Evolution of Pneumatic Cylinder Sensors

Today’s pneumatic cylinders are compact, reliable, and cost-effective prime movers for automated equipment. They’re used in many applications, such as machinery, material handling, assembly, robotics, and medical. One challenge facing OEMs, integrators and end users is how to detect reliably whether the cylinder is fully extended, retracted, or positioned somewhere in between before allowing machine movement.

A widely used method for cylinder position detection is to attach magnetically actuated switches or sensors to the sides of the cylinder using brackets, or by inserting them into a slot extruded into the body of the cylinder. Magnetic field sensors detect an internal magnet that is mounted on the moving piston through the aluminum cylinder wall.

The selection of which type of magnetic sensors to use depends on your application needs and specific data requirements.

Magnetic Sensor Types

Reed switches

The reed switch is the most simplistic and most often used end-of-stroke sensor available on the market. It consists of two flattened ferromagnetic nickel and iron reed elements enclosed in a hermetically sealed glass tube. The tube aids in minimizing contact arcing and prevents moisture from getting to the switch elements. As an axially aligned magnet approaches the switch element, the reed elements are magnetized and attracted together completing the circuit.

AMR and GMR sensors

Most cylinder manufacturers and OEMs use electronic sensors with either magnetoresistive technology (AMR) or giant magnetoresistive (GMR). Both versions are based on a change in resistance. One advantage of these sensors is that they will work with the axially magnetized magnet and, in some cases, the radially magnetized magnet. GMR sensors can be physically smaller than the AMR sensors. They are more sensitive, more precise and have a better hysteresis. Versions exist that provide reverse polarity protection, overload protection, and short circuit protection.

The initial cost of an AMR or GMR sensor may be slightly more than a reed sensor, however, this cost is increasingly less, especially if you figure the cost of downtime when the reed switch fails. AMR and GMR sensors are also three-wire devices, unlike the two-wire reed switches. In the end, the AMR and GMR sensors are the better solution since there are no moving parts and they typically last much longer than the reed switch.

Position detection sensors for both C-slots and T-slots

Pneumatic cylinders typically have either a C-slot or T-slot feature in the extrusion of the cylinder body. Many sensor housings have these same housing profiles and the sensor can either be dropped into the slot from above and tightened with a screw or slid in from the end of the cylinder provided there is no end plate. For round cylinders or tie rod cylinders, additional brackets are available that can use either a C-slot or T-slot sensor. This allows for commonality of sensors for end users and OEMs to meet the needs of many applications and reduce the number of sensor part numbers and inventory.

Today, there are more options than ever for piston position detection in pneumatic cylinders, including different housing styles to meet the cylinder extrusions. Also available are two sensors – one for extended and one for retracted – that share a single, four-pin connection. These magnetic sensors are also available now with weld field immunity for harsh welding applications.

Technology has advanced as well. Now cylinder sensors can be taught to trigger at certain points along the travel of the piston. The user simply moves the piston to a desired location and presses a button to set the switching location. This teachable sensor can also be connected to IO-Link, allowing up to eight switching points for flexibility in several applications.

Over the years, many users have abandoned reed switches, due to their failure rate, in favor of mechanical or inductive sensors to detect pneumatic cylinder position. AMR and GMR sensors are smaller, faster, easy to integrate, and are much more reliable. With the vast improvements in sensor technology, AMR and GMR sensors should now be considered the primary solution for detecting cylinder position.

Reed Switches vs. Magnetoresistive Sensors (GMR)

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).

Reed Switches

PrintThe 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)

PrintThe 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.

Reliable Sensors for Reliable Process Quality

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”

What is the hysteresis of your magnetic field sensor?


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?”

The Pros and Cons of End-of-Stroke Detection with Reed Switches

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”