Healthcare robots are increasingly being used and the way they are engineered they still have several challenges regarding reference models and validation. In this experience report we focus on the ArmAssist robotic system and how it can be modelled including safety considerations for validation in early design phases. ArmAssist is an upper-limb robotic system for stroke rehabilitation based on serious games. The open- source tool Papyrus for Robotics was used for modelling the robotic system in close collaboration with neurorehabilitation domain experts. Papyrus for Robotics includes new functionalities that we contributed for contract-based design at component and system level, allowing to make explicit and validate the safety considerations using formal languages. In our case, the assertions are expressed in OCL and Othello. We present the resulting model and a discussion from domain experts.