Common Signals for ECU/LRU Testers
In making ECU and LRU testers, we come across a lot of common signal types. We estimate that these common signals make up at least 80% of every such system. Here's what we see, where we tend to see it, and how we commonly approach acquiring or generating it.
(These are expressed from the tester's point of view. Our input is your ECU or LRU's output.)
What is it? An analog voltage or current signal. Typically less than 10V for voltage or 20 mA for current, the goal of this signal is not to convey power but information.
Where do we commonly see it? Sometimes ECUs or LRUs will have analog outputs for communicating an analog value, such as a conditioned sensor signal. Occasionally they may even drive very small loads.
Our typical approach: We most often use NI DAQ cards such as the 6363 for voltage signals. They are a good mix of performance and channel count, with good versatility. For higher speed requirements, we use NI's simultaneous samping cards. For current measurements, cards such as the NI 9203 work well.
What is it? This is usually an analog voltage signal, but occasionally current. Again, it is usually less than about 10V, and the goal is to convey information, not power.
Where do we commonly see it? Simulating conditioned analog sensors for ECUs requires simulating these. One example is the common O2 sensor.
Our typical approach: NI DAQ cards are appropriate, though we sometimes use R-series cards depending on the update rate needed. With the R-series cards, we can use FPGAs to simulate high-speed sensor models. LVDTs, etc. use signal conditioning where more current is needed than NI FPGA cards can provide.
"Discrete" Digital Inputs
What is it? A digital signal, but not at TTL (or chip voltage) levels. The voltage levels for a discrete signal are typically either zero or battery voltage. The system "battery" voltage, or positive rail, varies from system to system, but is generally nominally 12 V or 24 V in automotive systems, while being nominally 28V is common in aerospace systems.
Where do we commonly see it? This is common for discrete control signals (representing boolean values), safety relay loops, and occasionally for switching small loads like heads up display lamps.
Our typical approach: Use NI DAQ cards with standard TTL voltage interfaces (though many of them don’t output 5V, which is still most common in ECUs and LRUs). Putting the voltage-adjusting signal conditioning external to the card gives us modularity and flexibility. Most cards on the market that handle wider voltage ranges aren’t built for performance. Often sampled slower than the main system (10 to 100 Hz rather than 1 kHz), since ms timing on discretes rarely matters. Where it really matters, we have the option of choosing a DAQ card that gives us even higher performance. For those higher performance systems, we want to get a card with hardware-timed capability - in some cases, that means an FPGA-backed card such as the 7820 is the best choice.
What is it? Largely the same as Discrete inputs, but going the other way.
Where do we commonly see it? The ECU or LRU receiving button presses or digital signals between actuators, safety relay, or other ECUs.
Our typical approach: Since most of our digital IO cards backing the acquisition are reversible in direction, the approach looks the same as discrete inputs.
What is it? Often a discrete signal (though sometimes not at the full battery voltage), but rather than the on/off state being important, the frequency of the transition is important. Whether in terms of frequency, duty cycle, or ratio or absolute measure of high times and low times, these signals are typically the same for all but software.
Where do we commonly see it? Used to communicate a discrete value between ECUs, or to acquire a signal such as a tachometer.
Our typical approach: Like discretes, applying voltage shifting signal conditioning outside of the acquisition card is a best practice. We often use NI Counters or FPGAs. FPGAs have the advantage of handling 0% and 100% duty cycles better than DAQmx cards.
Resolver / LVDT / RVDT Simulation
What is it? An linearly variable differential transformer (LVDT) measures displacement(but is sometimes mechanically rigged to measure rotation), and rotary variable differential transformer (RVDT) measures rotation (often up to a fixed angle), and a resolver measures rotation (typically continuous rotation of a shaft). All three can be very precise and robust compared to alternatives.
Where do we commonly see it? Automotive and aerospace applications use them these days. Aerospace xVDTs and resolvers often operate at an excitation frequency of 400 Hz or less, and at moderately high excitation voltages compared to automotive xVDTs and resolvers. Automotive resolvers are often used in electrified motors, and tend to operate at much higher excitation frequencies, often from 10 to 20 kHz. But, these automotive resolvers also typically run at a lower excitation range of about 5 volts.
Our typical approach: These sorts of sensor simulations are not straightforward as supplying a simple voltage output. The tester receives an excitation and has to output a precise waveform in response to it, modulated by the position of a model. We can use NI FPGA cards for some xVDT and resolver solutions. For others that require higher current drive on the output, we can go with a transformer-based solution such as Bloomy's xVDT card for the SLSC <https://www.bloomy.com/products/slsc-and-crio-modules-and-accessories/vdtresolver-simulation-module-slsc>.
CAN / 1553 / ARINC-429
What is it? This is the standard digital communication bus for an ECU or LRU. They vary from application to application, but if you squint really hard at them they all start to look similar.
Where do we commonly see it? For automotive applications, CAN is dominant. For military aerospace 1553 is used, and ARINC-429 for civilian aerospace. We're painting with a broad brush here, because the prevalence of these technologies has led to other applications adopting their technologies. Additionally, there have been lots of rumblings about the move to ethernet-based technologies. While OEMs in both aerospace and automotive spaces are excited about the benefits, adoption has been slower than it seemed it would be several years ago, probably owing to the large technical development effort required to re-architect the many systems and interfaces involved.
Our typical approach: NI carries modulesfor CAN (which they call their XNET cards), and resell cards for ARINC-429 and MIl-Std-1553. Those are often our first choices, though other similar options exist on the market.
What is it? This is often a relay-switched power source to power things such as small lamps or small dc motors.
Where do we commonly see it? These abound in most application spaces.
Our typical approach: We treat these similar to discrete inputs, with the input card being high impedance. The load with the proper impedance is wired in parallel, and often stored in a vented portion of the rack for heat dissipation purposes.
We've seen these common problems before, and have dealt with many of them many times. We know how to build your tester and get your project done and done right.