Decentralized robust control of modular robot manipulators with harmonic drive transmission using RARKF-based joint torque estimation 2022 34th Chinese Control and Decision Conference (CCDC) | 978-1-6654-7896-0/22/$31.00 ©2022 IEEE | DOI: 10.1109/CCDC55256.2022.10034169 Tianhe Wang1 , Tianjiao An1 , Chongyang Wei1 , Yuanchun Li1 , Bo Dong*1 1. Department of Control Science and Engineering, Changchun University of Technology, Changchun, 130012 E-mail: dongbo@ccut.edu.cn Abstract: This paper presents a decentralized robust control of modular robot manipulators (MRMs) with harmonic drive transmission using redundant adaptive robust extended Kalman filter (RARKF)-based joint torque estimation. The dynamic model of MRMs is formulated via RARKF-based joint torque estimation method. A decentralized robust control is developed to guarantee the trajectory tracking error is uniform ultimate bounded (UUB). The closed-loop robotic system is asymptotic stability on the basis of Lyapunov verification. The experiments are performed to clarify the effectiveness of the proposed method. Key Words: Modular robot manipulator, Decentralized control, Harmonic drive, Redundant adaptive robust Kalman filter 1 INTRODUCTION A modular robot manipulator (MRM) is a robot composed of many modules with the same structure and function. Each module has a completely independent structure and function. The MRM can freely add and remove its own modules according to different task requirements and specific working conditions which perform different configurations of the structure. MRM is widely used in many fields, especially in the field of medical and industrial robots. In the middle of the 20th time, the harmonic drive was used, which has the advantages of high gear ratio and light weight. In the past few years, many researchers have studied the modeling of harmonic transmission devices[1-3]. After that, use these findings and solve the trouble encountered in torque estimation[4,5], motion control of robot system based on harmonic drive[6-9]. Installing torque sensors at the robot joints will complicate the structure of the joints, which will directly affect the rigid connection between the joint module and the robot connecting rod. The strain gauge in the joint torque sensor will change with the change of the external environment temperature, which will also directly affect the accuracy of the joint torque sensor. Therefore, a joint torque estimation scheme of the robot system is proposed, which uses the harmonic transmission device to estimate the joint torque value by using the joint position information, which is of great significance for improving the accuracy of MRM. This work is supported by the National Natural Science Foundation of China (Grant nos. 61773075, 62173047 and 61703055), the Scientific Technological Development Plan Project in Jilin Province of China (Grant no. 20200801056GH) and the Science and Technology project of Jilin Provincial Education Department of China during the 13th FiveYear Plan Period (Grant nos. JJKH20200672KJ, JJKH20200673KJ and JJKH20200674KJ). c 978-1-6654-7896-0/22/$31.00 2022 IEEE In order to eliminate the torque measurement fluctuation caused by gear tooth meshing, a standard Kalman filter is used for estimation. However, the standard Kalman filter can only guarantee the optimality of the torque estimation, but the robustness is also very important in the torque estimation.Because robustness and optimality are contradictory, as robustness increases, optimality will decreasey. However, robustness is not always required, and stronger robustness will result in loss of optimality. Therefore, in order to balance the robustness and optimality, an adaptive robust extended Kalman filter [10] algorithm is designed, which can make the filter perform between the robust mode and the optimal mode, the filter gain can be adjusted adaptively. However, the non-linear error in the measurement model will make the switching function in AREKF invalid. In response to this problem, we have introduced a redundancy factor to form a redundant adaptive robust extended Kalman filter (RAREKF). This allows the switching function to work stably and provides tolerance for modeling errors in torque estimation. In this paper, a redundant adaptive Kalman filter algorithm is used for robot joints with harmonic transmission. It can reduce the influence of nonlinear modeling errors and noise. The algorithm has adaptive robustness, it can self-adjust the filter gain and self-switching filter mode according to the load change, and realize the dynamic balance of robustness and optimality. The decentralized robust control method is used to ensure that the trajectory tracking error in the case of redundant adaptive Kalman filtering is consistent and ultimately bounded, and experiments are used to verify the effectiveness of the proposed method. 5496 Authorized licensed use limited to: Institut Teknologi Sepuluh Nopember. Downloaded on October 17,2025 at 09:40:07 UTC from IEEE Xplore. Restrictions apply. 2 DYNAMIC MODEL FORMULATION 2.1 Harmonic Drive Model changes. The local elasticity coefficient kf i of the flexspline can be expressed as Considering an MRM is made up of N modules, each module is made up of a rotating joint based on harmonic drive. According to the ideal input/output kinematics relationship, the position of the harmonic drive angle can be obtained. θwi = γi θf i (1) where i is the ith module, γi stands for deceleration ratio, θwi is the rotation angle of the wave generator , θf i is the rotation angle of the flexible wheel, the torque transmission relationship of the harmonic device is expressed as τwi = 1 τf i γi (2) where τwi is the input torque of the wave generator. τf i is the output torque of the flexspline, because the position of the circular spline in the harmonic drive is fixed and the circular spline itself is not flexible. So that, θci = 0. The output and input are not linear due to torsional compliance, motion error and nonlinear friction in harmonic drive. Describe the kinematic relationship between the motion and the force constraint in the harmonic drive, the remaining effects can be combined by introducing friction, flexibility, and motion error characteristics. θwOi is the external position of the wave generator, θwIi the input position of the wave generator, θf Oi is the angle position of the flexspline on the load side, θf Ii is the angle position of the flexspline measured by the gear. τmri stands for motor friction. τf ri represents the total friction torque of the harmonic drive. From the output side of the transmission. The elastic coefficient of the wave generator and the elastic coefficient of the flexspline are denoted by kwi and kf i respectively. We consider the friction loss in harmonic drive. 1 τωi = τi − τmri = (τf i − τf ri ) γi (3) where τi is the motor torque. The wave generator twist and the flexspline twist can be expressed as Δθωi = θωOi − θωIi (4) Δθf i = θf Oi − θf Ii (5) In the above two formulas, the input position θwIi of the waveform generator can be measured by the motor side encoder. The output position of the flexible wheel can be measured θf Oi by the connecting rod side encoder. Based on the measured position information, the following relationship can be used to calculate the total torsion angle of the harmonic device of the joint. Δθi = θf Oi − θwIi γi (6) According to the given harmonic drive stiffness and hysteresis performance, the results show that when the torque of the flexspline changes, the local elastic coefficient also kf i = dτf i dΔθf i (7) Considering the symmetry of the stiffness of the harmonic drive, kf i can be expressed as 2 kf i = kf i0 (1 + (cf i τf i ) ) (8) where kf i0 and cf i are undetermined constant values. If kf i0 = 0, the torsion angle of the flexspline can be calculated as follows τf i dτf i arctan(cf i τf i ) = (9) Δθf i = k cf i kf i0 fi 0 In addition, at rated torque, the harmonic drive deformation range drops rapidly to zero, which means that the wave generator stiffness increases rapidly. The elastic coefficient of the wave generator in the harmonic drive can be expressed as kwi = kwi0 ecwi |τwi | (10) where kwi and cwi are undetermined values. If kwi0 = 0, the torsion angle of the wave generator can be expressed as τwi dτwi sgn(τwi ) = (1 − e−cwi |τwi | ) (11) Δθwi = k cwi kwi0 wi 0 According to Eqs. (9), and (11), the total torsion angle of harmonic drive calculated as Δθi = arctan(cf i τf i ) sgn(τwi ) + (1 − e−cwi |τwi | ) (12) cf i kf i0 γi cwi kwi0 The total torsion angle Δθi of harmonic drive can be brought into Eq. (6) by measuring joint position information with dual encoders. The joint torque can be calculated by the above formula, Therefore, τf i can be expressed as: τf i = 2.2 1 sgn(τwi ) tan(cf i kf i0 (Δθi − (1 − e−cwi |τwi | ))) cf γi cwi kwi0 (13) RARKF-based joint torque estimation Consider the filtering model Xk = F (k, Xk−1 ) + wk (14) Yk = Xk + v k (15) where k ∈ N , Xk is the system state at time k and Yk is the measured value at time k.The dynamics of the system in state Xk are represented by system function F (k, Xk−1 ), w represents process noise, v means measurement noise. Covariance matrices W and V are Gaussian white. When estimating the torque of the joint, F (k, Xk−1 ) = Xk−1 ,and the filtered value of τf i is X, and the measured value of τf i is Y . In the torque model, the unknown change of the joint torque from k to k − 1 2022 34th Chinese Control and Decision Conference (CCDC) 5497 Authorized licensed use limited to: Institut Teknologi Sepuluh Nopember. Downloaded on October 17,2025 at 09:40:07 UTC from IEEE Xplore. Restrictions apply. is represented by w, and the random noise in the torque calculation result is represented by v. On the time step k Y k = τf i k P̄Y k = (16) The structure of the RARKF algorithm is as follows: (1) Predict the state X̂k|k−1 = f (k, X̂k−1|k−1 ) (17) Pk|k−1 = Pk−1|k−1 + Wk−1 (18) (2) Adjust the gain P̄Y (k) is defined as k|k−1 = C(Pk|k−1 ) (20) (21) −1 k|k−1 P Y k (22) X̂k|k = X̂k|k−1 + Kk Ŷk −1 (23) −1 k|k−1 + V −1 k) (24) In order to ensure the stability of the filter when there are modeling errors, the H∞ filtering algorithm is designed. If the following equation holds max ||X̂k|k ||22 ≤ δ2 2 ||wk ||2 + ||vk ||22 + ||E||22 ,k > 0 (28) 1/2 (29) -1 (k|k − 1) (30) ARKF filter can adaptively switch the filter mode to achieve a balance of robustness and optimality. When there is a modeling error that makes the condition PY k > βPY k hold, Eq.(27) is the same as Eq.(26), let β > 1. Therefore, the redundancy factor and compensation function are introduced into ARKF to form a redundant adaptive robust Kalman filter algorithm. When the modeling error follows the joint load changes, the redundancy factor β can be introduced to make the result more accurate. Because the size of the motor current can reflect the size of the load, the redundancy factor is defined as σIM (k) 2 , σIM (k) 2 > 1 (31) β= 1, otherwise The compensation function is defined as (25) Define δ as the attenuation factor, the gain scheduling operator can be determined as −1 −1 −2 T L k Lk ) (26) k|k−1 = (P k|k−1 − δ In order to change the matrix k|k−1 , we need to define a matrix Lk .The attenuation factor δ is related to estimation error, measurement noise and modeling error. When the attenuation factor δ is relatively small, the system needs robustness. We can change the attenuation factorto change the robustness and optimality of the system. When the attenuation factor δ becomes larger and larger, the robustness of the system getting smaller and smaller, when the attenuation factor δ = ∞, the filter at this time is Kalman filter. When the attenuation factor δ is given, the algorithm may not reach the optimality of filtering, and it will work in robust filtering mode. Therefore, in order to better switch the filtering mode, we use the adaptive robust Kalman filter (ARKF) algorithm. In the ARKF, the gain scheduling operator C in Eq. (19) is determined by Pk|k−1 and −1 (P −1 k|k−1 − δ −2 LT k Lk ) , P̄Y k > βPY k k|k−1 = Pk|k−1 , otherwise (27) 5498 k=0 The compensation factor ε̄−2 max I in L(k) satisfies the following conditions ε̄−2 max I ≤ (4) Update estimates Pk|k = ( ρPY k−1 +Ỹk ỸkT ρ+1 Lk = δ(P −1 k|k−1 − ε̄−2 max I) (19) Ŷk = Yk − X̂k|k−1 PY k = k|k−1 + Vk Ỹk ỸkT , where ρ is a forgetting factor. When the value of P̄Y k is greater than the measurement error variance PY k , the filter will be in robust filtering mode at this time. In order to make the filtering mode stable, let where C is a gain scheduling operator. (3) Update measurement value Kk = −1 −1 ε̄-2 k|k−1 max I = f β k P where the compensation factor is expressed as P̄Y k fβ k = β −1 diag PY k (32) (33) This can make the filtering stable when β = 1 .According to Eqs.(28)-(33) and Eqs.(19)-(23), the redundant adaptive robust Kalman filter algorithm can be expressed by Eqs.(17)–(24). 2.3 Dynamic Model Formulation Considering RARKF-based MRM dynamic model is Imi γi q̈i + fi (qi , q̇i ) + Imi i−1 T zmi zj q̈j j=1 + Imi j−1 i−1 j=2 k=1 (34) τf i zmi (zk × zj )q̇k q̇j + = τi γi T where i is the ith subsystem, Imi is the moment of inertia of the motor, γi represents the reduction ratio of the gear, qi is the position information of the ith joint, q̇i is the speed information of the ith joint, and q̈i is the acceleration information of the ith joint. fi (qi , q̇i ) represents the concentrated friction of the harmonic device and the friction of the 2022 34th Chinese Control and Decision Conference (CCDC) Authorized licensed use limited to: Institut Teknologi Sepuluh Nopember. Downloaded on October 17,2025 at 09:40:07 UTC from IEEE Xplore. Restrictions apply. motor. The friction term fi (qi , q̇i ) is a function related to qi and q̇i . 3 DECENTRALIZED TROLLER DESIGN ROBUST CON- The overall control of each joint is defined as No filter RARKF filter 4 Joint torque estimation(Nm) fi (qi , q̇i ) = (fci +fsi exp(−fτ i q̇i2 ))sgn(q̇i )+fqi (qi , q̇i )+bi q̇i (35) where fci , fτ i , fsi and bi are uncertain parameter values in the friction model. 5 3 2 1 0 -1 -2 -3 τf i + ui τi = γi (36) i−1 j−1 i−1 T zmi zi q̈j 30 2 e = q − qd , r = ė + λe, a = q̈d − 2λė − λ e (38) where λ is any positive constant. Assume that the inertia of the motor is known, but some system parameters are unknown, and its reference trajectory and the first and second derivatives are bounded. Use the decomposition-based friction compensation method to compensate the friction of ith joint, where Y (q̇i ) and F̃ i are denoted as Y (q̇i ) = [q̇i , sgn(q̇i ), exp(−fˆτ i q̇i )sgn(q̇i ), − fˆsi q̇ 2 exp(−fˆτ i q̇i )sgn(q̇i )] 60 70 80 fˆτ i − fτ i ] τf i + b̂i q̇i + (fˆci + fˆsi exp(fˆτ i q̇i2 ))sgn(q̇i ) γi i−1 Ij2 (uijc + uijv ) + uiu + Y (q̇i )(uipc + uipv ) + τi = j=1 + j−1 i−1 (41) i i i Jkj (Vkjc + Vkjv ) + Imi γi ai j=2 k=1 where Imi is expressed as the inertia of the motor, τf i is the estimated joint torque, b̂i , fˆci , fˆsi , fˆτ i is the nominal friction parameter, uiu is used to compensate for nonparametric uncertainty fqi (q, q̇). For parameter uncertainties F̃ci and F̃vi ,uipc and uipv compensation can be used. For each joint, friction is compensated in the same way. Compensators uipc , uipv and uiu are defined as uiu = −ρf i |rrii | |ri | > εi −ρf i |εrii | |ri | ≤ εi (42) t uipc = −k( T However, the friction parameter models bi , fci , fsi and fτ i are not accurate, where b̂i , fˆci , fˆsi , fˆτ i is the estimated value of bi , fci , fsi , fτ i . Due to the change of temperature, the uncertainty of the parameter model is not constant. (40) where F̃ci is an uncertain constant vector, F̃vi is a variable i | < ρin . An adaptive comand has defined meanings as |F̃vn pensator is designed to compensate the constant parameter uncertainty by using the control design method based on decomposition, robust compensator to compensate for F̃vi . The control torque τi of the ith joint is (39) i fˆsi − fsi 50 F̃ i = F̃ci + F̃vi T zmi (zk × zj )q̇k q̇j = ui fˆci − fci 40 F̃ i can be decomposed into According to the above formula, the control input ui can be designed for a single joint. Because the motion of the lower joint causes the uncertainty of the model, a robust controller can be used to compensate for the uncertainty, in order to achieve the desired performance with a small feedback gain. So use the decomposition-based control design method Using the basic strategy of decomposition-based system modeling and control methods, a separate compensator is designed to compensate for each different type of uncertain parameters and variables. For some uncertainties that cannot be measured in real time, robust compensators can compensate, and these compensators work together to form a controller. Define the error as F̃ = [b̂i − bi 20 (37) j=2 k=1 i 10 Figure 1: The estimated torque of joint 1 is unfiltered and RARKF filtered j=1 + Imi 0 Time(s) where ui is the control input for joint i. For the ith joint, according to (34) and (36) Imi γi q̈i + fi (qi , q̇i ) + Imi -4 Y T (q̇i )ri dτ + ri ) (43) 0 uipvn = ⎧ T ⎨−ρi Y (q̇i )T ri , |Y (q̇i )T ri | > εi n |Y (q̇i ) ri | T pn ⎩ −ρin Y (q̇ii) ri , |Y (q̇i )T ri | ≤ εipn n = 1, 2, 3, 4 εpn (44) where εi and εipn are positive control parameters. 2022 34th Chinese Control and Decision Conference (CCDC) 5499 Authorized licensed use limited to: Institut Teknologi Sepuluh Nopember. Downloaded on October 17,2025 at 09:40:07 UTC from IEEE Xplore. Restrictions apply. 1.5 0.4 Joint torque estimation error(Nm) Joint torque estimation(Nm) No filter RARKF filter 1 0.5 0 -0.5 -1 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 Time(s) Time(s) Figure 2: The estimated torque of joint 2 is unfiltered and RARKF filtered Figure 4: Joint torque estimation error of joint 1 after second-order low-pass filtering 0.3 Joint torque estimation error(Nm) Joint torque estimation error(Nm) 0.4 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 0.2 0.1 0 -0.1 -0.2 -0.3 0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 Time(s) Time(s) Figure 3: Joint torque estimation error of joint 1 after RARKF filtering Figure 5: Joint torque estimation error of joint 2 after RARKF filtering Decompose Imi i−1 j=1 T zmi zi q̈j into Uzic and Uziv , where Uzic represents constant uncertainty, Uziv stands for variable parameter uncertainty, using a decomposition-based control design method, design an adaptive compensator uijc for the constant uncertainty, and robust compensator uijv for variable part. t i (45) ujc = -k2 k2 ri dτ + ri 0 uijvn = −ρDj |kk22 rrii | |k2 ri | > εiDn , −ρDj kεi2 ri |k2 ri | ≤ εiDn n = 1, 2 (46) Dn εiDn is the positive control parameter. i−1 j−1 T Similarly, for the ith joint,Imi zmi (zk × zj )q̇k q̇j j=2 k=1 can be decomposed into Vzic and Vziv , where Vzic stands for constant uncertainty, Vziv stands for variable parameter uncertainty. Using the decomposition-based control design i is designed for conmethod, an adaptive compensator Vkjc i stant uncertainty term Vzic . and a robust compensator Vkjv 5500 0.3 is designed for the variable part Vziv . t i Vkjc = −k3 ( k3 ri dτ + ri ) (47) 0 i Vkjvn = −ρV k ρV j |kk33 rrii | , |k3 r| > εiV n , −ρV k ρV j kεi3 ri , |k3 r| ≤ εiV n n = 1, 2 Vn (48) Theorem: Consider a modular robot manipulator with the subsystem dynamic model formulated in (34), the model uncertainties existed in (39). The error of closed-loop MRM system is UUB under the decentralized robust control law proposed by (41). Proof: The candidate Lyapunov functions are selected as follows: 1 2 r (49) Vir = 2Bi i The derivative of (49) can be obtained as: V̇ir = ri ((−fpi −Y (q̇)(F̃ic +F̃iv )−(Uzic +Uziv )−(Vzic +Vziv ) (50) 2022 34th Chinese Control and Decision Conference (CCDC) Authorized licensed use limited to: Institut Teknologi Sepuluh Nopember. Downloaded on October 17,2025 at 09:40:07 UTC from IEEE Xplore. Restrictions apply. 0.6 0.2 0.4 Position tracking(Rad) Joint torque estimation error(Nm) 0.3 0.1 0 -0.1 Trajectory error Actual trajectory Desired trajectory 0.2 0 10-3 -1 -0.2 -2 -0.2 -0.4 -0.3 -0.6 -3 11 0 10 20 30 40 50 60 70 80 0 11.1 11.2 10 11.3 20 11.4 30 11.5 40 50 60 70 Time(s) Time(s) Figure 6: Joint torque estimation error of joint 2 after second-order low-pass filtering Figure 8: Joint 2 trajectory tracking 5 0.6 10-3 Position tracking(Rad) 1 0 0.2 11.4 11.6 11.8 12 0 -0.2 -0.4 REFERENCES Trajectory error Actual trajectory Desired trajectory -0.6 0 10 20 30 40 50 60 70 80 Time(s) Figure 7: Joint 1 trajectory tracking If the Lyapunov function satisfies the following conditions: ri | ≥ 4 i i n=1 (ρn εpn ) 4 (51) The position tracking error ei and velocity tracking error ėi can be UUB. 4 CONCLUSION A decentralized robust control of MRMs with harmonic drive transmission using RARKF-based joint torque estimation is proposed in this paper. The dynamic model of MRMs is formulated via RARKF-based joint torque estimation method. A decentralized robust control is developed to guarantee the trajectory tracking error is UUB and the experiments are performed to clarify the effectiveness of the proposed method. 2 0.4 80 EXPERIMENT In order to verify the effectiveness of the proposed control scheme, a 2-DOF MRM is used to conduct experiments to verify the effectiveness of the algorithm. In the experiment, redundant adaptive robust Kalman filter and second-order low-pass filter are used to filter the estimated joint torque. Experiments show that after the estimated joint torque passes through the redundant adaptive robust Kalman filter, the estimated torque of the harmonic drive device is smoother and more in line with the requirements. 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Liu, Adaptive robust extended Kalman filter for nonlinear stochastic systems, IET Control Theory Appl. 2 (3) (2008) 248–250. 2022 34th Chinese Control and Decision Conference (CCDC) 5501 Authorized licensed use limited to: Institut Teknologi Sepuluh Nopember. Downloaded on October 17,2025 at 09:40:07 UTC from IEEE Xplore. Restrictions apply.
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