Hermes Giberti

A Practical Approach to the Selection of the Motor-Reducer Unit in Electric Drive Systems

The selection of a motor-reducer unit in electrical servo-systems has a profound impact on the dynamic performance of a machine. This choice must be made considering all the limits imposed by each component of the system and all the operational constraints. This article proposes a useful and practical methodology for the correct sizing of a motor-reducer unit. The relationships between motor and transmission are investigated by introducing some easily calculated factors useful for comparing all the available motor-reducer couplings and selecting the best solution. The article suggests an innovative approach for the selection of a motor-reducer unit that involves solving the problem with the use of graphs that would allow showing all the possible alternatives.

A model predictive protection system for actuators placed in hostile environments

In most applications where motors control several degrees of freedom of a mechanical system there is the necessity to implement a protection which is able to stop the motors when the mechanical system reaches the extreme positions of its working space. When the mechanical system is located in an hostile environment and/or when the limit positions define a complex surface this simple protection strategy is not always viable, since its not possible to place sensors and limit switches in the proximity of the limit positions. We will present in this paper a solution for the actuator protection problem based on a model predictive algorithm capable to estimate in real time the actuator state variables (positions and speeds). The idea of the model predictive protection scheme is to implement a model of the mechanical system forced by the emergency braking actions in order to predict, using as initial condition the current observed state of the system, the final rest position together with its uncertainty. If the probability that the predicted rest position lays outside the physical boundaries of the working space exceeds a predefined threshold the protection trips. With this approach we will also reach the maximum exploitation of the systems working space, since it is adapted to the current system state. We will show the application of this concept to the protection of a set of steering microwave antennas placed in an hostile environment (hard vacuum condition, strong neutron and gamma radiation, strong magnetic fields) inside a nuclear fusion reactor.

On the functional design of the DTU10 MW wind turbine scale model of LIFES50+ project

This paper illustrates the mechatronic design of the wind tunnel scale model of the DTU 10MW reference wind turbine, for the LIFES50+ H2020 European project. This model was designed with the final goal of controlling the angle of attack of each blade by means of miniaturized servomotors, for implementing advanced individual pitch control (IPC) laws on a Floating Offshore Wind Turbine (FOWT) 1/75 scale model. Many design constraints were to be respected: among others, the rotor-nacelle overall mass due to aero-elastic scaling, the limited space of the nacelle, where to put three miniaturized servomotors and the main shaft one, with their own inverters/controllers, the slip rings for electrical rotary contacts, the highest stiffness as possible for the nacelle support and the blade-rotor connections, for ensuring the proper kinematic constraint, considering the first flapwise blade natural frequency, the performance of the servomotors to guarantee the wide frequency band due to frequency scale factors, etc. The design and technical solutions are herein presented and discussed, along with an overview of the building and verification process. Also a discussion about the goals achieved and constraints respected for the rigid wind turbine scale model (LIFES50+ deliverable D.3.1) and the further possible improvements for the IPC-aero-elastic scale model, which is being finalized at the time of this paper.

Drive System Sizing of a 6-DOF Parallel Robotic Platform

In this work, it is considered a 6-DoF robotic device intended to be applied for hardware-in-the-loop (HIL) motion simulation with wind tunnel models. The requirements have led to a 6-PUS parallel robot whose linkages consist of six closed-loop kinematic chains, connecting the fixed base to the mobile platform with the same sequence of joints: actuated Prism (P), Universal (U), and Spherical (S).As is common for parallel kinematic manipulators (PKMs), the actual performances of the robot depend greatly on its dimensions. Therefore, a kinematic synthesis has been performed and several Pareto-optimal solutions have been obtained through a multi-objective optimization of the machine geometric parameters, using a genetic algorithm.In this paper, the inverse dynamic analysis of the robot is presented. Then, the results are used for the mechanical sizing of the drive system, comparing belt- to screw-driven units and selecting the motor-reducer groups. Finally, the best compromise Pareto-optimal solution is definitely chosen.Copyright

A nonlinear controller for trajectory tracking of pneumatic cylinders

This work presents a nonlinear control strategy for pneumatic cylinders that uses two three way independent proportional valves to control the position of a pneumatic cylinder. The control algorithm is based on a sliding mode controller that allows efficient and robust trajectory tracking at any speed of the end-effector and a wide variance range for pay loads. The overall performance of the controller has been achieved with a minimum cost regarding sensors; in fact we only use one linear encoder feedback signal for closing the position loop. The performance of this control strategy has been tested on a experimental device.

Dynamics Modeling and Accuracy Evaluation of a 6-DoF Hexaslide Robot

Parallel Robots are commonly preferred in place of Serial Robots for tasks in which the working volume is limited, compared to the overall extension of the machine, but where the accuracy requirements are very high. In fact, since the errors do not sum together in series, the positive effect of the closed-loop linkages arrangement is a lower overall sensitivity of the end-effector positioning to the various sources of error. The precision depends on many factors among which the flexibility of the components plays a fundamental role. This paper investigates the contribution of belt-driven and screw-ball-driven linear transmission systems to the overall flexibility of the HexaSlide parallel architecture, using an ADAMSⓇmodel. The six Degrees-of-Freedom (DoF) of its mobile platform are provided through the actuation of the same number of Linear Transmission Units (LTUs), fixed to the base, while its links have fixed length. The LTUs are integrated together in the robot. The first eigenfrequencies are mapped in a systematic manner over the whole workspace, using a discretized grid of poses.

On Brushless Motors Continuous Duty Power Rate

The “continuous duty power rate” of a brushless motor is a coefficient that characterizes its ability to accelerate a load. For this reason it is also called “accelerating factor” and playes a considerable role in the choice of the motor to be used in an automatic machine. This work is focused on the analysis of this parameter, defined for each motor as the ratio between the square of the motor nominal torque and its momentum of inertia: it is exclusively defined by parameters related to the motor and, therefore, it does not depend on the machine task. The research is carried out through a database containing the main electromechanical features of some commercial motors and their “accelerating factors”. Using the collected information, related to more than 300 brushless motors, some graphs are produced showing how motors having the same size can have different accelerating factors or, conversely, how same accelerating factors can be obtained through different electro-mechanical design.Copyright

A genetic algorithm approach to the kinematic synthesis of a 6-DoF parallel manipulator

The main applications where parallel robots are used remain until today very limited. The best known examples include flight simulators, large vibrating tables for seismic tests, pointing systems of antennas and telescopes mirrors. Common to all these robotic devices are: (1) high accuracy, (2) high velocities and accelerations, and (3) high loading capabilities: the robot itself becomes the supporting structure. This paper presents the kinetostatic synthesis for the realization of a six degrees of freedom robotic platform with Hexaglide parallel architecture. The kinetostatic synthesis was achieved through a multi-objective optimization of the geometrical parameters with a genetic algorithm. The objectives to be achieved are concerned with: (1) the coverage of the desired workspace, (2) the static forces multiplication, (3) the longitudinal size, (4) the link-to-link interference, and (5) the link-to-rail interference.

Error Analysis and Adaptive-Robust Control of a 6-DoF Parallel Robot with Ball-Screw Drive Actuators

Parallel kinematic machines PKMs are commonly used for tasks that require high precision and stiffness. In this sense, the rigidity of the drive system of the robot, which is composed of actuators and transmissions, plays a fundamental role. In this paper, ball-screw drive actuators are considered and a 6-degree of freedom DoF parallel robot with prismatic actuated joints is used as application case. A mathematical model of the ball-screw drive is proposed considering the most influencing sources of nonlinearity: sliding-dependent flexibility, backlash, and friction. Using this model, the most critical poses of the robot with respect to the kinematic mapping of the error from the joint- to the task-space are systematically investigated to obtain the workspace positional and rotational resolution, apart from control issues. Finally, a nonlinear adaptive-robust control algorithm for trajectory tracking, based on the minimization of the tracking error, is described and simulated.

A non-contact optical technique for vehicle tracking along bounded trajectories

This paper presents a method for measuring the non-controlled trajectory of a cart along a bounded rectilinear path. The method uses non-contact measurement devices to identify the position of a movable laser scanner working in helical mode in order to reconstruct the 3D model of bridges. The main idea of the proposed method is to use vision systems in order to identify the coordinates of the laser scanner placed on the cart with respect to the global reference system. A fit-to-purpose vision system has been implemented: the system uses three CCDs cameras mounted on the cart to identify the relative rotations with respect to the environment. Two lasers pointers and a laser distance meter are fixed at the starting point of the trajectory and pointing in the direction of motion of the cart, creating three dots on a plane placed on the cart. One of the camera detects the cart displacements and rotations in the plane using a blob analysis procedure. The method described in this paper has a constant uncertainty and the measurement range only depends on the lasers power. The theoretical accuracy of the measurement system is close to 1 mm for the translation along the motion direction and around 0.5 mm along the other two directions. Orientations measurement have a theoretical accuracy of less than 0.1 °. The solution has been implemented for the 3D reconstruction of concrete bridge; preliminary experimental results are presented and discussed.