The Impact of Control Delay upon the Performance of a DC-Motor Control: Comparison of a Centralized vs. a Network-Based Approach
Abstract
An important aspect when dealing with computer control systems is the impact of control delay upon the performance of the controlled item. Such control delay may be a consequence of the computer controlled implementation or, for the case of a network based control system, can be the consequence of the use of a communication network to interconnect sensors and actuators with the controller. Basically, control applications require a predictable delay between measurement and actuation, with small variability, i.e., small jitter. In this paper we present some simulation results concerning the impact of a varying control delay upon the performance of a DC-Motor, both for a centralized computercontrolled setup and for a network based control system. The results for the centralized setup were also experimentally confirmed. For the network-based setup it was used the h-BEB real-time communication protocol. The evaluated scenarios were simulated using the TrueTime simulator that is based in MATLAB/Simulink. The experimental tests were developed in LabVIEW, where the controller task was executed as a software routine. The tests consisted in the variation of the sampling period and control delay to determinate the steady region of the system.
Keywords-computer control system, network-based control system real-time, control delay and DC-motor
I. INTRODUCTION
Computer control systems are widely used in applications as automotive, aerospace, automation and others. The design of a discrete controller can be done following two different approaches: a) design of the controller in the time domain and then its discretization; b) total discretization of the system and then its design in sampled domain. For a real implementation of a controller project, both the sampling period (hk) and the delay/jitter between measurement, controller and actuation, usually referred as control delay (τk), can have a direct impact upon the system performance. Whenever a computer control system is implemented, the control delay may vary significantly. Two different approaches that yield opposite results in what concerns the control delay are the centralized and the network-based implementation approaches. * Work partially supported by UnilesteMG, and FAPEMIG Brazil A Networked Control System (NCS) is a feedback control system, where the page link between sensors, controller and actuators is closed through a communication network. As the control-related messages are exchanged through a communication network, extra timing delays are introduced in the control loop. The control performance may be degraded by such communication-related delays, which can compromise the performance of the application. Figure 1 shows an example of an NCS. The network must provide a predictable communication service, i.e., an upper bounded access delay with reduced jitter. More precisely, it is required an infrastructure able to support periodic message streams, in order to convey the control-related periodic data between the controller and the set of related sensors/actuators; guarantee upper-bounded response times for the message transfers, in order to cope with the control-related delays

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