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THE PROJECT
I developed this gripper robot concept from scratch as part of a design challenge for a robotics startup in the bay area. The task was to design a robot to transport a ping pong ball from within beaker 1 (600mm x 150mm) into beaker 2 (150mm x 150mm). The inital robot volume is meant to exist within a 300mm x 300mm x 150mm space.
Included deliverables: Drawings of robot concept, 2x CAD models in submitted files (robot initial state and robot final state with beaker), renderings of robot assembly and mechanism showcase (most renderings done in Blender and some within CAD suite), information on robot operation, analysis of major components, dimensional requirements technical drawing, further considerations, next steps
OVERALL DESIGN CONCEPT
Options
Arm that is actuated upward, dispensing ball out of the backside of the robot
Gripper that rotates beaker in plane (couple issues with this)
Best Option: Upward Actuating Arm — Translates beaker up and over robot
For overall simplification of robot operation, we want to minimize the available degrees of freedom (rotation and translation0 that are included into the robot design. With the Upward Actuating Arm design there are two major degrees of freedom (gripper end effector for the beaker and rotation of the arm). With option 2 (gripper rotating in plane), there is an additional degree of freedom that would be necessary, which is translating the beaker upward in the Z-axis so as to allow proper in-plane rotation of the beaker. For these reasons I chose to design Option 1.
DRAWINGS (Initial concepts, calculations for feasibility, design features)
DESIGN LANGUAGE
Given the nature of the challenge (including robot footprint limits and environmental scale), I made the decision to pursue a design language similar to the company's flagship robot. Looking at the drive mechanism, 2 motor operated wheels towards the front of the robot (weight concentrated around wheels) with a rear pivot made the most sense in terms of beaker translation, especially given the beaker’s 450mm extension over the top of the robot that would increase weight at the rear of the robot when the arm is at full extension.
MAJOR COMPONENTS
BODY PANELS
Injection Molded
All panels shown in CAD and renderings are designed for injection molding with a wall thickness of 2.5mm. The renderings themselves are shown in a silver metallic material for the sake of imagery, but the assembly itself is shelled to a plastic injection molding specification (not including structural ribbing)
Sheet Metal - potential option to stay within the 5kg weight limit
Stamped/Formed sheet metal base with ribbed areas and holes for fasteners
Similar in construction to a sheet metal adapter for vehicle chassis systems
LINEAR ACTUATOR (options)
Servo Electric or Mechanical (servo-actuated rack & pinion)
Pros - Cheaper system to maintain, greater precision, movable, modular
Cons - Higher initial cost than hydraulic
Servo Hydraulic
Pros - Higher force capability, better hardware management (bulk of system can be placed away from main frame)
Lower startup cost than other systems
Cons - Much higher weight
More expensive to maintain than other available systems
System contains many components (greater room for error)
Pneumatic
Overall less capable than hydraulic or electric for the sake of linear motion due to pressure losses and comparative compressibility of air.
TELEOPERATION: Wheels, Motors, and support at robot rear
Front wheels & Motors
Wheels: Rubber outer tread and injection molded hub (similar to Matic flagship bot)
Each wheel is controlled by an individual stepper motor. Choice of stepper motor was made due to space limitations and reasonably assumed minimal RPM requirements
Wheel @ rear
Caster Wheel
The flagship Matic robot seems to use a caster wheel based system
Omni-Directional Wheel - chose this for space savings and modeling ease
Also a good option, although there might be inconsistencies with robot translation as omni-wheels don’t have continuous contact patches with the ground in some cases.
URLs (for external parts used in assembly)
Stepper Motor, 2.1” Overall Length
Omni-directional wheel for Rear support
ARM OPERATION
As shown above, maximum extension of the arm is slightly past vertical. This allows the beaker to have a slight angle downwards relative to the opening when the arm is at max extension.
This will cause the ball to roll out into beaker #2
CALCULATIONS
To the left is an analysis of gripper operation/mechanics based on the Beaker OD of 150mm. The gripper is designed for geared operation with a servo motor but this is not designed into the model at the moment. The movement of each gripper arm is sound however and a next step would be to design the gearing and servo motor interface. This gripper end effector was custom designed due to a very wide maximum opening of ~160mm— a specification not generally available for sale on the market.
PERCEPTION DESIGN
Parts: two cameras at the front of the robot that are used to map the environment and identify the locations of beaker 1 and beaker 2
Operation
Robot navigation is calculated relative to the length of the robot (~303 mm from arm hinge to center point of end effector) and the distance between both beakers.
IF distance between the beakers is less than the nominal length of interest of the robot (303 mm), then calculate and curve/spline path for the robot to traverse
WEIGHT ANALYSIS
The robot’s Center of Gravity (while carrying Beaker 1) must be to the left of the pivot (omni-wheels) in order for the system to not tip over.
Beaker Properties (Glass)
CG: 284.18 mm in Z-direction
Motors (Aluminum 6061 @ 2.700 g/cm^3) : 0.988 lbs x 2 = 1.976 lbs
Main Body (PC/ABS plastic @ 1.100 g/cm^3): 0.813 lbs
Gripper Assembly (PC/ABS plastic @ 1.100 g/cm^3): 0.390 lbs
Arm Assembly (arm + flange x 2) (PC/ABS plastic @ 1.100 g/cm^3): 0.954 lbs
Wheel Assembly (hub + tread) (PC/ABS @ 1.100 g/cm^3 & Rubber @ 0.930 g/cm^3): 0.512 lbs
Linear Actuator (Aluminum 6061 @ 2.700 g/cm^3): 0.1 lbs
Omni-directional wheels (PC/ABS @ 1.100 g/cm^3): 0.078 lbs Total: 2.191 kg
***Total weight doesn’t include hardware, wiring, cooling, fasteners, or adhesives***
Shown to the right: the robot system Center of Gravity (yellow circle) is to the right of the omni-wheels that acts in this case as a pivot. With this configuration, the robot will tip over. Adjustments to the robot geometry must be made so that the pivot is positioned to the right of the system CG. The ‘system’ in this case references the robot having suspended Beaker 1 up and over its rear.
[CG calculation shown with reference to a simulated overall robot weight — slightly less than 5kg]
Adjustments to be made:
Extend omni-directional wheels further to the rear of the robot. Goal is to have the omni-wheel axle behind the arm assembly axle.
Push arm pivot forward and decrease arm length to push CG forward + decreases distance traveled by beaker + decreases force output requirement of linear actuator (less torque load)
Extend the base piece of gripper assembly further out front of the robot in order to allow for an initial state of the gripper that is pulled further back from its initial state shown above.
This allows further optionality of adjustment for the initial state of the robot
RESULT OF CHANGES MADE BASED ON WEIGHT & CG ANALYSIS
Shown to the right: System Center of Gravity is now in front of the omni-wheel pivot point.
Changes made:
Arm pivot moved forward by 35mm, arm length decreased by 35mm
Omni-wheel axle moved rearward by 40mm
Top piece modified to accommodate new arm length and retain design language
Linear actuator shortened by 20mm
To consider:
Impulse force generated by actuation of the arm and deceleration of arm movement prior to settling at its extended position
Widen the position of omni-wheels for lateral stability—separate into individual slots
Updated General Dimensions Drawing
RENDERS OF UPDATED MODEL
CONSIDERATIONS
Effectiveness/Repeatability of Task & Speed of Task
Although hard to gauge at this stage of the design, effectiveness and repeatability is possible due to minimal mechanical operation and redundancy mechanisms (arm actuation for example)
Due to minimal operable degrees of freedom and nature of two-wheeled teleoperation, the robot will be able to quickly go about the task, especially if it is operator controlled.
Rotational limit for Arm assembly
The rear upper quarter of the arm is designed in such a way to act as an additional mechanical limit for the entire system. This flat surface makes contact with the rear of the top cap/piece when fully extended. Mechanical redundancy is achieved with both the linear actuation system and this mechanical limit acting in congruence.
Gripping Force on Beaker
Due to the weight limit of 5kg, a force feedback system might not be ideal/possible. Regardless, force feedback is not particularly necessary for this application. If the environment included multiple beakers of unknown thicknesses, then some feedback system would be necessary to ensure the beaker doesn’t break.
Gripper end effector will be slightly compliant → instruct the gripper to contract a certain distance (based on iterative testing of gripper force on the beaker)
Motion of Linear Actuator
I have some concerns with the movement of the linear actuator when extending the arm. Due to limited space on the inside of the robot, the fore and aft motion of the actuator seems to be irregular. With more time I would have shown this in an animation for further analysis.
End effector servo motor location and aesthetic considerations
The servo motor for the gripper could be placed underneath the gripper assembly base and initially sit within the walls of the robot.
As the servo will naturally stick out of the gripper, it makes sense to design the servo location to be hidden in order to maintain a clean, congruent design aesthetic.
Central cross-section showing rear arm and mechanical limit at rear
POTENTIAL NEXT STEPS
FEA analysis of injection molded outer structure and subsequent design of structural ribbing
Impulse analysis of arm operation relative to robot weight, center of gravity, and lower pivot
Tolerance analysis and assembly designed with fasteners
Analysis of linear actuation motion to test for effectiveness and repeatability
3D Animation of glass beaker with ball inside of it & simulation of a test case including navigation; digital twin for CAE
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