Guide to Selecting the Ideal Pneumatic Cylinder Positioning Application
What are the Sensor Options for Pneumatic Cylinder Positioning?
A magnet is embedded within the piston of a pneumatic cylinder. This magnet is clearly visible from a cutaway standpoint for theory or on paper, but in practical application, technology is needed to view the magnet and determine its position through the cylinder wall, meaning the magnet isn’t visible to the naked eye.
There are several technologies available to accomplish this task, some of which are quite outdated and some of which have overcome the trials of the past to provide a reliable, cost-effective solution.
From a very low first-cost and durability to simplicity in application, pneumatic cylinders have many advantageous over other options.
With integration into an automated system, the control system must be supplied with electrical signals. This tells the controller the pneumatic cylinder’s position status. There a number of ways this is accomplished and extension and retraction are detected.
Techniques for Position-Sensing
An inductive proximity switch or external electromechanical limit switch may be installed to sense the pneumatic cylinder’s position. These detect metal targets on mobile parts of the machine, but they do hold several disadvantages:
• Everyday impact or contact, for example, leaves the external hardware more apt to become misaligned or damaged.
• There are also a number of problems due to adjustment difficulties, overall assembly, the cost of the brackets, and the complexities and size.
It’s such problems that make it more popular to attach sensors or magnetically actuated switches directly into or on to the cylinder. A magnet is mounted in the moving piston and magnetic field sensors detect the magnet. In most cases, the magnetic sensor is able to convey end of stroke detection. Multiple positions can be detected by installing multiple sensors along the cylinder. End of stroke sensing has traditionally been accomplished via two top proximity switches that detect specific targets. Mounting the magnetic field sensors at the bottom provides a more comprehensive and reliable solution. Reed switches can be attached to the tie rods.
Magnetic field sensors usually use one of two different types of cylinder magnets. The axially magnetized magnet is the first option to actuate a reed switch, and it’s perhaps the most widely utilized of the two. It places both poles aside each other in the axial plane, which can be viewed from the side. Option two is the radially magnetized cylinder magnet. It’s more often used with hall-effect sensors. It’s poles are on opposite inner and outer diameters.
Types of Magnetic Sensors
1) Reed Switch
The reed switch, which consists of two flattened ferromagnetic nickel and iron reed elements, is the simplest magnetic sensor option. The elements are enclosed in a glass tube that’s hermetically sealed. To reduce contact arcing, the glass tube is evacuated to a vacuum. Magnets need a strong Gauss rating of at least 50 to overcome return forces. The electrical circuit is completed by the reed elements attracting the magnetic flux lines of an approaching axially aligned magnet and using magnetic force to draw together.
Reed switches do not require standby power, perform under ac and dc electrical loads, and are low-cost to operate, all of which make them adequate magnetic sensors. That said, there are also a number of disadvantages as well:
• They may not respond quickly enough for all high-speed applications because they’re slow to operate.
• They have a limited life expectancy with a limited number of operational life cycles because they’re mechanical devices and because they’re sensitive to high-current electrical load switches.
• They may deliver multiple switching points.
• They can be damaged or experience contact bounce if the application involves a lot of vibrations and shock.
• The lack of dependability within the above can cause costly unexpected downtime and productivity losses; facilities using dozens of reed switches , for example, can expect continual maintenance from multiple failures occurring around the clock.
These are solid-state electronic devices with a voltage amplifier and a comparator circuit driving switching output, meaning there aren’t any moving parts. They have dc current steadily passing through a thin Hall-effect chip. Electrons are distributed uniformly across the element. Current moves in a straight line. Outputs on the sides of the Hall-effect chip will not experience a current differentiation. The magnetic field is perpendicular to the Hall elements’ current flow when the radically oriented magnet makes its approach. The magnet unbalances the electrons, pushes them off course, and in the direction of one side of the Hall chip. This creates a small micro-voltage impact across the element in proportion to the magnetic field’s strength. Once the comparator circuit has found a sufficient voltage, then the sensor output switches on. The pneumatic cylinder’s embedded magnets inside the piston transmit the orientation – axial or radial.
With no moving parts, response time isn’t reliant upon mechanical inertia being overcome, which makes for faster operation. It also means operation is more reliable because shock and vibration isn’t a consideration.
However, there are drawbacks to the Hall as well:
• Sensitivity and preexisting magnetic field orientation could make it difficult to just go in a replace reed sensors with Hall-effect sensors. The pneumatic cylinder is likely already oriented for the axial sensor of a reed vs the radial sensor of a Hall.
• Double switching, where both the magnet’s poles are sensed, is another a drawback of the Hall.
3) Anisotropic Magnetoresistive Sensors (AMR)
The rugged AMR, like the Hall-effect sensor, doesn’t have moving mechanical elements. It has a bias current running through its sensor element that undergoes a change in resistance under a magnetic field. Current changes are detected by a comparator circuit, which in turn switches the output of the sensor. It’s a very simple operating premise.
Specifically compared to the Hall, the AMR element is less prone to false tripping and has a greater noise immunity because of its better sensitivity. Compared to a Hall-effect sensor, AMRs are typically around 200 times more responsive, with most only requiring a 15 Gauss magnet field strength.
This sensor doesn’t have any double switching points. It remains in the the on state while the low-strength aspect of the magnetic field passes the censor. This is thanks to a higher sensitivity of the magnetoresistive sensor. Meanwhile, a Hall-effect frequently drops just by sensing a weaker portion within the strong areas of the magnetic field.
They’re also diverse in that they can detect both radially and actually magnetized magnets.
Of note, the AMRs have a smaller physical size, improved immunity to noise, and a lesser mechanical hysteresis. High quality AMRs even have added output protection circuits for robust electrical overload, reverse connection, and short-circuit protections.
Some AMR sensors have weld field immunity, allowing them to reliably and efficiently operate in at welding fields 200kA/m without suffering damage or false signals. Metallic housing is another option with some AMRs that afford it protection from hot weld spatter that would otherwise melt lesser sensors with plastic bodies.
4) Giant Magnetoresistive Sensors (GMR)
These are the 2.0, updated version of AMRs. They feature at least a 10% improvement over the robust action of AMRs when it comes to reaction to magnetic fields. Chip materials are lesser because of the improved sensitivity, and the resulting smaller packaging and housing means that they’re ideal for mini pneumatic grippers, short-strike cylinders, small-bore cylinders, and other small applications.
Advanced output protection circuits (reverse-connect, overload, and advanced output protections) are also features of quality manufactured GMRs.
The Bottom Line
Reed switches are often abandoned because they’re prone to high failure rates. Instead, many have been using inductive or mechanical sensors for their pneumatic cylinder position detecting needs.
GMR and AMR have been left to overcome a lot of bad legacy that their predecessor switches have created. However, GMR and AMR hold significant improvement over their predecessors, and they indeed offer a faster, more reliable, more integrative, and smaller solution to cylinder position detection. As such, they should now be considered the standard.