Adding position feedback to a hydraulic cylinder provides several benefits which include increasing the efficiency of a process, automating a function, and adding safety to a machine. Most manufacturers of cylinder sensing products offer both discrete and proportional outputs to achieve the cylinder feedback required of the application. Of the proportional types, there’s been a few technologies utilized through the years which include resistive potentiometers, glass scales, linear Hall effect, optical readers, linear variable displacement transducers (LVDT’s), and magnetostrictive transducers. Of these many technologies, magnetostriction continues to be the technology of choice for many absolute position feedback applications due to its non-contact sensing, repeat accuracies, linearities within a few micrometers, and robust mechanical assemblies.
The phenomenon of magnetostriction was first discovered by James Joule in 1842. Joule found that a ferromagnetic material, such as an iron rod, would change dimensions slightly when subjected to a magnetic field. Today’s magnetostrictive transducers use special ferromagnetic alloys and utilize Joule’s effect as a position marker. Additional electronics, including time-of-flight circuitry, are then used to define the position and/or velocity of the marker. While the technology of the magnetostrictive transducer is sophisticated today, the general principal remains the same and is well proven.
Magnetostrictive transducers are widely used in steel mills, sawmills, tire manufacturers and many other industrial processes. They are also widely used in mobile equipment in industries such as construction, agriculture, and rail maintenance of way vehicles.
One strong application for cylinder feedback in mobile equipment is for operator safety. Large mobile elevated work platforms (MEWPs, aka boom lifts, man lifts, cherry pickers, etc.) do not utilize outriggers to stabilize the machine due to the machine’s ability to drive while the basket (and operator) are at height. These machines are also likely to be rented, leaving the skill of the operator in question. A quality cylinder transducer provides precise position feedback to the electronic control module which determines if the operator is approaching an unsafe working condition. One such scenario is when the boom is at 45 degrees and telescoping further out from the side of the machine. In this case, the joystick controls will limit the operator inputs to keep the machine from extending any further out, keeping the machine within a predetermined working “envelope.” Another popular application would be as a memory function. A good magnetostrictive transducer will allow the operator to “teach” a specific position. The operator can return to the programmed position automatically. Memory functions are useful for repeat actions such as returning a bucket to a specific height. If trucks to be filled are all the same height, the memory function can save time and reduce mishaps, allowing the operator to concentrate on other functions such as turning and driving. In the rail industry, maintenance of way machines uses magnetostrictive transducers to determine the depth of hydraulic tines that are used to compact ballast, or to raise the track to a specific height.
No matter what the application, when reliable feedback of a cylinder is needed, magnetostrictive transducers provide reassuring feedback on mobile machines, even in harsh conditions.
But not all magnetostrictive transducers are found within a cylinder housing. Some manufacturers offer both internal and external products. The arguments for an internal approach center around added protection for the transducer from rocks, dirt, heat, etc., while advocates for an external approach speak of less downtime in the event of a transducer mishap, and the reduced costs and delivery times of using a standard cylinder. A reputable manufacturer with technical experts can help guide your choice.
Whether internal or external, industrial or mobile, the phenomenon of magnetostriction will continue to be the technology chosen for reliable, accurate detection of hydraulic cylinders.
