Industrial Application Case Study - Data Acquisition Industrial

Industrial Applications

Galvannealing Phosphor Thermometry

Customer Case

During the process of galvanizing, steel is dipped in a bath of liquid tin to increase its resistance to oxidation and moisture. After galvanizing, a precise heat treatment allows control of the mechanical properties of the final galvanized steel. This heat treatment is known as galvannealing.

During galvannealing, precise temperature control is necessary to promote the desired grain growth in the steel's tin-rich coating. Galvanneal thermometry is difficult. The rolling of the hot steel makes contact thermocouples and optical pyrometry impractical.

A customer's galvanneal furnace is equipped for phosphor thermometry. This novel technique uses the decay of radiated light by an excited phosphor. This effect can be observed on the night-glow faces of older analog watches. A quicker phosphor coating lines the inside of a television tube. A ceramic phosphor is sprayed on a hot rolling steel sheet during galvannealing. The phosphor coating is then excited by a laser pulse piped through a fiber optic cable. Light radiated by the phosphor passes through another cable to a photomultiplier tube which measures its intensity.

The intensity of the light radiated by the phosphor decays exponentially with a characteristic time constant. This time constant decreases with temperature and can therefore be calibrated to function as a thermometer. Just as chemical reaction rates generally increase with temperature, the decay of florescence in phosphors is more rapid at higher temperatures.

The customer's galvannealing system is equipped with four phosphor thermometers. Thermometer decay constants can become as low as 1 us at the highest galvanneal temperatures. For high resolution of the time constant, the customer wants to digitize the signal from the photomultipliers so that 50 points are acquired per time constant. This implies a sampling rate of 50 MS/s on each of the four thermometry channels. To reduce signal noise, the customer wants to average 10 decay signals. The averaged signal will be fitted to an exponential decay to yield the decay time constant and ultimately, through a calibration table, the temperature of the steel sheet.

GaGe Case Solution

The GaGe solution is two CompuScope 8012A boards operating in Master/Slave Mode. The CS8012A is a 12-bit ISA board that is capable of sampling at 50 MS/s on two channels. The Master/Slave configuration will allow the two boards to operate effectively as one board that simultaneously samples four channels at 50 MS/s. The setup of the galvanneal furnace is illustrated below:

Setup of Galvanneal Furnace Using CompuScope 8012A Master/Slave System Diagram

At the highest temperatures, the florescence decay time constant is 1 us and the boards will be sampling at 50 MS/s. Sampling will be triggered by a TTL pulse whose rising edge is synchronized with the short excitation pulse from the laser. The intensity signals will be amplified so that they are 1 Volt high immediately after the laser pulse and then decay to 0 volts. Since the 12-bit CS8012A has bipolar inputs, the unipolar decay signals are 11 bits high and span 2 11 = 2048 quantization levels. Once the florescence signals have decayed to below one quantization level, then further signal capture is unnecessary. The time it will take for the signal to decay to 1/2048 of its peak value may be calculated from the known form of the simple exponential decay: V/V MAX = e (-t/t) . When V/V MAX = 1/2048 and if t = 1 ms, then the time, t, is determined from:

1/2048 = e (-t/1 us)
t = 1 us x ln(2048) = 7.625 us

The customer wants to digitize for twice this time so that the baseline will be unambiguously determined when the data is fitted to an exponential form. The data capture time is therefore 15 us, which gives 50 MS/s * 15 us = 750 samples per channel.

While the data capture is simultaneous on all four channels, the data from each channel must be separately transferred from on-board memory to PC RAM through the ISA bus. The CS8012A can transfer data through the ISA bus at 1 MS/s. The transfer of 750 samples will take 750 S / (1 MS/s) = 750 us. Transfer of the data from all four channels will therefore take 3 ms.

The total time necessary to completely handle the data from one decay is equal to the sum of the data capture time, the data transfer time and the overhead time which includes, for instance, the time necessary to rearm the boards for the next trigger and the time necessary to set up the ISA bus for data transfer. Since the 3 ms is much longer than the 15 us capture time and the 30 us overhead time, the total time may be approximated as 3 ms.

The time necessary to capture 10 decay signals for averaging is, therefore, only 30 ms. If we ignore the averaging and curve fitting times, then the customer should be able to make over 30 temperature measurements per second. At lower temperatures, the decay of florescence will take longer so that the measurement rate will drop. This is due to the intrinsic limitation of the measurement and not of the CompuScope 8012A boards, which will still be able to transfer all on-board data in only 30 ms.

If more temperature channels are necessary in the future, more Slave CS8012A boards may easily be added to the system. Currently, the customer only uses the phosphor thermometers as a diagnostic tool. In future, however, the customer may use the thermometer readings as feedback to regulate the galvanneal furnace heaters in a control loop. This type of operation is impossible in an oscilloscope-based system and requires the high data transfer capability of GaGe's ISA based A/D cards.

GaGe Case Recommended Products

  • 2 x CompuScope 8012A Master/Slave System - 12-Bit, 50 MS/s Dual-Channel (100 MS/s Single-Channel) A/D Card

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