Electronic Controls for Mobile Hydraulics

July 10, 2008
Standard hardware and software are critical for efficient and economical systems.

Andrew J. Smith Brian Van Batavia
Eaton Hydraulics
Eden Prairie, Minn.

Edited by Kenneth Korane

As the mobile-equipment industry evolves toward more-sophisticated systems, it is integrating more electronics with traditional hydraulics to address customer demands for higher performance and greater efficiency. The trend of increasing electronic content in mobile applications has been apparent for years, but initial adoption has been somewhat slower than one would expect.

Two issues that may have kept electronics from becoming more mainstream until now were uncertainty over standards (or lack of standards), and whether an open or proprietary control architecture approach is best.

Open architecture and predefined industry standards are two keys to successful electrohydraulics. And this holds for both software and hardware. It lets engineers choose from a wider range of components when designing machines, and helps ensure interoperability of different components as a system.

Software platform
The first step is a standardized approach to software development, as well as one robust enough to address different application, systems, and user preferences. Eaton engineers recently demonstrated the value of this approach in redesigning a mobile-logging trailer, used to pick up and load timbers for transport. For instance, the IEC 61131-3 programming standard does both, and is becoming increasingly prevalent in mobile applications.

The standard defines common data structures and programming methods that let engineers collaborate on software development. For instance, it provides five graphical and textual languages for programmers to choose from. The choice of languages helps when engineers come from different backgrounds. Someone with industrial experience might use ladder diagram, while a software engineer who knows C programming may prefer structured text.

IEC 61131-3-compliant programs are completely scalable and can be updated or enhanced as cirneeded. The standard approach also helps programmers create high-quality code with fewer defects, as the programming system itself alerts the user to syntax and other errors.

An Eaton EFX1624 electronic-control unit (ECU) controls the trailer hydraulics. The ECU reads inputs from valves, joysticks, and sensors; executes control logic; and delivers outputs. Run time for this program is typically about 5 msec.

Engineers used Eaton’s Control F(x), a standard platform based on IEC 61131-3, to create the software that coordinates system functions. Control F(x) offers several programming languages: ladder diagram, instruction list, structured text, sequential-function chart, function-block diagram, and continuous- function chart — each with certain advantages. For this system, the control strategy relies on continuous input signals processed by the control algorithm, which produces continuous outputs. To accomplish this, the continuous function chart language was chosen as the primary programming language for nearly all modules.

One significant strength of Control F(x) programming is its ability to save, modify, and reuse previously developed logic, which helps speed control design. Taking and proven logic not only increases programming efficiency, but reliability in the field as well.

Programs are easily captured in a library for subsequent use in other applications. One example is a hardware library that accompanies each EFX controller. It contains function blocks that let users quickly set up the controller, for instance, initializing hardware such as the CANbus interface. Other libraries handle interface details between the controller and a growing list of complementary products, including control valves, joysticks, switch modules, and displays. Well-tested and documented function blocks let users concentrate on applications, rather than worrying about specific details of a hardware interface.

And general-purpose function blocks have been created to perform routine control tasks. For example, every work circuit must translate user input to valve flow through a “gain” factor. The specific value used on a particular machine might be unique, but the general algorithm inside each block is preprogrammed and only needs to be tweaked for each circuit. This minimizes the effort required to test and validate control software and improves maintainability of the code.

Open architecture
Working hand in hand with the IEC standard is the concept of open architecture — building components that understand and run on common software. Although some suppliers still champion proprietary solutions that lock users into their hardware, a strong trend is running in favor of standardsbased products.

Using IEC 61131-3 along with an open architecture lets users combine different components from various manufacturers — each best suited to a particular task — to design a system. There are no restrictions, any standard device can be chosen. And an open architecture generally means wider variety and lower costs, compared with proprietary systems.

Yet open architecture gives developers the flexibility to customize applications to suit an OEM’s needs. Although the same basic device works on any machine, suppliers can differentiate performance via refined onboard electronic controls.

For example, the trailer has an articulated robot arm with three joints and a grapple end-effector. Each arm link has a hydraulic actuator, and all actuators connect to an Ultronics ZTS16 mobile control valve. It uses open architecture, onboard electronics for valve control, and lets users communicate with and configure the valve as needed.

The valve has a pressure-conditioning input section, followed by six configurable work sections. Each has a mechanical pressure relief that regulates maximum pressure in each work circuit. All valve sections are spring centered to block flow to or from the doubleacting cylinders in the absence of a valid command signal.

Distributed control
The Ultronics valve provides “inner-loop” flow control through each work circuit. Each valve section contains a DSP that executes proprietary control algorithms, letting it work as a stand-alone system capable of wide-ranging, application-specific control tasks.

In the logging trailer, the valve inlet senses the load on each work section and controls pressure to meet instantaneous system requirements. This helps maintain optimum efficiency by ensuring power consumed is only that required at any given moment.

The ECU provides “outer-loop” control of system movement. It interprets user inputs and sends appropriate commands to the valves. The ECU also monitors feedback information and takes appropriate action when necessary, and sends data to the display panels.

Software-based electrohydraulics also lets the operator select two distinct operating modes: open or closed loop. Open-loop control routes operator inputs directly to the valves, meaning the user controls system movements. With closedloop control, the system controller manages flow and motion.

In open-loop mode, one can adjust the relationship between user input and resulting flow (flow gain). The system has three preset gain settings: off, economy, and power. The off setting forces all flow commands to zero regardless of joystick input. Economy produces 20% of maximum system flow for a corresponding maximum joystick input command. The power setting gives 80% of the maximum flow at maximum input.

Users can also modify the preset gains in increments of 1% via rocker switches. And an “inching” control produces work-circuit flow that is 1/10th the corresponding joystick input.

In closed-loop mode, the operator simply presses a button on the joystick and the ECU automatically directs the machine to a preprogrammed position. The controller processes position feedback from proportional proximity sensors and adjusts machine position by changing valve flow. Mechanical limit switches prevent unwanted movements.

Specialized control
One advantage of using software to control hydraulics is that algorithms are easy to change and adjust. For example, even though it is possible to build a proportionalcontrol circuit using only hydraulic and electronic components, software is much more easily adapted — making it the logical choice for advanced control. Straightforward reprogramming can change the system from a proportional controller to a proportional-integral controller, for example.

Software is also generally cheaper to develop, because control algorithms are readily copied. Finally, as programs grow more complex, algorithms can be described using a combination of symbols and text, and quickly tested. Several functions of the timber trailer are particularly suited to this type of programming.

Single degree of freedom, closedloop control. One advanced feature is the ability to control the absolute position of a machine axis. Each of the arm’s three axes has sensors that provide angular position feedback over a range of motion of about 180°.

Properly calibrated, the system software can determine position and control the orientation of the three primary machine joints. For instance, the operator could position the machine by simply entering the desired joint angles via the control panel.

Perhaps the best use of this feature is a simple “home” function that moves the machine to a preset position from any arbitrary starting point, without operator input. This automatically returns a machine to a “ready” position with minimal operator effort and maximum efficiency. It can significantly reduce operator fatigue and increase reliability and safety. What may not be as obvious is the ability to easily adapt the machine to different applications. For example, the arm’s speed could be adjusted to suit a particular material by simply changing a few software parameters. This lets identical mechanical and electronic components produce significantly different results.

Multiple degree of freedom, coordination control. This type of control would also be extremely difficult with only hydraulic components and electrical circuitry. Again, software opens up a broad area of advanced capabilities that enhance value to the OEM.

One significant example is coordinating several axes to precisely control the motion path, such as in a common telescopic lifting system. On these vehicles, one hydraulic circuit typically controls the boom angle with respect to the ground; another circuit controls the boom extension; and yet another controls the tool angle at the end of the boom. These vehicles are commonly used to move materials to and from stacked shelving.

Without servohydraulics, the operator must precisely coordinate several input devices simultaneously to keep the load parallel to the ground and moving along a path that’s most efficient in terms of speed and power consumption. However, software can reduce user input to a simple task, such as moving a joystick in one direction, while the software monitors machine position and ensures proper end-tool orientation.

In more advanced versions, a machine could be programmed to automatically lift materials to their intended spot when the geometries of the shelving and material are known. Or, if the machine commonly moves through a repeatable path, like hydraulic arms that load railcars or cargo ships, software can also automate these movements. The range of applications that could benefit from software controls is practically limitless.

On the trailer, new capabilities are now available at the flip of a switch, and users of different skill levels can efficiently operate the vehicle. The electrical and hydraulics components retain a standard interface but, thanks to modular and scalable open architecture, the on-board electronics and control software differentiate the system from competing products.

Environmental Resilience

Mobile equipment typically faces harsh environments, a fact that has to some extent restricted the use of electronics in control systems. This obviously must be addressed before electronics can be used with confidence.

Electronics must be protected if they are to enhance mobile applications. Shock and vibration protection are obvious requirements, but just as important is how enclosures and housings protect against dust, dirt, humidity, and fluids.

Ingress-protection IP67 rating has become the expectation for mobile electronics. This rating translates to a total resistance to dust and temporary immersion in water. Some companies are setting their sights on IP68 and IP69K-rated components that protect against indefinite submersion to 10 m and withstand high-pressure washes to 140 bar.

The Ultronics ZTS16 mobile valve features twin spools and independent meter-in and meter-out flow control. Integrated pressure and spool-position sensors coordinated with open-architecture, onboard electronics provide closed-loop control as well as diagnostics for efficient troubleshooting. The valves have up to six independent sections rated at 300 bar and 130-lpm flow.

EFX electronic controllers are programmed with Control F(x) software, based on the IEC 61131-3 standard. The ECUs offer analog and digital I/O capabilities, pulse-width modulation, and current-controlled outputs. The units support CANopen and J1939 communication protocols and resist harsh environments with an IP67 rating.

A logging trailer upgraded with advanced electrohydraulics showcases the advantages of supplementing hydraulic components with electronics and software. Rather than relying on highly skilled operators, the new system offers features such as open and close-loop control, economy and power settings, and automatic homing.

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