PC/104 Data Acquisition for Industrial Applications
Computer-based measurement and control is based upon analog input and output variables from sensors, representing parameters such as temperature, pressure, acceleration humidity and others. In the “real world” a variety of analog sources must be accurately digitized for automation and control to be effective
ROBERT A. BURCKLE, WINSYSTEMS
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Personal Computer (PC)-based automation and data acquisition have become entrenched over the years in applications such as industrial process control, factory automation, test equipment and scientific instruments. In the “real world” of industrial applications, a variety of analog sources must be accurately sampled and digitized for automation and control to be effective. Transducers, sensors and thermocouples present data in the form of analog voltages that represent parameters such as temperature, pressure, acceleration, humidity, frequency, vibration, weight, flow rates and current. Typically the analog source changes relatively slowly compared to audio or video signals so that very high-speed data acquisition is not required.
The earlier widespread usage of the desktop x86-based PC platform has given way to enormous markets for peripheral card and cable-connected (dongle) acquisition devices. A wide variety of solutions are possible because the computing platform is rich and standardized. These solutions include ISA-, PCI- and USB-based systems or even embedded industrial systems such as PC/104 or CompactPCI. The result is that a system designer is faced with the decision of which platform to use and then with selecting the corresponding analog I/O interface. For industrial and deeply embedded applications, a standard desktop PC approach can be unreliable because of its susceptibility to vibration, humidity, temperature extremes and even the rapid market-driven obsolescence associated with the consumer world. In other cases, it is simply too bulky to package into an instrument or OEM application.
Enter PC/104. Not only is the tiny 3.6” x 3.8” industry standard form-factor a great size for compact, highly integrated data acquisition systems, but it is powered by a wide range of PC-compatible CPU modules from 133 MHz to 1 GHz and beyond. These processors run Linux, Windows XP Embedded and other x86-compatible real-time operating systems with networking support. A designer or system integrator can stack two boards (CPU and analog) with an industrial CompactFlash device for data logging, which can be integrated into a single enclosure as small as 4” x 4” and only 2” high. These minuscule systems are designed to tolerate shock, vibration, dust, humidity, and operate over an extended temperature range without a fan, depending on the processor speed.
But even with a small, rugged solution like PC/104, there are still important analog signal design issues to be considered. The data acquisition system must be configurable to handle a variety of full-scale voltage ranges and be accurate over a broad range to ensure integrity of the data.
Ideally, the system should not require user calibration to maintain data integrity. Such a “No-Cal” implementation has great advantages. Old technology boards with trimpots (potentiometers) are prone to time- and temperature-related drift. Unpredictable and untraceable errors render data questionable and perhaps unusable. Additionally there is down time and the costs of a technician required to measure and adjust the system. User-initiated auto-calibration is better, but “No-Cal” solutions ensure the accurate results that are demanded by OEMs.
Precision Analog Input
In order to achieve good accuracy and resolution, a 16-bit analog-to-digital (A/D) converter is desirable. However, such precision comes with a cost. It takes specially designed circuits to mitigate noise and drift along with matching- and leakage-related inaccuracies over temperature. Frequent calibration helps, but the optimal implementation would feature either automatic onboard calibration or no calibration (No-Cal) at all. The challenge to board and systems designers is to shrink all of this circuitry into a space-saving size while improving operation and cost of ownership over the long haul.
To minimize the effects of drift error, analog board designers must approach calibration and drift from the ground up through careful design and component selection. The single most important component in any analog converter design is the analog voltage reference. Reference voltage drift directly affects analog conversion accuracy, expressed as full-scale (gain) error. The reference voltage tends to vary over temperature as well, although well-matched and compensated implementations keep the variations to a minimum (Figure 1).
Drift contributions from any other source, whether from the converter itself or signal conditioning circuitry, can affect either zero (offset) or full scale trim (gain). Drift errors are predominantly functions of component drift over time and how changes in temperature affect components in the system.
Previous generation analog design techniques moved to onboard auto-calibration. However, in some manufacturers’ designs, these auto-calibration circuits would drift more over time and temperature than the analog converters they were intended to calibrate. At their best, auto-calibration circuits effectively compensate for drift error between calibration intervals over a limited temperature range on products with highly stable references. The worst auto-calibration circuits do little more than add marketing weight to a datasheet, and will likely reduce the functional accuracy in the field.
The best solution today is to select an integrated data acquisition system on a chip. Linear Technology has integrated the key analog system design elements onto a single die in their “Soft Span” series of ADCs. By doing this they can match and trim each subsystem to compensate for errors introduced in the entire conversion process.
With the small PC/104 board stacking architecture in mind, the Linear Technology’s LTC1859 is part of a family of analog-to-digital converters (ADCs) that lends itself to 8-channel applications with 16-bit conversions at fast sample rates. The device includes input multiplexer, range select, sample and hold, analog-to-digital converter voltage reference and associated control logic. Each high-resolution, high input voltage range ADC in this family has an on-chip, temperature compensated, curvature corrected, band gap reference that is factory-trimmed to 2.50V.
In addition, the use of precision, laser-trimmed thin-film resistors eliminates the need for user calibration. Zero error, zero error match, full-scale error, full-scale error match, linearity, reference voltage and conversion time are trimmed during production. The LTC1859 series attains the desired No-Cal implementation—no user calibration is needed.
This ADC uses an easy serial interface for configuration, and can be software programmed for 0V to 5V, 0V to 10V, ±5V or ±10V input ranges. The 8-channel multiplexer can be programmed for single-ended inputs or pairs of differential inputs or combinations of both. This replaces external analog switches, amplifiers and attenuators. In addition, all channels are fault protected to ± 25V for high reliability and low cost-of-ownership considerations (Figure 2).
A fault condition on any channel will not affect the conversion result of the selected channel. An onboard high-performance sample-and-hold and precision reference minimize external components.
Precision Analog Output
Thanks to the highly integrated input conversion circuitry, there is sufficient room available on a PC/104-size board for several output channels of precision analog voltages. Similar to the analog input section, it is important that no calibration be required for the analog output as well. In the past, designing a universal output module was a difficult task since the cost and board space consumed were problematic. However, with the new multiple output range DACs, all of this complexity is unnecessary.
As a further example of a highly integrated D/C converter, let’s examine the Linear Technology LTC1588. It is a 12-bit D/A with all the standard industrial output ranges (0V to 5V, 0V to 10V, ±5V, ±10V). All of the ranges are accurate with low drift, fast settling and low glitch operation. The LTC1588 DAC incorporates all the switches and precision resistors. A full implementation is PC/104-friendly, using less than 0.5” x 0.5” of board space including the dual operational amplifier, bypass and compensation. This analog output subsystem can be reconfigured under software control in real time.
An advanced analog I/O module facilitates the migration from PC-centric automation to small self-hosted stand-alone DAQ systems, in the industry standard PC/104 form-factor. This is possible thanks to the availability of No-Cal integrated circuits. Such a high-density analog and digital I/O card can operate from -40° to +85°C. This PC/104-compliant card includes a 16 channel, 16-bit analog-to-digital (A/D) converter, 8 channel, 12-bit digital-to-analog (D/A) converter and 48 lines of digital I/O. Using Linear Technology’s fully integrated A/Ds and D/As eliminates the need for all of the outboard analog circuitry used in older designs, which causes the errors and offsets that lead to the former need for calibration. There are no missing codes and the measurements are monotonic over the full temperature range from -40° to +85°C.
An example of such a card, the PCM-MIO from WinSystems shown in Figure 3, has been designed to minimize drift error effects while simply containing ultra low-noise power supplies and Linear Technology SoftSpan A/D and D/A integrated converters. The module optimizes converter accuracy over time and temperature while avoiding the pitfalls of trimpots and other conventional calibration techniques. It is compatible with isolated signal conditioners that will protect, filter and isolate the analog input and output signals from electrical transients for rugged industrial applications. There are many models available from third-party vendors to interface to a wide variety of voltage, current, temperature, position and other analog-based instrumentation.
There was even room left over for 48 lines of digital I/O for a very complete digital acquisition system. Each line is individually programmable for input, output, or output with read-back. Edge detection can also be programmed to generate interrupts for each event change without polling. The lines are TTL-compatible and can source and sink 12 mA, which allow them direct connection to industry-standard (Dataforth, Opto-22, etc.), optically isolated AC and DC signal conditioners.
Using state-of-the-art low-noise 16-bit A/D and D/A converters with No-Cal auto-calibration gives a shot in the arm to the DAQ market. This clean, simple design yields smaller size, lower cost and much better accuracy by avoiding error-prone manual calibrations. The PC/104 Bus platform provides an efficient and long-lifecycle, 16-bit data path for these converters. The small form-factor is attractive to integrators and OEMs who need to integrate the PC and the DAQ circuitry together for their next designs.
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