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.
One trend we see today in many applications is the need for smaller low profile proximity sensors. Machines are getting much smaller and the need for error proofing has ultimately become a must for such applications in the Stamping and Die industry. Stamping Die processes can be a very harsh environment with excessive change overs to high speed part feed outs when running production. In many cases these applications need a sensor that can provide 5mm of sensing range however they simply do not have the room for an M18 sensor that is 45 to 50mm long. This is where the “FlatPack” low profile sensor can be a great choice due to their low profile dimensions.
Proximity sensors have proven time and time again to reduce machine crashes, part accuracy and proper part location. Sensors can be placed in multiple locations within the application to properly error proof “In Order Parts” (IO) for example detecting whether a punched hole is present or not present to ensure a production part is good. All of this adds up to reduced machine downtime and lower scrap rates that simply help a plant run more efficiently.
So when selecting proximity sensors and mating cables it is very important to select a sensor that A) mechanically fits the application and B) offers enough sensing range detection to reliably see the target without physical damage to the sensor. Remember, these sensors are proximity sensors not positive machine stops. Cables are also key to applications, it is important to pick a the proper cable needed for example an abrasion resistant cable may be needed due to excessive metal debris or a TPE cable for high flex areas.
Below both sensors have 5mm of sensing range:
Below both sensors have 2mm of sensing range:
You can see that in certain process areas “FlatPack” low profile sensors can provide benefits for applications that have space constraints.
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o Low variation of switch point from sample to sample
o Tight repeat accuracy of switch point
o Low temperature drift of switch point
o Low maximum hysteresis (distance between switch-on to switch-off)
An inductive proximity sensor is a non-contact device that is used to detect a metal target. When power is applied to an inductive proximity sensor the sensor’s coil will generate an oscillating electromagnetic field out of the face of the sensor. This field will vary in shape and size depending on the diameter of the sensor and whether the sensor is a shielded or non-shielded model. For example, a M12 size sensor will generate a smaller electromagnetic field than an M30 size sensor. When the metal target gets close enough to the sensor’s face it begins to penetrate the electromagnetic field. When this happens, eddy currents are generated on the surface of the metal target. As the metal target gets closer to the sensor face – the eddy currents increase – which in turn decrease the amplitude of the electromagnetic field. Once the electromagnetic field’s amplitude is reduced to a certain level – the sensor will activate indicating it has detected the metal target.
This explanation is a little wordy and, as in most cases, a visual demonstration can be of great help. Watch this short video explaining the basic functionality of an inductive proximity sensor.
For more information on inductive proximity sensors, click here.