Disruptive sensor for smart hydraulics p. 112 Synchronization with multi-axis controller p. 118 Industrial compressed air best practices p. 124

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Mobile versus PAGE 106

industrial hydraulics PAGE 106

Coveragepages 37-76

C Y L I N D E R S E N S O R SC Y L I N D E R S E N S O R SC Y L I N D E R S E N S O R S

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Alexander Bertsch • Head of Product Line Sensor TechnologyLiebherr Elektronik GmbH | Lindau, Germany

• 2020 www.fluidpowerworld.com

• Head of Product Line Sensor Technology

The adoption of elec tronic c ontrols into mobile eq uip ment has led to game-changing producti vity enhancements precision excavati ng and grading, remote maintenance capabiliti es and autonomous machine operati on, ust to name a few. ut for such systems to operate seamlessly, the sensors in components like hydraulic cylinders must supply data that are precise and reliable.

Modern measurement systems in mobile machinery face daunti ng challenges like severe vibrati on and extreme temperatures. ased on extensive know-how with demanding operati ng conditi ons, Liebherr has developed LiView, an innovati ve stroke transducer for hydraulic cylinders that uses the piston itself as a robust sensing element. The new measurement concept can be easily integrated into cylinders large and small, is universally applicable and rugged and reliable. It is parti cularly advantageous in heavy-duty applicati ons because no moving parts that serve as sensing elements are installed inside or outside the cylinder.

System basicsLiView technology measures the physical state of a hydraulic cylinder by making real-ti me electrical measurements of its mechanical structure to determine cylinder positi on and velocity. The basic confi gurati on involves two probes mounted in the piston rod bearing that are connected to an electronics module via a

conventi onal coaxial cable. Each measuring cycle, the electronics generates a predefi ned high-frequency electromagneti c signal that the probe at one port in ects into the cylinder structure. The transmitt ed signal propagates to the piston, where it is refl ected back and intercepted at the second probe port. This analog signal is digiti zed in the electronics and processed to calculate positi on. The working principle is based on viewing the cylinder as a multi -port electrical system that can be thoroughly characterized by means of its of scatt ering parameters a technique that is used in high-frequency electronics for circuit characterizati on. The two probes, termed port and 2, are used for in ecti ng a sti mulus at a given frequency and simultaneously receiving the cylinder response. rom an electrical point of view, the system is fully described by a 3 3 scatt ering matrix, which defi nes the relati onship between in ected and refl ected electromagneti c waves at either of the two ports. The transmission factor between port and port 2 relates to the impedance of the cylinder tube that can be modelled as a transmission line. Piston positi on defi nes the length of this transmission line, producing a direct relati onship between the phase of the electrical signal and positi on informati on. Liebherr engineers have developed a mathemati cal model that precisely desc ribes the elec tric al resp onse of the hydraulic cylinder.

The LiView is an innovative position transducer for hydraulic cylinders that uses the piston itself as a sensing element.

A disruptive sensor forintelligent hydraulics

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LiView positi on transducers are built to withstand extreme environments and severe operati ng conditi ons typical of mobile machinery.

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LiView positi on transducers are built to withstand extreme environments and severe operati ng conditi ons typical of mobile machinery.

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It’s used to transform the measured scatt ering parameters and calculate absolute piston positi on. This makes LiView an absolute stroke transducer, allowing the calculati on of positi on at each new measurement cycle and without requiring starti ng-positi on informati on.

LiView componentsAs menti oned above, LiView consti tuent parts include the electronics unit and two probes. It also has a high-frequency passive element that operates as a guard ring isolati ng the cylinder cavity from the external environment. The guard ring ensures a stable boundary conditi on for electrical measurements. The LiView Electronics Module (LEM) mounts either on the cylinder or on the surrounding structure, depending on applicati on constraints. The only caveat is that cable length should be limited to not degrade signal characteristi cs by att enuati on. Typical maximum suggested length for opti mal performance is 2 m, although longer cables are possible if proper att enuati on characteristi cs are specifi ed. Two diff erent secti ons make up the LEM. A high-frequency front-end generates the signals used for cylinder measurement, acquires the cylinder response and digiti zes it. The digital back-end applies algorithms to calculate positi on and velocity, and runs safety surveillance functi ons. Each single measurement from signal generati on to processing takes approximately 300 µsec. This allows real-ti me positi on measurements in high-dynamics applicati ons with very low latencies compared to most other established measuring devices. Each measuring cycle is repeated at diff erent frequencies based on specifi c cylinder characteristi cs, thus providing redundant positi on informati on that can be used to compensate for instabiliti es and other eff ects. The LEM is considered a universal measuring device. Calibrati on parameters for diff erent cylinder types are generated offl ine and downloaded during initi al set-up. Cylinder length has no impact on system calibrati on; parameterizati on mainly depends on the rati o between piston and rod diameters.

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The system includes an electronics unit, two probes and a guard ring. Each measuring cycle, a signal generated at one port refl ects off the piston and is intercepted at the second probe port. This analog signal is digiti zed in the electronics and processed to calculate positi on.

LiView involves making real-ti me electrical measurements of a cylinder’s mechanical structure to determine piston positi on and velocity.

Guard ring

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The LiView probes ensure capaciti ve coupling between electronics and piston rod. They are integrated in the high-pressure side of the hydraulic cylinder rod bearing, behind the primary seal. The probe’s contact element rides on a fl oati ng bearing. It ensures a stable electrical connecti on in a dynamic system, permits relati ve displacements along all three main axes and prevents wear to both the contact element and piston rod. The probes handle multi ple functi ons. They permit a sealed high-pressure cable feedthrough for routi ng a coaxial cable inside the cylinder structure. They couple the signal into the cylinder inner cavity. And they compensate for relati ve displacement between piston rod and rod bearing, thus ensuring opti mal signal transmission during cylinder operati on even in harsh environments. This is essenti al for heavy-duty applicati ons, where strong lateral forces can conti nuously stress a cylinder.

Real-world considerations An important factor is sensiti vity to oil instabiliti es. Measurement accuracy is infl uenced by the dielectric characteristi cs of the cylinder subsystem. Any change in oil permitti vity (ε) due to temperature, pressure and aging eff ects on the oil or its consti tuents will aff ect dielectric characteristi cs. Pressure variati ons inside the cylinder will produce positi on-dependent changes within a few msec, while aging

might impact measurements over weeks or months. That’s important because any parameter infl uencing the oil dielectric constant infl uences the propagati on speed of electric signals inside the cylinder cavity. A change in signal speed generates a phase error of the measured signal, thus producing a systemati c error on the calculated positi on. This error depends on the length of the travel path inside the dielectric. Techniques that counteract this behavior are essenti al for acceptable performance in real applicati ons. Correcti on algorithms that run in real ti me during every measurement cycle compensate for temperature, pressure and aging eff ects of the hydraulic

fluid. As a result, LiView has a total absolute accuracy specification of 0. (peak-to-peak) or 0.3 , which represents the maximum expected absolute accuracy error. These values confirm the suitability of the stroke transducer for mobile machine applications.

epeatability of the measured position is another important performance parameter in real applications. epeatability, o en referred to as precision, is evaluated as the total deviation (minimum vs. maximum) recorded at each position during dynamic cylinder operation.

or conventional stroke transducers the stated performance typically refers to ideal testing conditions in a laboratory. This is not possible with the LiView system, because measurements require the presence of the cylinder structure, which represents the ma or source of inaccuracies. These include both dielectric constant changes of the fluid and from displacements and deformations induced by forces and temperature gradients acting on the cylinder during operation.

e performed field tests on various types of mobile machines in different environments, to evaluate overall system performance. e tracked the precision on a plunger cylinder of a mobile machine where working conditions included varying temperatures and loads. ata on dynamic cylinder performance were compared to a wire potentiometer (including its inaccuracy). esults confirm signal precision well below 2 mm peak-to-peak over the ma or part of the cylinder stroke.

Functional Safety LiView also meets demanding functional safety requirements. It has been developed to fulfill Performance Level d ( I E I 3 ) and

IL 2 ( I E I 0 ) requirements. The main di culty with our approach relates to the intrinsic different nature of the LiView sensor concept compared to conventional transducers. The latter are based on a well-defined measuring path. ( or example, in magnetostrictive transducers the measuring path is encapsulated within a predefined

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The LiView system, shown here on an excavator arm cylinder, is available to both cylinder manufacturers and EMs. It’s currently being tested in various applications, including shredders, agricultural equipment and material-handling machines. eries production of LiView-equipped crawler tractors and loaders is set to begin shortly.

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metal bar that c an be p rec isely manuf ac tured and c ontrolled by the sensor p rovider. ) I n c ontrast, L iV iew p erf orms elec tric measurement of an external c y linder struc ture, not ty p ic ally under c ontrol of Liebherr. hile this set-up off ers advantages in terms of robustness and servic eability in the fi eld, compared to other opti ons, it introduces additi onal complexity for safety. The main ob ecti ve of the safety functi on is to ensure the measured positi on is correct. It is addressed by introducing surveillance functi ons operati ng at the component and sy stem levels. At the component level, the LEM verifi es signal characteristi cs that are produced and measured. T hat inc ludes hardware and so ware monitoring functi ons. ardware tasks range from tracking circuit-board temperature to monitoring all power supplies and core hardware elements through a complex surveillance circuit that includes a watchdog. o ware monitors analyze both the generated and acquired signal characteristi cs. The fi rmware and all data-processing algorithms run on a lockstep processor with two independent computati onal cores that are conti nuously compared through the hardware surveillance functi on. This identi fi es and c omp ensates f or most p ossible errors, or notes that they require countermeasures.

S y stem- level saf ety is imp lemented as a surveillance algorithm running in the LEM. It checks cylinder response to a known sti mulus and compares the measured scatt ering parameters to the expected ones. C y linder behavior c an be p redic ted with a high degree of accuracy on the order of

. Measurements at diff erent frequencies produce intrinsic redundant informati on. The scatt ering parameter measuring technique conti nuously monitors the probe cables’ state and compensates for dri s due to mechanical, temperature and aging eff ects. This is p erf ormed every c y c le. I n c ase of anomalies, positi on informati on is fl agged as potenti ally dangerous but is sti ll broadcasted on the bus.

Condition monitoringO ther imp ortant f eatures of L iV iew inc lude conditi on monitoring capabiliti es and reliable fault diagnosis. Thanks to its permanent self-monitoring, LiView identi fi es any deviati on between its current and opti mum state. Thus, the sy stem reliably detec ts f aults suc h as undervoltage or defecti ve probes. I t also has an internal memory that saves previously diagnosed faults. The content of this memory is transmitt ed via the standardized diagnosti c communicati on protocol ( nifi ed iagnosti c ervices) as specifi ed in automoti ve standard I 22 . The fault

memory is easily read out with a standard off -board diagnosti c tool, or downloaded with so ware Liebherr can supply to EMs. F or users, L iV iew allows a detailed diagnosis of system faults for the fi rst ti me at the sensor level. B ec ause L iV iew already detec ts the c ause of the f ault, the O E M does not have to c onduc t a c omp lex and exp ensive fault analysis. This simplifi es and accelerates ap p rop riate rep airs, avoids extended downti me and increases e ciency. Cylinder manufacturers also benefi t. As soon as the intelligent positi on transducer is integrated into the cylinder, conditi on monitoring can check for proper functi oning. The basic idea of using the cylinder as sensor is a promising approach for many heavy-duty applicati ons, where both sensor performance and robustness are criti cal and for which conventi onal sensors are not a viable opti on. The limited envelope required for the integrati on of the probes in the piston rod bearing makes LiView suitable for small-diameter c y linders. Moreover, this innovati on opens a new approach to system-level state monitoring. by providing important cylinder state informati on to higher-level controls. FPW

Liebherr | www.liebherr.com/liview

LiView as a disruptive technologyLiebherr engineers have noted the main error sourc es caused by changing oil dielectric properti es and developed suitable compensati on strategies. owever, cavitati on is also frequently encountered in real applicati ons. In a bucket cylinder for instance, an abrupt variati on of the mechanical load produces a f ast p iston- rod disp lac ement that the hy draulic sy stem c annot follow with su cient oil fl ow. As a consequence, fl uid pressure suddenly drops and bubbles or voids form in the fl uid. LiView is able to detect this conditi on because it is extremely sensiti ve to changes in fl uid electrical properti es that aff ect measuring accuracy. The presence of air in fl uid impacts the dielec tric c onstant of oil and would result in an undetec ted positi on error. urveillance algorithms identi fy this in real-ti me. The system notes a diff erence between measured and

predicted positi ons and generates a signal that correlates to the presence of air inside the cylinder chamber. The LEM generates a warning signal and provides important informati on to the machine c ontroller, and thus sup p orts more- stable hy draulic s c ontrol. This capability can also be exploited to generate state informati on on the component and hydraulic system that includes the cylinder, which would not be possible using conventi onal technologies where the sensor is a well-confi ned subsystem. In this context, LiView can be understood as a technology pla orm allowing real-ti me monitoring of the cylinder system state. The technology is based on complex measuring hardware, on which so ware functi ons can be added to implement new functi ons and provide further insight on the cylinder health.

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