One of the prevalent piston position measurement technologies used for mobile hydraulic cylinders is based on the Magnetostrictive properties of ferromagnetic metals. The physics of these sensors is quite complicated, taking advantage of something called the Villari and Wiedemann effects to produce an ultrasonic wave that travels down a hollow tubular waveguide at the speed of sound. A diagram of this physics is shown below.
How Do Magnetostrictive Position Sensors Work?
To measure the position of a piston within a hydraulic cylinder, the cylinder has to be carefully designed to accommodate both the movable position magnet, and the length of the waveguide rod for which a hole must be carefully bored through the center of the cylinder. The location of the position magnet is determined by first applying a current pulse to the waveguide while simultaneously starting a timer. This current pulse causes a sonic wave to be generated at the location of the position magnet (Wiedemann effect). The sonic wave travels along the waveguide until it is detected by the pickup. This stops the timer. The elapsed time indicated by the timer can then be calibrated to represent the position of the piston.
Magnetostrictive Sensor Challenges for Mobile Hydraulics.
Magnetostrictive sensors are highly accurate and are a technology that field mechanics understand how to service. They are designed into many hydraulic cylinder applications including non-automotive vehicular industries like Construction, Drilling, Mining equipment, and long stroke hydraulics.
The problem is, they are one of the most intrinsically vulnerable position sensing technologies for these robust applications. Their high resolution is complete overkill for most long-stroke applications, and their vulnerability increases both costs and complexity of field servicing.
A Drop-In Magnetostrictive Position Sensor Replacement.
We get it: You’ve used magnetostrictive sensors for years; you know them, you like them. When they break or stop operating you know how to replace them.
But what if there was another technology that could be a cost neutral move to providing more robust position measurement in the field? What if this technology was a field proven, drop-in replacement for most existing sensors and electronic interfaces?
In our next blog, we’ll talk about how CPI was able to engineer a superior solution to linear position measurement in large scale mobile hydraulics, and why its time to get out of your comfort zone and try something new.
Magnetostrictive Linear Position Sensors are a common type of position sensor for hydraulic cylinders where high resolution is of primary concern. The sensor incorporates a stainless steel tubular probe (ie. “the Rod”) and is surrounded by a short toroidal magnet installed in a counterbore in the piston. Magnetostrictive sensors can consume a fair amount of power and are not the most mechanically rugged sensor technology. One of the key weaknesses of the solution for heavy duty hydraulic applications is the sensitivity of the hollow waveguide tube when faced with the high temperatures, shock and vibration commonly found in factory or outdoor environments. Even a small deformity of the tube compromises the basic operation of the waveguide and will cause a failure of the sensor.
Magnetostrictive Sensor Field Replacement in Hydraulic Cylinders
Plant managers operating steel mills, Oil & Gas refineries, waste processing facilities or almost any type of large scale manufacturing may have literally dozens of powerful hydraulic systems that employ position sensors. And the larger the stroke length, the more likely that high heat and gravity will make that long magnetostrictive waveguide sag over time until one day when the shaft bends sufficiently and gets hammered into a corkscrew by the retraction of the piston. A plant manager then has to maintain stock in each and every stroke length of sensor used on the premises in order to insure quick replacement is available.