GB2211334A - Transportation system having an inter-vehicle distance control arrangement - Google Patents

Abstract

Autonomous vehicles, of e.g. a post-office railway, control their speed to maintain the inter-vehicle gaps at a minimum substantially equal to the braking distance for any speed up to the system limit. Each vehicle may carry optical devices for monitoring the gap in front and for receiving speed information from the vehicle in front. The optimum gap gOPT at each speed may be determined from look-up tables, while the actual gap may be found using an up/down counter comparing the two speeds. <IMAGE>

Description

TRANSPORTATION SYSTEM HAVING A PLURALITY OF VEHICLES AND AN INTER-VEHICLE DISTANCE CONTROL ARRANGEMENT The present invention relates to a transportation system having a plurality of vehicles and an inter-vehicle distance control arrangement. The invention may be used in a multi-vehicle packet sorting system in a Post Office environment.

The system requires multiplicity of autonomous vehicles, which share a common track, to route objects from loading stations to individual unloading destinations. The number of destinations can be very high, in the order of many thousands. Each vehicle may transport a single or several objects to a specific destination, and returns to the loading station, under total self control.

The system requires a specified high throughput rate, minimal floor space, and lowest cost (hence minimal number of vehicles). In order to achieve the smallest system size, with so many destinations it is necessary to introduce branches into the looped track configuration. However, the use of branches introduces problems into the system because the use of branches implies merging and diverging junctions in the tracking system, both of which, necessarily cause a slow down or stop of individual vehicles. Similarly loading stations, unloading at destinations and bends in the track also slow down vehicles. Hence the average rate of vehicles passing any (main loop) point, "P", Figure 1 in the tracking system must diminish.Thus, in order to maintain the required throughput, either more vehicles must be utilised or higher acceleration, deceleration and maximum velocity, must be engendered in the vehicles. The cost increases drastically with increasing vehicle specification requirements, and track space requirement increases if more vehicles are used.

In a system where individual vehicles must change veleocity, some form of queue or platoon will naturally occur. The individual controlling features of vehicles will determine how well this is done, but the most important requirement is that no vehicles can collide.

Each vehicle must, then, incorporate a sensing means to detect a leading vehicle, and decelerate when it senses one, as shown in Figure 2. If the detection distance is equivalent to the vehicle braking distance, gb, from its maximum velocity (Vmax) to zero, then a vehicle will stop directly behind another stationary vehicle, with no gap, as shown in Figure 3. Queues of many vehicles can build up at junctions etc. Considering now the start-up sequence of the queue, each vehicle must wait until its leader is out of range before it can start. It then has to accelerate to Vmax before its follower can start (assuming acceleration rate = deceleration rate). Vehicles starting from a zero velocity queue will form a platoon at Vmax, where intervehicle gap is twice the braking distance, as shown in Figure 4.

The problem with this type of vehicle control is to cause system throughput to decrease drastically. Also the dynamic response of platoons is underdamped.

The present invention provides the advantages of minimal platoon length and critically damped platoon dynamics.

Accordingly, an aim of the present invention is to provide a transportation system having inter-vehicle distance control arrangements which does not suffer from the above mentioned problems.

According to the present invention there is provided a transportation system having a plurality of vehicles and an inter vehicle distance control arrangement, said arrangement including means for maintaining inter-vehicle gaps at a distance substantially equal to the actual braking distance of each vehicle, at any speed, up to the system's limiting velocity.

An embodiment of the present invention will now be described with reference to the accompanying drawings wherein: Figure 1 shows a plurality of vehicles travelling along a track and passing a main loop point P, Figure 2 shows each vehicle having a detecting means for sensing the leading vehicle, Figure 3 shows a pair of stationary vehicles with zero inter vehicle gap, Figure 4 shows vehicles starting from zero velocity forming a platoon where the inter-vehicle gap is twice the braking distance, Figure 5 shows vehicles having a reduced inter-vehicle gap when the leading vehicle is forced to slow down, Figure 6 shows each vehicle equipped with a retro reflector and a retro transmitter/sensor, Figure 7 shows each vehicle equipped with a back light projector and a forward mounted sensor, Figure 8 shows each vehicle equipped with alternative measuring means such as an intrinsic distance measuring means, Figure 9 shows a minimised platoon length obtained by vehicles equipped with the alternative measuring means, Figure 10 shows the circuitry necessary for providing a vehicle control loop function, and, Figure 11 shows the circuit means for deriving in instantaneous gap g'.

The following description should be read with reference to Figures 5-11 of the accompanying drawings.

It is an essential feature of the transporation system that the instantaneous inter-vehicle gap is optimised in conformation with system parametrical requirements.

The parametrical requirements are as follows: 1. Vehicle Safetv In the event of an instantaneous stop of a leading vehicle, the inter-vehicle gap should allow safe braking of the following vehicle prior to actual collisions. Optimally, no violent collisions should be propagated through the following vehicles.

2. Oueue Formation/Deformation In the event of stop/go behaviour in a leading vehicle, signalled to the leading vehicle by external means, (eg Traffic lights at merging junctions), queue formation/deformation should be optimal.

In the event of a STOP being imposed on a leading vehicle, following vehicles should close the inter-vehicle gaps to zero in a controlled fashion. In the event of a subsequent START being imposed on the leading vehicle, following vehicles should start and assume the minimum inter-vehicle gap consistent with 1 above and at no time should the inter-vehicle gap exceed the safety requirement, or else inefficiency in queue length will occur.

3. Queue Slow Down (Mean Velocitv) In the event of a SLOW DOWN of a leading vehicle for any cause, eg under-specified velocity or merging junctions etc, following vehicles should close up inter-vehicle gaps to a distance (gout) compatible with safety as shown in Figure 5.

These optimising conditions are best met by maintaining an inter-vehicle gap consistant with the safety requirement at all times and at all velocities.

The optimal inter-vehicle gap can be related to the instantaneous vehicle velocity v, and the available safe retardation "a", by: Inter-vehicle gap, gaps (v - Vc)2 - (A) 2a Where Vc = safe collision velocity.

Thus if a following vehicle is instantaneously aware of its velocity v2 and the inter-vehicle gap g', the optimal spacing, can can be maintained by acceleration or retardation of v2 according to an error signal e, where: e = g' = g' - (v2 - Vc)2 2a ie. when e = 0 g' = gout and v2 = vl, as shown in Figure 5.

The function of the system can therefore be optimised if the autonomous vehicle can sense its distance from a leading vehicle, and this is achieved by the following means: The vehicle is equipped with a retro reflective optical sensor to quantise gap sensing between gopt (max) and zero.

To this end a retro reflector, 1, is placed on the rear of the leading vehicle, and the following vehicle is equipped with a retro transmitter/sensor, 2, which projects a beam of light, 3, at angle , as shown in Figure 6. When the projected beam is incident upon a retro reflective surface the sensor yields a response. A sufficient length of suitably disposed retro reflective material can yield a signal, Lg = 1, from the sensor for the condition: < g < gopt (max) where gopt (max) is the inter-vehicle gap required for safe operation at Vmax, determined by available vehicle retardation "a".

The leading vehicle is equipped with a back light projector, 4, the back light being modulated in intensity according to the vehicle velocity as shown in Figure 7.

The following vehicle is equipped with a forward mounted sensor means, 5, which views the back light projector of the leading vehicle for all values of: O < g < gopt (max) The following information is now available to the following vehicle.

Its own velocity = v2, the velocity of the leading vehicle, vl and the gating signal, Lg, indicating that the leading vehicle is spaced away by g', where 0 < g < gopt (max) Thus by simple, dead reckoning, interpolative means a signal, s, corresponding to the instantaneous gap g' can be derived. The signal, S, is set to gopt (max) (ie when Lg = 0).

For improved accuracy an additional sensor may be used to generate a signal, Lo, obtained by sensing direct vehicle contact, (ie zero gap) to reset s to zero when g' = 0.

Alternatively, an intrinsic distance measuring means, 6, may be incorporated into the following vehicle sensor scheme, the back light velocity indicating signal, and the specific distance sensor means could be dispensed with, and performance characteristics improved in specific situations, as shown in Figure 8.

The overall characteristic of platoon formation, whether using the specific distance and velocity measurement means as shown in Figure 7 or an intrinsic distance measuring means as shown in Figure 8 is to minimise platoon length and provide smooth, well controlled acceleration/deceleration characteristics, as shown in Figure 9 when v2 to vn = vl.

An example of the vehicle control loop function is shown in Figure 10, in respect of velocity v2. The output g', generated from the distance measuring means is applied to one input of a comparator A. The other input of the comparator A receives a signal indicative of the optimum inter-vehicle gap gOpt and is generated from a look-up table addressed by a signal representing the velocity v2. The output of the comparator N, Ae is used to control the velocity v2.

Figure 11 shows the means by which the instantaneous gap g' is derived. An up/down counter receives a signal indicative of velocity vl at its up input, and a signal indicative of velocity v2 at its down input. The counter receives a load input, Lg representing the signal generated from the retro reflective material 1, shown in Figure 6. The counter is reset by signal Lg which represents a zero gap between vehicles. The output of the counter g' represents the instantaneous gap.

The above description is not intended to limit the scope of the present invention. For example, alternative means for measuring distance may the used, other than those described, and such means will readily be appreciated by those skilled in the art.

Claims (9)

1. A transportation system having a plurality of vehicles and an inter-vehicle distance control arrangement, said arrangement including means for maintaining inter-vehicle gaps at a distance substantially equal to the actual braking distance of each vehicle, at any speed, up to the system's limiting velocity.

2. A transporation system as claimed in claim 1, wherein the inter-vehicle control arrangement comprises, for each vehicle, a rearward facing retro reflector and a forward facing retro transmitter/sensor arranged to project a beam of light at a predefined angle, and when the beam becomes incident upon the reflector of a leading vehicle the reflector generates a signal indicative of the distance between the vehicles.

3. A transportation system as claimed in claim 2, wherein the inter-vehicle control arrangement further comprises, for each vehicle, a back light projector arranged to generate a light beam, and a forward mounted sensor arranged to detect the light beam generated from the projector of a leading vehicle, said light beam being arranged to be modulated in accordance with the velocity of the vehicle.

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4. A transportation system as claimed in claim 1, wherein the inter-vehicle distance control arangement comprises a forward facing intrinsic distance measuring means for each vehicle and used to detect a leading vehicle and generate a signal in accordance with the detection of the leading vehicle.

5. A transportation system as claimed in claims 3, or 4 wherein the inter-vehicle distance control arrangement includes, for each vehicle, sensors arranged to detect direct contact between adjacent vehicles.

6. A transportation system as claimed in claim 5, wherein the inter-vehicle distance control arrangement includes means for controlling the velocity of each vehicle, said means comprises a comparator arranged to generate an output control signal in dependance upon a signal indicative of an optimum inter-vehicle gap generated from a look-up table addressed by a signal indicative of the velocity to be controlled.

7. A transportation system as claimed in claim 6, wherein the inter-vehicle distance control arrangement includes means for generating a signal indicative of a distance representing an instantaneous gap between vehicles, said means comprises an up/down counter arranged to receive a signal indictive of the velocity of a leading vehicle and a signal indicative of the velocity of the vehicle being controlled, the counter being further arranged to receive a load signal generated by the retro reflector of the leading vehicle, and a reset signal generated by the sensors which detect direct contact between the adjacent vehicles.

8. A transportation system substantially as hereinbefore described.

9. A transportation system substantially as hereinbefore described and with reference to the accompanying drawings.