Trolley vehicle assisted guidance
Although entitled historically as "trackless", trolleybus technology essentially provides overhead wiring that might be considered as a track to provide position indication that can be used to achieve accurate vehicle docking.
Why?
In providing a trolley vehicle concept that can convincingly compete with light rail systems, the advantages of guidance need to be included. To provide the ability to draw up within 50mm of a raised kerb, to enable passenger convenience, and to provide minimal infrastructure costs on segregated bus-ways, for maximum service speed, guidance is essential, as driver steering ability can not be sufficiently accurate.
What?
Using standard, industrial components, widely in use, it is proposed that the relationship between the overhead line and the vehicle position be automated. This relationship is based on the distance between the overhead and the raised kerb edge. This can be fixed uniformly as a specific distance found at every stop on the trolley vehicle network or 'understood' by reference to a 'bus docking model' database carried on board. With optical absolute angle encoders feeding information to a PC-based embedded computer, using real-time software that controls stepper motor driven assisted steering, the system would be low cost, rugged and effective. All equipment would be vehicle based. No additional infrastructure, such as specially designed kerbs, manual steering tracks, underground inductive loops or optical indicators, would be required.
How?
As the vehicle approaches a stop, the booms trigger the assisted guidance system by passing a micro-switch installed on the overhead, fixed to interact with the trolleyhead. The absolute angle encoder under the trolley base immediately relays the boom angle relative to the direction of the overhead to the embedded computer. This angle is compared in software with a series of desired angles, which represent the "S" curve that takes the trolley vehicle into the raised kerb, after first aligning the vehicle to correct any deviation at the point of engagement. The computed deflection angle is transmitted to the stepper motor that drives the steering to turn the front wheels to equal this deflection angle. This results in a change of boom angle that is expected to increase. A feedback loop is established that continues until the vehicle comes to a halt. The system self tunes itself via an encoder that measures the actual deflection angle. If stop sections include differing configurations that are not parallel to the kerb, an additional model reference control can be imposed on the system that understands the specific attributes of each stop on a route, either by being selected as sequence at the start of service or by a coded trip on the overhead at each stop section that transmit an identity to the software.
Safety
The system triggering, by physical switching installed on the overhead line, can be overridden at any time by the driver. By using highly accurate, sealed angular encoders that read absolute degrees, rather than incremental degrees that need counting, not only obviates any initiation computation but also safeguards against power breaks by instantly knowing the current angle relative to the 'straight ahead' datum point. Alarms that discontinue the system are built into the encoder electronics.
Refinements
Because of the normally flexible nature of overhead, that which is used for the assisted guidance system would need to be rigid, otherwise the accuracy of docking would not be sufficiently achieved. This would need to be installed for a distance before triggering, to eliminate boom "rattle". Stops with curves, if unavoidable, would need the addition of fuzzy logic within the software to dampen the effects of the overhead having a series of straight segments that could result in uncomfortable jerks while trying to "follow" the wiring. To be able to re-engage the system, for example after having to manually avoid an obstruction, the over-ridding of the docking operation by the driver would effect the stepper motor output, not the monitoring and selection of actual and desired angles. This would enable the re-engagement of the guidance operation if the stop can still be drawn up to. The system is not dependent on an accurate alignment in distance along the road. There is, however an assumption in the algorithm that the docking operation is normally achieved within a certain minimum distance, by reference to the clock synchronisation of the encoder.
A.Bruce
The Electric Tbus Group. 1st Draft. 25/10/00