Today’s pneumatic cylinders are compact, reliable, and cost-effective prime movers for automated equipment. Unfortunately, they are often provided with unreliable reed or Hall Effect switches that fail well before the service life of the cylinder itself is expended. Too often, life with pneumatic cylinders involves continuous effort and mounting costs to replace failed cylinder position switches. As a result, some OEMs and end users have abandoned magnetic cylinder switches altogether in favor of more reliable — yet more costly and cumbersome — external inductive proximity sensors, brackets, and fixed or adjustable metal targets. There must be a better way!
One position sensing technique is to install external electro-mechanical limit switches or inductive proximity switches that detect metal flags on the moving parts of the machine. The disadvantages of this approach include the cost and complexity of the brackets and associated hardware, the difficulty of making adjustments, and the increased physical size of the overall assembly.
A more popular and widely used method is to attach magnetically actuated switches or sensors to the sides of the cylinder, or into a slot extruded into the body of the cylinder. Through the aluminum wall of the pneumatic cylinder, magnetic field sensors detect an internal magnet that is mounted on the moving piston. In most applications, magnetic sensors provide end-of-stroke detection in either direction; however, installation of multiple sensors along the length of a cylinder allows detection of several discrete positions.
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. The glass tube is evacuated to a high vacuum to minimize contact arcing. 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. The magnet must have a strong enough Gauss rating, usually in excess of 50 Gauss, to overcome the return force, i.e. spring memory, of the reed elements.
The benefits of reed switches are that they are low cost, they require no standby power, and they can function with both AC and DC electrical loads. However, reed switches are relatively slow to operate, therefore they may not respond fast enough for some high-speed applications. Since they are mechanical devices with moving parts, they have a finite number of operating cycles before they eventually fail. Switching high current electrical loads can further cut into their life expectancy.
Hall Effect sensors are solid-state electronic devices. They consist of a voltage amplifier and a comparator circuit that drives a switching output. It might seem like an easy solution to simply replace reed switches with Hall Effect sensors, however, the magnetic field orientation of a cylinder designed for reed switches may be axial, whereas the orientation for a Hall Effect sensor is radial. The result? There is a chance that a Hall Effect sensor will not operate properly when activated by an axially oriented magnet. Finally, some inexpensive Hall Effect sensors are susceptible to double switching, which occurs because the sensor will detect both poles of the magnet, not simply one or the other.
Today, solid-state magnetic field sensors are available either using magnetoresistive (AMR) or giant magnetoresistive (GMR) technology. Compared to AMR technology, GMR sensors have an even more robust reaction to the presence of a magnetic field, at least 10 percent.
The operating principle of AMR magnetoresistive sensors is simple: the sensor element undergoes a change in resistance when a magnetic field is present, changing the flow of a bias current running through the sensing element. A comparator circuit detects the change in current and switches the output of the sensor.
In addition to the benefits of rugged, solid-state construction, the magnetoresistive sensor offers better noise immunity, smaller physical size, less susceptibility to false tripping, speed and lower mechanical hysteresis (the difference in switch point when approaching the sensor from opposite directions). Quality manufacturers of magnetoresistive sensors incorporate additional output protection circuits to improve overall electrical robustness, such as overload protection, short-circuit protection, and reverse-connection protection. Some manufacturers also offer lifetime warranty of the sensors.
Over the years, many users have abandoned the use of reed switches due to their failure rate and have utilized mechanical or inductive sensors to detect pneumatic cylinder position. AMR and GMR sensors are smaller, faster, and easer to integrate and are much more reliable however; they must overcome the stigma left by their predecessors. With the vast improvements in sensor technology, AMR and GMR sensors should now be considered the primary solution for detecting cylinder position.