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.
The reed switch is the most simplistic end of stroke sensor available on the market. It has several advantages and even more disadvantages. However before we look at the laundry list of pros and cons let’s review how the reed switch operates.
The reed switch consists of two flattened ferromagnetic nickel and iron reed elements, enclosed in a hermetically sealed glass tube. The hermetically sealed 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 are attracted together completing the circuit.
The pros of the reed switch:
- LOW cost is the number one advantage of a reed switch, which would explain its wide use.
- No power consumption – the reed switch is a mechanical switch that provides a “dry” contact closure.
- It is a two-wire device – it can be used in either AC or DC applications, and it’s easy to integrate into a PLC. All that is required, is to apply your supply voltage on one reed and connect the other reed to the PLC input module or any device that has a low power consumption rate.
- Ummm…well that’s about all of the pros.
The cons of the reed switch:
- Remember that earlier I mentioned it was mechanical… well with mechanical devices and moving components the results are a finite life.
- If you apply a high current load, you will shorten that finite life even more.
- Oh did I mention it was mechanical…well mechanical devices are typically slow; I mean we are talking several milliseconds. Reed switches may not respond fast enough in high-speed applications.
- The magnet used must have a strong enough Gauss rating, usually in excess of 50 Gauss, to overcome the return force or spring memory, of the reed elements.
- In some applications with high vibration and shock, the reed switch may be damaged or exhibit contact bounce.
- Reed switches can also demonstrate double switch points. What’s this? As the magnet, passes underneath the sensor mounted on a pneumatic cylinder, the twin lobes of certain magnets may cause the reeds to open and close.
- Reed switches will not work with a radially magnetized magnet because only one pole of the magnetic field is presented to the reeds and both poles are required to magnetize the reeds.
Reed switches are commonly used in a wide variety of applications. They certainly do have their place; however, they also have caused many production lines large amounts of down time and lost productivity. Next, we will look at some better alternatives for pneumatic cylinder end of stroke detection.
For more information on magnetic field sensors, click here.