Evolution of a data logger

4USB solves some connectivity challenges, while Ethernet solves others. A combination of the two technologies with the right software behind it makes a powerful data acquisition system. The thinking inside the architecture shows how today's data acquisition systems have evolved.

More than a decade ago, Prosig based a new family of systems on USB hardware and improvements in data acquisition software. USB 2.0 was just around the corner, promising high throughput of up to 480 Mbps and high flexibility with plug-and-play connectivity. The software user interface at that time was adequate, but the prospect of handling aggregate data rates of several million samples per second within the existing software structure was not at all feasible. With faster USB, faster memory speeds and the latest SATA drives, and redesigned data acquisition software, high-throughput rates in excess of 20 MSps for logging applications are now readily achievable.

The underlying structure of the acquisition software is based on a multithreaded architecture with global memory buffers providing the transport media between the main control threads. Data acquisition is a straightforward business of capturing the raw data, storing it safely (if required for the application), and then performing some analysis processing.

There are two main control processes in the acquisition software. The first takes responsibility for retrieving data from the USB front end and optionally logging this data to disk. This process has the highest priority to ensure that the USB device is serviced fast enough to avoid buffer overrun. The second control process manages the user interface and carries out the processing required to display the data. This dual-process arrangement has the advantage of being configurable, either to be biased toward capture and storage for very high throughput applications or to be biased toward display responsiveness if the application demands it.

USB short, Ethernet long

The use of USB hardware for device communications has many advantages, but it also has a downside, namely a maximum cable length of 5 meters (Figure 1). When a long-standing customer wanted to use the system as an unattended logging system in a wind tunnel application, the design team had to think about how to overcome this restriction.

21
Figure 1: Using USB for device communications has the disadvantage of involving long cable lengths.
(Click graphic to zoom by 1.9x)

The nature of the application meant that the data acquisition unit would be slaved to the main tunnel control computer and mounted in the instrumentation bay, while the user interface would ideally be located in the control room 100 meters away. It was also desirable to have a local control mechanism, as data needed to be captured on command from the tunnel control computer.

The solution was to introduce a low-cost SBC with USB 2.0 and GbE to act as a remote control. This system would run Windows XP or Windows Embedded and implement the data capture and logging process of Prosig’s Data Acquisition Front End Software. The process of replacing the memory as the transfer medium between the capture and logging thread and the display thread with Ethernet was straightforward. Once complete, this subsystem provided a media converter that used Ethernet to extend the cable length between the host PC and the USB device transparent, as shown in Figure 2.

22
Figure 2: A distributed data acquisition system uses Ethernet to extend the cable length between the host PC and the USB device.
(Click graphic to zoom by 1.9x)

Storing and analyzing data

To meet the customer requirements, the design team also had to provide the tunnel control computer with the means to control data capture. Each data capture lasted for several seconds and captured data at a rate of 10,000 samples per second for each of the 90 channels.

It was therefore decided that the data should be stored locally on an SBC, which was configured with sufficient storage capacity to hold one day’s data captures. A serial port was also provided to allow the tunnel control computer to handle the data capture. Because it was important to record the tunnel conditions at the time of each capture, the serial link accepted other commands that provided this information.

As the tunnel conditions changed, this information was captured by the logger software, and when a command was received to commence capture, the current conditions were also logged in the same data file. This made it possible to access the data files after several runs using Windows file sharing, ensuring that the data captured had been encapsulated with the prevailing tunnel conditions. Data files were provided in a format compatible with the software so that when the tunnel data was post-processed, the tunnel conditions were readily accessible to allow grouping and automatic report annotation.

Embedding the intelligence

Prosig has now developed a more refined version of the logger called Prolog, which can work as a stand-alone unit, operated only from manual controls on the front panel or trigger signals. The final refinement to the logger unit was to provide the ability to locally store the acquisition setup.

When the unit is powered up, it detects if there is a locked configuration available, and if so, configures the external USB hardware to match that stored configuration. If the configuration is successful, the unit arms and awaits a capture request, which might come from a button press or trigger condition. At any time in this process, the acquisition software on the host can connect to the logger over Ethernet and either reconfigure the acquisition parameters or monitor the inputs of selected channels.

Remote controllers

The Prolog unit shown in Figure 3 uses a fanless Intel Atom-based SBC that requires less than 5 W of power. It also provides GbE and USB 2.0 connectivity and a solid-state SATA drive to supply additional robustness.

23
Figure 3: Powered by the Intel Atom processor, the Prolog controller enables remote operation of data acquisition hardware.
(Click graphic to zoom by 1.9x)

Prolog controllers replace a laptop or PC to allow remote, unattended, or stand-alone operation of a P8000 system. The system can include one or more P8000 chassis. Prolog can also be used to operate P8000 hardware from a remote location using a PC or laptop. It has a GbE port that allows the P8000 hardware to be controlled over a LAN, WAN, or VPN. The P8000 acquisition software can control a P8000 system over USB or Ethernet with the help of the Prolog unit without any loss of functionality.

Prosig’s P8000 capture hardware and DATS analysis software focuses on the capture of noise and vibration and associated data for mobile and laboratory applications. The P8000 series features a range of 24-bit measurement hardware. These systems can be configured with 4 to 1,024 channels and have options for accelerometers, microphones, strain gauges, thermocouples, and powered transducers. Additional options offer support for capture of data from CANbus, GPS, and IRIG inputs. The DATS software package has more than 35 years of development behind it and integrates capture from the P8000 systems with analysis and reporting capabilities. DATS also offers a wide range of data import and export from other measurement and analysis systems. IES

Jim Marshall is the managing director of Prosig Ltd. and has more than 30 years of experience in developing software for data acquisition applications.

Prosig Ltd. +44 (0)1329 239925 jim.marshall@prosig.com www.prosig.com