Ivan Godler

Design guidelines for disturbance observer's filter in discrete time

Control by using a disturbance observer is one of the widely recognized servomotor robust control techniques. It is generally known that higher robustness is achieved by higher bandwidth of the low-pass filter, which is used in the disturbance observer. However, control engineers in practice face problems with realization of the high bandwidth filter. It was considered that the high frequency noise, which is amplified by the time derivative in the observer, causes instability. However, in our paper we show that the vibration and instability of the control system are rather caused by inappropriate sampling time than by the generated noise. We study both the low pass filter and the disturbance observer in discrete time, and give the guidelines to set the filters cut-off frequency, nominal parameters of the observer, and gain of the speed loop controller, all in relation to the sampling time, to achieve stable and nonvibrational response. Results are verified by simulation and experiments.

Position and force control of a robotic finger with twisted strings actuation

Actuation and control of robotic hands is one of the difficult problems in mechatronics that still needs to be solved. In this paper we present a kinematic analysis of a robotic finger that is actuated by twisted strings mechanism, which we named Twist Drive. The principle of actuation is briefly described in the paper and a structure of the developed robotic finger is presented. Forward kinematics for a finger with nonlinearly coupled joints is given. The obtained Jacobian is used in position control and in force control on the fingers tip. Experimental results are presented. Force control in two directions is successfully demonstrated.

Robotic finger with coupled joints: A prototype and its inverse kinematics

Without an external load or without a special effort to overcome the coupling, human fingers move in such a way that DIP and PIP joints of each finger except of a thumb move simultaneously in a coupled fashion. Similar motion of robotic fingers can be achieved by independent control of each joint, but to reduce number of needed actuators, a mechanical coupling of respective joints is frequently used. In this paper we derive an algebraic solution of inverse kinematics for robotic finger with linearly coupled PIP and DIP joints. The calculation involves solving a polynomial. A prototype of robotic finger with coupled joints that uses newly proposed mechanism called Twist Drive is presented. The principle of proposed joints actuation mechanism and its main characteristics are explained. A prototype of robotic finger and basic experimental results of joint positioning control are presented.

Concept of Mechatronics Modular Design for an Autonomous Mobile Soccer Robot

In this paper, we describe the concept, design and implementation of a series of autonomous mobile soccer robots, named Musashi robot, which have a mechatronics modular architecture, to participate in the RoboCup middle-size league. In this design methodology, we show that the selection of a proper moving mechanism, a suitable vision system and a mechatronics modular architecture design can lead to realize a reliable, simple, and low cost robot comparing with most of car-like robots including many kinds of sensors and a complex design structure.

Control of robotic joint by using antagonistic pair of Twist Drive Actuators

In this paper we present a control method for robotic finger that uses twisted strings actuator called Twist Drive. A five fingered robotic hand prototype was developed by using Twist Drive actuators to control 14 active degrees-of-freedom. This paper focuses on control of a thumbs CM joint, which is the only joint in the robotic hand prototype that uses two Twist Drive actuators configured in an antagonistic pair of actuators. Positioning accuracy and smoothness of the joints motion is influenced by nonlinear characteristics of the actuators and by friction in the mechanism. To compensate for the disturbances, we used disturbance observer, and additionally enhance smoothness of motion by controlling the unwinding torque of opposing actuator. The robotic hand and the proposed control algorithm are explained in the paper, experimental results of CM joint positioning control are presented.

Stability, gain and sampling time of discrete time acceleration control loop

Servo control is realized by position, speed and current control loops. To further improve the robustness of the servomechanism, acceleration control in the minor loop of a speed control loop was effectively proposed. However, the necessary gain of acceleration control loop and necessary bandwidth of acceleration sensing has not been studied and no practical guidelines for the design of acceleration control loop were given. In this paper, the authors present the guidelines to design the acceleration control loop and analyze stability and damping of the acceleration control loop realized in discrete time. Conditions for the gain and sampling time to achieve stable and non-vibrational response are given. It is shown that short sampling time is crucial to realize sufficient gain and damping of acceleration control loop. Influence of current control loop to the acceleration control loops response is also studied. It is shown that difference in the bandwidths of current control and acceleration sensing is indispensable for nonvibrational response.

A System Design Concept Based on Omni-Directional Mobility, Safety and Modularity for an Autonomous Mobile Soccer Robot

In this paper, we describe the concept, design and implementation of a series of autonomous mobile soccer robots, named Musashi robots, which are designed referring ISO safety standards and have mechatronics modular architecture. The robots are designed to participate in the RoboCup Middle Size League. Using a modular design philosophy, we show that the selection of a proper moving mechanism, a suitable vision system and a mechatronics modular architecture design can lead to the realization of a reliable, simple, and low cost robot when compared with most car-like robots that include many kinds of sensors and have a complex design structure.

Performance evaluation of twisted strings driven robotic finger

Actuation and control of robotic hands is one of the problems in engineering that still needs to be satisfactorily solved. In this paper we present a robotic finger actuated by twisted strings mechanism that we call Twist Drive. A principle of the proposed actuation is described and its basic characteristics are presented. The structure of a developed robotic finger with the proposed actuation method is explained. Forward kinematics of a finger with non-linearly coupled joints was solved and the obtained Jacobian was used for two directional force control on the fingers tip. Experimental results of the force control and measured frequency characteristics of the force control are presented in the paper. The results show that the developed robotic finger has capabilities similar to that of a human finger. Additionally, because there is no gear reducer used in the transmission, the finger is of lightweight, low-cost, and provides zero mechanical impedance when the strings are untwisted.

Multi-fingered robotic hand employing strings transmission named “Twist Drive” — Video contribution

A goal of our research is to produce a light-weight, low-cost five fingered robotic hand that has similar degrees of freedom as a human hand. The joints in the fingers of the developed robotic hand are powered by a newly proposed strings transmission named “Twist Drive”. The transmission converts torque into a pulling force by using a pair of strings that twist on each other. The basic characteristics of the transmission are given in the paper. A robotic hand prototype with 18 joints of which 14 are independently powered by Twist Drives was produced. The size of the hand is equal to the size of an adult humans hand and its weight including the power circuits is approximately 800 grams. The mechanical and the control systems of the hand are presented in the paper.

Power and propulsion systems design for an autonomous omni-directional mobile robot

In this paper power and propulsion systems design for an autonomous omni-directional mobile robot which is developed for RoboCup compaction, is presented. For robot energy storage unit, lithium ion polymer battery is chosen because of its high power and energy densities, and a protection circuit against deep battery discharge was designed for it. Because the power flow between energy storage units and motors and also speeds of motors have to be controlled at the same time, we proposed a two cascaded cell modules consisting motor speed control and power flow control modules. The other parts of our robot power system are dc-dc converters and kicker circuit. The simulation and experimental results show proposed scheme is valid and energy management and speed control can be achieved properly using this method. The filed experiments show robot mobility functions to perform the requested motion is enough and it has a high maneuverability in the field.