Inductive Proximity Sensors: Understanding Specifications – Part 1
In automation design, it is necessary to understand the precise technical definition of a component’s behavior. The following definitions, if not the terminology itself, are unique to inductive proximity sensors. Therefore, it is important to describe and comprehend definitions before implementation of the inductive component into your application. Upon reviewing an inductive proximity sensor data sheet, you’ll find that it displays many specifications that describe how to implement the inductive device for the purposes of detecting a specific object.
Standard Detectable Object
When an inductive proximity sensor’s data sheet refers to a standard detectable object, it tells the specified shape, size, and material which is used as the standard to check the performance of the proximity sensor. This understanding is important because the detection distance of the inductive proximity sensor differs according to the shape and material of an object. Typically, the standard detectable object will be an iron plate with a thickness of 1mm and height and width of equal length to the diameter of a shielded inductive sensor and 1.5 times the diameter of non-shielded inductive sensor.
The standard sensing object is the smallest size object for which the sensing range becomes constant.
Detection distance is the position at which the inductive proximity sensor operates when a standard detectable object is moved in front of the sensor in a defined way. For an Inductive proximity sensor with an end (or “front”) detection surface, the detection distance is determined by aligning the center line of the Inductive sensor with the center line of the standard detectable object. The standard detectable object is moved towards the face of the inductive proximity sensor until the proximity sensor changes states and the detection distance is determined. This will give the Maximum Operating Distance.
For a sensing range for which the sensor can stably detect the standard sensing object even if there is an ambient temperature drift and/or supply voltage fluctuation. Normally, it is a 70 to 80% of the maximum operating distance.
For temperature ranges between -25C° to +70C°, a difference between +/-10C° can be expected.
The same change can be expected when the supply voltage varies by +/-10%.
A Target Size Correction Factor can be applied when targets are smaller than the standard target. To determine the sensing distance for a target that is smaller than the standard target (Snew), multiply the rated sensing distance (Srated) times the correction factor (T). If, for example, a shielded sensor has a rated sensing distance of 1 mm and the target is half the size of the standard target, the new sensing distance is 0.83 mm (1 mm x 0.83).
Snew = Srated x T
Snew = 1 mm x 0.83
Snew = 0.83 mm
This can also be calculated by using a chart provided by most manufactures and found in most inductive proximity sensor data sheet engineering section.
A. Standard Iron Target of 10 x 10mm
On the horizontal axis of the chart, find the 10-mm point. Trace the line upward until it intersects with the line representing iron. The intersecting point is the detecting distance of the target.
B. Rectangular iron target of 3.6 x 10mm
1. The area is 3.6 x 10mm^2
2. Find the Square root of (1.) √36 = 6mm
Read the line representing iron on the chart as described above.
Detecting distance of (i) is approximately 2 mm.
Detecting distance of (ii) is approximately 1.8 mm.
Therefore, while detecting (i), the detection of (ii) is enabled only when the target is placed as close as 1.8 mm.
Thickness of the target is another factor that should be considered. The sensing distance is constant for the standard target. However, for nonferrous targets such as brass, aluminum, and copper a phenomenon known as “skin effect” occurs. Sensing distance decreases as the target thickness increases. If the target is other than the standard target a correction factor must be applied for the thickness of the target.
Hysteresis or Reset Distance
The reset distance refers to the distance at which the inductive proximity sensor releases its output when the standard detectable object is removed from its field of detection. The difference in distance between the detection distance and the reset distance is called the “Distance differential.” Typically, the distance differential is from 10% to 20% of the overall detection distance. The distance differential is incorporated into the design of the inductive proximity sensor to prevent the proximity sensor from having its output chatter due to noisy environments or detectable object vibrations.
Today’s quality inductive proximity sensors can have trigger points that are repeatable to 1/10,000ths of an inch. Customers must be aware, however, that desired detectable object must approach the face of the inductive sensor to trip the output and then be removed from the inductive sensor’s field of detection by the distance differential before another precise object trigger can occur.
The detecting distance tolerance range when a standard target is subjected to repeated detection under set conditions.
Normally Open – N.O.
The operating mode that permits the sensor to output an ON signal when a target enters the detecting range.
Normally Closed – N.C.
The operating mode that permits the sensor to output an ON signal when a target goes out of the detecting range.
Proximity switches respond to an object only when it is in a defined area in front of the switch’s sensing face. The point at which the proximity switch recognizes an incoming target is the operating point. The point at which an outgoing target causes the device to switch back to its normal state is called the release point. The area between these two points is called the hysteresis zone.
The size and shape of the response curve depends on the specific proximity switch. The following curve represents one type of proximity switch.
Maximum Response Time / Frequency
As shown in the figure below, a rotating plate having the standard sensing object pasted at constant intervals is placed in front of the proximity sensor. The plate is rotated while observing the sensing output. The maximum number of times per second at which sensing can be done, for which the corresponding sensing output can be obtained, is the maximum response time / frequency.
This article is part one of a three part series about selecting and using inductive proximity sensors. If you have any questions or would like clarification on any of the topics covered, please leave a comment below. Read parts 2 and 3, Sensor Design and Selection and Mounting Tips & Tricks or download the entire article in PDF format by clicking here.