Abstract
This work presents performance comparison of different Tuning schemes applying the optimization technique named as Fitness Based Adaptive Differential Evolution (FBADE) for a prosthetic arm. From the dexterous point of view, the mathematical modelling is the fundamental requirement to improve the performance level. Here, we define the optimum transfer function applying fuzzy logic to establish the link of the robustness and the controllability metrics between amputated body parts and prosthetic arm. A proportional integral derivative (PID) controller is chosen for the purpose of continuously modulated control. Also, the role of most influential tuning parameters have been illustrated to measure the performance level of different tuning schemes. In the end, Lyapunov method is applied to analyze the stability of the designed model.
Keywords
Introduction
The development of prosthetic [9, 21] arm modelling is continuously growing over the decades, and many important and fruitful research work already been made till date. One of the recent trends of research is to control a biomechanical arm connected with the nervous system by linking electrodes to human nerves. The dynamic mathematical modelling of prosthetic arm controlled is demonstrated in [4] and it is acquired in this study to extend further. The four sets of transfer functions are developed for the prosthetic arm model and they are tuned through Ziegler-Nichols (ZN) rule [30], Genetic Algorithm (GA), Particle Swarm Optimization (PSO) rule [10, 30] and Bacterial Foraging Optimization (BFO) methods [11, 23]. The schemes in this work are initially passed through Fitness Based Adaptive Differential Evolution (FBADE) technique, and then comparative step response analysis and control parameter analysis have been performed applying Fuzzy Logic to obtain the most appropriate transfer function. Besides, to achieve more suitable result, the stability analysis is also performed applying Lyapunov method.
Block diagram of the proposed Prosthetic arm gripper.
Mechanical prosthetic arm used for control modelling determination.
The mechanism of the experimental process of Prosthetic arm gripper is presented in Fig. 1. The force experienced by the gripper is fed to the error detector to compare with the reference input signal at the input end. The error produces at each and every instant of time is processed with Fuzzy controller. Thereafter, the corrected output of the controller is applied on the dynamometer of the gripper. The physical movement of the arm gripper is operated as per the controller corrective action. During the functioning of this mechanism friction, the shaft movement may cause some disturbance to the system. The mechanical arm used for the present study is shown in Fig. 2. The displacement of fingers of the prosthetic arm is measured with change in load using dynamometer. Figure 3 illustrates how more force can be applied to the fingers to move away from each other in step by step manner [4]. The four sets of transfer functions from the dynamic modelling of the prosthetic arm are as follows:
Experimental set up with mechanical arm using dynamometer.
The soft computing techniques are adopted for setting the optimized PID controllers constants. The satisfactory decision making is achieved through the PID tuning approaches with simple control configuration. In this work, 4 sets of transfer functions with unity feedback are considered and then ZN rule [6], GA [9], BFO methods are applied to tune the transfer functions. The ZN tuning rule has a large impact in making PID feedback controls [1] acceptable to control engineers [10, 21, 30]. In genetic algorithm, a population of candidate solutions to an optimization problem is evolved toward better result. When the optimized PID parameters [14] are highly correlated, GA cannot provide proper efficiency, whereas the PSO rule optimizes a problem by iteration and trying to improve a candidate solution with regard to a given measure of quality. Also, the tuning with BFO has been widely accepted for distributed optimization and control [5]. In the present study, another optimization technique that is FBADE technique is applied for better efficacy and comparative result analysis. Measured tuning procedures are transcendent for the selection of proportional, integral and derivative constants. This will be furnished further to obtain efficient mathematical model for prosthetic arm.
The different PID tuning methods such as ZN rule, PSO method, GA, BFO and FBADE technique tuning parameter values are observed for the previously mentioned transfer functions in Table 1.
For the selected transfer function, the corresponding step responses are illustrated in Figs 4–7 applying the conventional and advanced tuning methods respectively to take on characteristics performance analysis. All the graphical plots are given to show the output and scale adjustments are chosen according to the requirement. For each graph within the specified scale, the system is showing its stability. The parametric analysis is adopted as it is difficult to select the best suited transfer function applying graphical analysis for the proposed model.
Step responses of set 1 transfer function with different tuning.
Step responses of set 2 transfer function with different tuning.
Step responses of set 3 transfer function with different tuning.
Step responses of set 4 transfer function with different tuning.
The different control parameters such as rise time, settling time, overshoot and peak time (see Figs 8–11) are observed to determine the most suitable model through the application of PID tuning via various types advanced methods of tuning.




By reviewing the control parameters, more than one preferable case can be observed, but it is difficult to select the most optimum case. Therefore, fuzzy logic concept is incorporated to select the most suitable mathematical model.
In the recent age of technology, the applications of fuzzy logic have increased in huge number. This work is concentrated on the development of software package for the automatic tuning of myoelectric prosthesis [7, 12, 13]. To improve the response of the flexible arm depending on the vibration feedback, PID is introduced for controlling of a single-link flexible manipulator [26, 27, 28]. Point to be noted, Fuzzy logic is a multi-valued logical system. Fuzzy logic (FL) relates classes of objects with uncap boundaries in which membership is a matter of degree [15, 16, 17]. Here a flexible-link robot arm control using fuzzy logic and a singular perturbation approach are considered. A brief on singular perturbation approach is incorporated to derive the slow and fast sub-systems, and thus reduce the effect of spillover [18, 19, 20].
To find the best stable method, the five tuning methods are compared applying the suitable rule as given below:
where, L stands for Low, M stands for Medium, H stands for High, RT stands for Rise Time, ST stands for Settling Time, OV stands for Overshoot, PT stands for Peak Time.
The different control parameter status for various tuning methods are illustrated in the following Tables 3–7.
Control parameter analysis for different tuning method
Control parameter analysis for different tuning method
Control parameter status for ZN method
Control parameter status for PSO method
Graphical presentation of fuzzy membership function of tuning parameters.
Control parameter status for GA method
Control parameter status for BFO method
Control parameter status for FBADE method
ZN method of set 2, PSO Method of set 3, and GA Method of set 4 are concluded as the best models from the analysis. Hence, they are again considered for further selection of most optimum one. In Figs 13–15, the step responses of the selected three models are demonstrated. According to step response and control parametric analysis of the selected mathematical models; for set 2 model using ZN, the settling time is too high among others, so it will take more time to reach to steady state value that would affect the speed of the response and also it is showing undershoot characteristic. Between remaining two transfer functions, set 3 transfer function using PSO method, the overshoot value is much higher compared to another. Increasing overshoot will degrade the stability. Reduced overshoot shows how quickly the response is changing over time. Finally, set 4 transfer function using GA method is opted for its most appropriate characteristics as it is nearly satisfied with the rule mentioned in Table 2 compared with other two methods ZN and PSO.
Step response of closed loop transfer function for set 2 using ZN method.
In Section 6, fuzzy logic is applied to achieve the optimum transfer function among all other methods. As per fuzzy rule base selection, the control parameters such as rise time, settling time, peak time should be low and overshoot should be medium which have taken into account to select the most suitable method. All the specified tuning parameters of different methods are verified with the suitable fuzzy rule based methods and the results are shown in Tables 3 to 7 respectively. The ZN method of set 2, PSO method of set 3, and GA method of set 4 are perfectly matched with the expertise knowledge based system for achieving the outcome. As the remaining sets are entirely matched, thus they have not incorporated for further study.
The tuning parameters of PSO method
Step response of closed loop transfer function for set 3 using PSO method.
Step response of closed loop transfer function for set 4 using GA method.
The Lyapunov Stability method for open loop transfer function have been implemented for paraplegic and fatigued condition of human arm with control representations [8]. The learning and exploiting the power of Control Lyapunov Function (CLF) scheme have been incorporated to ensure global asymptotic stability of nonlinear dynamic system [24]. Here, the block diagram representation of the optimum model is illustrated in Fig. 16. for further study through Lyapunov Stability method. Now, the information about Lyapunov basic is given for the controlling of a robot [22]. The qualitative study between optional and prescribed prosthetic model is analyzed using Lyapunov logic [25, 29].
Control block of the optimum model.
The analysis of the Lyapunov stability of the closed loop linear system can be followed as: From Fig. 16, we get:
Applying inverse Laplace transform, we get:
Let,
Thus,
Equation (8) can be re-expressed as:
where
Now, scalar positive definite function can be expressed as:
where
Now, taking the derivative with respect to time t, we get:
For the positive definite function
Now we take the coefficients in such a manner that,
Thus taking,
Here we show,
Substituting Eq. (17) into Eqs (13)–(15), we get:
Now, substituting Eq. (22) into Eqs (19) and (20), we get:
Now substituting Eqs (17), (21)–(24) into Eq. (10) we get:
Let
From Eq. (26), we clearly see that
As
Hence, from Eqs (18) and (27), we get:
which is negative semi-definite and
From Eq. (26), we obtain:
By showing that
The equilibrium state is at the origin of the system. So that the system is asymptotically stable.
This work is a basic approach for generating efficient prosthetic arm model for adopting different advanced tuning procedures. As the comparative step response analysis is not adequate to decide the optimum case. therefore control parameter selection by fuzzy logic approach is implemented to formulate the best result of the present study. Also the stability analysis by Lyapunov Stability method is adopted to emphasize on the selected model. The further extension of this work is definitely going to be a benchmark in the field of Limb prosthesis.
