The Sled Mangled the Coupling

DN Staff

November 15, 2013

3 Min Read
The Sled Mangled the Coupling

In the 1980s, I joined a seven-person startup with the idea to design a compact disk recorder. The leader had a background in optical media -- we obtained half a million in venture capital and proceeded to ramp up. I bought some lab benches and set up an electronics lab. We bought 20 or so CD players and dissected them. We were doing reverse engineering.

About a month after conception, we observed the EFM signal, which is an RF waveform correlated to the bit patterns on the disk. This was a milestone. At the time, Phillips had a Red Book of CD design, but it was very expensive. We avoided buying it and went our own way.

We found out about a company in England that was working on optical media. They had the beginnings of a recorder, but it had never worked. It was the size of a desk.

I was given the task of designing the R/W laser tracking system. We had scanty information that a digital pulse train would drive a stepper motor and then a gear reduction to a screw drive. A very low frequency error signal could be derived from the tracking optics.

I read up on filters and analog and decided to visit our local National Semiconductor sales office over lunch one day. I guess the apps engineers were out to lunch, too, as I was greeted by a friendly receptionist who told me, "The data books are back there. You can have anything you want."

I was like a kid in a candy store. All kinds of awesome data books and tons of sample chips were stacked neatly. I grabbed some linear data books and a couple of each kind of IC. I looked through my haul and found some switched capacitor ICs -- MF10. I used one of these as a low pass filter whose output went to an absolute value circuit. I used this to drive a VCO. I took the VCO output and drove a three-decade divide by n. I mounted BCD switches on the front panel. The divide by n drove the stepper pulse in. I used the three decades, since I had no idea of the gear ratio I would encounter.

The last few days before we flew to England we all worked around the clock to get our subsystems done. We got to England and went to work. I was happy when my design closed loop and phase locked the write servo. We actually were able to copy a disc after a bit. But one big problem came up.

One of us had bypassed the limit switches on the optical sled drive. It had a fast slew switch that sent the sled cruising to the end of travel and then mangled the brass flexible coupling used to correct slight shaft misalignment. We all sat around for a couple of weeks before we could get a replacement. It was very discouraging.

Over the years, I have heard and read about mishaps incurred by defeat of limit or lockout switches. When you see one of these, do not dismiss it lightly. To do so may threaten your life, your job, or your happiness.

Steve Lindberg has loved electronics since he received his first Weller soldering gun when he was 12. He has 35 years of experience in test, debug, and design.

Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.

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