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.