By RICHARD MANDEL, senior editor

Automated spot-welding can be optimized with integrated pneumatic controllers

Welding cycle time is critical for high-volume automotive manufacturers. With thousands of sheet-metal spot welds made per shift, shaving seconds off weld-cycle time can mean a huge increase in productivity.

Time savings can be realized through the use of weld-control systems that optimize spot-welding applications. Such a system can control critical dynamic system characteristics by rapidly moving electrodes in their approach to a workpiece, providing a soft, low-impact electrode/workpiece contact, and instantly developing the desired contact pressure required for welding.

Rapid-approach and low-impact
Many auto manufacturers in Europe and America have implemented integrated pneumatic weld-control systems from Parker Hannifin (www.parker.com/pneu) in their automated spot welding lines.

At the core of the system is Parker's DH valve block, which controls the pneumatic cylinder that moves the weld-gun. The block has two solenoid-operated valves — one to control the pre-stroke that brings the weld-gun to weld position at high speed, and the other controlling the slower, low-impact stroke.

Rapid pre-stroke is particularly advantageous with welding guns that have a large opening between electrodes and when longer strokes are required, such as on C-type welding guns with a stroke greater than 2.36 in. (60 mm).

Movement of a piston in a pneumatic cylinder is basically a matter of filling a chamber on one side of the piston with air, while the other side of the piston is exhausting air at the same time. A quick exhaust feature in the Parker DH double-stroke spot welding cylinder allows the cylinder to reach its pre-stroke extension faster, rapidly positioning the electrodes at the weld site while reducing each weld-cycle time by 100 msec or more (depending on cylinder stroke distance). A proximity sensor signals the controller when to switch the system to its reduced-speed, low-impact weld function, at a point with enough clearance to avoid contacting, and possibly damaging, the metal sheets to be welded.

The low-impact function then advances the electrodes with low force and speed (low kinetic energy) until they contact the metal sheets. This soft electrode touch reduces noise and eliminates electrode bounce, which allows spot welding to start immediately after first contact. In conventional welding systems, welding cannot begin until the electrodes come to rest on the sheets. Such waiting, or squeeze time, can be as long as ¹ /2 sec, so its elimination is an immediate benefit to cycle time. The addition of an optional piezo-electric switch provides an electrical feedback signal when 80 percent of desired welding force is reached, to indicate that welding can begin.

The low-impact weld movement is stroke independent, which means that the system works regardless of the position of the weld-gun cylinder. Turning a flow control screw located in the valve block regulates impact force and closing speed.

The low-impact function also reduces electrode wear, resulting in a measured lifetime improvement of 30 percent and a doubled life for the electrode-cap. Another important benefit in robotic applications is the reduction of abrupt, jerking movements transmitted to water hoses and electric cabling, resulting in longer life and less maintenance.

Pressure during welding
Immediately after touch down of the electrodes, the controller raises the contact force to the desired welding pressure, ensuring proper contact between the electrodes and sheets. At this point, spot weld current is delivered to the electrodes. When the current shuts off, clamping pressure is maintained as the water-cooled electrode tips cool and the weld nugget solidifies.

In many applications, two levels of air pressure are required to the valve block. One — weld schedule pressure — drives the low-impact strokes. The other — at line pressure — drives both retract strokes and the weld return. The second, higher pressure ensures enough force is issued for the weld tips to completely separate from the welded sheets before the robot moves to the next weld site. This dual-pressure feature also allows the valve block to close the tips under extremely low pressure for proper tip dressing.

A digital Parker Par 15 series air-regulating valve is suggested by the company for use with the valve-block system. The electro-pneumatic device is capable of dividing a single inlet pressure into any of 15 equally spaced output pressures to control the valve block, by energizing and de-energizing any of 4 solenoids in the valve housing that, in turn, permit different pressures of air to pass through. The high-flow characteristics of the Par 15 unit produce an instantaneous and repeatable response, within 20 msec, that helps minimize weld-cycle time.

In use
At one vehicle manufacturing facility, a robot-mounted Parker weld-control system was used in an application requiring 19 welds. Cycle time was cut from 59 sec to 53 sec, a reduction of nearly 10 percent, while meeting stringent quality requirements. With each car or truck body requiring several thousand spot welds, and with hundreds of robots in a plant each doing multiple spot welds, reducing weld-cycle time by 10 percent has the potential of producing a huge boost in productivity.

The weld-control unit also can be set up to control manual-welding units. The low-impact, low-noise, low-sparking characteristics of the weld-control units are important operator advantages resulting in less stress and fatigue.