SPECIAL SECTION FEBRUARY 1999

he typical DA system requires signal conditioning to amplify low-level signals or filter unwanted high-frequency noise. With notebook computers and PCMCIA A/D cards, engineers can digitize sensor signals and store the data on the computer for rapid analysis. But PCMCIA A/D converter cards are too small and thin to provide multichannel signal conditioning. And the card's size makes it impractical for direct sensor-input wiring.

For applications requiring an instrument that provides the multifunction/multichannel benefits of PC-based systems and a small enough form factor to make the unit portable, you have four options: a notebook expansion chassis or specialized portable PC with internal full-size ISA/ PCI slots, parallel port­based units, special-purpose instruments, and external signal conditioning systems.

A number of DA vendors have modified conventional docking-station technology and developed expansion stations for mobile DA applications. These chassis connect to the notebook PC via the same high-speed port normally used to attach a notebook to a desktop docking station. The configurations accept a wide variety of ISA-/ PCI-compatible cards and run any software written for the PC without alteration.

But not all notebook computers have an expansion-chassis option. The chassis limits you to a supported notebook computer and adds to the notebook's cost and bulk. In addition, when it is equipped with plug-in DA boards, the unit typically requires an AC outlet, prohibiting use for portable DA. Finally, new boards can conflict with other devices competing for address space on the ISA bus.

  Parallel Port Units. Many DA vendors favor the standard parallel port and the enhanced parallel port. For example, Strawberry Tree's DATAshuttle portable systems offer thermocouple, RTD, strain gauge, bridge, and piezoelectric termination options for signal conditioning, as well as analog and digital I/O. Each unit uses one parallel port cable to connect to the PC's parallel port.

  Special-Purpose Instruments. This dedicated instrumentation is usually limited to only one type of signal conditioning, which makes it costly if additional hardware is required for other types of signals. These instruments target a range of applications and functions that may be too broad or too narrow to accommodate your specific needs. Also, they don't retain multifunctionality with PC-based systems.

  External Signal Conditioning Systems. Offering considerable advantages in size and the ability to expand, external units now perform signal conditioning. These units allow multichannel monitoring of a variety of parameters and typically connect to the DA device by way of a PCMCIA card, a proprietary interface, or a standard interface (e.g., RS-232 and IEEE-488). The units contain a CPU card and nonvolatile memory, and they can operate on batteries or an AC power source.

Photo 1. The SCS-800 signal conditioning system can be used with laptop data acquisition computers equipped with a PCMCIA A/D converter card. The SCS-800 provides built-in multiplexing for 64 channels, simplifying connection to the A/D card. You can also wire each channel individually for simultaneous sampling. Signal conditioning modules perform amplification, anti-alias filtering, strain-gauge excitation, and bridge completion.

Alligator Technologies offers one such system. Its SCS-800 modular system accommodates 64 channels in an external chassis the size of a notebook computer (see Photo 1). The system offers the same functions as standard ISA plug-in boards in a portable, expandable serial-controlled package. The system offers several advantages:
  • It enables multichannel monitoring of voltage, current, temperature, force, frequency, vibration, and other parameters.
  • It can be connected to a laptop, notebook, or desktop computer.
  • It keeps costs down by not duplicating the function of the computer.
  • It connects to a computer via a standard serial port. The system controls signal conditioning, provides analog I/O services, and does not store data onboard during high-speed applications, eliminating the necessity for high-speed communications buses.
  • It offers good noise immunity and signal integrity because all critical signal conditioning circuitry is in the external chassis outside the PC. In addition, the electronics can be placed close to the operation being measured, significantly reducing noise to low-level analog signals.
  • It features an onboard microprocessor and memory that stores configuration data for programming key characteristics between power interruptions.
  • It offers a variety of filter types and cutoff frequency ranges, uses standard Windows and LabVIEW software, and conforms to the Eurocard 3U form factor specifications.

An SCS-800 portable chassis system consists of as many as eight multichannel modules (see Figure 1).

Figure 1. The SCS-800 includes a control module containing nonvolatile memory. This module communicates parameter settings to the multifunction signal conditioning modules. Communications through RS-232 or RS-485 interfaces allow external control of the system by a remote computer running the SystemView 800 utility software. A programmer's interface provides for easy control of the system from custom application software or LabVIEW, HP-Vee, DASYLab, Diadem, or other data acquisition control platforms.

The nonportable SCS-801 rack-mount chassis can hold 18 multichannel modules. An optional multidrop RS-485 link allows 126 chassis to communicate with the control PC. The flexible architecture lets you configure a variety of modules to meet the requirements of specific applications. To expand the system, you simply plug additional modules into the chassis as test requirements change. The operator makes all I/O connections from the front of the unit, making operation and maintenance easy.

The SCS-800 chassis has the same footprint as a typical notebook computer. The modules are housed in an impact- and corrosion-resistant case that features a carrying handle, a front Plexiglas cover, tilt feet, and a front-panel LED that indicates power on/off status. The system's built-in RS-232 interface connects to the host computer's serial port to implement remote control.

The SCS-800 can be powered from a variety of sources. For stationary applications, the system comes with a low-noise AC power supply that provides regulated DC power to all boards at positive and negative voltages. For portable applications, a rechargeable battery provides 16 hr of operation with eight installed channels and eliminates the possibility of ground loops and power-supply noise commonly found with AC-powered systems. A companion dual-stage battery charger recharges the battery in 8­11 hr while the system is operating. With an optional cable, the SCS-800 can also be powered from a standard 12 V car battery.

Screen 1. SystemView 800 software provides point-and-click control of the SCS-800 from a remote PC operating Windows 95, 98, or NT. The screen shows how a strain gauge control module can filter, excite, and auto zero as many as four bridge-completed strain gauges. Up to 127 SCS-800 chassis–each containing as many as 64 channels–can be controlled simultaneously from a single computer running SystemView 800.

With bidirectional communications, the software verifies correct system configuration during operation. A controller card inside the SCS-800 stores the last selected configuration in nonvolatile memory between power interruptions. Multiple configurations can be saved on the host PC and applied again by selecting the file and sending the data to the SCS-800 over the RS-232 link. Available for Windows 95, 98, and NT and popular third-party software, the SCS-800's software preserves multivendor compatibility.

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