This is a case study of an actual engineering project in the Irvine Spectrum area of southern California, a 2-mile strip of land in south Irvine with a high density of medical instrument companies. A medical laser company was having some technical problems with their NdYAG (neodymium yttrium aluminum garnet) laser development. This is a laser that can output 5 W of optical power. Protective goggles are worn when working around these lasers. Over a kilowatt of electrical power was required to operate the laser, and with the electrical-to-optical conversion inefficiency, the thermal design was substantial, with heat exchangers and a water cooling system.
Upon arrival, I immediately encountered various consultants working on different parts of the system. (I was brought down through the laser developer in Vancouver, WA.) Two digital engineering partners were doing the microcomputer hardware. It contained three microcontrollers, each a commercial single board product. One controller ran the main functions and commanded the other two. One was the user interface and the third controlled the laser system. The interfaces between the slave controllers and the main one were via serial ports, run through RS-485 interfaces on a motherboard. At power-on, the system would sometimes become catatonic; the user interface would be unresponsive. The software engineers could not find any clues to the malfunction in their programming. The hardware design partners would repeatedly test each of the controller boards and they performed flawlessly. By swapping in different controller units, the behavior would change, but the starting problem did not go away.
The fact that power-on behavior was not repeatable was the key clue. Each of the controllers worked correctly and each had their own power-on, power-off circuitry. However, the controller power-on reset timing had some variability among controller units. This is unavoidable and normally it is not a problem if a controller takes t versus t +
μs to negate the reset waveform to the controller after the power-on threshold has been crossed by VDD. This variability will mainly be caused by the varying threshold voltage values in the controllers.
In this case, it was essential that the main controller not start sending commands to the slave controllers before they were properly initialized and ready for commands. By placing a start-up delay in the main controller, variations in reset times among the controllers would be accounted for by making this delay long enough to ensure that the slave controllers were always ready and waiting for a serial-port command string. The software delay quickly solved the problem after the hardware designers had been wrestling with it for some time. I chuckled to myself when one of the partners grinned as he said to me: "Beginner's luck." I was new to the project.
In the medical NdYAG laser, the laser flashlamp currents were commanded to control the output power of the laser. The system-level loop is shown below, scanned from my project notebook.
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Feedback was accomplished by using a dichroic mirror. Most of the power reflected from the mirror into the output optical path, but a small, fixed fraction went through the mirror. This was sensed by a photodiode and amplified by a photodiode amplifier (PDA). The problem that arose was that of maintaining a stable feedback loop over the power range of the laser. At low power, the accuracy was lacking but if the loop gain was increased, then at high power, feedback-loop instability resulted.
The key to the problem was in characterizing each of the functional blocks in the feedback loop. One of these was the flashlamp-laser combination. Upon examination, it was found to have a nonlinear light output as a function of current. The transfer function was parabolic ("square-law"), as shown below from data taken on a prototype unit for two commanded levels of output power. The loop-gain increased superlinearly with flashlamp current and laser output power.
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