October 14, 2006
Written by Dave Bird
Sensors in automatic, robotic or semi-automatic welding cells are used to indicate that parts are in the appropriate position, that particular features are present, and that components are aligned and nested properly before any welding occurs. When all mechanisms in the cell perform as designed, parts are welded and inspected in post-weld check stations or on the fly with amazing speed, accuracy and quality. When problems occur with any of the system components, miss-welds, machine down time, maintenance headaches and the potential for re-work and sorting can occur.
Sensors are integrated into a welding system for specific reasons, but if aspects like proper mounting, "bunkering," protection from process hostilities such as weld spatter (a.k.a. slag), parts loading impact damage or proper sensor gapping/alignment in the fixture aren't seriously considered, the life of these non-contact devices will be considerably shortened.
The mean time before failure for an inductive proximity sensor is generally in excess of 100,000 hours when used under a manufacturer's stated specifications. However, rarely do we witness anywhere near that kind of life expectancy with sensors in weld cells because of their exposure to robust manufacturing conditions. Consumption rates for misapplied or unprotected sensors can also be significant. It's common to see large end-users spend anywhere from $5,000 US to $60,000 US per month on material replacement costs. Much of this cost is preventable.
Sensors are often damaged because of their exposure to robust manufacturing conditions. There are generally five categories of sensors most commonly integrated into welding cells:
1. Inductive proximity sensors
These non-contact devices are designed to sense metal with the ideal target being mild steel. It's important to note that these devices are almost always application-specific. In a welding 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, it's imperative to avoid flimsy plastic mounting brackets. Instead, the user should encapsulate sensors in metal mounting hardware and only use materials that repel weld spatter. Coated sensors (face and enclosure) as well as coated mounts lengthen maintenance intervals while allowing for mechanical removal of accumulated slag during scheduled maintenance periods.
2. Photoelectric sensors
Diffuse-reflective (with and without background suppression), retro-reflective (used with a dependable target, a reflector) and through-beam types (contains a matching energized emitter/receiver pair) are all found in many welding applications. Regardless of the category, the same mounting/bunkering methodology should be incorporated as with inductive proximity types, but with a twist. Just like a pair of glasses, if the optical lens becomes damaged or occluded, it becomes increasingly difficult to dependably sense a target. It's important to note that photoelectric sensors are application-specific as well. Putting the right photocell in the right place for the right application requires special attention if we're to dependably and repeatedly detect part features in a weld cell. Fiber optics can also be found in weld cells, but their dependable function is dependent on constant cleaning, routine maintenance and alignment. One speck of debris, and the fiber optic’s function is generally rendered useless.
3. High-pressure cylinder clamping sensors
In high-pressure hydraulic or pneumatic clamping cylinders, embedded inductive proximity sensors sense the cushion or "spud" of a cylinder piston to indicate clamped or unclamped position (end of stroke). The sensing face of these high-pressure inductive types (generally rated to 3,000 psi, with European types rated to 7,250 psi) are buried deep in the cylinder and are generally protected from process hostilities. Sensor electronics are contained in a metal head that sits on either end of the cylinder. This category of sensor has WFI built into its electronic circuitry.
4. Power clamp sensors
Newer generations of power clamps from a wide range of manufacturers grasp metal pieces to be joined. Although sensors in these devices are used to detect "clamp" or "unclamped" position, they are not high-pressure rated and are not embedded inside a pneumatic or hydraulic cylinder. 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’re hidden well inside the clamp body.
5. Pneumatic cylinder 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 function due to temperature, board-level degradation, etc., over time). Hall Effect sensors are generally not short circuit-protected or reverse polarity-protected, something to consider in weld cells. There are alternative technologies available, such as magnetoresistive magnetic field sensors. This category of cylinder sensor eliminates many of the undesirable characteristics found in Reed and Hall sensors.
Cable and connector protection
It's also important to note that all of these sensor types are generally hard-wired 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. PVC jacket material on connectors should never be used in a weld environment. PVC burns through quickly or can become extremely brittle in a short period of time. PUR-styles (polyurethane) offer a better degree of knick resistance, flex characteristics and resistance to welding debris, but a new generation of thermoplastic elastomer (TPE) takes the positive aspects of PUR to a higher degree of positive performance. Most weld cell users have learned to further encapsulate their connectors with opaque, medical-grade silicone that resists weld slag and "weld berries," and significantly prevents cable burn through. Newer materials such as woven ceramic fibers are also being investigated.
Improving the weld cell sensor system
Metal mounting brackets for tubular types and adjustable flat-mount brackets with a positive stop protect inductive sensors and photoelectric sensors from hostility, act as a heat sink, resist weld debris and provide a method of rapid change out.
In areas where parts-loading in the cell cause impact damage to sensors, heavy solid aluminum bunkers with rapid change-out capability eliminate damage to tubular style sensors. With cube-style sensors, heavy bunkering resists loading impact.
The tiny ceramic particles suspended in heat-resistant industrial epoxies used in weld sensor coatings provide a thermal barrier, protect sensor faces, and allow for slag removal; PTFE coatings on enclosures resist weld spatter accumulation.
Bunkering photoelectric sensors increases performance, ensures alignment, and protects sensor bodies. New high excess gain photoelectric sensors can sense through dense weld smoke and their metal bodies resist impact.
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.
TPE jacketed connectors with medical-grade silicone protective sleeves resist the hostilities found in weld cells and lengthen connector life.
If you are experiencing the heavy consumption of sensors used in your day-to-day welding process, or you believe maintenance time is out of the ordinary, an audit of each individual sensor in every weld cell location may be warranted. In almost every instance, it’s a possible - even highly probable - that you’ll dramatically increase production, reduce machine down time, reduce material and maintenance costs, and increase profitability by integration of even a few of these recommended weld cell improvement methods.
Dave Bird is the automotive business development manager at Balluff Inc.
Reference: Manufacturing AUTOMATION