How to choose the right proximity sensor?

Choosing the right proximity sensor capable of detecting nearby objects without any physical contact depends on the size, target material, and spacing of the target and its sensing distance.

Proximity sensors operate by generating an electromagnetic field and detecting the eddy current losses generated when ferrous and nonferrous metal target objects enter the field.

The maximum sensing distance is the distance in which the sensor is close enough to detect a ferrous target whose diameter is equal to or greater than the sensor diameter. Typically, the sensing distance should be between 50 to 80% of the maximum sensing range to assure reliable detection.

The maximum sensing distance is estimated from the curve for target sizes smaller than the sensor diameter. Ideally, the spacing between adjacent targets should be at least one sensor diameter so that the first target completely leaves the sensor field before the next target appears.

Individual targets can still be resolved as separate objects if the spacing is reduced to 70 or 75% of the sensor diameter. However, this can introduce a minimum limit on sensing distance, making the adjustment more critical.

All applications have specific needs, but, in general, the following steps will help you choose the right proximity sensor for your application.

Step 1: Determine the sensing distance

The sensing distance is the distance between the sensor and the object. Usually, each sensor family comes with a specification table as the selection guide to determine their appropriate sensing distances.

In many applications, placing the sensor as far as possible from the sensing object is very important due to temperature concerns. If a sensor is placed too close to a hot source, the sensor will fail quicker and require more maintenance.

The greater distance can be achieved with extended and triple-range sensors. Using an extended distance sensor to get the sensor farther away from the detected object is beneficial to the sensor’s life. Mounting the sensor farther from the detection object will eliminate unneeded contact with the sensor, thereby extending the sensor’s life.

You must use different proximity sensors, depending on the materials being sensed (i.e., brass, copper, aluminum, steel, etc.) If you are sensing a non-metallic object, you must use a capacitive sensor to measure virtually all materials at the specified sensing distances.

Step 2: Determine the nature of the object being detected.

Depending on the nature of materials to be detected, such as solid, liquid, granular, or metallic, the proximity sensors are divided into inductive proximity sensors, capacitive proximity sensors, and magnetic inductive proximity sensors. For detecting metals, the inductive proximity sensor is used, and for non-metallic materials, the capacitive proximity sensor is used. When detecting magnetic signals, the magnetic induction proximity sensor is used.

Step 3. Consider environmental placement concerns

Will the sensor be placed underwater, in a high-temperature environment, continually splashed with oil, etc.? This will determine the type of sensor you may use. Look at the specification tables for each sensor family to understand the environmental protection degree ratings.

Step 4: Decide if you need a shielded or unshielded sensor

Shielded and unshielded sensors are also referred to as embeddable and non embeddable. Unshielded sensors allow longer sensing distances, but shielded sensors allow flush mounting.

Step 5: What is the sensor output connected to?

The type of output required must be determined (i.e., NPN, PNP, or analog). Most PLC products will accept either output. If connecting to a solid-state relay, a PNP output is needed. If you need analog outputs, this is determined by the sensor application and what the sensor will be connected to. Devices with analog outputs produce an analog output signal approximately proportional to the target distance.

Step 6: Determine output connection type.

Determine whether you want an axial cable factory attached to the sensor (pigtail) or a quick disconnect cable. There are many advantages to using a quick-disconnect cable, such as easier maintenance and replacement. All proximity sensors will fail in time, and using a Q/D (quick disconnect) cable allows for simple replacement. Go for factory attached axial cables, since they have several advantages. First, the cable is integrated into the sensor and included in the price. Since the cable is sealed into the sensor, there is less chance of oil, water, or dust penetration into the sensor, which could cause failure.

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