Introduction

Since resistance weld monitors come in many configurations, answering a few questions will help you decide the kind of monitor your application requires.

Which weld parameters are critical to your process? Since resistance welding machines tend, by nature, to exhibit constant current characteristics, current monitoring alone may not be an adequate indicator of weld quality. Tests show that some welders may produce nearly the same current under tip-to-tip conditions as when the work is present between the tips. Voltage measuring capability helps a monitor accurately determine the weld quality.

Will displacement information play a critical role in determining weld quality? As described in the case of an incorrectly positioned projection weld fastener, measuring displacement may save you money by helping to reduce back charges. Displacement information will also indicate inadequate setdown, which results in a low-strength weld.

By monitoring electrode force - either directly as with a load cell or strain gauge or indirectly as related to airline pressure - will you be able to spot problems before rejects occur? Too much pressure may produce unwanted part deformation. Too little pressure results in expulsion and the inability to form a weld nugget.

Do you need to measure only? For machine setup or when records need not be kept, measurement alone may be adequate. On the other hand, do you need to set high and low tolerances? Tolerances allow you to define accept and reject limits. With these properly set, the monitor instantly alerts the operator to weld problems and identifies reject welds.

Many monitors have built-in electrical interlock relays that can be wired to your welding control to prevent further operation if a reject is detected. You may also be able to divert rejects to a different bin when they are ejected from an automatic welding machine.

Will the monitor work with your process? Most monitors work well with alternating current (AC) spot welders. However, frequency converters, high-frequency direct current (DC), capacitor-discharge (stored energy), and seam welders impose unique requirements on the monitor.

Can one type of monitor handle all the resistance welding processes you use in your plant? For example, seam welds usually exceed the 99-cycle limit imposed by most monitors. You may need a monitor designed to record welds lasting several thousands of cycles.

What will you do with the monitored data? If it is for temporary use only, during machine setup, for example, storing the data is not very important. You should still consider whether or not it would be helpful to print a record of the weld for future reference. Does the monitor print waveforms, numeric data, or both? Does it provide a time and date stamp for future reference?

How does the monitor handle the data? If you routinely weld coated materials, you may want to ignore or "blank" the first few weld cycles during which expulsion may occur. When monitoring an impulse weld, are cool times ignored or averaged in with heat times? Are the current (and voltage) readings in root-mean-square (RMS) or average units?

Since each weld has a distinct pattern, does the monitor look for a single average current for the entire weld, or can it recognize the weld's signature, including slopes or pulsation? Some monitors are not able to distinguish such weld features. Others apply the specified tolerance to each half-cycle of the weld.

Can the monitor learn what a good weld looks like and then store the parameters for future recall as a master weld? If you plan to use the monitor with several welding heads, can the monitor store and recall a different signature for each head? Do you have to be a rocket scientist to use the monitor? Some monitors have almost no setup requirements. Others display English language prompts which give helpful instructions during setup. Still others require a skill level beyond that of the typical user. Next>>