Manufacturing battery cells is a complex, demanding process with significant challenges. In part 1 of our series, we highlighted the key challenges in developing a fully automated battery manufacturing line. Now, we look at how DWFritz and Bosch-Rexroth partnered to identify and overcome these challenges and develop a comprehensive system to meet our client’s needs.

Flexibility and Scalability

Early in the design and planning process, we identified the range of different cell sizes that will be produced so we could design a tool to scale from the smallest to the biggest part expected. To accommodate this range, DWFritz developed the DWFritz Flexible Platform (DFP) 5000. This is a standard assembly module with multiple configurability options. The dual-sided process stations offer flexible, configurable conveyance systems to handle products infeed and return lines, as well as quick-change hardware such as pallets, nests, webs, conveyors, or tray stackers. The DFP 5000 creates a base system within one frame that then allows us to focus on the critical assembly components, such as end-effectors and other modules.

As you can see in Figure 1, our tool included two processes and two buffer/cure stations within one 2×3 meter frame. With the two process stations positioned side-by-side, we could leverage the controls, PLC, kinematics, frame, and electrical distribution across multiple stations. To move the material in and out of the tool, the DFP 5000 was configured with the Bosch Rexroth VarioFlow+ conveyance. The VarioFlow+ conveyor’s compact size facilitated efficient material flow through the various assembly processes and allows for traffic management within the dense tool.

DFP 5000 standard module and Rexroth VarioFlow+ conveyance.

Figure 1. DFP 5000 standard module and Rexroth VarioFlow+ conveyance.

The DFP 5000 uses a precision dowel assembly method for quick change hardware, such as a nest, tooling, or end-effector. In this way, assembling the DFP 5000 does not require mechanical realignment, but rather a simple hardware switch, which optimizes the tool’s uptime and further minimizes the footprint.

Packing a lot of motion into a limited space for web-handling was a significant challenge. To solve this problem, we used Rexroth’s Mi family of smart servo motors with onboard servo drives. As seen in Figure 2, the tool daisy chained multiple Mi motors to significantly reduce cabling and the overall control panel footprint. 

Servo motors daisy chained together.

Figure 2. Servo motors daisy chained together.

When solving throughput challenges, speed is king. To meet these demands, Rexroth supplied their TS+ pallet conveyance system. Using servo-driven elevators with precise pallet positioning using clamping and lift-and-location mechanisms, the system facilitated a short path, overhead pallet return system to accelerate the pallet return speed, which minimized the number of pallets required. The tight radius of the VarioFlow+ conveyors allowed for very tight snaking turns of the conveyor throughout the assembly processes using diverters and merging to optimize the material flow through the tool. These two systems working in conjunction resulted in smaller much machine footprint.

All of the servo drives and controls use standard open protocols and interfaces for ease of engineering and integration. The main controller uses the latest generation of Rexroth’s controls platform, ctrlX, which affords IoT connectivity via Message Queuing Telemetry Transport (MQTT) protocols to the Manufacturing Execution System (MES) using OPC UA clients and servers and includes expansion options through customer-specific IoT applications. The new platform has a very configurable controller that is very easy to program and includes cloud-based device management via the ctrlX Device Portal, as well as security updates, bug fixes and new function upgrades to improve the controller’s performance over time.

Precision Inspection of Incoming Materials

Everything starts with defect detection of incoming materials, and DWFritz brings years of experience developing our own vision-based inspection systems, algorithms, precision handling, and lighting techniques to detect defects. With such tight assembly tolerances, we leveraged these technologies to enhance the positioning accuracies of our inspection sub-assemblies, and the systems at large. We used machine vision cameras to locate fiducials or datums on the parts so we could perform corrections based on the pick of the part or the incoming assembly.

In the case of web-handling, for example, maintaining precise positions, velocity, and tension control is critical to handling these brittle materials. We used closed-loop sag controls to monitor the web to ensure the proper payout of material across multiple rolls and materials. Bosch-Rexroth high-speed, multi-axis servo drives and motion controllers ensured roll-to-roll or within-roll, cascaded synchronization. Independent rolls were then synchronized to create a more modular material payout that allowed for any number of rolls to be added or removed from the process. This design methodology minimized reconfiguration or reprogramming of the system.

Each material payout roll used optical sensors to perform on-the-fly calculations, measuring the linear speed of the material and performing sag control to accommodate different roll changes with no adverse performance impact on the system. In conjunction with the vision systems, we used high-speed position latching and capture to tell the machine controller where defects are located. The servo-controlled gearing and camming algorithms enabled high-speed material extraction.

Meeting Precision Tolerances Across the Line

To meet the stringent tolerances across various modules, the project considered many parameters. The importance of fixturing, for example, cannot be overstated as precision handling and fixturing go hand in hand. Fixturing using precision kinematic mounts helped in obtaining micron-level repeatability across various modules. We used precision mounts that referenced back to material datums or were designed into the part or the fixtures to help, for instance, laser welding operations or threading operations.

To account for different product type while minimizing changeover and alignment setup time, we used kinematic or precision mounts on the tooling that could be aligned offline and changed over when switching between products.

Bosch-Rexroth supplied various hardware solutions to meet the requirements of this project, including precision linear modules, which have ±5μm repeatability with very precise motion and an XYZ cartesian system used for pick-and-place. Rexroth assembled several different families of high precision linear modules into one solution to meet the requirements of this project, using servo motors with high resolution encoders at ±50 arc-second accuracy. These single cable, single connection solutions feature power and encoder signals on a single cable, reducing the amount of cabling, cost, and complexity of the system. Additionally, servo drives and controllers used advanced tuning algorithms to minimize vibration and settling time, resulting in faster positioning and higher throughputs.

Other options also were considered and used to improve tolerances, including advanced vision systems to both detect parts and for alignment. Figure 3 shows a vision capture of an edge card connector used to locate the pins so we could pick them with high accuracy. If we can detect where we picked the part, then we can remove the part variation for a better assembly tolerance. And, while performing location and alignment, we can also inspect materials. By detecting a part that is bent or misshapen prior to assembly, we could improve yield, and keep the tool running more smoothly throughout the production process.

Vision alignment of components.

Figure 3. Vision alignment of components.

Finally, we used 2D/3D lasers and machine vision systems to perform quality control, checking final assembly at almost every step of the assembly process. As seen in Figure 4, we measured anode to cathode offsets with a 2D laser and matched them with customer specifications to identify defects and send upstream feedback into the assembly process to further tighten up process parameters.

Measurement of anode to cathode offsets.

Figure 4. Measurement of anode to cathode offsets.

Electrical Interconnections

Electrical interconnects (busbar) that physically connects the various layers of the anode and cathode present significant challenges due to the variety and size of raw materials. To accurately locate the busbar, we again turned to a vision-guided alignment system to locate the busbar and identify where it needed to be on the cell. We then used the same system to locate the exposed electrical interconnections and guide the laser when positioning the welds.

Busbars typically enter the system on a roll and need to be straightened, cut, formed or shaped. We used in-process sensing equipment and quality controls to measure the thickness or width (typically ~50μm-wide soft substrate) and length of the final material. This process needed to be very flexible, accommodating different materials that may vary in thickness, material type, and differing mass while requiring minor adjustments to handle the different busbars.

During welding, we needed to properly position the weld and ensure a good electrical connection. To ensure this outcome, we used mechanisms to clamp multiple layers together for welding to create a zero, or near-zero, gap prior to welding. Leveraging relationships with leading laser vendors and our customer, we outlined specific process parameters and used proven laser settings that produce high quality welds. Finally, in-situ inspection and traceability of the welds confirmed all welds were complete, positioned correctly, and of good quality. The inspection data also was stored and traced back to individual cells via barcodes and incoming material lots to reduce scrap and skip subsequent process steps for non-conforming material.

Throughput and Speed Solutions

To achieve the high speeds and throughputs required by the client, Bosch-Rexroth utilized the XM42 Motion Controller. These high-end motion controllers use CODESYS-based PLC Open Programming, which can be programmed in various languages. A high number of servo motors running on a real-time, deterministic communication bus enables high-speed, synchronized motion. One of the machines featured 54 axis of coordinated motion running from one controller and high-speed motion bus.

To simplify safety and minimize component recovery, integrated network safety modules provide rapid recovery from an operator intervention event (E-Stop) without re-homing of axis or losing positioning, minimizing the impact and potential scrap. Servo tuning is critical to enable high-speed performance with accuracy and minimal settling time. Together with electronic gearing and camming mechanisms, this affords, high-speed punch-out and extraction of web materials.

Key Learnings

The importance of selecting a supplier with a global footprint, the right capabilities, and willingness to collaborate cannot be overstated. Bosch-Rexroth and DWFritz have manufacturing and support facilities all over the globe, and having this extensive footprint becomes increasingly important when installing systems in different countries.

Partners also bring different key capabilities to a project, complimenting each other and filling gaps that might otherwise delay a project or result in substandard outcomes. DWFritz and Bosch Rexroth engineers worked closely together to develop and refine design concepts to improve efficiency and reduce costs on numerous occasions, resulting in more efficient use of modules, reduced system footprint, and lower associated costs.

Collaboration also must extend to your customer. We often talk about Design for Manufacturing (DFX) with customers early in the process, so we can influence subtle process design details, discuss specifications, technologies, and other critical elements that go into designing and manufacturing the final systems.  This is critical to developing a seamless, high-performing tool with high yield and high throughput while minimizing costs and meeting your schedule.

In-situ inspections were critical to meet precision assembly requirements across modules and to ensure quality. Tolerances are getting tighter and tighter as we look to hit GR&R levels of 15% or even lower, which require inspection driven by advanced sensors, machine vision algorithms, and innovative lighting. Our years of experience in precision inspection helps us perform these quality inspections at a high level, ensuring that we catch and segregate defects to maximize product quality.

Capturing this defect data also leverages into Industry 4.0, using that inspection information as feedback upstream in the process to improve incoming materials and assembly tools. All of this improves product quality and reduces costs.

Partnering with a supplier with deep application knowledge along with pertinent technical solutions also proved invaluable. Having Rexroth’s engineers and their wealth of expertise in many key areas available to us made this a great partnership.

Finally, the modular design approach provided a key ability to leverage the same hardware for future products and scale. Perhaps we needed to add more stations, change a process station, or change product sizes? By designing this production line for flexibility, it can now be configured and adapted for product changes without the customer needing to purchase all new hardware, which makes it more scalable and able to keep pace with the market.