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Die Protection Tip Of The Month for April

Using Inductive Ring Sensors

As the name implies, an inductive "ring" sensor is a type of inductive proximity sensor shaped like a ring.   The sensor actuates when a metallic object passes through the hole in the sensor.   These sensors are popular for detecting parts or slugs ejected through the bottom of the die, as well as for detecting some air ejected parts.   This article will provide some tips on their proper use.

Inductive Proximity "Ring" Sensor

Minimum Object Size
Sensor selection typically begins with the object that sensor needs to detect, and inductive ring sensors are no different.   The first specification to consider when selecting an inductive ring sensor is the sensor's minimum object sensitivity.   This spec indicates the minimum sized object that the sensor can detect when the object is dropped anywhere through the ring.   The part to be detected must be larger than the sensor's minimum object sensitivity.   Note that in most cases, the specification will refer to a certain diameter steel ball.   Since most stamping applications do not involve detecting a steel ball, you'll have to approximate the size of your part to the specification.

Shielded vs. Unshielded
Like other proximity sensors, inductive ring sensors are available in both shielded and unshielded versions.   A shielded inductive ring sensor can be mounted so that one or both sides of the ring is flush against a metal object (like a die shoe or die block).   When unshielded sensors are used, a minimum distance must be maintained between the sensor and any surrounding metal objects (this information can be found in the sensor specifications).   Shielded sensors are more convenient to mount (especially when space is tight), but can be significantly more expensive than unshielded sensors.

The sensing field surrounds an unshielded sensor. Metallic objects in this field will actuate the sensor.

If you must install the sensor flush against (or very close to) a metallic object, you must either use a shielded sensor, or select a 'dynamic' sensor.

Static vs. Dynamic Sensors
A static inductive ring sensor acts like a typical inductive proximity sensor: When a metallic object of sufficient size enters the sensing field, the sensor will actuate.   The sensor will stay actuated as long as the metallic object stays in the sensing field, and will only turn "off" when the object leaves the sensor field.

With a dynamic sensor, the sensor will actuate (turn "on") when a metallic object enters the sensing field, but will turn "off' after a specified amount of time if the object remains stationary.   The sensor will actuate again when another object enters the sensing field, even if the first object is still present. In other words, a dynamic sensor will detect objects that pass through the sensing field, but ignore stationary ones.   In this respect, a dynamic sensor can be used in place of a shielded sensor - such as installing it flush against the bottom of the die - as long as the nearby metallic objects are stationary.

Protecting the Sensors
The inside (ring) portion of the housing of these sensors is made from plastic.   Even if it is a carefully selected, high quality plastic, it is no match for a sharp burr on a metal part.   If the parts in your application repeatedly contact the inside of the ring, you will eventually destroy the sensor.   The simplest way to protect the sensor is to install the ring sensor on the outside of a plastic tube, and to drop/eject the parts through the tube.   The tube cannot be made of metal, otherwise the sensor will detect the tube and not the parts. Also, the tube should be inspected periodically and replaced when the inside becomes worn out. = - otherwise you're only delaying rather than preventing the failure of the sensor.

Bench testing for inductive proximity rings is easy: Simply drop your parts through the ring, and ensure that it actuates.   Although simple, it is still very important to ensure that the sensor will reliably detect your parts.

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