The quality of products can improve by using robots.
The quality of products can be improved by using robots.
In which sentence is the verb "improve" used correctly?
#2 seems better to me because "the quality of products" in the sentence is an object.
The first sentence essentially says that the quality itself will be using robots to improve [itself].
The second sentence does not specify who is going to use robots to improve the quality, only that their use can affect the improvement in quality.
Generally speaking, since quality is a passive trait, it can't invoke help of other things or processes. We do say "the quality has improved" but that does not make quality active, just indicates a change of state.
Given the above, I think the second is closer to correct semantically (rather than grammatically), and I suppose you've asked it in that sense.
RE latest edit: The "quality" is the subject in both sentences.
The second option is a perfectly understandable use of passive voice construction.
The first rendering seems odd to a native speakers ear because we tend to think of improve as a transitive verb, which needs an object. And, with the robots being mentioned, it sounds as if they are acting on the product to improve its quality. But no, the robots are being used—by whom? That is left unsaid. So the sentence feels queasy (uneasy and unsatisfied.)
To express it in a more straightforward way, we might say;
But remember that improve can also be intransitive. This is the "improving itself" sense that Victor Bazarov alluded to. (We say, for example, "the weather is improving" and nobody asks "improving what?".)
So we could say, for example:
This might not mean quite the same as the original sentence. It does state a correlation between the use of robots with the improvement of quality, but it does not explicitly state a causal relationship—we are left to infer that.
"Robots can improve the quality of products."
"The product quality can be improved."
In both cases "quality" is the subject, not "product."
