Product category:
Automatic and robotic welding systems
News Release from: Balluff UK | Subject: Sensors and arc welding cells
Edited by the Manufacturingtalk Editorial
Team on 20 August 2007
Properly mounted sensors resist arc
welding
Review sensor choice, the way they are located, how they mounted and how they are connected and the result will be higher welding cell efficiency and lower costs, said Balluff UK.
Review sensor choice, the way they are located, how they mounted and how they are connected and the result will be higher welding cell efficiency and lower costs, said Balluff UK One of the most hazardous places to put a sensor is in the working area of an arc welding cell, where sensors have to be properly mounted and protected to avoid sensor failure, said Balluff
This article was originally published on Manufacturingtalk on 24 Oct 2000 at 8.00am (UK)
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Balluff UK said that the rsistance and arc welding cells are one of the toughest places to put a sensor.
Weld spatter, high temperatures, impact damage, and incorrect mounting techniques all contribute to premature sensor failure.
Failing sensors adversely effect cell performance.
Further reading
Sensor resolution down to 20 microns
The BOD series Laser Measurement Sensors utilizes the accuracy of laser light to produce an analogue distance measurement sensor with resolution down to 20 microns.
Miniature inductive proximity sensors get smaller
Miniature inductive proximity sensors with long sensing distance are made now with an even smaller package for smaller automatic handling systems, special assembly machines and grippers.
Compact ultrasonics sensors are easy to operate
Compact ultrasonics sensors with numerical digital display needs the operator only to input parameters and then read measuring values in plain text during the running operation.
When all mechanisms in the cell perform correctly, said Balluff, then welding cells can operate with remarkable speed and accuracy.
They produce consistently high quality parts and products.
* Sensors in welding cells - sensors are used to verify that parts to be joined are in the correct clamped or unclamped position (fit-up) and that components are aligned and seated properly before welding.
Sensors are also often used toy enjoy anywhere near that kind of life expectancy.
Consumption rates for misapplied or unprotected sensors can be significant.
It's common for large end-users of welding cells to spend thousands of punds/month on material replacement costs alone.
Much of this cost is preventable, said Balluff.
** Sensor replacements - many sensor suppliers compound the problem, said Balluff, by not attacking the 'core issues' within the weld cell.
Instead, they increase the supply of replacement sensors, and place spares stocks nearby.
Balluff reckoned that such practice makes it easier for maintenance teams to replace destroyed sensors rather than fix the reasons for their premature failure.
Improving the supply chain process does not improve the core welding process.
It only puts up a 'smoke screen' over the problem, increasing basic costs, and masking the real path to increased welding cell productivity.
There are several categories of sensors commonly integrated into welding cells, with the most common by far being the inductive proximity sensor.
** Inductive proximity sensors encapsulation - originally these sensors were used to predominately sense a mild steel target, mild steel facilitating maximum sensing range, now however, so called multi metal or Factor 1 sensors perform equally well on all metals and are becoming increasingly used.
In a welding cell environment and in the presence of strong electromagnetic fields emitted by a weld gun, it's imperative to use electronically weld field immune (WFI) inductive sensors to prevent false triggering or 'chatter'.
The sensor's circuitry is designed to ignore electromagnetic fields.
Moreover, if the sensor is located in the presence of hot weld debris, flimsy plastics mounting brackets must be avoided.
Instead, the user should encapsulate sensors in metal mounting hardware and only use materials that repel weld spatter.
Heavy duty Teflon/ceramic coated sensors (face and housing) as well as coated mounting brackets lengthen maintenance intervals while allowing for mechanical removal of accumulated slag during scheduled maintenance periods.
** Photoelectric sensors - there are three types used in welding applications and cells, as follows.
* Diffuse-reflective (with and without background suppression).
* Retro-reflective (used with a dependable target, a reflector).
* Through-beam types (comprising emitter/receiver pair).
Regardless of the category, the same mounting and protective methods, should be incorporated as with inductive proximity types, but with a difference.
Just like a pair of glasses, if the optical lens becomes damaged or pitted from weld spatter, it becomes increasingly difficult to dependably sense a target.
Putting the right photocell in the right place for the right application requires special attention if part features in a weld cell are to dependably and repeatedly detected.
Consider also physical protection with application specific brackets, deep seating the sensor into a protective cowl.
Fibre optics can also be found in weld cells, but their function is dependent on constant cleaning, routine maintenance alignment and most importantly, physical damage.
One speck of debris and the fibre optics function is generally rendered useless.
** Pneumatic cylinder jig/fixture work clamping sensors - 'through-the-wall' Reed and Hall Effect sensors are commonly found mounted to rod-style, profile, dovetail, slot or cylindrical styles of pneumatic clamping cylinders in the weld cell.
These are also used to indicate 'clamp' or 'unclamped' position.
Generally, failure rates in this environment with these two technologies are significant.
Damage to lightweight mechanical mounting systems occurs regularly.
Reed switches are generally inexpensive to replace, but these mechanical devices are failure-prone.
Hall Effect sensors are solid-state devices, but generally possess their own set of issues regarding drift (movement away from normal, dependable electronic functions due to temperature, board-level degradation etc over time).
Hall Effect sensors also are generally not short circuit-protected or reverse polarity-protected There are successful alternative technologies available, such as magneto-resistive magnetic field sensors.
This category of cylinder sensor eliminates many of the undesirable characteristics found in Reed and Hall sensors.
** Power clamp sensors - newer generations of power clamps from a wide range of manufacturers grip parts to be joined.
In these devices, sensors are used to detect 'clamp' or 'unclamped' position.
These inductive proximity types (generally a pair that are joined into a common connector) sense mechanically actuated components inside the power clamp to indicate extended or retracted position of the cylinder powering the clamp jaws.
These are generally protected from welding hostilities, as they are hidden well inside the clamp body.
** Improving the weld cell sensor system - coated metal mounting brackets with a positive stop protect inductive sensors and photoelectric sensors from hostility.
The brackets also act as a heat sink, resist weld debris and enable quick replacement.
The positive stop ensures that the sensor is always installed at correct target distance.
* Cubic brackets - the use of heavy solid aluminium cubic brackets, again with positive stop and quick change facility, eliminates damage to tubular style sensors.
With block-style sensors, heavy shielding resists loading impact as operators load parts to the fixtures, jigs or manipulators.
* Cushion clamps - spring return cushion clamps, with up to 15mm of over-travel, can be used to minimise sensor damage in instances where target impact on sensor face is unavoidable.
* Heat-resistant epoxies - the tiny ceramic particles suspended in heat-resistant industrial epoxies used in weld sensor coatings provide a thermal barrier.
The barrier protects sensor faces and allows for slag removal.
PTFE coatings on enclosures resist weld spatter accumulation.
* Shielding - shielding photoelectric sensors increases performance ensures alignment and protects sensors bodies.
New high excess gain photoelectric sensors can sense through dense weld smoke, metal housings resist impact.
* Cylinder position sensors - robust magneto-resistive cylinder position sensors with heavy mounting hardware can be installed on essentially any pneumatic cylinder style regardless of magnet orientation or Gauss strength.
These have the benefit of being available with Weld Field Immune circuitry and a quick replacement connector.
By utilising a number of specifically designed brackets it is also possible to use one common sensor across a variety of differing cylinders.
** Cable and connector protection - it's also important to note that all of these sensors types are generally plug and play connection to M12 DC Micro or M8 Nano-style connectors.
One of the largest problems with sensors in weld cells revolves around the issue of cable /connector burn through.
* Don't use PVC - standard PVC jacket material on connectors should never be used in a weld environment.
PVC burns through quickly and can become extremely brittle in a short period of time.
SJTO rated connectors, with a protective metal braid, have been used successfully for many years and provide a robust solution to weld spatter.
Where exaggerated movement occurs sacrificial strategically positioned, and hence quickly replaceable cables, can be used.
Irradiated PUR styles (polyurethane) offers a better degree of knick resistance and flex characteristics.
There is a 'new generation' of thermoplastic elastomer (TPE), which takes the positive aspects of PUR to a higher degree of positive-performance.
Splitter boxes which marshal sensor signals also contribute to quick-change connection via the double ended moulded connector.
** Get a weld cell audit - if the welding sections are experiencing what is believed to be heavy consumption of sensors and/or excessive maintenance down-time, then it may warrant an audit of each individual sensor in every weld cell.
In almost every instance, said Balluff, it's possible - even highly probable - that the welding sections will experience the following.
* Dramatically increase production.
* Reduce machine down time.
* Reduce material and maintenance costs.
* Increase profitability by integration of even a few of these recommended weld cell improvement methods.
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