Developing Three World-Class Robotic Test Systems for 3D Dynamic Test

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"Control and monitoring system design based on NI products made the overall system programming friendly, highly configurable and expandable, functionally robust, and cost-effective. We were able to use a single software platform (LabVIEW) and a single reconfigurable hardware platform on these three quite different systems, thereby reducing cost and risk considerably."

- Dr. Boyin Ding, The University of Adelaide

The Challenge:
Developing an advanced 3D dynamic load test facility for mechanical and civil engineering research applications.

The Solution:
Applying NI host-target (real-time and FPGA) architecture to achieve robust and high-precision motion/load control of the robotic test system, versatile functionality of the test system for a wide range of research applications, deterministic critical safety control that ensures operation safety, a user-friendly interface for manipulation and monitoring, and high-speed DAQ for post-processing.

Author(s):
Dr. Boyin Ding - The University of Adelaide
Prof. Benjamin Cazzolato - The University of Adelaide
A/Prof. John Costi - Flinders University
A/Prof. Karl Sammut - Flinders University
Dr. Stuart Wildy - Flinders University

About This Project

This is a joint project between the University of Adelaide and Flinders University, both of which are based in South Australia (SA), Australia. We initiated the project with a South Australian Government funding grant in 2009 to develop a high-precision six degree of freedom (6 DOF) test robot for biomechanical research as shown in Figure 1. This initial platform was completed and fully functional within two years of commencement. The project won two awards for engineering excellence from the SA Division of Engineers Australia and was nominated for the Engineers Australia National Award. In 2014, the two universities jointly secured an Australian Research Council Linkage Infrastructure, Equipment and Facilities Grant ($400k) with the objective of developing two large, complementary hydraulic-driven robotic test systems with a loading capacity of 4 tonnes for 3D dynamic test. These large-scale platforms were developed based on the same control systems concepts used on the smaller biomechanics platform. Both robotic test systems have been commissioned and are being configured for their designated research applications. The system based at Flinders University is shown in Figure 2.

Figure 1. The 6 DOF biomechanical robotic test system tests an artificial knee joint.

Figure 2. The 6 DOF Hydraulic-Driven Robotic Test System Based in Flinders University (left), NI Hardware (right top), and the Hydraulic Pump (right bottom)

Limits of 3D Test Systems

The 3D test systems on the market have many limitations such as relatively low loading capacity, poor dexterity, and limited functionality for research. Aside from these limitations, existing platforms can easily cost well over 1 million Australian dollars, which puts them out of reach of most academic institutions. For this project, we developed three custom-built robotic test systems based on the structural concept of the Stewart platform and NI products. Robust PID control algorithms were developed for the Stewart-platform-based system, which ensured high-precision motion of the robotic test system under extremely high loading conditions. A novel dynamic 6 DOF load control algorithm was implemented to enable research that required the replication of 3D loads on the test samples. The functionalities of the robotic test systems were designed for a wide range of advanced research applications. Consequently, they made the system extremely user friendly for researchers. We integrated sophisticated critical safety control in the system to ensure the safety rating of the whole system. With a user-friendly interface, users can easily manipulate and monitor the system following a short training process. High-speed deterministic DAQ allows the user to save a large amount of data from sensors for offline analysis.

Limits of Test Facilities

In Australia, and even globally, research on advanced materials, structural vibration, human factors and biomechanics, marine platforms, and earthquake-resistant structures is typically limited by the lack of adequate test facilities. This prevents researchers from conducting essential experimental validation to prove new theoretical work. It confines validation to the computational simulation realm or the extrapolation of single DOF measurements. The mathematical models on which these computational simulations are based incorporate simplifying assumptions to represent the complex 6 DOF behavior of physical test structures. They consequently do not faithfully replicate the real behavior of the test structure. Some particular research applications even lack the availability of a high-fidelity model to start with. For example, human bodies are more complex mechanisms compared with modern machines, and their behaviors are poorly understood both clinically and mathematically. Therefore, a specifically designed 3D test facility is of paramount importance to boost multidisciplinary research to a higher level and develop better mathematical models of complex structures.

Application Details

The robotic test systems are controlled and monitored by NI hardware programmed using LabVIEW. The overall system applies an NI host-target (real-time and FPGA) architecture, where the real-time controller handles robot trajectory generation, kinematics computation, state-machine control, safety-critical control, and DAQ while the FPGA reconfigurable I/O (RIO) devices run six robust PID control loops that control the movement of the six Stewart-platform legs. A user-friendly GUI has been designed to operate the robot and monitor the status of the robot and hydraulic system. A virtual reality model of the robotic test system can be displayed on the GUI for remote visualization of the system both in real time and offline to check the displacement envelope prior to operation.

Our Switch to NI Products

Before we selected NI products for the development of the robotic test systems, we used commercial test machines for our experimental test activities. Most of these test facilities were single-axis machines that limited the research purely to 1 DOF studies. The 6 DOF test machines were extremely expensive, not modular, and not programming friendly.

NI Hardware and Software

We integrated several generations of PXI controllers, RIO devices, and DAQ devices in the three robotic test systems. In general, we used PXI-8106, PXIe-8133, and PXIe-8135 PXI Controllers to execute the higher level control algorithms (for example, trajectory generation, kinematics computation, state- machine control, safety-critical control, and DAQ). We used RIO devices to execute the lower level leg motion control. The PXI-6528 PXI Digital I/O Module was used to connect to the industrial electronics. We used the PXIe-6363 PXI Multifunction I/O Module for standard analog and digital I/O. The PXI-4497 PXI Sound and Vibration Module provided ICP supply for the accelerometers (for vibration measurements). The PXIe-4330 PXI Strain/Bridge Input Module provided bridge and inputs for strain gages. For software, we used several LabVIEW versions (ranging from 8.6 to 2014) to program the robot control systems. LabVIEW Real-Time, FPGA, SoftMotion, VISA, and signal processing express packages were the most often used features.

The LabVIEW Real-Time and LabVIEW FPGA modules were essential for the application. The add-ons were simple to use even for new LabVIEW programmers. Measurement I/O and Instrument I/O express functions made DAQ much simpler and more configurable in real time. They also enabled fast development.

With products ranging from myRIO to high-performance PXI systems, NI is cost-effective for real-time control and data monitoring. NI PXI systems have high modularity, which allows us to expand system capacities in the development stage. From the application point of view, many control systems are C program based, which is less intuitive than modern graphical programming languages such as LabVIEW. Compared with hardware that is specifically designed for teaching, NI products are more versatile. They suit not only teaching but also industry-standard applications.

The 40 MHz deterministic clock rate and reconfigurable nature of the FPGA modules ensure effective lower level control performance as well as the accuracy of essential safety systems. The PXI system is highly modular, so we can easily expand it for more complex research objectives. LabVIEW coding is so intuitive and easy to document that we can assign coding tasks to university students. A rich driver library developed by the NI community allows easy handshaking between NI products and other electronics (for example, Ethernet communication between robotic test systems, DC motor servo amplifiers, and hydraulic power units).

Challenges

During the development phase of the robotic test systems (particularly for the first one), the primary challenge was the organization/arrangement of the coding structure. The robotic test was designed for a variety of research applications, so the functionality of the code had to be upgraded and extended frequently. Therefore, it was important to use a modular coding structure from the commencement of code development. The development of such a modular coding structure was based on not only the coding skill but also a deep understanding of the research requirements in applications. Based on our experience with the first robotic test system, we carefully planned and documented the codes for the two hydraulic-driven robots for future development. We addressed many small challenges in code development, for example, parallel robot control involved solving an optimization problem. LabVIEW has no adequate solver for our problem of interest. We addressed this by compiling a C++ based solver to a dll, which was then called from within LabVIEW. Another significant challenge we faced was the development of the PID controllers on the 16-bit FPGAs. Because of the exacting requirements of precision and speed, the signals of the individual PID controllers had very different magnitudes, so they required a 32-bit emulation to avoid overrun.

Application Benefits

The three robotic test systems are the first of their kinds. Developed for a wide range of advanced 3D dynamic test research, they offer extremely high-load capacity, ultra-high precision under extreme loading condition, high versatility for a variety of advanced research, configurable and expandable natures, and novel 6 DOF dynamic load control. Each robotic test system complements the others in terms of functionality. The initial platform was designed for biomechanics research, with an absolute accuracy of 10 µm and a loading capacity of 1 tonne. The two larger hydraulic platforms have a loading capacity of up to 4 tonnes and an absolute accuracy of less than 1 mm, one being designed mainly for 6 DOF acceleration and vibration test and the other having the capacity for large rotations (with reduced load capacity) to suit marine and aerospace applications. The three systems are highly configurable and programmable for a variety of research applications. Such flexibility was achieved by the use of NI products. An innovative Stewart platform structure design combined with an online correction algorithm ensures the accuracy of the robot even under extremely high-load applications. In addition, a novel 6 DOF load control enables the robot to replicate 6 DOF dynamic loads on the specimen. Both of these features are the first of their kind and are robust given the deterministic real-time performance of NI products.

Test System Impact

The three robotic test systems will significantly contribute to a wide range of advanced research applications in materials, biomechanical, aerospace, maritime, and civil engineering for the two universities and their research partners at other universities and industry across Australia. The facilities will enhance the fruition of new ideas from theory to practice. The developed systems will also have an impact on the 6 DOF test machine market, leading to more versatile and robust yet cost-effective 6 DOF test systems.

Conclusion

Control and monitoring system design based on NI products made the overall system programming friendly, highly configurable and expandable, functionally robust, and cost-effective. We were able to use a single software platform (LabVIEW) and a single reconfigurable hardware platform on these three quite different systems, thereby reducing cost and risk considerably. The coding work for the three robotic test systems will continue to allow the further development and evolution of the algorithms to meet new requirements as more projects are identified. Their functionalities will keep growing as the research work and industry activities continue to expand into new areas. It is anticipated that the NI products used in the robotic test systems will need to be upgraded to support the new functionality requirements for the systems.

 

Author Information:
Dr. Boyin Ding
The University of Adelaide
Australia
boyin.ding@adelaide.edu.au

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