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Data Acquisition with Small Modules

Capturing Elusive Data Ensures Reliable Results

Data acquisition systems are continually advancing in functionality to provide practical and cost-effective solutions in those applications that generate elusive events. Event controlled recording can greatly increase the accuracy and ease the analysis of complex test scenarios.

BEN HAEST, QED, SA AND KLAAS A. VOGEL, ELSYS INSTRUMENTS

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Modern electronics systems, whether found in industrial, medical, automotive or other applications, are continually evolving to include additional functionality, to cram more components into a smaller space, and to provide better reliability for fast access to the most accurate data. Regardless of the environment, in most cases the information transfer is happening at a breakneck pace, and testing these advanced systems to ensure data integrity becomes paramount since a system is only as reliable as its weakest data packet. If information is lost or delayed, the whole system can quickly fall apart.

For example, in the medical field, information gathering is increasingly moving toward more data-based systems versus traditional photo imaging systems. Some of the techniques that rely on accurate data measurement and recording include electroencephalography (EEG), magnetoencephalography (MEG) and electrocardiography (EKG). Improper information gathered from these critical tests can lead to image distortion that prevents proper diagnosis or could even mask an underlying problem resulting in a missed diagnosis.

Another critical area where human safety is paramount is in the modern automotive industry where reliable component function is a top priority. The increasing amount of electronic components present in today’s cars not only need to coexist, but they need to operate autonomously from some and in conjunction with others without impeding safe operation…while withstanding a multitude of harsh environmental factors over an extended period of time.

For example, over the lifetime of virtually any car, some lasting more than 200,000 miles, the electrical contacts within a connector have to withstand vibrations, heat, cold, humidity and dust under many different driving and parking conditions.  

Ensuring Connections 

Since electronics have entered the automotive scene, several standards for testing electrical contacts have been developed. The aim is to produce and to manufacture reliable connection systems between the different control and peripheral units within a vehicle. Reliability is of utmost importance to assure the proper functioning of the Engine Control Unit (ECU), anti-lock brake system (ABS) and airbag systems to name a few. 

In the early days of testing, when the first electrical contacts were being incorporated into a car, they only carried the current for headlights and other basic components.  Nowadays, additional signals, such as audio, HD TV, phone and GPS, are passed through the contacts of multi-pole connectors along with the critical operating signals. In addition, many of the actual signals are high frequency and low current, so they demand especially stringent testing criteria. 

For each environmental parameter—vibration, temperature, dust, humidity, etc.—different standards, typically conducted in a laboratory, have been developed to simulate the damaging influence of the environment on the quality of the electrical contacts during a vehicle’s lifetime.

A complete test specification will define the level, frequencies or shock level and test time for the excitation vibrations on a shaker table and the simultaneous temperature cycles in a climatic chamber. In a typical automotive test, the connector is set up in exactly that way. The combination of the simultaneous environmental parameters—vibration and temperature—is more rigorous than two separate tests with only one environmental condition, while being both more realistic and more time efficient.

Different automotive standards also accurately define how the connector should be mounted onto the shaker table. Because the cables are mounted to the connectors, the different electrical contacts are also heavily mechanically stressed through vibrations that come through the cables as well. It is important to note that the connectors and the cables must be fixed in a very specific way to ensure tests’ repeatability and guaranteed comparable results. 

One very common test requirement is the SAE/USCAR-2 standard: a DC current of 100 mA flows through each electrical contact. The standard specifies that as soon as the contact resistance is more than 7 ohm for a minimum of 200 nsec (0.2 μsec), an electrical contact is interrupted and does not function properly anymore. The standard uses a 12 V/DC voltage source with a series resistor of 120 ohm. The voltage across the electrical contact is measured to identify the increase of contact resistance during an environmental test.

Trigger conditions can be freely defined according to the test specifications, and many car manufacturers have developed their own test specifications over time. A good example is the FlexRay Communications System, a robust, scalable, deterministic and fault-tolerant digital serial bus system designed for use in automotive applications. Developed under an industry consortium comprised of several leading automotive manufacturers, this network communications protocol features high data rates of up to 20 Mbyte/s. Trigger conditions are 1 ohm contact resistance and 100 nsec (10-7 sec) interruption time (Figure 1). 

Figure 1
Contact resistance goes up when connector pin comes loose due to shaking, temperature fluctuations, humidity, etc. When surpassing pre-programmed conditions like level, window, other, the particular channel in the transient recorder will trigger and the events will be stored together with time stamps.

For non-automotive tests, quite often the temperature cycles are omitted. Only vibration conditions are simulated. Also the contact interruptions are defined differently: the maximum voltage across a contact should not be more than 10 V/DC. An interruption is defined as a voltage increase across the contact for more than 50% (5 V/DC) and 1 μsec (10-6 sec). These examples show just how versatile data recorders need to be to acquire accurate information under a variety of conditions.

Acquiring Elusive Data

Luxemburg-based QED, S.A., which develops customer-specific solutions in the fields of measurement, regulation and control technology for testing laboratories and production lines, was finding that many standard measurement systems failed to perform the necessary testing required by several industry standards. 

It has since developed an automotive testing method, using Elsys Instruments’ high-speed, high-resolution transient recorders with the Event Controlled Recording (ECR) mode, which allows targeted acquisition of cyclic or sporadically arising events, to gather the required test data.  QED’s contribution to a bay of test equipment consists of vibration tables, environmental chambers and a 20-channel transient recorder system with 40 to 100 Msample/s sampling rate per channel at 14-bits vertical resolution and software (Figure 2). 

Figure 2
The QED Gn?stic64 (Gnostic) measurement set-up depicted here is repeated for each pin in the connector under test with maximum 20 channels per system and up to 64 channels networked together.

Event Controlled Recording

The advanced ECR functionality of the recorder made it easy to set a great number of trigger filters that could respond to certain waveform behaviors, enabling the recorder to function as an “Intelligent Streaming Digitizer System” (ISDS).  The registration of measuring data in ECR mode only occurs if certain signal conditions (trigger, time window, repetitions, etc.) are fulfilled, ensuring unwanted and unneeded signal data will not be stored. 

Take an arbitrary interruption in the connector testing application as an example. While the underlying slower signal phenomenon is being recorded at slow sampling rates for the duration of the test cycle (which may be many hours to days or even weeks), when a fast event occurs that fulfills the preprogrammed trigger conditions, it will be recorded at a high sampling rate, complete with time stamp. In a way, this is comparable to an oscilloscope with a triggered delayed time base. Also the mode forms an intelligent way of “streaming” waveform data to disk, as only the events of interest are being stored. 

ECR enables zero dead time between events, so no events are lost, even with many channels active at maximum scanning rate over a long period of time, since each channel possesses its own signal buffer with up to 128 Msamples (Figure 3). With such a large buffer available so that data only needs to be transferred now and then to the onboard hard disk, data rates as high as 80 Msamples/s can be achieved. The trigger conditions can be individually set for each channel, with individually set complex trigger signal criteria such as pulse width/interval/height, slew rate, window-IN/OUT, etc.

Figure 3
Transients are recorded in a circular memory buffer as large as 128M samples (14 or 16 bits) per channel, including pre and/or post trigger delay and time stamp information. Data recordings are off loaded to system hard drive files for further analysis. Data blocks recorded under ECR trigger modes are usually small in size, hence an enormous number of events can be gathered over a long period of time without supervision. The system can be programmed to automatically off-load recorded data to host hard drives from time to time. Thanks to LAN connectivity the host computer may be remotely stationed.

ECR supports three modes to enable extreme versatility in acquiring data for various testing parameters (see sidebar “Three Modes of Event Controlled Recording”).  The Multi-Channel Mode allows different channels to be associated with each other, saving test time and critical resources. 

Single-Channel Mode, used in connector testing, allows different trigger conditions and trigger events per channel, where a trigger event for an oscilloscope or standard transient recorder typically starts the acquisition on all channels.  While a micro-second interruption on one channel triggers the high-speed data acquisition on that channel for the recording of the time signal, all other channels stay armed. They remain ready for the high-speed data acquisition of the eventual micro-second contact interruptions on these channels. Each channel can essentially operate as an independent transient recorder. 

Also in this mode it is possible to associate channels with each other. For example, excitation frequency and temperature are measured simultaneously during the interruption of an electrical contact. The recorded frequency and temperature are then linked to the event of the contact interruption. After the test procedure has been finished, this information can be used to analyze the correlation between the excitation frequencies and the number of contact interruptions.

While the contact interruptions are recorded with a very high sampling rate—with a time resolution up to less than 10 nsec (10-8 sec)—various standards require that the electrical contacts be monitored during the entire test time. The Dual Mode, in conjunction with Single-Channel Mode, is especially useful by defining a second slower sampling rate. The high-speed sampling clock is divided by a specific parameter to obtain a low-speed sampling rate for monitoring. For example, a sampling rate of 40 Msamples/s can be divided by a factor of 4 x 107 to obtain a sampling rate of 1 Sample/s. A continuous, seven day test would take less than 1.2 Mbyte per channel for a trend analysis of the contact resistance after the test. 

In the case of automotive connector testing, without ECR, QED’s process of gathering data for analysis on the quality and durability of the electrical contacts would have required many parallel channels of high-speed streaming recorders, with massive buffer memories and complex data processing. To seek out such failures and quality issues would have required much more computing power and specialized software for pattern recognition. 

Three Modes of Event Controlled Recording

All Elsys test systems have embedded PCI digitizers as their core, are LAN controlled, have single ended and differential inputs, sampling rates up to 240 Msamples/s at 14- and 16-bit resolution and extensive smart trigger modes, including the ECR method (below) of acquiring data. Clock synchronization up to 1024 channels is foreseen. Apart from housing PCI digitizer hardware, the instrument enclosure features server capabilities and hard drive, running in a Windows or Linux environment. ECR operates in three distinct modes:

ECR Multi-Channel Mode

The signal data is being recorded parallel on all active channels. After the initial start command, all the actual signal curves are constantly written into the module memory, which is configured as a ring buffer. Subsequently, each trigger event marks the range where the signal data will be read from the module memory and finally stored to the file. This kind of recording corresponds mainly to the well-known Multiblock Acquisition Mode, the way a digital oscilloscope records data in segmented memories. However in ECR, the stored time periods can overlap each other. Only with this method can a real zero dead time be guaranteed (also at acquisitions with a pre-trigger setting).

ECR Single-Channel Mode

The signal data is being registered separately for each channel. In this mode each selected channel serves as a single trigger source, which marks the area where the signal data is selected from and stored into the file. Only data from the corresponding channel is being registered. However, you can associate further channels parallel to a single channel. The signals of those associated channels are then stored in the same time frame as the triggering channel.

ECR Dual-Mode

ECR offers a third mode called Dual-Mode. This parameter in ECR defines a second sampling rate to digitize usually the underlying slower signal on which the transients ride. In ECR Dual-Mode the high-speed sampling clock is divided by a specific parameter to obtain a low-speed sampling rate for monitoring. For example, a sampling rate of 60 Msamples/s can be divided by a factor of 105 to obtain a sampling rate of 600 Samples/s (10 times the AC frequency). At those rates, continuous testing of principle signal and the fast phenomena can go on for a very long time. 

Elsys Instruments
Niederrohrdorf, Switzerland. 
+41 56 496 01 55. 
[www.elsys-instruments.com].

QED
Echternach, Luxembourg. 
26 95 78 90. 
[www.editus.lu].