The competitive nature of the automotive industry generates constantpressure to boost product quality while driving down costs. Hence,demanding but useful activities such as electronic functional test areoften tagged as “necessary evils” that must guarantee a high return oninvestment.
System designers responsible for testing automotive electronics thenrequire architectures that can maximize performance, minimize cost andplan for the future. An example of these test systems is the LANeXtensions for Instrumentation (LXI) solution (Figure 1, below ).
Yearly changes in car models require manufacturers of automotiveelectronics to bring new products to market in a very short time.Factors that impede rapid development of test systems or decreaseproduction uptime are not tolerated.
Thus, rapid system creationdepends on getting instruments connected and systems running as soon aspossible. This enables manufacturers to focus on verifying thefunctionality of a module and its subassemblies.
Most of these systems are created with VXI -or PXI-based hardware. As they arecontrolled with either an embedded PC or a standalone PC connectedthrough an interface card and cable, developers face four key problemsthat LXI addresses:
Interface. Rather than an MXI or GPIB interface, LXI uses the Ethernet. Thus, it does not require the installation of an additional interfacecard in the PC, proprietary cables and software. The solution can runsystems in minutes instead of hours.
PCconfiguration. Because a PXI cardcage is an extension of the PCbackplane, the whole system must be rebooted every time a card isinserted or removed. This is not an issue with LXI. Its LAN connectionmakes it unnecessary to reboot the PC when connecting or disconnectinginstruments. Moreover, several modular LXI instruments allow for”hot-docking” or the insertion and removal of cards while power is on.
Drivers. When a PXI system reboots, the PC uses an instrument discovery processto identify newly connected devices – and this requires downloading andinstallation of device drivers, which can take up time.
The LXI Standard specifies the use of IVI-COM drivers, which make iteasier to work in different development environments. However, some LXIinstruments can be programmed directly through Standard Commands for ProgrammableInstruments (SCPI) when greater functionality or performanceis required. SCPI can be used with any computer language through simpleVISA function calls.
User interface. With no frontpanel interface, it can be difficult to use the PC-basedsoftware to diagnose problems in PXI and VXI devices. Using benchtopLXI instruments, the front-panel interface makes it easy to experimentwith an instrument and learn how it works.
Most modular LXI instruments lack a front panel and instead have abuilt-in Web interface that allows engineers to learn devicecapabilities by opening a Web browser on the connected PC.
The browser function also enables engineers to view the equipment'sdevelopment from anywhere in the world, simplifying system support andensuring greater system uptime.
Thousands of tests
Testing of automotive electronics includes complex powertrain controlmodules that require thousands of tests and simple airbag modules thatmay require large data transfer.
These tests often challenge the speed of GPIB, which has up to 1MBps data rate. Using LAN, I/O transfer speed is now a non-issue, with1Gbps connections becoming commonplace and 10Gbit versions on the way.
I/O performance is not an issue for LXI devices in automotiveapplications that require both transactional programming and transferof large data blocks.
The speed of LAN allows the transfer of large data blocks such as awaveform captured by a digitizer. In transactional programming, LANfaces the issue of latency, which is also an issue for storagenetworks. To reduce the number of required communication cycles,instructions are preloaded to LXI devices.
Minimizing the overall cost of test requires fast, reliable testingat the lowest possible price. There were suggestions that functionaltest adds no value – at that late stage of the manufacturing process,most manufacturers have inspected incoming parts, performed X-rayinspection and completed in-circuit test.
These steps do improve product quality, but do not eliminate theneed for functional test because they cannot detect faults due toinfant mortality, design errors and inaccessible nodes.
All this is compounded by conflicting requirements of automakers asthey can impose penalties for both late shipments and high defectrates. To solve this, instruments must deliver optimum capabilities andperformance for the money. It also requires careful consideration ofboth initial hardware cost and recurring costs such as spares,warranties, local versus return-to-factory repair options, andavailability of rental equipment.
It is also worthwhile to account for the learning-curve costs ofcardcage instruments vs. LXI. Cardcage instruments require differentsoftware drivers for each development environment – LabVIEW , Visual Basic, C++and so on.
LXI instruments generally offer a choice, enabling use of eitherdrivers or SCPI. For developers familiar with SCPI, the instrumentlearning curve is typically short. The IntelliSense help functions andonline documentation of the .NET environmenthelp simplify programming with drivers.
A typical automotive electronic functional test system built with LXIdevices contains expandable reed relay matrix, armature-relay loadswitches, channels of arbitrary waveform output and channels of D/Aconversion (Figure 1, above ).
In a cardcage-based system, these devices can quickly fill everyslot and then additional devices will require another cardcage andcomputer interface. For simple systems that need just a few cards, thecardcage adds cost and consumes space, though the empty slots allow forfuture expansion.
LXI instruments provide the exact functionality needed and makes iteasy to add devices without requiring another cardcage or computerinterface. At most, the system may require the addition of a low-costLAN switch to provide more ports for added LXI devices.
There has been continuous improvement in LAN performance over theyears while maintaining backward-compatibility. The widespread adoptionof LAN suggests that it will continue to be a dominant force in thecomputer industry for a long time.
The extensions designed into the LXI Standard ensure that it willmeet industry needs for test and measurement for a long time. The needfor longevity is important for the automotive electronics industry,which supports an active aftermarket and long product lifetimes.
In terms of flexibility, cardcage- based solutions limit the optimalplacement of instrumentation in a test rack. For example, it is usefulto put switching in one low-cost subsystem and stimulus/ measurementinstruments in another. This simplifies service and also avoids theinefficient use of costly backplanes to control slow relays.
LXI instrumentation also enables a new approach for flexibility. Forexample, an LXI-based device that has internal DMM and selection ofswitching cards can offer a low-cost, dedicated method to create aswitching subsystem. This feature also enables the LXI-basedinstrumentation subsystem to be placed elsewhere.
Functionality can also be an issue. Few cardcage-based powersupplies meet the current requirements of automotive electronicmodules. This requires the use of external power supplies based ondifferent architectures such as those that are LXI-compliant.
|Figure2. Some LXI power supplies have size and functionality advantages overGPIB and PXI models.|
Modern designs have additional improvements, such as fast up/downprogramming, power waveform creation and monitoring, and compactenclosures. Examples of this are shown in Figure 2 above.
In automotive applications, an LXI-based functional test system canbe assembled in a 400mm tall rack (Figure3 below ). Some LXI-based devices can partly contribute to thisspace efficiency. To achieve maximum density, system developers oftenuse cardcage-based instrumentation.
With VXI, a C-size cardcage can hold up to 12 high-performanceinstruments in about 6U – but this is often a high-cost solution. PXIalso provides high density, but its compact 4U size has four keyshortcomings:
Card size. The size of PXI cards sometimes requires the use of more than one slotto achieve the needed functionality. LXI instruments can be created invarious sizes to fit their intended use.
Shielding. PXI cards suffer from different interference issues. For example, anSCXI power supply that emits high magnetic interference can lower theperformance of an adjacent PXI DMM, potentially lowering DMMperformance by a full digit of resolution.
VXI avoids such problems because it requires that all cards haveshielded enclosures. Likewise, LXI devices are inherently shieldedbecause they are fully self-contained.
Cooling andpower. Cardcages must provide sufficient cooling andpower-supply capacity to handle a maximum number of instruments orrelays at one time. In demanding systems, it may be necessary toupgrade to one or several higher-cost mainframes that can provide therequired cooling and power.
Furthermore, applications in automotive electronics often requireinstrumentation output voltages that exceed the voltage capability ofmany PXI mainframes. Most LXI instruments can provide the requiredpower, voltage and cooling for their target application.
|Figure3. With LXI, a functional test system can fit into a rack that is just400mm tall.|
Automotive production test systems typically co-locate all of theirinstruments. However, durability test systems, R&D test systems andproduction validation systems can benefit from placing LXI instrumentswhere the measurement must be made.
Production test systems can also benefit from a remote test-head.With off-the-shelf LXI switch modules, it is possible to create a testfixture that automatically adapts to any engine control module comingdown the line, whatever the pinout.
This could be mounted inside an enclosure and attached inside arobotic final test cage. LXI modules have the ability to put thestimulus and measurement instruments where they are needed – withminimal or even no cabling back to the core of the system. Modules suchas the Agilent L4400A series -1U high and no front panel – are designedfor this type of remote or distributed application (Figure 4 below ).
|Figure4. LXI switching modules enable the creation of powerful remote testsystems.|
Another factor that favors LXI is remote debugging andtroubleshooting. Service technicians with remote access privileges candiagnose a test system from anywhere in the world simply by using a Webbrowser. If a LANconnected Webcam is added to the system, the remotetechnician can see what's happening as they troubleshoot the systemfrom afar.
Furthermore, in high-volume production lines, the ability to shaveoff 1s of test time per module can be worth thousands of dollars. Insuch case, any change to hardware or software that causes an increasein test execution time is totally unacceptable.
LXI addresses this through extensive triggering capabilities. Itstarts with a standardized trigger bus in Class A LXI instruments. LXIgoes farther, providing a new way to improve test execution time:self-triggered measurements based on a precise real-time clock aresynchronized from instrument to instrument. With this capability basedon IEEE-1588,complicated and time-consuming measurements are performedwithout intervention from the host computer.
This can minimize or eliminate trigger wiring a test system andreduce I/O bottlenecks. This new capability is yet to be available onall LXI devices.
As cars become rolling hubs of Internet, cellphone and GPSconnectivity, they use multiple types of wirelesscommunication.Consequently, test requirements in the automotiveindustry are beginning to merge with those of the telecom andaerospace/ defense industries – and more RF test sets will likely maketheir way into automotive electronic test systems in the future.
For example, the U.S. aerospace/ defense industry is demandingdiscrete instrumentation building blocks – RF amplifiers,up- anddown-converters,digitizers – thatcan be easily arranged and rearrangedon the fly to provide the functionality of oscilloscopes, networkanalyzers and spectrum analyzers .