DAQ system records rocket engine test results

Greg Reed, Contributing Technical Editor -- Test & Measurement World, 8/1/2006

At its facility in San Bernadino, CA, Wyle Laboratories tests rocket engines with thrusts up to 50,000 lb. When the company needed a data-acquisition system to capture engine thrust data, it turned to VI Engineering.

VI developed a turnkey PXI-based system that uses six computers—five of which are PXI controllers running National Instruments' LabView Real-Time (RT). Four NI PXI-4472 signal-acquisition boards reside in one chassis and acquire data from dynamic sensors at 32 kHz.

A second chassis includes three NI PXI-6250 data-acquisition modules. A digital trigger is used to synchronize the start of all analog input tasks. A NI PXI-6511 isolated digital input board is reads microswitch valve inputs to confirm valve actuation.

Wyatt Meek, project manager at VI Engineering, explained, "Our primary concerns centered around safety to the facility and the test article, reliability and accuracy of data recording, and flexibility and efficiency of configuration of the system. The channel count, data rates, and event recognition goals presented significant performance challenges."

Every millisecond, deterministically synchronized acquired data is packaged into a contiguous block or frame and passed to the master controller PXI system. Reflective memory transfers the data from controller to controller.

The master system reads all input data and evaluates the control sequence and alarms to determine the output state for up to 192 devices. The output values are written to three NI PXI-6512 digital I/O boards and added to the data frame in reflective memory immediately after the input data.

The real-time logger system reads all input and output data from reflective memory and then streams data to disk at the predefined rate. A server component provides engineering unit data to the host PC. The server also evaluates calculated channels for the display, such as pressure transducer differences, integrals, or derivatives of signals.

One PXI controller is a watchdog system that monitors the functioning of the master controller. This slave controller features parallel digital outputs that follow the master's values until it detects a failure of the master to update a watchdog signal. After a 3-ms timeout, the slave takes control and executes a user-defined safe shutdown sequence. Should both master and slave software fail, a third level of safety control is invoked—the hardware watchdogs on the 6512 boards return each valve to its user-specified fail-safe state.

A LabView interface on the host PC provides system control for test configuration, sequence specification, transducer calibration, test preparation, test execution and real-time data display, and post-test review. The program has three graphical user interfaces (GUIs) that schematically mimic the facility layout for oxidizers, fuel systems, and the test article environment. A fourth screen provides an event log and digital indicators and graphs for transducer data.

Data files are saved in National Instruments TDM (time-division multiplexing) format, which provides a mechanism to attach metadata to the raw values and associate groups of multi-rate data files. Automatic quick-look reports and ad-hoc data analysis capability are provided by NI's DIAdem.

Ana Cool, JRETS/E DAC technical lead at Wyle, commented, "The VI Engineering-developed system provides reliable and accurate data acquisition and control for the rocket engine test facility. The software and graphical user interface provides robustness and flexibility to accommodate customer/user-defined test sequences, red-lines (alarm conditions), and data simulations for cold-flow sequences."

A key benefit is that the system can generate an event-driven, user-defined test sequence that allows for multiple red-line alarm channels. Up to 20 simultaneous red-line channels can be defined per event in the automated test sequence, which has a maximum duration of 64 events.

Automatic alarm evaluation with complex alarm clause logic allows for tight multi-level alarms during the high-risk activities and more open criteria during the low risk phases of a hot-fire test. This provides fewer falsely aborted tests and increases the facility throughput.