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25 September 2001

Inductive Proximity Sensor Strategy

by Guerrino Suffi

Here's a guide to the technology and its specification and implementation.

When the detecting distance¹ is 1 inch or less and your application calls for metal sensing, the inductive proximity (IP) sensor provides the needed solution. Choosing this sensor seems natural, due to its rugged metal housing and enclosed internal circuitry. Its epoxy potting gives it an advantage over other sensing technologies in harsh environments, such as those found in machine tool sensor applications, where dust and dirt buildup are an issue.

Nevertheless, the IP sensor has advantages and disadvantages, both of which you should consider before committing to any brand of sensing solution. In addition, proximity sensors have common pitfalls that lead designers to implement them improperly. Knowledge of certain proximity sensor idiosyncrasies can help prudent designers avoid these pitfalls. Among the issues that should be considered to ensure a successful sensing application are object detection capabilities, simplicity of mounting and setup, and effects of the sensor's environmental surroundings, whether they be mechanical or electrical.

Inductive Proximity Sensor Technology

Evaluating an IP sensor's object detection capabilities first requires us to understand the sensing technology's design fundamentals. This will, in turn, lend insights into the technology's effectiveness vis-à-vis the object task at hand.

An IP sensor has four components: the coil, the oscillator circuit, the detection circuit, and the output circuit. The coil is a tightly wound, multiturn strand of copper wire located within the sensor's face. The oscillator circuit generates a fluctuating current through the copper wire and induces a magnetic field in the coil. This field extends outwards from the sensor's face in a rough doughnut shape.

This magnetic field induces eddy currents in the target detectable object (DO). These currents themselves magnetically push back and dampen the inductive sensor's own oscillation field, which makes metal objects detectable. Further, because IP sensor sensing requires this eddy current buildup, we find the conductive metals that disperse current to be poor DOs. Conversely, thin materials such as foils (e.g., aluminum) allow eddy current buildup and are excellent DOs.

The detection circuit monitors the eddy current dampening; when the magnetic effect becomes sufficiently damped, it triggers the output circuitry.

Inductive proximity sensors come in two varieties: shielded and unshielded. In the shielded type, the sensing coil is sheathed in a metal (ferrous) casing. This kind of sensor is less affected by surrounding metal and can be embedded in a metal base, but it has a shorter sensing distance.

An unshielded sensing coil, as you might guess, isn't metal shielded. Consequently, while it provides a longer detecting distance, it's easily affected by surrounding metals. Thus, no object other than the target should be present near the sensor face's end (see Figure 1).

Object Detection Capabilities

Eddy currents build differently within various types of materials. Typically, examining an IP sensor's specification sheet reveals technical information referring to the "standard detectable object." This object is usually a square made of an iron (ferrous) material, with a 1-millimeter (mm) thickness and a side equal to the diameter of the rated proximity sensor's face.

Other materials (e.g., stainless steel, brass, aluminum, copper) have different specification ratings. These are shown in multipliers that are applied to the inductive sensor's specifications. For example, to calculate an IP sensor's sensing distance when it's detecting stainless steel, multiply the standard distances by a reduction factor of 0.8. Other multipliers are 0.5 (brass), 0.4 (aluminum), and 0.3 (copper).

Furthermore, an IP sensor's detecting distance changes with the shape and size of the DO. If a DO of the same material as the standard DO is of a greater size, you'll obtain a detecting distance nearly the same as the standard. If a smaller object is to be detected, however, the IP sensor requires the distance indicated in the specification sheet: "size of detectable object vs. detecting distance."

Detection distances for irregular objects may not be estimable from the manufacturer's data. These objects must be tested independently and measured so that the irregular object's detection distance is determined for the inductive sensor in question.

An irregular object's "setting distance"² is calculated thus:

where

Alternately, the IP sensor's "resetting distance" refers to the distance to a position at which the proximity switch releases (when measured from the reference position) by moving the DO away from the proximity switch after it operates (see Figure 2).

Inductive proximity sensors created within the last decade have been designed with superior coil windings and detection circuits. The detection trigger points are repeatable to 0.0001 inch when sensing. Obtaining such precision, however, requires the DO to approach the sensing face in the same precise manner.

IP sensors usually feature a wide range of acceptable input voltages; however, one needs to meet the specified power ratings. Reduced line voltages can lead to weakened magnetic fields and hence to smaller than expected sensing distances.

You can expect the life of a properly used IP sensor to be in the realm of 100,000 hours.

Simplicity of Mounting and Setup

Considering the high costs of labor in the U.S., a sensor's ease of setup and mounting becomes a paramount concern for machine designers. This is particularly critical when evaluating the finished design's long-term maintenance potentials. For the purposes of ease of installation and replacement, the IP sensor's CENELEC housing standardization makes it an excellent choice. Seventy-plus percent of all IP sensors sold worldwide are of the cylindrical housing variety. These housings are specified by their diameter and thread pitch.

For example, a popular inductive proximity is the nickel-plated brass barrel specified at M12x1. Ergo, the sensor has a 12-mm cylindrical housing (and roughly a 12-mm sensing face) threaded at a 1-mm pitch. The 1-mm pitch means that adjusting the M12 by one full rotation will move the sensor laterally by 1 mm. Armed with this information, your installation technician can quickly and effectively set up a new or replacement IP sensor.

We can imagine a M12x1 IP sensor with a sensing detection distance of 5 mm. For the sake of argument, let's say the application calls for a setting distance of 3 mm. First we'll adjust the IP sensor clockwise, using its housing's wrench flats, until its sensing face touches the target DO. Then, to get a reliable and precise object detection trigger point, we'll adjust the sensor back so its sensing face is the setting distance away from the DO. Thus, we turn the sensor counterclockwise three full rotations, moving its sensing face laterally away from the DO by 3 mm.

Environmental Effects

Environmental conditions will, of course, affect the capabilities of any sensor in a given application. Although IP sensors are temperature resilient, extremes can reduce their operating life and could cause premature failure. Higher temperature extremes can cause them to become more sensitive, while extreme cold temperatures lower their resistance to shock. What's more, the IP sensors' internal semiconductors may begin to behave erratically under extreme temperatures, either producing unexpected outputs or not responding at all. A good IP sensor temperature rating is -40° to +85°C.

If you need something beyond this, there are special IP sensors with separate amplifiers. The sensing head (coil only) is placed in the high-temperature environment, while the circuit amplifier is kept remotely in a cooler climate. These separate sensing heads can be resilient to 200°C.

In addition, the sensing faces of the separate amplifier type of IP sensor can be miniaturized, allowing for mounting in tight applications.

While shielded IP sensors can be embedded into metallic mounting fixtures up to the sensor's detection face, you must be aware that incidental (nontarget) metals should be clear of the sensing face by three times the sensor's standard detection distance. In addition, shielded IP sensors shouldn't be recessed into any metal mounting surfaces because of false triggering risks.

Unshielded IP sensors can't be embedded into metal mounting fixtures completely. They're susceptible to the influences of the surrounding metals because their magnetic sensing fields protrude from the sides of their sensing faces. This is what gives them their extended sensing capabilities. Unshielded IP sensors must be mounted into a metal-free area.

For unimpeded operation, the free area on each side of the sensor must be equal to the sensor's diameter. The depth clearance from the device's sensing face must be twice the sensor's standard detection distance. By following these rules, you can reduce the chances of false detection and reduced sensing distances.

When using several IP sensors in close proximity to one another, you must be aware of an effect called mutual interference. This occurs when a proximity sensor's magnetic field couples with that of another sensor, producing a beat frequency in one or both devices. This may lead to a triggered output. Beat frequency false triggering can be intermittent and difficult to reproduce.

Many IP sensors are available in alternate frequency models. Alternate frequency inductive sensors oscillate their magnetic fields at frequencies different from those of standard inductive devices. Because the frequencies are substantially different from one another, they don't interfere or cause any unwanted beat frequencies.

If alternate frequency sensors can't be obtained and you still need to mount the inductive sensors close together, you'll need to employ a multiplexed design. Switch the inductive sensors on and off while taking alternate reads. You'll eliminate mutual interference, while the sensor response time will go up. Consult your manufacturer's specifications to determine allowable on/off switching times.

Inductive Proximity Sensor Applications

We've learned quite a bit about IP sensors so far. The following summary of features enables us to envision some suitable sensing applications for the devices:

• Detecting distances are small—typically 1 inch or less.
• Circuitry is protected by a rugged, epoxy-full housing.
• Inductive proximity sensors detect metals.
• They have shortened sensing distances for conductive metals.
• Inductive proximity sensors are known to be repeatable to 0.0001 inch.
• Inductive sensors have a wide range of input voltages; 10-30 volts DC and beyond are typical.
• Starting at 100,000 hours, the inductive electrical life is long.
• The cylindrical-bodied inductive sensors are easy to set up.
• The devices are relatively strong against temperature extremes from -40° to +85°C.
• Inductive sensors are weak against the influences of surrounding metals.
• Inductive sensors are difficult to mount in close proximity to one another without the possibility of false triggering.

Here are several proved inductive sensor applications.

Lead frame position detection. Inductive proximity sensors detect the position of integrated circuit lead frames. The proximity sensor detects the position of the alignment hole. In electronic packaging, space is an important issue. This makes the separate amplifier proximity sensor a good solution (Figure 3).

Injection mold closure detection. Plastic injection machines can generate significant heat due to energy dissipations from molten materials. Thus, the IP sensor must be resistant to high temperatures. Again, the separate sensing head can be used. The sensor detects when the injection mold tool is completely closed (Figure 4).

Lathe control. Depressing the start button when the cutting tool is at the TL₃ position engages the tool post M₂ motor, and the cutting tool moves quickly. When the cutting tool reaches TL₂, M₂'s speed reduces, causing the cutting tool to move slowly. At TL₁, M₂ reverses, causing the cutting tool to return quickly to TL₃, as shown in Figure 5.

Float detection for flow control. The needed flow value can be maintained via inductive proximity sensing external detection methods, using a tapered pipe and a float covered in aluminum foil (Figure 6).

We've covered inductive proximity sensor designs and technologies, their object detection capabilities, their mounting and setup, and the effects of their environmental surroundings. I'm hopeful that these discovery issues and our brief excursion into the world of inductive sensing applications will garner the spirit of sensor problem solving in the complex world of machine automation. MC

Footnotes

1. The distance to a position at which the proximity sensor operates when measured from the reference position with the standard detectable object.

2. The distance from the detecting surface to the passing position of the DO which permits positive detection, even when the detecting distance is decreased due to temperature or voltage fluctuations.

Author Information

Guerrino Suffi is a graduate in electrical engineering from the University of Illinois at Urbana-Champaign. While there, he studied quantum electronics and wave theory. Guerrino has worked at Omron Electronics LLC since 1996. He is a safety product marketing specialist. Contact him at One Commerce Drive, Schaumburg, IL 60173; tel: (800) 55-OMRON; reno.suffi@omron.com; www.info.omron.com/oei.