By Simon Yardley - Director of Strategic Business Development & Ben Holter - System & Automation Engineering Manager, Husco
The project aim was to design a system that enabled automated and digital electrohydraulic control of any pilot-operated excavator. The system would have to integrate with the existing hydraulic circuit on any machine and not replace the main control valve. Our performance target was to produce a full-speed automated grade with a precision of ±25 mm. And the number-one deliverable had to be safety.
Husco designs and manufactures hydraulic components ranging from small solenoids to large main control valves and complete turnkey sub-systems. Husco is a direct supplier to OEMs like Caterpillar, John Deere, Ford and General Motors. Everything Husco produces is customized, so we have very deep engineering relationships with all our OEM partners.
Problems with Productivity in Construction
The problems that Husco’s OEM customers and the entire industry face today have been anemic gains in construction productivity. While productivity of industrial manufacturing has doubled in the last 20 years, thanks to innovations like robotics and automation, the construction industry has seen only small gains.
One reason is the lack of skilled labor. Hiring reasonably priced skilled labor is a real challenge worldwide. Low productivity growth also increases pressure on capital utilization. On a macroeconomic scale, a one percent increase in productivity brings a $100 billion global annual saving to the construction industry. So small gains have big effects.
On the positive side, there is greater acceptance of technology on the worksite: drones for surveying, remote monitoring on sites, RFID tracking of equipment and materials, and use of building information modeling (BIM) on projects are all on the increase.
End users are embracing these technologies and are demanding more from their systems. Consider the effect of adding an electrohydraulic valve to a motor grader to automate blade tip control. If that increases working speed from three to six mph and saves one pass, that's worth a lot.
Control Challenges Husco has been successful helping OEMs upgrade to electrohydraulic (EH) systems. On dozers Husco supplies the top three manufacturers globally with electrohydraulic systems, replacing the pilot and mechanically operated system that proceeded them, on motor graders it is a similar story. In this project the challenge was to develop a retrofit system suitable for the next primary piece of equipment - the excavator, all to enable productivity gains to be made and help users improve their ROI. See Figure 1. for an example of one of the control experiments that was used.
Our test machine was a standard 21-ton excavator, a four-year old rental machine that’s seen 4,000 hard operating hours. It had a negative-control hydraulic system and was a good representation of machines found on a typical excavating site. Probably 95 percent of all excavators in the 15 to 70-ton range have pilot-operated systems. Not many EH systems are in use. How do we bring robot-type kinematic control to these machines? Industrial robotic end-effector positioning using G-code for programming and autonomous control with servo-actuators is mature technology. But with one motor per effector on the robots, positioning is relatively easy from a geometric standpoint. Excavators are different. Mobile equipment has been developed for human control using hydraulic actuators. We have to consider the boom, arm and bucket trajectories all moving at different velocities. How they all balance contributes to precise bucket-tip positioning. Engineers face the difficult task of considering all these elements when planning motion paths and developing control strategies. We instrumented the test unit to obtain precise working data. We tracked pump flow versus pressure and saw that the human operator was a big compensator, always over-commanding beyond the machine’s actual capabilities (see Figure 2). Nearly all of these excavators have two-pump systems (although smaller models can have three pumps). Operator commands must balance pump pressure, flow and power in proportion to engine power and torque, to successfully complete a movement.
During grading the boom changes direction — it raises and then lowers — and relies on a linear cylinder which has a greater area one side to the other. This is a really complicated movement. (The fact that operators perform these actions seamlessly is thanks to refinements in designs and tuning over that last 20 to 30 years.) We then had to overlay the data with a software interface to bring in our digital functions. Another challenge is the speed at which you can control the system, so we looked at the amplitude versus natural frequency of different elements of the excavator. As speed increases, amplitude decreases, a natural effect of hydraulics. At some point amplitude drops below the machine’s natural frequency, and you either get resonance or loss of control. The other element is reaction speed, or latency. The human brain reacts quickly. Our system had to exceed that to over-command the system.
The Developed System The complete integrated system, named Exacto, encompassed Husco high performance digital-to-hydraulic valves, an integrated controller and proprietary work-function algorithms. The all-in-one subsystem connects with 14 hydraulic hoses and one electrical harness. The three-function system controls boom, arm and bucket movements. Safety is absolutely paramount. The safety logic circuit ensures any breakdown of electrical or other systems will immediately switch the system to manual control. Pressure sensors track operator commands and main control valve signals to provide closed-loop feedback. Various control modes were developed from operator-only to full automated control. It's important to know where the Exacto fits into the entire machine-control landscape. Operating information from a site plan or BIM system, or direct operator input, is sent to the machine HMI. The machine interface tells the operator what’s going on and sends signals to an automation controller or ECU that defines the functions needed for boom, arm and bucket movements. The controller transmits commands to the Exacto, which converts digital information to hydraulic control, and sends pressure signals to the main control valve. The valve, in turn, controls the machine hydraulics. IMU, GNSS or other sensors provide feedback to the ECU, to close the loop and ascertain exact end-effector position. So the Exacto has only a small part in this whole landscape. But it's a key part, serving as the digital interface that ultimately controls the system. The exciting parts, obviously, are the results. With the Exacto system, we were able to push out past the natural frequency of all of the different elements, ensuring that we had the required amplitude for total control. Also, latency of the system improved. In hands-off Exacto operation, results showed an excellent match between command versus actual pressure and velocity of the boom, arm and bucket. But what’s really important was comparing command position versus actual bucket tip position. We met our target of ±25 mm across the entire grade, and the first 70 percent was within ±10 mm. We were very pleased with the result. Movements when the boom cylinder changes direction are particularly difficult to control. As it moved over center we had a bit of a wobble — but still within our performance window — and expect improvements in future iterations of this project. Our other goal was to produce a 5-m long grade with 100 percent of the velocity available from the machine — the best the operator could achieve pulling the sticks as fast as possible. To overcome a bit of jerkiness we had to slightly slow the machine to 90 percent and, thus, generated the grade in 5.6 seconds. We expect to address this with additional tuning. As well as the flat grade we were also able to return to the control performance in drawing the perfect circle. Figure 3 shows what the team were able to achieve, note there is no driver in the cabin!
Future Challenges Since the project was completed the Exacto has developed further and been installed globally on numerous customer machines ranging from 14-45 ton class. We can now announce that the Exacto system is going to go into serial production in September this year. The next steps are to further optimize the system performance and increase the features offered by the technology. We are looking into automated commissioning and working on adding further functions such as tilt buckets and tilt rotators. Areas such as remote-controlled and fully automated vehicles are also under consideration. Finally, we see opportunities for implementing this approach and learning on other machine forms. The value added to the end user and the acceptance of higher productivity benefits is clear. The Exacto should be suitable for any pilot-operated machine.
Simon Yardley is the Director of strategic business development responsible for matching the mobile market needs in motion control and how Husco’s expertise to add value, performance, and productivity improvements to the machine.
Ben Holter has been at Husco for over eight years working on various advanced engineering projects with customers in North America, Europe, China, Korea and Japan. He has a background in mechanical engineering and a Master's in Predictive Analytics. Ben currently heads up a team of software, controls and hydraulic engineers who are at the forefront of developing the tools and products that are core to Husco's globally evolving technologies.
Yardley and Holter presented the education session Machine Control Automation of Functions on the Machine at IFPE 2020. IFPE, the International Fluid Power Exposition is the leading North American exhibition bringing together the fluid power, power transmission and motion control industries. For more information on automated grading with hydraulic excavators, listen to Simon and Ben’s interview on NFPA's Fluid Power Forum podcast.
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9/8/2020