Future-proof position measurement in hydraulic cylinders

Supplier: SICK Sensor Intelligence By: Alexander Hirt, Linear Encoders Manager, SICK STEGMANN GmbH, Donaueschingen
16 April, 2018

The new linear encoders in the MAX product family from SICK STEGMANN boast a compact design and compatibility, precision and performance, reliability and raised levels of availability thanks to status monitoring.

The absolute path measurement systems for non-contact and fully integrated position measurement in hydraulic cylinders within mobile machines have been redesigned from scratch. They operate in line with the robust operating principle of magnetostriction, provide extensive diagnosis functions as the first sensors of their kind, and thereby also offer added value for mobile machines that will continue to be relevant as we head into the future.

There is an increase in the demand for linear sensors for mobile automation in general and even more so for hydraulically driven machines. But the quantitative aspect is just one side of the story. More and more vehicle manufacturers, integrators, and operations managers are setting their sights on top-quality innovations. Everyone wants the added value that became standard within industrial automation long ago: the encoder needs to gather status and process information. In future, precise, high-performance position measurement is set to come with the added option of performing detailed analyses based on sensor information. This will make it possible, for example, to provide support for assistance systems, avoid sudden machines failures, optimise maintenance routines, and improve machine availability on the whole. Within the field of hydraulic measurement technology for mobile automation, the linear encoders in the MAX product family are currently the only devices of their kind to offer this added value.

A compatible yet innovative new development

One key feature of the new SICK linear encoders is their full mechanical compatibility with existing cylinder constructions that are already prepared for integration into a path measurement system. Integration into hydraulic cylinders is therefore guaranteed to be a quick and easy process. The product family also offers the most compact linear encoder for hydraulic cylinders around at the moment in the form of the MAX30. The reduction of the diameter of the sensor head to 30 millimetres does not impact negatively on the housing volume, allowing for space-saving integration into hydraulic cylinders that are in turn smaller too.

Inside these linear encoders, you will find a number of different technologies and innovations compared to the market standard. Based on more than 25 years of experience in linear path measurement technology for mobile machines, SICK STEGMANN started off by studying the market requirements and then went on to develop a radical new sensor concept from there. The main features include the use of magnetostriction, optimisation of the power density by minimising measurement dead zones, and integration of diagnosis and prognosis functionalities that can be used to avoid machine downtimes and improve operational plans and maintenance routines for entire fleets.

Non-contact, interference-free, and maintenance-free magnetostriction with micrometre accuracy

Magnetostriction is a time-of-flight measurement method. The distance between two points is measured using the travel time of a short current pulse and its partial reflection within a waveguide. The components of the MAX linear encoder are a basic module with a sender module and signal transducer, a waveguide as the sensor element, and a movable permanent magnet. The emitted current pulse generates a circular magnetic field around the waveguide. The permanent magnet on the cylinder rod acts as the position encoder – its magnet field lines run at a right angle to the pulse-induced magnet field of the waveguide. At the point where the two magnetic fields overlap, magnetostriction brings about elastic deformation in the micrometre range. This creates a mechanical wave that partly continues to travel along the waveguide and is partly “reflected” back to the MAX basic module. When the magnetostrictive wave arrives, the signal transducer uses it to produce an electrical signal. By the known propagation speed of the wave in the waveguide, the time between the current pulse being sent, and the returned magnetostrictive wave being detected, the distance of the permanent magnet is calculated, as is the position of the hydraulic piston in turn, with a resolution in the µm range.

Magnetostriction technology offers multiple benefits when linear encoders are used in hydraulic machines. Position measurement is absolute – the precise piston position is detected as soon as on-board voltage is supplied when the machine is started up. There is no need for a reference run as is the case with incremental sensors. During operation, magnetostriction is exceptionally reliable, wear-free, and even robust, proving particularly resistant to mechanical stress and extreme electrical influences, such as those caused by radiated or coupled faults in the on-board power supply of a vehicle. The high operational safety level of the MAX linear encoder within vehicles that require approval is certified by the E1 approval mark, issued by the German Federal Motor Transport Authority.

Added value with MAX technology

With the MAX product family, SICK STEGMANN is breaking new ground in a number of ways. Regarding product technology, the measured value resolution of 0.1 millimetres given measuring ranges of up to 2.5 meters with the MAX48 and 1.5 meters with the MAX30 is certainly notable. Another special feature of the linear encoder is the significant minimisation of the dead zone that results from the null and cushion zones being reduced down to 30 millimetres. The measuring ranges are much larger than other position sensors available on the market, this allows for better utilisation of the piston stroke in the tight installation space of the cylinder. Last but not least, the MAX sensors are designed for a wider fluid temperature range of between -30 °C and +95 °C.

Real market innovation: the integrated diagnosis functions of the MAX linear encoders for hydraulic cylinders

What wear are steering cylinders and spring systems subjected to; when exactly should lifting and pressing mechanisms in garbage trucks be serviced; what causes a hydraulic cylinder for load torque restriction on mobile cranes to fail; how many operating hours have stroke, swivel, and tilt cylinders on telescopic cranes completed? The MAX linear encoders are the first sensors of their kind in a position to provide valid answers to all of these questions. This issue of status monitoring is nothing new to SICK STEGMANN as a manufacturer – it has not just been among the standard requirements for sensors and sensor systems within industrial applications since Industry 4.0 became a hot topic. Monitoring and counting piston strokes, recording operating hours and peak loads, and checking maximum permitted oil temperatures, are all ways of deriving information about the current status of a machine, its cost-optimised operation, and potential external disturbance variables.

Some manufacturers have already realised that the MAX is offering them a unique way of improving their mobile machines, making them stand out from the competition. Their business partners – those responsible for procurement and operations on behalf of their customers – value the opportunity to reduce operating costs for vehicles fitted with this equipment, while improving their capacity and availability at the same time. Plus, the MAX linear encoders make it possible to automate and monitor work processes performed by mobile machines. When digging, the option to control depth of excavator shovels using position measurement, avoids damage being caused to cables, pipes, and foundations. Excavators can also be fully automated and operated with extreme precision when removing highway embankments. Measuring the piston position for the presses in refuse vehicles reveals the current load. Arrangements can be changed so that a vehicle that is still half-empty is sent to empty garbage cans at further collection points, thereby avoiding half-empty vehicles making inefficient journeys back to the recycling centre.

The list of advantages up for grabs extends to almost all applications within mobile automation. But that is not all! The application possibilities for intelligent, future-proof linear encoders working with magnetostriction are also seemingly endless within industrial settings – the wood industry, rubber and plastics processing, wind energy, packaging technology, and mechanical engineering are just a few examples. These product families are ready to show the world what they can do!