DISTRIBUTED CONTROL OF AN XY BOARD VIA ETHERNET Lilantha Samaranayake, Mats Leksell Chammika Mannakkara* Department of Electrical Engineering Department of Microelectronics and Information Technology* Royal Institute of Technology (KTH) Stockholm, Sweden email: lilantha@ekc.kth.se Sanath Alahakoon Department of Electrical and Electronic Engineering University of Peradeniya Sri Lanka email: sanath@ee.pdn.ac.lk ABSTRACT With the recent IPv6, Power over Ethernet (PoE) and ro- bust hardware designs, Switched Ethernet technology is currently reaching the status of a complete fieldbus system for harsh factory floor applications. However, the investors still hesitate on its field level operation for time critical ap- plications, due to the non-deterministic delays in the closed world of a switched network. The objective of this paper is to practically prove an adaptive sampling scheme to handle network delays and to highlight the potential of Switched Ethernet in implementing time critical real-time distributed control system applications. Experiments are carried out on a distributed XY board, in which the speed and position control loops on each axis, are closed through a 100Mbps Switched Ethernet network. KEY WORDS Adaptive sampling, Distributed XY board, Switched Eth- ernet. 1 Introduction In operating a system in the distributed control mode, the control delay plays the most important role. Because whether it is constant or time varying, a delay from output measurement to control signal input, i.e., control delay (D), contributes to a phase lag [1]. This phase lag reduces the phase margin of the system and therefore can result in a deteriorated or even an unstable output. Considering vari- ous negative points in the design and especially in the im- plementation of standard delay compensation techniques as described in the survey paper [2], the authors have derived a predictor based delay compensation technique in [3]. How- ever, to realize this technique the condition D<h, where h is the sampling period has to be satisfied. If not, with any delay compensation technique, the plant will effectively be updated at D and not at h. This has theoretically been verified using a graphical approach in [4] by the same au- thors and have proposed an adaptive sampling scheme to change the sampling rates at the sensors to ensure D<h. This paper is aimed to practically verify that result using an XY board which is distributed real-time controlled via a Switched Ethernet network. The paper is arranged as follows: Section II is on the hardware details of the sensors, actuators, motor drives and their Ethernet connectivity. System identification and controller design is described in section III. Section IV de- scribes the adaptive sampling scheme used to adapt the sys- tem according to the changes in the network. Software im- plementation of the server comprising the controller, ref- erence signal generator, round trip time estimator and the adaptive sampling scheme are given in Section V. Section VI presents the results of the practical experiments and Sec- tion VII concludes the paper with some proposed future work. 2 The XY Board The XY board consists of two BrushLessDC (BLDC) mo- tor and their inverter fed servo amplifier and gate drive cir- cuits, linear actuators on X and Y axes, Ethernet Ready Sensor Actuator Modules [6], 100Mbps Switched Ethernet network [12] and the Ethernet Switch and the control com- puter, where the control algorithm is executed. This section describes the construction details and operational aspects of the XY board. 2.1 BLDC drive motors Each axis of the XY board is driven by a 0.4kW BLDC motor fed by an inverter. The block diagram is shown in Fig. 1. The outer speed/position control loop can be closed externally or internally within the servo amplifier before coupling to the inner current controller as shown in the fig- ure. In the XY board, its closed through the Ethernet net- work. 2.2 Actuators The rotating motion of the BLDC motor is converted to lin- ear motion by the linear guide actuators in each direction. The one-piece structure of the guide consists of U-shaped outer rails on both sides of the inner block and the ball screw of 6mm pitch in the center of the inner block. This construction ensures high rigidity and accuracy, at minimal