GB2191310A - Conveying system - Google Patents

Description

GB2191310A SPECIFICATION be made to the trolley on reaching a process ing

station, thereby considerably increasing the Conveying system cost of the conveying system.

The present invention seeks to provide an The present invention relates to a conveying 70 automated conveyor in which the trolley can system for transporting vehicle bodies be- be positioned with accuracy in each station, is tween stations in an automated production brought from high speed to rest in as short a process, and is applicable more generally to time as possible, and in which the accuracy any system where it is required to transport a does not vary with the load presented by the load from one location to another with a given 75 conveyed vehicle body.

degree of accuracy within a limited time According to the present invention, there is In a totally automated production line, the provided an automated conveyor for driving a vehicle or vehicle body must be positioned plurality of trolleys through a plurality of sta quickly in each station with considerable accu- tions, comprising a digital programmable com racy and for this reason complex mechanical 80 puter connected to receive positional informa arrangements have been used to ensure the tion from each trolley at least when the trolley correct positioning of the trolley at the end of is in the vicinity of a processing station and each cycle. operative to process the positional information In the known systems, a programmable to produce a desired speed signals for the logic controller (PLC) controls the sequence of 85 respective trollies, and wherein the motor driv operation of the conveyor to synchronise the ing each trolley is controlled by a speed servo movement of all the vehicles carried on the control loop in which a signal indicative of the conveyor from one station to another. When a actual speed is compared with the desired trolley enters a station, the position of the speed signal received from and generated by trolley is first adjusted coarsely at the over- 90 the computer and the motor is controlled in head conveyor, for example by means of a such a manner as to minimise the resulting motorised bar with roller follower driven be- error.

tween two centralising cams on the carrier Preferably, the digital programmable com side. Once the trolley is positioned correctly puter is a programmable logic control serving cones are moved upwards and locate in coni- 95 additionally to control the sequencing of the cal seats in the base of the trolley to achieve movement of all the trollies on the conveyor fine positioning of the vehicle body in the pro- between the different processing stations.

cessing station. Any suitable drive configuration may be The reason for the need for both a fine and adopted in the present invention. For example, a coarse means of positioning the trolley in 100 the motor may be conventional or linear and each processing station is that the motors the drive may be by friction or by rack and driving the trolley could not locate the trolley pinion.

with sufficient accuracy to ensure that the fine Where the drive is through rack and pinion, positioning cones would line up with the conithe position transducers to indicate the posi cal seats in the base of the trollies under all 105 tion of the trolley may be a shaft encoder conditions. In the prior art, position sensors fitted on the motor output shaft. If the drive is detected when a trolley approached a profrictional, the positional information may be cessing station and reduced the current to the provided by means of a shaft encoder con motor driving the trolley in discrete steps, nected to a wheel in frictional engagement usually four steps, to bring the trolley to a 110 with the overhead track.

standstill at the desired position. However, the The zero position of the shaft encoder can position transducers are not constantly moni- be reinitialised at each station since the trolley tored but only at certain sampling intervals. is mechanically brought to rest in a preset On occasions, detection of the position of the position at every station. The risk of cumula trolley would occur so late in the sampling 115 tive errors is therefore obviated and each cycle that the deceleration applied would be transducer need in practice only be accurate very abrupt. Furthermore, the load presented over the distance between stations.

by the vehicle varies as more components are The desired speed signal is computed as a mounted on the vehicle in the production line continuous reducing function of the remaining and the ideal deceleration curve for one 120 distance to be travelled by the trolley before weight of vehicle would not be the same for reaching the desired end position. Because of another weight. Consequently, the control cir- this the retardation to which the trolley is sub cuit in the prior art would occasionally cause jected is gradual and the jerkiness in the unacceptable jerkiness in the motion of the movement is avoided. Furthermore, as the de conveyed body and would also result in the 125 sired speed varies with distance the trolley trolley overshooting or possibly not quite travels ever slower as it reaches the end posi reaching the station. In the event of under-run, tion. Because of this, the slope of the dis an unacceptable creep time penalty is incurred. tance versus time graph is reduced in the vi These positional inaccuracies resulted in the cinity of the desired stopping point allowing need for both coarse and fine adjustments to 130 greater leeway to compensate for varying load 2 GB2191310A 2 on the motor and delayed detection within a into each motor 12 to vary its speed in accor sampling cycle. dance with a desired speed versus distance Even in the unlikely event of an overshoot, curve. The speed is in this way continuously the desired speed signal would change in sign reduced gradually so that the trolley should so as to reverse the current through the mo- 70 come to a stop at the precise desired posi tor and return the trolley to the desired posi- tion.

tion. The servo varies the motor current in such It is an important feature of the invention, a manner that the actual speed of the motor that the improved positional accuracy may be should match the desired speed represented achieved without any additional installation 75 by the command signal received from the PLC costs since the control of the trolley motors 10. The control is relatively independent of may be effected by the same digital computer the mass of the trolley as any inability of the as must in any event be provided for the pur- servo control loop to follow the command sig pose of sequencing and synchronisation of the nals from the PLC 10 due to excessive mass movement of all the trolleys in the system. 80 will be accompanied by an error in the next The invention will now be described further, measured position of the trolley along the by way of example, with reference to the ac- conveyor and this will in turn cause the PLC companying drawing which shows schemati- to vary the next command signal in a sense cally a preferred embodiment of the invention. to reduce the cumulative error. In other The mechanical construction of the conveyor 85 words, though the speed control is open loop may be conventional in design and for this controlled by the PLC, the positional feedback reason is not being illustrated in the drawing. from the position sensors constitutes an inte The system illustrated comprises a central gral feedback path which is used to reduce PLC 10 connected to several motors 12a to the cumulative errors in speed control which 12n each associated with a respective posi- 90 can result from variation in the mass of the tion sensor 14a to 14n which sends back to trollies.

the PLC 10 signals indicating the actual posi- The trollies may be driven by the motor tion of the overhead trolley driven by the mo- having a toothed pinion engaging a rack ex tor. Each motor is controlled by a speed tending along the length of the conveyor. In servo 13a to 13n which is connected to the 95 this case, there being no slip, the position of position sensor and varies the motor current the trolley can be determined from a shaft in dependence upon the difference between encoder on the motor. If however the trolley the desired and actual speeds of the motor is driven by a friction wheel, then the position such that the trolley should move at the transducer can be a further wheel which rolls -speed dictated by the command signal from 100 on the conveyor during movement of the trol the PLC. ley and itself associated with an angular posi A PLC is present in conventional systems to tion transducer.

control the sequencing of the movement of When the trolley enters a processing sta the trollies between processing stations but in tion, the control system described above is the present invention the PLC 10 serves the 105 sufficient to ensure that the trolley is posi additional function of speed control of the in- tioned with accuracy in aligenment with the dividual trollies during their transit between centering devices, such as conical seats, used processing stations so as to improve the posi- to locate the trolley precisely at each station tional accuracy and reduce the transit time. and dispensing with any course means of con- Because of the limitations on processing all 110trol. Once the centering devices have been the available data in real time, it is not practic- brought into operation, then the position of able to relay positional information from all the trolley is known with absolute precision trollies at all times to the PLC 10. However, it and at these time the sensors may be re is only necessary for the PLC 10 to have pre- initialised in order to avoid any cumulative cise control over the speed and position of 115 measurement errors.

the trollies as they approach a processing sta- It is possible to construct an overhead con tion and limiting the data reaching the PLC 10 veyor such that each trolley includes its own in this manner enables the PLC 10 to process motor which remains with the trolley at all the data in real time without calling for a sig- times. It is alternatively possible for there to nificant increase in its processing power. Thus 120 be provided shuttles each moving forward and it is preferred to sense proximity to a pro- backwards between one pair only of process cessing station either by means of a separate ing stations the shuttles being selectively en sensor or by means of the the position sen- gageable with the individual trollies. Either sors 14a to 14n associated with the motors construction may be adopted in a system ac- and to send position data to the PLC 10 only 125 cording to the invention.

when the trollies are approaching a processing

Claims (6)

1. station. CLAIMS

   The PLC 10 processes the signals from the 1. An automated conveyor for driving a plu position sensor 14a to 14n and transmits a rality of trolleys along a track through a plural- signal to the servo control mechanism 13 built130 ity of stations, comprising a digital program- 3 GB2191310A 3 mable computer connected to receive posi tional information from each trolley at least when the trolley is in the vicinity of a processing station and operative to process the positional information to produce a desired speed signals for the respective trollies, and wherein the motor driving each trolley is controlled by a speed servo control loop in which a signal indicative of the actual speed is com- pared with the desired speed signal received from and generated by the computer and the motor is controlled in such a manner as to minimise the resulting error.
2. 2. A conveyor as claimed in claim 1, wherein the digital programmable computer is a programmable logic control serving additionally to control the sequencing of the movement of all the trollies on the conveyor between the different processing stations.
3. 3. A conveyor as claimed in claim 1 or 2, wherein the drive from the motor to the track is effected through a rack and pinion, and the position sensor for indicating the position of the trolley is a shaft encoder fitted on the motor output shaft.
4. 4. A conveyor as claimed in claim 1 or 2, wherein a frictional drive couples the motor of each trolley to the track and each position sensor comprises a shaft encoder connected to a wheel in frictional engagement with the track.
5. 5. A conveyor as claimed in any preceding claim, wherein in the programmable logic control the desired speed signal is computed as a continuous reducing function of the remaining distance to be travelled by the trolley before reaching the desired end position.
6. 6. A conveyor as claimed in claim 1 and constructed substantially as herein described with reference to and as illustrated in the accompanying drawing.

   Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd, Dd 8991685, 1987. Published at The Patent Office. 25 Southampton Buildings, London. WC2A lAY, from which copies may be obtained.