring is flexed)

Open Figure 14. Finger grasps a larger object (Spring is unflexed)	Open Figure 15. Finger grasps a smaller object (Spring is flexed)

Prototype Bill of Materials

While our prototype only entails the development of a single compliant finger mechanism to demonstrate its unique motion profile, we use a variety of materials and off-the-shelf components to create a working assembly. The following draft BOM documents the parts, relevant dimensions, quantities, prices, and sources that make our prototype come to life.

Note that for this BOM, we list electronic components along with the torsional springs as we plan to implement these for testing beyond manual actuation. However, due to delays in shipping and the requirements of our prototype demonstration, both the spring and electronics are not yet physically implemented into our assembly. These items are italicized in the BOM for clarity.

From this draft BOM, we can see that our prototype primarily uses available parts from our class’s resources and from TIW, keeping our total costs relatively low.

Plans to Transition from Prototype to Final

With the current prototype complete, we have identified several key improvements for the final iteration of our project. First, we will integrate the torsional spring into the mechanism and conduct testing to validate its functionality.

Secondly, we plan to refine the link designs to enhance contact surfaces, making them more anatomically finger-like while also improving durability. These updates aim to achieve more effective and reliable grasping.

Lastly, we will expand the system from a single finger to a multi-finger configuration, targeting a total of two to three fingers to form a simplified robotic hand. To maintain design efficiency, all fingers will be actuated using a single motor. We anticipate implementing either a gear-based transmission system, belts, or a linkage mechanism to distribute the rotary input across multiple fingers.

With these changes, the final system will demonstrate a functional, simplified hand composed of compliant finger mechanisms capable of adaptive grasping across different object sizes.

2. Final Product Phase

Following the development of our initial prototype, the focus of the remaining weeks is to refine our design for improved grasping performance and integrate the actuation system. This includes geometric updates to better mimic the anatomical finger structure, the addition of a second finger to form a simple gripper, and the implementation of a single motor transmission system for simultaneous finger control.

Improving Anatomical Finger Designs and Adding a Second Finger

The first major design update focused on improving the geometry of the compliant fingers to enhance grasping performance. The original links were made to demonstrate the motion profile, but for our final product, we updated the links to increase contact surface area during grasping. This not only made our linkage mechanism more anatomically accurate in terms of appearance but also in performance. This change allowed for improved force distribution and more stable object interaction.

Specifically, link 3 and link 4 were redesigned to ensure the screw connecting the links would not collide during actuation and to create a larger finger tip respectively. These updates are shown in Figure 16 and Figure 17.

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Figure 16. Link 3 Updated Design to Avoid Fastener Interference

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Figure 17. Link 4 Updated Design to Increase Contact Surface

In addition to modifying the finger’s individual links, a second identical finger was introduced to create a simple two-finger gripper configuration. To accommodate this change, the base was redesigned to support both fingers while maintaining proper alignment and motion. The distance between the inner grounded joints of the two fingers was set to 3 inches to provide sufficient spacing for our test object in both long and short orientations.

A key feature of the updated base is the vertical offset between both fingers, which allows both fingers to operate within the same plane. Due to our team only having access to “right-handed” torsional springs, we had to reuse the finger mechanism and flip an identical copy upside-down to allow the spring to be oriented properly. This ensures the fingers are contacting the test object at the same height to avoid twisting the object during grasping. The two-finger configuration is shown below:

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Figure 18. Isometric View of Two-Finger Setup

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Figure 19. Top View of Two-Finger Setup

The updated components were all 3D printed and assembled to verify its intended functionality. Preliminary testing showed that we may want to experiment with the spacing between the fingers to optimize the grasping performance. Additionally, we noted that links 2 and 3 would occasionally collide with the object, preventing the distal segment of the finger to make a full closure. This seemed to result from where the proximal link 5 would be grounded, so we made note to revisit this issue to ensure proper grasping.

Implementation of Transmission System (Gears + Belts)

The next stage of development focused on implementing a transmission system that allows a single motor input to simultaneously control both fingers during opening and closing. After evaluating multiple concepts, a combination of gears and timing belts was selected. Two identical gears with a 1:1 ratio enable the fingers to rotate in opposing directions, while timing belts transmit this motion to each finger across the base.

To support this system, the base was redesigned to accommodate additional shafts and bearings, as shown in the CAD model below. A spacer (Figure 20) was also added above the right finger to ensure that all timing belts remained aligned within the same plane.

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Figure 20. New Base for Gears and Belts

The updated components were fabricated using 3D printing and assembled with the purchased hardware. Initial testing showed that while the gears meshed correctly, the timing belts lacked sufficient tension. This resulted in slipping, particularly during the second stage of motion when the spring begins to flex and higher torque is required.

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Figure 21. 30T Gears in mesh; Belts are loose

To address this, the positions of the lower shafts (where the gears are mounted) were adjusted to increase the center distance, introducing greater belt tension. This modification successfully eliminated slipping, but in return, the increased tension caused the shafts to bend inward toward each other. To resolve this issue, additional structural support was introduced by incorporating braces with bearings above the shafts as seen in the figures below. These supports stabilized the shafts, maintaining proper alignment while preserving the necessary belt tension for reliable operation.

Open Figure 22. Short Support Shaft (58 mm)	Open Figure 23. Long Support Shaft (120 mm)

Open Figure 22. Short Support Shaft (58 mm)	Open Figure 23. Long Support Shaft (120 mm)

Improving the Grasping Performance

With the transmission system in place, we shifted our focus back to improving grasping performance. Although the motor was not yet integrated, the implemented gears and timing belts allowed us to effectively simulate how both fingers would respond to a single input.

The first update targeted link 5, the proximal segment of the finger. As noted earlier, links 2 and 3 were making unintended contact with objects, preventing proper closure. By increasing the thickness of link 5, this segment engages the object earlier in the motion, effectively blocking the other links from interfering. This modification is shown in the CAD model below.

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Figure 24. Updated Link 5

Additionally, despite using the smallest torsional spring available (6 mm diameter, 120° angle), the distal segment (link 4) did not rotate as far as expected. To address this, link 4 was redesigned to be larger and thicker, enabling earlier contact with the object and reducing the amount of spring deflection required for motion. This update is also shown below.

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Figure 25. Updated Link 4

As a result of these changes, the grasping performance of the under-actuated finger improved significantly. The mechanism now achieves the intended motion profile, including clear and distinct two-stage behavior. The fully functional design (without motor or electronics) is shown below.

Open Figure 26. Updated Mechanism with New Links	Open Figure 27. Mechanism Grasping Object (More Information in 19.5 Final Demonstration)

Open Figure 26. Updated Mechanism with New Links	Open Figure 27. Mechanism Grasping Object (More Information in 19.5 Final Demonstration)

Plans for Implementation

With the core mechanism finalized and validated, the next phase focuses on full system implementation. This includes integrating the motor by making minor adjustments to the base and adding a coupler to directly connect the motor shaft to the transmission system. As shown in the CAD models below, the motor is mounted beneath the assembly and positioned to remain accessible from the rear.

Open Figure 28. Updated Base with Cavity for Motor	Open Figure 29. CAD Model with Motor and Coupler In Place

Open Figure 28. Updated Base with Cavity for Motor	Open Figure 29. CAD Model with Motor and Coupler In Place

The next figures show the coupler that was made for the motor and 5 mm shaft. A hole on the side of the coupler allows a set screw to press into the shaft, and the additional holes allow screws to pass through it and into the steel arm of the motor’s input.

Open Figure 30. CAD Model of Motor-Shaft Coupler	Open Figure 31. Image of Coupler Connected to Motor

Open Figure 30. CAD Model of Motor-Shaft Coupler	Open Figure 31. Image of Coupler Connected to Motor

In parallel, the electronics will be incorporated using Arduino-based components and a battery power supply to enable active control of the fingers. A joystick interface will be implemented to control opening and closing through simple programmed inputs. Additional details on the final assembly, fabrication process, and electronics and coding implementation are provided in Section 19.4 (Implementation).

Final Bill of Materials

These updates collectively improved both the functionality and robustness of the compliant finger mechanism. The geometric design updates increased contact surface area and enabled a more reliable two-stage grasp, while the addition of the transmission system allowed synchronized motion of both fingers from a single input. Structural improvements, including added supports and optimized tensioning, further increased stability and consistency during operation. The final Bill of Materials shown below reflects these design updates, incorporating the additional shafts, bearings, gears, and timing belts.

Note that, italicized items in the BOM have not yet been implemented in this Section 19.2 Design Process. Instead, their integration is discussed further in Section 19.4 Implementation, where the electronics and full system assembly are detailed.

Overall, the total project cost remains under $50, due to the use of readily available components and materials provided through class resources. We kept our costs low by minimizing the need for additional purchases while still achieving a functional and effective design.