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Flow sensors in newspaper production
OMEGA Int. J. of MgmtSci., Vol. 20, No. 5/6, pp. 631-639, 1992 Printed in Great Britain. All rights reserved
0305-0483/92 $5.00+ 0.00 Copyright© 1992PergamonPressLtd
Flow Sensors in Newspaper Production SAMEER K U M A R SANT A R O R A University of Minnesota, USA (Received October 1991; in revisedform December 1991) The paper evaluates potential savings in a newspaper production operation achieved by reducing errors in flow sensor counts at stackers, where newspaper bundles are made for distribution to customers. The reduction in count errors is achieved by installing extra flow sensors at various transfer points in the production flow network and using their counts along with using the deducible information from flow conservation equations. As the product flows in the production network, more diversity within the product fine is introduced due to variations in inserts. Stop rules for a product inflowing into multiple reservoirs are developed which would minimize expected over production. More precise projections for product completion times are generated, which allow timely corrective actions. State sensors are installed, which give real time information about state of each equipment as to whether it is in an operational or failed mode. Knowledge of the state of the system provides ability to promptly generate, on a real time basis, alternative system configuration substituting redundant operational units for the failed units.
Key words--control processes,information, optimization, control, inventory, production, materials
INTRODUCTION NEWSPAPER PUBLISHING is a time-critical operation. Several hundred thousand copies of the newspaper have to be published and distributed to consumers within a span of a few hours. Furthermore, there are multiple products involved in a daily edition due to variations in the sections, supplements and other inserts that are included in the newspaper for different zones. Product switch-overs are time consuming and expensive. Unnecessary product switchovers should be avoided. Any section of a product printed in a given shift (advance or live, defined in the next section) should ideally be produced in a single uninterrupted run. Determination of "when to stop production of a p r o d u c t " so that availability of the desired number of good copies of the product is ensured without excessive overruns would require flow sensors which could count number of copies with reasonable accuracy. Errors in these coun-
ters would require producing some extra papers as safety stocks. Even small errors result in substantial overruns as the product line is moving at a speed of about 25 miles per hour; and these overruns end up being a part of waste. The study was conducted at Star Tribune, a leading daily newspaper in the state of Minnesota. Currently flow sensors are provided at all outflow points of presses, and at inflow points of stackers. At stackers, papers are stacked into bundles. Counting process at stackers input points is somewhat more difficult than counting at the press outflow points. Current press counters are quite accurate, however, current acujet stacker counters, which were installed four years ago, do not give accurate counts. As shown in Fig. 1, papers are transferred from the main press conveyor line onto the laydown belt conveyor prior to their release to the stackers. Papers overlap each other. The airflow ejection counts the overlap 631
Kumar, Arora--Flow Sensors in Newspaper Production
632
Flow sensor®
> Outflow to other stackers,drums or winders
I
e vey°I Press
Press
®
Flow sensor at release point of each stacker >
~
.....
~
.........
~_.
I <
Laydown conveyor
Acujet counter ®
~-\ \
Stacker Fig. 1. Product flow and flow sensors locations.
points. A correct count requires the intertime between two consecutive overlap points to synchronize with the airflow ejection timings from the acujet counter. The counts are missed if overlap points flow timings across the count point do not coincide with the airflow ejection timings. Currently, there are no flow sensors provided at the front end of stackers. These additional sensors would help in timely monitoring the deviation between the product count released to a stacker and the product count reaching the stacker, and control charts monitoring this deviation will be used for re-adjusting of the overlap setting among consecutive papers by adjusting speeds of press conveyor and laydown belt conveyor. This paper estimates annual expected savings that would be realized with more accuracy in stacker inflow counting. Additional flow sensors at transfer points of the product flow network are proposed, which would help quantify waste being generated at various locations. Some of the sensors are redundant, if counters are assumed to be accurate and conservation flow equations are assumed to hold. However, with a possible scope of errors in counters, redundant counts will provide a check against count errors. Production and distribution operations for a newspaper involve a large number of interdependent activities which are carried out in a dynamic environment where many unforeseen events do happen. A great deal of co-ordination among various operations can be achieved through good forecasting models, sound company-wide data bases and knowledge bases
and sound intermediate and long-term planning based on the statistical performance information. Static policies and decisions, however, do not completely substitute for dynamic controls. Dynamic controls in a newspaper production operation require continuous monitoring of product flow and an on-going knowledge of the state of the system components [2, 4, 7, 8] as a pre-requisite. State monitoring sensors [5] will provide information about the status of the system components. Ideally, dynamic controls should operate within the framework of static optimization, and dynamic controls should strive to handle dynamic events without too excessive penalties. Most importantly dynamic controls should be procedurized, as far as possible, rather than these being carried out in a fire-fighting mode. DESCRIPTION OF THE NEWSPAPER PRODUCTION OPERATIONS
The production of a newspaper may be viewed as a product flow through a network comprised of nodes and arcs analogous to the traditional PERT network [6]; nodes representing events signifying completion of various operations and arcs representing the flow from one event to another. Different facilities are involved in carrying out various operations. Examples of facilities are automatic guided vehicles, presses, inserting drums, stackers, winders, unwinders, conveyor lines, and examples of arcs are operations such as printing, inserting, stacking, bundling, despatching, distribution, etc.
Omega, Vol. 20, No. 5/6
633
Daily newspaper I
Metro Dealer
Metro Home
I
Zone
l...Zone
6 Zone
State
Mail
I
I
1 .... Z o n e
6 Zone
21 Zone
l...Zone
i...Zone21
I
& Zone 1
..........
Zone23
Fig. 2. P r o d u c t l i n e s o f t h e daily newspaper.
The newspaper operation being studied has four editions--Metro dealer, Metro home, Mail, and State. Each of these editions has further several product lines for various state and city marketing zones as shown in Fig. 2. The two important factors that influence revenues of a newspaper company are, a timely delivery of the paper to the customers and more
inclusion of the important late occurring news. In order to accomplish these objectives efficiently, the production work is divided into advance and live runs. Sections which do not include late occurring news are published as part of advance run shift. Efforts are made to keep the size of live-runs as small as possible. The work flow through various work centers for a typical day is displayed in Fig. 3.
WORK FLOW Advance Run Shift
Live Run Shift
AM Tc=12 : 00
Desired completion for various editions M e r g e of Advance and live runs
TA=TB=4 : 30
I
M-C,C'
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I-B'
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1:30 PM
___
i0 PM
10:15 PM
11:15
12:30 AM
3 3:30
4:30 TIME
Fig. 3. PERT network flow diagram with earliest start times and latest completion times for various editions on a week day. The following notation is used: TR = desired completion time of edition R; f---denotes an operation. The entries following the hyphen denote the operands. The operands cover advance or live runs of various editions. The following is the list of operations: P--printing; W---winding; /--inserting of supplements and/or merging; S--stacking papers into bundles; L--labeling addresses on mail edition; L D - - l o a d i n g and despatching of papers to remote depot locations; M - - m e r g i n g of advance and live runs of an edition. The following is the list o f operands: A - - a d v a n c e run o f home delivery edition; A '--live run of home delivery edition; B--advance run o f dealer edition; B'--live run of dealer edition; C - - a d v a n c e run of mail edition; C'--live run of mail edition; D - - a d v a n c e run of state edition; D'--live run o f state edition.
634
Kumar, Arora--Flow Sensors in Newspaper Production Waste
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Fig. 4. Copy flow through the production network. [® Point of measurement (flow sensor); ---, points of waste; <> flow distribution point.] P--press; T--transfer point; D l ~ r u m I; D2---drum 2; Wl--winder 1; W2--winder 2; S1---stacker 1; S2--stacker 2.
Two important factors which determine work load on the press operations are size of the newspaper and size of its circulation. Size of the newspaper varies from day-to-day, and it does not follow a weekly pattern. Size o f circulation also varies from day-to-day but it stays stable from week to week. Each press has multiple units. The pages of a newspaper are distributed among various units of a press. The printing of a paper is constrained within the units of a press. Multiple presses will be used when number of copies of an edition exceed the capacity of a press within the available production time span. Hence, variation in the newspaper size affects the number o f units that are required to be operated per press, whereas variation in circulation affect the number of presses to be operated. Variations in both the number of operating units per press and the number o f operating presses influence the work force required. Start times and completion times for various editions are prescribed from marketing considerations. However, small adjustments in start times and completion times are considered if these adjustments result in lowering peak loads significantly [6]. Some important long-term decisions are: • Designing o f the product lines. • Determining the total number of presses needed in the system. • Planning to reduce delays due to noncritical failures.
• Planning to reduce the quantity of waste copies produced. • Conducting sensitivity analysis to prescribe earliest start times and latest completion times of various products. Some important operational decisions are: • Determining the number of presses to run during live and advance runs on any day. • Scheduling crew for the day. DESIGN AND LOCATION OF SENSORS The system has two types of sensors--flow sensors and state sensors: • A flow sensor counts the number of copies o f a product that pass the sensor. A state sensor is provided for each important equipment. It gives information about the state of the equipment. An equipment can be in one o f the following three modes--operational active, operational non-active, and failed mode (critical and non-critical). A wide variety of sensors are available in the marketplace which include retro-reflective photoelectric cells, proximity inductive sensors,
Omega, Vol. 20, No. 5/6
635
Table 1. Categories and sub-categories of waste Categories I. Waste not affected by sensors
II. Waste affected by sensors
Sub-categories 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 1. 2. 3. 4.
Brief explanation
Reel wrapper Reel core White Paster Dock spoils Tray spoils Truck spoils Startup Plate quality Color quality
Paper removed along with kraft wrapper Paper left on reel core Paper used to web press Papers dropped during paster cycle Papers spoiled as loaded into truck Papers spoiled by tray system Papers spoiled in truck--wet, etc. Waste during press startup Printed waste because of bad impression Printed waste because of bad color
Overruns Mail room Shutdown Circulation
Papers printed in excess of desired quantities Papers spoiled or lost in mail room Papers dropped during press shutdown Padding to make-up for the deficiencies in some short bundles and also cover-up wastage during handling
magnetic sensors, load cells, lasers, circuit breakers, limit switches, etc. [5]. These sensors may be classified into basically three types: optical sensors, mechanical-electrical sensors, and proximity detectors. Optical sensors are used in the prepress and mailroom operations. Photoelectric cells and filters are used in color separation systems to determine the degree of correct exposures. Photoelectric cells, which read laser beams, are also used to detect the presence of newspapers in a belt-stream conveyor. Retro-reflective sensors have been used in the bundle distribution system to count bundles, and in bottomwrapping machines to sense the bundle. They are also used in belt-stream conveying systems to detect gaps between products going through the inserting process. Limit switches are mechanical-electrical sensors. Once a moving object comes into contact with a limit switch, it engages and completes a circuit, the movement of the object ceases. Such sensors are used on presses, for instance, to verify whether ink-adjustment has been made. Load cells are another type of mechanical-electrical sensor. Count verification equipment in the mail room use these sensors. A finger switch has a finger that hangs down. It makes an excursion when it touches a newspaper in the belt stream and a count is registered. Proximity detectors use sound, light, or air to sense the presence of something. There are sensors which rely on magnetism. Magnetic proximity sensors have been used in platemaking system to count the plates being made and to determine how long it is taking. These sensors are also used in the arms of reel stands
of presses to monitor the speeds o f both the expiring roll and the roll to be spliced. Infrared sensors are used in automatic roll-loading systems with newsprint mounted on reel stands. There are numerous systems where combinations of sensors are used. For example, retroreflective photoelectric cells are used in mailroom inserting equipment to spot instances of misses. Limit switches are used to detect double insertions, and proximity sensors detect the height of the preprint inserts loaded into the input hoppers. Figure 4 shows the key locations for flow and state sensors. Some conservation equations hold for flow. An adequate redundancy is provided while determining the location of flow sensors, so as to provide a means for checking for flow sensor count errors. SOURCES OF WASTAGE Newsprint waste is generated at various stages [9]. Waste may be classified into two categories--waste of types whose quantities are not going to be reduced by sensors, and waste of types whose quantities are going to be reduced by sensors. Table 1 gives waste categories and subcategories along with a brief explanation of each subcategory. IMPACT OF FLOW AND STATE SENSORS IN OPERATIONAL MANAGEMENT DECISIONS Stop-production decisions for different products, carried out without flow sensors will result in more frequent under or over production of a product [10] and this will cause unnecessary
Kumar, Arora--Flow Sensors in Newspaper Production
636
Table 2. Possible production equipment combinations for various production operations utilizing various presses Operations/equipment
Presses Red press
White press Blue press
Yellow press
Print, insert, stack
Print, insert, wind
RI, DI, S2 R1, D2, $6 WT2, D3, SI0 WT2, D4, S14 B3, D1, $2 B3, D2, $6 B3.4, D3, SI0 B3.4, D4, S14 Y3.4, D3, SI0 Y3.4, D4, S14
R1, DI, W1 RI, D2, W2 WT2, D3, W3
Print, wind
Print, stack
RI, WI RI.1, W5
R1, S2 RI.I, S15
WT2, W3
WT2, $6
B3, W2
B3, $7
B3.4, W4
B3.4, S10
Y3.4, W4
Y3.4, S10
G5, I¥5
G5, S18
WT2, D4, W4 B3, D1, W1 B3, D2, W2 B3.4, D3, W3 B3.4, D4, W4 Y3.4, D3, W3 Y3.4, D4, W4
Green press
Rl--Red press conveyor 1; Rl.l--red press conveyor 1.1; WT2--white press conveyor 2; B3--blue press conveyor 3; B3.4---blue press conveyor 3.4; Y4--yellow press conveyor 4; G5---green press conveyor 5; Dn---drum I-4; Wn--winder 1-5; Sn--stacker 1-18.
product switchovers in cases of under production. Furthermore, absence of flow sensors will not allow giving advance preparatory signals for product changeovers. Advance signals help in reducing product switchover times. Flow sensors also help in more precisely projecting timings of important events, as for example, product completions, truck loadings of products, and in comparing actual truck departure times with scheduled departure times. Statistical information collected from flow and state sensors will assist in improving long-term planning decisions from static optimization point of view. FAILURE MINIMIZATION AND CONTROL OF THE PRODUCT FLOW NETWORK
A flow sensor is installed in the flow network at nodes where product flow distribution occurs. Control charts [1, 3] monitoring quantities of waste generated at various locations will allow timely corrective actions when waste quantities start exceeding their normal rates. State sensors reporting equipment status will allow configuring alternative paths promptly, substituting alternative operational units (as shown in Table 2 and in Fig. 5) for the failed units. It will be possible to determine more appropriate levels of redundancies for various equipment from an overall economic consideration. MATHEMATICAL MODEL FOR STOP RULES
The most critical element in the dynamic control of flow is establishing production stop rules for various products. The model considers
inflow of a product coming from different paths into multiple finished reservoirs. For simplicity, we consider flow along two paths into two finished reservoirs. Flow sensors are provided at the input points to the reservoirs as shown in Fig. 6. The extension to a case with multiple reservoirs can be carried out easily. The objective of the model is to develop production stop rule which would ensure the availability of the required number of good copies of a product with a specified level of reliability, with an uninterrupted production run while minimizing the expected production overrun quantity. The model is described in the Appendix. REDUCTION OF WASTE WITH ERROR-FREE FLOW SENSORS AT RESERVOIRS
In the current system, flow sensors installed at the press outflow points give an accurate master count of copies going into mailroom. There are no flow sensors at all the transfer points in the flow network. Flow sensors in the reservoirs are not accurate. We illustrate below expected reduction in waste due to overruns for the metro edition, achieved in stacker counts with flow sensors measuring inputs coming into two reservoirs. The following values are estimated for the parameters in the model. In the standard deviations expressions, to denotes the average completion time for the metro edition. R1 = 50,000 copies/hour,
copies/hour,
R2 = 45,000
Omega, Vol. 20, No. 5/6
637
Press Failure
Critical I v Before cutoff point
Non-critical After cutoff point
v J u m p to available press otherwise wait
Before cutoff point
v No action
v
v Wait till fixed
After cutoff point v No action
Press Conveyor Failure
I
V Critical I v Before cutoff point n
v
After cutoff point
Non-critical v After cutoff point
Before cutoff point
h
Alternate conveyor with winder available
Alternate conveyor with w i n d e r not available
.Use alternate conveyor .Unwind on same drums to p r o d u c e final product .Later jump to another available press
No Alternate [v~ Alternate No action conveyor | | c o n v e y o r action with I I with winder I I w i n d e r not available
availablel I
v ~ .Use Wait alternate till fixed conveyor .Unwind on same drums to produce final p r o d u c t .Later when problem fixed, get back to normal
v Jump to available press otherwise wait
Drum/Drum Conveyor Failure
1 Critical 1 v Before cutoff point v .Partly or totally wind w i t h o u t insert .Unwind w o u n d p o r t i o n and insert at other available drum(s)
INon-critical After cutoff point v No action
Before cutoff point
v
After cutoff point
v .Partly or No t o t a l l y wind action without insert .Fix the p r o b l e m .Unwind w o u n d p o r t i o n and insert at other a v a i l a b l e drum(s)
Fig. 5. Operational decision trees used on equipment failure.
638
Kumar, Arora--Flow Sensors in Newspaper Production
v ®
o
2
1
J
J
I
I
$2 S1 Fig. 6. Flow of product j along two paths (® flow sensors).
A = 0.25 hour, Nj = 370,000, z~ = 2, atV/~0 = 5000, a2V/~0 = 4000, a'l = 500, at = 300 where al x/~0, and a2x/~0 are standard deviations of the total observed counts for the two reservoirs. In these estimates, we have assumed A = 0. It may be recalled the A denotes the stopping time difference for the two presses. Putting values of various variables in equation (11), (16) and (17), we have a(t0)=6430, k l = 196,000, and k 2 = 187,000 giving an expected waste estimate of 8000 copies which translate into 2.13%. At $0.25 per copy, daily expected waste reduction in dollars on a typical week day for the metro daily edition amounts to $2000. Annually, the expected waste savings in dollars due to overrun waste reduction for all the products is estimated at $1.42 million. The efficiencies achievable with state sensors are being reported in a separate paper. The sensor project is a part of the mailroom totalizer project. The totalizer project is aimed at improving overall integration among operations within pressroom, mailroom, distribution and despatch. The project is in the final stages of planning and design, and is expected to be implemented within the following 18 months.
to overrun are expected. The flow sensor would also allow generating production completion time projections more precisely and improve planning for corrective actions in cases of delays. When different component failures are reported, alternative system configurations would be generated, substituting available redundant operational units for the failed units. As a consequence, scheduling, sequencing, and despatching operations would improve significantly. Long-term statistics generated from the flow sensors will allow improving both longterm planning and day-to-day operational management decisions. Programs have been initiated within the company to lower non-critical failures and reduce waste category which is not affected by sensors. APPENDIX
Mathematical Model for Estimating Waste Redaction In the current system, sensors are not provided at all transfer locations. Furthermore, sensors in stackers are not accurate. There is a discrepancy between the observed counts and the true counts. Let the observed and the true counts at any time t of product j into reservoirs S 1 and $2 be given by the following equations: nl(j, S1, t) = n~(j, SI, t) + el(t )
(1)
n2(j, $2, t) = n°(j, $2, t) + e2(t)
(2)
where nl(j, S1, t), n2(j, $2, t) are the observed counts and n°(j, SI, t), n°(j, $2, t) are the true counts of inflow at time t into reservoirs S 1 and $2 respectively. The errors el(t), e2(t) are assumed to follow normal distribution with means zero and standard deviations alx/~ and a2x/~ respectively. The time is measured from the start time of production for the product. Let Xj, I, Xj,2 be the observed counts of the product in the pipelines of paths 1 and 2
CONCLUSIONS Integration among various inter-dependent functions can be improved by continuously monitoring product flow with flow sensors and monitoring equipment status with state sensors. Real time information about the state of the system would allow more appropriately matching operational decisions with the state of the system. With increased accuracy of the stacker flow sensor, significant reductions in waste due
>
A B <--z~o--> 0 0 Nj Vj (t) +Xj,I+Xj, 2
Y(t)
Fig. 7. Normal distribution of total real product counts. Points A and B are given by A = N j and B = V j ( t ) + X°, + X° 2.
639
Omega, 1Iol. 20, No. 5/6
respectively. The pipeline quantities are time independent random variables. The observed and true counts of the pipeline quantities are related by the following equations:
Pr[{ Vj(t) - z,a(t )} >>-Nj - X° l - X~j,2] = 1 - c~ P r [ ~ ( t ) t> N j - X°~ - X°2 + z,cr(t)] = 1 - ~
Deterministic equivalent of constraint (12) is given by
Xj, l = X ° , +e~
(3)
Vj(t) = N i -- X°I -- X°2 + z,a(t)
Xj.2= X°2 + e'2
(4)
Assuming R 1 and R2 to be the inflow rates in the pipelines along paths 1 and 2, we have
where X),0 l, ~.0j. 2 are true counts of product j in pipelines of paths 1 and 2 respectively and e',, e~ are errors with means zero and standard deviations a~ and o ~respectively. Approximate minimization of the objective function is achieved by following the rule that a A time difference is kept between the sequential stopping of the two inflows, and the inflow to the reservoir with larger counter error is stopped first. The true number of total units of product j , Y ( t l , t 2 ) , in the two reservoirs and the pipelines is given by
R l ( t -- A) + R2(t) = Vj(t) = Uj - X ° , - X~2 + z~o(t)
or (RI + R2)t -- R I A = N j - X ° 1 - )(°2 + z,a(t)
or t = { N j - X°~ - X°z + z,a(t) + R 1A}/(R1 + R2) (13)
We have,
Let Nj be the number of copies that are desired to be produced. The stopping rule is concerned with finding numbers k 1, k2 for the stacker counters into reservoirs S1 and $2 respectively at which production of product j will be stopped. These numbers should minimize expected overrun size E ( Y ( t , , t2) - Nj)
(6)
and satisfy the following constraints Pr(Y(t~, t2) >/Nj) = 1 - ~
(7)
nl(j, S l , tl) >t k l
(8)
n2(j, $2, t2) >1k2
(9)
The minimization of the expected overrun size is approximately achieved by following the A difference rule in the stopping times of the two flows. The problem thus reduces to finding feasible solution of the constraints (7), (8), and (9). Equivalently, we want to find the values of k 1, k 2 in terms of observed counter values which satisfy constraint (7). Putting tt = t - A, and t2 = t, constraint (7) can be re-written as Pr[{nl(j, SI, t - A) + n2(j, $2, t) --e,(t-A)--e2(t) }>~uj-X°l-)(°21=l-~
(10)
Assuming Wiener process for the observed flow counts into reservoirs S1 and $2, the variance az(t) of the total product counts in the system at time t (as shown in Fig. 7) is given by t r 2 ( t ) = t r ~ . ( t - A ) + t r 2 . t +a~2+tr~2
(11)
Putting n~(j, S 1 , t - A) + n2(A $2, t) = Vj(t) the constraint (10) may be written as
k l = R l ( t - A)
(14)
k2 = R2(t)
(15)
and
Y(fi, t2) = n°(j, S I , t.) + n°(j, $2, t2) + X °, + X°2 (5)
OME 20/5-~-G
(12)
Substituting the value of t given by expression (13) into equations (14) and (15), we have k l = R I ( N j + z , a ( t ) - R2A)/(RI + R2)
(16)
k2=R2(Nj+z~a(t)+RIA)/(RI+R2)
(17)
and As mentioned earlier, these stopping rules can be easily extended to the case with multiple reservoirs. REFERENCES 1. Baer T (1990) Advanced interfaces show information, not data. Managing Automation (January), 26-28. 2. Burg L (1990) Information processing in the mailroom. Ferag Company's Customer Note, 4-5. 3. Gould L (1990) Process control expands outward and upward. Managing Automation (January), 22-24. 4. IDAB Inc. Product Brief (1989) Newspaper Production Totalizing System. Publication A2151-10/89. 5. Kruglinski P (1986) Largely hidden from view, sensors are newspaper sentries. Presstime (February), 1t-13. 6. Kumar S and Arora S (1991) Improved manpower utilization in live-run shift of newspaper printing operations, l i E Trans. J. Forthcoming. 7. Monitor displays computer-controlled mailroom operations (1974) Newspaper Prodn (February), 4~47. 8. Owen E (1990) L A Times counts on totalizer. Newspapers and Technol. (February). 9. Scheidler PJ (1990) Newspaper conservation at the Providence Journal. Newspaper Systems Group's Report, Newport, RI. 10. Truitt RC (1990) Shortages and overruns plague accountability. Presstime (August), 6-8. ADDRESSFOR CORRESPONDENCE: Professor S Arora, Department o f Mechanical Engineering, University o f Minnesota, 111 Church Street, Room 125, Minneapolis, M N 55455, USA.
0305-0483/92 $5.00+ 0.00 Copyright© 1992PergamonPressLtd
Flow Sensors in Newspaper Production SAMEER K U M A R SANT A R O R A University of Minnesota, USA (Received October 1991; in revisedform December 1991) The paper evaluates potential savings in a newspaper production operation achieved by reducing errors in flow sensor counts at stackers, where newspaper bundles are made for distribution to customers. The reduction in count errors is achieved by installing extra flow sensors at various transfer points in the production flow network and using their counts along with using the deducible information from flow conservation equations. As the product flows in the production network, more diversity within the product fine is introduced due to variations in inserts. Stop rules for a product inflowing into multiple reservoirs are developed which would minimize expected over production. More precise projections for product completion times are generated, which allow timely corrective actions. State sensors are installed, which give real time information about state of each equipment as to whether it is in an operational or failed mode. Knowledge of the state of the system provides ability to promptly generate, on a real time basis, alternative system configuration substituting redundant operational units for the failed units.
Key words--control processes,information, optimization, control, inventory, production, materials
INTRODUCTION NEWSPAPER PUBLISHING is a time-critical operation. Several hundred thousand copies of the newspaper have to be published and distributed to consumers within a span of a few hours. Furthermore, there are multiple products involved in a daily edition due to variations in the sections, supplements and other inserts that are included in the newspaper for different zones. Product switch-overs are time consuming and expensive. Unnecessary product switchovers should be avoided. Any section of a product printed in a given shift (advance or live, defined in the next section) should ideally be produced in a single uninterrupted run. Determination of "when to stop production of a p r o d u c t " so that availability of the desired number of good copies of the product is ensured without excessive overruns would require flow sensors which could count number of copies with reasonable accuracy. Errors in these coun-
ters would require producing some extra papers as safety stocks. Even small errors result in substantial overruns as the product line is moving at a speed of about 25 miles per hour; and these overruns end up being a part of waste. The study was conducted at Star Tribune, a leading daily newspaper in the state of Minnesota. Currently flow sensors are provided at all outflow points of presses, and at inflow points of stackers. At stackers, papers are stacked into bundles. Counting process at stackers input points is somewhat more difficult than counting at the press outflow points. Current press counters are quite accurate, however, current acujet stacker counters, which were installed four years ago, do not give accurate counts. As shown in Fig. 1, papers are transferred from the main press conveyor line onto the laydown belt conveyor prior to their release to the stackers. Papers overlap each other. The airflow ejection counts the overlap 631
Kumar, Arora--Flow Sensors in Newspaper Production
632
Flow sensor®
> Outflow to other stackers,drums or winders
I
e vey°I Press
Press
®
Flow sensor at release point of each stacker >
~
.....
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Stacker Fig. 1. Product flow and flow sensors locations.
points. A correct count requires the intertime between two consecutive overlap points to synchronize with the airflow ejection timings from the acujet counter. The counts are missed if overlap points flow timings across the count point do not coincide with the airflow ejection timings. Currently, there are no flow sensors provided at the front end of stackers. These additional sensors would help in timely monitoring the deviation between the product count released to a stacker and the product count reaching the stacker, and control charts monitoring this deviation will be used for re-adjusting of the overlap setting among consecutive papers by adjusting speeds of press conveyor and laydown belt conveyor. This paper estimates annual expected savings that would be realized with more accuracy in stacker inflow counting. Additional flow sensors at transfer points of the product flow network are proposed, which would help quantify waste being generated at various locations. Some of the sensors are redundant, if counters are assumed to be accurate and conservation flow equations are assumed to hold. However, with a possible scope of errors in counters, redundant counts will provide a check against count errors. Production and distribution operations for a newspaper involve a large number of interdependent activities which are carried out in a dynamic environment where many unforeseen events do happen. A great deal of co-ordination among various operations can be achieved through good forecasting models, sound company-wide data bases and knowledge bases
and sound intermediate and long-term planning based on the statistical performance information. Static policies and decisions, however, do not completely substitute for dynamic controls. Dynamic controls in a newspaper production operation require continuous monitoring of product flow and an on-going knowledge of the state of the system components [2, 4, 7, 8] as a pre-requisite. State monitoring sensors [5] will provide information about the status of the system components. Ideally, dynamic controls should operate within the framework of static optimization, and dynamic controls should strive to handle dynamic events without too excessive penalties. Most importantly dynamic controls should be procedurized, as far as possible, rather than these being carried out in a fire-fighting mode. DESCRIPTION OF THE NEWSPAPER PRODUCTION OPERATIONS
The production of a newspaper may be viewed as a product flow through a network comprised of nodes and arcs analogous to the traditional PERT network [6]; nodes representing events signifying completion of various operations and arcs representing the flow from one event to another. Different facilities are involved in carrying out various operations. Examples of facilities are automatic guided vehicles, presses, inserting drums, stackers, winders, unwinders, conveyor lines, and examples of arcs are operations such as printing, inserting, stacking, bundling, despatching, distribution, etc.
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Daily newspaper I
Metro Dealer
Metro Home
I
Zone
l...Zone
6 Zone
State
I
I
1 .... Z o n e
6 Zone
21 Zone
l...Zone
i...Zone21
I
& Zone 1
..........
Zone23
Fig. 2. P r o d u c t l i n e s o f t h e daily newspaper.
The newspaper operation being studied has four editions--Metro dealer, Metro home, Mail, and State. Each of these editions has further several product lines for various state and city marketing zones as shown in Fig. 2. The two important factors that influence revenues of a newspaper company are, a timely delivery of the paper to the customers and more
inclusion of the important late occurring news. In order to accomplish these objectives efficiently, the production work is divided into advance and live runs. Sections which do not include late occurring news are published as part of advance run shift. Efforts are made to keep the size of live-runs as small as possible. The work flow through various work centers for a typical day is displayed in Fig. 3.
WORK FLOW Advance Run Shift
Live Run Shift
AM Tc=12 : 00
Desired completion for various editions M e r g e of Advance and live runs
TA=TB=4 : 30
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Fig. 3. PERT network flow diagram with earliest start times and latest completion times for various editions on a week day. The following notation is used: TR = desired completion time of edition R; f---denotes an operation. The entries following the hyphen denote the operands. The operands cover advance or live runs of various editions. The following is the list of operations: P--printing; W---winding; /--inserting of supplements and/or merging; S--stacking papers into bundles; L--labeling addresses on mail edition; L D - - l o a d i n g and despatching of papers to remote depot locations; M - - m e r g i n g of advance and live runs of an edition. The following is the list o f operands: A - - a d v a n c e run o f home delivery edition; A '--live run of home delivery edition; B--advance run o f dealer edition; B'--live run of dealer edition; C - - a d v a n c e run of mail edition; C'--live run of mail edition; D - - a d v a n c e run of state edition; D'--live run o f state edition.
634
Kumar, Arora--Flow Sensors in Newspaper Production Waste
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Fig. 4. Copy flow through the production network. [® Point of measurement (flow sensor); ---, points of waste; <> flow distribution point.] P--press; T--transfer point; D l ~ r u m I; D2---drum 2; Wl--winder 1; W2--winder 2; S1---stacker 1; S2--stacker 2.
Two important factors which determine work load on the press operations are size of the newspaper and size of its circulation. Size of the newspaper varies from day-to-day, and it does not follow a weekly pattern. Size o f circulation also varies from day-to-day but it stays stable from week to week. Each press has multiple units. The pages of a newspaper are distributed among various units of a press. The printing of a paper is constrained within the units of a press. Multiple presses will be used when number of copies of an edition exceed the capacity of a press within the available production time span. Hence, variation in the newspaper size affects the number o f units that are required to be operated per press, whereas variation in circulation affect the number of presses to be operated. Variations in both the number of operating units per press and the number o f operating presses influence the work force required. Start times and completion times for various editions are prescribed from marketing considerations. However, small adjustments in start times and completion times are considered if these adjustments result in lowering peak loads significantly [6]. Some important long-term decisions are: • Designing o f the product lines. • Determining the total number of presses needed in the system. • Planning to reduce delays due to noncritical failures.
• Planning to reduce the quantity of waste copies produced. • Conducting sensitivity analysis to prescribe earliest start times and latest completion times of various products. Some important operational decisions are: • Determining the number of presses to run during live and advance runs on any day. • Scheduling crew for the day. DESIGN AND LOCATION OF SENSORS The system has two types of sensors--flow sensors and state sensors: • A flow sensor counts the number of copies o f a product that pass the sensor. A state sensor is provided for each important equipment. It gives information about the state of the equipment. An equipment can be in one o f the following three modes--operational active, operational non-active, and failed mode (critical and non-critical). A wide variety of sensors are available in the marketplace which include retro-reflective photoelectric cells, proximity inductive sensors,
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Table 1. Categories and sub-categories of waste Categories I. Waste not affected by sensors
II. Waste affected by sensors
Sub-categories 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 1. 2. 3. 4.
Brief explanation
Reel wrapper Reel core White Paster Dock spoils Tray spoils Truck spoils Startup Plate quality Color quality
Paper removed along with kraft wrapper Paper left on reel core Paper used to web press Papers dropped during paster cycle Papers spoiled as loaded into truck Papers spoiled by tray system Papers spoiled in truck--wet, etc. Waste during press startup Printed waste because of bad impression Printed waste because of bad color
Overruns Mail room Shutdown Circulation
Papers printed in excess of desired quantities Papers spoiled or lost in mail room Papers dropped during press shutdown Padding to make-up for the deficiencies in some short bundles and also cover-up wastage during handling
magnetic sensors, load cells, lasers, circuit breakers, limit switches, etc. [5]. These sensors may be classified into basically three types: optical sensors, mechanical-electrical sensors, and proximity detectors. Optical sensors are used in the prepress and mailroom operations. Photoelectric cells and filters are used in color separation systems to determine the degree of correct exposures. Photoelectric cells, which read laser beams, are also used to detect the presence of newspapers in a belt-stream conveyor. Retro-reflective sensors have been used in the bundle distribution system to count bundles, and in bottomwrapping machines to sense the bundle. They are also used in belt-stream conveying systems to detect gaps between products going through the inserting process. Limit switches are mechanical-electrical sensors. Once a moving object comes into contact with a limit switch, it engages and completes a circuit, the movement of the object ceases. Such sensors are used on presses, for instance, to verify whether ink-adjustment has been made. Load cells are another type of mechanical-electrical sensor. Count verification equipment in the mail room use these sensors. A finger switch has a finger that hangs down. It makes an excursion when it touches a newspaper in the belt stream and a count is registered. Proximity detectors use sound, light, or air to sense the presence of something. There are sensors which rely on magnetism. Magnetic proximity sensors have been used in platemaking system to count the plates being made and to determine how long it is taking. These sensors are also used in the arms of reel stands
of presses to monitor the speeds o f both the expiring roll and the roll to be spliced. Infrared sensors are used in automatic roll-loading systems with newsprint mounted on reel stands. There are numerous systems where combinations of sensors are used. For example, retroreflective photoelectric cells are used in mailroom inserting equipment to spot instances of misses. Limit switches are used to detect double insertions, and proximity sensors detect the height of the preprint inserts loaded into the input hoppers. Figure 4 shows the key locations for flow and state sensors. Some conservation equations hold for flow. An adequate redundancy is provided while determining the location of flow sensors, so as to provide a means for checking for flow sensor count errors. SOURCES OF WASTAGE Newsprint waste is generated at various stages [9]. Waste may be classified into two categories--waste of types whose quantities are not going to be reduced by sensors, and waste of types whose quantities are going to be reduced by sensors. Table 1 gives waste categories and subcategories along with a brief explanation of each subcategory. IMPACT OF FLOW AND STATE SENSORS IN OPERATIONAL MANAGEMENT DECISIONS Stop-production decisions for different products, carried out without flow sensors will result in more frequent under or over production of a product [10] and this will cause unnecessary
Kumar, Arora--Flow Sensors in Newspaper Production
636
Table 2. Possible production equipment combinations for various production operations utilizing various presses Operations/equipment
Presses Red press
White press Blue press
Yellow press
Print, insert, stack
Print, insert, wind
RI, DI, S2 R1, D2, $6 WT2, D3, SI0 WT2, D4, S14 B3, D1, $2 B3, D2, $6 B3.4, D3, SI0 B3.4, D4, S14 Y3.4, D3, SI0 Y3.4, D4, S14
R1, DI, W1 RI, D2, W2 WT2, D3, W3
Print, wind
Print, stack
RI, WI RI.1, W5
R1, S2 RI.I, S15
WT2, W3
WT2, $6
B3, W2
B3, $7
B3.4, W4
B3.4, S10
Y3.4, W4
Y3.4, S10
G5, I¥5
G5, S18
WT2, D4, W4 B3, D1, W1 B3, D2, W2 B3.4, D3, W3 B3.4, D4, W4 Y3.4, D3, W3 Y3.4, D4, W4
Green press
Rl--Red press conveyor 1; Rl.l--red press conveyor 1.1; WT2--white press conveyor 2; B3--blue press conveyor 3; B3.4---blue press conveyor 3.4; Y4--yellow press conveyor 4; G5---green press conveyor 5; Dn---drum I-4; Wn--winder 1-5; Sn--stacker 1-18.
product switchovers in cases of under production. Furthermore, absence of flow sensors will not allow giving advance preparatory signals for product changeovers. Advance signals help in reducing product switchover times. Flow sensors also help in more precisely projecting timings of important events, as for example, product completions, truck loadings of products, and in comparing actual truck departure times with scheduled departure times. Statistical information collected from flow and state sensors will assist in improving long-term planning decisions from static optimization point of view. FAILURE MINIMIZATION AND CONTROL OF THE PRODUCT FLOW NETWORK
A flow sensor is installed in the flow network at nodes where product flow distribution occurs. Control charts [1, 3] monitoring quantities of waste generated at various locations will allow timely corrective actions when waste quantities start exceeding their normal rates. State sensors reporting equipment status will allow configuring alternative paths promptly, substituting alternative operational units (as shown in Table 2 and in Fig. 5) for the failed units. It will be possible to determine more appropriate levels of redundancies for various equipment from an overall economic consideration. MATHEMATICAL MODEL FOR STOP RULES
The most critical element in the dynamic control of flow is establishing production stop rules for various products. The model considers
inflow of a product coming from different paths into multiple finished reservoirs. For simplicity, we consider flow along two paths into two finished reservoirs. Flow sensors are provided at the input points to the reservoirs as shown in Fig. 6. The extension to a case with multiple reservoirs can be carried out easily. The objective of the model is to develop production stop rule which would ensure the availability of the required number of good copies of a product with a specified level of reliability, with an uninterrupted production run while minimizing the expected production overrun quantity. The model is described in the Appendix. REDUCTION OF WASTE WITH ERROR-FREE FLOW SENSORS AT RESERVOIRS
In the current system, flow sensors installed at the press outflow points give an accurate master count of copies going into mailroom. There are no flow sensors at all the transfer points in the flow network. Flow sensors in the reservoirs are not accurate. We illustrate below expected reduction in waste due to overruns for the metro edition, achieved in stacker counts with flow sensors measuring inputs coming into two reservoirs. The following values are estimated for the parameters in the model. In the standard deviations expressions, to denotes the average completion time for the metro edition. R1 = 50,000 copies/hour,
copies/hour,
R2 = 45,000
Omega, Vol. 20, No. 5/6
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Press Failure
Critical I v Before cutoff point
Non-critical After cutoff point
v J u m p to available press otherwise wait
Before cutoff point
v No action
v
v Wait till fixed
After cutoff point v No action
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v
After cutoff point
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Before cutoff point
h
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Alternate conveyor with w i n d e r not available
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No Alternate [v~ Alternate No action conveyor | | c o n v e y o r action with I I with winder I I w i n d e r not available
availablel I
v ~ .Use Wait alternate till fixed conveyor .Unwind on same drums to produce final p r o d u c t .Later when problem fixed, get back to normal
v Jump to available press otherwise wait
Drum/Drum Conveyor Failure
1 Critical 1 v Before cutoff point v .Partly or totally wind w i t h o u t insert .Unwind w o u n d p o r t i o n and insert at other available drum(s)
INon-critical After cutoff point v No action
Before cutoff point
v
After cutoff point
v .Partly or No t o t a l l y wind action without insert .Fix the p r o b l e m .Unwind w o u n d p o r t i o n and insert at other a v a i l a b l e drum(s)
Fig. 5. Operational decision trees used on equipment failure.
638
Kumar, Arora--Flow Sensors in Newspaper Production
v ®
o
2
1
J
J
I
I
$2 S1 Fig. 6. Flow of product j along two paths (® flow sensors).
A = 0.25 hour, Nj = 370,000, z~ = 2, atV/~0 = 5000, a2V/~0 = 4000, a'l = 500, at = 300 where al x/~0, and a2x/~0 are standard deviations of the total observed counts for the two reservoirs. In these estimates, we have assumed A = 0. It may be recalled the A denotes the stopping time difference for the two presses. Putting values of various variables in equation (11), (16) and (17), we have a(t0)=6430, k l = 196,000, and k 2 = 187,000 giving an expected waste estimate of 8000 copies which translate into 2.13%. At $0.25 per copy, daily expected waste reduction in dollars on a typical week day for the metro daily edition amounts to $2000. Annually, the expected waste savings in dollars due to overrun waste reduction for all the products is estimated at $1.42 million. The efficiencies achievable with state sensors are being reported in a separate paper. The sensor project is a part of the mailroom totalizer project. The totalizer project is aimed at improving overall integration among operations within pressroom, mailroom, distribution and despatch. The project is in the final stages of planning and design, and is expected to be implemented within the following 18 months.
to overrun are expected. The flow sensor would also allow generating production completion time projections more precisely and improve planning for corrective actions in cases of delays. When different component failures are reported, alternative system configurations would be generated, substituting available redundant operational units for the failed units. As a consequence, scheduling, sequencing, and despatching operations would improve significantly. Long-term statistics generated from the flow sensors will allow improving both longterm planning and day-to-day operational management decisions. Programs have been initiated within the company to lower non-critical failures and reduce waste category which is not affected by sensors. APPENDIX
Mathematical Model for Estimating Waste Redaction In the current system, sensors are not provided at all transfer locations. Furthermore, sensors in stackers are not accurate. There is a discrepancy between the observed counts and the true counts. Let the observed and the true counts at any time t of product j into reservoirs S 1 and $2 be given by the following equations: nl(j, S1, t) = n~(j, SI, t) + el(t )
(1)
n2(j, $2, t) = n°(j, $2, t) + e2(t)
(2)
where nl(j, S1, t), n2(j, $2, t) are the observed counts and n°(j, SI, t), n°(j, $2, t) are the true counts of inflow at time t into reservoirs S 1 and $2 respectively. The errors el(t), e2(t) are assumed to follow normal distribution with means zero and standard deviations alx/~ and a2x/~ respectively. The time is measured from the start time of production for the product. Let Xj, I, Xj,2 be the observed counts of the product in the pipelines of paths 1 and 2
CONCLUSIONS Integration among various inter-dependent functions can be improved by continuously monitoring product flow with flow sensors and monitoring equipment status with state sensors. Real time information about the state of the system would allow more appropriately matching operational decisions with the state of the system. With increased accuracy of the stacker flow sensor, significant reductions in waste due
>
A B <--z~o--> 0 0 Nj Vj (t) +Xj,I+Xj, 2
Y(t)
Fig. 7. Normal distribution of total real product counts. Points A and B are given by A = N j and B = V j ( t ) + X°, + X° 2.
639
Omega, 1Iol. 20, No. 5/6
respectively. The pipeline quantities are time independent random variables. The observed and true counts of the pipeline quantities are related by the following equations:
Pr[{ Vj(t) - z,a(t )} >>-Nj - X° l - X~j,2] = 1 - c~ P r [ ~ ( t ) t> N j - X°~ - X°2 + z,cr(t)] = 1 - ~
Deterministic equivalent of constraint (12) is given by
Xj, l = X ° , +e~
(3)
Vj(t) = N i -- X°I -- X°2 + z,a(t)
Xj.2= X°2 + e'2
(4)
Assuming R 1 and R2 to be the inflow rates in the pipelines along paths 1 and 2, we have
where X),0 l, ~.0j. 2 are true counts of product j in pipelines of paths 1 and 2 respectively and e',, e~ are errors with means zero and standard deviations a~ and o ~respectively. Approximate minimization of the objective function is achieved by following the rule that a A time difference is kept between the sequential stopping of the two inflows, and the inflow to the reservoir with larger counter error is stopped first. The true number of total units of product j , Y ( t l , t 2 ) , in the two reservoirs and the pipelines is given by
R l ( t -- A) + R2(t) = Vj(t) = Uj - X ° , - X~2 + z~o(t)
or (RI + R2)t -- R I A = N j - X ° 1 - )(°2 + z,a(t)
or t = { N j - X°~ - X°z + z,a(t) + R 1A}/(R1 + R2) (13)
We have,
Let Nj be the number of copies that are desired to be produced. The stopping rule is concerned with finding numbers k 1, k2 for the stacker counters into reservoirs S1 and $2 respectively at which production of product j will be stopped. These numbers should minimize expected overrun size E ( Y ( t , , t2) - Nj)
(6)
and satisfy the following constraints Pr(Y(t~, t2) >/Nj) = 1 - ~
(7)
nl(j, S l , tl) >t k l
(8)
n2(j, $2, t2) >1k2
(9)
The minimization of the expected overrun size is approximately achieved by following the A difference rule in the stopping times of the two flows. The problem thus reduces to finding feasible solution of the constraints (7), (8), and (9). Equivalently, we want to find the values of k 1, k 2 in terms of observed counter values which satisfy constraint (7). Putting tt = t - A, and t2 = t, constraint (7) can be re-written as Pr[{nl(j, SI, t - A) + n2(j, $2, t) --e,(t-A)--e2(t) }>~uj-X°l-)(°21=l-~
(10)
Assuming Wiener process for the observed flow counts into reservoirs S1 and $2, the variance az(t) of the total product counts in the system at time t (as shown in Fig. 7) is given by t r 2 ( t ) = t r ~ . ( t - A ) + t r 2 . t +a~2+tr~2
(11)
Putting n~(j, S 1 , t - A) + n2(A $2, t) = Vj(t) the constraint (10) may be written as
k l = R l ( t - A)
(14)
k2 = R2(t)
(15)
and
Y(fi, t2) = n°(j, S I , t.) + n°(j, $2, t2) + X °, + X°2 (5)
OME 20/5-~-G
(12)
Substituting the value of t given by expression (13) into equations (14) and (15), we have k l = R I ( N j + z , a ( t ) - R2A)/(RI + R2)
(16)
k2=R2(Nj+z~a(t)+RIA)/(RI+R2)
(17)
and As mentioned earlier, these stopping rules can be easily extended to the case with multiple reservoirs. REFERENCES 1. Baer T (1990) Advanced interfaces show information, not data. Managing Automation (January), 26-28. 2. Burg L (1990) Information processing in the mailroom. Ferag Company's Customer Note, 4-5. 3. Gould L (1990) Process control expands outward and upward. Managing Automation (January), 22-24. 4. IDAB Inc. Product Brief (1989) Newspaper Production Totalizing System. Publication A2151-10/89. 5. Kruglinski P (1986) Largely hidden from view, sensors are newspaper sentries. Presstime (February), 1t-13. 6. Kumar S and Arora S (1991) Improved manpower utilization in live-run shift of newspaper printing operations, l i E Trans. J. Forthcoming. 7. Monitor displays computer-controlled mailroom operations (1974) Newspaper Prodn (February), 4~47. 8. Owen E (1990) L A Times counts on totalizer. Newspapers and Technol. (February). 9. Scheidler PJ (1990) Newspaper conservation at the Providence Journal. Newspaper Systems Group's Report, Newport, RI. 10. Truitt RC (1990) Shortages and overruns plague accountability. Presstime (August), 6-8. ADDRESSFOR CORRESPONDENCE: Professor S Arora, Department o f Mechanical Engineering, University o f Minnesota, 111 Church Street, Room 125, Minneapolis, M N 55455, USA.