Modern digital technology and a little imagination have led to today’s most popular devices, like the MP3 player — a tiny digital music player that you can carry in your shirt pocket, and yet which can hold countless songs, TV shows, and movies. And they have led to cell phones that are serving as cameras and PDAs. Thankfully, this same technology “push” from the last 10 years has had its effect on the data acquisition system, albeit in a slightly more serious way.
The classic data acquisition system digitizes voltages, and displays and records them onto a hard drive. The better ones provide high-isolation signal conditioning so that the front-end is isolated and immune from ground loops. If you’re lucky, the software interface is intuitive and easy to learn, while capable and robust. But, this is analogous to a really great record player — it doesn’t add any new capabilities other than the storage of data in a random access digital storage medium.
Thankfully, the convergence of emerging technologies has enabled a real evolution in data acquisition instruments, unlocking their potential to do more than ever before. These evolutionary, or revolutionary, steps include the following technologies:
- Digital video recording.
- GPS receivers and timing technology.
- Digital data buses, such as CAN for automotive, and PCM for aerospace.
Imagine the case of an automotive test engineer, who finds himself faced with recording all manner of analog sensors in order to test his car’s performance under a variety of conditions, but who also can benefit greatly from recording hundreds of parameters from the car’s CAN bus. And GPS can provide exact position information, which can be used to calculate displacement, velocity, and even acceleration if enough time axis resolution is available. Video cameras can be used to monitor and record various test conditions, putting the data into context.
Truly Synchronized Data
Even today, too many engineers are faced with using four different acquisition systems to record the analog, CAN (digital), GPS, and video data sources. This presents a terrible challenge when it comes time to make sense of it all. If the data are not synchronized to begin with (which is the norm, since the systems have no common time source), making a cohesive analysis of the data from all these sources is difficult at best, and impossible at worst. If you add in other important sensors and technologies, such as infrared cameras for measuring temperature on surfaces that you can’t easily make contact with (like a disk brake in a moving car, for example), or even high-speed video cameras running at hundreds or even thousands of pictures per second, the challenge is exacerbated exponentially.
But so, too, is the opportunity for greatness. Imagine if you could do all of that in a single, integrated platform, and that all the inputs were recorded in a synchronous way. This would clearly result in more meaningful recordings, and more importantly, a richer understanding of test results. Figure 1 shows the integrated display of analog sensor outputs, CAN bus data, synchronous video, and 200-Hz speed and distance outputs from GPS.
Tests like this formerly required multiple instruments for the acquisition, and an incredible amount of post-processing time due to the nearly impossible task of trying to synchronize the recordings from these disparate devices after the fact. But with today’s convergence of video, GPS, and analog data, all of these inputs are recorded and displayed in a truly synchronized way.
The same challenge affects aerospace engineers, who need to record data while physically connected to the test subject, as well as those recording data from spacecraft and aircraft that are disconnected from wires (see Figure 2). In the latter case, the data must be packed into a PCM stream and then sent back to earth using FM or another wireless band.
For this application, a special interface called a bit and frame synchronizer, along with a decommutator, are used to receive and reconstruct the transmitted data. Normally, the digital data stream coming from the aircraft or spacecraft has hundreds of channels of information, at all different rates and bit depths. In this sense, PCM data is similar to the CAN bus data found in most cars. It is normally much faster and contains more parameters.
In today’s space and flight test ground stations, the ability to record this PCM data stream into a convenient data recorder has powerful benefits, including the elimination of costly and resolutionlimited programmable D/A converters, which have commonly been needed in order to make the PCM data analog again for display and acquisition by recorders that commonly have only analog inputs.
There is a further requirement to extract the time code from this PCM stream and use it to synchronize the data to real-world events, not to mention any analog data coming in parallel (often from tape recorders, when space mission data are replayed from a variety of recorders for further analysis). Today’s “convergence” data acquisition instruments can do all of this, and more, including recording multiple video data streams at the same time, when required.
Acquisition Without Boundaries
And don’t think that this technology convergence has positive benefits only for high-end applications in testing cars and aircraft. Data acquisition knows no boundaries, and there are more factories and power plants than all of the other types combined. Can you imagine the testing of pressure relief devices in power plants? This testing is essential and even mandated by safety protocols, and yet continues to be problematic in many cases. The simple addition of synchronized video acquisition, so that the test engineers and technicians can watch the valve start to leak fluid or gas, in perfect sync with the pressure and temperature outputs from the test stand, improves throughput and repeatability in all areas (see Figure 3).
Would you believe that temperature is the most often measured parameter of all those acquired by recorders and data acquisition systems worldwide? More channels of temperature are recorded than any other kind. It may not be the most glamourous parameter, but it is one of the most important, as well as one of the most underestimated. Temperature is easy to measure, but not easy to measure accurately. There are several key technologies employed, starting with the most popular: thermocouples. These sensors are inexpensive and easy to mount and connect. They are passive, requiring no excitation or power. But at the same time, they require an excellent cold junction compensation and linearization in order to produce accurate and meaningful results.
RTD sensors provide more accuracy over a narrower temperature range, but are more expensive and harder to wire. And being powered, resistance-based sensors, the excitation power provided actually can cause a shift in readings due to the “self-heating” phenomenon. This must be compensated for in order to ensure accurate readings.
And today, infrared sensors are becoming more and more mainstream and affordable. These “array” sensors offer more resolution and range, and the ability to see, in effect, thousands of points at the same time. Finally, being “contactless,” they allow you to measure temperature without touching the test subject: important when the subject is going to explode or move very quickly, and attaching wires is problematic or even impossible.
Imagine that you can record the same test with a conventional optical camera and an IR camera at the same time, and still add analog sensors, GPS, or whatever else is needed. And all the inputs are completely synchronized, no matter that their update rates might be.
The convergence of these technologies has blurred the lines between the “classic” instruments, and the creation of more flexible and versatile instruments. This has been well underway since the turn of the century less than 10 years ago. There is no doubt that based on the pace of these new technologies, and the seemingly boundless imagination of today’s test engineers for finding new and better ways to tackle test and measurement applications, these advancements will only continue.
It used to be that engineers would have a scope, a data recorder, a power analyzer, a video acquisition system, an FFT analyzer, a sound power meter — all separate instruments. But now this has converged (or is in the process of) into a single, more flexible device, or devices with multiple personalities. That is the real power of technology convergence as it has come to affect data acquisition today.
This article was written by Grant M. Smith, President of Dewetron, Inc., Charlestown, RI. For more information, Click Here