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Abstract System Implementation Literature Review – Try Once Discard (TOD)

Simulation Results

In many networked control systems (NCSs) only one node is allowed to use the shared medium at any given time. This network constraint can adversely affect the performance of the system and its stability. There are two types of network schedulers, static and dynamic. Static schedulers have problems handling large-scale systems. On the other hand, dynamic schedulers, such as try-once discard (TOD) cannot guarantee good quality of service for each node, especially the low-priority ones. In this paper, a hybrid scheduler is proposed. We refer to this scheduler as the traffic-division arbitration (TDA) protocol. The network-induced delay error bound and the system stability of the NCS using the proposed scheduler are investigated. Simulations illustrate the performance of the proposed scheduler and difference from TOD are shown.

Why Networked Control Systems?

NCS: control system that closes its control loops through a network

One node can only use the shared medium at any given time

Affects the stability and performance of system Requires a scheduling protocol for NCS Various applications in Control Systems

Robotics, automation, machinery Air conditioning, cooling, elevators Medical equipment, remote surgery Automobile industry

Two kinds of scheduling protocols: static and dynamic Static: predetermined channel access Dynamic: channel access determined during network

access Maximum Error First-Try Once Discard (MEF-TOD)

protocol Arbitrates between multiple nodes Does not give equal access to all nodes Message with largest error wins the medium access Error due to the network-induced delay Packet discarded if it fails to win the competition One node may hog the network for long period of

time

Design Objectives – Motivation

Objective: analyze the effect of increasing traffic Each subsystem is an armature-controlled DC motor State-feedback and PID controllers to stabilize the

system and eliminate transient response

Traffic Division Arbitration (TDA) Protocol

Design Objectives:

Protocol that provides more even access for all nodes Addresses network fairness issue Maintains closed-loop system stability Targets different types of systems

Protocol Description:

TDA has two arbitration levels; dynamic and static Dynamic portion: network traffic divided into

transmission cycles Threshold determines which messages passes to second

level Static portion: messages from the first level transmitted

according to a pre-determined error priority Error threshold depends on the system nature

TOD and TDA Operation Comparison

Stability Analysis

Error Decision Functions

Scheduling protocols work properly in low traffic situations

Network-induced time delay increases as network load increases

Low priority messages will be delayed more than once Main objective: a hybrid static-dynamic scheduling

protocol that improves QoS

_________________________________Walsh et al, Stability Analysis of Networked Control Systems.

General NCS Setup

Overall State:

Network-induced error state:

Combined NCS State:

Without a network, Dynamics reduced to:

Stability Analysis

Number of nodes: k Maximum error growth: β Networked induced error bounded by:

TDA Error Bound

First decision function computes relative error rate:

computes the rate of convergence as the error gets small Denominator included to normalize tracking error that

comes from different types of subsystems

Second decision function: One step predictor of the tracking error Approximates future tracking error

Step response comparison using first decision function

Step response comparison using second decision function