The Resistance Spot Welding Advantage
CHRIS ANDERSON (chris.anderson@motoman.com) is associate chief engineer at Yaskawa America Inc., Motoman Robotics Division, Miamisburg, Ohio.
Learn how this technology drives transformational change
A cost-effective process that yields solid welds for long-term performance, resistance spot welding (RSW) remains a top joining method for a variety of structural parts. RSW was one of the first applications leveraged by robotics, and advances in robot hardware and software have enhanced it. With its robust arm designs and intuitive digital interfaces, RSW is widely supported and continues to be a go-to method for a range of thin-sheet metal fabrications. The robotic welding process features a variety of innovations, such as midfrequency (1000 to 2000 Hz) inverters for direct current welding that reduce transformer size and weight for improved spot gun design and servomotors that actuate spot guns for improved energy efficiency and precision over pneumatic guns.
Acquiring Productivity Gains
RSW technology is especially beneficial for automotive advances, such as electric vehicle manufacturing, as it fosters efficient floor space utilization, fast cycle times, and high-quality parts. To achieve these results, workspaces are being integrated with flexible high-performance robots in optimal configurations that are designed to leverage servo-controlled spot guns, weld controls, and other highly capable peripherals. That said, what specifically is being implemented to drive this change?
High-Performance Robots
Quick robot axis speeds and acceleration capabilities are key to reducing air-cut time and achieving faster cycle times. Robot design advances include hollow arm construction and integrated spot harnesses that reduce cable interference for better range of motion and floor space utilization. Moreover, lightweight guns allow lighter-payload robots (80–120 kg [176–265 lb]) to be used with faster speeds and smaller footprints. Typically, heavier-payload robots (150–200+ kg [331–441+ lb]) have longer reaches, with servo control allowing them to accelerate faster with guns that may be less than rated payloads. Whether carrying a spot gun to a part or moving parts to a stationary RSW machine, robots can synchronize servo-controlled spot guns with weld processes. Likewise, robots carrying parts can be used to combine processes such as arc or projection welding.
Servo Spot Guns
To achieve higher forces for RSW, it has become popular to use actuators that combine the motor and ball screw elements into a single unit. Gun manufacturers may include an actuator or a servomotor coupled to a gear box along with other gun design elements to satisfy a customer’s application. Aluminum gun arms and other advances are aiding the design of lighter-weight guns with smaller profiles and higher clamping forces.
Servo Gun Functions
Allowing precise motion control via encoders that monitor shaft position, servomotors can be controlled to apply a specified torque consistently, making them an ideal fit for spot guns and allowing for robot control. Key functions controlled from the robot’s teach pendant include the following:
Position-Related Functions
Gun Equalization: This function uses position control to move the robot’s arm while applying gun pressure and can offset gun arm flexing and induced forces, reducing the potential for part damage.
Tip Wear Compensation: Utilizing an encoder to measure tip wear when the gun is closed, this function automatically compensates the position of gun tips for greater weld consistency, as even a marginal shift in travel distance can impact the quality of the weld. Worn tips are dressed or refinished to remove mushrooming of the tip and ensure weld consistency, or they can be replaced based on tip wear feedback.
Workpiece Thickness Detection: This feature is ideal for preventing errors from missing parts and enables the gun opening to be measured during clamping to confirm the material stack-up is within tolerance.
Torque-Related Functions
Controlled Clamping Pressure: Calibration of the electrode clamping force vs. servomotor torque allows users to specify a desired clamping force, and the robot will drive to the required torque. Robot controls allow different clamping profiles with variations in pressure and time.
Weld Sequencing with Pressure: The robot will initiate the weld control to start the weld when the electrodes are at clamping pressure. This can eliminate the squeeze time associated with pneumatic guns.
Pressure Compensation Function: This function allows an outside value to change spot gun pressure, where a weld control with provisions to output electrode force can set or change the force during the welding cycle. Growing in usage for aluminum spot welding, this process is known as force forging and helps prevent cracks in the weld nugget.
Workpiece Search Function: This function simplifies programming, especially where the part or tooling may obstruct line of sight, and allows the gun to close on the workpieces until it senses contact based on torque feedback. It can also be activated during automatic operation to search for weld locations that may vary due to fluctuation in part batches.
Other Useful Functions
With the robot controlling the servomotor on the gun axis, it can provide additional useful functions:
Parameter Graphing: Using the help of an oscilloscope function on the robot, this function allows for the gun’s motor speed and torque as well as spot timer sequencing to be monitored for process control and troubleshooting.
Servo-Driven Tip Dressers: Tip dressers are a peripheral to help resurface worn spot tips. They recut a small layer of the copper cap using a pneumatic or electric motor to drive the cutter. A servomotor provides an added level of torque and speed control that can be used to power through heavier cuts, program limits to avoid damaging electrodes, and change pressure and speed to generate a polish to smooth the surface.
Digital Interface Weld Controls
Offering feature-rich functionality for optimized programming, weld controls with digital interfaces help manufacturers simplify connections via single-wire Ethernet cables. Controls can program and monitor information with more than 255 weld programs supported for each control. Similarly, multiple weld controls can be networked with multiple robot controllers or multiple guns operating from a single robot controller. Some of the more advanced features in weld controls include the following:
Adaptive Feedback: The weld control generates a high-voltage waveform on the primary side of the transformer to get the desired current on the secondary side at the gun. A timer with this feature uses sensing leads on the electrodes to monitor the output and make adjustments to ensure the proper current output from the transformer.
Programming Software: The spot timer will have some type of software interface to allow the schedules to be programmed with weld commands. Some may have an open protocol that allows programming to be performed from the robot’s teach pendant, eliminating the need to plug in a PC.
Data Monitoring: The weld control supervises the welding process and can log the results of each weld for troubleshooting or quality purposes. This can include electrode force with the addition of a transducer on the gun.
Achieving Feature-Rich Operations
Whether the industry is agriculture, automotive, construction, or something else, these methods and technologies can work together to improve task capabilities for operational change. Decision-makers looking for greater reliability and accuracy for operational efficiency and weld quality should reach out to an experienced robot supplier or integrator to learn more.
Fig 1 — High-performance robots with hollow-arm designs use resistance spot welding technology to maximize floor space while improving cycle times and product quality
