Parameterising automotive low voltage waveforms for test
When trying to test automotive systems and sub-systems a common and very effective method is to parametrise these signals, essentially breaking complex waveforms down into a just sufficient number of voltage steps and ramps and corresponding time periods.
This technique essentially simplifies the process of testing while maintaining the key features of the waveforms likely to cause issues during test. This then allows you to focus on producing repeatable waveforms and applying them to the device(s) being tested.
Some of the key features known to cause issues during low voltage testing are shown below:
- Voltages near in internal electronic threshold (such as CPU supply voltage)
- Fast rising edges (can cause in-rush currents)
- Fast falling edges (can pull current out from the PSU or other circuitry)
- Voltages between known thresholds (eg: between the thresholds of two different CPUs in a system)
- Voltages above the nominal working voltage
- Voltages regularly varying around a known threshold
Each of these effects occur in commonly encountered scenarios, such as loss of function failures during engine cranking (or start-stop cycles), low voltage drop-out pulse testing and battery discharge ramp testing.
What does parameterising waveforms really mean?
By looking at a waveform carefully, and taking into account the types of common failure modes listed above, it is possible to approximate parts of a waveform as constant, other parts as a linear ramp and others as oscillations of a particular frequency and magnitude.
Thus, for example, the voltage waveform exhibited by a vehicle supply, developed by the combination of battery wires and starter motor during engine start, can be simplified into a specifiable format to the extent it can then be demanded in standards testing.
Tests such as ISO 16750 Part 4.6.3, Ford CI230 Part 17, Chrysler DC-10615 Part 7.4, GM GMW3172 Part 9.2.17, JLR CI265 Part 16.1 and many others all use these techniques to attempt to specify a waveform that is easy to define and reproduce.
‘Meet The Standards’ reference document
We have compiled a list of many of the automotive low voltage testing standards into a downloadable form, detailing which hardware is required to test to these standards.
This document provides a very useful reference on many of the published standards and what is required to meet these.
How do I generate these parameters?
Once these waveforms have been defined, they need to be played by equipment that can easily load the parameters and calculate a waveform to accurately reproduce the waveform using high power electronics.
Engine starting or cranking waveforms.
Use cases: Standards testing for all electronic control units (ECUs), stop-start system verification and testing, general functional testing for automotive systems.
Relay or contactor interference.
Use Cases: Standards testing for all electronic control units such as JLR CI 265, ECU power supply robustness tests, general functional testing for automotive systems.
Simulating unattended battery discharge issues.
Use Cases: Testing for long-term unattended use, such as airport parking, standards testing such as battery dropout tests.
Brief interruption of supply issues.
Use Cases: Looking to simulate poor electrical contact or very fast rise and fall times on the power supply.