Friday, December 16, 2011

Simulating Power Transients for Testing ECUs

Electronic control units (ECUs) used in automotive and aerospace/defense applications need to be immune to the harsh power systems in which they operate. Power system surges and drop-outs are common, so you need to thoroughly validate your ECU to assure proper operation. To assist ECU designers, standard ISO test specifications have been developed that replicate the power transients seen in automotive applications. These test specifications are rigorous, and the test equipment required to generate these transients is specialized and expensive. As a result, this equipment typically remains in the quality control (QC) lab, and it may not be accessible to the design engineers who need it most.

In this post we will look at how modern high performance power supplies and their arbitrary waveform (arb) generation capability provide an easy to use and capable platform for power transient testing of ECUs. To do this we will generate two example waveforms common in the automotive industry. The focus of each example will be ease of use (no code) and the power supply's arb performance. To generate the waveforms we will use the N6705B DC Power Analyzer. N6705B is a modular high performance power supply with up to 4 supply outputs and over 20 modules to choose from. Outputs can be put in series or parallel for higher voltage and current needs. For creating power arbs 50 W and above the N675xA series of modules is the best choice for ECU power transient testing. The N675xA series can generate arbs with edge rates of ~33V/ms into a full resistive or capacitive load up to 680 uF.

The first figure below shows a power supply reset test pulse train that is commonly used for ECU test. In this case, the N6752A module is used to create a simple pulse-train using a sequence of pulses (created right from the front panel). The device under test (DUT) was a load of 100 Ω in parallel with 10 μF. A close up of the final pulse shows a rise time of 553 μs (second figure below). Fall time (not shown) was measured to be 206 μs. Rise and fall times of approximately 1 ms are commonly called for in these types of tests. 

Next let’s generate a transient waveform for engine crank immunity testing using a high performance power supply like the N6705B. We will use the Starting Profile waveform in the ISO 16750-2 specification, which is pictured below.

At first glance this waveform seems complex but really it can be divided into four common waveforms, three ramps and one repeating sinewave. Using the N6705B and its built-in waveforms along with its waveform sequencing capability, we can easily build the engine crank waveform. Below are some screen shots (click on to enlarge) that show building the four waveforms and sequencing them together all from the N6705B's front panel.

And finally below we get to the resulting engine crank output waveform into a load of 100 Ω in parallel with 10 μF.

If you have to capture and recreate or just create complex custom waveforms this can be done fairly easy too. Typically you can capture a waveform on a scope or generate it using software like Matlab and then transfer it to the power supply via a CSV file using a remote connection or a USB memory stick. In some cases the power supply may have its own waveform editing software. For instance the N6705B has accompanying software (model number 14585A) that provides waveform editing as well as other measurement features. 

In the post we looked at how modern power supplies like the N6705B can be used to generate complex power transients for ECU testing. They provide an easy to use power transient testing alternative to design engineers who can't easily access expensive test setups in the quality control lab or who want to avoid expensive back and forth trips to compliance testing labs. If you have any personal incites or comments you want to add please use the comments section below.


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