I did not regard any of the situations that I encountered using optical isolated relays as a drawback. They never gave me any problems, with one small exception. A solid state relay that controlled s solenoid valve had one of it's twi anti-parallel SCR devices fail open. So the indicator light would show that the output was active but the valve would not shift. Once I realized that the bulb was at reduced intensity the replacement effort quickly solved the problem.
My primary use of the opto-isolated relays has been as a computer interface in industrical test systems. 100,000 hours of on time before a 3% reduction in illumination, and the associated increase in turn-on time would be no problem, since the production lifetime of the product tested may not be that long. And for those very long-life products another few milliseconds of response time would not be a problem either.
The other use of opto-relays is for on-off cycling of heaters in temperature controlled systems, where electrical heater powr is cycled to control temperature. For those applications with the zero-crossing switching the problem is even less likely to be noticed or to cause problems. It might possibly have some effect in motion control systems,but that is a different realm completely.
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For industrial control applications, or even a simple assembly line, that machine can go almost 24/7 without a break. But what happens when the task is a little more complex? That’s where the “smart” machine would come in. The smart machine is one that has some simple (or complex in some cases) processing capability to be able to adapt to changing conditions. Such machines are suited for a host of applications, including automotive, aerospace, defense, medical, computers and electronics, telecommunications, consumer goods, and so on. This discussion will examine what’s possible with smart machines, and what tradeoffs need to be made to implement such a solution.