Microsoft has added two more companies to the list of Android device manufacturers paying it royalties over mobile patents: Acer and Viewsonic. The agreement with Acer covers Microsoft's patent portfolio for their tablets and smartphones running Android, while one with ViewSonic covers tablets and mobile phones running Android or Chrome OS. No exact financial terms were disclosed by the companies involved.

In a statement, Horacio Gutierrez, Microsoft's deputy general counsel of intellectual property and licensing, said the company is pleased that Acer and ViewSonic entered into the deals as part of a program "to help companies address Android's IP issues."

Neither Acer nor ViewSonic are big players in the Android world, but like everyone else, they are trying grab a piece of the tablet market. Microsoft on the other hand is using its patent arsenal to deter Android's growth, with added costs and legal complexity, while making its Windows and Windows Phone platforms more attractive to OEMs.

The company has already managed to successfully extract royalty fees from HTC, Onkyo, U.S. defense contractor General Dynamics Itronix, Velocity Micro, and Wistron. They are also seeking $15 per Android device from Samsung and have sued Motorola and Barnes & Noble for selling products with Android software.

Google announced plans to buy Motorola Mobility and its huge collection of patents for $12.5 billion. Whether that will give the company enough ammo to defend its mobile platform and partners from these threats remains to be seen.