Measuring Displacement: Absolute vs Reference

In resistance welding, “displacement” is the term used to define the distance that parts compress or collapse during the welding process, and monitoring this displacement is critical for ensuring weld quality.

There are two methods for measuring displacement – reference and absolute – however, the definition and usage of these terms varies between different companies. In this article, we follow the terminology outlined using PECO equipment and in doing so help support our customers using these products.

Many manufacturers default to the absolute method because of a widespread lack of information on how to interpret displacement values. But we believe what we call the reference method might yield better results for some applications, especially those where the component needs to be welded to a specific desired thickness such as tang welds, fork welds, and projection welding. The following is an overview of the two displacement methods and considerations for selecting the right one for a particular application.

Resistance Welding Event Dynamics

In order to understand displacement, it is important to understand the fundamentals of the resistance welding process. Fig. 1 provides an overview of the timing of electrode force and weld energy firing.

The electrodes start in the open position. An actuation mechanism (mechanical, pneumatic, or servo motor) moves the electrodes onto the part and, once in contact, applies force. Initially, the force “rings” – think of a mallet coming in contact with a bell – but as force continues to be applied (SQUEEZE), the ringing dampens and reaches the desired force setting. Then the current flows through the parts (WELD) stopping at the preset weld time. The weld head continues to apply force during the current flow and for a time thereafter (HOLD). Then, at a set time after the weld has occurred, the electrodes retract and return to the home position.

Understanding the dynamics of the basic welding schedule will help facilitate understanding of the methods to measure displacement. SQUEEZE, WELD, and HOLD time are essential concepts for determining when to measure.

Why Measure Displacement?

Measuring and controlling displacement in the resistance welding process is critical in order to ensure reliable results. By measuring the amount of collapse during the weld, it is possible to achieve a consistent weld quality that can overcome part-to-part variations. This is especially useful when welding two wires or parts with projections.

Displacement can also be used to detect part presence or lack thereof, which, in turn, can prevent firing the weld energy and incurring scrap of partial parts.

Displacement Method Basics

So, where does displacement come into play? There are three key location definitions which need to be understood before we discuss displacement. Fig. 2 provides a high-level overview of displacement measurement.

“Zero” or “Reference” point- as outlined above, the electrodes are moved under force to a defined position and a zero reference point is set. For opposed electrodes, the reference point is most commonly considered to be the electrode-to-electrode interface (Fig. 2a), but sometimes it can be set including part stack-up and would be equivalent to initial part thickness. For parallel gap electrodes or series welding, the reference point may be set to either of the two electrodes with reference to tooling or top surface of the part prior to weld.

Initial Part Thickness

The initial part thickness is the displacement measurement taken under force at the end of SQUEEZE time and before current flow is initiated. This value is most commonly greater than zero and is a measurement of the thickness of the part stack-up (Fig. 2b). For opposed electrodes, it is the distance from the top of the part to the referenced lower electrode face.

As noted above, some people may use this as the reference point as it is determined just before the weld.

Final Part Thickness

For final part thickness (Fig. 2c), the measurement is taken after current flow has stopped at the end of HOLD time. The displacement measurement can be either the actual distance the electrodes have moved (absolute) or the distance from the top of the part to the lower electrode (reference) during weld and hold time.

 Weld-to-Displacement

One other special term to know when talking about displacement is “weld-to-displacement.” Often, it is desirable to achieve a certain displacement because it indicates a good process result: the system is set up to continue to apply current and force until a defined displacement is reached. This is referred to as “weld-to-displacement.” In the next section, we will compare the absolute and reference displacement methods and the effect on “weld- to-displacement” features for each.

Absolute vs Reference Displacement Measurement Method

Fig. 3 illustrates an example of the difference between the absolute method and reference methods. In the graphic, the weld to displacement value is set to 100 microns (μm) for both methods. In the absolute measurement method, the electrodes will move only 100 μm before the energy shuts off. In the reference method, however, the electrodes will move to 100 μm above the zero or reference point

before energy shuts off. In the reference method, the final displacement is displayed as the position in reference to the lower electrode; to find the absolute distance the electrodes moved during welding, you can simply subtract the final displacement value from the initial displacement value.

Looking Closer at the Absolute Method

Fig. 4 shows displacement using the absolute method illustrating the start and end of the absolute displacement measurement. Beginning on the left, the graphic shows the electrodes at rest, in the start position. The active electrode moves down to apply force to the parts before welding. (SQUEEZE). The initial part thickness measurement (A) is taken at the end of squeeze time, if the part stack-up is within tolerance the welder passes current causing the parts to compress. The electrodes follow the compression maintaining constant force. The actual absolute distance the electrodes move during welding is referred to as absolute displacement.

At first glance, Fig. 5 looks identical to Fig. 4. However there are some key differences. Fig. 5 shows weld-to-displacement using the absolute method, including the start of displacement measurement (A), the cutoff value (D), and the final resting point (E).  It shows the electrodes at the start position at rest, the active electrode moving to the parts, and application of force. Initial thickness measurement is taken at the end of squeeze time. Current is then passed through the parts and will remain on until the “weld to” value is met. This “weld to” value is entered as a programmed value by the user and is the absolute distance the electrodes move during welding. Once this absolute value is met the current will shut off and the parts and electrodes will come to rest at a value slightly greater than the set cutoff value due to heat remaining in the electrodes and parts just after the current is shut off.

A Closer Look at the Reference Method

In the reference method, shown in Fig. 6, the active electrode meets the part under force and the initial thickness is measured at the end of squeeze time. Current is passed, the parts compress, and then, when the electrodes are at rest, the final thickness measurement is taken. Unlike the absolute method where the final displacement value is the actual distance the electrodes have moved, reference method final displacement is taken in reference to the zero point or, in the case of Fig. 6, the lower electrode.

Fig. 7 shows the weld–to-displacement results for the reference method, the illustration shows the electrodes at the start position at rest. The active electrode moves to the parts, applies force, and the initial thickness measurement is taken at the end of squeeze time (B-C). Current is then passed through the parts and will remain on until the “weld to” value (B’-C) is met. This “weld to” value is entered as a programmed value by the user and is in reference to the zero point, the electrodes in this case. Once the cutoff value (B’-C) is met the current will shut off and the parts and electrodes will come to rest at a value (E-C) slightly smaller than the set cutoff value due to heat remaining in the electrodes and parts just after the current is shut off. The final displacement (E-C) reading is taken and the end of hold time.

For both absolute and reference measurement methods, it should be noted that the user must enter enough weld time into the weld control to ensure the “weld to” displacement value is met.

 Which One is Right for Your Application?

Operators looking to utilize displacement monitoring as an additional means of process control should carefully consider which method would work best for their application.

The absolute measurement method works well in most cases where the part thickness and presentation are consistent; it allows the user to correlate displacement with other important weld parameters such as current, voltage and time. This includes applications like battery tab welding, sheet metal welding, or cross wire welding where material tolerances are usually tight.

The reference measurement method, on the other hand, ensures the amount of collapse and that the final distance from the lower electrode, or zero point, is kept in tight tolerance.

Manufacturers should consider using the reference method for tang welds, fork welds, rail projection welds, and cross wire welds. Why? Because the tolerance of the part dimensions in these kinds of applications can vary and thus the height of the part stack up will vary. In other words, the fork may be too open or too closed.  The tang angle may vary. Or, since it’s a stamping process, the rail projection may not be the same height.  This variation creates a problem for the absolute method as the final part height will vary with the initial stack up of parts.

Displacement Measuring Equipment

Now that you know the definitions of the absolute and reference measurement methods, and when to use displacement measurements, what equipment do you need and how do you apply this to your equipment set-up?

First you need a displacement sensor that can provide a signal of its position to a reading device. Popular brands for displacement sensors in resistance welding are Heidenhain and Ono Sokki. Generally, for micro-welding, a stroke of 30 mm is required and a resolution in microns.

The displacement sensor is typically mounted to the weld head in a secure location.

The signal from these sensors can be displayed on different weld checkers and monitors. Software on these monitors allows the user to set limits for initial and final displacement and offer part detection and weld-to-displacement.

Weld Process Monitors for Displacement

Basic monitors that measure just current and voltage have been around for many years and are effective when monitoring welding of resistive materials (e.g. Nickel, Stainless Steel, NiChrome), because part misplacement or lack of force will cause a noticeable spike in the voltage which will readily be detected by a monitor. However, when monitoring welds of conductive materials (e.g.Copper) the change in voltage is so minimal that it may not be picked up by the weld monitor unless specific limits have been set. In both cases, adding displacement improves knowledge of what is occurring during the welding process.

Displacement monitoring makes it possible to identify and correlate good welds from poor welds with known displacement. Displacement monitoring works with most “soft” metals like copper, nickel, and stainless steel but is less effective with hard materials like carbon steel, molybdenum, and tungsten. Fig. 11 shows displacement monitoring system screens that help capture the weld process and identify potential welding issues. Just like the other weld parameters, upper and lower limits can be set for displacement and alert an operator or send a signal through the PLC that the weld was good or out of limits

A variety of advanced displacement monitoring systems are available in the marketplace that enables operators to monitor displacement, including initial and final displacement, displacement cutoff (weld to) for up to 3 pulses, part detection, and allows the user to choose between the absolute or reference methods.

A desktop resistance weld monitor outputs peak and RMS data for every weld and is ideal for manufacturing settings where it will be dedicated to a single workcell. However, often times, it is preferred to have one weld checker that can be used to spot check multiple workcells. It is then important that it is hand-held and easily transportable.

For advanced analysis of weld properties and product development, advanced data analysis monitors can review and manipulate waveforms and analyze large amounts of data, making it the perfect monitor for R&D and product development.

 Summary

AMADA WELD TECH utilizes two methods to measure displacement during a resistance welding process: absolute and reference. Selection of the better method for a particular process depends on the parts and the tolerance in presentation of these parts during the welding process. Monitoring the displacement values – in addition to other process parameters – provides another check on the welding process and can be used to further ensure manufacturing success.  There are several weld checkers and monitors that are available on the market that provide these capabilities. Using these devices regularly can improve throughput and manufacturing quality, while providing the ability to trace product manufacturing.