4.1 Project Proposal
Introduction
For our project, we wanted to incorporate some form of human-robot interaction. In order to do this, we decided to create a robot arm mechanism that can play rock paper scissors autonomously with a human. Our motivation for this idea was to make something that could keep younger siblings or children engaged in an interactive activity. We also thought this mechanism could easily be extended to have a broader positive impact than simply being used for the rock paper scissors game. For example, sign language and technology operation are two other valuable use cases of robotic hand movement and recognition. Additionally, this project interests us as it will be challenging to make our robot appear realistic and functional, without making it as mechanically complex as an actual human hand.
Problem Statement and Description
Our problem centers around designing and building an automated entertainment option, and will primarily be defined by the motion profile. This requires the formation of rock, paper, and scissors shapes with a hand- and wrist-like form factor. The scope will also most likely include an up and down motion of the arm and wrist before initiating the rock/paper/scissors hand motion. Each of these motions is individually planar but the total motion is 3D. For example, from a top down view, the rock will be planar, but the wrist motion will be planar from a side view. Therefore, at least two separate motion systems will need to be coordinated to form cohesive and distinct motion. There will be some element of a force profile and investigating the torque transmission and mechanical advantage, as stability of the hand must be maintained throughout any up and down motion of the arm. From a design standpoint, the system should also likely be structurally stable enough to withstand a small force from someone hitting it. As an added scope, velocity can be considered as well to make the motion realistic and not unnerving.
The primary complexity of this system involves connecting multiple motions together for fluid motion with minimal digital control, as the up and down motion of the arm should match with hand motion and changes. Existing robots simulating hand motion typically struggle to execute both fluidity and precision. So, along with this, choosing appropriate link lengths will be essential to ensuring seamless motion with minimal discontinuities.
Though some separate motor actuation will need to occur for the fingers to independently articulate, this can generally not be accomplished with a series of simple joints. The independent articulation of fingers with multiple degrees of freedom and various states requires the need for linked mechanism systems. Overall, joint activation needs to be sequenced correctly and mechanically connected to create a realistic image in a more effective manner than individual control.
Description of Proposed Mechanism
Our proposed mechanism will be inspired by the structure of the human arm, particularly the relationship between the elbow, wrist, and fingers. The elbow will feature a revolute joint anchored to a static base, allowing the forearm to pivot in planar motion. This actuation will simulate the hand pumping motion of the introductory chant rock, paper, scissors… shoot! To drive this motion, a crank will serve as the input, creating a 4-bar closed mechanism that introduces a more complex motion profile to the system.
At the end of the forearm, the wrist will act as a revolute joint, securing the hand linkage and enabling it to move along the same plane as the elbow joint. The hand itself will be designed with a wider yet shorter structure to allow space for finger articulation. Positioned at the outermost edge of the hand linkage, three cylindrical joints will align three finger links in a row, permitting independent yet coordinated movement.
We chose to incorporate three finger linkages that will either be positioned inwards or outwards to correspond to the distinct hand formations of the rock-paper-scissors states. The rock state illustrates all three fingers positioned inwards. The scissors state depicts the top two fingers in the outward direction while the bottom finger remains inwards. The paper state will present all three fingers fully extended outwards.
Proposed Scope of Work
We will build a mechanism capable of actuating hand positions that are easily recognizable as the signs in rock-paper-scissors. First, we will analyze the position of the finger mechanism and arm mechanism to position itself in multiple states. Then we will design the mechanism considering weight, packaging, and aesthetics. We will need to prototype finger linkages to ensure proper tolerance and ROM of finger and arm linkages. Finally, we will build and integrate mechanisms together and with motor electronics In addition to our mechanism, we have group members with a CS interest that can integrate the mechanism with the MediaPipe computer vision library to make the mechanism interactable with wins and losses. Extra steps we can take to further this project is to verify with a velocity analysis of the mechanisms to find the desired actuation speed of the hand, add more joints on the fingers or the arm, or program celebration or defeat hand reactions in response to the game.
Preliminary Design
The below diagrams and preliminary analysis show various sections of our initial design. As shown by the image in “Description of Proposed Mechanism”, there will be a linkage system that controls the up and down motion of the arm, and a separate system that controls the fingers (specifics not shown in above picture).
4-bar Arm Mechanism Analysis
First, we consider the arm mechanism to be a 4-bar crank.
The Greubler and Grashof conditions are both satisfied for our four bar mechanism. The Grashof indicates that this is a Class I kinematic chain, so L2 (the shortest link) can complete a full revolution.
Arm Alternative: Adding Wrist
Below, an alternative to the above 4-bar arm mechanism is presented. This transforms that mechanism into a 6-bar that includes an additional coupler for the wrist. The output is still actuated by the crank (L2). As shown by the attached video, the wrist and forearm couplers move at different angles to create a more realistic motion that still simulates the arm pumping motion of rock paper scissors. The Gruebler analysis below shows that the mechanism has 1 DOF. Grashof is not completed because this is a 6-bar mechanism.
Fingers Analysis
We will not look more in depth at the fingers. The three main fingers (purple) can use a slider crank system that is “backdriven” where L4 is the input and the angle between L2 and horizontal is unknown. This will allow us to drive the L4 in another mechanism to be able to couple multiple fingers together.
The diagram below shows the multiple finger mechanisms coupled together. Because the pointer and middle finger are coupled in every motion (out for both scissors and paper, in for rock) we can couple them together so they don’t move independently. This simplifies the outputs we need to control. We can designate the outputs crudely as position booleans where 0 is retracted and 1 is extended. For rock, all fingers need to be in position 0 (a) and for paper, all fingers need to be in position 1 (b) For scissors, fingers 1 and 2 need to be in position 1 while finger 3 is in position 0 (c). These three outputs can be achieved through a cam following mechanism that can have 3 angles where those position outputs are clearly defined. An optional “thumb” can be included (red).
Fingers Alternative: Mechanical Switching for Fingers
The finger actuation mechanism relies on a spring-loaded system that keeps the fingers in the default "rock" position unless acted upon by a moving platform. This platform, positioned beneath the hand, has three distinct states that determine the final finger configuration. In its neutral state, the platform remains disengaged, allowing all fingers to stay curled inward in a fist. To form the "scissors" gesture, the platform rotates to a position where two prongs selectively push against the outer two fingers, forcing them to extend while the bottom finger remains curled. For the "paper" gesture, the platform moves into its final configuration, where all three prongs make contact with all three fingers simultaneously and extend them outward. Once the platform rotates away, the springs naturally return the fingers to the rock position. This design ensures a simple, passive reset mechanism without the need for additional motors or linkages in the hand itself, relying solely on the platform’s controlled movement to define the hand’s state.
Sources
Human hands are astonishing tools. Here's why robots are struggling to match them
3D Printed Robot Hand Structure Using Four-Bar Linkage Mechanism for Prosthetic Application