Inductive proximity sensors have been around for decades and have proven to be a groundbreaking invention for the world of automation. This type of technology detects the presence or absence of ferrous objects using electromagnetic fields. Manufacturers typically select which inductive sensor to use in their application based on their form factor and switching distance. Although, another important factor to consider is how the sensor will be mounted. Improper mounting conditions can cause the sensor to false trigger, decreasing its reliability and efficiency. Since inductive proximity sensors target metal objects, surrounding the sensor with a metal mounting will cause unintended consequences for the user. Understanding these implications will help you select the correct inductive sensor for you specific application. There are several mounting options available for this type of sensor, including flush mount, non-flush mount, and semi-flush mount. We will dive into each type in more detail below.
Flush mounting, also known as embeddable mounting, is exactly what the name describes. The sensor is flush with the mounting surface. The advantage of mounting the sensor in this way is that it provides protection to the face of the sensor. The opportunities are endless for how sensors can be damaged but with the flush mounting style, these factors are reduced. The way a flush mounted sensor is designed causes the magnetic field to only generate out of the face of the sensor (see below). This allows the sensor to work properly by avoiding triggering from the mount as opposed to the target. The disadvantage of this is that it creates shorter switching distances than other mounting types.
A non-flush inductive proximity sensor is relatively easy to spot because it extends out from the mounting bracket and also uses a cap that surrounds the sensor face. Non-flush sensors offer the longest sensing distance range because the electromagnetic field extends from the sides of the sensor face as opposed to the edges or strictly the front of the face. There are some consequences to consider when selecting this style. The sensor head is exposed to the external environment. These sensors are more susceptible to being hit or damaged, which in turn, can cause failures within the process and cost the company money for replacements. It is important to understand these potential problem factors so they can be avoided in the design phase if you require the longer switching distance.
The semi-flush, also known as quasi-flush, is similar to that of the flush mounting style but requires a metal-free zone around the sensor face to achieve the optimal sensing range. Thus, this sensor is protected and offers a larger sensing field than a flush mounted sensor. The disadvantage is that if metal is touching the edge of the sensor face, this will dramatically decrease the sensing range.
Each style offers advantages and disadvantages. Each style uses a specific technology and design to allow it to adapt to different applications. Understanding these pros and cons will allow you to make a more informed decision for which to use in the application at hand.
The simplest magnetic field sensor is the reed switch. This device consists of two flattened ferromagnetic nickel and iron reed elements, enclosed in a hermetically sealed glass tube. As an axially aligned magnet approaches, the reed elements attract the magnetic flux lines and draw together by magnetic force, thus completing an electrical circuit.
While there are a few advantages of this technology like low cost and high noise immunity, those can be outweighed by the numerous disadvantages. These switches can be slow, are prone to failure, and are sensitive to vibration. Additionally, they react only to axially magnetized magnets and require high magnet strength.
Magnetoresistive Sensors (GMR)
The latest magnetic field sensing technology is called giant magnetoresistive (GMR). Compared to Reed Switches GMR sensors have a more robust reaction to the presence of a magnetic field due to their high sensitivity, less physical chip material is required to construct a practical GMR magnetic field sensor, so GMR sensors can be packaged in much smaller housings for applications such as short stroke cylinders.
GMR sensors have quite a few advantages over reed switches. GMR sensors react to both axially and radially magnetized magnets and also require low magnetic strength. Along with their smaller physical size, these sensors also have superior noise immunity, are vibration resistant. GMR sensors also offer protection against overload, reverse polarity, and short circuiting.
One of the basic differences is that detection method that each solution utilizes. Magnetic field sensors use an indirect method by monitoring the mechanism that moves the jaws, not the jaws themselves. Magnetic field sensors sense magnets internally mounted on the gripper mechanism to indicate the open or closed position. On the other hand, inductive proximity sensors use a direct method that monitors the jaws by detecting targets placed directly in the jaws. Proximity sensors sense tabs on moving the gripper jaw mechanism to indicate a fully open or closed position.
Additionally, each solution offers its own advantages and disadvantages. Magnetic field sensors, for example, install directly into extruded slots on the outside of the cylinder, can detect an extremely short piston stroke, and offer wear-free position detection. On the other side of the coin, the disadvantages of magnetic field sensors for this application are the necessity of a magnet to be installed in the piston which also requires that the cylinder walls not be magnetic. Inductive proximity sensors allow the cylinder to be made of any material and do not require magnets to be installed. However, proximity sensors do require more installation space, longer setup time, and have other variables to consider.