Waveforms of current, voltage, power and weld displacement can be displayed together on a single screen and show the values of selectable parameters.

A time history of force and current can be used to optimize the resistance welding process. The left trace indicates firing before force is reached, the right shows timing is optimized.

Mounted displacement sensor on a weld head.

Good Weld(left), Low Force(right)

Low Current(left), Misaligned electrodes(right)

Running blind is an expression that can be applied to resistance welding operations throughout the manufacturing world. Many engineers and operators do not have clear information on what is actually occurring during the weld or a means to verify that the equipment they are using is performing as required. The welding monitoring tools that will help manufacturers to minimize downtime, reduce scrap and provide tracking data for customers,now are available in a range of products.

By providing time based waveform data on current, voltage, resistance, power, force and displacement as well as peak and RMS values external weld monitors are able to provide the necessary data that can be used to understand, optimize, benchmark the weld and verify that equipment is performing.

The electrical data can be used to confirm that the weld resistance change is consistent, and the mechanical data, based on weld displacement, can be used to verify that the expected deformation has occurred. This data can be used to indicate process drift, troubleshoot and, ultimately, to provide data that a process is in control.

There are three primary steps in moving toward a controlled and documented welding process:
Machine set-up and equipment benchmarking.
Process optimization.
And, establishing process limits.

Machine Set-Up
There are two steps to machine setup: initial verification that the equipment is performing as expected, and, when the weld is optimized, to benchmark the equipment.

The equipment has three parts. The control is measured for current output with time, assessing rise time and stability. The secondary circuit is measured for the voltage drop across each connection to ensure that a cable is not worn or a connection is not oxidized. The weld head is checked for force response and basic timing with the weld control.

An example of the way to check equipment setup is given in the graph below. It shows the timing of force (in blue) and current (in yellow).

The first graph shows the current firing before constant force has been reached. Based on this information, the squeeze time was extended to enable the force to reach a constant level before firing, and the success of the adjustment is shown in the second trace.

If a welding process is run with insufficient squeeze time, the force at firing is variable, and produces inconsistent welds. This also will make it impossible to optimize the welding process.

All of the machine setup information then can be referred to for verification or for troubleshooting purposes.

For example: hard copies of power supply waveforms and peak – or RMS – values can be posted by the machine to enable quick reference. If the process begins to require more current, the secondary circuit can be checked at each connection and checked with the original specification. Any deviation from the specs will immediately indicate the faulty connection or degrading cable.

Weld Optimization
With the option to display time-based waveforms of the current, voltage, force, displacement, resistance or power on a single screen, the monitor can be used as a tool to optimize the weld. The timing of events within the weld can be linked to visual observations during welding and that of the finished weld. This correlation of the cause and effect of weld parameters improves understanding and reduces the time to optimize a welding process.

Utilizing Weld Displacement
If current and force are the key input parameters for the weld, then weld displacement is one of the key measurable weld output parameters. The level and signature of the weld collapse provide a clear indication of the heating rate withinthe weld, and whether the correct amount of energy was used to complete the weld.

The displacement is measured with a linear, glass slide encoder that can be mounted on the weld head to provide data on initial part thickness, final displacement and the displacement profile.

Depending upon the weld control I/O features there may be an option to use a pre-determined displacement value to terminate the weld. This means that the weld energy is controlled by a direct physical output from the weld.

There are number of displacement measurements that can be used to monitor the weld.

The initial part thickness measurement determines whether the correct parts and the correct number parts have been loaded. The weld may be aborted as a result of this measurement.

The final thickness indicates the level of weld collapse that can, in some cases, be directly correlated to a weld strength parameter such as pull or peel strength.

A time based displacement profile also is related directly to the heating rate of the parts, and can be used in conjunction with current, weld time and force to dial in the collapse profile. This this is particularly useful for projection welding.

In projection welding the displacement generally needs to occur quickly and smoothly. The displacement profile, current, force and weld time can be tuned accordingly, avoiding problems of insufficient set down and excessive weld expulsion.

Welding to displacement is a function that enables the monitor to terminate the weld control when a preset displacement has been reached. The actual value is determined by experimentation, and is set slightly above the desired final displacement due to the thermal inertia of the weld continuing to collapse after the current has been stopped. The additional displacement after termination is consistent and highly repeatable.

Using this method the input weld energy varies according to a direct weld output factor that encompasses variations in all the other weld factors.

It should be noted that the termination signal from the monitor needs to be wired directly into the weld control and not through a PLC, because instantaneous termination is needed.

Establishing Limits
It is very useful to know when the weld moves outside of the process window after the process has been optimized. This is true for an individual weld or for a slow process drift. In either event the monitors have the capability to provide instant recognition of the change and to provide an out-of-process alert to prevent excessive scrap and enable intervention.

Setting High and Low limits is an iterative process that is initially determined through weld optimization. As the process matures in production, these limits usually require fine-tuning based on electrode wear and on batch to batch part variations.

Data Output
Recording weld data provides an indication that the process is in control. For certain industries, such as automotive, data recording is required by many customers. However, there is no doubt that the requirement to record data is quickly spreading across many other industries. Data output is in two forms; hard copies and electronic.

Hand-held checkers are usually supplied with a printer option, whereas mountable units usually have a printer built in.

With a printer, hard copies of data can be produced, analyzed and shared immediately, and that ability provides immense value to the operator.

Mounted displacement monitors can print a waveform or screen when a process drift, an error or an out-of-limit event occurs. Electronic data collection can be extracted via a communications port and can be sent with the desired time stamped information directly into a spreadsheet. The data also can be interrogated on line and archived to databases.

Along with the raw data screen capture, software for the monitor additionally allows electronic records of each of waveform, peak and RMS values and weld history. Because of the size of the records and speed of downloading, this data mostly is used as reference for the engineers rather than part of the data collection records. Further, network connection of multiple monitors also is possible.

Types of Weld Monitors
The success of any monitor revolves around the ease of use and functionality.

The operation should be straightforward, with easy screen navigation and intuitive data analysis. The presentation of data should be clear and concise.

The display screen should have multiple color functionality to provide easy interpretation, and have a good viewing angle for quick reading and to allow group interactions. The recorded data should be accurate and repeatable.

There are two basic types of monitors: hand-held and portable, machine mountable.

The hand held versions, more known as checkers, primarily are used for spot verification of machines. This may occur prior to welding a batch of parts for a customer or after a shift change.

By comparison, the machine mountable units either are permanently attached or are used for extended machine testing, and typically offer more features such as process limits, I/O and displacement measuring capability.

Hand-held units may be used with a weldthrough sensor. This sensor is comprised of copper jaws that have an internal load cell to allow the current and force to be measured at the same time between the electrodes. This quick and immediate method is ideal for spot verification of machines.

When the electrode configuration does not allow the use of such a sensor, the hand held unit should allow for the connection of a toriodal coil and input from an external load cell. The operational lifetime of the hand held checker should allow it to operate for at least one full shift between charges. Most units have an automatic shut off feature that prevents wasting battery life.

Selecting the type of monitor is specific to the manufacturing environment and number of welders. One fact is the value of external weld monitoring.