EuroSun2008-2

The Virtual Prototype of the Tracking System

The literature presents some constructive solutions of tracking mechanisms [6-10], but a general approaching for the conceptual design and the structural synthesis of these mechanisms is missing. Thus rises the necessity of a unitary modelling method of mechanisms, and in our opinion this method is based on the Multi Body Systems (MBS) theory, which may facilitates the self- formulating algorithms, having as main goal the reducing of the processing time. According with the MBS theory, a mechanical system is defined as a collection of bodies with large translational and rotational motions, linked by simple or composite joints. In this way, the structural design of the tracking systems consists in the following stages [11]: identifying all possible graphs, taking into account the space motion of the system, the type ofjoints, the number of bodies, and the degree of mobility of the multibody system; selecting the graphs that are admitting supplementary conditions imposed by the specific utilization field; transforming the selected graphs into mechanisms by mentioning the fixed body and the function of the other bodies, identifying the distinct graphs versions based on the preceding particularizations, transforming these graphs versions into mechanisms by mentioning the types of geometric constraints.

In the structural synthesis, there can be taken considered general criteria, for example the degree of mobility of the mechanism (M=1 for the single-axis trackers, and M=2 for the dual­axis trackers), the number of bodies, and the motion space (S=3 in the planar space, and S=6 in the general spatial case), as well as specific criteria, for example the type of the joint between the base and the input/output body. In this way, the structural synthesis method was applied and a collection of possible structural schemes were obtained.

The solution for system used in the study was selected from the multitude of the structural solution by using of the Multi Criteria Analysis. The evaluation criteria of the solutions were referring to the tracking precision, the amplitude of the motion, the manufacturing and implementation. The solution corresponds to an equatorial (polar) tracking system, at which the daily motion is directly driven by a rotary motor (fig. 1). The solar collector is rotated relative to a support on which the motor is disposed. The support can be rotated relative to the sustaining frame for the seasonal tilt angle adjustment, but the seasonal motion is not considered in paper.

For blocking the system in the stationary positions between actuatings, when the motor is stopped, the model includes an irreversible transmission, and in this way there is no energy consumption in these positions. The solid model of the tracking system was realized using the CAD environment CATIA. The geometries of the parts were transferred to the MBS environment ADAMS using the STEP file format (via ADAMS/Exchange Interface). The virtual model of the tracking system takes into consideration the mass forces, the reaction in joints, and the joint frictions, which are modelled by the coefficient of dynamic friction, the friction arm, the bending reaction arm, the radius of the pin, the stiction transition velocity, the maximum stiction deformation, and the preload friction torque.

For simulating the real behaviour of the tracking system, we developed the control system in the concurrent engineering concept, using ADAMS/Controls and EASY5. For connecting the mechanical model and the control system, the input & output parameters have been defined. The control torque represents the input parameter in the mechanical model. The output transmitted to the controller is the daily angle of the solar collector (in fact, the angular position of the rotor). For the input state variable, the run-time function is 0.0 during each step of the simulation, because the control torque will get the value from the control system. The run-time function for the input variable is defined using a specific ADAMS function that returns the value of the given variable: VARVAL(control_torque).For the output state variable, the run-time function returns the angle about the revolution axis: daily_ angle - AZ(collector. MAR_1, support. MAR_2), which returns the rotational displacement of one coordinate system marker attached to collector about the Z-axis of another marker attached to support.

The next step is facilitating the exporting of the ADAMS plant files for the control application. The input and output information are saved in a specific file for EASY5 (*.inf); the export also generates a command file (*.cmd) and a dataset file (*.adm) that are used during simulation. With these files, the control system diagram was created in EASY5 (fig. 2). The input signal block represents the database with the daily angles of the solar panel (i. e. the imposed motion law); this subject is described in the next section of the paper (optimizing the mechatronic tracking system from the motion/control law point of view).

From the controller point of view, for obtaining reduced transitory period and small errors, we used a general PID controller. The specific parameters of the controller have been established having in view the following conditions: the increasing of the proportional term generates the decreasing of the transitory period from the dynamic response of the system, and of the position error, respectively; the integral term generates a class of dynamic responses and attenuates the error history; the derivative term generates a class of dynamic responses and amortizes the error; the system is considered with a critical amortization. In the mechatronic model, ADAMS accepts the control torque from EASY5 and integrates the mechanical model in response to them. At the same time, ADAMS provides the current daily angle for EASY5 to integrate the control system model.