Robotic Resistance Welding: Improving Productivity
by Chris Anderson, Market Segment Manager, Welding
, and Mary Kay Morel, Staff Writer/Editor
Motoman Robotics Posted 06/03/2002
Sometimes a New Solution is Right in Front of You
The same servomotors used to control robot motion are now being applied to spot welding guns with excellent results. Precision servo-control improves quality, reduces cycle times, and extends tip life. This improved gun control has been further enhanced by integrating the spot welding timer inside the robot control. The integrated spot timer offers an improved user interface, lower cost, and significant reduction in weld time.
Spot Welding Process Overview
Resistance Spot Welding (RSW) has the basic parameters of pressure, heat, and time. The spot gun electrodes close and squeeze two or more parts together. High current is passed through the electrodes to create I2R heating which creates the weld nugget. The size of the nugget also depends on how long the current is applied. The squeeze pressure used is critical to keeping the molten weld nugget contained and making a quality weld. The gun must also continue to squeeze the material after the current is removed so the molten nugget can solidify. Achieving a quality weld requires the gun and welding current to be properly sequenced and controlled.
Servo Gun Advantages
Pneumatic or hydraulic cylinders actuate most spot welding guns. The electrodes move the entire range of the cylinder when the gun opens and closes. Clamping force is normally fixed by a pressure regulator, and there is usually no means to provide feedback regarding the actual clamping pressure. The motor-controlled servo gun provides variable electrode openings and programmable regulated pressure. This reduces cycle times and improves weld quality while reducing tip maintenance.
Servomotors can be controlled to regulate position or torque. The robot axes are position controlled (for repeatability). A servo gun switches back and forth between a position control loop and a torque control loop.
Pneumatic guns often have two cylinders; one is used for short open and the other creates a full open space between electrodes. The servo gun (in position control) provides programmable electrode opening anywhere between the full stroke of the gun. The electrode opening can be programmed to move simultaneously with other axes of the robot. Application flexibility cycle time savings are realized by the servo-gun's ability to open the electrodes only a short distance, or a larger amount, to provide the exact clearance needed around tooling or parts.
During the weld, the servomotor switches to torque control and provides a uniform calibrated clamping force. This is easily programmed in the robot control and is expressed as a unit of force. The force can be stepped during an individual weld cycle or varied from weld to weld for different material thickness stack-ups.
Pneumatic guns close at full clamping force, which creates high impact on the tips. The servo gun controls the rate at which the electrodes close and ramps up to the clamping force. This controlled process extends the life of tips and is a major reason auto manufacturers have been using them. The controlled clamp force also improves quality and cosmetics, allowing welds to be made on Class A surfaces. Honda has been changing to servo guns as they upgrade or add new lines.
Where can you get a servo gun? Most major spot gun manufacturers have servo guns in their product lineup. They are typically available in C and X type configurations. Since robot manufacturers need to control the motors, gun manufacturers will adapt motors from various robot companies to their drives. Servo guns can provide clamping forces in the 400-600 kgf range.
Tip wear detection is an added bonus of the servo guns. The robot can store the position of the electrode with new tips in place. During the course of production, the electrodes can be closed and the electrode position compared to new tips. The resulting wear amount can be used as an offset to shift the robot program or to initiate a tip change routine.
Servo guns initially cost up to $5,000-20,000 more than similar pneumatic guns. Unless you have many robots in production, the increased tip life and improved quality alone might not be enough to justify the added cost of servo guns. The full benefit of the improved gun control is realized through reduced cycle times achieved through digitally integrated spot welding controls.
Integrated RSW Software
The classic RSW interface uses separate controls for the robot and the weld timer. Each set of controls is programmed independently and uses simple relay I/O to communicate. Now, interfaces are controlled with digital communication and in some cases, the timer board is plugged into the robot card cage. The result is cycle time reduction (documented at .15 sec/weld) due to elimination of hardware I/O.
Additionally, robot software allows spot weld parameters to be entered directly from the robot teach pendant as opposed to using separate controls. This simplifies programming and reduces the cost of the spot welding controller.
Digital interfacing of RSW controls is done at different levels. Some controls use a standard digital protocol, such as DeviceNet®, to communicate with the robot. This normally means fewer commands are available to program, but it involves less software programming. In some cases, the user can do this programming.
Other interfaces use robot software to mimic the spot welding controller's programming terminal. This provides the same 'look and feel' to programming the spot timer, but it is done from the robot teach pendant.
Another approach is to provide a simplified user interface on the robot pendant and let the robot communicate spot timer language in the background. This is less flexible, but is simplified and restricts the variables that can be changed by floor personnel.
It is important to note that in each of these cases, the spot timer is still in control of the welding process. The robot is used to change parameters or programs in a user interface mode. During welding, it tells the timer board which program to execute, starts the process, and then waits for the timer to indicate that the weld is complete. Spot control manufacturers have proprietary features and sophisticated controls for regulating the power that would be difficult to duplicate in robot software.
In the case of a pneumatic gun, the programmer 'guesses' at the amount of time it will take for the gun to close and reach full clamping pressure. This normally results in conservative times being entered into the spot timer before applying weld current. The integration of servo gun and spot timer means that the robot will 'fire' the weld at the precise moment the gun reaches the exact programmed pressure.
The level of interfacing is partially a plant preference. Many companies already use their own spot welding programs and use PLC interfaces with simplified screens to input parameters. Providing the user interface through the robot pendant reduces the cost of hardware in the spot timer. However, it is the reduction of weld time that will generate the cost savings required to justify the added cost of servo guns.
Summary: Easy Justification for Servo Guns
Resistance spot welding has benefited greatly from recent developments in weld gun and control technologies that are creating higher quality, and consistent spot welds faster and more economically than ever before. These developments have decreased cycle time and added process flexibility while increasing safety, uptime, and productivity. The initial added investment of servo gun technology can be justified by reduced costs and cycle time of the integrated spot timer. For multiple robot applications, the reduced downtime, extended tip life, and improved quality (resulting in the need for fewer safety spots), can easily justify the added investment in servo guns.