CA1299703C - Control system for an endless belt conveyor train - Google Patents

Abstract

"CONTROL SYSTEM FOR AN ENDLESS BELT CONVEYOR TRAIN"

ABSTRACT OF THE DISCLOSURE
A system is provided for controlling the feed rate of material being loaded onto a conveyor train of variable configuration to prevent overloading. The system involves monitoring the power drawn by the various drive motors of the train to establish which is the limiting conveyor under prevailing operating conditions. The instantaneous weight of feed material loaded onto the conveyor train, at its input end, is continuously measured and the current average feed rate is computed from the instantaneous weights measured over a pre-set time interval. A rolling average of the total load to be carried by the limiting conveyor downstream is computed from the average feed rate, taking into account the length of the limiting conveyor. The feed rate is then adjusted in response to the computed rolling average to thereby avoid overloading the limiting conveyor and optimize loading of the conveyor train.

Description

FIELD OF THB INVBNTION

2 The invention relates generally to a system for

3 controlling the loading of feed material onto an endless belt

4 conveyor train.

BACKGROUND OF THE INVENTION
6 The present invention has been developed in 7 connection with a conveyor train used to transport oil sand 8 from a mine site to a hydrocarbon extraction plant. It will 9 now be described in connection with that application, however the system can easily be adapted for use in other 11 circumstances. It is to be understood, therefore that the 12 invention is not to be considered limited solely to the 13 application described herein.
14 Typically, a conveyor train system comprises a number of endless belt conveyors, hereinafter termed 16 ~ conveyors~, serially arranged whereby the output load from 17 one conveyor is transferred as the input load to the next 18 conveyor in the train. Each conveyor, however, comprises a 19 discrete unit.
The individual conveyor comprises lengths of 21 flexible belts spliced together end to end to form the 22 endless belt. In high capacity systems, each such belt is 23 commonly formed of upper and lower rubber layers having 24 reinforcing steel cords sandwiched therebetween. The belt is supported on a plurality of spaced, cushioned, anti-friction 26 idlers. One or more driven pulleys is provided at the 27 conveyor end for driving the belt. The driving pulleys are 28 normally powered by conventional electrical motors.

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1 Additional drive motors may be included, i required to cope 2 with variations in load. Tension means are lncorpoxated to 3 keep the belt taut.
4 For a system comprising belts of fixed length, it S is common practice to run the conveyor train in a manner 6 such that each conveyor draws the same percentage of its 7 total rated power.
8 Where the load-carrying portion of any one belt 9 can vary however, a major disadvantage in such a system is that overloading of a single conveyor can cause it to stall.
11 When this occurs, the whole train must be brought to a halt.
12 Restarting the train is difficult due to the great power 13 needed to get the loaded conveyors under way again.
14 Frequent stalling and restarting reduces productivity and increases equipment wear.
16 In applicants' commercial open-pit oil sand mining 17 operation, carried out in the Fort McMurray region of 18 Alberta, about 300,000 tonnes per day of oil sand is mined 19 and conveyed to the extraction plant. Mining is carried out using large draglines whlch excavate the oil sand to a depth 21 in the order of 40 metres. The draglines deposit the oil 22 sand in elongate windrows along the edge of the rectangular 23 pit. A bucketwheel reclaimer transfers the oil sand from 24 each windrow to a bridging conveyor, which feeds it onto the first conveyor of a train. The train to the stacker may 26 consist of 2 to 4 conveyors extending around the perimeter 27 of the pit. In applicants' case, there are 4 draglines in 28 use, each supplying a separate train. The 7C~3 1 trains all terminate at a common zone adjacent the extraction 2 plant. Here the load from each train is transpoxted by an 3 inclined conveyor (or ~stacker~J and deposited on an arcuate 4 stack of oil sand. The extraction plant draws its feed from these 4 stacks.
6 It will be noted:
7 - that the bucketwheel reclaimer works its way 8 along the length of the windrow and thus the g point, at which its bridging conveyor deposits lo the oil sand onto the first conveyor of the 11 train, varies;
12 - that the bucketwheel reclaimer deposits the oil 13 sand in discrete spaced bucket loads on the 14 transfer conveyor; and - that the various conveyors of a train vary in 16 length. Exemplary lengths may range from 150 17 to 2500 metres.
18 Stated otherwise, the weight of the feed is not 19 distributed uniformly along the conreyors and the load-bearing length of the conveyors in a train is also not 21 uniform.
22 It is a requirement of the train concept that the 23 velocity at which the various conveyors travel should be the 24 same. So any resistance to motion will automatically require that the power drawn by the drive motors be increased, so as 26 to maintain a constant belt speed. There exists a maximum 27 safe level of electrical power that can be drawn from each 28 motor without causing it to stall. One seeks always to 29 optimize the loading of conveyors by running them at their 1 maximum safe rated power draw. This is a power draw somewhat 2 below the maximum (stall) power level. ~aximum safe rated 3 power correlates with the upper limit of the load that can be carried ~y the conveyor.
However, it is not difficult to inadvertently 6 exceed this rating.
7 Conveyor power draw will fluctuate depending upon a 8 number of factors. For example~ the internal frictional g losses associated with the conveyors are subject to numerous 10 variations. Additionally, external factors such as 11 fluc~uations in ambient temperature or differences in the 12 properties or grade of the oil sand per se will give rise to 13 variations in power draw requirements. Expanding further on 14 this latter point, the adhesive properties of the oil sand vary with its grade. If the oil sand is sticky, it will 16 build up on the belt. This in turn affects the drag 17 characteristics of the belt and hence the power draw. Or 18 there may be density variations due to snow or rainfall pick-19 up, which will alter the weight and adhesivity of the load.
20 It is to be emphasi~ed that the power draw fluctuations 21 caused by the above factors are not insignificant.
22 However, the single predominant factor leading to 23 rapid alterations in the power draw requirements resides in 24 the inherent inconsistency and intermittent nature of the bucketwheel loading technique itself.
26 In using a bucketwheel reclaimer for loading the 27 bridging or first conveyor of the train, the operator has 28 only his experience and visual observation to guide him as to 29 the rate at which the feed material should be deposited on i7~3 1 the conveyor.
2 When the system was irst put into use, the only 3 method of control was full on/full off. An observer in the 4 control tower at the systems delivery end simply radio-signalled that the bucketwheel reclaimer should start or stop 6 adding feed to the firs~ belt of the system.
7 In order to assist the reclaimer operator to more 8 accurately gauge the optimum conveyor loading rate, a prior 9 art control system was utilized. This method involved lo attaching wattage meters to the conveyor drive pulley motors 1l and monitoring the power draw requirements thereof. An 12 operator, located in a control tower positioned at the output 13 end of the conveyor train monitored the wattage draw signals 14 and instructed the reclaimer operator to 0ither reduce or increase the feed rate depending upon whether an overload or 16 reduced power draw was observed.
17 However, this prior art method allows of only crude 18 control. The method fails to take into account the 19 following:
- the irregularity and intermittent nature of the 21 rate of loading the feed from the individual 22 buckets of the wheel;
23 - the time lag between the actual loading of the 2~ feed material onto the belt, its transportation along the conveyor train, and the relaying of the 26 feedback message to adjust the loading rate 27 accordingly (this time lag may be of the order 28 of twelve minutesJ;

9t7 [?3 1 - the variation of effective length of the first 2 conveyor actually in use. This length will 3 change depending upon the position of the 4 reclaimer as it moves along the windrow; and - the combined effects of irregular feed rate 6 and power draw fluctuations on the optimum 7 load each individual conveyor can carry.
8 It is to be noted that, in the present instance, g the feed point of the system varies according to where, in the mine, the oil sand is loaded onto the system. As well, 11 belt lengths are changed as the system is expanded to 12 accommodate an increase in the size of the pit. The conveyor 13 system is thus dynamic and variable. This differs from a 14 static system where the feed point and belt lengths are fixed-16 There exists, therefore, the need for a control 17 system functional to adopt itself to systematic fluctuations 18 and to optimize the load carried by the conveyor train whilst 19 not exceeding the permissible rated power draw for the drive mOtors-22 The present invention was based on the recognition 23 that:
24 - In every conveyor train, there exists a limiting conveyor. That is to say, there is one 26 conveyor in the train which approaches 27 its maximum power draw requirements more 28 closely than do any of the remaining ~ ~5t~

1 conveyors. This conveyor is referred to 2 herein as the ~limitin~ conveyor'. As conditions 3 change, the conveyor in the tl-ain, which .is the 4 limiting conveyor, can chan~e (that is, at one point in time the limiting conveyor may be the 6 first conveyor - at another point, it may be the 7 third conveyor);
8 - ~t is necessary to identify the limiting g conveyor and to provide means for continuously lo monitoring the power draw thereof to ensure 11 that overloading is avoided;
12 - Instantaneous weight readings of the feed 13 material being loaded onto the conveyor have to 14 be averaged out over pre-determined time periods, in order to provide meaningful data. Due to the 16 irregularity of the deposition of feed material 7 onto the train from the b~lcketwheel reclaimer, 18 instantaneous weight readings as an indicia of 19 feed rate are valueless. Additionally, the weight of feed has to be measured at the input 21 end of the train, to minimize errors arising from 22 time lags within the system; and 23 - A 'rolling average' of the total weight of feed 24 material on the limiting conveyor can be computed in advance. By 'roll.ing' is meant that 26 several data points are averaged and, at fi~ed 27 intervals, the oldest point is withdrawn from 28 the computation and the most recent added. Thus 29 by computing a 'rolling average' it is possible 1 to predict the total load of feed material which 2 would be carried ~y an~ pre-determined section of 3 the train downstreamt given the current average 4 feed rate, and hence the current digging rate of the reclaimer. A substantially direct cor-6 relation exists ~etween the computed 'rolling 7 average' and the predicted power draw of the 8 limiting conveyor. As a result, a feedback 9 means can then be applied for controlling the feed rate of material onto the train to there~y 11 optimize loading of the system and avoid 12 overloading of the limiting conveyor thereof.
13 ~roadly stated, the invention in an apparatus aspect comprises: first means, associated with each endless belt conveyor drive means, for substantially continuously 16 measuring the power draw by such drive means and producing 17 signals indicative thereof , whereby the conveyor in the 18 train which is the limiting conveyor may be identified;
19 second means, associated with the initial conveyor in the 20 train, for instantaneously measuring the weight of the feed 21 material deposited thereon and producing signals indicative 22 thereof; third means, associated with the weight measuring 23 and signalling means, for computing the average feed rate of 24 material deposited on the initial conveyor over a pre-set period of time, utilizing the instantaneous weight 26 measurements sensed and producing signals indicative thereof;
27 fourth means, associated with the first and third means, for 28 projecting the computed current average feed rate to the 29 total load the limiting conveyor downstream will carry at 1 that feed rate, calculated as a rolling average o~ the load, 2 and producing signals indicative thereof; and fi~th means, 3 associated with the manually operated loading machine, for 4 receiving and displaying the signal produced from the fourth means, whereby the feed rate addition to the first conveyor 6 may be altered in response to the fourth means signals, to 7 thereby control the rate at which feed material is being 8 deposited onto the conveyor train to avoid overloading the g limiting conveyor.
In a method aspect, the invention comprises:
11 measuring the individual power draws of the drive means of 12 each conveyor of the train and utilizing said measurements to 13 establish which is the limiting conveyor of the train;
14 substantially continuously measuring the instantaneous weight of feed material being added at the input end of the train 16 and producing signals indicative thereof; averaging, over a 17 pre-set period of time, the rate at which feed is added to 18 the initial conveyor; computing from said weight-added 19 signals a rolling average of the total load on the limiting 20 conveyor; and adjusting the feed rate in response to the 21 computed rolling average to ensure that the limiting conveyor 22 will not be overloaded.

24 Figure l is a schematic showing the mining equipment used at applicants' open-pit oil sand mine site;
26 Figure 2 is a schematic of apparatus employed in 27 the practice of the present invention;

1Figure 3 is a flow chart showing the steps of the 2 method carried out by the apparatus of Figure 2;
3Figure 4a, 4b and 4c are a series of histograms 4 included to demonstrate the operability of the invention.
They show plots of the frequency (% of operating time) versus 6 the conveyor load (kt/h). The rate capacity varies depending 7 on ambient conditions. For the examples of Figures 4a and 4c 8it is 6400 kt/h, and for 4b it is 7300 kt/h. Figures 4a and 9 4b exemplify cases where the conveyor is operated using conventional methods. In 4a, output is fairly high ~79%J but 11 the rated capacity is often exceeded. In 4h, to keep maximum 12 load at or below the rated capacity, production is reduced 13 (64%). Figure 4c shows an example of the conveyor being 14 operated using the method of the present invention. In this 15 case, the histogram shows that most events occur at or just 16 below the rated capacity. Production is high (86%) but the 17 system is never overloaded; and 18Figure 5 shows plots of weightometer response 19 versus time as (a) raw load cell output (b) filtered load 20 cell output (c~ filtered load cell output averaged over 4 21 second intervals to eliminate the effect of intermittent feed 22 as it is deposited by individual buckets of the BWR, and (d) 23 is a rolling average produced from the average readings to 24 predict the load that will be carried by the limiting 25 conveyor.

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DE:SCRIP ION OF l'HE PREFERRED EMBODIMENl' 2 ~aving reference to Figure 1, it shows a 3 conventional oil sand mine in which the present invention may 4 be utili~ed. It should be understood that although the mine system set forth is operated in four essentially equal 6 quadrants, it is described herein particularly with respect 7 to a single quadrant.
8 Nore particularly, the mine system comprises a 9 dragline 10 which excavates the oil sand at the pit face to generate a windrow 12. A bucketwheel reclaimer 14 transfers 1l the oil sand from windrow 12 onto the endless belt conveyor 12 train 16. Conveyor train 16 carries the oil sand to a 13 stacker 17 which transfers the material onto a stack 18. A
14 control tower 13 is centered within the stacking area. The mined sand is conveyed from the stacking area to the 16 hydrocarbon extraction plant (not shown)~
17 The conveyor train 16 shown comprises four 18 conveyors 16a, 16b, 16c, 16d, serially arranged as shown in 19 Figure 1. Each conveyor was driven ~y a driven pulley powered by one or more electric motors 19. Typically, the 21 train layout comprises the following:
22 Length Travel l'ime Number of 23 (Meters) (Minutes) Motor Drives 24 Conveyor 16a2,500 9 4 Conveyor 16b 300 26 Conveyor 16c2, 500 8 4 27 Conveyor 16d1,000 4 3 g~

l In accordance with the invention, and as shown in 2 Figure 2, watta~e meters 22 were electrically connected to 3 each drive motor l9 (only one of which is illustrated~ to 4 monitor the power drawn by each drive system. I'he wattage meters 22 were conveniently located in the control tower 13.
6 From the tower, a radio transmitter 24 sent a signal 7 indicative of power draw to a radio receiver 26 located in 8 the buc~etwheel reclaimer 14. The operator in the bucketwheel g reclaimer 14 would instruct a computer as to which conveyor 10 in the pertinent train was the limiting conveyor.
ll A continuous-sensing weightometer 23 was associated 12 with the bridge conveyor lS of the bucketwheel reclaimer 14.
13 More particularly, roller 30 was arranged to continuously 14 contact the underside of the belt 32 of the bridge conveyor 15. A pivoting arm 3~ supported the roller 30 and l6 communicated with a load cell 36. As roller 30 moved l7 upwardly and downwardly in response to the amount of material 18 loaded on belt 32, a signal indicative of the weight was 19 transmitted from the load cell 36. The weightometer 23 was constructed by the inventors. Meters serving the same 21 purpose are widely available in commercial form. The load 22 cell was manufactured by Strainsert as model FL7.5U (CJ-23 2SKWT.
24 A signal conditioner 38 received the output signal transmitted from the load cell 36. Signal conditioner 38 26 electronically filtered out noise and variables which would 27 influence the true weight readings. It further functioned to 28 amplify the output signal. The signal conditioner 38 used 29 was model TSC 17-1121 option l manufactured by Acrotech.

1 Slip rings 40 were mounted in the bucketwheel 2 reclaimer 14. The amplified signal from the conditioner 3~
3 was passed via slip rings 40 to a computer 42. Attached to 4 computer 42 was a monitor screen 44.
It is to ~e noted that the computer 42 waB
6 "associated" with or operatively interconnected with the 7 wattage meters 22 (through the medium of the reclaimer 8 operator) and with the load cell 36.
9 ~ithin the internal memory of the computer were two programs which enabled the computer to process the incoming 11 data to provide a feedback arrangement for controlling the 12 rate at which feed was loaded onto the conveyor train and 13 hence for controlling the digging rate.
14 More particularly, the first program caused the computer to average the instantaneously sensed weight of oil 16 sand loaded onto the bridge conveyor 15 over a pre-determined 17 time interval. This produced a measure indicative of the 18 average current feed rate .
19 The second program was adapted to utili~e the average current feed rate data and the transit time of the 21 limiting conveyor as the input data, to compute a 'rolling 22 average' of the load on the limiting conveyor. The transit 23 time was determined by dividing the length of the conveyor by 24 its velocity.
Both first and second programs were written 26 utilizing conventional programming steps. Programs of the 27 second type should take account of any variations in belt 28 slope.

1 The steps executed are illustrated in Figure 3.
2 ~ith load added to the system held constant for a length of 3 time required to fill the entire system, the wattages drawn 4 by the drive motors of the individual conveyors of the conveyor train were monitored or measured, using meters 22.
6 The conveyor which drew the highest percentage of the maximum 7 power available from its drive motor(sJ 19, was established 8 as the limiting conveyor of the train. rhe bucketwheel 9 reclaimer operator was advised of the identity of the limiting conveyor. The length of the loaded portion of the 11 limiting conveyor was utilized as input to the second 12 program.
13 'The instantaneous weight of feed material deposited 14 on the bridging conveyor 15 was determined as described hereabove. The current average feed rate was computed 16 utilizing program 1. The computed rate was utilized as input 17 data for program 2.
18 As stated earlier, the second computer program then 19 computed the projected rolling average of the weight o~ feed material onto the limiting conveyor from the input data.
21 The results from the first and second computer 22 programs were displayed to the bucketwheel reclaimer 23 operator. The power draw, monitored in the control tower, 24 indicated whether the limiting conveyor was loaded below capacity. Based on this, the operator was instructed to 26 adjust the digging rate if required. The correctness o~ the 27 adjustment was determined by the new wattage draw. rrhe 28 adjustment was determined by the effect of a discrete change 29 in digging rate on the load carried by the limiting conveyor 1 as given by the rolling average program.

2 Example of Operation 3 l. Determine critical conveyor:

4 B~R operator digs at constant average feed rate until entire conveyor train is loaded at that feed rate.
6 Control tower power meters are examined to identify 7 conveyor which is draw.ing the highest percent of its 8 allowed power. This is the critical conveyor.

9 2. Set rolling average period~

Identity of critical conveyor is radioed to B~R
11 operator, who inputs corresponding loaded length to 12 program Z. Computer now displays rolling average of 13 feed on loaded length of critical conveyor.

14 3. Set desired digging rate:

If power draw is low compared with available power, 16 control tower radios BWR operator to increase digging 17 rate~

18 or 19 If power draw is too high or if less feed is required, control tower radios BWR operator to crease digging 21 rate.

,3 1 BWR operator increases or decreases digging rate las 2 required) and maintains new average rate until critical 3 conveyor has been loaded at that feed rate.

4 Control tower radios BWR operator with further corrections as a few iterations may be required.

6 4. Naintain desired digging rate and conveyor power draw:

7 The desired digging rate, as determined by step 3 is 8 maintained by the B~R operator keeping a constant g rolling average feed rate.

Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a solid materials transportation system wherein discrete spaced loads of material are intermittently fed by means of a manually operated loading machine onto the first of a plurality of serially arranged individual endless belt conveyors which in combination form a conveyor train moving at a substantially constant velocity and wherein each endless belt conveyor is provided with separate drive means, apparatus for optimizing the load carried by the conveyor train which comprises:
first means, associated with each endless belt conveyor drive means, for substantially continuously measuring the power drawn by such drive means and producing signals indicative thereof, whereby the conveyor in the train which is the limiting conveyor may be identified;
second means, associated with the initial conveyor in the train, for instantaneously measuring the weight of the feed material deposited thereon and producing signals indicative thereof;
third means associated with the weight measuring and signalling means, for computing the average feed rate of material deposited on the initial conveyor over a pre-set period of time, utilizing the instantaneous weight measurements sensed, and producing signals indicative thereof;
fourth means, associated with the third means, for projecting the computed current average feed rate to the total load the limiting conveyor downstream will carry at that feed rate, calculated as a rolling average of the load, and producing signals indicative thereof; and fifth means, associated with the manually operated loading machine, for receiving and displaying the signal produced from the fourth means, whereby the feed rate addition to the first conveyor may be altered, in response to the fourth means signals, to thereby control the rate at which feed material is being deposited onto the conveyor train to avoid overloading the limiting conveyor.

2. A method for controlling the rate of loading feed material onto a train of individually driven conveyors, to optimize the loading thereof so as to avoid overloading the limiting conveyor of the train, which comprises:
measuring the individual power draws of the drive means of each conveyor of the train and utilizing said measurements to establish which is the limiting conveyor of the train;
substantially continuously measuring the instantaneous weight of feed material being added at the input end of the train and producing signals indicative thereof;
averaging the instantaneous weight measurements over a pre-determined period;
computing from said weight-added signals a rolling average of the total load on the limiting conveyor; and adjusting the feed rate in response to the computed rolling average to ensure that the limiting conveyor will not be overloaded.

3. A method for controlling the rate of loading oil sand with a bucketwheel reclaimer onto a train of individually driven conveyors, to optimize the loading thereof so as to avoid overloading the limiting conveyor of the train, which comprises:
measuring the individual power draws of the drive means of each conveyor of the train and utilizing said measurements to establish which is the limiting conveyor of the train;
substantially continuously measuring the instantaneous weight of oil sand being added at the input end of the train and producing signals indicative thereof;
computing from said weight-added signals the current average feed rate of oil sand being added for a pre-determined period;
utilizing said current average feed rate values to compute a rolling average of the predicted total load on the limiting conveyor, which will occur when oil sand currently being added reaches and loads said limiting conveyor; and adjusting the reclaimer feed rate in response to the computed rolling average to ensure that the limiting conveyor will not be overloaded.