KineticSystems ::. Data Acquisition (DAQ)

Testimonials

Magazine: Aerospace Testing International
Issue Date: July 2002

On-cart data system reduces test installation times in AEDC's 16 Foot Transonic Wind Tunnel

Robert E. Lindeman
Program Manager
Sverdrup Technology, Inc.
Arnold AFB, TN

KEYWORDS

Data acquisition and control, VXI, VME, PLC, reflective memory, fiber optic network

ABSTRACT

Reducing the amount of time it takes to prepare for a wind tunnel test has significant benefits for customers. This paper describes an innovative approach to a new on-cart data acquisition and control system in AEDC’s 16T Wind Tunnel that provides major improvements in the installation time required.

INTRODUCTION

Arnold Engineering Development Center's 16-foot transonic wind tunnel (16T) is a variable density, continuous-flow tunnel. 16T operates at Mach numbers from 0.06 to 1.6. Stagnation pressures can vary from 120 to 3,800 psfa with the maximum stagnation pressure a function of Mach number. 16T was designed for aeropropulsion testing and is equipped with scavenging systems to remove combustion products. A key design feature of 16T is the use of a removable 16-foot square by 40-foot long test section. This feature allows the build-up of a test program in a remote building, the Model Installation Building, without tying up the tunnel. Since two test sections are provided for 16T, a high-angle sting support cart (referred to as the HAAS cart) and a multi-purpose cart (referred to as the CTS cart), one test can be conducted in 16T while another test is in build-up.

THE TEST PROCESS

Many different types of wind tunnel tests are conducted in 16T. These include Force tests, Pressure tests, Propulsion tests and Captive Trajectory tests. In each case, a similar process is followed.

The project engineer receives a test inquiry. The customer and the project engineer begin the planning process by determining the high-level test requirements. This determines what tunnel equipment, such as test support system, plant equipment, and data and control systems, will be needed. From here the current tunnel schedule is consulted to determine when the test can be run. Once this is agreed to, specific model and instrumentation needs are determined and a testing sequence developed. These efforts involve the customer and support services such as facility, instrumentation, controls, and data reduction engineers. The time critical work begins in earnest with the arrival of the test model. Once delivered to the Model Installation Building (MIB), the build up process begins. 16T uses removable test carts that are physically removed from the tunnel circuit and moved via a transport cart to the MIB. The model is instrumented with various sensors and mounted to a sting support system on the test cart. All instrumentation is hooked up to disconnect panels and to special systems in the MIB for checkout and calibration. This checkout includes operation of the model positioning systems and any model control surfaces. After this checkout the instrumentation is disconnected from the MIB test equipment and the test cart moved to the tunnel. Once installed in the tunnel, the hookup, checkout and calibration effort is repeated, this time with the tunnel data and control systems located in various control rooms. It is here that displays and data reduction checkout is performed. Once completed, the tunnel is activated, test conditions set and the test program executed. After all data has been taken, the test cart is disconnected and moved back to the MIB. The model is removed and shipped back to the customer.

INSTALLATION TIME

AEDC is continually looking for ways to improve our services to our customers. The ability to execute a test program more efficiently and thus reduce time and cost is of prime importance. Reengineering efforts at AEDC have identified installation time as one one of the big hitters in the time it takes to execute a test program. While this time variesous from test to test, it typically averages 50 hours from the time the test cart leaves the MIB until air is blowing in the tunnel. By reducing this time, several benefits can be realized, including tunnel availability, scheduling flexibility, customer access, and cost reduction. A stretch goal of cutting this installation time from 50 to 4 hours was established.

Closer examination of the steps involved in installing a typical test revealed that the majority of the time spent during installation was consumed by the hook up, checkout and calibration of the model instrumentation and control equipment. Further, it was discovered that most of these efforts were performed twice, first in the MIB and then in the tunnel after the cart was moved to the tunnel. This was required to verify the hookup and operation of the systems in the tunnel since different data and control systems were used and since all signals had to be disconnected while the cart was moved and then reconnected in the tunnel, a process prone to error. Earlier efforts to improve on this process included a pre-test checkout system in the MIB that duplicated the tunnel data systems and thus reduced a portion of this duplication. However, this proved to be ineffective as differences in the interconnection of the instrumentation and in the data reduction and display system configuration required repeat of most checkout and calibration in the tunnel. It was clear a revolutionary approach was needed to meet our goal of 4 hours.

After consulting the literature, benchmarking against several other wind tunnels and conducting several brainstorming sessions, two approaches to minimize installation time were considered. Both approaches focused on using the same DACS for the MIB checkout as well as for testing thus eliminating the redundant work. The first consisted of a second test cart that would house the data and control system and be connected to the test cart by an umbilical cord. The idea was to permanently install the data and control systems on a separate test cart that would move with the model cart from the MIB to the tunnel after all systems were hooked up, checked out and calibrated. The test cart would be installed in the tunnel and the data cart left outside the tunnel under ambient conditions. The second approach was to mount the data and control systems in environmentally controlled enclosures as integral parts of the test cart - better known as the On-Cart Data Acquisition and Control System (On-Cart DACS).

ON-CART DATA ACQUISTION AND CONTROL SYSTEM

The The approach utilizing an umbilical cord was dismissed based on cost considerations. Additionally, there was opposition to the approach of locating the instrumentation within the tunnel. The opposition stemmed from belief that hardware reliability would be such that ready access to the system would be required for either repair or adjustment. Also, it was believed that the on-board system would have limited channel capacity and that this would limit test capabilities. After consideration of the improved reliability of today's systems, it was decided that the risk of a component failure necessitating shutting the tunnel down was acceptable in order to gain the benefits of reduced installation time as well as improved data quality attributable to shorter cables and fewer disconnects. Regarding channel capacity, test entries for the previous five years were analyzed to determine the number of channels as well as the mix of signal conditioning. The decision was made to design and build an on-board cart system that accommodated 80% of tests with an auxiliary system located in the control room environment for those tests whose requirements exceeded the on-board cart system . approach selected was the On-Cart DACS. This consists of four systems the Steady-State Data Acquisition System (SDAS), a Pressure Scanning System (PSS), a Dynamic DAS (DDAS), and a Test Article Control System (TACS).

Steady-State Data Acquisition System (SDAS)

The SDAS is based on VXI architecture incorporating multi-channel A/D cards with on-board and external signal conditioning. Force balances have high quality signal conditioning external to the VXI chassis with constant current and voltage excitation capabilities. Strain gages, thermocouples, RTDs, and miscellaneous analog inputs are conditioned by VXI based signal conditioning.

SDAS system specifications are:

Force - 0.1% FS
Temperature - ±2 degrees F
Pressure - 0.1% FS
Channel Scan Rate - 100 Hz
Bandwidth - 2 to 10 Hz
Data Update Rates - 50 msec

The SDAS software performs the following functions:

A/D and signal conditioning calibrations
Data Acquisition
Engineering unit data calculation and storage in reflective memory
Channel integrity monitoring

Dynamic Data Acquisition System (DDAS)

The DDAS is also based on a VXI system. The system uses a sigma-delta analog-to-digital converter per channel. All digitized samples for each channel are placed in reflective memory, using a multi-buffered technique, and are recorded to disk.

System specifications for the DDAS are:

Force - + 1.0 % FS
Pressure -0 - 15 PSI, + 1.0% FS
Bandwidth -100 Hz to 10 kHz
Max Scan Length - 60 sec
Channel Time Corr. -100 Hz to 2 kHz, + 1 °

2 kHz to 20 kHz, + 2.5 °

The DDAS software performs the following functions:

A/D and signal conditioning calibrations
Data Acquisition
Digital filtering and storage of raw data in reflective memory
Channel integrity monitoring

Pressure Scanning System (PSS)

Measurement of a large number of pressures is required for many tests at AEDC. The PSS uses electronically multiplexed pressure scanners for this function. Each scanner is capable of measuring 64 pressures with semiconductor, temperature-compensated pressure transducers. These scanners and the digitizer are miniaturized and are normally placed inside the model in order to minimize pressure lags. Digitizers send digital data back to a host computer over fiber-optic cables.

The PSS provides the following measurement capabilities:

Pressure Range - 1 PSID to 500 PSID
Measurement Uncertainty - 0.05% FS
Data Update Rates - 50 msec

Test Article Control System (TACS)

Position and velocity control of the various 16T model support positioning systems is provided by the TACS. The TACS is a PC-based, multiprocessor, VME system using digital position/velocity control. A PLC-based safety monitoring subsystem monitors all limits and conditions that could be hazardous to both personnel and equipment.

HAAS Cart

4 Main Support Control Channels (HAAS Pitch, Roll, Aux. Roll, & Pitch Table)
8 Miscellaneous DC Motor or Hydraulic Control Channels

CTS Cart

10 Main Support Control Channels- CTS, Pitch Table, Pitch Boom, Std Roll, HP Pitch
8 Miscellaneous DC Motor or Hydraulic Control Channels

Functional requirements for the above measurements include:

A/D and signal conditioning calibrations
Automatic and Operator Control of Test Article Positions and Velocities
Engineering unit data calculation and storage in reflective memory
Channel integrity monitoring
Test Article equipment and personnel protection

NETWORK

The on-cart DACS includes a fiber optic network that provides all data and control signal routing. The main components of this network are a 10Mbyte/sec link between the SDAS and DDAS front ends located on the test cart and their host computers located in an instrument room, and a reflective memory ring linking all on-cart systems and associated data reduction, analysis, recording, and display processors.

All network connections from the test cart are made via a 12 fiber military style connector. This is the only data and control connection required to be completed when a test cart is installed in the tunnel.

CONFIGURATION

Each system is mounted in an environmentally controlled enclosure that provides cooling, vibration isolation, and maintains atmospheric pressure - essentially a control room environment. All systems remain powered up from original hook up through test completion.

RESULTS

The new on cart DACS has been successfully used on three test programs with very impressive reductions in installation time. The average for these three test programs is 5 hours. As we become more familiar with the new system and streamline some remaining mechanical alignment procedures, we fully expect to reach our 4-hour requirement.

SUMMARY

The use of a revolutionary approach to providing the instrumentation and control systems needed to support wind tunnel tests in 16T, the on-cart DACS, has proved to be very successful. The hardware reliability after extensive checkout has proven to be acceptable and not requiring frequent access for adjustment or repair. Additionally, the reflective memory has proved to be an acceptable method for networking systems in a real-time environment. We have achieved significant reductions in the time required to install a test, which provides more tunnel time availability, lower costs, and better value for our customer. This same approach is currently being implemented on the CTS cart and in AEDC's 16-foot supersonic wind tunnel (16S).

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