S.A.T.U.R.N.Simulation and Analysis Tools for Urban Automated Rapid Transit Networks.

What is Automated Rapid Transit (ART)?

• ART refers to the broad class of transportation systems making extensive use of automation to move people or goods.

• In this project, we deal with a subset of ART:– Personal Rapid Transit (PRT).

• PRT is a relatively new and experimental concept for public transportation.

Small Modules

• Advantages:– Privacy – Personal Transit– Riders travel alone, or grouped by

choice (family or friends, for example).– Fast Acceleration – riders are seated.– Light-weight – 250-350kg loads

Use Dedicated Infrastructure

• PRT Infrastructure:– Slim, light guide-ways – made possible by light vehicles.– Grade separated, to prevent competition with other

modes of transportation.– Narrow rights of way, to facilitate integration into the

urban environment.– Network of unidirectional loops provides maximum

coverage of the network area.– Bi-directional guide-ways are wider and heavier, which

makes them more intrusive. Station design is also significantly complicated by two-way traffic.

– Tunnels are also a possibility, though less popular because of higher implementation costs.

Off-Line Stations

• First, an on-line station:– A typical LRT station is shown.– It is on the main line.– Only one train can pass.

• So, an off-line station:– A PRT station is not on

the main line.– Therefore, multiple vehicles

can pass the station.

… …

Station

Station

Main Line

Main Line

Vehicles

Train

Use Off-Line StationsOff-Line Station Design

• “SkyTran” puts the main line above the station line. Other designs usually put the station beside the main line.

• Incoming modules switch off of the main line and slow down on a deceleration ramp.

• Then, they accelerate back to nearly full speed on an acceleration ramp, before merging back onto the main line.

• Because modules do not stop on the main line, the average main-line speed is higher.

Use Off-Line StationsUsing Off-Line Stations to Improve Performance

• Higher main-line speeds are achieved when modules do not stop on the main line.

• In an on-line system, the main-line speed drops as the distance between stations decreases (the vehicle must accelerate and decelerate from / to zero, at each station).

• As stations get further apart, passengers must travel further to access them. This impacts the quality of service.

• Because of their higher main-line speeds, off-line systems can put stations closer together, making them more convenient for passengers.

Provide Non-Stop Trips

• Non-stop trip: A rider is taken directly from the source station to the destination station.

• Contrast with a bus service:– Everyone stops at every stop, even if only one or two

people get off the bus. The same goes for a train.– Also, one commonly has to transfer between buses or

trains to reach a destination, which is inconvenient.

• PRT is more like a taxi service: trips are direct.• Non-stop trips are made possible by off-line

stations, because only modules stopping at a given station actually have to stop at the station.

Be Demand-Responsive

• Contrast: Demand-Static System– Vehicles run on fixed schedules.– Passengers are responsible for meeting vehicles.

• So, in a Demand-Responsive System, vehicles respond to trips as requested by passengers.

• Again, this is more like a taxi service than what we think of as public transportation.

• Off-line stations and small modules make demand-responsive systems possible.

• Demand-responsive systems place heavier loads on control systems.

Operate using Computers

• Control Paradigms: Degrees of Centralization– Asynchronous: anarchy– Synchronous: fully centralized authority– Quasi-Synchronous: somewhere between the two

• Quasi-Synchronous control is most popular.– Retains a central authority, but modules have on-board

computer systems that enable them to perform some tasks.

– We end up with a “constitution” that determines what computers do what tasks.

Operate using ComputersQuasi-Synchronous Control

• Power-Sharing Agreement in SATURN

• Communication distances are limited – e.g. modules communicate mainly with zone controllers near them.• Modules have sensors (radar / IR, etc.) that help it remain safe from unexpected obstacles.

Central Computer Zone Controllers Module Controllers

Trip Requests

Network Health

Statistics

Propulsion System

Safety – Checks

(Receives input from onboard sensor instruments.)

Module Interaction

(Receives input from sensory equipment in nearby guide-ways.)

PRT’s Main GoalCompete with the Personal Automobile

• Small modules give privacy.• Off-line stations allow for high line speeds (resulting

in lower trip times) and closely spaced stations, resulting in higher accessibility.

• Non-stop, direct trips make travelling easier and reduce waiting times.

• PRT’s demand-responsive nature also reduces waiting times.

• Automation allows for high line density (modules can be quite close to one another without jeopardizing safety).

SATURN: An experiment in the deterministic simulation of a SMART system.

• General facts about SATURN:– Written in C++.– ~ 23 000 lines of source code (including comments).– Fully object oriented design promotes abstraction and

code re-use to accurately simulate objects in the system.– Program core is isolated from Windows GUI to facilitate

portability.

• SATURN is not the end-all of PRT simulators, but it provides a good basis for demonstrating the technology and techniques for PRT control systems and simulators.

SATURN: System Overview: Block Diagram

Infrastructure User Demand Simulation

Programmed Modules

Simulator

Junctions Sections

Stations

Simulator Output

DemandPoints

Classes

Instances

System Display

Object Display

DemandCoefficients