The U.S. has set a goal to reach grid parity (the point where solar electricity is equal to grid electricity) by 2015. Yet, other nations claim to have reached it last year. No matter what your thoughts are on regulatory involvement, it’s clear there will be a resurgence in investment, development, and innovation within the PV manufacturing community throughout the world. The resurgence will largely be driven by technology.
Finding the most effective tools and processes to increase productivity and decrease costs within a set capital plan is paramount. While it’s easy to see the significance of robot automation in solar cell manufacturing, choosing the appropriate robot and kinematics for each process is more challenging. For instance, which solar manufacturing areas offer the greatest ROI for robotic automation? Which robot type is best for a given manufactuing task and how does vision fit in? This article is a primer targeting these issues and discussing how the solar industry can best maximize factory throughput, drive down costs, and improve efficiencies with robotic automation.
Robotic automation’s impact
Robots in photovoltaic manufacturing are important because they can significantly reduce costs, while increasing their attractiveness over manual labor. Richard Swanson, CTO of SunPower Corporation (a manufacturer of solar technology) frames automation’s impact in an interesting light by discussing the economies of PV manufacturing in terms of labor. He explains that producing 1 GW of solar power requires 250 to 500 laborers to make poly silicone, 250 to 500 more 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. That’s 8,000 to 16,000 laborers required to produce 1 GW of photovoltaic capacity. Therefore, producing 500 GW of solar power per year equates to roughly 4 million people. With more automation of appropriately applied robotics the solar industry can cut 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 when designing a production line.
Selecting the right kinematics
A handful of considerations offer good direction in selecting a correct robot. First and foremost, what is the payload requirement for the robot? Frequently, people only consider the products being handled. However, it is important to also consider the end of arm tool (EOAT).
Evaluating motion requirements is also critical. One must consider the simple motion of picking and placing, as well as the interferences between the robot, its linkages, and other items that may be in dynamic motion within the cell. Also important is considering how parts are produced and throughput requirements―how repeatable must the robot be? It’s also important to recognize that robot manufacturers tend to speak in terms of repeatability, while engineers and designers tend to view from the standpoint of accuracy. A robot’s repeatability outlines the machine’s ability, once taught, to return to that learned 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. Thus, a good understanding of a process requirements in combination with the capabilities of a given robot requires careful evaluation. For instance, do 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 protection from elements such as slurry ingot processing?
Major robot types
Robot kinematics fall into four major categories: cartesian, SCARA, articulated, and delta/parallel.
Cartesian kinematics are highly configurable, because the platform includes everything from a single degree of freedom or unidirectional travel to numerous axis of motion. Given the simplicity of this kinematic, adjusting strokes or lengths and configuration is relatively easy when compared to this model’s counterparts. Multiple drivetrains can be 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 are available to accommodate small part assemblies to extremely large part transfers such as overhead gantry cranes often found in manufacturing facilities.
Cartesians have numerous applications in the PV industry. For instance, they can be applied to small and large work spaces. Cartesians typically serve applications where the substrate remains in the same plane. This means that if you were to pick a part off a table or conveyor, it need not be flipped nor its configuration changed other than a rotation in the same plane as the table or conveyor (an X-Y plane). An example of a job using a small cartesian is dispensing sealing material on the flange of a junction box. The sorting and placement of solar cells in a large rectangular area is also a suitable application for cartesians. Other appropriate applications include solar cell sorting into multiple stacks in a large work area, and processes such as stringing up and lay up within a large cubic area where robots are required to reach with good repeatability.
SCARA stands for Selective Compliance Assembly Robot Arm. It offers a cylindrical work envelope and typically provides higher speeds than cartesians and articulated robots for picking, placing, and handling processes. SCARA robots also deliver greater repeatability by offering positional capabilities that are superior in many cases than articulated arms. This class of robot is usually used for sub 10-kg payloads, applications such as assembly, packaging, and material handling.
These robots are best suited for high-speed and high repeatability handling of cells in smaller work spaces. For example, junction box handling and panel assembly are good applications for SCARA robots. Stringing is unmanageable with manual labor because of its increasingly tight tolerances. As wafers migrate to thicknesses of 150 micron and thinner, the propensity for damage is greatest with human labor. As wafer thicknesses decrease over time—as forecast—the thermal expansion of the silicon when soldering will also become an issue. Thus, it will be increasingly important to maintain yields in stringing by controlling and automating the soldering operations—even in low-cost labor markets—through use of mechanisms such as SCARA 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. Articulated robots are the largest segment available on the market, and so come in a wide range from tabletops to large 1,000 kg plus solutions. Articulated robots are frequently applied to process-intensive applications where they can use their full articulation and dexterity for welding, painting, dispensing, loading, assembly, and material handling.
Articulated robots are applied to a wide variety of solar applications. One example is in handling heavy silicon ingots in an area where the robots might require industrial protection, or handling wafer cassettes where the orientation of the carrier might differ from pick-to-place using the full dexterity of the robot. Also, an articulated arm’s flexibility is useful in handling glass, sub assemblies, and assemblies where the products are introduced to the cell in a different configuration and presented to the system again. 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. Another use of these robots is within PV manufacturing, in an operation called edge trimming and also in module assembly where tool change and other process considerations dictate the use of articulated arms.
Parallel robots provide a cylindrical work envelope and are most frequently applied to applications where the product again remains in the same plane from pick-to-place. The design uses a parallelogram and produces three purely translational degrees of freedom, which require working within the same plane. Base-mounted motors and low-mass links allow for fast accelerations and greater throughput when compared to their peer groups. Mounting the robot overhead maximizes its access and minimizes its footprint. These units work at high-speed handling of lightweight products and offer lower maintenance due to the elimination of cable harnesses and cyclical loading.
Parallel robots work in many solar-cell operations. They offer high-speed transfer of solar cells through manufacturing lines. Three examples of many tasks include diffusion of process equipment, wet benches, and anti-reflective coating machines. In these applications, tables and trays have large placement opportunities that could be equally serviced by a cartesian. However, the parallel robot out performs the cartesian from a throughput standpoint. For example, the Quattro parallel linked robot from Adept Technology Inc. recently reached 300 cycles per minute, illustrating the capabilities such machines to handle products at high rates.
Where robots work best
PV process steps are broken into four basic groups where high concentrations of robots are deployed. Ingot processing predominantly uses cartesian gantries and large articulated arms due to the requirement for heavier payloads and large workspace optimization. Wafer manufacturing uses a variety of arms depending on volume and process requirements. Cell processing tends to use gantries, SCARAs, and parallel-linked robots. The decision is usually based on reach and repeatability requirements. Module build uses a variety of arms with a high concentration of articulated and cartesian for reach and flexibility. But some specific tasks use SCARA and parallel robots.
Let’s compare the four robot categories by considering their use in an anti-reflective coating load and unload process. If we look at a cartesian robot, we see it’s optimized from a reach standpoint. However, most solutions here would be too slow and require more than a single head EOAT. This complication would drive the need for pre-alignment and further complications in pre-conditioning the product, so a cartesian solution may be considered less flexible.
• Too slow for loading/unloading using a single-head EOAT
• Require cell pre-alignment, because multi-head EOAT is often used
• Less flexible when reconfiguring for different size wafers is required
SCARA robots would provide increased speeds and prove more flexible than a cartesian. However a traditional table-top version limits the workspace and so may not be optimal in reaching all points on the load and unload areas of this machine.
• Faster and more flexible than cartesians when used with vision guidance
• Table-mounts can limit work space
• Multiple robots required to cover pallet/matrix
Articulated robots would be pedestal mounted but may prove too slow in increasing complexity of the installation.
• Too slow for loading/unloading with single-head EOAT
• Have a spherical work envelope that isn’t ideal for covering pallet/matrix
Therefore a delta or parallel style robot might be best for a number of reasons. First, the overhead mount is ideal for reducing the footprint of the automation cell. It can reach all places on Plasma-enhanced chemical vapor deposition (PECVD) pallets. Also, when we combine the benefits of the delta with vision, it gives a flexible solution that meets throughput requirements. As noted below, vision is an enabler not only for parallel linked robots, but it provides the same benefits to all categories of robots.
Delta / parallel robots:
• Overhead mount design is ideal for loading/unloading equipment
• Larger delta robots can cover the width of most PECVD pallets
• Enable good positioning when used with vision guidance
• Excellent flexibility and are quickly reconfigurable
• Design is optimal for handling cells (lightweight) at high speeds
Flexibility with vision
Vision has become a highly adopted tool to improve the productivity of robot automation in all industries and facets of placement. Vision systems offer great flexibility for applications that don’t require fixtures or trays for part location. Vision-guidance is a feature that lets the system take a picture, 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. It allows flexibility and lowers costs because parts don’t have to be fixtured. Parts can be randomly presented for the robot without pre-orientation or alignment, or put into a tray which also reduces cost. These systems frequently incorporate line tracking, which lets the robot pick parts from a moving belt. This further optimizes production. Robot-integrated vision allows incorporating inspection into the handling process. This puts the inspection or quality control 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 into a single cell. This offers tremendous power and flexibility for solar manufacturing.
Robotic solar manufacturing summary
The common goal for solar manufacturers is to drive down the cost per watt. As the solar industry strives for grid parity, manufacturers must be knowledgeable about modern robotics, automation technologies, and the value they contribute to reducing the cost of solar cells.
Automation has played a significant role in reducing manufacturing costs in many manufacturing industries. Considering the costs associated with higher quality and yields, the benefits of automation offer an even more appealing value proposition. While robotics and automation may be viewed by some industries as mature technologies, industry leaders are continuing to develop products and new technologies that are suitable for solar manufacturing operations.
It would be prudent for solar manufacturers to look outside of their industry for the best practices in high-volume manufacturing with automation and robotics to achieve their cost reduction initiatives.
-Rush LaSelle/Director of Sales and Marketing/Adept Technology, Inc. www.adept.com