Practical Gripper Performance for Intelligent Active Force Control of a Robot Arm Actuated by Pneumatic Artificial Muscles

Practical Gripper Performance for Intelligent Active Force Control of a Robot Arm Actuated by Pneumatic Artificial Muscles ARIA ENZEVAEE, MUSA MAILAH, SUHAIL KAZI Department of Applied Mechanics & Design Faculty of Mechanical Engineering Universiti Teknologi Malaysia 81310 Skudai, Johor MALAYSIA a_enzevaee@yahoo.com, musa@fkm.utm.my, skazi@fkm.utm.my Abstract: - In this paper, an intelligent Active Force Control (AFC) of a single-link Pneumatic Artificial Muscle (PAM) actuated robot arm employing Fuzzy Logic (FL) element has been applied and tested through an experimental study. The robot arm is desired to move along a one radian circular trajectory as a joint angle tracking control in the wake of the introduced disturbances. To demonstrate the practicality of the robot arm, a radial gripper has been physically designed, developed and attached to the arm. The experiment was conducted using Hardware-in-the-Loop-Simulation (HILS) strategy, taking into account the variation in the payload masses.the results clearly show the capability of the proposed controller to counter the disturbances in the system effectively and robustly. Key-Words: - Pneumatic artificial muscle, Single link robot arm, Active force control, Fuzzy logic, Hardware-in-the-loop simulation 1 Introduction Numerous types of actuators are generally used in the robotic systems, each presenting special performance characteristic. Electrical motor, hydraulic drive and pneumatic drive are among the major ones. For any application based on the system and performance requirements, there are a series of selection criteria for the actuator including high power density, high power-to-weight ratio, accurate, repeatable control, rapid response, cleanliness, low cost and high efficiency. The pneumatic artificial muscle (PAM) actuator possesses almost all of the mentioned advantages and is therefore considered as one of the best candidate for robotic arm applications. However, due to undesirable characteristics such as inherent nonlinearities, high sensitivity to payload and time-varying performance parameters, force and position control of this class of actuators are challenging. PAM, typically known as McKibben muscle, was invented in the 1950s by the atomic physician, Joseph L. McKibben which used the pressure of gas to contract and actuate as a real muscle [1]. PAM generally consists of a flexible rubber tube cross-weave sheath material and two connection flanges. The contraction force depends on the applied pressure and the muscle s length, ranging from an extremely high value at maximum length (i.e., zero contraction) to zero at minimum length (maximum contraction). Numerous researches have been done on the control of PAM actuated robots. Zhu et al. applied a discontinuous projection-based adaptive robust controlmethod to control a three-pneumatic-muscles-driven parallel manipulator through simulation and experimental studies to achieve the precise posture trajectorytracking control [2]. Tondu et al. used the principle of virtual work to derive the muscle force model [3]. A similar work was done by Chou and Hannaford to model McKibben actuator using the virtual work principle [4]. Repperger et al. proposed a typical model in terms of a spring and damper system for pneumatic artificial muscles [5, 6]. Thanh and Ahn proposed the use of neural networks for the tuning of the nonlinear PID controller parameters [7]. Recently, Ahnand Anh developed their work using genetic algorithm to identify the parameters of a 2-link pneumatic artificial muscle manipulator by an ARX model. They also proposed an adaptive recurrent neural network which they implemented for the position control of the joint angle of a two link manipulator actuated by pneumatic artificial muscles [8]. Chan et al. carried out a series of simulation studies on the control of pneumatic artificial muscle using fuzzy ISBN: 978-960-474-371-1 134

PD+I for position tracking of the vertical movement of a mass suspended from a pneumatic muscle [9]. Lilly et al. implemented an adaptive tracking technique to pneumatic muscle actuators arranged in bicep-tricep configuration [10]. A simple, effective and robust control scheme that can be applied for PAM actuated robot control is the Active Force Control (AFC) algorithm. Hewit and Burdess were the first researchers who applied AFC in their works and conducted extensive research on the subject [11]. Mailah and fellow researchers extended and consolidated their works by applying the AFC concept to a number of dynamical systems [12-15]. where, and. Also, r 0 is the initial radius of the PAM and is defined as the angle between the PAM axis and each thread of the braided sheath before expansion 3 Dynamics of Robot Arm The general configuration of the robotic system under study is depicted in Fig. 2. Fig. 2: A representation of the single link robot arm 2 Pneumatic Artificial Muscles Artificial muscle actuators were developed for biorobotic systems by Klute [16] and a pneumatic muscle hand therapy device by Koeneman. However, the fluidic muscles (PAM) inherently possess non-linear loading characteristics, hysteresis behaviour, and are susceptible to temperature changes [14]. Also, time variance plus non-linearity behavior of gas, friction, viscosity and etc., which cause disturbances and non-linear functions, make it hard to control the high speed system precisely. Fig. 1 illustrates the virtual work principle applied to PAM actuator. Fig.1: Application of virtual work principle to McKibben actuator From the virtual work theorem, it can be deduced for the PAM as follows: (1) (2) where r and P denote the radius of the muscle and the internal pressure, respectively. The expression for the force produced by the PAM can be obtained in terms of the control pressure, P and the contraction ratio ε, as follows: (3) The robot arm in this study is supposed to move in horizontal plane only and hence, the gravitational effect is neglected. Therefore, the governing equation for the actuator torque for a single link arm is reduced to: where I is the robot arm inertia and angular acceleration. (4) is the 3.1 Antagonistic PAM Configuration The robot arm is actuated by a pair of PAMs in antagonistic configuration designed to emulate the human arm biceps-triceps anatomy. The two muscles are connected by a timing belt and pulley mechanism for the transmission of actuator force. The differential pressure and the resultant force difference between the agonist and the antagonist produce a clockwise or counter-clockwise torque. To obtain more linear behavior of the PAM, both actuators were initially pressurized at P 0 and an initial contraction ratio, ε 0. When the agonist muscle is inflated with pressure P 1 different from the antagonist muscle pressure P 2, a rotation of angle θ is produced at the joint as illustrated in Fig. 3. Fig. 3: Antagonistic PAM configuration for a single link robot arm. Denoting the agonist actuator force by F 1 and that of the antagonist by F 2 and the pulley radius by ISBN: 978-960-474-371-1 135

s, the torque produced by the actuator can be formulated as: T = (F 1 (ε 1, P 1 ) F 2 (ε 2, P 2 ))s (5) where ε 1 = ε 0 + rθ/l 0 and ε 2 = ε 0 - rθ/l 0. Considering a monovariable approach of the PAM actuator which can be called symmetrical cocontraction by analogy with the neurophysiological terminology, the symmetrical pressure variation is noted as P. This is the pressure difference applied from initial pressure P 0 in both muscles as P 1 = P 0 + P and P 2 = P 0 - P. Then the actuator torque model becomes: T = 2K 1 P 2K 2 P 0 θ (6) where K 1 = (πr 0 2 )r[a(1-kε 0 ) 2 b] and K 2 = (πr 0 2 ) 2ra(1- kε 0 )kr/l 0. The static relationship between the pressure difference and the corresponding joint angle can then be obtained as: θ = G P (7) where G represents the open-loop gain, G= K 1 /K 2. 4 Control Scheme Active Force Control with Fuzzy Logic (AFCFL) scheme has been implemented to eradicate the negative effects of the disturbances. Note that a proportional-integral-derivative (PID) control is first designed prior to the AFC-based system which is expected to produce good overall system performance. The sub-schemes are described in the following sections. a. PID control The transfer function of a typical PID controller is: G c = K P + K I /s + K D s (8) where K P, K I and K D are the proportional, integral and derivative gains, respectively. These are set and tuned by the heuristic method, employing One-Value-At-a-Time (OVAT) technique, i.e., each time changing one parameter while the other two were fixed. The values corresponding to the least track error are selected for the PID controller parameters and these remain unchanged when the AFC-based scheme was implemented. b. Active force control The first successful implementation of AFC to a robot arm was done by Hewit and Burdess in 1981 [11]. They proposed a practical and robust technique to compensate for the internal and external disturbances of a dynamical system by employing an internal force error feedback control based on real-time acceleration and force measurements. The essential mathematical representation of the AFC controller can be written as: (9) where IN is the estimated inertia of the link. The only computational burden in the AFC algorithm is the estimation of the inertia which is to be multiplied by the acceleration of the arm and then compared with the actuator torque to eliminate the undesirable torque caused by the disturbances in the system. In this study, fuzzy logic (FL) was selected and implemented. (c) AFC with FL implementation AFC can perform much more efficiently if a method is used to provide good estimated of the inertia of the arm at any arbitrary moment of time. Thus, the proposed AFCFL algorithm exploits the FL as the inertial parameter estimator in the AFC loop. The track error and angular acceleration of the arm are used as inputs to the FL system whereas the estimated inertia of the arm, IN is configured as the output. The proposed scheme is depicted in Fig. 4. Fig. 4: Schematic of the proposed AFCFL controller For simplicity of computation, triangular membership functions (MFs) for fuzzy sets have been utilized as illustrated in Figs. 5(a) to (c). (a) Track error (b) Angular acceleration ISBN: 978-960-474-371-1 136

fabricated from aluminium and eventually assembled and attached to the robot arm. A 3D sketch of the gripper is illustrated in Fig. 6. (c) Inertia Fig. 5: Membership functions of inputs ((a) and (b)) and output (c) The fuzzy rules have been defined and implemented based on the previous works on the same project [15, 17]. The rules are defined in a logical manner as follows: 1. If Trackerror is NL and Acceleration is NL Then IN is VB 2. If Trackerror is NL and Acceleration is NS Then IN is VB 3. If Trackerror is NL and Acceleration is Z Then IN is VB 4. If Trackerror is NL and Acceleration is PS Then IN is B 5. If Trackerror is NL and Acceleration is PL Then IN is M 25. If Trackerror is PL and Acceleration is PL Then IN is M Table 1 presents the defined fuzzy rules between the inputs and the output of the inertial parameter estimator. It should be noted that the MATLAB Fuzzy Logic Toolbox has been used to define the MFs and fuzzy rules. Table 1: Fuzzy rules Estimated Inertia Angular Acceleration (rad/s 2 ) (kgm 2 ) NL NS Z PS PL NL VB VB VB B M NS VB VB B M S Z VB B M S VS PS B M S VS VS PL M S VS VS VS Track error(rad) Finally, a crisp output is obtained through a defuzzification process using an average technique called centroid or center of gravity method that is proposed in the toolbox as a defuzzification tool and is described by the equation: (10) 5. Development of Gripper To enhance the previous PAM robot system by making it more industrially practical, a 5-member radial gripper was developed, fabricated and integrated into the robot arm for a simple pick-andplace task. Initially, the 3D modeling was done using Solidworks software. The parts were then Fig. 6: A 3D sketch of the gripper It was presumed that the gripper is supposed to move and handle objects of maximum load of 150 g and a servo motor driver was intended to be used as the gripper actuator. From static force analysis, a value of 3.34 N was calculated as the required force by the servo motor. Using a rack-and-pinion power transmission unit with 16 mm pitch diameter, the calculated actuator torque was obtained as 0.3 kgcm. A DC servo motor (C55R) was selected as it was able to provide the required torque. 6 Experimentation Fig. 7 presents a view of the mechanical rig and the attached gripper. For the Hardware-in-the-Loop Simulation (HILS), a data acquisition card (DAQ) employing the PCI-1710HG model from Advantech has been used which collects the analog data from the rotary encoder and through Analog-to-Digital (A/D) conversion sends them to the personal computer (PC) to be processed. Implementing MATLAB Real Time Workshop (RTW) feature and Simulink, the mechanical rig was integrated with the control system and the digital control signals were then sent from the PC to the DAQ through the Digital-to-Analog (D/A) conversion. A pair of PAM (model: DMSP-10-N-100-RM-CM) manufactured by FESTO were used as the actuators. To control the pressure, proportional pressure regulators (FESTO, MPPES-3-1/8-10-010) were utilized to control the pressure inside the PAMs. An Omron E6B2-CWZ6C rotary encoder was used to measure the joint angle and an ADXL 335 accelerometer to measure the acceleration of the robot arm. Table 2 summarizes the details of the hardware employed in the experimental study while Fig. 8 illustrates them. Fig. 7: Top view of the robot arm and the gripper attached ISBN: 978-960-474-371-1 137

Table 2: Hardware employed in the experimental study Data Acquisition card (DAQ) Hardware Model Manufac turer DMSP- FESTO PAM 10-N- 100-RM- CM Accelerometer ADXL Analog 335 Devices Rotary encoder E6B2- Omron CWZ6C Proportional MPPES- FESTO pressure 3-1/8- regulator 10-010 PCI- 1710HG Advantech Operating Range 0 bar to 8 bar 630 N ±3 g 5V to 24V DC 0 10 bar -10V to 10V 100MHz (a) (b) PAM Proportional pressure regulator Rotary encoder Accelerometer DAQ Fig. 8: Illustration of the main hardware employed in the experimental study The experimental study undertaken included the joint angle tracking control of the single-link robot arm under the variation of payload masses, m pl = 35, 50 and 55 and 150 g (maximum payload). The arm was assigned to follow the 1 rad prescribed trajectory. The Simulink model with HILS for the experimental study is presented in Fig. 9. Fig. 9: Simulink model with HILS 6.1 Experimental results and discussion Prior to applying the controller, both the PAM actuators were pressurized to 4 bars. Due to the same pressure in each PAM, the torque is zero implying that there is no motion. The experiment was carried out four times, considering different payload masses. For each test, the joint angle tracking performance was studied. The results show that the system performance is satisfactory. Figs. 10(a) to (d) present the actual joint angle outputs of the robot arm for each of the payload masses. (c) (d) Fig.10: Actual joint angle for various payloads (a) 30 g (b) 50 g (c) 55 g (d) 150 g As expected, the increase in the payload mass significantly influences the system performance and causes the track error to increase. For maximum payload, the robot arm failed to completely follow the prescribed trajectory and stopped at about 4/5 of its path. This may be due to the fact that PAM actuators are sensitive to the variation in the payload masses. It can also be caused by other reasons such as PAM nonlinearity, inconsistent pressure supplied by the air compressor and noises in the control signals or accelerometer/encoder signals. Nevertheless, the system presents relatively satisfactory performance which clearly shows the superiority of the proposed AFCFL. 7 Conclusion The proposed AFCAFL controller was implemented for the real-time control of a single link robot arm driven by PAM. The practical aspect of the research was carefully investigated in which the HILS was ISBN: 978-960-474-371-1 138

successfully applied to integrate the mechanical rig with the control system. The system has been shown to readily operate in real-time. The experimental findings exhibit lower levels of accuracy due to a number of physical constraints. The integration of the gripper into the robotic system allowed for studying the performance of the system based on variation of the payload masses. Future works may consider different sets of trajectories as well as the coordinate motion control of a more complex PAM system that will subsequently require more hardware/software integration. References: [1] G.K. Klute, J.M. Czerniecki, B. Hannaford. McKibben artificial muscles: pneumatic actuators with biomechanical intelligence, Procs. of IEEE/ASME International Conference on Advanced Intelligent Mechatronics, 1999. [2] X.C. Zhu, et al., Adaptive robust posture control of a parallel manipulator driven by pneumatic muscles, Automatica, Vol.44, No.9, 2008, pp. 2248-2257. [3] B. Tondu, P. Lopez, Modeling and control of McKibben artificial muscle robot actuators. Control Systems, IEEE, Vol.20, No.2, 2000, pp. 15-38. [4] C.P. Chou, B. Hannaford, Measurement and Modeling of McKibben Pneumatic Artificial Muscles, IEEE Transactions on Robotics and Automation, Vol.12, No.1, 1996, pp. 90-102. [5] D.W. Repperger, K.R. Johnson, C.A. Philips, Nonlinear feedback controller design of a pneumatic muscle actuator system. Procs. of the American Control Conference, 1999, 1999. [6] D.W. Repperger, et al., Power/energy metrics for controller evaluation of actuators similar to biological systems, Mechatronics, Vol.15, No.4, 2005, pp. 459-469. [7] T.U.D.C. Thanh, K.K. Ahn, Nonlinear PID control to improve the control performance of 2 axes pneumatic artificial muscle manipulator using neural network, Mechatronics, Vol.16, No.9, 2006, pp. 577-587. [8] K. Ahn, H. Anh, Comparative study of modeling and identification of the pneumatic artificial muscle (PAM) manipulator using recurrent neural networks, Journal of Mechanical Science and Technology, Vol.22, No.7, 2008, pp. 1287-1298. [9] S.W. Chan, et al., Fuzzy PD+I learning control for a pneumatic muscle, FUZZ '03, 12 th IEEE International Conference on Fuzzy Systems, 2003. [10] J.H. Lilly, Adaptive tracking for pneumatic muscle actuators in bicep and tricep configurations, IEEE Transactions on Neural Systems and Rehabilitation Engineering, Vol.11, No.3, 2003, pp. 333-339. [11] J.R. Hewit, J.S. Burdess, Fast dynamic decoupled control for robotics, using active force control, Mechanisms and Machine Theory, Vol.16, No.5, 1981, pp. 535-542. [12] H. Jahanabadi, M. Mailah, M.Z.M. Zain. Active Force Control of a fluidic muscle system using Fuzzy Logic, Procs. of IEEE/ASME International Conference on Advanced Intelligent Mechatronics, 2009 (AIM 2009), 2009. [13] M. Mailah, H.H. Mun, S. Kazi, H. Jahanabadi, Experimental Implementation of Active Force Control and Iterative Learning Technique to a Two-Link Arm Driven by Pneumatic Artificial Muscles, Procs. of 10th WSEAS International Conference on Computational Intelligence, Man-Machine Systems and Cybernetics (CIMMACS '11), Jakarta, 1-3 Dec. 2011. [14] H. Jahanabadi, M. Mailah, M.Z.M. Zain, Active Force with Fuzzy Logic Control of a Two-Link Arm Driven by Pneumatic Artificial Muscles, Journal of Bionic Engineering, Vol.8, No.4, 2011, pp. 474-484. [15] M. Mailah, N.I.A. Rahim, Intelligent active force control of a robot arm using fuzzy logic, Procs. of TENCON 2000, 2000. [16] G.K. Klute, Artificial Muscles: Actuators for Biorobotic Systems, University of Washington, 1999. ISBN: 978-960-474-371-1 139