Autonomous functions for wheelchair-mounted robotic manipulators (WMRMs) allow a user to focus more on the outcome from the task - for example, eating or drinking, instead of moving robot joints through user interfaces. In this paper, we introduce a novel personal assistive robotic system based on a position-based visual servoing (PBVS) approach. The system was evaluated with a complete drinking task, which included recognizing the location of the drink, picking up the drink from a start location, conveying the drink to the proximity of the user's mouth without spilling, and placing the drink back on the table. For a drink located in front of the wheelchair, the success rate was nearly 100%. Overall, the total time of completing drinking task is within 40 seconds.