# Current State of Robotics in Hand Rehabilitation after Stroke: A Systematic Review
## 2 Materials and Methods
### Control Strategy of Hand Rehabilitation Robot
#### 2.3.1 Interactive Control Based on Force Signals  
In force signal-based interactive control, the force signal specifically refers to the force acting on the mechanical structure due to the contraction of the limb muscles, the interactive force. It can be measured directly by a force/moment sensor through clever mechanical design or estimated by a dynamic model of a human-machine hybrid system. Compared to biomedical signals, force signals have better determinism and are a more direct reflection of the patient's active movement intentions, making force-based interaction control relatively reliable and stable. However, since the acquisition of interaction forces usually depends on mechanical structures, which is not as convenient and flexible as the detection of biomedical signals, the application of this interaction control method is limited. Two of the most widely used force control strategies for interaction between rehabilitation robots and patients are force-position hybrid control and impedance control [83]. Force-position mixing control: The force-position hybrid control approach was first proposed by [PERSON] et al. to solve the problem of controlling a robot in a constrained environment [84], which can be simply described as the need to control the robot's position in some directions and the need to control the interaction forces between the mechanism and the external world in other directions. Therefore, in force-position hybrid control, when the robot is in contact with the external environment, its task space is naturally partitioned into two subspaces, namely, the position subspace and the force subspace, and the tracking control of position and force is accomplished in the corresponding subspaces [85]. The goal of interactive control of hand rehabilitation robots is to create a safe, comfortable, natural and actively sample training environment for paralyzed patients, and precise force trajectory tracking is rarely required. Therefore, the following literature review on force-position hybrid control is also related to upper limb rehabilitation robots, and the approach is also useful for the control of hand rehabilitation robots. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Drive Types** & **Definition** & **Advantages** & **Disadvantages** & **Representative Works** \\ \hline New & Smart materials that respond to changes in external environmental conditions or internal states, convert their own energy into mechanical energy and actuators for hand rehabilitation robots & (1) Light weight, malleable, flexible, low noise & (1) Harsh driving conditions & [72; 77; 81; 82] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._Impedance control: The control research of hand function rehabilitation robots is mainly focused on finger position control at this stage. The research on the control of the contact force between the functional hand rehabilitation robot and the human hand is less. In the 1980s, [PERSON] [86] proposed the well-known theory of impedance control method in the study of contact control between robot end and environment. Impedance control incorporates force and position into the same control system and has the advantages of less computational effort as well as greater robustness, making it an efficient method for dealing with machine human control that has been widely studied. Impedance control is essentially an indirect force control, characterized by not directly controlling the desired position and force, but by adjusting the corresponding dynamic relationship between the position (or velocity) and the force acting at the end of the robot in real-time, thus achieving a soft and compliant control of the robot.