Differentiable Compliant Contact Primitives for Estimation and Model Predictive Control

Control techniques like MPC can realize contact-rich manipulation which exploits dynamic information, maintaining friction limits and safety constraints. However, contact geometry and dynamics are required to be known. This information is often extracted from CAD, limiting scalability and the ability to handle tasks with varying geometry. To reduce the need for a priori models, we propose a framework for estimating contact models online based on torque and position measurements. To do this, compliant contact models are used, connected in parallel to model multi-point contact and constraints such as a hinge. They are parameterized to be differentiable with respect to all of their parameters (rest position, stiffness, contact location), allowing the coupled robot/environment dynamics to be linearized or efficiently used in gradient-based optimization. These models are then applied for: offline gradient-based parameter fitting, online estimation via an extended Kalman filter, and online gradient-based MPC. The proposed approach is validated on two robots, showing the efficacy of sensorless contact estimation and the effects of online estimation on MPC performance.Comment: Submitted ICRA24. Video available at https://youtu.be/CuCTcmn3H-o Code available at https://gitlab.cc-asp.fraunhofer.de/hanikevi/contact_mp

Hypertensive Disorders of Pregnancy : Complications and Obstetric Outcome at Princess Marina Hospital, Gaborone, Botswana

OBJECTIVES: To determine the prevalence, maternal complications, foetal outcome and characteristics of patients with hypertensive disorders of pregnancy. SETTING: Princess Marina Hospital, a tertiary and referral Hospital in Gaborone, Botswana. MATERIALS AND METHODS: A retrospective chart review of all hypertensive women who delivered at Princess Marina Hospital from December 2002 to April 2003 was done. Information from patients’ records was entered on a compilation sheet which was entered in SPSS program and analyzed. Prevalence rates (per 1000 deliveries) were estimated by type of hypertension. Demographic characteristics, maternal complications and perinatal outcome were also determined. RESULTS: Of 1919 deliveries at Princess Marina Hospital during the study period, there were 100 cases of hypertensive disorders of pregnancy giving a prevalence of 52.1 per 1000 deliveries. Twenty one cases had chronic hypertension of which 11 (52%) developed superimposed pre-eclampsia while 10(48%) had no proteinuria. Of 79 patients with pregnancy induced hypertension, 36 (46%) had hypertension without proteinuria, 42(53%) had pre-eclampsia and 1(1%) had eclampsia. Age ranged from 18 to 47 years with a mean age of 29.5 years. Teenagers were 8 cases out of 100. Maternal complications were HELLP syndrome (3 cases), acute renal failure (2 cases), and disseminated intravascular coagulation (1 case). There were no cases of abruptio placentae, cerebral haemorrhage or maternal death. Of all deliveries, perinatal complications included preterm deliveries (45%), low birth weight (41%) and still births (17%). Conclusion: The prevalence of hypertensive disorders of pregnancy was high but there were few maternal complications

고급 로봇 힘 제어 알고리즘의 개발

Robot controlIn this thesis, advanced model-based robot force control algorithms are developed exploiting the availability of multisensor information towards attenuating the sensor noises and suppressing the effects of force disturbances. For joint-space single-dof application, a reduced-order multisensor-based force observer (RMFOB) for accurately estimating the force exerted on a load is developed. To suppress the effects of force disturbances in a robust way, a disturbance observer known as model-based force disturbance observer (FDOB) is proposed. Then, the RMFOB and FDOB are combined in a closed-loop setting to form a twofold observer-based force control system. Design methodology and systematic parameter tuning criteria for this double observer-based force control is developed. And lastly, towards high-performance motion control and contact stability, a novel integrated DOB (IDOB) is also developed and its effectiveness evaluated. The IDOB design concept is applied to a multi-dof system where an outer-loop integrated DOB-based admittance control method in task space (OIDOBt) is developed. This is implemented in task-space and outside the inner position/velocity control loop for the 6-DOF industrial manipulator.본 논문에서는 다중 센서 정보를 활용한 고급 모델 기반 로봇 힘 제어 알고리즘을 제안한다. 관절 공간에서 단일 자유도를 가지는 어플리케이션에 적용하기 위해, 로봇의 외력 측정 정확도를 높이는 다중센서 기반 축소차원 힘관측기(RMFOB) 알고리즘이 개발된다. 그리고 힘 외란의 영향을 강인한 방식으로 억제하기 위해 모델 기반 힘외란관측기(FDOB)로 알려진 외란관측기가 제안된다. 또한, RMFOB와 FDOB가 폐쇄 루프 설정에서 결합되어 이중 관측기 기반 힘 제어 시스템을 형성한다. 이 이중 관측기 기반 힘 제어 시스템을 (복잡하지 않게) 개발하기 위한 설계 방법론과 체계적인 매개 변수 조정 방법을 제안한다. 마지막으로 고성능 모션 제어와 접촉 시의 안정성을 위해 새로운 통합외란관측기(IDOB)가 개발되고 그 효과 또한 평가된다. 다자유도 시스템을 위해서는, 작업 공간의 외부 루프 통합외란관측기 기반 어드미턴스 제어 방법(OIDOBt)이 개발된다. 이 방법은 6자유도 산업용 로봇를 위한 작업 공간과 내부 위치/속도 제어 루프 바깥에 구현된다.I. Introduction 1 1.1 Research Background 1 1.2 Related Literature 2 1.2.1 Force Measurement and Estimation 2 1.2.2 Force Control 5 1.3 Thesis Contributions 7 1.4 Thesis Flow and Overview 10 1.5 List of Publications 11 1.5.1 Journal Papers 11 1.5.2 Conference Papers 12 II. Force Control Problem Description For a Single-DOF System 14 2.1 Generalized Single-DOF System 14 2.2 Modeling of Force Sensor-Based Force Servo System 14 2.3 System Description Including Noise Characteristic 19 2.4 Disturbances in Force Control System 21 III. A Reduced-Order Multisensor-Based Force Observer 22 3.1 Design and Analysis of the RMFOB 22 3.1.1 State Transformation and State-space Derivation 24 3.1.2 Extraction of Force and Measurement Offsets States 27 3.2 Discussion of the RMFOB Characteristics 29 3.3 Verification of the Proposed RMFOB 30 3.3.1 Experiment Design 31 3.3.2 Validation of Offsets Compensation Using Simulations 33 3.3.3 Experimental Verification 36 3.4 Conclusions on RMFOB Design 42 IV. Twofold Observer-Based Precise Force Control 43 4.1 Proposed Twofold Observer-Based Force Control System 43 4.2 Estimation of Accurate Interaction Forces with RMFOB 43 4.3 Estimation and Suppression of Disturbances with FDOB 44 4.4 Development of Parameter Tuning Criterion 45 4.4.1 Step 1: Derivation of the output equation 46 4.4.2 Step 2: Effect of noises and disturbances on force output 47 4.4.3 Step 3: Quantitative formulation 47 4.5 Analysis of Proposed Force Control System 50 4.5.1 RMFOB and FDOB Performance Analysis 50 4.5.2 Robust Stability Analysis 51 4.5.3 Assessment of Effect of Observer Cutoff Frequency 53 4.6 Simulation and Experiments 54 4.6.1 Experiment Design 54 4.6.2 Simulation Study 55 4.6.3 Experimental Validation 55 4.7 Conclusion on Twofold Observer Control 58 V. Integrated Disturbance Observer-Based Robust Force Control 60 5.1 Introduction 60 5.2 Design and Analysis of Robust Force Control Systems with Force-Based DOBs 62 5.3 Design of Force-Based Disturbance Observers 64 5.3.1 CFDOB and FDOB 64 5.3.2 Integrated Disturbance Observer 65 5.4 Structural, Tracking Performance, and Stability Analyses of Force-DOB-Based Robust Force Control Systems 67 5.5 Contact Stability Analysis 69 5.6 Influence of Force Sensor, Motor Encoder, and Motor Current Noises to Disturbance Estimation Performance 71 5.7 Analysis of Influence of Model Uncertainty 72 5.7.1 Disturbance Suppression Performance Analysis 73 5.7.2 Analysis of Stability Against Model Uncertainty 74 5.8 Experimental Verification 76 5.8.1 Experiment Design 76 5.8.2 Experiments 77 5.9 Conclusions on IDOB Design 86 VI. Application of DOB to Admittance Control of Multi-DOF Industrial Robot 87 6.1 Introduction 87 6.2 Problem Description 89 6.2.1 The Robot and its Basic Admittance Control Method 90 6.3 Limitations of Basic Admittance Control Method 94 6.3.1 Desired Admittance Rendering Limitation 95 6.3.2 Contact Stability Limitation 95 6.3.3 Constraint of Inner-loop Velocity Control 96 6.4 Design of Proposed Control Method 97 6.4.1 Proposed Control Architecture 97 6.4.2 Detailed Design of the DOBs 97 6.5 Closed-Loop Analysis 100 6.5.1 Rendered Admittance Limitation Evaluation 100 6.5.2 Contact Stability Limitation Evaluation 101 6.5.3 Robustness to Payload Mass Variations 103 6.5.4 Disturbance Rejection and Norminalization 104 6.5.5 Influence of Sensor Noises on Disturbance Estimation Performance 105 6.6 Experimental Validation 106 6.6.1 Parameter Determination For Robot and DOB Models 106 6.6.2 Controller Implementation 108 6.6.3 Exp 1: Admittance Rendering in Free Space 109 6.6.4 Exp 2: Contact With Stiff Surface 110 6.6.5 Exp 3: High Speed Force Control 114 6.7 Conclusions on DOB Application for Admittance Control 116 VII. General Conclusion 119 7.1 Conclusion 119 7.2 Future Works 120 References 122DoctordCollectio

A Comparative Study of Force Observers For Accurate Force Control of Multisensor-Based Force Controlled Motion Systems

This paper presents a comprehensive comparative study of the multisensor-based force observers for accurate force control. A force controlled system which contains a force sensor for measuring force transmitted to the load by the motor and an encoder for measuring motor position is considered as the general multisensor-based motion system in this study. Even though these multisensor-based motion systems are emerging as potential motion systems as the demands for collaborative robots increase, there has been few studies that investigate their advantages and limitations. to address this issue, three types of observer-based force controllers that utilize the multisensors are designed and implemented. These controllers exploit the availability of force sensor, motor encoder, and motor torque information from the multisensor-based motion system to estimate accurate force which is later utilized to close the feedback loop. Mathematical and quantitative analyses are conducted to compare performances of the proposed observer-based force control and through this, their advantages and limitations are pointed out. Finally, simulation and an experimental case study with an actual robot are conducted to validate the force tracking performance of the designed force control systems. © 2022 IEEE

FDOB-Based Robust Impedance Control of Force Sensor Implemented Force Servo System

Instability which occurs when the robot's end effector contacts a very stiff environment is a challenge in designing control systems for safe physical interaction and cooperation of robots with environment. One of the reasons for the instability is force disturbances caused by the mechanical factors of the robot system. To this effect, this paper presents the design, analysis, and implementation of a robust impedance controller for a force servo system. To suppress the force disturbances, a force disturbance observer (FDOB) is implemented in the impedance-controlled system. For comparison purposes, impedance control system when the FDOB is not implemented is also designed and analyzed. Further, using the passivity approach, coupled stability conditions of the designed impedance control systems are derived and analyzed to assess the effect of FDOB on passivity and overall control performance. Simulations and experiments are conducted to evaluate performance of the designed impedance control systems and it is found that the FDOB-based control system shows superior performance by improving contact stability compared to direct force sensor feedback control system. © 2021 IEEE

High-Performance Admittance Control of An Industrial Robot Via Disturbance Observer

Safe physical interaction using admittance control on an industrial robot with inner-loop motion control remains challenging. This is partly due to the low intrinsic admittance and stability issues from inner-loop motion control limitations (e.g. bandwidth). To increase the admittance at an interaction point with the user/environment, this paper proposes a robust admittance control architecture. A disturbance observer (DOB) is used to improve effective inner-loop motion control, suppressing the effects of velocity disturbances. The DOB uses the robot's closed-loop task space velocity control as the nominal model, compensating disturbances between the commanded robot velocity and realized robot velocity output. An admittance controller uses measured force to generate robot velocity commands. Detailed analyses are carried out to theoretically evaluate the proposed control system. Experiments conducted on a COMAU RACER-7-1.4 industrial robot verify the effectiveness of the proposed admittance control scheme and stability in environmental contact. Moreover, the proposed method is simple to implement on the existing robot system. © 2022 IEEE

Performance Comparison of Position Controlled Robotic Stage When Force-and Position-Based Disturbance Observers are Implemented

Position control of the robotic stage when position-based and force-based disturbance observers (DOB) are implemented and their performances compared is presented in this paper. Implementation of the DOBs is aimed at improving the quality of force control by suppressing the disturbances within and into the robot mechanical system. This is because the accuracy of position control is of paramount importance since bad position control affects the reproducibility of the position perturbation which in-turn affects the production of reliable force readings for balance assessment function with the robotic stage. The overall control and disturbance suppression performance is analyzed and compared for all the control strategies. Simulations and experiments are conducted to evaluate the position control strategies. Moreover, the obtained reaction force recorded when the DOB-based position control strategies are utilized is also analyzed to point out their differences which are caused by the designed controllers. © 2021 ICROS

Force disturbance observer-based force control for compliant interaction with dynamic environment

Disturbances are one of the major challenges that should be dealt with when designing high performance force control systems for robots that interact with unknown environments. To achieve high performance dynamic interaction, this paper presents a robust force control system that implements a force disturbance observer (FDOB). Dynamic compliance with the environment is greatly improved with this control technique. The whole force control structure consists of a servo system with a force sensor, the proposed FDOB, feedforward and feedback controllers, and the low-pass filter for attenuating measurement noises of the force sensor feedback signal. The nominal model of the proposed FDOB is obtained by nonparametric system identification method. The FDOB then estimates disturbances by utilizing the motor torque and force sensor measurement signals as its inputs. Theoretical analyses of the FDOB and the overall force control system are conducted. To validate the proposed control structure, experiments are conducted while considering various scenarios from where it is found out that it shows superior performance over the conventional force control method. © 2021 IEEE

Twofold Observer-Based Precise Force Control

Force sensor measurement noises and mechanical disturbances make realization of stable and robust force control systems challenging. With regard to this, a precise force control system that implements a combination of force and disturbance observers (DOBs) is proposed in this brief. The observers are a reduced-order multisensor-based force observer (RMFOB) to attenuate sensor measurement noises and a model-based force DOB (FDOB) to suppress the effects of mechanical disturbances. These are combined in a closed-loop setting to form a twofold observer-based precise force control system. A novel systematic tuning criterion for the RMFOB and FDOB cutoff frequencies utilizing sensor noise and force disturbance intensities derived from the proposed closed-loop system is developed. In addition, analysis of robust stability against nonlinearity and parameter variations of the proposed system is conducted using RMFOB cutoff frequency. Finally, the effectiveness of the proposed method is verified by theoretical analyses, simulations, and experiments