Distance Measurement with Inductive Sensors

When we think about inductive sensors we automatically refer to discrete output offerings that detect the presence of ferrous materials. This can be a production part or an integrated part of the machine to simply determine position. Inductive sensors have been around for a long time, and there will always be a need for them in automated assembly lines, weld cells and stamping presses.

We often come across applications where we need an analog output at short range that needs to detect ferrous materials. This is an ideal application for an analog inductive proximity sensor that can offer an analog voltage or analog current output. This can reliably measure or error proof different product features such as varying shapes and sizes. Analog inductive sensors are pure analog devices that maintain a very good resolution with a high repeat accuracy. Similar to standard inductive sensors, they deal very well with vibration, commonly found in robust applications. Analog inductive proximity sensors are also offered in many form factors from M12-M30 tubular housings, rectangular block style and flat housings. They can also be selected to have flush or non-flush mounting features to accommodate specific operating distances needed in various applications.

Application Examples:

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For more specific information on analog inductive sensors visit www.balluff.com.

Back to the Basics: How Do I Wire a DC 2-wire Sensor?

In one of my previous post we covered “How do I wire my 3-wire sensors“. This topic has had a lot of interest so I thought to myself, this would be a great opportunity to add to that subject and talk about DC 2-wire sensors. Typically in factory automation applications 2 or 3 wire sensors are implemented within the process, and as you know from my prior post a 3 wire sensor has the following 3 wires; a power wire, a ground wire and a switch wire.

A 2-wire sensor of course only has 2 wires including a power wire and ground wire with connection options of Polarized and Non-Polarized. A Polarized option requires the power wire to be connected to the positive (+) side and the ground wire to be connected to the negative side (-) of the power supply. The Non-Polarized versions can be wired just as a Polarized sensor however they also have the ability to be wired with the ground wire (-) to the positive side and the power wire (+) to the negative side of the power supply making this a more versatile option as the sensor can be wired with the wires in a non – specific location within the power supply and controls.

In the wiring diagrams below you will notice the different call outs for the Polarized vs. Non-Polarized offerings.

PolarizedDiagramsnon-polarized diagramsNote: (-) Indication of Non-Polarized wiring.

While 3-wire sensors are a more common option as they offer very low leakage current, 2 wire offerings do have their advantages per application. They can be wired in a sinking (NPN) or sourcing (PNP) configuration depending on the selected load location. Also keep in mind they only have 2 wires simplifying connection processes.

For more information on DC 2- Wire sensors click here.

When is a Weld Field Immune Sensor Needed?

When the topic of welding comes up we know that our application is going to be more challenging for sensor selection. Today’s weld cells typically found in tier 1 and tier 2 automotive plants are known to have hostile environments that the standard sensor cannot withstand and can fail regularly. There are many sensor offerings that are designed for welding including special features like Weld Field Immune Circuitry, High Temperature Weld Spatter Coatings and SteelFace Housings.

For this SENSORTECH topic I would like to review Weld Field Immune (WFI) sensors. Many welding application areas can generate strong magnetic fields. When this magnetic field is present a typical standard sensor cannot tolerate the magnetic field and is subject to intermittent behavior that can cause unnecessary downtime by providing a false signal when there is no target present. WFI sensors have special filtering properties with robust circuitry that will enable them to withstand the influence of strong magnetic fields.

WFIWFI sensors are typically needed at the weld gun side of the welding procedure when MIG welding is performed. This location is subject to Arc Blow that can cause a strong magnetic field at the weld wire tip location. This is the hottest location in the weld cell and typically there is an Inductive Sensor located at the end of this weld tooling.

So as you can see if a WFI sensor is not selected where there is a magnetic field present it can cause multiple cycle time problems and unnecessary downtime. For more information on WFI sensors click here.

Do’s and Don’ts For Applying Inductive Prox Sensors

Inductive proximity sensor face damage
Example of proximity sensor face damage

In my last post (We Don’t Make Axes Out of Bronze Anymore) we discussed the evolution of technologies which brought up the question, can a prox always replace a limit switch?  Not always.  Note that most proxes cannot directly switch large values of current, for example enough to start a motor, operate a large relay, or power up a high-wattage incandescent light.   Being electronic devices, most standard proxes cannot handle very high temperatures, although specialized hi-temp versions are available.

A prox is designed to be a non-contact device.  That is, it should be installed so that the target does not slam into or rub across the sensing face.  If the application is very rough and the spacing difficult to control, a prox with more sensing range should be selected.  Alternately, the prox could be “bunkered” or flush-mounted inside a heavy, protective bracket.  The target can pound on the bunker continuously, but the sensor remains safely out of harm’s way.

If direct contact with a sensor absolutely cannot be avoided, ruggedized metal-faced sensors are available that are specifically designed to handle impacts on the active surface.

Be sure to consider ambient conditions of the operating environment.  High temperature was mentioned earlier, but other harsh conditions such as disruptive electrical welding fields or high-pressure wash-down can be overcome by selecting proxes specially designed to survive and thrive in these environments.

Operationally, another thing to consider is the target material.  Common mild carbon steel is the ideal target for an inductive prox and will yield the longest sensing ranges with standard proxes.  Other metals such as aluminum, brass, copper, and stainless steel have different material properties that reduce the sensing range of a standard prox.  In these cases be sure to select a Factor 1 type proximity sensor, which can sense all metals at the same range.

We Don’t Make Axes Out of Bronze Anymore

Every technology commonly in use today exists for a reason.   Technologies have life cycles: they are invented out of necessity and are often widely used as the best available solution to a given technical problem.  For example, at one time bronze was the best available metallurgy for making long-lasting tools and weapons, and it quickly replaced copper as the material of choice.  But later on, bronze was itself replaced by iron, steel, and ultimately stainless steel.

When it comes to detecting the presence of an object, such as a moving component on a piece of machinery, the dominant technology used to be electro-mechanical limit switches.  Mechanical & electrical wear and tear under heavy industrial use led to unsatisfactory long-term reliability.  What was needed was a way to switch electrical control signal current without mechanical contact with the target – and without arcing & burning across electrical contacts.

Example of an inductive proximity sensor
Example of an inductive proximity sensor

Enter the invention of the all-electronic inductive proximity sensor.  With no moving parts and solid-state transistorized switching capability, the inductive proximity sensor solved the two major drawbacks of industrial limit switches (mechanical & electrical wear) in a single, rugged device.  The inductive proximity sensor – or “prox” for short – detects the presence of metallic targets by interpreting changes in the high-frequency electro-magnetic field emanating from its face or “active surface”.  The metal of the target disrupts the field; the sensor responds by electronically switching its output ON (target present) or OFF (target not present). The level of switched current is typically in the 200mA DC range, which is enough to trigger a PLC input or operate a small relay.

In my next post, I will explain the do’s and don’ts for applying inductive prox sensors.

Visit Automation Tradeshows for Free!

I am experiencing the future of tradeshows; a networking & educational conference without the travel, the expense, and the suit!  I can sit at my desk and make contact with future vendors and customers.  The online database GlobalSpec hosts multiple times per year industry specific virtual tradeshow events.  There are presentations and exhibitors.  A place to sit and drink virtual coffee with your peers and of course the token giveaway raffles.

Today I am working the Balluff booth in the Sensors and Switches Virtual show.  It is a collection of companies and attendees from many different industries.  I really enjoy these events because we can contact quickly with potential customers and potential vendors right from the comfort of our conference room and at a much reduced cost. Here you can see our hard working staff chatting with customers.

Check out the Balluff booth at the  Sensors & Switches Virtual Tradeshow, it will be available to visit for 90 days from today.

Are Limit Switches Obsolete?


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Being the “product guy” for mechanical or limit switches I am often told that I have the obsolete products. Well I am here to say that mechanicals are still around and definitely have their place in automation.

Mechanical switches, at least the ones I deal with, are precision limit switches. How can a mechanical switch be a precise device? These switches use a cam or trip dog and once the switch and cam are secured in the application, the repeatability, with a chisel plunger, can be .002mm – that’s two microns. Applications for these switches include actuators for automatic controls, positioning and end of travel for machine tools, transfer lines, transport equipment, and gantries.

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Save Space with Miniature Rectangular Proximity Sensors

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Historically the most popular selling housing style for an inductive proximity sensor has been the tubular style.  The more popular sizes tend to be M8, M12, M18 and M30.  Smaller tubular sizes of 3 mm, 4 mm, M5, and 6.5 mm are also available and have seen increased sales in the most recent years.  One issue that may affect a tubular sensor’s application is its length.  Most standard models are 50 mm to 65 mm long while some shorter body types may be in the 30mm range.  What if your application requires 1.5 to 3 mm of sensing range, but you only have 10mm of depth to allow for the sensor?  Try looking at a block or rectangular style inductive proximity sensor.

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Inductive Proximity Sensors for Special Applications

Written by: Jeff Himes

Inductive proximity sensors can be used in a variety of applications.   Many special use inductive sensors exist in the market, but you have to be aware these unique models are available.

An overview article addressing this topic was just published in Design World Magazine.  Check out this article to learn more about these unique models:  Special Application Inductive Proximity Sensors

Inductive Sensor Protection and Positioning Made Easy – Use a Prox Mount

Written by: Jeff Himes

“Downtime” is never a good word in any manufacturing facility.  It means something has malfunctioned or broken, parts are not being made, production is reduced, and money is being lost.  In some cases this downtime may be caused by a physically damaged inductive proximity sensor.  If this failure mode is happening on a regular basis to the same location, it may be time to look at the advantages a prox mount can provide.

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