Customer Solutions
Actuator Performance and Reliability Test System
Author(s):
Seong Kang, University of Texas at Austin; Chetan Kapoor, University of Texas at Austin
Industry:
University/Education
Product:
Data Acquisition, LabVIEW, PXI/CompactPCI
The Challenge:
Automating the testing of electromechanical actuators to collect and analyze data regarding their performance, reliability, and endurance.
The Solution:
Developing a PC-based system using a dynamometer coupled with a brake, load motor, PXI modular instrumentation, and LabVIEW.
Introduction
The Robotics Research Group at the University of Texas at Austin focuses on the design and development of intelligent machines - primarily robotics - with emphasis on modularity, distributed control, performance, and condition-based maintenance. The standardized intelligent actuator, analogous to the microprocessor in the computer industry, is the key building block of these machines. These actuators are electromechanical systems composed of a motor, gear-train, brake, clutch, up to 10 sensors, an embedded digital motion controller, and embedded control software. For these actuators to be successful, we must characterize their endurance, reliability, and all other performance data. This data helps us design better systems and develop control algorithms that we can use to enhance system performance. The following parameters of the actuators are especially important to us:
To create the actuator testbed, we needed a mechanical and electrical design that could sustain a motor used to apply varying programmed loads on the test actuator. (The dynamometer comes equipped with a brake that can apply only static loads.) We also needed to design mechanical fixtures to test actuators with different shapes, sizes, and capacities. Finally, we needed instrumentation hardware and software for data acquisition (DAQ), analysis, and archiving. This is where we achieved success using National Instruments PXI modular instrumentation, DAQ boards, and LabVIEW software.
System Applications
Torque smoothness is an essential requirement in a wide range of high-performance motion control applications. For example, the quality of the surface finish achievable with metalworking machine tools is directly dependent on the smoothness of the instantaneous torque delivered to the rotary tool-piece. Similarly, the performance specifications of servomotors embedded in equipment ranging from robots to satellite trackers require minimization of all sources of pulsating torque. Accordingly, our first experiment was to measure and characterize the torque ripple in an actuator. The torque ripple is basically a power-electronic switching variation in the output torque of an actuator. It is unique for each motor-drive pair. The torque ripple is a function of the motor speed, current, and temperature. We can use the data gathered from measuring the torque ripple in the form of look-up tables during real-time actuator control to compensate for the torque ripple.
System Considerations
We placed requirements on both hardware and software for the actuator testbed. On the hardware side, we preferred the use of a PC-based system with a standard commercial operating system, a rugged platform, at least four expansion slots for interfacing hardware, and Ethernet connectivity. The test consists of a PXI-8156 controller and a PXI-6040E DAQ module housed in a PXI-1020 chassis with an SCB-68 connector block, dynamometer, dynamometer electronic controller, servoamplifier for controlling the test motor, and servoamplifier for controlling the load motor to perform precise testing of many types and sizes of robot actuators. On the mechanical side, a stiff one-piece frame was manufactured for the dynamometer assembly. A test table was designed and machined to support the weight of whole equipment, reduce vibrations, aid precision, and produce reliable materials. To eliminate the time required to machine new actuator fixtures, we designed a flexible fixture to accommodate various types and sizes of actuator modules.
With the PXI-6040E DAQ module, we used at least four analog I/O channels and four digital I/O lines. We used the standard NI-DAQ driver for the PXI-6040E module to reduce our development time. We also needed a programming environment compatible with data acquisition, graphing and charting, a graphical user interface (GUI), data archiving, analysis, and ease of programming suitable to mechanical engineers. Therefore, we chose LabVIEW for the testbed software development environment because it met all of our requirements. For this testbed, we used LabVIEW virtual instruments (VIs) for both analog input and output. These VIs gathered actuator speed, torque, and temperature information. We used analog output to control voltage on the servoamplifier modules to control the speed of the test actuator and also the load motor. We also used LabVIEW to develop the graphical control panel of the testbed, which simplified experimentation. The GUI displayed torque, speed, voltage, current, and temperature values, hardware input, control command, and load control on the front panel.
Results
PXI modular instrumentation, an NI-DAQ board, and LabVIEW were instrumental in the development of a fully automated robot actuator testbed. We finished our project within our budget primarily because of the completeness of the available systems, their ease-of-use, and the excellent documentation and technical support from National Instruments. The data collected from our tests has shown reliable and correct operation. Further work will involve software development to withstand additional tests.
For more information about the Robotics Research Group a the University of Texas at Austin, visit www.robotics.utexas.edu/rrg
robotics.pdf
View the entire user solution in Adobe Acrobat PDF format.
