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Inductive Sensors in Welding: Weld-Immune and PTFE Technology
Inductive Sensors in Demanding Welding Environments – Resilience, Proper Mounting, and Fault Diagnostics
Welding processes are among the most demanding areas in industrial automation. Elevated ambient temperatures, glowing molten metal spatters, extreme welding currents (from 10 to even 60 kA), and intense electromagnetic fields cause standard automation components to degrade instantly.
For Maintenance (UR) departments, every sensor failure on a robotic workstation means downtime that generates high financial losses. To ensure production continuity when welding steel and aluminum, modern lines require specialized solutions with enhanced resistance.
Application Engineer's Opinion:
"In most welding applications, sensor failure does not result directly from the ambient temperature, but from the long-term, destructive impact of microscopic metal spatters and electromagnetic interference. Therefore, when choosing a solution, one must analyze the entire operating environment, not just the declared maximum operating temperature of the device."
Operating Principle of an Inductive Sensor – Physical Detection Scheme
A classic inductive sensor utilizes the phenomenon of electromagnetic induction. Inside the housing, there is a coil with a ferrite core, which is part of an oscillator that generates an alternating electromagnetic field in front of the active face.
When a metallic object enters the zone of operation, the field induces eddy currents within it. The generation of eddy currents in the metal causes attenuation of the oscillator's amplitude, which is detected by the electronic circuit and switches the output state (most commonly a PNP transistor output operating in NO mode – normally open), sending a signal to the PLC controller. The absence of moving mechanical parts theoretically ensures a long lifespan – provided that the external environment does not destroy the sensor's structure itself.
Why Do Standard Sensors Fail in Welding Environments?
Standard proximity sensors have an active face made of plastic (e.g., PBT). In welding shops, they fail due to several specific technical reasons:
- Spatter Burn-In: Hot droplets of molten metal (steel or aluminum) melt into the plastic face. Once solidified, they form a permanent conductive layer that constantly attenuates the field. As a result, the sensor "sees" the spatters as the target part and remains permanently locked in a high state.
- Thermal Microcracking: Rapid temperature fluctuations induce material stresses, leading to cracks in the plastic. Through these fissures, moisture, cooling fluids, or technological oil penetrate the internal electronics, causing a short circuit.
- Magnetic Core Saturation: Welding currents of several tens of kiloamperes generate an external magnetic field that saturates the sensor's ferrite core. The standard component then loses its ability to generate its own flux and stops responding to the presence of the part.
Industrial Application Examples (Case Studies)
Case Study 1: Robotic Workstation on a BIW Line (Steel Welding)
Application Description: A robotic cell (MAG welding method) at a manufacturer of steel frames and automotive seat components. In this process, the ambient temperature in the part nesting zone was a stable 55–70°C, but the problem lay in hundreds of microscopic, glowing spatters. Standard M12 sensors failed on average every 2 months because the melting particles accumulated on the face, generating false steel profile presence signals to the PLC.
Implementation: Professional, PTFE-coated and spatter-immune Weld-Immune sensors from Nexotec were deployed in the part nesting locations.
Result: Thanks to the anti-adhesive properties, welding spatters stopped adhering to the active face. The sensor operated for 12 months without a single failure, and the number of unplanned maintenance interventions dropped by 80%.
Case Study 2: Welding of Aluminum Structures (MIG Method)
Application Description: A robotic welding line for aluminum structural body components (MIG method) at an automotive supplier's plant. Although aluminum spatters cool down faster than steel ones, they tend to form a dense, hard-to-remove coating on nesting sensors. Additionally, high welding currents generated interference, triggering false states on the PLC input cards.
Implementation: PTFE-coated, electromagnetic field-resistant sensors from the Nexotec welding series were applied.
Result: The sensors became completely immune to aluminum spatters and EM interference. The need for manual cleaning of the nesting fixtures after each production shift was eliminated.
Case Study 3: Resistance Spot Welding Station for Sheet Metal
Application Description: A welding nut presence sensor mounted just 5 cm away from the welding spot of an automotive component. Electromagnetic fields induced by a welding current of 40 kA generated severe noise voltages in the signal cables, causing random input errors on the PLC controller card and interrupting the cycle of a 6-axis industrial robot.
Implementation: The component was replaced with integrated, monolithic Full Metal sensors featuring enhanced resistance to EM interference.
Result: The PLC signal interference problem was completely eliminated. The process became 100% repeatable, eliminating downtime and quality rejects.
What Does a Sensor Failure Look Like? Symptoms in Maintenance Practice
Under the specific conditions of a welding shop, failures produce very repetitive symptoms that maintenance engineers look for during machine diagnostics:
- Random Switching and LED Flickering: The input signal on the PLC jumps (values 0-1-0-1), which usually indicates structural microcracks and the penetration of moisture or cooling liquid into the electronics.
- Permanent High State (LED Constantly On): The sensor signals object presence even when the nesting slot is empty – this is the effect of a burnt-in, conductive layer of spatters on the plastic face.
- Sensor Works Correctly Only Outside the Welding Phase: When the robot is not welding, the sensor detects the part perfectly. The moment the arc ignites or the welder electrodes close, the signal instantly disappears. This is an evident symptom of magnetic saturation of the ferrite core in a standard sensor lacking welding filters.
- Operation Only After the Fixture Cools Down: The sensor shuts down after several minutes of intensive line operation and returns to normal only after an extended standstill of the workstation.
Diagnostics: How to Check if the Sensor Is the Problem? Checklist
- ☐ Does the fault appear only during actual welding/spot welding?
- ☐ Does the problem disappear after moving the sensor away from the field source and testing manually with a part?
- ☐ Are large, burnt-in metal spatters visible on the active face?
- ☐ Does the signal cable run directly alongside high-current welding cables?
- ☐ Is the sensor flush-mounted to protect its sides from impact?
- ☐ Is a properly grounded shielded cable used in the installation?
Troubleshooting Table for Common Problems
| Problem | Probable Cause | Suggested Technical Solution |
|---|---|---|
| False signals during transformer operation | Strong EM (electromagnetic) field | Application of Weld-Immune technology |
| Face destruction by glowing particles | Steel or aluminum welding spatters | PTFE (Teflon) coating on the housing and face |
| Housing cracking, mechanical damage | Part impacts, sheet metal friction during nesting | Monolithic steel housing (Full Metal) |
| Overheating of internal electronics | Extreme pinpoint temperature | Sensor with remote electronics module (Remote) |
| Too small or unstable detection range | Large mounting distance from the sheet metal path | Long Range sensor versions |
How to Select a Welding Sensor? Decision Process and Criteria at the Manufacturer
Step by Step: Selection Algorithm for the Right Technology
During professional selection, application engineers analyze a set of workstation parameters:
Are large spatters (steel or aluminum) present? → YES → PTFE-coated and spatter-immune execution required.
Are mechanical impacts from sheets present? → YES → Monolithic Full Metal housing required.
Are strong EM interferences present (e.g., spot welding)? → YES → Electromagnetic field-resistant technology (Weld-Immune) required.
Does the temperature at the measurement point exceed 100°C? → YES → Necessity of using a version with Remote electronics.
Most Common Mistakes When Choosing a Welding Sensor:
- Choosing a regular M12 sensor: Using universal MRO catalog sensors due to a lower purchase price ends in quick failure.
- Lack of EMC resistance: Ignoring specifications regarding electromagnetic compatibility near spot welders, which causes interference in signals sent to the PLC.
- Too small nominal range (Sn): Not taking into account that safe mounting requires moving the sensor away from the direct arc ignition zone.
- Routing cables in a single bundle: Laying sensor signal cables together with welding cables without applying separation or appropriate shielding.
Rules for Correct Sensor Installation in a Welding Shop
Even a technically advanced sensor will suffer premature destruction if improperly mounted. The following guidelines must be strictly observed:
- Distance from the electrode: The sensor should be moved as far away from the source of the direct arc/electrode as its rated operating zone allows. Mechanical shields are used if necessary.
- Cable Routing: Signal cables must not run parallel to robot welding cables. If they must cross, they should do so at a right angle to avoid inducing noise and interference in the PLC.
- Shielding: In zones with a high level of EM interference, it is necessary to use shielded cables, with the shield grounded on one side (usually on the control cabinet side).
- Distance and Mounting Adjustment: In areas exposed to lateral impacts, flush mounting (sensor embedded level with the mounting plate surface) is safer. A torque wrench must be used during mounting so as not to exceed the permissible tightening torque of the body.
Sensor Technology Comparison – How to Choose the Right Protection?
The choice of the appropriate sensor execution depends on which destructive factor dominates at a given welding workstation. There is no single universal sensor for everything, which is why in engineering practice three main protective technologies are used, which can be found, among others, in the product offering of solutions such as Full Metal sensors or a dedicated welding series.
| Cell Threat Specificity | Standard Version (MRO) | Weld-Immune Technology | PTFE (Teflon) Coating | Full Metal Execution |
|---|---|---|---|---|
| Strong EM field (welding) | ❌ None (sensor loses signal) | ✅ Maximum protection | ⚠️ Medium (depends on body) | ⚠️ Basic |
| Hot spatters (Steel / Aluminum) | ❌ None (burns into plastic) | ⚠️ Basic | ✅ Maximum protection | ✅ Very high (steel) |
| Mechanical plate impacts | ❌ Low (face cracking) | ⚠️ Medium | ⚠️ Medium | ✅ Maximum protection |
| Coolants and oils | ⚠️ Medium (risk of leakage) | ✅ High (IP67) | ✅ High (IP67) | ✅ Maximum (IP69K) |
Modern design approaches allow for combining these features. If the application requires it, optimal choices become advanced electromagnetic field-resistant sensors that are simultaneously PTFE-coated (completely immune to spatters) or feature a reinforced steel monolithic housing of the całometalowe (Full Metal) type, allowing the measurement point to be protected against several threats simultaneously.
Overview of Structural Variations Used in Welding
The modern automation market has moved away from a rigid division into simple product catalogs. Advanced protective technologies are integrated directly into standardized threaded housings, such as the popular M8 or M12 series. The choice depends on physical space limitations and required detection distances.
In places with extremely limited space, such as miniature pneumatic grippers and clamps, a compact long-range M8 Sn=4mm flush sensor works ideally. It ensures precise positioning while maintaining small dimensions and protection against mechanical shearing.
On the other hand, the most universal standard in body-in-white assembly lines and steel and aluminum subassemblies remains the long-range Sn=8mm M12 sensor, combining an optimal distance with solid mechanical resistance.
All these execution variants are designed with direct integration into industrial robots operating in the difficult conditions of welding cells in mind. They work in grippers, welding positioners, clamps, and indexers, guaranteeing reliable verification of component presence before starting the sheet joining process. All basic build types feature professional solutions, such as universal inductive sensors, which can be easily adapted to standard mounting sockets.
Which Parameters are Truly Important? Expert Specification:
- Switching Frequency:
