The AED-40/4 model belongs to the Sensophone AED40 family of acoustic-emission diagnostic equipment. Signals originating from the detectors on the investigated object are processed using four identical and completely independent channels. This model may be upgraded to contain a maximum of 12 AE channels. Environmental parameters that may influence the measurement, as well as parameters of the measurement itself can be recorded using the 16 analog input channels provided. In addition, 16 digital I/O lines (each programmable as input or output) support the implementation of automatic or semi-automatic measurements. All of the so-called primary AE parameters of the detected AE events are acquired and stored from each channel. Derived AE parameters may be calculated in real-time, or at any convenient time after the measurement.
Preparation and implementation of measurements as well as the analysis of measured data are fully supported by the hardware configuration and by the system software running under Windows. Using the AE sensors in test mode it is possible to determine important characteristics of the investigated object relevant to the measurement. In this mode it is also possible to test the proper functioning of the whole equipment, including the cables and the external amplifiers. During the preparation phase unused channels may be switched off, the threshold level of AE channels may be adjusted, the coordinates of the sensors and the parameters of waveguides (if used) may be specified. Automatic adjustment of the threshold levels of channels may also be selected. The sampling frequency of environmental variables may be adjusted, and – to facilitate later interpretation – linear transformation of the analog signals (i.e. a relationship between the physical quantity and the electrical signal measured) may be specified for each analog channel. The functions used for linear transformations may be saved into a separate disk file for later use or inspection. During the preparation of the real-time measurement process, digital filters based on almost any measured or calculated parameter may be defined. If any filters are active (switched on) during measurements, only those data records which satisfy the conditions of the filters are passed to the analysis module. In order to localize AE signal sources, channels may be assigned to different groups. The sizes of maps showing localized sources may be specified, and files containing sketches of the investigated object may be assigned by name to these maps (these sketches may be conveniently produced by commonly used CAD programs). For listing the measured or calculated parameters, groups of parameters may be defined. It is also possible to define in advance distribution functions as well as the functions Y1 = f(X) and Y2 = f(X) for the graphical representation of data, together with the method of graphical representation. Equipment settings can be saved into a configuration file.
During both the preparation phase and the actual measurement, the occurrence of acoustic events may be continuously followed by the aid of acoustic and visual signals provided. In case if any of the active channels detects an event, a buzzer is activated and the LED assigned to the channel is flashed. The buzzer may be disabled by a front panel switch. The image of the LED array located on the front panel is displayed on the screen.
During measurement the values of environmental parameters sampled and the measured parameters of AE events detected are saved to a data file opened in advance. Parallel to this, in real time, processing of AE events which pass the enabled digital filters is also carried out: derived parameters are calculated, parameter groups are listed, the graphs defined are updated, and AE sources are localized and placed on the maps. All this activity can be followed on the screen of the device. The majority of operations listed in the section where the preparation of the device was discussed may also be carried out during the measurement itself. Channels may be switched on and off, threshold values of individual channels may be modified and the sampling time of analog channels for environmental parameters may be adjusted. It is possible to simulate an active AE source by sending test pulses to a selected channel. New filters may be installed, or one from the already installed set may be selected or modified. New localization maps, parameter groups and functions may be defined, and the display status (visible or hidden) of previously defined graphs may be freely changed. Any changes in the settings of the instrument are immediately stored in the data file, together with any text comments entered.
It may happen that, during the measurement, events disturbing data recording occur on and around the object investigated: it may have to be inspected or repaired and the resulting noise may render measurement impossible. In this case it is possible to suspend data recording without terminating the measurement, and resume it when the disturbing noise is no longer present.
The most important task of the device is recording and saving to file all measured data without loss. Usually this process is not hindered by the real-time calculations required for the parallel analysis of measured data. However, if acoustic activity is significantly increased, analysis may lag behind data recording. The extent of the lag is shown on the monitor by a progress indicator. Analysis can be speeded up by temporarily disabling the display of raw data and the calculated functions.
At the time of measurement termination the data file is closed. Analysis of data may be performed again at any later time, either by using the instrument settings saved at the start of the measurement, or by using a modified set of parameters. Obviously, during the analysis of data recorded in a previous measurement the threshold levels of AE channels and the state of channels (active or inactive) cannot be modified. The buzzer and the LEDs indicating AE events are also inactive in this case. The progress indicator on the screen shows the proportion of data records successfully processed so far. When the analysis of data is finished, arrows and legends may be added to the figures generated. Figures may be automatically numbered, then printed or inserted one by one into the measurement log.
The computer that controls the measurement and performs data analysis is not supplied as a part of the AED-40 equipment. Any personal computer equipped with a 100/10 Base-T Ethernet card and running Windows 9x/NT/XP may be used for this purpose, after installing the software supplied with the measuring system.
The main unit of AED-40 is enclosed in a special industrial style case.
The unit contains a Pentium MMX single-card computer, RAM, a floppy disk drive and a 256 MB flash drive (operating system is DOS 6.22). The main unit is connected to the controlling computer through an Ethernet interface. You may attach a monitor, a keyboard and a mouse to the main unit, but these components are not required for proper functioning. The single-card computer is inserted to the outermost slot of a passive ISA backplane having 4 slots (the size of the backplane depends on the actual configuration). The 16-channel, 12-bit A/D converter that records environmental parameters and handles digital I/O is inserted into a special connector on the single-card computer. One of the ISA backplane slots contains the measuring module with the circuits of four complete AE channels. The AEC401 measuring module provides power (60 mA @ 12V) for each external amplifier attached. The connectors of analog channels (two connectors with 25 poles each) and those of the AE channels (four BNC connectors) are located on the backside of the rack. The number of AE channels may be increased to 12 by the addition of more AEC401 modules.
There is an LLA102 programmable external amplifier unit for each AE channel. The amplifiers are connected to the case using 75W coaxial cables. The length of the cable must not exceed 300 meters. Power is supplied to the amplifiers through this connecting cable, and programming is also done by sending parameters via the cable. In normal operation mode, logarithmic gain (slope 0.5 V / decade, dynamic range 100 dB) or linear gain (adjustable by 10 dB steps, maximum output amplitude 3.178 V) may be selected. In test mode one may choose the type of test signal (continuous symmetric square wave or single pulse), as well as the amplitude, frequency and width of test pulses. The connector to which the cables of the sensors are connected can function either as an input (normal measuring mode) or as an output (test mode). Switching the connector between these two modes is performed by a reed relay. The frequency range of the amplifier is 30 kHz to 1 MHz (filter shorted); the maximum noise on the input is 30 mV RMS. In normal operation mode the amplifiers provide power (10 mA @ 7.5 V) for the preamplifiers attached.
The AE sensor is connected to the input of the amplifier using a 5-meter long Teflon insulated 75W coaxial cable. This cable is driven by the preamplifier installed on the sensor end. Power is supplied to the preamplifier via the same coaxial cable. Gain is 20 dB, the frequency range is 30 kHz to 1 MHz and input noise is 3 mV. In test mode, test pulses having a negative polarity bypass the amplifier and go directly to the sensors. Type A-11 piezoelectric transducers are used with the following characteristics: resonance frequency approx. 150 kHz, operating temperature range -25 to 85 oC. The sensors can be attached to the surface of the investigated object by magnetic hold-down devices or by any other suitable means. The use of acoustic coupling material between the surface and the sensors is recommended.
For the storage and transportation of the AED-40 and its accessories an aluminium carrying case is provided.