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Background
This design project seeks to improve the solar cell manufacturing process conducted by the Solar Energy Innovation Lab (SOLEIL) at UCSD. The automation of the process via the introduction of a electromechanical pick-and-place system will improve device consistency, increase throughput, and minimize manual labor. A picture of a completed solar cell is provided in Figure 1.
A viable design solution for this project must consider several constraints.
The entire manufacturing process takes place within an acrylic enclosure measuring 200 cm x 80 cm x 70 cm in x, y, and z directions. As such, the automated system must be designed such that it does not exceed any of these dimensions.
It must also be compatible with the equipment being used in the lab; this includes the spin coater, liquid handler, and hot plates.
The solar cell manufacturing process involves coating the top of glass substrates with solvents. As such, our gripper must be able to grip the glass slides from the sides or from below to avoid damaging the devices. Furthermore, these solvents and the nitrogen-rich environment inside the enclosure require that components be able to withstand continued exposure to chemicals.
Figure 1: A completed solar cell sample
Project Goal
The project goals consist of the design, prototyping, testing, and documentation of a robust electromechanical gripper system that can reliably pick and transfer the glass substrates used in solar cell device manufacturing from station to station. Specifically, the system moves the samples from the spin coater located inside the liquid handler to the hot plates for annealing and later to the storage station for subsequent analysis. The steps are shown below in Figure 2.
Figure 2: Steps involved in the solar cell manufacturing process
Functional Requirements
The electromechanical gripper system must meet several functional requirements, including:
General
Fit inside the acrylic enclosure (200 cm x 80 cm x 70 cm) in x,y, and z
Produce a batch of twenty or more devices without user intervention
Maintain a service life on the scale of years
Be resistant to solvents used in the manufacturing process
Pick and place samples without causing damage
Gripper
Provide a gripping force that holds samples without cracking or breaking
Be able to mount onto the cantilever arm
Be compact and light enough to be supported by the cantilever arm and translation system
Pick glass samples from the sides without smudging the coating
Cantilever Arm
Withstand a downward force up to 2 N where the gripper is mounted without noticeable deflection
Contain cutouts to mount the gripper
Contain holes to mount onto the gantry plate for the z-translation
Translation System
Support the combined weight of the cantilever arm and gripper system
Allow for the gripper to reach each station in the manufacturing process
Include limit switches to stop the motion of the system
Maintain a precision of 0.1 mm to ensure accurate positioning
A top view of the final design solution is provided below in Figure 3. The electromechanical system moves from left to right. The gripper first reaches into the liquid handler to access the spin coater. After collecting a sample from the spin coater, the translation system deposits the sample onto the hot plate. Following the annealing process, the translation system deposits the sample in a storage cell.
Figure 3: Top view of final design solution
The major components of the electromechanical system are shown in the table below. Key features are also highlighted.
Gripper Assembly
Cantilever Arm
Translation System
Storage Unit
Direct-drive gripper using a microservo
Two parallel grippers move along slot in base plate
Springs are attached to maintain grip on samples
Bearings are used to reduce friction as grippers move
along the slots
Prototype made from PLA 3D filament
Allows gripper to reach into liquid handler
Cut-outs and holes to mount onto gripper and z-translation
Attach to cantilever arm and gripper assembly
Allows for linear movement of the gripper
Operated by NEMA 23 motors
Belt driven in x- and y-directions
Screw driven in z-direction
Each unit holds up to twenty samples
Composed of a base tray, heat-distributing material, and a frame to separate samples into cells
Easy to clean and stack
Analysis for Proof of Concept
The undergraduate team carried out several calculations and simulations of the components that comprise the electromechanical system to help ensure that the
system would work as intended during implementation.
Translation System - Beam Deflection
Appreciable deflection the x-translation beams would inhibit the linear motion of the entire translation system. As such, beam deflection analysis for an intermediate
load on a simply supported beam was conducted.
The assumptions included:
Worst case loading of 50N on either beam
Simplifying the cross-section of the beam as a rectangle
Either end of the x-translation was fixed
Maximum deflection would occur at the midlength
The results are provided below in Figure 4. They suggest that deflection along the beams for the x-translation are negligible as they are on the scale of 10-6 m
Figure 4: X-beam deflection results
Cantilever Arm Deflection
Static simulations of the cantilever arm were also carried out to determine if the arm would appreciably deflect or break under expected loading conditions.
The assumptions included:
The back of the arm was fixed against the gantry plate
The cantilever is isotropic
A downward load would be placed where the gripper attaches to the cantilever arm
The maximum load will not exceed 5 N
The results are provided below in Figure 5. They suggest that maximum deflection occurs along the end of the arm with a magnitude of approximate 0.24 mm. The von Mises
stresses are relatively low and result in a factor of safety of approximately 35. This deflection is considered acceptable for the prototype as it should not affect the
gripper's functionality.
Figure 5: Cantilever arm deflection results
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