Requirements

The system is designed to perform life tests of aerospace products, hydraulic power steering, including drop tests. The measurement is performed on a pair of independent stands.

System records signals from up to 32 channels with sampling up to 2 kSa/s on each one. It is assumed that all 32 channels for strain gauge can be used. Reserve of some channels for measurement of other signals, eg. temperature or position (at the expense of strain gauge channels) is present.

In addition, signals for a pair of load hydraulic motors are generated. The signal generating device provides feedback so that system makes corrections to the generated electrical signal according to the actual value of the load.

The task of the system is to generate load signals, monitor measured analog signals and evaluate whether they meet the specified conditions. If so, they can be forgotten in most cases. Otherwise, they are recorded on a large disc. The system takes into account the possibility of one site reconfiguration while measuring at the other one.

System description

Modules are placed in the VXI frame of VX1410 type from Tektronix. The frame has 13 positions; 7 of them are occupied by measuring modules and a computer, so 6 positions remain available as a reserve for further expansion of the system. The frame is equipped with a fault monitoring module (eg. drop in power supply, overheating, etc.) that reports detected faults, as well as eg. the status of the air filter. The report is transmitted either via the RS232 serial line to one of the computers in the workplace or directly to the built-in measuring computer. This monitoring suits for long-term unattended tests.

The frame is designed so that in the event of a failure, the faulty module can be replaced as quickly as possible. As example: the power block can be replaced within three minutes. This feature minimizes the consequences of failure on long-term and unrepeatable measurements.

VX1410

DBS8700 module is used for the measurement. It has a 16-bit resolution at 200 kHz sampling. It contains logic to control coupled multiplexers (max. number of channels is 255). Assuming full speed measurements on all 32 channels simultaneously, it allows sampling at up to 6,250 samples/s, meeting the measurement requirements with sufficient reserve. The module contains a pair of DMA channels designed for hardware data transfer via the VXI bus at full speed, without computer intervention. There is a possibility of independent sampling of individual channels. The module has built-in memory for repeated sequence of channel sampling and amplification (scan list) consisting of up to 1024 steps. The module is equipped with a FIFO buffer for data with a depth of 1024 samples. Importantly, the module allows automatic start of the measuring sequence (without computer intervention) when the preset level on a given channel is exceeded – this feature can be advantageously used for automatic start of measurement only if the limit value of some signal is exceeded.

Input signals are multiplexed using a pair of 16-channel DBS8710 multiplexers. The multiplexers are equipped with sampling circuits on all input channels, allowing voltage sampling at all inputs simultaneously. This eliminates the problems arising from the gradual measurement of individual channels (the usual problem in many measuring tasks).

Generation of motors is ensured by a pair of DBS8751 modules. Each module contains four channels, each channel is equipped with a memory of up to 64,000 samples. The contents of the memory can be changed both by writing from the computer and by direct access to memory modules (DMA). With the set sampling frequency, the data is transmitted by D/A converter with 16-bit resolution to the analog output, where it is adjusted by a low-pass filter with a cut-off frequency of 21 kHz.

PC-VXI module is key part of the system for test control, data evaluation and generation of waveforms. It is a Pentium Pro 200 MHz, inserted into the VXI system frame. The computer works in Windows NT environment, it has 32 MB memory, Ethernet port, PCMCIA, SCSI-2 and GPIB buses. It provides VXI bus control, allows block transfer of DMA speeds of up to 20 MB/s and mapping address spaces of VXI modules to their address space. Included is a VISA library with fully programmable trigger support and a set of configuration programs. The PCI/ISA expansion slot in the module is equipped with an IRC position sensor board. The computer is very reliable.

VXI

System is complemented by a custom-made measuring program that is created in the TestPoint development environment.

Program

This program provides measurement at two sites, evaluation and visualization of data, as well as control of excitation actuators. The control signal is force, but the system allows regulation to a selected level of any quantity (ie. even such that is calculated, not measured directly). The system is connected to the corporate LAN.

The system supports generation of excitation curves. Due to its resistance to environmental influences and small dimensions, it is used to measure data in various modes of real operation. The program, then performs statistical processing of empirically determined values ​​and creates “typical” characteristics for the given mode of use. With possibility of graphical modification of the excitation waveform, and with manual input of fault signals, modification of this characteristics is supported.

Program
The solution has the following main advantages:
  • highly reliable solution,
  • system grows very slow old,
  • delivery as a whole, warranty for functionality of the system as a whole without the risk of undetected design errors,
  • one partner for solving possible complaints of technology and programs, domestic service,
  • an open solution allows to combine the most suitable modules into one unit,
  • expandable to 128 channels,
  • up to 6,250 samples / s on all channels simultaneously,
  • feedback allows keeping the shape of excitation pulses not on the electrical output, but directly as angle or displacement values, including the possibility to generate load signals of any course,
  • 2 channels of DMA (= hardware data transfer via VXI at full speed, no computer intervention),
  • possibility of independent sampling of individual channels,
  • built-in memory for repeating channel sampling and scan lists of up to 1024 steps,
  • FIFO data buffer,
  • automatic start of the measuring sequence (without computer ,intervention) when the set level on the channel is exceeded,
  • includes a dedicated 3 channel IRC counter that counts back and forth with a zero mark,
  • one system for testing in the testing room and for data acquisition from real operation,
  • connection to the environment via a computer network, data available anywhere in the company,
  • modules are VXIplug & play compatible, with full VXIplug & play support from National Instruments (ie. de facto from the standard creator),
  • measuring program based on user requirements, developed with participation of the user’s staff according to the mutually agreed project,
  • possibility of further modification of the program,
  • direct contact to program authors, hot line,
  • with the exception of drivers, there is not customized code in any language (C, Pascal), everything is programmed in a graphical development environment instead,
  • detailed documentation,
  • possibility of lending modules during repair or calibration period,
  • all VXI modules are self-test equipped.

More information request here.