223Data acquisition system

To measure the naphthalene sublimation depth locally on the naphthalene blade surface, a microcomputer-controlled four-axis automated data acquisition system is used, which is also described by Wang (1997) in detail.

The four-axis data acquisition system consists of a four-axis table, a motor-controller (Superior Electric MODULYNX) with four terminals, a signal conditional (Schaevitz ATA-101), a Linear Variable Differential Transformer (LVDT) depth gauge (Schaevitz PCA-200-010 LVDT), a Digital multimeter (Fluke model 8840A), and an IBM-XT microcomputer with UNIX operating system and GPIB card. A flow chart of the system is shown in fig. 2.6.

The four-axis (X — Y — Z — 0) table described in fig. 2.7 is used to position the naphthalene blade and LVDT gauge. The XY-table (Design Components Inc. DC-1212) is similar to that described in Jin (1998). The rotary table is a model 6R90 of Design Components. The Z-table (Velmex B4015) holds the LVDT gauge while the rotary table controls the orientation of the naphthalene blade. All the tables are driven by step motors connected to four terminals of the motor-controller from Superoior Electric.

An LVDT is an electromechanical device that produces an electrical output proportional to the displacement of a separate movable core. The LVDT depth gauge made by Schaevitz Engineering (PCA-220-010) has a standard deviation of 225rim, in a linear range of ±0.25mm as shown by calibration and mock measurement. The LVDT gauge is calibrated by measuring a series of precision gauge blocks at 25.4^m intervals (fig. 2.8(a)). The random errors of the LVDT gauge and the four-axis table positioning system are estimated by mock sublimation depth measurement on the metal rims of the naphthalene blade. The blade is aligned and scanned twice as for a real naphthalene sublimation testing. Due to the random errors in electronic signals as well as in repositioning, the sublimation depth is not ideally zero, instead it has an Gauss distribution as shown in fig. 2.8(b).

The signal from the LVDT is sent to the Schaevitz signal conditioner (model ATA-101) and amplified to a DC voltage within range of I 13V. and is then input to the Fluke (model 8840A) digital multimeter.

The IBM XT computer is connected to the four-axis system and the digital multimeter through an IEEE-488 (GPIB) interface. The command issued by a data acquisition program from the computer is transferred through the IEEE bus to position the gauge and the naphthalene blade. The measured signal from the LVDT gauge is then converted by the conditioner and displayed by the multimeter, and finally collected by the computer through the IEEE-488 (GPIB) interface.

0 0

Post a comment