I have developed a new device used in automobiles and applied for a patent (as personal, not from a company). After producing commercially and releasing into the market, what may I do if someone copies my design with some small changes, and produces and sells the device in the market?
What is protected is defined by your patent's claims. If someone implements every step of even one of your claims, you have grounds to sue. It is up to you to defend your patent through law suits (or the threat of a suit). This can be very expensive if the infringing company is large and well funded. Some law firms may represent you on a contingency basis.
check if those changes are falling within the scope of your claims. Suing is one option if your claim cover modified product (if not literally under doctrine of equivalence) but through understanding of costs involved is required.