It’s worthwhile to mention that because the test system can be situated at some distance from the DUT, good engineering practices should be observed in wiring the system together. Twisted-pair interconnects work best in these cases to reduce noise, crosstalk, and ground loops.
Because functional test is switch intensive, care must be taken in terms of selecting switches for the matrix. Designers have three main options: FET switches, reed relays, and armature relays.
FET switches are great if you need very high-speed switching and can tolerate higher on resistance. They have virtually infinite lifetimes, but being solid-state devices, they’re more prone to static damage than mechanical relays and can’t tolerate as high a voltage or current as relays since there is no additional isolation between the control circuitry and the signal path.
Reed relays offer high switching speeds and have higher voltage and current handling capability than FETs, while also providing for a low contact resistance. Armature relays are popular for their ruggedness and support for higher currents and voltages than the other options. They also offer a low resistance, but generally have slower switch times. Generally speaking, relays switch more slowly than do FETs and are mechanically larger.
Hardware In The Loop
Having examined a typical test setup, we’ll now take a closer look at the explosion in software in the automotive arena and how it has impacted test. As the number of lines of code grows, it’s safe to assume that the number of bugs will grow as well. “These are really defects in the vehicle, implemented in software,” says Chris Washington, National Instruments’ senior product manager for hardware-in-the-loop (HIL) and real-time test.
As a result, the exploding amount of software in vehicles presents significant challenges to test engineers for quality and safety. HIL test has proved to be a viable means of mitigating that growing challenge.
Testing of ECU firmware can be accomplished sooner, more thoroughly, and with more repeatability. By replacing the hardware components with models, you can test scenarios that would result in catastrophic failure of the real-world hardware. A common scenario in the aerospace world is an aircraft experiencing total engine failure. The automotive corollary would be running an engine with no oil.
A typical example of an HIL testing situation would be for an engine position sensor that would feed its signal to an ECU, which in turn is running firmware that calculates engine timing. In HIL testing, you can replace the sensor with a software model and run a simulation.
“The system doesn’t know if it’s connected to the real world or a simulated world,” says Washington. “It’s virtual reality for the controller being tested by simulating the sensors and actuators.” There’s no slowing down the real world, so you need real time to execute simulations very deterministically.
If you were testing a braking controller, one way to do it would be to have a dynamometer to simulate the inertial load on the axle. A real hardware brake can be applied to the dynamometer to facilitate test of the antilock braking algorithm to see how fast it can stop the system.
But with an HIL strategy, the brake-control ECU is connected to a simulated environment and a hardware brake is no longer necessary. The HIL is connected to a wheel speed sensor and to the ECU’s actuator outputs, which can then pump the simulated brake as in an ABS system. All that is fed back into the simulation.
“You can now have multiple terrains, multiple types of tires, swap out any variable in the system,” says Washington. Simulations can be run overnight to test all possible combinations of axles or brakes you might want to verify for the fleet. The combinations can be queued up to learn how an ABS algorithm works for each combination.
For HIL purposes, National Instruments’ Veristand product provides a sophisticated test framework within which to perform real-time simulation, data logging, and more. “With LabVIEW, test engineers have a development environment that’s graphical, quick, and with connections to hardware I/O,” says Washington. “But what we found is customers didn’t want to write and maintain this level of application themselves.” NI Veristand gives them an off-the-shelf framework for real-time HIL testing.
Meld Measurement, Analysis
In a typical scenario in almost any test-lab environment, data is gathered on instruments, typically oscilloscopes, and then transferred to a secondary environment for analysis, which is often Matlab from the MathWorks. But users of Agilent scopes have the option of acquiring their instruments with Matlab already installed.
Rather than forcing engineers to go through a two-stage work flow of gathering data with the instrument and dumping it to a PC for analysis, Agilent’s instruments now run Windows, which enables installation of Matlab for immediate analysis.
“You can do some monitoring and filtering on the fly in real time,” explains Wensi Jin, automotive industry marketing manager at the MathWorks. “But it’s typically a large amount of data, so it’s not a good idea to filter the data as it’s being collected.”