Basic Operating Principle of an Inductive Proximity Sensor

Did you ever wonder how an Inductive Proximity Sensor is able to detect the presence of a metallic target?  While the underlying electrical engineering is sophisticated, the basic principle of operation is not too hard to understand.

At the heart of an Inductive Proximity Sensor (“prox” “sensor” or “prox sensor” for short) is an electronic oscillator consisting of an inductive coil made of numerous turns of very fine copper wire, a capacitor for storing electrical charge, and an energy source to provide electrical excitation. The size of the inductive coil and the capacitor are matched to produce a self-sustaining sine wave oscillation at a fixed frequency.  The coil and the capacitor act like two electrical springs with a weight hung between them, constantly pushing electrons back and forth between each other.  Electrical energy is fed into the circuit to initiate and sustain the oscillation.  Without sustaining energy, the oscillation would collapse due to the small power losses from the electrical resistance of the thin copper wire in the coil and other parasitic losses.

 Inductive proximity sensor cutaway with annotation Read more of this post

Are you taking a chance with low-cost sensors?

Don’t take chances with low-cost sensors. Some companies have been severely scaling back on sensor quality to meet price targets. Be on the lookout for these telltale signs of poorly engineered or manufactured sensors:

  • Varying sensing distance: to drive out costs, some manufacturers are eliminating the final distance calibration step. This means the actual sensing range can vary up to 30% from the specifications.
  • Temperature compensation: affecting mostly inductive proximity sensors, this is one of the more technical areas of sensor design. Special circuits and design methods eliminate the large operating distance variation seen with some low-cost sensors.
  • Adequate electrical protection: there are numerous methods to protect a sensor’s output circuit, not all are created equal. Many do not take into account overvoltage, overcurrent, short-circuit, reverse supply polarity, mis-wiring, and energy backfeed from the load.
  • EMI resistance: influence from electro-magnetic interference (EMI) noise can cause false triggers leading to machine malfunctions. It takes years of experience and testing to make sensors that will operate reliably near motors and drives.

Fortunately, there is an answer to these potential problems: the Global line of sensors offered by a reliable sensor manufacturer with decades of proven experience. These products are not built down to price, but instead are built up to the highest standards in the industry. By utilizing highly automated product lines and funneling usage to fewer part numbers with broader application potential, the Global line is one of the most cost-effective sensors programs available today, and without sacrificing any quality or reliability. Bottom line? You don’t need to sacrifice quality or reliability in order to meet your cost budget. For more information, see the entire Global line here.

global

Meeting the Challenges of Precision Sensing: High Acceleration Machinery

Challenge: High Acceleration Machine Movement

Fundamental application problem: Anything mounted to the moving mechanism must be low mass

  • Added mass reduces acceleration capability of a given motor & drive system
  • Added mass increases motor and drive size requirements to meet acceleration specs, driving costs higher
  • Larger motors increase energy consumption, which makes the machine less competitive in the market
  • Any space taken up by sensors reduces space available for tooling and work-in-process
  • Conventional prox sensors and brackets are much too large and heavy to address these requirements

Solution: Incredibly miniaturized, self-contained inductive proximity sensors

  • Tiny size = inherently low mass
  • Correspondingly tiny mounting brackets = inherently low mass
  • Totally self-contained electronics = zero space taken up by separate amplifier
  • Miniaturization of sensors allows no-compromise installation in compact tooling
  • Additional tooling sensors enhance the level of high-end machine automation/control that can be achieved

Stay tuned to this space for more precision sensing challenges and solutions. Miniaturized sensors are also available in photoelectric, capacitive, magnetic cylinder, ultrasonic, and magnetic encoder. Click here to see the whole mini family.

Sensors Reduce Downtime in Welding Applications

Sensors in welding cells are subject to failure because, although they are intended to be non-contact devices, they tend to be located directly in the middle of the welding process. Conditions such as damage by direct mechanical impact, erosion by hot welding slag, false tripping by accumulated slag, and high intermittent heat cause conventional sensors to fail at an excessive rate. In a previous blog post we discussed our three-step protection process.

bunkerproxProperly bunkering and protecting sensors will prolong their service life and reduce downtime. Ideally, this strategy is implemented during the design and construction of the weld cell by the equipment builder in response to buyer demands for increased process reliability. But what about currently existing production equipment that originally was built to a lower standard that is plagued with issues? It can be very difficult for a plant to find the time and personnel resources to go back and address problematic applications with better sensor mounting solutions. The job of retrofitting an entire weld cell with proper sensor protection can take two experienced people up to eight hours or more.

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4mm Is The New 2mm

When I began working in the industrial sensor industry back in the mid-’90s, the standard sensing range for a size M12 flush-mount inductive proximity sensor was 2.0 mm.  Advances in sensor technology later brought about so-called “Extended Range” proxes, with M12 flush-mount proxes rated at 4.0 mm nominal sensing distance.  Another popular term for these extended range proxes is “2X”, as in “Two Times” the standard sensing range.

Today, competition and time has brought down the prices of most 2X sensors close to or equal to the prices for standard “1X” sensors.  As a result, there’s a large and growing industry trend to just go ahead and standardize on 2X sensors.  And why not?  If the cost is essentially the same, dollar for dollar a sensor with more range is a more versatile sensor.  The availability of longer range in smaller housings is driving migration from M18 bodies to M12 bodies, and from M12 housings to M08 housings.  This saves money and helps reduce the size and weight of modern machinery.

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Inductive Sensor Correction Factor

Some applications have multiple materials that have to be detected. When specifying a standard inductive proximity sensor the first question asked is, “what is the target material that will need to be detected.” In my previous post, I indicated that the ideal target for an inductive sensor is a target made from mild steel. This is correct; however, an inductive sensor can also detect non-ferrous materials but a correction factor has to be determined into the rated operating distance of your selected sensor. For example, if you select a sensor that has 4mm of operating distance (Rated Operating Distance), and the target is aluminum, we would multiply a correction factor of 0.30-0.45 to get the new rated operating distance of your sensor (1.2mm -1.8mm). Due to the aluminum’s non-ferrous material we can no longer achieve the 4mm rated operating distance in proximity to the aluminum target.

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Analog Inductive Sensors

In his post, When Do You Specify An Inductive Sensor?, Shawn Day (Market Manager, Inductive Sensors) discusses selection criteria and application for inductive proximity sensors.  In that article, Shawn focuses on what are sometimes referred to as discrete sensors – sensors that detect the presence of a metal target, and then turn on (or turn off).  As Shawn points out, there are many, many applications for this type of discrete sensing.

But what if just indicating the presence or absence of a part is not enough?  What if you need to know not only if a part is in a particular position or not, but rather you need to know exactly where the part is at any given point along its entire range of travel?  That’s where analog, or continuous, inductive position sensors come into play.

Analog inductive sensors employ basically the same technology as discrete proximity sensors.  That is, they use inductive coils to generate eddy currents that respond to a metal target.  But, unlike discrete sensors, analog inductive sensors provide a continuously variable output, not just an on/off change of state.

Tubular Analog Inductive Sensors

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When Do You Specify An Inductive Sensor?

Many times when you look at an application you ask yourself, what is the best type of   sensor for my application? Let’s assume that we have an application that calls for a metal target. In this case an Inductive Sensor is going to be our first choice if the operating range is relatively short (Typically less than two inches). Inductive sensors are great for applications that require a rugged sensor that can withstand vibration.

Main Applications for Inductive Sensors

  • Machine Position Verification…
    • Are the machine components in the proper position?
  • Part Position Verification…
    • Is the part itself in the correct location?
  • Part Feature Verification…
    • Did a process happen that should have?

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Why would anyone pay more for an M18 Inductive Proximity Welding Sensor?

Answer: Because it has the extreme potential to save a lot of money.   The general mentality these days, with regards to inductive proximity sensing, has been, “Lowest price wins the business”.   Some manufacturers and industrial consumers alike have been accused of treating these devices as true commodities.  Some salespeople have also caved in over the years with regards to price pressures in exchange for the big win.  We’re all guilty to a degree, for leaving money on the table and hastening price degradation for this category of automation device over the years!

Maybe a little of this is justified.  As electronic device manufacturing volume increases, prices for sub-components used to make these sensing devices decrease while manufacturing methodologies become more streamlined.  The result is that cost comes out, prices drop and the game becomes more globally competitive.   But with regards to application specific, hostile sensing applications, there must be a paradigm shift otherwise consumption can become gargantuan, both for material and for labor costs in the real world of factory automation. Using “generic” non-application-specific sensors in rotten environments, like welding for parts presence or Poke-Yoke applications, creates a problem.  “Generic” sensors fail with regularity, change out becomes a massive maintenance issue, machine down time becomes costly and even bad parts can potentially be made (a really bad problem….audits and everything associated with shipping bad parts must obviously be avoided as much as possible).

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You can be doing MORE with Your Sensors!

Recently Hank Hogan published an article in Control Design titled “Sensor, Diagnose Thyself.”  (To be honest, I really wanted to steal his title for my blog entry.)   I think Hank did a great job dissecting the key benefits of smart sensors and the amazing things you can do with them.  Utilizing the technology IO-Link (that we have discussed in many past Blog Entries), sensors can communicate more with the controller and provide more data than ever before.

Some of the key points that I really thought are useful to maintenance and engineers at end-user facilities or machine builders:

  • Being able to detect and notify about pending failures; for example a photoeye’s lens is dirty and needs to be cleaned.
  • A failed sensor needs to be swapped out quickly; IO-Link allows for the smart sensors settings to be cloned and the swap to be executed super fast.
  • Configure a sensor before installation; program with your laptop: sample rate, response time, measurement settings, on/off switch points, anything!
  • One platform can be used for many sensor types;  this gives familiarity to a single interface while using multiple sensor types and technologies.
  • In the future sensors in a wireless cloud would self-heal;  this is an amazing concept and if we can figure out the price for radios and batteries to make it cost-effective, I think this could be a game changer someday.

But all that being said, it really comes down to the total cost of ownership doing it the standard sensor way versus the smart sensor way.  I think you will pay more upfront in capital but down the line there will be less cost in maintenance and downtime.

I have a great line of IO-Link products and smart sensor devices, please check them out!

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