A workhorse for over a half-century, the robotic spot welding process continues to benefit from technology and control advances. Especially helpful for joining a variety of structural parts for Tier 1 suppliers in the automotive, agriculture and construction industries, these improvements are bringing greater reliability and accuracy for operational efficiency and weld quality.
The use of mid-frequency inverters for DC (MFDC) welding, as well as higher frequency inverter switching (1,800 Hz to 2,000 Hz), also eases the spot welding process by reducing transformer size and weight for improved spot gun designs. Add in the utilization of servo motors to actuate spot guns integrated with the robot controller, and electrode wear can be minimized for increased uptime. Clamping force can also be made consistent for improved quality, and “squeeze time” can be eliminated for reduced cycle time.
Increased aluminum focus
Today, with the advances in lightweighting and electric vehicles, there is an increased focus on robotic spot welding for aluminum. Because aluminum is one-third the resistance value of steel, it takes about three times more welding current to make the aluminum spot. This requires a transformer that is 200 kVA (or more), while most steel applications can be performed with a 100-kVA transformer. Fortunately, the MFDC power can keep the weight of the transformer to approximately 40 kg.
This higher power may require larger conductors to the transformer. Heavy payload robots (225-plus kg) with hollow wrists have been introduced to allow these heavier conductors to be run to the spot gun. This improves cable management and reduces downtime due to issues with the utility cables and hoses. These include gun power, water lines, air lines, servo motor cables and peripheral signals.
It is also possible to use a large-payload robot to manipulate the part to a pedestal welder. The fixed weld station may offer improved cable and utility routing for reduced wear and interference. Freeing the robot from excessive external dress, this can sometimes simplify the robot programming process for optimal uptime.
Spot welding advancements
As trends for vehicle lightweighting and electric vehicle production progress, manufacturers are learning how important advanced spot welding technology can be for application success. So much so, a mix of hardware and intuitive functionality is optimizing application management.
This is especially seen with servo spot guns that are integrated with a robot controller – where guns can easily be programmed to fulfill unique design requirements with greater mobility and subsequent part accessibility. The precise motion control that servo gun function provides is accomplished through encoders that monitor shaft position. This control enables various key functions as well as the ability to apply specified torque consistently.
Spot gun manufacturers often incorporate actuators that combine the motor windings and ball screw elements into a single unit. These provide more force in a compact, lighter weight drive unit. This is important for aluminum because the clamping forces are nearly 20 percent higher than steel to prevent the formation of cracks in the weld nugget. The actuator may need different encoders for different brands of robots, so it is important that the gun manufacturer knows to which robot/controller the gun will be mounted.
From position-related functions, such as spot gun equalization, tip wear compensation, workpiece thickness detection and coordination of the robot and gun motion, to torque-related functions, such as workpiece search function, controlled clamping pressure, weld sequencing with pressure, pressure compensation function and more, there are many proven processes that facilitate a controlled spot welding process. The following functions have been found to boost productivity and weld quality for the aluminum welding process.
Pressure compensation function: In traditional spot welding, the force applied by the electrodes is constant. The robot control of the servo motor can set and maintain a torque value to achieve a constant pressure during the welding cycle. A simple calibration of the spot gun allows the user to specify a force value in Newtons or kilograms (Kgf), and the robot will set the necessary torque value. The robot can vary the pressure profile to different levels for various times based on its program.
More recently, pressure compensation function, which allows an outside value to change spot gun pressure, has received special attention where the spot welding of aluminum is concerned. Allowing bias to be set for a program value in the pressure file, this function facilitates the adding or subtracting to program pressure, enabling an outside value to effectively change what the gun is doing.
Taking the concept of pressure compensation a step further is a timer function that enables a process coined force forging. Ideal for preventing cracks in the weld nugget, force forging involves applying an additional pressure to the electrodes during the weld sequence.
To accommodate this, a strain gauge is added to spot guns and monitors the force profile along with spot parameter data. This allows the spot gun to sense when the material is hitting a plastic or molten state so the weld timer can trigger the additional “forge force” or clamping pressure required as the nugget forms. This helps to effectively seal the weld at the opportune moment to improve weld integrity and close any voids to prevent cracking.
Pendant oscilloscope function: The weld timer can monitor spot gun pressure with the addition of the strain gauge, but the robot also provides feedback on the gun pressure. Used to monitor gun pressure, the pendant oscilloscope (O-scope) function is a monitoring tool used to track the speed and torque of each robot axis, as well as an external gun axis, on an oscilloscope when needed.
Ideal for also checking the status of spot welding signals, such as weld control signals for start and end of the weld, this function is very helpful in verifying or troubleshooting proper weld timing. A user-friendly interface with a waveform display window and a condition setting panel typically allows several processes to be performed simultaneously.
The robot signals can be exported to a database where monitoring software can analyze the results over time. Monitoring torque values of robot axes can help to detect wear and give advance notice for when maintenance might be required. The monitoring to tip position has also shown promise as a potential indication of poor welding conditions that are generating expulsion.
While these functions are not necessarily exclusive to aluminum welding, they help to tackle tough challenges on the production floor where the fabrication of this lightweight metal is concerned. Overall, implementing robotic automation and being able to leverage peripheral process control tools is key to staying competitive in demanding markets. From reduced cycle times to higher quality welds, manufacturers that continue to find unique ways to leverage advanced technology will be better poised for long-term success.