1、Decentralized, Modular Real-Time Controlfor Machining Applications.AbstractSeveral different architectures for a control system for a reconfigurable machining system are considered, in terms of the inherent delays and their effects on the system performance. It is shown how time delays associated wi
2、th distributed architectures impact the performance of the control algorithms, and how different types of communication protocols could be implemented to meet the required deadlines. A two-axis contouring system is considered in some detail, and the effects of various delays on the contour error are
3、 determined by simulation. These analytic and simulation results can be used to specify the maximum allowable communication delays in the system. Control requirements for a manufacturing control system can thus be mapped to temporal constraints on the data managed for the environment.1. Introduction
4、The Engineering Research Center for Reconfigurable Machining Systems at the University of Michigan is developing the necessary theory and technology to enable the next generation of machining systems to be quickly and easily reconfigured in response to changing market demands and new technology inno
5、vation. In order to realize the vision of reconfigurable manufacturing systems, the machine tool hardware, as well as the software which controls it, must be constructed in a modular fashion. We envision that each hardware module, be it a single spindle, linear or rotary axis, or a multi-axis groupi
6、ng, will have its own sensors as well as control hardware and software modules. When a set of modules are grouped together to forma machine, the control task may demand not only certain requirements for individual axes, but also have constraints on the coordination of interacting axes (as in a conto
7、uring application). Thus, the axis-level control modules, running on distributed processors with communication over a network, must be coordinated in an appropriate way to ensure that the desired task is completed with the highest possible speed and accuracy. This coordination gives rise to stringen
8、t constraints on both the control execution as well as the data communication between control modules.In order to design and build modular controllers for modular machine tools, the issue of modularity itself must first be examined. It must be understood how different control algorithms should modul
9、arized (what is the appropriate granularity) and how they should be combined. In this paper, we assume that the modules are defined according to their purposes: position servo, cross-coupling, process control, etc. We consider the issues of how the control algorithm modules should be allocated among
10、 the many processors and how the communication between processors should be scheduled; we also discuss the performance tradeoffs associated with the different choices.The accuracy of a machine tool is often described in terms of the axis errors and the contour errors (i.e., various deviations from t
11、he precisely required trajectories). While in a machining system there are many sources of errors, in this work we focus on the errors associated with realtime, distributed communication and computation. We consider different controller architectures and the impact of these types of errors on the pe
12、rformance of the data and coordination of control system. Therefore, we assume that there are no errors associated with imperfect modeling of the axis motions, and no disturbances such as electrical noise, thermal deformation, or sensor inaccuracies associated with the machine. The interactions betw
13、een the control system and the manufacturing environment, involving large quantities of realtime data, need to be managed correctly and efficiently. The states of the machine tools, the sensor readings,actuator signals, and control variables, together represent the data to be managed. The various ma
14、nufacturing activities are coordinated in real-time, and in turn, they impose certain consistency and temporal constraints on the managed data. Yet, it is also necessary to allow autonomous executions in the distributed environment to allow for modularity and reconfigurability. There are currently s
15、everal related real-time control and coordination efforts at varying stages of development and deployment 1, 2, 3, 4, 11; an overview can be found in 15. Our research on the software for distributed coordination protocols has identi.ed the coordination constraints in manufacturing systems that need
16、support from a computing system, see 13, 14.2. Machine tool control structureFollowing Koren et al. 8, in this paper we consider three basic types of control modules for a multi-axis machine tool controller: servo, interpolation, and process. First we consider the axis-level controllers, also called servo controllers, which track reference inputs. Servo controller operates in discrete-time with a sampling time of Ts. An interpolator coordinate
