In my 45+ year career as a professional (from when I co-opted summers during school to my current situation) I have NEVER worked where signing a 'patent agreement' was NOT a condition of employment, period! At least from my experience I can't see how anyone could set-up an arrangement where he could retain 'ownership' of the work he did for his employer.
Now don't get me wrong, I'm not saying that you couldn't negotiate some sort of deal where you get to share in the benefits derived from your 'inventions'. My first employer, for whom I worked 14 years (this included my summer co-opt time), offered an incentive program which encouraged it's engineers to come-up with new ideas which could be patented. For each idea submitted which, upon research, was deemed beneficial to the corporation and which proved to be unique enough that it actually COULD be patented, you would receive $100 (after taxes), and for each idea for which a patent was subsequently issued, another $100 (after taxes). Now remember, this was back in the 60's and 70's when a $100 could actually buy you something ;-)
BTW, this was a much better deal than a friend of mine who worked for Dow Chemical, also a company in Michigan, who eventually had 23 patents issued while he was there, for which he received 23 Silver dollars encased in Lucite (all Patent Agreements must provide a minimum payment of one dollar to make it a legal transfer of ownership since patents are real property). Granted, if he's still alive (he was much older than I) those 23 silver dollars may be worth much more than a dollar today, but I know back then he wished he could have gotten a $100 like I was getting where I worked.
And as for my current employer, I don't really know if there is any additional remuneration since I've never had anything patented since I started to work here (I had 2 patents at that original company along with 4 other ideas which were never patented but for which I did collect my $100), however I do know that you at least get a plaque to hang in your office since one of guys I work with has had a patent while working here.
John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
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To an Engineer, the glass is twice as big as it needs to be.