# Upper Limb Bionic Orthoses: General Overview and Forecasting Changes
## 6 Control System Feedback  
Feedback of different signals was not used in control systems at early exoskeletons development. In those times, robots were built with master/slave control systems. A breakthrough in the field of exoskeleton system control was the use of an interaction force between the human body and the exoskeleton system [131]. Nowadays, most of rehabilitation devices do not use feedback. Instead, they rely only on the physiotherapist's judgement and actions, sometimes being able to pass information about the patient's condition (based on biochemical signals) and current parameters of the device. This situation can be deemed inadequate, because rehabilitation systems should use biochemical signals obtained from the rehabilitated person to adjust their operation. In this case the physiotherapist could only perform a supervisory function (Triggered Passive Control \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Sensor** & **Main Advantage** & **Main Drawback** & **Usefulness in Bionic** \\  & & & **Orthoses** \\ \hline Touch sensor & Feedback improvement & Complex & Optional \\ Force/torque sensor & Could be estimated by the current & Difficulties in measurement & Yes \\ Encoder & Simplicity & Relatively big & Yes \\ Accelerometer & Versatility & Limited accuracy of & Yes \\  & & determining device & \\  & & orientation & \\ Inclinometer & Simple posture control & Usefull in specific & Optional \\  & & conditions & \\ Gyroscope & Precision & Requires additional & Yes \\  & & electronics & \\ Distance sensor & Protection against breakage & Need to use several sensors & Optional \\ Camera for shape & Increases rehabilitation & Expensive & Optional \\ recognition & efficiency & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Sensors used in bionic orthoses. Model) [132; 133; 134; 135; 136]. For this reason this type of feedback bears often the name of a full biological feedback (Figure 5). There are many measures to register change in a person's physiological state and the most common are: electromyography (EMG), electroencephalography (EEG), electrodermal activity (galvanic skin response--GSR or electrodermal response--EDR), slow cortical potential (SCP), hemoencephalography (HEG), heart rate variability (HRV), respiratory sinus arrhythmia (RSA), as well as other respiratory and temperature biofeedback [27]. The most frequently used signal to recognise stimuli is the EMG. For years, EMG has been used to control an exoskeleton in tasks, like supporting movements [137] or rehabilitation [138]. High efficiency of EMG signals used in control is presented by [PERSON] et al. [139; 140]. However, interferences, which occur during signal processing, can lead to system malfunction. For this reason, different types of filtration are used. The most common of them are: Hilbert transform [141], Fourier transform [142], and continuous wavelet transform [143; 144]. EEG is used as a control signals in a brain-computer interface (BCI), which allows the patient to control the rehabilitation process directly by using brain waves. An extensive literature review of EEG-based BCIs has been prepared by [145]. High performance of computer control systems based on EEG has been proven, i.e., by [PERSON] et al. [146]. The results give hope for faster and more effective rehabilitation for people with cerebral palsy [147; 148]. Along with many advantages of using EEG to control a bionic orthosis, this method has one basic defect--control is very complex and requires long-term preparation from a patient [149]. Scientists are trying to solve this problem by using hybrid BCI systems [150]. The popularity of EEG as a control signal is increasing, which can be depicted by how many studies are focused on it every year (Figure 6). Figure 5: Types of rehabilitation systems: (**a**,**b**) no feedback—open loop, (**c**) classical feedback—closed loop, (**d**) full biofeedback—closed loop. Figure 6: Publications on bionic orthosis control signals.