BACKGROUND OF THE INVENTION
This invention relates to an automatic conveyor system and more particularly to a conveyor system having a plurality of sensors to detect carrier location.
Conveyor systems, the type for which the present invention is adapted, such as the United States Post Office Department (now the U.S. Postal Service) Model Nos. 120 and 121 of the Letter Sorting Machine (LSM) presently in use by the U.S. Postal Service, are designed to sort mail, parcels and other objects which are received from a plurality of input consoles for conveyance to a plurality of output receptacles. An explicit requirement of such an automatic conveying system is that the station address of the receptacle to which the conveyed object is destined must somehow be correlated with the conveying carrier and stored until the time that the carrier reaches the destined receptacle. A particular conveying system which has been successful in the past is one in which the respective carriers were provided with mechanical escort memories or code wheels which ride along a code track. When a particular set of code wheels correspond to the code of a particular receptacle, the carrier is dumped or otherwise unloaded. The mechanical escort memory presently used in the LSM comprises code wheels associated with each cart compartment of the carrier and code bars at each of the output bins or receptacles for decoding the wheels. The code wheels are attached to the carrier and move on tracks. Obviously, it is extremely important that the letter cart door be opened at the correct position with respect to the letter receptacle in order to insure that the mail pieces will properly enter the receptacle. The LSM carts are held in position and transported by a pair of roller-chain conveyors. In the particular models of the LSM referenced hereinbefore, the conveyor chains are each approximately 282 feet long and twelve operator input consoles for loading mail into the carts are provided. Along the conveyor chain, 161 letter carts are spaced on 21-inch centers. Thus, each cart is eighteen inches in length and the carts are separated from each other by a distance of three inches. Each letter cart contains twelve compartments, spaced 1.5 inches from each other. The memory system must have the capability of keeping track of twelve mail pieces per letter cart in 161 letter carts per machine, or a total of 1,932 mail pieces. As a letter cart compartment advances through the LSM it passes each of the 277 letter receptacles located along the 282 feet of conveyor. Taking into account the relationship between the door latches of adjacent mail compartments and the solenoid-actuated door link, together with the physical dimensions of the solenoid actuator arm itself, it can be shown that in the LSM the solenoid actuator arm must operate within a "window" distance of about 1.19 inches of conveyor chain travel, if it is to open the correct letter cart compartment door and not interfere with or open the door on either side of it. Simply stated, the problem is to know precisely where any letter compartment is to within 1.19 inches along a 282 foot conveyor chain. The mechanical escort memory system provides excellent reliability since there are no effects on performance due to variations of machine speed or in the location of the carrier. However, reliability decreases as the size of the system is increased since the larger the number of mechanical components involved, the greater failure rate becomes. For example, in the LSM such a system includes 1,932 code wheel assemblies, or 23,184 code wheels, and 278 code bar assemblies, or 3,058 code bars. In addition there are the mechanical supports and guides required along a track of approximately 244 feet.
As distinct from mechanical escort memories described above, other types of magnetic and electric memories have been provided for automatic conveyor systems. Such memories may take the form of a rotating drum or an electronic shift register where the receptacle address is moved or shifted through the memory as the conveying carrier moves along the conveyor system. When the address reaches a location in the memory corresponding to the destined receptacle, the carrier is unloaded. Again, the reliability of this type of system decreases as the size of the system increases since chain or conveyor slack or stretch can result in nonuniform carrier spacing such that the array of addresses moving through the memory is no longer analogous to the exact model of the conveyor system.
Certain automatic conveyor systems of the prior art have attempted with some success to maintain some analogy between the memory cycle and the conveyor cycle by synchronizing the starting positions of the respective cycles. For example, a counting scheme may be employed wherein a counter is started when a mail piece is inserted into a certain letter cart and compartment. Since each letter receptacle is a fixed distance from a certain operator console, the motion of the letter cart conveyor chain can be used to generate distance-travelled pulses. A shaft encoder can be attached to the drive chain sprocket of the letter cart conveyor to generate a pulse to correspond to any desired linear advancement of the chain. The distance from a console to any letter receptacle can be expressed by the number of pulses needed to reach the receptacle. Such techniques are successful for relatively small conveyor systems. However, as the size of the conveyor system increases, clack due to wearing or stretching of the conveyor belt or chain can approach the pitch or distance between successive carriers in the conveyor system thereby substantially reducing the reliability of the system. If the slack becomes greater than the pitch, the system becomes inoperable. In connection with the LSM, it has been found that the length of the conveyor chain of a fixed number of links varies significantly because of manufacturing tolerances, change in chain tension (stretch), and wear between pins and bushings. Moreover in the afore-mentioned counting scheme, occasional system recalibration as to distance counts from the operator console to the letter receptacles would be ineffective, because the chain wear would probably not be uniform along the LSM 282 foot conveyor length.
Another possible solution to the problem involves locating a position sensor at each of the letter receptacles as well as one sensor at each operator input console. Each sensor at the operator console establishes the cart compartment of one of the plurality of carts which is to receive a certain mail piece. A memory is provided to retain this information and make it available to the sensor located at the letter receptacle. The receptacle sensor then waits for the correct letter cart and compartment to arrive at the receptacle so that it can trigger the door actuator mechanism to open the door. This type of system has the advantage that the correct letter cart and compartment can be accurately located by a letter receptacle regardless of chain stretch, wear or manufacturing tolerances. In connection with the LSM chosen for purposes of illustration, the disadvantage of this system is the need for the large number of position sensors which total 289 and the associated logic circuits and memory needed to service 1,932 letter cart compartments. The cost of such a system is prohibitive and the large number of sensors and associated circuits effectively decrease the overall system reliability.
It is, therefore, an object of the present invention to provide improved automatic conveyor system.
It is another object of the present invention to provide an automatic conveyor system wherein error will not result from slack due to wearing or stretching of the conveyor belt or chain.
It is a further object of the present invention to provide an automatic conveyor system which will be error free irrespective of the length of the conveyor belt or chain. It is a still further object of the present invention to provide an electronic system which will operate reliably and economically over the entire life-span of the equipment.
SUMMARY OF THE INVENTION
The above objects of the present invention are accomplished by providing a counting scheme in combination with a plurality of sensors distributed along the conveyor path in the area of the respective receptacle bins such that the position of the individual conveyor carriers can be specified as the movement of each carrier relative to its nearest sensor. The spacing between the sensors is chosen to be less than that distance in which the maximum tolerable error can occur due to wearing, stretching or sagging of the conveyor belt or chain. With relation to the models of the LSM system referenced hereinbefore, the invention contemplates dividing the 282 foot long conveyor-chain distance and the receptacle bins it services into a plurality of zones, each monitored by a conveyor position sensor. As indicated hereinbefore, assuming that in the LSM system, a 1.19 inch "window" of distance must be furnished to operate a letter cart compartment door properly, nine zones each having a conveyor position sensor are required. Further assuming that twelve operator consoles are present, 12 console position sensors are also required. Obviously, the total number of sensors, that is, 21, contemplated in this particular LSM system modified in accordance with the present invention, is but a small fraction of the 289 sensors required in the system described hereinbefore.
Each receiving bin is specified by the position at which the corresponding carrier will be when adjacent to that bin when it is required that the carrier deposit a package, letter or other article therein. This position designation, along with the carrier designation, is stored in a multi-word memory at the time that the article is placed in the carrier at an input console and the contents of this memory are periodically scanned for comparison with counters associated with each of the sensors. When the particular count associated with a particular sensor corresponds to designated position stored in the memory, a signal is sent to actuate a trip mechanism associated with the respective receiving bin to trip the carrier causing it to deposit its article. Provision can be made for storing positions of alternate receptacles or receiving bins should the first specified bin be full or otherwise incapable of receiving an article.
A feature, then, of the present invention resides in a conveyor system having a plurality of sensors mounted along the conveyor path in the area of the receiving bins and counters associated with the respective sensors to designate the relative position of the individual carriers as they pass each sensor.
Another feature of the present invention resides in the provision of a multi-word memory to store the delivery designation for each of the respective carriers and the means to compare the count associated with each sensor with said delivery designation.
Still another feature of the present invention resides in the means to provide for alternative designations for each of the respective carriers should the first specified designation refer to a receiving bin that is not capable of receiving additional articles.
DESCRIPTION OF THE DRAWINGS
The above and other objects, advantages and features of the present invention will become more readily apparent from a review of the following specification when taken in conjunction with the drawings wherein:
FIG. 1 is a representation of a conveyor system employing the present invention;
FIG. 2 is a schematic representation of the input, memory, and comparator logic employed with the present invention;
FIG. 3 is a representation illustrating the manner in which carrier positions are designated according to the present invention;
FIG. 4a, b and c are representations of memory word formats as employed in the present invention;
FIG. 5 is a flow diagram of the input console polling sequence as employed in the present invention;
FIG. 6 is a flow diagram of the computation routine employed in the present invention; and
FIG. 7 is a flow diagram of the scan and compare routine as employed in the present invention.
GENERAL DESCRIPTION OF THE SYSTEM
As illustrated in FIG. 1, which is a representation of the reference LSM transport system, conveyor system 10 is adapted to traverse zones or receptacle areas 11, . . . , 19 each of which comprises a plurality of receptacle bins that are adapted to receive parcels and other articles that were loaded into the conveyor system from input consoles 31, . . . , 42. Corresponding zone sensors 21, . . . , 29 are associated with the receptacle areas 11, . . . , 19 respectively. Input console sensors 51, . . . , 62 are associated with input consoles 31, . . . , 42 respectively. It should be readily apparent that the representation of FIG. 1 is a simplified one. Carrier carts each having at least one, and usually more than one, compartment while not shown in FIG. 1, are assumed to be affixed to and transported by the conveyor medium 10. In the LSM referenced herein, each cart comprises twelve compartments and the conveyor 10 is of the roller-chain type. Similarly, the zones or receptacle areas numbered 1 through 9 include all of the receptacle bins -- each zone comprising approximately the same number of bins.
Each of the individual carrier carts is provided with a sensor actuating device. The respective sensors may just be photo-cells in which case the sensor actuating device for each cart would be a flag of sufficient dimension to interrupt the photo-cell. Or the respective sensors may be mercury-wetted reed switches in which case the sensor actuate devices would be magnets. Except for the first carrier cart, each carrier would be provided with one sensor actuating device; however, the first carrier cart is provided with two such actuating devices to provide two closely spaced pulses to synchronize the position of the first cart relative to the nearest sensor. The distance of each receiving bin from its associated zone sensor is then measured as a number of incremental units. As will be explained in greater detail hereinafter, the increments or incremental units represent a predetermined fixed advancement of the carrier conveyor medium. The actual increment distance chosen determines the system resolution, that is, the precision required in locating the carriers. The smaller the bias increment, the faster the memory access cycle that will be required. In the LSM, an increment of one-sixth inch was found to be satisfactory.
Perhaps a better understanding of the manner in which each receiving bin is addressed will be obtained by reference to FIG. 3 which is a diagrammatic representation of the position of the respective receiving bins relative to the carrier carts as they traverse the conveyor path. In FIG. 3, it is assumed initially for purposes of explanation that each of the carriers contains only one compartment adapted to hold only a single piece of mail and that the parcel discharge occurs when the leading edge of the carrier cart is aligned with the right edge of the receiving bin. Thus, as illustrated in FIG. 3, carrier cart number 2 is in a discharge position over bin number 8 and carrier cart number 5 is in a discharge position over bin.
The mail carriers (C) are numbered sequentially, for example, from 0 to 160 inclusive in the LSM, relative to a reference carrier which has the "0" designation. The only system requirement is that the reference carrier be uniquely identified, for example, by means of its double sensor actuating devices, and that on the basis of this identification the numbers of subsequent carriers be recognized.
The base distance (D) is a set of numbers which represent the physical distance of the right edge of each bin or receptacle relative to the nearest zone sensor which is the bin reference point depicted in FIG. 3. As indicated in the table in the upper part of FIG. 3, the distance measurements are made in terms of numbers of nominal carrier pitches (distance between carriers) and fractions of carrier pitches (carrier increments). In terms of the one-sixth inch increments referred to hereinbefore in connection with the LSM, the carrier pitch expressed in increments is 126, corresponding to the 21-inch spacing between carriers.
The reference carrier position (R) defines the location of the reference carrier relative to the bin reference point.
In FIG. 3 the carrier pitch scale used for the measurement of the various parameters has been indicated. It should be noted that the division of the carrier pitch into ten parts has been made solely for ease of explanation. As indicated hereinbefore, the incremental distance would normally be measured by monitoring carrier motion, such as by the shaft encoder 20 depicted in FIG. 1. It will be observed from FIG. 3 that the reference carrier position R will have a relation to the positions of each of those carriers whose right edges are in alignment with respective right edges of bins, such that the difference between the reference carrier position and the number designation of a given one of the last-mentioned carriers is the base distance of the bin aligned therewith. Thus, the base distance D3 of bin number 3 is just one carrier and four carrier increments. When carrier number 5 is in a position adjacent to bin number 3, the reference carrier position (position of carrier 0) will be five carriers plus (one carrier plus four increments) or six carriers plus four increments. Similarly, the base distance D8 of bin number 8 is four carriers and four increments relative to the nearest sensor. Again, the reference carrier position R will be two carriers plus (four carriers plus four increments) which is six carriers plus four increments. In this manner, when the sum of the carrier number designation and the base distance of its destined receptacle bin are equal to the reference carrier position R, the given carrier is in the appropriate discharge position adjacent to the destined receptacle bin.
The description of FIG. 3 has thus far not taken into account the fact that each of the carriers may comprise a plurality of compartments. Consider the LSM wherein each carrier comprises 12 compartments displaced from each other by 1.5 inches or nine bias increments, assuming that the selected bias increments are each one-sixth inch. Under the conditions illustrated in FIG. 3, it has been assumed that the first compartment of carrier 2 is in the proper position to discharge into bin 8. If it is desired to empty the second compartment of carrier 2 into bin 8, it is necessary to advance the carriers in the direction of the arrow by nine bias increments. At this point, actuation of the door on the second carrier compartment will cause the contents thereof to be deposited in bin 8. Similarly, if the twelfth compartment of carrier 5 is to be deposited in bin 3, the carriers must advance 99 increments to the right of the position depicted. This operation is described in detail in connection with FIG. 2. At this point, however, the following considerations will be helpful to the reader. If information is placed in the 12th compartment of carrier 5 by console 12, a compartment increment fixed bias of 99 increments, provided by unit 621 in FIG. 2, will be added to the cart count of the console counter 620 in FIG. 2. The cart count, namely 5, and increment bias, 99, of the compartment are then added to the base distance of the destined receptacle. In FIG. 2 such addition is performed by the computing logic 82. If the destined receptacle is the bin 3 of FIG. 3, it will be assumed that this bin has a base distance of one carrier pitch and four increments. The total of six carriers and 103 increments from this addition is referred to as the "computed conveyor position" . When the count in the zone counter associated with the bin reference sensor of FIG. 3 is identical to the "computed conveyor position", the 12th compartment of carrier 5 will be in position to unload its contents into bin 3.
DETAILED DESCRIPTION OF THE SYSTEM
A better understanding of the system will perhaps be obtained from the review of the various memory word formats as illustrated in FIGS. 4a, b, and c. The memory is designed to consist of 8,192 addresses each containing a 36-bit word. This memory is divided into three segments. The first segment as illustrated in FIG. 4a consists of addresses 0 to 1,023 and is adapted to contain the numbers of three separate receiving bins designated as primary, first alternative, and second alternative. In the event that only one receiving bin is to be assigned to the keyboard address, the remaining two receptacle numbers will be a repeat of the assigned number. The second segment of memory is used to contain the base distance data as illustrated in FIG. 4b. This data includes the zone position sensor number controlling the particular receiving bin, the carrier cart count and the carrier increment count, the sign (plus or minus) to indicate whether the receiving bin is downstream or upstream from the sensor, and redundancy bits to provide a "no go" signal if the memory should lose its word. The third segment of memory contains the corresponding computed conveyor position data as illustrated in FIG. 4c. This data represents the count that will be reached on the specific conveyor position counter when it is time to open the carrier cart compartment door when the cart is located above a specific receiving bin. Since with the particular embodiment described, there can be up to 1,932 parcels in transit toward the receiving bins and since each parcel can be directed to go to any of three different receiving bins it is necessary to store 5,796 words for this purpose.
Having described the particular memory word formats, the system itself will now be described with reference to FIG. 2. As illustrated therein, console position sensors 51, . . . , 62 which were discussed briefly in regard to FIG. 1, are connected to a corresponding binary counters 510, . . . , 620 which are adapted to be reset to zero upon the receipt of a double-pulse. It will be remembered that the reference carrier 0 is provided with a double trip mechanism for this purpose. This resetting action is used to indicate the position of the reference cart relative to a particular console sensor. Since the carriers or carts are usually designated by successive numbers, it is convenient, although not necessary, to arrange the count in the cart counter of a give console to be the same as the cart number designation of the cart being loaded thereby. With this arrangement, if cart 124 is being serviced by a console, actuation of the console position sensor by cart 124 will cause the count in the associated console cart counter to also be 124.
In normal operation, the first console loads the first compartment in the carrier cart, the second console loads the second compartment of the carrier cart and so on where each carrier cart is provided with a plurality of compartments. Since the second compartment of each cart is a fixed distance downstream from the first compartment, it is necessary to add a fixed bias to the cart counter for each of the respective consoles upstream from the first console. As indicated in FIG. 2, the incremental bias to be added to the first console's cart count is provided by unit 511. The incremental bias to be added to the 12th console count is provided by unit 621. It will be understood that corresponding units would be provided for each of the other consoles.
The output signals from the respective consoles are then applied to cart counter multiplexer 81 for presentation to computing logic 82 in a sequential manner.
At the time that a parcel is placed in a conveyor cart compartment the operator enters the designation of the destined receptacle bin into the console keyboard. The respective signals from each console are then supplied to keyboard multiplexer 70 for ultimate presentation to memory 79. The entries from the respective console keyboards will be in the form of binary coded decimal signals which are converted to binary signals by converter 71. Translator 72, which is programmed by control unit 78, recognizes and separates the respective receptacle designation as primary, first alternative and second alternative for appropriate placement in the respective registers 73, 74 and 75 as indicated in FIG. 2. Format control information is entered into control unit 78 by card reader 77 or other appropriate input devices.
The first two segments of memory as described in relation to the memory word formats are stored in memory 79. The third segment of memory which contains the computed conveyor position data is stored in memory 80. This memory receives the computed conveyor position from computing logic 82 which is essentially an adder-subtractor that adds or subtracts the particular base distance for a specified receptacle bin to the polled console conveyor data. That is to say, the computed data position for a particular parcel destined to a particular receptacle bin is the cart count and increment bias of the cart compartment in which the parcel has been placed plus the base distance of the destined receptacle bin. The result is a number that will appear on the specific conveyor position counter when it is time to release the parcel into the destined receptacle bin. It will be remembered from the discussion of FIG. 4b that the base distance of the destined receptacle bin may be added or subtracted as required by the sign that appears in the first bit of the base distance memory word format.
Conveyor position computing logic 82 also has the ability to handle cart count overflow. For example if the console cart count is 160 carts and 99 increments and if the base distance of the destined receptacle bin is 3 carts and 51 increments, the total would be 163 carts and 150 increments. Assuming that the incremental distance between adjacent carts is 126 increments, the incremental count would be corrected to 1 cart and 24 increments and thus the total would be 164 carts and 24 increments. Further assuming that the total number of carts in the conveyor system is 161, the final conveyor position would be obtained from subtracting that number which final result would be three carts and 24 increments. This is the number that is to be stored in memory 80. To arrive at this result, computing logic 82 receives the base distance data from memory portion 79 and the console cart count and increment bias from counter multiplexer 81 as illustrated in FIG. 2.
As the respective carrier carts traverse the conveyor system, their presence is detected by zone sensors 21, . . . , 29 as illustrated in both FIGS. 1 and 2. It will be remembered that the first carrier cart is a reference carrier and induces a double pulse in each of the sensors to reset the respective counters. The system is provided with an increment position sensor 20 which may just be a rotary incremental shaft encoder connected to the shaft that drives the conveyor system. This encoder will provide the pulse for each incremental advancement of the conveyor system.
Each of the zone sensors 21, . . . , 29 is adapted to step corresponding zone conveyor position counters 210, . . . , 290 at the time that a cart is detected by the sensor. This step also resets the corresponding increment counter. The respective counters, thus, provide both a cart count and an increment count for each of the respective zone sensors which are in turn supplied to conveyor position counter multiplexer 83. The particular zone counter data to be received by multiplexer 83 is determined by the zone number presented to multiplexer 83 from memory 80 as illustrated in FIG. 2. At the same time, the corresponding computed conveyor position is transferred to solenoid bias logic 84 for comparison with the received count from multiplexer 83 which comparison is made by comparator 86. At the same time, the receptacle bin number associated with the computed conveyor position is presented from memory 80 to receptacle solenoid decoder 85. Should comparator 86 indicate identity in the data being compared, solenoid decoder 85 will actuate the respective solenoid driver 87 to open the corresponding carrier cart door.
A particular function of solenoid actuate-deactuate bias adder logic is to maintain the solenoid in an open position for sufficient time to trip the cart compartment door latch. To this end, when a comparison is first achieved by comparator 86, zero is added to the conveyor position count by adder logic 84. Logic 84 then adds nine increments to the count going to the comparator so that comparator 86 will find a comparison for those nine incremental counts thus maintaining the corresponding solenoid in an open position for the appropriate period of time. At the end of the ninth count, the solenoid will be released so as not to open subsequent carrier compartment doors.
Memory 80 of FIG. 2 is adapted to be accessed through all of its word locations in sequence so that all of the computed conveyor positions will be compared with corresponding conveyor position counters during each operation cycle. The maximum number of accesses required for the system described would be 5,796 scan and compare cycles plus 5 accesses per console for each of the 12 consoles. For a resolution increment of one-sixth of an inch and a velocity of 21 inches per second, an appropriate memory would be one with an access time of 1.2 microseconds. Thus, each operational cycle of the scan and compare routine can be accomplished in 7.3 milliseconds. As the size or speed of the system is increased or the increment resolution is made smaller, thereby requiring a shorter operational cycle, faster memories are available for this purpose.
OPERATION OF THE SYSTEM
The system control which is not expressly illustrated in FIG. 2 can best be described in relation to FIGS. 5, 6, and 7 which are flow diagrams respectively of the input console polling sequence, the computation routine and the scan and compare routine. Both the polling of the input console and the scanning and comparing of the computed conveyor position with the zone conveyor position counter are carried out during each incremental advancement of the conveyor chain. Upon detection of the first incremental pulse from increment sensor 20, the polling counter (not shown in FIG. 2) is preset to one, keyboard multiplexer 70 is checked to see if the console specified by the polling counter inserted a parcel. If no such parcel was inserted the polling counter is checked to see if it equals the total number of consoles. If that result is false the polling counter is incremented and the subroutine of checking to see if the parcel has been inserted is repeated.
As indicated to FIG. 5, if a parcel had been inserted before the next poll, the system enters a computation routine in which the keyboard input is translated into the primary, first alternate, and second alternate destination receptacle registers. Then, the terminus conveyor position counts for each of the destination receptacles are computed and stored in memory according to the routine illustrated in FIG. 6 and the polling sequence is reentered to see if the polling count is equal to the total number of consoles. When a polling counter does equal the total number of input consoles, the scan and compare routine of FIG. 7 is entered.
Referring now to FIG. 6, the computation routine will be described. As illustrated therein, the translator memory of translator 72 of FIG. 2 is read at the addresses specified by the console keyboard input and the contents thus obtained are placed in respective primary, first alternate and second alternate registers. The base distance is also read therefrom and added to the polled console conveyor position data. If the cart count in the sum is greater than the total number of carts in the system then that cart count is adjusted to accommodate overflow. Otherwise, the results of the conveyor position computation, the associated receptacle number and the zone number are stored in computed conveyor position memory 80. At this point, memory 80 is checked to see if the first alternate receptacle conveyor position count has been computed. If it has been, the the second alternate receptacle conveyor position count is checked to see if it has been computed. If that check is true, then the sequence reenters the polling sequence as described in reference to FIG. 5. If neither of the alternate receptacle conveyor position counts have been computed or only one of them has been computed, then base distance memory 79 is read at an address specified by one or the other of the alternate receptacle registers 74 or 75 and the system reenters the computed conveyor position routine as indicated in FIG. 6.
Referring now to FIG. 7, the scan and compare routine will be described. It will be remembered from the discussion of FIG. 5 that the scan and compare routine is entered each time after all of the input console counters have been polled. At the beginning of the routine, the memory address counter of memory 80 of FIG. 2 is preset to the first address thereof to obtain a receptacle number, a zone sensor number and a computed conveyor position. As was explained above in reference to FIG. 2, the conveyor position counter associated with the zone sensor of the zone specified is then sampled and the counter reading is compared to the computed conveyor position. If there is identity between the counter reading and the computed position, the solenoid of the receptacle specified is actuated. If the sampled conveyor position count equals the computed conveyor position count plus nine increments, the solenoid associated with the receptacle is deactuated but not until then. The memory address counter memory 80 is then checked to see if it specifies the end address of that memory. If it does not then the address counter is incremented and the scan and compare routine is repeated. Once the memory address counter does equal the end address, the scan and compare routine is exited and the input console polling sequence is reentered.
With the system thus described, the number of zone sensors can be so chosen that the distance between adjacent sensors is less then the minimum distance over which maximum tolerable error can occur due to wearing, stretching or slack in the conveyor belt or chain.
Data is available to indicate the order of magnitude of such error. Normal manufacturing tolerance for a 105 inch section of chain is plus or minus 0.015 inches for a maximum variation of 0.965 inches in a 282 foot long chain. For the same length of chain, chain stretch under a starting load of 300 pounds can amount to 1.125 inches. Wear at each pin and bushing can reach up to 0.005 inches per pin over the entire life span of a chain for an amount of 11.2 inches of variation in the same length of chain. The accumulation of these variations in a 282 foot chain could amount to a total of 13,290 inches. It will be observed that a major portion of this variation is due to wear between pins and bushings.
A typical carrier cart as anticipated for the present invention would have compartments spaced 1.5 inches apart. Assuming that the door-opener solenoid is to be actuated for 1.5 inches of cart compartment advancement, the maximum variation of chain length that can be tolerated without the solenoid actuating the door catch of either a later cart or earlier cart is approximately 0.732 inches. Comparing this variation to the maximum variation for 282 foot length chain, indicates that it is sufficient to place monitoring sensors approximately 15.5 feet apart. These calculations are for carrier velocity of 21 inches per second where the solenoid rise time is 15 milliseconds and the fall time is 20 milliseconds.
While but one embodiment of the present invention has been disclosed, it will be apparent to those skilled in the art that additions and modifications may be made therein without departing from the spirit and scope of the invention as claimed.