# Review of Advanced Medical Telerobots
## 1 Introduction
### The Critical Issue of Stability and Transparency of Telerobots  
One of the main performance metrics for any telerobotic system is the transparency of the coupling. Transparency means that the degree to which the operator would feel the environment and deterioration of transparency would result in deterioration of perception. This can be caused by a violation in velocity tracking at the environment side or force tracking at the operator side. One of the main challenges for transparency is the dynamical behavior of the robots in the loop of teleoperation. To address this challenge initially, [PERSON]'s four-channel teleoperation (see Figure 1 with \(C_{5},C_{6}=0\)) was introduced in the literature [80], which would require the sending of both force and velocity from one side to the other side. However, due to the complexity of the four-channel design and the low stability margin of the architecture, an extended version of the four-channel design has been proposed by adding a local force control loop on both operator's side and the environment's side [81] (see Figure 1). The extended [PERSON] architecture can drop one channel and achieve three-channel teleoperation while guaranteeing transparency (see Figure 1 with \(C_{3}=0\)). A modification of extended [PERSON] architecture has been proposed by [PERSON] et al., which allows for achieving perfect transparency only with two communication channels, reducing the complexity of the system and increasing the stability margin (see Figure 1 with \(C_{3}=C_{4}=0\)). Although the transparency of the system can be achieved using the aforementioned architectures, stability has always been a major concern for the designers of telerobotic systems. Stability is one of the main bottlenecks of adding haptic feedback to surgical telerobotic systems. Using hybrid transfer matrix evaluation of a transparent teleoperation system, it can be shown that stability and transparency are two conflicting criteria in telerobotic systems. As a result, any stabilizer would result in deterioration of the transparency of the system. More advanced stabilizers optimize the amount of waveform deterioration, minimizing transparency degradation. It should also be noted that some features of telerobotic systems such as scaling, can affect the stability of the system since it directly affects the loop gain, and it can be shown that based on the small gain criterion, increasing the feedforward gain of force from the environment side to the operator's side results in reducing the stability margin of the system. This should be considered if force magnification is considered as one of the features of the system. One of the main challenges of haptics-enabled telerobotic systems is the communication delay. It can be mathematically shown that the delay would inject extra energy into the system resulting in accumulation of energy in the closed loop system which can (under some circumstances) result in instability and divergence. There has been a wide range of research and development activities to design and implement various stability frameworks for robotic and haptic systems to deal with the instability caused by the delay. Most of the existing approaches function based on Weak Passivity Control Theory (W-PCT). For this, the environment and the operator are usually considered to be passive, while the communication network is considered to be the source of nonpassivity due to the communication delays. Based on W-PCT, by making the communication passive, a telerobotic system will be converted to a negative interconnection of passive subsystems (Human, Communication, Environment), which will be passive and thus stable. The existing approaches usually provide local (on one side) or distributed (on two sides of the communication) damping to dissipate the extra energy introduced into the system by a nonpassive interconnection link and guarantee global passivity and thus stability.