With all the electronic subsystems packed into today’s vehicles, testing can be tricky. Here are some switch-system design techniques to help avoid testing pitfalls.
The sophistication of today’s automobiles continues to increase, a trend driven by an ever-expanding suite of electronic control subsystems. As a result, fast, accurate testing of these systems is critical, calling for extensive test and analysis not only during production, but also during validation and qualification.
A critical component of any effective test system is tightly integrated, flexible switching. Space constraints and test-stand footprint also are key issues, driving the need for high-density switch designs without compromising signal integrity.
The validation and test process for various electronic-control modules used in the automotive and truck industries requires monitoring and controlling a large number of data points. The synchronization of test sequences involving the application of stimulus and measurement of each response is key.
A sample listing of typical vehicle control modules consists of the engine control module (ECM), the transmission control module, anti-lock brakes, the instrument panel, the electronic system controller, and the onboard computer. It is not uncommon for these modules to exceed a combined I/O requirement of 200 channels, so signal routing is of paramount importance.
Fault Condition Testing
A typical validation test sequence involves configuring a number of relays to simulate a component failure, including headlights, taillights, and feedback devices such as door closures. As a result, the test system must provide high-density, high-current switching paths.
One common method for performing this test is to connect a number of SPDT relays to the control module, with signals from the module routed through the normally closed contacts. The fault condition then is simulated by opening the specific relay connected to the input or output under test; for example, opening one relay might simulate a broken headlight filament.
Event Monitoring
Event monitoring involves generating a serial message on the bus commanding a certain event, such as left-blinker activation, and then monitoring the results. This typically is performed by connecting the ECM output response line to an I/O card and an event monitor in parallel.
The relay must route the signal without degradation; consequently, bandwidth specifications are essential when evaluating hardware solutions. The parallel connection of these two devices will permit event confirmation as well as a deterministic record of when the event occurred (Figure 1).
Load Testing
Load testing requires generating worst-case inductive load conditions, energizing solenoids and motors for example, and characterizing the control module response to the instantaneous load change. The same switching requirements still apply, but additional current-carrying capability is needed. Parameters such as duty cycle and rise and fall times also may affect the choice of switch routing.
When routing signals to other instruments, such as a universal counter, digital multimeter, oscilloscope, or digitizer, noise generated by high-level signals can be coupled to adjacent relays and circuit components, resulting in crosstalk. VXI Technology addresses this issue by shielding all relay boards, providing superior crosstalk rejection and isolation performance. Other sources of error can include thermal offsets, capacitance, and insertion loss. All these parameters are characterized by the manufacturer and should be considered during the design phase.
The solution to these test problems is far more complicated than simply connecting several relays and digital I/O lines to a module. The issues facing the design and test engineer now become how to most efficiently and cost effectively perform this task.
Switching Is the Key
One thing most testing scenarios have in common is the need for tightly integrated, dense switching solutions. Innovations in device miniaturization have enabled manufacturers of VXI-based hardware to provide the optimal test solutions for these and other similar applications.
Traditional IEEE 488-based switch solutions, while providing low to medium channel-count capability, are hampered by several serious issues. IEEE 488-based devices generally operate at slow speeds and the limited bandwidth switching capacity will manifest itself when evaluating test-execution times. This is even more critical in production environments.
Additionally, the use of proprietary backplanes in IEEE 488 switching devices can severely limit the selection and availability of switching configurations and support. The end user now is tied to the development schedule of a single vendor, who may charge exorbitant, nonrecurring engineering fees or simply decide the additional switch development no longer is part of the vendor’s business plan. This is a consideration when long-term support or product development cycles are important.
PXI offers a PC-based backplane supported by multiple vendors, but this choice also is faced with limitations. Density, triggering, cooling, and power are concerns with PXI.
Compared with the constraints of PXI, VXI hardware is the clear choice for this application. The VXI standard resolves many issues including high-density, high-power consumption; heat dissipation; and synchronization.
The introduction of high-density modular switching has provided the capability to insert as many as 960 2-A relay points into two VXI slots. This modular approach permits a variety of switching configurations, such as individual SPST, SPDT, multiplexers, and matrices, to be installed in a single C-size carrier board.
As many as six individual switch modules can be inserted into a two-slot main card carrier, allowing extreme flexibility when configuring the system.This approach not only increases the configuration flexibility, but it also simplifies the task of the maintenance technician if a failure occurs. Instead of being faced with many cables and connectors on a card, a single switch module can be isolated and quickly replaced, reducing downtime.
Additional concerns for most test and production systems are physical space and the footprint of the completed system. The high density of this modular switch approach decreases the number of slots required in the chassis, which will reduce system size appreciably. The end results are a smaller footprint and increased cost-savings due to lower switch and instrument hardware investment costs.
Signal-Routing Requirements
A wide variety of signals must be simulated, controlled, and measured during a test sequence, including dashboard indicators, lights, switches, sensors, solenoids, relays, and motors. Signals that have relatively low current requirements can be routed using standard 2-A SPST or SPDT relays. Examples of this type of signal include indicator lights or door switches.
Other signals require much higher current-handling capacity. Steady-state current levels may reach 20 A, with some surge currents reaching 70 A. The rated relay capacity must accommodate these signal levels, and the chassis must dissipate the heat generated when several switch paths are active. The VXI specification addresses this issue, and VXI mainframes are designed to perform under these load conditions.
VXI hardware has evolved to the point where the solution to all three measurement tasks in this article can be addressed in a single slot. A modular approach to instrumentation allows a combination of up to three similar or different instruments to be installed on a single carrier card.
Future Expansion
Another key concern of the engineer is the ability to quickly and easily expand a system to meet future product development requirements. A modular system provides a variety of configuration possibilities. If the number of switch points increases with next year’s automobile models, a new card can be installed and the software updated.
Incorporating next-generation modular switch solutions also will help resolve traditional expansion challenges. If, for example, three switch modules were housed in a two-slot carrier to satisfy today’s requirement, you could expand the system in the future with three additional modules without using any additional VXI slots (Figure 2).
Software
Most VXI manufacturers provide standard VXI plug-and-play drivers with their switching solutions. Some manufacturers simplify the programming task by providing a standard software driver that is used across the entire family of switch cards. This greatly reduces the development time involved by leveraging a single command set across the range of switch products rather than requiring switch-card-specific drivers.
Another approach provides direct register access to the switch controllers. This has additional speed advantages by permitting direct hardware access and bypassing traditional levels of a message-based software driver. If execution speed is of concern, this is one approach to consider.
Conclusion
Electronic control modules are an integral part of any automobile or truck control system, and because public safety is a major concern, these modules must perform flawlessly. As a result, each module has to be tested and characterized thoroughly before it is released for general use.
Dense, multipoint switch solutions are the key to performing these tests. Although different platforms are available, VXI provides cost and performance advantages. When the chassis is configured with high-density instrumentation, such as DMMs, universal counters, and digitizers, a complete test solution can be implemented in one high-power, flexible package.
About the Author
Jon N. Semancik is the field applications manager at VXI Technology and holds a B.S.E.E. from Fenn College of Engineering. He has more than 20 years of automated test and data acquisition design experience in the automotive and aerospace industries, previously holding positions with DaimlerChrysler, Westinghouse Electric, and Northrop Grumman. VXI Technology, 17912 Mitchell St., Irvine, CA 92614, 949-955-1894, e-mail: [email protected]
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November 2002