Horizontal-axis wind turbines (HAWT) consist of the tower column, and the nacelle mounted on top, which houses the generator assembly that holds the rotor and connects to the gearbox-generator. Tower heights are approximately twice to triple the blade length and help to balance the material costs of the tower structure against better use of the more expensive moving parts. (Note: If you were to double the height of a wind turbine, it would provide about a 35% improvement in energy efficiency but would increase the material cost more than eight times.) The tower column may mount on a concrete base for land-based installations or on pilings if installed on the ocean floor. Many different types of tower structures exist but most large wind turbines are designed with tubular steel towers, which are manufactured in sections. Tower columns are tapered to handle the excessive loads. This saves material and makes them more aesthetically pleasing. Taller towers — approximately 80 m in height — place the rotor at an elevation that supports stable wind speeds, and therefore produce higher output with capacities of more than 1.5 MW.

Wind tower fabrication starts out with flat plates of steel that are rolled into sections to form a conical shape. The cone section is closed and the joints welded on both the inside and outside. The conical sections are joined end to end into larger sections 15 to 30 m in length with flanges at either end so the assemblies may be shipped over the road and bolted together on the site to form the tapered tower. The welded sections are 100% inspected for quality, weld size, and structural integrity. Many times the welds are shaved smooth through a grinding or milling process to further save weight and eliminate unnecessary stress in the section elements. Based on regional weather conditions, wind tower loads become quite variable due to unexpected forces of nature so every countermeasure to maximize safety is critical. For example, in the Great Plains of the United States installations have to survive winter’s freezing conditions as well as higher sheer winds produced from tornados.

Robotic systems mimic their hard automation counterparts and are adapted to the large positioning structures with booms and transporter manipulators to move the robot’s torch near the work area. See Robot Mounted on Boom Manipulator. Intelligent robotic sensors such as through-arc joint tracking or vision guidance are often required due to the inconsistent shape of the parts and the large weld joint configurations. Robots with multiple axes provide for additional servomotors integrated with the manipulator and turning rolls. The integration of servomotors allows all axes to coordinate together to provide a greater working space for the robot and to turn the parts into the correct position for welding.

Challenges can exist for the control and safety configurations of the robotic systems. Robot software-based dual check safety provides a safe mechanism to limit the robot’s working area to the weld locations, providing even greater flexibility for operators to work alongside intelligent robots completing pre- and postprocess preparations. Dual check safety is the most up-to-date technology and has been employed on hundreds of robots. It is more cost effective and flexible than large fenced-in safety enclosures that limit the robots’ working area. This technology enables the operator and robot to safely coexist in the welding environment.

Tower Design

Designed with three-dimensional computer- aided design (CAD) and simulation programs, tower columns must meet structural requirements. 3-D design programs allow for exporting of the geometric data, utilized for finite element analysis as well as direct robot programming. Once the design criteria for a particular weld is established, the geometry and weld path are downloaded to the robot’s simulation environment (virtual robot controller) where reach, accessibility, and collisions are determined for the particular assembly in the robotic welding system.

Wind tower sections tend to be modular in design and therefore lend themselves to robotic off-line programming. Robot programs and weld paths can be quickly adjusted to another tower section. The robotic off-line programming environment enables the virtual robot controller to develop the perfect work and travel angles. By placing the PC cursor on the weld joint, the virtual robot controller can determine the perfect work and travel angles for the weld and then download the information to the robot. This saves the operator from needing to be 6 m above the shop floor to program the robot. Intelligent robot sensors like through-arc joint tracking can then adapt these perfect robotic weld programs to the imperfect weld joint on the conical section. Joint tracking adapts the weld position based on the current feedback while weaving, achieving appropriate fill based on the variable weld joint geometry.

Building the Doors

Wind turbines require regular maintenance and manned access to the tower column and nacelle all accessed through a door near the base. This poses a design and manufacturing challenge because of the localized stresses in this area. Ideally, the location should have minimal impact on the structure design, but the application of the column design does not warrant this. Placing the door in the lower section eases the maintenance accessibility at the expense of fabrication complexity. The thickest material of the tower is at the base, approaching 180 mm in thickness, and is tapered in shape. Thus far, manual operators have had to cut and weld the door due to the thicker material and the imperfect shape of this rolled section.

The latest robotic solution is through laser scanning of the shape of the conical section profile and building the cutting path for the door’s hole size, shape, and bevel angle. See Automatic Profile Scan Generates Accurate Cutting and Welding Paths. The robot moves the laser scanner along a predescribed path and measures the offset at very minute slices, generating 3-D data of the area for the door. This data virtually projects the door support ring into the tube and makes robot path adjustments based on the actual shape. Optimized robot programs automatically cut the opening, exceeding a manual operator’s skill due to the size and shape of the parts. Therefore, the hole is custom profiled according to the radius of the conical tube at the location to be cut and beveled. In addition, weld sizes are optimized by laser scanning the profile and robotic beveling, taking into account the tube’s profile shape. Matching the door intersection to the main column tube provides consistent root openings, reducing material waste, lower welding cycle time, and improved structural quality.

Steel is selected based on the specific mechanical properties for the application and then laser cut to a minor banana shape that is rolled into a taper to form the conical sections of the tower column. Weld areas are prepped based on the welding process including the bevel angle and root face. The parts are then arranged for the longitudinal weld by setting the root opening and then tack welding. Complete penetration is a requirement; the rings are joined lengthwise from the inside (see Robot Reaching Inside the Tower Section) and out, and then placed on turning rolls to position the components in the optimum location for the robot (see Turning Roll for Part Rotation Optimization). Robots with more than 3 m of reach are typical for these longitudinal welds, and they produce multipass welding with a single setup. After the long joints are welded, the conical sections are placed on turning rolls and the same basic steps mentioned previously for preparation and setup are carried out. The difference is the turning rolls should be set up so that the smaller diameter is placed toward the top of the conical section so that it can spin the column smoothly. Welding multiple smaller cone sections together grows the tower in length up to the allowable size of the available transporter.

Using Submerged Arc Welding

The submerged arc welding (SAW) process allows for greater deposition rates, some as high as 45 kg/h, much more than the typical single and twin GMA welds. The higher deposition rates are attributed to many factors such as the powdered flux cover to shield the weld and improve current transfer. Flux is delivered to the weld joint just ahead of the arc and while some is consumed in the weld process, most of it can be recovered. Other advantages that improve deposition rates are the ability to run on AC/DC welding machines where the polarity and current type (AC or DC) are switchable and can be modulated through variable balance AC current. Twin wires offer an effective improvement and allow for combinations on the leading and trailing arcs. Modern inverter welding power supplies increase welding efficiency due to the electronics. A side benefit is dynamic switching with no requirement to change weld torch leads based on the output desired as the machine is software controlled. Microprocessors monitor the welding process through state-of-the-art DSP control and communicate through networks across an Ethernet port , supporting data collection and reporting as well as sequence control. One feature that stands out is the welding network control, which allows direct control and sequencing through another computer or motion planning device. Robot controllers synchronize the welding machine and offer improved capabilities such as through-arc joint tracking and remote control and data monitor and collection. The new SAW power supplies with these capabilities opens up improved performance and higher throughput with intelligent welding control from the robot.

Submerged arc welding applied to a robot is a relatively new development in the industry and is equally capable for long continuous joints that require high deposition rates. The modern intelligent inverter power supply more readily connects to the robot controller and offers new application with SAW. Applying robotic SAW to the wind tower is a win-win as the robot can manipulate many more degrees of freedom than a typical mechanized transporter, improving the capabilities for many more applications on the tower. The robot has the capability to adapt to the weld location based on the welding current feedback signal providing a sense of direction to lead the arc into the weld joint. Robot controls handle multiple welding torches with ease, such as twin wire, so the operator can simply select the lead wire based on the weld direction and the appropriate through-arc joint tracking sensor function. The robot manipulates the SAW process, welding the door’s curved profile while coordinating the turning rolls. Robotic coordinated motion provides 1G orientation, which is difficult for mechanized hard automation systems

Normal production methods rely on cutting or shaving the weld to save weight and reduce propagation of stress risers. Operating a manual weld shaver is heavy work and with the long welds on the tower column, it becomes a time-consuming process. Robots have utilized machining equipment with specialized force control to manage the bead profile, reducing it to a smooth transition from each side of the weld. Weld sizes such as a 45-mm butt joint are routinely shaved by the robot. These automated weld shavers provide force feedback to the robot control so that appropriate material is removed with each pass. A force of 35 kg is applied to the work while the robotic auxiliary servomotor controls the velocity of the slot-milled cutting tool, cleanly and quickly removing the weld bead convex shape. Robots can remove this type of material at speeds of 10 to 12 mm/s providing continuous performance, making the robotic controlled weld shaver a necessity for wind tower production.

Testing the Welds

Nondestructive examinations such as ultrasonic testing (UT) are normally carried out on all main structure welds of the tower including the longitudinal weld joints as well as the conical sections and mounting rings. See Robotic Automatic Ultrasonic Weld Inspection. Typically, this is a tedious process where the scanning head is moved along each weld by an operator. Robotic automation provides the capability to handle the UT sensor with greater precision and allow it to accurately travel along the welds at greater distances than possible with manual scanning. The interface is simple and the robot’s accurate speeds provide excellent data feedback for the monitoring and validation of each weld, maintaining high quality standards and records for liability.

The large size of the parts places the robot and the welding torch far from the operator’s view. Remote control is available on robots to allow for setup, operation, programming, and monitoring the weld. Remote access through the robot’s teach pendant is achieved through standard PC office tools such as a Web browser, which lowers the cost of monitoring and control. Welding equipment settings and systems functions like the flux hopper control can be set and adjusted from the PC. Optional viewing cameras can be integrated through the remote PC for viewing the actual robot system, closing the loop for the operator.

Worldwide energy demands have been increasing at rates that will require developments of alternative sources in a larger scale. Wind energy appears to be an immediate technology offering lower risks because of the leveraged global installed base and experience. Manufacturing large wind generators utilizes much of the technology that has been developed over the years, including robotic automation. Many factories are already applying robotic automation for tower manufacturing, but some still utilize a manual method for production, and accrue higher production costs. Large volumes of wind towers are required to meet the energy demands of tomorrow and the taller, more efficient sizes are becoming commonplace, so manufacturers will have to adopt robotic automation to be competitive. Improved production volumes and robotic automation will likely lower the overall cost of manufacturing and therefore the kWh for energy produced. Most of the discussions to date have been tailored around developed nations, but if the costs can be lower, then developing countries may be able to take advantage of the clean energy provided by wind generators. Even future applications like wind to hydrogen become more viable when the cost to produce the equipment is reduced, making these storage technologies more practical.

Wind power generation can provide cost-effective energy with short-term payback but robotic automation of wind tower generators can make this an even shorter payback through improved efficiencies. Robots are extremely powerful and flexible and well suited for wind tower manufacturing.

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This article was republished in its entirety in the Welding Journal, August 2009.