Written by: Jeff Himes
Selecting the correct inductive proximity sensor for an application can be an intimidating process. There are literally thousands of models available from various vendors so having a good starting point to narrow down the field is essential.
At this point is will be assumed that an Inductive Proximity Sensor is the type of sensor being selected. If you are at the beginning of your selection process, please read and earlier blog post pertaining to your initial sensor selection.
8 Selection Criteria for an Inductive Proximity Sensor
1. What sensing distance is required to detect the target? Inductive sensors typically have a sensing range starting from around 0.8 mm (3 mm tubular style) up to 40 mm (30 mm tubular style). Models having a sensing distance longer than 40 mm do exist, but the majority of applications fall under the 40 mm detection distance. The sensing distance needed will also dictate the sensor size available for the application. In general as the sensor coil diameter increases, the sensing range increases as well. For example the standard sensing range for a M8 inductive sensor is 1.5 mm while a M30 size sensor has a sensing range of 10 mm.
2. What is the Target Material? Since we are discussing inductive sensors, the target must be metal – but what kind of metal? The standard sensing distance of an inductive sensor is based on a target of ferrous mild steel (Fe 360). An inductive sensor will detect non-ferrous metals such as aluminum, brass, copper or stainless steel – but at a shorter detection distance. See an earlier target material blog for more detailed information. The main concept to remember is that if the target material is not mild steel – the sensor will detect the target at a shorter distance. This sensing reduction can be eliminated by using specialized sensors called Factor 1 which detect ferrous and non-ferrous targets at approximately the same distance.
3. What form factor and mounting style are needed? The most popular form factors are tubular models however compact rectangular housings are also available. If limited space (depth) is available, a rectangular form factor may be the best choice as some of these are only 5 mm in depth. The type of mounting required will also need to be considered. A shielded bodied sensor can typically be mounted flush in metal whereas a non-shielded sensor cannot be mounted flush in metal. Some applications may require a flush mount sensor due to the restricted space available for the sensor. More information on shielded and non-shielded sensors can be referenced in this related blog.
4. What is the size of the target? Per the IEC specification there are two basic rules for the proper target size. Rule 1 indicates the target size should be flat, square in shape, and have sides at least as long as the sensor coil’s diameter (12mm sensor = 12mm x 12mm target). Rule 2 indicates the target size should be equal to 3 times the rated operating distance (Sn) of the sensor. The target should then be the larger result of these two rules. An inductive sensor will detect a target smaller than these recommendations, but at a reduced sensing distance. More information and examples for target size can be reviewed by clicking on target size information.
5. What is the shape of the target? As referenced above, a flat metal target of the appropriate size is the best target for an inductive proximity sensor. If the target is a bent piece of metal or has a concave or convex shape the rated sensing distance may not be achieved. Once again, at some point the inductive sensor will detect the odd shaped target, but more than likely at a closer range. A flat metal target will provide the best detection distance.
6. Do any unique environmental conditions exist? Will the sensor be subject to low temperatures (below -25°C) or high temperatures (above +70°C)? Are welding process or servo motors being ran close by? Are caustic chemicals or high pressure wash-down cleaning requirements stipulating a stainless steel housing? Most inductive sensors are rated to IP67 liquid ingress protection which is appropriate for the majority of applications. Sensors with an IP69K rating also exist for high pressure wash-down needs as well. Make sure you understand what environmental conditions the sensor will be subjected to and then ensure the sensor has the proper ratings for that environment.
7. What electrical requirements are needed? For this answer the control interface will need to be known. What electrical voltage is needed to interface to the controller and what output type is needed? Most of the inductive proximity sensors sold in the U.S. are 3-wire DC types. These can have outputs of PNP N/O, PNP N/C, NPN N/O or NPN N/C. 4-wire models having both a N/O and N/C output are also possible. The more popular output in the U.S. is the 3-wire PNP N/O version. 2-wire DC (polarized and non-polarized) and 2-wire AC/DC sensors are also available, but they are used to a lesser degree. Your PLC or interface controller will dictate the voltage and output type needed – then you need to ensure the chosen sensor can match that need.
8. What connection method is required? The majority of inductive sensors sold are quick disconnect type. This means they have an integrated quick disconnect and must have a mating cable connected. For DC sensors that connector is usually a M5 x 0.5, M8 x 1, or M12 x 1.
Other connection options are a pre-wired cable with flying leads (maybe 2 meters or 5 meters long) or a pre-wired cable with a molded on connector (sometimes called a “pigtail”). Any of these connections methods are acceptable; it’s normally the customer’s preference that drives the requirement.
Hopefully these 8 selection criteria will help you narrow down your search for the correct inductive proximity sensor. If after answering these questions you are still unsure of your sensor selection, your vendor of choice should have knowledgeable Customer Service or Technical Support personnel to assist in selecting the right sensor for your application.