The United States has set 2015 as a goal to reach grid parity (that point where solar electricity is equal to grid electricity). No matter your thoughts on regulatory involvement, it is clear there will be a resurgence in investment, development, and innovation within the PV manufacturing community throughout the world, and it will largely be driven by technology. Finding the most effective tools and processes to gain more productivity and decrease costs within a set capital plan is paramount.

Cartesian robots with flexibility in configuration allow custom robotic systems to be built in complex workspaces.

While the significance of robot automation in the manufacturing of solar cells is obvious, which robot types and kinematics fit each unique process may not be as obvious. Which solar manufacturing areas offer the greatest return opportunities for robotic automation? Which robot type is best for a given solar application task and how does vision fit in? This article should act as a primer targeting these issues, and discusses how the solar industry can best maximize factory throughput, drive down costs, and improve efficiencies with robotic automation.

Automation’s Impact

Robots in the photovoltaic manufacturing process are important due to their ability to significantly reduce costs while continuing to increase their attractiveness compared to manual labor. Richard Swanson, CTO of SunPower Corporation, a leading manufacturer of solar technology, framed automation’s impact by discussing the economies of PV manufacturing in terms of labor. He explained that to produce one gigawatt of solar power, it requires 250 to 500 laborers to produce poly silicone, 250 to 500 laborers to process ingots, 3,000 to 6,000 people to manufacture the cells, 1,500 to 3,000 for the panel lamination and associated applications, and 2,500 to 5,000 for the solar system integration. In total, that’s 8,000 to 16,000 laborers required to produce 1 gigawatt of photovoltaic capacity. Therefore, to produce 500 gigawatts of solar power per year, that equates to roughly 4 million people. With more automation, inclusive of appropriately applied robotics, the solar industry can cut that labor to 1 million people, realizing a 75% savings in direct labor costs alone. Given this magnitude, it is critical that robots receive ample consideration in line design.

Selecting the Right Kinematic Solution

SCARA tabletop robots offer high speed and repeatability within a cylindrical work envelope, making them suited for handling delicate solar cells in small workspaces.

A handful of considerations will provide good direction in selecting the correct robot. First and foremost, what is the payload requirement for the robot? Frequently, people only consider the products that are being handled. However, it is important to also consider the tooling solution or end-of-arm tool (EOAT).

Evaluating the motion requirements is also critical — not only the simple motion of picking and placing, but also what interferences exist among the robot, its linkages, and other items that may be in dynamic motion within the cell.

Consideration must also be given to how parts are produced and throughput requirements. How repeatable does the robot need to be? It’s also important here to recognize that robot manufacturers tend to speak in terms of repeatability, while engineers and designers tend to look at it from the standpoint of accuracy. A robot’s repeatability outlines the machine’s ability, once taught, to return to that taught position. Accuracy references the ability to input a given location digitally and have the robot move to that point in space “accurately.” This encompasses offsets and other digitally inputted motion parameters, and often varies within a given mechanical unit’s work envelope. A good understanding of a process’s requirements in combination with the capabilities of a given robotic solution requires careful evaluation.

Do your processes require special environmental considerations? Do you require a robot designed to eliminate the generation of particulates that might degrade the product? Or, does the robot need to be protected from process specific elements like slurry ingot processing?

Major Robot Types

Robot kinematics can be divided into four major categories: Cartesian, SCARA, Articulated, and Delta/Parallel.

Articulated robots are an ideal solution for complex work cells, delivering the dexterity of manual labor with the throughput of machine tools.

Cartesian -- The Cartesian kinematic solution is highly configurable as the platform includes everything from a single degree of freedom or unidirectional travel, to numerous axes of motion. Given the simplicity of this kinematic, adjusting strokes or lengths and configuration are relatively easy when compared to this model’s counterparts. Multiple drive trains exist that are optimized to provide high throughput or precise motion as characterized by whether the drive might be a ball screw or a belt-driven mechanism. Platforms exist that accommodate small-part assembly to extremely large part transfer such as overhead cranes that might be observed in a manufacturing facility.

Cartesian solutions have numerous applications within the PV industry. They can be applied to both small and large workspaces. Cartesians are typically called upon to serve applications where the substrate remains in the same plane. This is to say that if you were to pick a product off of a table or a conveyor, it does not need to be flipped or change its configuration other than a rotation in the same plane as the table or conveyor (X-Y plane). An example of a job using a small Cartesian might be dispensing sealing material on the flange of a junction box. The sorting and placement of solar cells in a large rectangle is also an optimal application for a Cartesian solution. Solar cell sorting into multiple stacks in a large work area and processes such as stringing up and layup within a large cubic area where robots are required to reach with good repeatability are optimum applications for Cartesian.

SCARA -- SCARA stands for Selective Compliance Assembly Robot Arm. It offers a cylindrical work envelope and this category of robot typically provides higher speeds for picking, placing, and handling processes when compared to Cartesian and articulated robotic solutions. They also deliver greater repeatability by offering positional capabilities that are superior in many cases to articulated arms. This class of robot is used for lighter payloads in the sub-10-kilogram category for applications such as assembly, packaging, and material handling.

Within solar manufacturing processes, these robots are best suited for high-speed and high-repeatability handling of cells in smaller workspaces. Where the workspace is constrained sufficiently, the SCARA is an excellent selection. For example, junction box handling and assembly of panels are good applications for this robot group. Stringing is a process that, with its increasingly tight tolerances, is unmanageable with manual labor. As wafers migrate to thicknesses of 150 micron and thinner, the propensity for damage is greatest when labor is applied. As wafer thicknesses decrease over time as forecasted, the thermal expansion of the silicon will also become an issue while soldering. So it’s going to become increasingly important to maintain yields in stringing by controlling and automating the soldering operations, even in low-cost labor markets, through the use of mechanisms such as SCARA robots.

Parallel robots with low inertia and high accuracy are capable of delivering a higher wafer throughput without damaging the product. Parallel robots also offer an extended long Zaxis.

Articulated Robots – Articulated robots have a spherical work envelope. These arms offer the greatest level of flexibility due to their articulation and increased numbers of degrees of freedom (DOF). This is the largest segment of robots available on the market, and therefore offers a very wide range of solutions from tabletops to very large 1000-kilogram-plus solutions. Articulated robots are frequently applied to process-intensive applications where they can utilize their full articulation and dexterity such as welding, painting, dispensing, loading, assembly, and material handling.

Articulated robots are applied to a wide variety of solar applications. Examples include handling heavy silicon ingots that are also in an area where the robots might require industrial protection, and handling wafer cassettes where the orientation of the carrier might differ from pick to place utilizing the full dexterity of the robot. Handling glass, subassemblies, and assemblies where the products are introduced to the cell in a different configuration than they are presented to the system again take advantage of an articulated arm’s flexibility. Articulated robots permit the optimum introduction of product into a cell that may be in a vertical orientation to maximize floor space while the assembly process is most efficient in a horizontal orientation. Edge trimming and module assembly where tool change and other process considerations dictate the use of articulated arms is yet another use of this class of robots within the PV manufacturing process.

For high productivity within a small footprint, a SCARA robot with embedded controls exceptionally handles solar wafers with vision-guidance.

Delta/Parallel Robots -- This kinematic solution provides a cylindrical work envelope and is most frequently applied to applications where the product again remains in the same plane from pick to place. The design utilizes a parallelogram and produces three purely translational degrees of freedom, driving the requirement to work within the same plane. Base-mounted motors and low-mass links allow for exceptionally fast accelerations and therefore greater throughput when compared to their peer groups. The robot is an overhead-mounted solution that maximizes its access but also minimizes footprint. These units are designed for high-speed handling of lightweight products, and offer lower maintenance due to the elimination of cable harnesses and cyclical loading.

Parallel robots are deployed into many solar cell processing steps. Again, they offer high-speed transfer of solar cells through manufacturer lines and a multitude of processes. Three examples are diffusion of process equipment, wet benches, and PECVD anti-reflective coating machines. In these applications, the tables and trays have large placement opportunities that could be equally serviced by a Cartesian; however, the parallel robot outperforms the Cartesian from a throughput standpoint.

Robot Comparison Example

Following is a comparison of the four robot categories when considering their use in an anti-reflective coating load/unload process. If we look at a Cartesian robot, it is optimized from a reach standpoint. However, the majority of solutions here would prove too slow and would require in excess of a single-head EOAT. This complication would drive the need for pre-alignment and further complications in pre-conditioning the product, and therefore may prove a Cartesian solution to be considered less flexible.

SCARA robots would give increased speeds and prove more flexible than a Cartesian. However, if we look at a traditional tabletop version, it would limit the workspace and therefore may not be optimal in reaching all points on the load and unload areas of this machine.

Articulated robots are too slow for loading/unloading with single-head EOAT, and a spherical work envelope isn’t ideal for covering pallet/matrix.

Therefore, a delta or parallel style robot might be optimal for a number of reasons. First, the overhead mount is ideal in reducing the footprint of the automation cell. And when we combine the benefits of the delta with vision, it provides a flexible solution that will meet the throughput requirements. As noted below, vision is an enabler not only for parallel linked robots, but also for all categories of robots.

Flexibility with Vision

Vision has become a highly adopted tool to improve the productivity of robot automation in all industries and all facets of placement. Vision systems offer tremendous flexibility for applications that don’t require fixtures or trays for part location. Vision-guidance is a feature that allows the vision system to take a picture and compute a part’s location and orientation, and guide the robot to the part using a computed robot-to-camera transformation obtained through an automated calibration process. It allows tremendous flexibility and cost savings because parts don’t have to be fixtured. Parts can be randomly presented to the robot without pre-orientation or alignment, or put into a tray, which also reduces cost.

These systems frequently incorporate line tracking, which enables the robot to pick these parts from a moving belt, further optimizing the production process. Robot-integrated vision allows inspection to be incorporated into the handling process. This puts the inspection or quality control process in parallel with handling, further reducing the overall cycle time and increasing throughput. Different part geometries only require vision re-training or the selection of a recipe instead of manual changes in fixtures and tooling. This increases the overall lifetime profit of the equipment by virtue of its optimization and improved throughput. Most robot manufacturers offer packages with multiple cameras and tracking solutions for integration into a single cell. This offers tremendous power and flexibility for solar manufacturing.

History has shown that automation has played a significant role in reducing manufacturing costs in many industries, and when the costs associated with higher quality and yields are considered, the benefits of automation offer an even more appealing value proposition.

This article was written by Rush LaSelle, Vice President and General Manager of Mobile Robots at Adept Technology, Pleasanton, CA. For more information, Click Here .


NASA Tech Briefs Magazine

This article first appeared in the February, 2012 issue of NASA Tech Briefs Magazine.

Read more articles from this issue here.

Read more articles from the archives here.