Yes, you can patent the new device as long as the improved device is novel and is not obvious over the previous device itself (e.g., common sense improvement) or in combination with other devices or disclosures in patents. If your improvement has something unexpected or superior property or advantage, that would certainly help. Utility may be the same but its performance can be better, e.g., greater efficicency. Please work with a patent attorney in determining whether it would be possible to patent it. Good Luck!
The foregoing does not constitute legal advice. No attorney-client relationship exists between me and you. Please consult a qualified attorney before making any significant decision.
Whether you can patent your new device depends on whether 1) someone else has previously made the same invention or, if not, whether 2) your invention is an obvious design in view of what has been invented before your invention.
Simply because your invention and the old design have the same utility does not necessarily mean you cannot get a patent. However, the more changes and more improvements you have made over the old design improves the likelihood of you getting a patent.
The answer provided is only for general information purposes and does not constitute legal advice.
What you describe is a good example of what the patent system is Constitutionally mandated to do .... promote the progress of useful arts. Find a patent attorney you like and .. go to town.
This posting is intended for general education and isn't "legal advice." It doesn't create or evidence an attorney-client relationship. You are encouraged to engage an attorney in the pertinent jurisdiction for confidential legal advice on matters of any importance.
In addition to agreeing with what these other attorneys have said, I would add that there is a time limit for filing the application. It would be best to consult an attorney sooner rather than later.
This answer is provided based on limited facts and does not create an attorney-client relationship, nor should it be relied on as legal advice. It is only a generalized, cursory examination and is not a substitute for retaining a licensed attorney.