Technical Highlight Vol.30

Vol.30: A new function for the ARCMAN™ robotic welding system: root gap detection by laser sensor

1.Preface

KOBE STEEL’s ARCMAN™ robotic welding systems are used by many customers in the building construction and construction machinery fields, which fabricate structural steels from middle to heavy plate thickness as shown in Figures 1 and 2. One of the advantages of robotic systems is to increase production efficiency. However, in order to achieve this in welding, it is essential to shorten tact time while maintaining welding quality. A robot simply performs whatever an operator teaches; however, depending on the accuracy of the workpieces and/or grooves, cases may occur in which a robot cannot perform precise welding under a set welding condition. Welding defects may result because large structures that employ plates of middle and heavy thickness can generate distortion and/or assembly errors. As a result, repair is required at the post-process stage, and overall production efficiency drops. During welding, welders keep their eyes on the workpiece shape and/or root gap and adjust their welding accordingly. To conduct high quality welding with a robot, then, it would seem necessary to provide the robot with eyes.

Currently, touch sensing offers the alternative to actually fitting a robot with eyes. It allows a workpiece position to be detected by utilizing the voltage applied to a welding wire that changes once the wire touches the workpiece. However, touch sensing is known to have weaknesses such as accuracy, the inability to function with certain groove shapes, and sensing movements that take more time than planned. In order to overcome these weaknesses, KOBE STEEL has developed a new system based on root-gap detection utilizing the company’s exclusive CB-Type Controller and a laser sensor. In this article, the features and actual usage of a laser sensing function will be discussed.

2.Outline of the laser sensor

2-1. Advantage of the laser sensor

There are three advantages when a laser sensor is applied for the robotic welding system.

① Wide range of joints

As shown in Figure 3, root gap detection has been applied not only to the conventional single-V and single-bevel grooves but also to the flare groove and T joint that could not be covered by the touch sensing method.

② Highly accurate measurement

Because the maximum measurement resolving power of the laser is 0.1mm or less, highly accurate measurement is possible, depending on the condition of measurement and workpiece.

③ Reduction of cycle time

Because the action of irradiating against a groove and measuring the root gap with the linear laser beam needs to occur only one time, the sensing cycle time is shortened. By contrast, the touch sensing method as shown in Figure 4, requires repeated operations for detecting the workpiece and measuring the root gap.

The robot’s operation orbit varies, depending on measuring and workpiece conditions.

2-2. Disadvantages associated with laser sensing

Despite the advantages of applying the laser sensor to the robotic welding system, some problematic issues have held back the adoption of laser sensing systems.

① Decreased operation ratio

Because the laser sensor must be placed close to the welding torch so that the laser sensor can additionally be mounted onto welding systems, the robot’s operation ratio may drop due to the laser sensor’s possible interference with workpieces.

② Necessity of safety measures

The installation and use of laser equipment require particular safety measures in accordance with standards by laser classification, because of the need to protect human bodies against harmful exposure to laser beams.

③ Influence of workpiece condition

As measurement is performed by reflecting the laser beam against a workpiece, the surface condition of the workpiece and/or any imperfections may have a large influence on the results.

2-3. A solution to laser sensing’s disadvantages

① Slim body allows the SFK350 to be used in confined spaces.

Table 1 compares the size of the custom-made SFK350 with that of the conventional laser sensor. It is clearly seen that the SFK350 has been reduced in size by about 50 % and weight by about 10 %; thus, the drop of operation ratio is surely restrained.

Table 1: Custom-made SFK350 laser sensor vs conventional laser sensor

	SFK350	Conventional laser sensor
Cubic volume (cm3)	368	749
Weight(g)	600	670

② Laser classification is less restrictive

According to the Safety of Laser Products (JIS C 6802: 2018), the conventional sensor is classified as Laser Class 3B, which requires isolation of the operating environment; however, the custom-made SFK350 is classified as Laser Class 2M, which does not, thereby allowing the system to be applied more widely.

Table 2 displays an excerpt from Hazards and Preventive Measures against Damages Caused by Laser Beam, provided by the Head of Standards Bureau of the Ministry of Health, Labor and Welfare (Japan) for reference.

Table 2: Action standards by class of laser equipment

	Class 2M	Class 3B
Protective equipment ・Safety glasses ・Working clothes with little exposure of skin	Not required	Required
Peripheral isolation	Not required	Required
Designated laser-controlled area	Not required	Required
Designated laser safety officer	Not required	Required

Because the conventional laser sensor has a strong output laser beam, it is classified as Laser Class 3B, and a number of hazard prevention measures are required. In addition to peripheral isolation, wearing protective equipment, and setting a designated laser-controlled area as well as a laser safety officer are required.

The specifications of SFK350 are listed in Table 3. A laser sensor with seam-finding function for detecting root gaps, it is enclosed in a strong casing that can tolerate any welding environment. Moreover, the algorithm that measures workpieces fabricated from steel plates of middle to heavy thickness steel plates that KOBE STEEL is known for is programmed in every groove configuration.

Table 3: SFK350 specification

	SFK350
Laser classification	Class 2M
Type of measurement	Seam-finding
Dimension (width x height x depth)	63mm x 139mm x 42mm
Depth of field	350mm
Stand off	200mm
Close plane (Field of view)	39mm
Far plane (Field of view)	111mm
Lateral resolution	0.07mm (@350mm)
Depth resolution	0.48mm (@350mm)

③ Reduction of influence on workpiece surface condition

In laser sensing, scanning is conducted to reduce the influence of the surface condition of the workpiece. Adhesions from welding fume or spatter to the surface of the workpiece or the inside of the groove, or even the scars left from processing or assembly, can change the apparent groove shape in a particular spot, resulting in incorrect measurements when one point only is measured by the laser sensor.

2-4. Comparison of laser sensing with touch sensing

Table 4 shows a comparison between laser sensing and touch sensing. Because both sensing methods have advantages and disadvantages, it is necessary to determine first whether the use of the laser sensor is applicable to a particular target workpiece.

Table 4: Comparison of laser sensing with touch sensing

	Laser Sensing	Touch Sensing
Applicable joint	Many	Few
Measurement resolving power	0.1 mm or less	About 0.5 mm
Sensing time	About 1 sec.(scanning time)	About 10 sec.(detection time)
Influence on operation ratio	Yes	No
Influence of surface condition	Influenced by reflection	Influenced by non-conductive parts
Influence from disturbance	Direct sunlight Arc light	No
Accuracy (i.e. difference in sizes and processing conditions between the workpiece and drawing)	Necessary for adding the sizes and measurement of the workpiece based on processing conditions.	The same setting can be applied even if there is some extent of dispersion.
Cost	High	Low

Note: Blue text indicates advantages.

3.System components

The laser sensing system is composed of a laser sensor, the ARCMAN™ robot and CB controller, shown in Figure 7.

When laser sensing is carried out, groove information on the target workpiece is sent from the CB controller to the laser sensor, and then sensing is conducted by means of an algorithm based on the groove information. The CB controller acquires the results measured by the laser sensor such as the distance up to the groove center, the root gap (or gap width) and other characteristics of the target groove. The accumulated information is displayed as laser sensing results on the screen of the teaching pendant as shown in Figure 8.

4.Contribution to higher quality welding

An important contribution to high quality welding is the gap sensing function, which automatically adjusts welding conditions according to the measured results of root gaps that vary in size in the groove. Changes in the root gap must be measured in advance of the welding of a workpiece.

5.Postscript

This report discussed the laser sensor’ s root gap detection function, which allows robotic welding systems to carry out higher quality welding. Specifically, the article examined some of the differences between laser sensing and touch sensing, and their respective features and discussed examples of actual application.

As the basis of developing this function, the following two functions are added:
① Exclusively programmed command for laser sensing
② Screen to display sensing results

These functions provide easier operation of the ARCMAN™ robotic welding systems that have a laser sensor installed.

KOBE STEEL will continue developing such products so that all of our customers will experience complete satisfaction with robot welding.

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