In a previous post we took a look at magnetic field sensors vs inductive proximity sensors for robot grippers. In this post I am going to dive a little deeper into magnetic field sensors and compare two technologies: reed switches, and magnetoresistive sensors (GMR).
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.
Here’s a real-world application where the reliability of the sensors is directly related to the reliability of the process in producing quality results.
Pictured below is a pneumatic actuator for a vacuum valve. Inside the actuator, a magnetic ring is installed around the moving piston by the manufacturer of the actuator (this is an option that must be requested when placing the order for the actuator). The magnet acts as a target to activate the sensors as it moves under them during operation. It’s important to note that the wall material of the actuator must be non-magnetic in order for this concept to work properly; typically aluminum or non-magnetic stainless steel is used.
Continue reading “Reliable Sensors for Reliable Process Quality”
There are numerous sensor technologies that can be used to provide liquid level feedback; ultrasonic sensors, capacitive sensors, and magnetic reed switches are but a few that spring to mind. For many, if not most, general liquid level applications, these technologies work just fine, and provide adequate precision. However, some liquid level applications, such as precision filling and dispensing applications, require higher precision. Continue reading “Linear Position Sensors for Precision Fill-level Applications”
I received a call the other day from a customer who wanted to use a magnetic field sensor on a cylinder, and evidently was requiring very precise results. He asked, “what is the hysteresis of your sensors? I notice that it is listed in your catalog as a percentage and I need to know the exact value in millimeters.” My response was, “well it depends”, upon which he was not overly pleased. I then continued to explain my answer which leads me to the contents of this posting.
Continue reading “What is the hysteresis of your magnetic field sensor?”
Pneumatic cylinders are used in many applications as prime movers in machinery, material handling, assembly, robotics, medical, and the list could go on. One of the challenges facing OEM’s integrators and end users is to detect reliably whether the cylinder has been fully extended or retracted before allowing machine movement. Solutions include the use of inductive sensors with some sort of target and internally mounted magnet (by the cylinder manufacturer) on the cylinder piston. In my previous blog, I discussed the two primary magnets, axially and radially magnetized magnets, used by cylinder manufacturers. Now, we will review one of the most commonly used magnetic field sensors to detect extension and retraction of the cylinder…the well-known reed switch.
Continue reading “The Pros and Cons of End-of-Stroke Detection with Reed Switches”