Engineers have developed several ways to detect extension or retraction of a pneumatic cylinder. Anisotropic magnetoresistive and giant magnetoresistive sensors are smaller, faster and reliable alternatives.
The pneumatic cylinder is a key component of automated assembly systems. Their advantages include low cost, simplicity and durability.
To integrate a pneumatic cylinder into an automated system, the controller needs to know the position status of the cylinder. Engineers have developed several ways to detect extension or retraction of a pneumatic cylinder and provide an electrical signal to the control system.
One position sensing technique is to install external electromechanical 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. Another problem is that the external hardware is prone to damage and misalignment due to everyday incidental contact or impact.
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 formed in the body of the cylinder. Magnetic field sensors detect an internal magnet mounted on the cylinder’s 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. There are two common types of cylinder magnets used with magnetic field sensors: axially magnetized magnets and radially magnetized magnets.
The axially magnetized magnet is ideal for actuating most reed switches. When viewed from the side, this magnet has the north and south poles next to each other in the axial plane.
The second most popular cylinder magnet is the radially magnetized magnet, which works well with Hall Effect sensors. Instead of the north and south poles next to each other, one is the inner diameter and the other is the outer diameter. The Hall Effect sensor only looks for a magnetic pole; it does not matter if it is north or south.
Over the years, many machine builders have stopped using reed switches, due to their failure rate. Instead, they use mechanical or inductive sensors to detect pneumatic cylinder position. Anisotropic magnetoresistive (AMR) and giant magnetoresistive (GMR) sensors are smaller, faster, more reliable and easier to integrate. However, they must overcome the stigma left by their predecessors.
Hall Effect sensors consist of a voltage amplifier and a comparator circuit that drives a switching output.
Magnetic Sensor Types
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. However, the magnet must have a strong enough Gauss rating, usually in excess of 50 Gauss, to overcome the return force of the reed elements.
Reed switches are inexpensive, require no standby power and can function with both AC and DC electrical loads. But, 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.
In addition, low-cost reed switches can sometimes deliver multiple switching points as the twin lobes of certain magnets pass by. Reed switches installed in high shock and vibration applications may also exhibit contact bounce or even become physically damaged. In many automated systems, reed switches are a major source of unplanned downtime. Failures often represent a continuous maintenance headache in plants with hundreds of reed switches.
Hall Effect sensors are solid-state electronic devices. They consist of a voltage amplifier and a comparator circuit that drives a switching output.
In a Hall Effect sensor, a steady DC current passes through the thin Hall Effect chip. The distribution of electrons across the element is uniform and the current moves in a straight line, with no potential difference generated at the outputs, which are located on the sides of the chip.
As a radially oriented magnet approaches, the magnetic field is perpendicular to the current flow through the Hall element. The presence of the perpendicular magnet pushes the electrons out of their straight-line path and toward one side of the chip. The imbalance of electron charge creates a potential voltage across the Hall Effect element.
The small micro-voltage that is created is proportional to the strength of the magnetic field. Once the voltage amplitude generated across the chip has satisfied the threshold level of a comparator circuit, the sensor output switches on.
Since Hall Effect sensors are electronic devices, they have no moving parts. Unlike a reed switch, their response time does not depend on magnetic force overcoming mechanical inertia. They operate faster and are more resistant to shock and vibration.
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. As a result, there’s a chance that a Hall Effect sensor will not operate correctly when acti-vated by an axially oriented magnet.
Another concern is that Hall Effect sensors typically have rather low sensitivity; the magnetic field strength must be in the 30 to 60 Gauss range. In addition, 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.
The practical magnetic field strength required to operate an AMR sensor can be as low as 15 Gauss.
AMR vs. GMR Sensors
Engineers also should consider the pros and cons of AMR and GMR sensors. The operating principle of AMR 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.
Compared to Hall Effect sensing technology, which generates a tiny microvolt-level signal, the magnetoresistive element responds with a more robust 3 percent to 4 percent change in bias current. This results in more noise immunity and less susceptibility to false tripping.
Magnetoresistive sensors are about 200 times more responsive than a typical Hall Effect sensor to a given magnetic field strength. The practical magnetic field strength required to operate an AMR sensor can be as low as 15 Gauss. Recent improvements in magnetoresistive technology now allow these sensors to detect both axially and radially magnetized magnets.
In addition to the ruggedness benefits of solid-state construction, AMR sensors offer better noise immunity, smaller physical size and lower mechanical hysteresis (the difference in switch point when approaching the sensor from opposite directions). Manufacturers of magnetoresistive sensors often incorporate additional output protection circuits to improve overall electrical robustness, such as overload protection, short-circuit protection and reverse-connection protection.
Unlike Hall Effect sensors, there are no double switching points, because the higher sensitivity of the magnetoresistive sensor allows it to remain in the “on” state as the low-strength portion of the magnetic field passes under the sensor. Hall Effect sensors, being less sensitive, will often drop out when they see a weaker portion of a magnetic field located between two stronger areas, then switch on again when the field strength increases.
Weld field immune versions can operate reliably in AC welding fields as strong as 200 kiloamperes per meter with no false signals or electrical damage. Many of these welding sensors are available with metallic housings to further guard against hot weld spatter that would melt a plastic-bodied sensor.
The GMR sensor is the most up-to-date magnetic field sensing technology available. Compared to AMR technology, GMR sensors have an even 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, very small-bore cylinders or miniature pneumatic grippers. Advanced output protection circuits, such as overload protection, short-circuit protection and reverse-connection protection, are also available. A