Proximity Switches for air cylinders. Things you need to know!

Many applications for air cylinders involve split second timing and the need to know exactly where the rod end is, or the tooling on the rod end is, at all times.

In the event that the machine timing went off, perhaps as the result of a stuck air valve not cycling a cylinder at the right time, or even that the cylinder itself has failed, then some rod end tooling on the end of one cylinder could impact the tooling on the end of another, possibly destroying both.

This scenario is expensive for the manufacturer both in replacing the components that were damaged at impact, and the even greater loss of revenue from having a machine down when it wasn't planned for and the resulting lowered productivity.

This could be avoided if proximity switches were used.

The cylinder manufacturers assist in position sensing by offering some of their cylinders as proximity switch ready by adding a magnetic strip around the circumference of the piston inside the cylinder. This usually comes at added cost, so a magnet on the piston may not be standard on every cylinder.

As the piston moves from end to end inside the cylinder barrel, so too does the magnetic strip. The magnetic field passes through the wall of the cylinder barrel, and can affect objects placed just outside that cylinder barrel.

This first graphic is a depiction of a simple reed switch inside a protective housing.

When the reed switch is installed on the outside of the cylinder barrel, as the magnet on the piston passed beneath it, the two small reeds shown in the drawing would be pulled together by magnetic force. When they touch, a circuit is made, and a low voltage signal is sent. As the magnet moves on, the reeds spring apart, and the outgoing signal stops.

Since the distance from the center of the piston to the end of rod tooling is known, the reed switch would be placed on the cylinder barrel in a location that it would be 'made' by the passing magnet when the rod end tooling was in the correct location.

Most cylinder manufacturers manufacture their cylinders with mounting points for reed switches on the outside. If an NFPA type air cylinder, the reed switch might clamp on the tie rod. In Rodless type band cylinders, the cylinder manufacturer will often machine or extrude a slot into the side of the cylinder barrel into which the reed switch can be slid.

Make sure that you discuss mounting options with your switch supplier.

The outgoing signal from the reed switch will not normally be powerful enough to do work, therefore, the signal will usually be sent to a relay in a simple circuit, or to a PLC which can trigger sequential actions.

Of the proximity switches available to the cylinder user, reed switches are normally the lowest cost.

Reed switches are mechanical, and have a life expectancy measured in the multi-million cycles before failure of the reeds. Reed switches are relatively low cost, simple and effective position sensors.

In a high cycle application, or the preference for a totally non-contact position sensing solution, the Hall Effect type of sensor might be selected.

This rather crude drawing depicts a barrel type proximity switch, and the dots represent the "field" generated by the switch.

Note that the Hall Effect switches for cylinders are designed with clamps for tie rods or to fit in slots machined in the cylinder barrel.

Hall Effect type switches have no mechanical moving parts, and as such, will last for many, many millions of cycles. They are more expensive that reed type switches but provide longer life in high cycle applications.

The outgoing signal when the Hall Effect switch is made is low strength, and will need to be amplified in some manner to make it useful.

The barrel prox shown in the drawing is the type that's frequently used to sense the position of the tooling at the end of the rod, rather than a magnet inside the cylinder barrel.

What to do when the cylinder barrel doesn't allow magnetic signals to pass, the environment may be exceptionally dirty, or the application demands position sensing at the tooling rather than from the position of the cylinder piston?

There are many options available to the system designer to help in position sensing their cylinder application.

Other types are optical, and pneumatic sensing devices.

All the various proximity switch or position sensing options have their strengths and weaknesses. There is enough variety that the machine designer should have no problem selecting the sensing option that fulfills these criteria: