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Material Testing: Monitoring Metal Fatigue Crack Growth

ADwin-Gold Real-Time Data Acquisition System

CAS DataLoggers recently supplied the data acquisition and control solution for a prominent manufacturer of material used in the aviation and aerospace industries. Characterizing the growth rate of cracks in metal samples under repeated cyclical stress was critical to the customer: typically, a material test system incorporating a closed loop servo-hydraulic system was employed to apply a repetitive stress to the sample while the growth rate was monitored by one of several methods. Variable amplitude-loading, conventional closed-loop servohydraulic systems using simple PID control were employed but became very difficult to tune to handle varying test conditions. Proper tuning became even more problematic as the cycling frequency was increased and dynamic limitations of the servo-hydraulic system became more apparent. Management realized the need for a stand-alone data acquisition system which could accurately generate the stress waveform and acquire the real-time data for force, displacement and crack monitoring, and which also featured advanced communications options for communication with a PC.

CAS DataLoggers custom-designed a system incorporating an ADwin-Gold Real-time Data Acquisition and Control System housed in a rugged and compact metal enclosure. A key feature in the stress signal generation was the use of a correction algorithm that used historical performance data from the command signal-feedback relationship to dynamically adjust the stimulus to reduce feedback errors to less than .1%, even in the presence of a large phase lag in the feedback signal. A PD500 system provided signal conditioning for DC potential drop measurements of the crack growth. Visual Basic provided the user interface to configure tests, view real-time data, and store the test results. Additionally, this mechanical test system could be retrofitted on a variety of commercially available test systems from MTS, INSTRON, etc., thus expanding its capabilities for this demanding application.

The ADwin-Gold system supported parallel, individually-controlled, real-time processes via its fast, local Real-Time DSP CPU while running independently of a PC’s operating system, providing deterministic operation with response times of 1 usecond or less. The system featured 16 multiplexed analog inputs connected to 2 16-bit and 2 14-bit ADCs, as well as 2 analog outputs (expandable to 8) with 16-bit resolution, along with optional counters enabling extremely low latency operation and CAN, SSI, and RS-232/485 interfaces. Signal acquisition was controlled by theADwin’s CPU, and operators could acquire two channels simultaneously without any phase-shift, which was critical for correlated signals. All measurements were recorded in real-time onto 16 MB of external DRAM for data storage, and the system also featured integrated Ethernet or USB interface for PC communication.

The ADwin software environment could be used under Windows (95/98/ME/NT/2000/XP/Windows 7) and LINUX or as a stand-alone data acquisition system. Multiple PCs could communicate with the ADwin-Gold concurrently, which proved useful during program implementation and commissioning. Conversely, one PC could also access multiple ADwin systems, for example in a network to provide centralized monitoring of various applications. Also, ADwin had drivers for many of the popular programming environments including VB, VC/C++, LabVIEW, TestPoint and others. The software also enabled calibration of the system’s analog inputs and outputs.

Real-time development was provided by ADbasic and ADtools: with ADbasic, users defined the processing sequences executed on the ADwinhardware, optimizing and compiling the program code on a simple mouse-click. After being loaded on the ADwin-Gold by ADbasic or a graphical PC user interface, the real-time processes executed independently. ADbasic contained the functions to access all inputs and outputs as well as functions for floating-point operations, process control and communication with the PC. A library was also provided containing standard functions, e.g. for filtering, various examples for counter use, closed-loop controllers, function generators etc., enabling a faster program implementation. By means of the easily configurable ADtools, users displayed their real-time data graphically or numerically to visualize process sequencings or to set input values via potentiometers, sliders, or push buttons. Additionally, ADtools always provided the current status of the ADwin system’s resources.

The manufacturer benefited in several important ways after installing the ADwin-Gold real-time data acquisition system. The ADwin-Gold provided more accurate, real-time control of sample loading when using variable amplitude loading, resulting in more accurate test results than other systems employing conventional PID controllers. The system handled stress waveform generation and recorded force, displacement and crack monitoring data in real-time. Additionally, the adaptive control capabilities of the ADwin system were far superior to conventional analog or digital PID controllers when operating over a range of frequencies and loads. The ADwin-Gold system also utilized its own DSP processor to operate independently of the Windows environment on the PC, allowing for much more stable and predictable operation, while its advanced communications capabilities simplified data transmission and accessibility.