Inductive Proximity Sensors: Design and Selection - Part 2

March 29, 2012
By Anonymous

Inductive Sensor Design

Inductive Proximity Sensors are best used when your application calls for metallic target sensing with a range that is within an inch of the sensing surface. These durable sensors are suitable for harsh environments. Industrial Inductive Proximity Sensors first came out in the early 1960s and today have a proven track record in the sensing industry.

Inductive Sensor Design

Inductive Proximity Sensors operate under the electrical principle of inductance. Inductance is the phenomenon where a fluctuating current, which by definition has a magnetic component, induces an electromotive force (emf) in a target object. In circuit design, one measures this inductance in H (henrys). To amplify a device’s inductance effect, the sensor twists wire into a tight coil and runs a current through it.

Inductive Proximity Sensor Design
Figure 1: Sensor Design

An Inductive Proximity Sensor has four elements: coil, oscillator, trigger circuit, and an output. The oscillator is an inductive capacitive tuned circuit that creates a radio frequency. The electromagnetic field produced by the oscillator is emitted from the coil away from the face of the sensor. The circuit has just enough feedback from the field to keep the oscillator going.  When a metal target enters the field, eddy currents circulate within the target. This causes a load on the sensor, decreasing the amplitude of the electromagnetic field. As the target approaches the sensor, the eddy currents increases, increasing the load on the oscillator, and further decreasing the amplitude of the field. The trigger circuit monitors the oscillator’s amplitude and at a predetermined level switches the output state of the sensor from its normal condition (on or off). As the target moves away from the sensor, the oscillator’s amplitude increases. At a predetermined level, the trigger switches the output state of the sensor back to its normal condition (on or off).


Figure 2: Trigger Circuit monitors the Oscillator's Amplitude
Figure 2: The Trigger Circuit monitors the Oscillator’s Amplitude

Consider two types when selecting an inductive sensor: shielded and unshielded. When current generates in the sensor’s coil, a doughnut effect it created which causes the proximity sensor to trigger when any object comes behind, along side or in front of the device. Shielding uses a ferrite core to direct the coil’s magnetic field to radiate only from the sensor’s detection face. Unshielded inductive proximity sensors are not completely unshielded. A peeled back ferrite core shielding in the unshielded case allows for a longer sensing distance, while still preventing sensing due to objects behind the detection face.


Figure 3: Shielded and Unshielded Sensors
Figure 3: Shielded and Unshielded Sensors

Understanding the operation, the magnetic nature, and the shielding of the inductive proximity sensor is helpful when considering the influences of target material, environment, and mounting restrictions on the sensor itself and in your application.

Inductive Sensor Selection

Inductive proximity sensors categorize in five specific types; cylindrical, rectangular, miniature, harsh environment, and special purpose. 70% of all inductive proximity sensor purchases are of the standardized cylindrical threaded barrel type. When one considers this statistic, it is easy to understand why you may want to specify into an application a general-purpose (or standardized) inductive proximity sensor. 70% of the time, you would be correct. Experience has shown, however, that applications in need of inductive sensing usually warrant the examination of a few additional design criteria. These conditional criteria eliminate the more special inductive proximity sensors available first before falling upon the general purpose inductive proximity sensor. The three guiding beliefs of inductive proximity sensor selection are target material, environment, and mounting restrictions.

Target Materials

In the world of inductive proximity sensors, not all metals are created equally. You find yourself looking for a quick fix to an inductive sensing problem that would vanish if a special inductive sensor was selected. The inductive proximity sensor specification that we have all become familiar with in technical data sheets worldwide references a “standard detectable object” made of an iron (ferrous) material. Other metallic materials, such as stainless steel, brass, aluminum, and copper have different influence over the inductive effect and are usually less detectable than iron.

Is the target material an iron object? Will the target material change in future runs of the application?

Figure 4: Sensing Object vs. Correction Coefficient
Figure 4: Sensing Object vs. Correction Coefficient

Examine the sensing distance reductions for typical inductive proximity sensors: Stainles Steel = Standard Sensing Distance X .76, Brass = Standard Sensing Distance X .5, Aluminum = Standard Sensing Distance X .48, and Copper = Standard Sensing Distance X .3.

Environment

Environmental conditions can have far and sweeping effects upon the inductive proximity sensor.  These effects specifically refer to sensor life, but can only be related to premature failure (false trigger or otherwise) of the inductive proximity sensor once installed into its component mounting position.

Is the application one in which metallic chips or filings are prone to build up on the side or face of the inductive proximity sensor?

Intelligent semi-conductor microprocessors found in some modern inductive proximity sensors have the ability to detect the slow build up of metal filings or “chips” over time and teach the inductive proximity sensor to ignore their effects. This type of specialized inductive proximity sensor is called a “chip immune” type.  Another type of inductive proximity sensor that is resilient against chip build up is the flat-pack proximity sensor. The slim profile of the flat pack proximity sensor when mounted with its sensing face exposed vertically is virtually unaffected by chip build up on its slim horizontal component.

Is the inductive proximity sensor exposed to cutting fluids or chemicals for prolonged periods of time?

In the face of cutting fluids or corrosive chemicals, a traditional inductive proximity sensor may become brittle and crack, shortening its life.  In such cases, the customer must again turn to a specialized inductive proximity sensor. Proximity sensors dipped, coated or shot from Teflon suffer no ill effects from the material in terms of performance or reliability. Teflon’s stability against cutting oils and corrosive chemicals outweigh the additional costs that come with a Teflon manufactured product. An additional benefit to a Teflon inductive proximity sensor is its ability to prevent any build up of weld slag.

Is the sensing application in a high temperature environment?

Inductive proximity sensors typically include their silicon amplifiers and detection circuitry inside the sensor head housing. These proximity sensors are called self-contained devices. Self-contained proximity sensors are practical for most applications until environmental conditions begin to exceed the normal operating parameters for a silicon-based circuit. Normal operating temperatures for silicon based circuitry is within the realm of -25 to 70°C (-13 to 158°F). Under any temperature conditions beyond these ranges, the circuitry becomes more prone to operating failure.  For temperature applications that exceed these requirements, look for inductive proximity sensors that use separate amplifiers. With separate amplifier inductive proximity sensors, the sensor head contains the inductive coil and little more. The intelligent amplifier and detection circuitry can be located safely away in a remote environmentally controlled area. Such sensors can resist temperatures as high as 200°C (392°F).

Mounting Restrictions

When it comes to miniaturization, few components so strongly represent the micro-electronic revolution that has occurred within the last 10 years as inductive proximity sensors. Today, Panasonic inductive proximity sensors have developed a 6mm X 6mm X 18mm rectangular proximity sensor with an extended sensing range of 1.6mm.

Does your application space constraint prohibit the use of an inductive proximity sensor with a traditional cylindrical based body?

Inductive proximity sensors come in a wide variety of body types. Rectangular style inductive proximity sensors range from the sub-miniature (6mm X 6mm X 18mm) to (18mm X 18mm X 28mm).

Does the inductive proximity sensor in question have a strong enclosure?

Cylindrical proximity sensor’s barrel housing thickness varies from manufacturer to manufacturer.  The thicker the barrel housing of the Inductive Proximity Sensor, the less likely it will be to break due to over zealous installation techniques or through incidental object collision.


Figure 5: Influence of Surrounding Metal
Figure 5: Influence of Surrounding Metal
 


  To learn more abou Inductive Proximity Sensors check out the first article in this series, Inductive Proximity Sensors: Understanding Specifications and the final installment, Inductive Proximity Sensors: Mounting Tips & Tricks.