A number of questions arise when operating mobile machines: What kind of wear are steering cylinders and suspension systems subject to? When is the right time to service lifting and press mechanisms on refuse collection vehicles? What peak loads are the hydraulic cylinders exposed to? What is the cause of a sudden failure? Has the maximum permissible oil temperature been observed? How many operating hours do lifting, slewing and tilting cylinders on telescopic cranes have?

The measured values and data of the linear encoder 'Max48' open up the possibility of automating individual work processes of mobile machines, for example to monitor the working depth during excavation.

© Sick

The 'Max48' from Sick can answer such questions with its diagnostic functions without incurring additional costs. The ability to carry out comprehensive condition monitoring of all aspects of the work processes of mobile machines via a non-contact positioning system in the hydraulic cylinder is a first in this sensor and application segment. The linear encoder counts switch-on cycles and operating hours, records the quality and consistency of the electrical supply voltage, measures the temperature of the hydraulic fluid, adds up the travel of the hydraulic cylinder and collects this data in an internal diagnostic memory. This provides valid information on the operating status, capacity utilization, operating conditions and possible causes of hydraulic cylinder failure.

The diagnostic data is collected as follows: At regular time intervals, the electronics of the linear encoder update the number of operating hours, the applied supply voltage, the temperature of the electronics (as a 1:1 indicator of the oil temperature in the cylinder), the number of switch-on cycles and the piston travel covered. The data is only updated if previous peak values are exceeded. This is logical for the operating hours, the switch-on cycles and the piston travel - these data cannot become less. Supply voltage and oil temperature are only overwritten if a new peak value has occurred since the last update cycle. So if the current values are lower, the higher old values are retained. This provides service personnel with valid information about the maximum load on the cylinder during operation during maintenance or repairs. If, for example, there is wear on sealing elements, this can be attributed to excessive operating/oil temperatures and used to make a warranty/goodwill/invoice decision to the user. The data is read out using an external diagnostic device, similar to that used for servicing cars. A version of the linear encoder with CANopen makes it possible to store the process information directly in the vehicle control unit and retrieve it from there. The evaluation can then be carried out in the vehicle manufacturer's central service tool, for example.

The measured values and data from the linear encoder enable both manufacturers and operators of agricultural and construction machinery, logistical lifting and conveying equipment, municipal vehicles and airport equipment to improve capacity utilization and availability and to automate and monitor individual work processes of mobile machines.

Based on magnetostriction

The non-contact position sensor consists of a magnetostrictive measuring element and a permanent magnet integrated in the piston. Its measuring range extends from 50 to 2500 mm with a measured value resolution of 0.1 mm. The sensor can be fully integrated, with no externally visible parts that are at risk of wear or damage. Mechanically, the linear encoder is compatible with hydraulic cylinders for mobile machines that are prepared for the integration of position detection. Modifications to existing cylinder designs are therefore not necessary. For electrical integration, the encoder is equipped with an analog interface as well as interfaces for CANopen, SAEJ1939 and PWM.

The linear encoder is based on the active principle of magnetostriction, a time-of-flight measuring method. The distance between two points is measured using the transit time of a short current pulse and its partial reflection within a waveguide. The emitted current pulse generates a circular magnetic field along the waveguide. A permanent magnet on the cylinder rod is the position sensor - its magnetic field lines run at right angles to the pulse-induced magnetic field of the sensor's waveguide. Where the two magnetic fields overlap, magnetostriction creates a micrometer-fine, elastic deformation. This generates a mechanical wave, part of which travels further along the waveguide and the other part of which is 'reflected' back to the base assembly of the sensor. When the torsional impulse arrives, the signal converter generates an electrical signal from the magnetostrictive wave. From the known propagation speed of the wave in the waveguide and the time between the transmission of the current pulse and the detection of the returning magnetostrictive wave, the distance of the permament magnet and thus the position of the hydraulic piston can be determined with a resolution in the µm range.

This technology offers various advantages for the use of linear encoders in hydraulic machines. The position measurement is absolute: the exact piston position is known immediately after the on-board voltage is applied by starting the working machine - a reference run as with incremental sensors is not necessary. During operation, magnetostriction also proves to be particularly robust against both mechanical stresses and extreme electrical influences, for example from radiated or coupled interference in a vehicle's electrical system.

Constructive details

With linear encoders such as the 'Max48' and its 'little brother' 'Max30', smart sensors are finding their way into mobile automation. The 'Max30' absolute displacement measuring system for non-contact position measurement in hydraulic cylinders has a measuring range of up to 1.5 m, while the 'Max48' can measure up to 2.5 m.

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In the 'Max48', the installation space for the electrical cables has been reduced and the metrological damping zone has been shortened to 30 mm. This makes better use of the available mounting space in the cylinder and reduces the costs for drilling the piston rod. During operation, the reduced damping zone ensures better kinematic and metrological utilization of the piston stroke. Thanks to intelligent material selection, the encoder's position magnet, unlike other linear sensors, does not require an additional spacer disk - meaning that cylinder manufacturers need to install and stock fewer parts.

The sensor is designed for oil temperatures in hydraulic cylinders from -30 to +95 °C and operating temperatures up to +105 °C and for nominal operating pressures of up to 400 bar. The shock and vibration resistance meets the tough requirements of mobile machinery. In addition, the linear encoder is interference-proof against electrical pulses in accordance with the basic standards for electrical systems and offers high resistance to interference pulses when charging vehicle batteries. Coupled electrical pulses, which can be transmitted through the vehicle electrical system when starting vehicles with combustion engines and alternators, do not impair the function of the sensor. As the linear encoder is specified in accordance with the EMC requirements of mobile machinery, manufacturers who produce mobile machines with electric and combustion engines can use the same linear encoder for both types of drive, thus reducing the variety of types in stock and service.

Author:
Peter Feucht is Product Manager Mobile Hydraulics Linear Encoders in the Global Business Center Motion Control Sensors at Sick Stegmann in Donaueschingen.