Up and Away: Poor calibration data for temperature
sensors results in increasing errors at high temperatures. Device-specific
calibration data in a TEDS lets software properly interpret temperature
Developments in instrumentation and data acquisition include the increasing
use of smart transducers, the adoption of USB 2.0 as a communication channel,
and advances in real-time operation of test software. When combined, these
technologies ease the setup and use of test systems that can offer real-time
Although "smart" transducers have existed for several years, devices that include a transducer electronic data sheet, or TEDS, offer the promise of better measurement accuracy. Transducer manufacturers traditionally provide written calibration data for their transducers, but that information may not let engineers take advantage of a transducer's inherent accuracy.
Printed calibration data often pertains to a batch of transducers or to a generalized transducer, rather than to individual devices. Transducers that provide a TEDS capability, however, let manufacturers use automated systems to calibrate individual devices and then save calibration data in a TEDS. As a result, that calibration data lets engineers obtain more accurate test results from a specific device. The increased accuracy available from TEDS-based transducers makes them attractive in exacting test applications, such as those in military, government, and safety test labs. The graph to the left compares errors in measurements taken with two types of temperature transducers: one with a TEDS and one without.
The calibration information stored in a TEDS has no value unless data-acquisition equipment can use it or pass it to a host controller or computer. (www.designnews.com/article/CA312628). Increasingly, data-acquisition systems offer the capability to interface with TEDS-based devices. Unfortunately, plug and pin standards don't yet exist for TEDS-based transducers, but standards will emerge.
Talk to Me
Communicating test data with a remote host computer also presents challenges. Some test engineers use Ethernet connections to link instruments and data-acquisition systems with a network or with a host PC. That arrangement works well in some situations. Test-equipment and data-acquisition suppliers also have adopted the Universal Serial Bus (USB) as a high-speed communication link.
The latest standard—USB 2.0—specifies three transmission rates: 1.5 Mbits/sec. (low speed), 12 Mbits/sec. (full speed), and 480 Mbits/sec. (high speed). Always check a manufacturer's data sheets for actual communication rates. Compliance with the USB 2.0 spec does not guarantee the high-speed rate. Suppliers may offer USB 2.0-compliant equipment that operates at only 1.5 Mbits/sec.
Perhaps the most significant advantage of USB centers on the ease of setup. Operating-system software in Windows-based PCs provides an "auto-detect" capability that senses new USB hardware attached to a host and properly configures drivers and associated application software. Performing the same operations for an Ethernet connection may require extensive program and driver development because Ethernet lacks an auto-detect feature.
USB also proves attractive because it facilitates real-time response and distributed data acquisition, and it simplifies equipment setup. The high speed offered by USB 2.0 lets a host PC easily stream test data to disk for temporary storage. The timely arrival of test data also lets a PC make decisions based on real-time information.
A data-acquisition application that requires an add-in board forces the user to open a host PC's case. During that process, a technician could dislodge a jumper, misconnect a signal header, or short out components. On the other hand, a USB cable simply plugs in to an open external connector, or port. The USB connections take the fear out of converting a PC into a data-acquisition system. And, today's PCs come with plenty of USB ports; even low-cost desk-top units may provide as many as eight.
A connection to a PC through a USB port also simplifies communications. That capability comes in handy when test engineers want to monitor results some distance from a test stand. Think of vibration testers, engine testers, and particle accelerators and you'll understand the need to put some distance between test apparatus and engineers. In other cases, it makes more sense to send digitized data to a host PC through a USB cable than to send many raw analog signals through an expensive multi-conductor cable.
High-speed communications and better transducer accuracy depend on underlying driver software. Drivers extract TEDS information and provide the code for USB communications. And in demanding applications, drivers must allow multi-threaded operations. Without this capability, low-priority tasks can block high-priority access to drivers. In a multi-threaded environment, a high-priority task can interrupt a low-priority task to gain access to key system resources. This type of priority-based operation is critical for real-time performance in data-acquisition and control systems.
Driver software also must handle single-point I/O tasks—those operations that require a fast response to a single data point. This type of response lets a data acquisition system take a single pressure value from an engine cylinder, for example, process it, and quickly adjust the timing of an engine valve to optimize the engine's operating performance. Without single-point I/O software, the acquisition system might have to evaluate many points on a waveform before it would be able to transmit a command to an engine valve.
You can expect to see more vendors combine smart sensors, USB-based data-acquisition systems, and sophisticated drivers as users "push the envelope" of technology with their test needs.