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A New Control Technique for Cross Machine Control of Basis Weight
Cop yright © IFAC Inst ru mentation and Automa tion in the Pape r , Ru bber, Plastics and Polymerisation Indust ries, Antwer p, Belgium 1983
A NEW CONTROL TECHNIQUE FOR CROSS MACHINE CONTROL OF BASIS WEIGHT A.
J.
Wilkinson* and A. Hering**
*AccuRay Co., H igh W.vcombe, Buckingham.shire, UK **AccuRay Co., Columbus, Ohio, USA
Abstract. Recent advances in automatic control theory have led to the development of self tuning regulators - regulators which adapt to the process they are controlling to produce the best control. One of the most interesting applications of self tuning regulators has been in the field of cross machine basis weight control on a paper machine. Process control systems which perform cross machine basis weight control using self tuning regulation techniques are now in routine use both in Western Europe and North America. This paper covers the following topics in relation to the cross machine control of basis weight using self tuning regulator techniques :1.
The operating principles used in the control system.
2.
A review of the actuators currently in use or under development for moving the paper machine slice lip.
3.
Results from operating systems
Keywords. Paper industry; weight control; cross machine engineering computer applications; self adjusting systems; robots
direction;
control
INTRODUCTION control of the basis weight with cross direction variation improvements of 40-60% being achieved. The same quality and economic imperatives which brought about the present widespread use of computer based process control systems will ensure that most of the Western World's paper machines become equipped with cross machine weight control systems within 10 years, if not very much sooner.
Since the first introduction to the Paper Industry of sensors for the continuous measurement of weight per unit area (BASIS WEIGHT), the need for automatic weight control has been appreciated. Very soon after the introduction of the first radioactive isotope-based sensor in the 1950's, simple closed loop control systems were being used to control the average basis weight in the machine direction of the paper machine. The need to control the basis weight in the cross machine direction has been just as great, but its implementation has had to wait 25 years. It is only recently that the advent of micro processors and advanced control theory have made cross machine direction basis weight control possible.
This paper outlines some of the recent developments which have led to cross machine weight control becoming a reality and some of the results which have been achieved by AccuRay from recent cross machine basis weight control installations.
Seventy percent of paper machines in the Western Economies are now equipped with process control systems providing scanning measurement of the paper basis weight with closed loop automatic control in the machine direction.
WHY HAS CROSS MACHINE BASIS WEIGHT CONTROL BEEN SO DIFFICUL T TO ACHIEVE? In the past 20 years or so a number of attempts have been made to control the cross direction basis weight profile of a paper machine automatically. These early attempts failed because the complexity of the control problem was not fully understood. In order to achieve successful cross machine weight control, the following pre conditions are necessary:
This measurement and control has reduced the machine direction product variability by 50 % or more, which in turn has produced improved paper runnability, improved quality and better utilisation of raw materials. The last 18 months have seen very rapid progess in the implementation of cross machine direction
175
A.J. Wilkinson and A. Herin g
176
Pre Condition 1 Accurate stable sensors. An accurate sensor is fundamental to any process control because the control can be no better than the measurement on which it is based. The cross machine direction control of basis weight imposes additional demands on the sensors being used. Sensors which are adequate for machine direction control may be inadequate for cross machine direction control. F or satisfactory machine direction control, sensors need only provide accurate information on the average value of the basis weight. Cross machine direction demands accurate cross direction resolution as well. Providing the appropriate resolution requires careful sensor design from both the sensor physics and signal processing standpoints. A sensor suitable for cross machine weight control must be insensitive to outside influences such as sheet pass line changes and be adquately compensated for any thermal or mechanical misalignment of their scanning mechanisms. The sensor must also be very stable and provide a high signal to noise ratio measurement.
Fig. 1 shows the stock flow distribution pattern which results when the position of slice screw is changed. This distribution pattern causes interaction between slice screw positions in the cross machine direction as illustrated in Fig. 2. This coupling between adjacent control positions can change with both the position across the machine and with time. A successful control strategy must adapt to the response changes. Pre Condition 3 Precise information on the position of the slice in the cross machine direction. As already indicated the head box slice lip is the most appropriate means of achieving cross machine weight control. If the slice lip is to be used successfully, however, care must be taken to avoid causing damage by forcing the slice into a position where it becomes permanently deformed and to prevent continuous cycling of the slice position which can eventually lead to fatigue failure. actuator n
Pre Condition 2 Any strategy An appropriate control strategy. adopted to perform cross machine control must take into account the individual characteristics of both the process and the control method being used. The traditional method of performing cross direction basis weight control is to change the position of the slice lip of the paper machine head box in the cross machine direction. Adjusting the shape of the slice lip has been by manual repositioning of the slice adjusting screws.
actuator n - 1
actuator n + 1
Fig. 2 Weight profile response to a single actuator It is, therefore, essential that an accurate and continuous measurement of the slice position be fed back to the control system at each slice screw position and that the control system only makes infrequent adjustments to the slice. We shall return to the choice of actuator for mov ing the slice la ter in the paper.
Fig. 1 Headbox, slice and wire of paper machine Despite being the most suitable method of cross direction control the slice lip has a number of characteristics which make it difficult to use and any successful control strategy must be designed to overcome these characteristics.
OPERA TING PRINCIPLES USED IN A SUCCESSFUL CROSS MACHINE BASIS WEIGHT CONTROL SYSTEM Fig. 3 illustrates the block diagram of a cross machine direction weight control loop. If is the vector representing the basis weight profile across the machine and is the vector representing the head box slice position across the machine. The fundamental problem which must to solved is the interaction which occurs between adjacent actuators. In other words a change in one element
Control Technique for Cross Machine Control adjustments.
Disturbance
Basis Weight Profile
Fig. 3 CD weight control loop of the u vector. Fig. 2 illustrates the typical response. To obtain satisfactory control, decoupling of the response of adjacent actuators must be achieved. The decoupling problem may be solved by describing a model of the process using matrices. A detailed treatment of the model and the control theory applied in producing this system has already been published in the work by Richards and Wilhelm Ref 1 and 2. A brief summary follows.
A complication in producing CD weight control is that the profile response changes as a function of machine speed, wire conditions, stock drainage and head box conditions. This means that the elements of matrix A are not constants but change with operating conditions. As a result self tuning becomes an essential ingredient of any control system which is to provide satisfactory control without the need for frequent retuning. PROFILE MANAGER The commercial embodiment of the principles outlined briefly above is the AccuRay Profile Manager, the system shown in block diagram form in Fig. 4. It consists of: a)
A scanning basis weight measurement which provides information on the cross direction basis weight profile
b)
Slice position feedback from each slice screw on the paper machine headbox. This information is essential both to prevent slice damage from excessive bending and to make precise slice positioning possible.
c)
A process identification which identifies the response of the process to a slice screw position change. The process identifier estimates the parameters in Matrix A which was described previously. It is the ability to identify the changing process response which provides this system with its self tuning ability.
d)
A process model which models the paper machine. This is in essence the model described in equation 0). The model is continually updated by inputs from the process identifier as the process response changes. The model permits the determination of the position of each slice screw to obtain the desired basis weight profile.
e)
A means of moving the slice to its new position. The slice screw can be moved manually or automatically. The AccuRay Profile Manager as the self tuning cross machine software is known, is available in two forms:
The model may be described by a matrix equation,
A /::, u
{Hj
where the elements /::, u represent changes in slice position at each actuator position and the elements /::, tj represent changes in the weight profile of corresponding positions. If the responses to all actuators are similar and symetric about the actuator positions, the matrix A will be band diagonal and symetric. For example :-
A
=
(a b cOO 0 ••• 0 ) (b a b cOO ••• 0 ) (c b a b ca ••• 0 )
(0 c b ab c ••. 0 ) (. (. (. (0 • • •
)
) ) 0 c b a
)
(2)
Where a, band c represent the profile response to a unit change in one slice screw centered at that slice screw, the adjacent screws and the screws two positions away respectively. The inverse of this matrix may be used to define the control action required. This control action will completely level the cross direction profile provided the matrix A accurately describes the process response.
/::, u
=
tjA -1 /::' tj *
177
(3)
where /::, tj* represents the desired change in profile and tj is a scaler gain factor generally less than unity provided to allow more conservative
i)
Guidance Where a digital display on the paper machine headbox informs the machine operator of the position to which he has to move each slice screw. See Fig. 5. In this way it is possible for the operator to level the profile with one set of slice screw adjustments. This represents a great time saving with much better results than present trial and error methods.
A.J. Wilkinson and A. Hering
178
ii)
Full automatic control The control drives the slice actuator to the precise position required. The slice is driven to a new position and then no further adjustment is made until required. This avoids continuous slice movement which was the major disadvantage inherent in many of the early attempts at cross machine weight control.
Protection of Slice The Profile Manager system ensures that no change in slice position is ever requested which would damage the slice. Attention is paid to the absolute position of each slice screw to ensure that it is not out of range, and to the relative position of each slice screw with respect to adjacent screws to avoid excessive slice bending.
Motors Many attempts have been made to use motors for cross direction basis weight control in the past and most attempts failed. The result is that there is a widespread but mistaken impression by many in the paper industry that motors should not be considered for cross direction control. In fact, the reason for failure was not the motors, but the control software or the basic measurement on which control was based. Motors are excellent slice actuators when correctly installed and many recent installations using the AccuRay Profile Manager software have been outstanding successes. Systems are commercially available from the following manufacturers: Devron, Beloit, Voith, Duff Norton and many others. Fig. 6 shows a photograph of a recent installation of the Beloit BeJcomp System and Fig. 7 shows a photograph of the Devron Autoslice Unit.
Control Objective The Profile Manager control objective is normally to reduce the cross direction weight variation to a minimum. It is, however, possible to modify this objective to take the moisture profile into account. In this case the control objective could be minimisation of moisture profile variation rather than weight profile variation, or some compromise between the two. The equation describing this optimisation is,
J
+
. . . ..
(4)
Where the function J the least squares error is minimised and a + b = 1. To minimise only weight variations a = 1, b = o. To minimise only moisture variations a = 0, b = 1 or a compromise of the two is possible.
Thermal and Hydraulic Actuators Two different types of Thermal actuators are available on the market. One is the Equilex system produced by Chleque Frote, which relies on the thermal expansion of a metal rod to move the slice. A more recent development is the Thermal Hydraulic unit produced by Devron which relies on the thermal expansion of a fluid to move the slice. This unit has a greater range of movement and is somewhat more compact in size than units relying on the thermal expansion of a metal rod. Fig. 8 shows a photograph of the Devron Thermal Hydraulic Actuator. It includes integral position feedback and a locking mechanism which holds the unit in its last position when power is removed. Motorised Robots
A REVIEW OF ACTUA TORS CURRENTLY IN USE OR UNDER DEVELOPMENT FOR MOVING THE SLICE LIP The recent developments in control technology which have made cross direction basis weight control possible have provided additional impetus to the development of actuators for moving the slice of the paper machine headbox. Many manufacturers are developing and introducing actuators, and at such a rapid pace that any review is likely to need revision within a matter of months. No one actuator can be expected to be the ideal solution in every situation. The variety of actuators now available on the market make it possible to select an appropriate actuator for most headboxes in use today. The actuators available fall into three main categories, namely Motors, Thermal and Hydraulic Actuators, Motorised Robots.
Great success has been achieved recently using a motorised robot. In this system a single motor on a traversing rail is directed to the slice screw to be adjusted by the process control system; it engages with the existing slice screw and drives the screwto the position desired. This system holds out the prospect of being one of the least expensive slice control actuators. Fig. 9 provides a photograph of the robot in position on a headbox. In summary, a wide variety of actuators are available and the prospect is that many more will become available in the near future. RESUL TS FROM OPERATING SYSTEMS At the time of described in this paper machines. have been taken
writing, May 1983, the system paper was operational on over 10 Over 20 additional system orders and will be installed in the near
Control Technique for Cross Machine Control
179
, MlN.JT(.5 ArT(R 1ST CON TRa.. ACTJ()f'oI CClfo4'l.ETE
future. Initial results have been very encouraging. The following is a summary of some of the results which have been obtained from the use of this system.
1)
Wiggins Teape, Fort William Mill, UK
PROOLIC T
. .... oRONT
BASIS W£IChT Hlqn
lU
h'V£t
2'.2
An • •
lid
Low
IS.'
•
1
Consolidated Papers PM15, Wisconsin Rapids, Wisconsin, USA See Fig. 5. Initial results with Profile Manager guidance. Base sheet weight range 40 - 60 / 3,300 ft 2 - Valley Airpadded Headbox. Cross direction variability reduction (2 sigma CD) of 55% obtained by operator using system. Nymolla PM1, Sweden
3)
AccuRay Actuator Robot - see Fig. 9. Initial results from this machine - Voith Headbox 80 gsm copying grade 54om/min. Reduction of CD 2 sigma variation by 54%. Accompanying moisture reduction of 40%. 4)
REEL 1tool
2600
•
BACK
.. r
'
.-
.......
l.OLBS/OIV
Initial results with Profile Manager guidance. Base sheet weight range 45 - 225 gsm - Beloit Walmsley Headbox. Cross direction variability reduction (2 sigma CO) of 46% obtained in a single set of adjustments by the operator. 2)
PAC( 1
COMPOSITE PROFILES
Proprietary Customer AB Initial results from site using Beloit Belcomp system - see Fig. 6. 40% reduction in basis weight peak to peak variation. 30.4% reduction in moisture peak to peak variation.
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Fig. 4 (b) These figures demonstrate the reduction in cross direction weight variations when the automatic control is switched on. Note the total elapsed time between the two projections is only 12 minutes. CONCLUSIONS A combination of accurate sensors, the computing power of microprocessors, self tuning control software and well designed reliable actuators have made the automatic cross direction control of the basis weight profile on a paper machine a reality. The improvements in paper machine performance which this control produces will ensure its rapid application to very many paper machines in the next few years. This is only one example of the many exciting developments under way in the field of process control computers applied to paper production. A number of equally spectacular advances can be expected in the next few years using similar techniques. REFERENCES
Proprietary Customer XY
5)
Richards, G.A. (November 1982). Cross direction weight control. Japan Pulp and Paper
Initial results from a Oevron Autoslice - See Fig. 7. Basis weight 243Ibs/3,000ft 2. 44% reduction in cross direction 2 sigma weight variations. BEFORE CXlNTRCL
PRClflL£ MANAG£R CONTROl. OF BASIS 'WEJ()-fT
MACHlr'£ I O}·III."
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COMPOSITE PROFILES
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Wilhelm, R.G. (1982). Self tuning control strategies. Multifaceted solutions to paper machine control problems. I S A Wilkinson, A.J. (July, August 1982). The role of automatic cross machine control in improving runnability. Paper Technology and Industry
180
A. J . Wi l k in son a nd A. He rin g
ER Guidan ce System to level the Fig. 5 A machin e operato r using the AccuRa y PROFIL E MANAG The operato r adjusts each slice Mill. slice on the machin e at Wiggins Teape, Fort William above the headbox which he is display digital system the by d provide ions followin g the instruct watchin g.
under the cover at right drive shafts Fig 6 Photog raph of the Beloit Belcom p System . Motors indicati on is provide d on each slice position coupled to individu al slice screws. Dial microm eter screw.
Control Technique for Cross Machine Control
181
Fig. 7 The Devron Autoslice. A machine operator lifting the cover over the electric motors which position the slice screws as instructed by the fully automatic AccuRay PROFILE MANAGER. Provision for manual adjustment exists. Dial micrmeters indicate the position of each slice screw.
Fig. 8 Devron Thermo Hydraulic Actuator with integral dial micrometer.
182
A.J. Wilkinson and A. Hering
Fig. 9 AccuRay Robot installed at Nymolla in Sweden with the covers removed. The unit is driven to the appropriate slice screw fitted with a knurled top where the motor driven "hand" engages to adjust the slice.
A NEW CONTROL TECHNIQUE FOR CROSS MACHINE CONTROL OF BASIS WEIGHT A.
J.
Wilkinson* and A. Hering**
*AccuRay Co., H igh W.vcombe, Buckingham.shire, UK **AccuRay Co., Columbus, Ohio, USA
Abstract. Recent advances in automatic control theory have led to the development of self tuning regulators - regulators which adapt to the process they are controlling to produce the best control. One of the most interesting applications of self tuning regulators has been in the field of cross machine basis weight control on a paper machine. Process control systems which perform cross machine basis weight control using self tuning regulation techniques are now in routine use both in Western Europe and North America. This paper covers the following topics in relation to the cross machine control of basis weight using self tuning regulator techniques :1.
The operating principles used in the control system.
2.
A review of the actuators currently in use or under development for moving the paper machine slice lip.
3.
Results from operating systems
Keywords. Paper industry; weight control; cross machine engineering computer applications; self adjusting systems; robots
direction;
control
INTRODUCTION control of the basis weight with cross direction variation improvements of 40-60% being achieved. The same quality and economic imperatives which brought about the present widespread use of computer based process control systems will ensure that most of the Western World's paper machines become equipped with cross machine weight control systems within 10 years, if not very much sooner.
Since the first introduction to the Paper Industry of sensors for the continuous measurement of weight per unit area (BASIS WEIGHT), the need for automatic weight control has been appreciated. Very soon after the introduction of the first radioactive isotope-based sensor in the 1950's, simple closed loop control systems were being used to control the average basis weight in the machine direction of the paper machine. The need to control the basis weight in the cross machine direction has been just as great, but its implementation has had to wait 25 years. It is only recently that the advent of micro processors and advanced control theory have made cross machine direction basis weight control possible.
This paper outlines some of the recent developments which have led to cross machine weight control becoming a reality and some of the results which have been achieved by AccuRay from recent cross machine basis weight control installations.
Seventy percent of paper machines in the Western Economies are now equipped with process control systems providing scanning measurement of the paper basis weight with closed loop automatic control in the machine direction.
WHY HAS CROSS MACHINE BASIS WEIGHT CONTROL BEEN SO DIFFICUL T TO ACHIEVE? In the past 20 years or so a number of attempts have been made to control the cross direction basis weight profile of a paper machine automatically. These early attempts failed because the complexity of the control problem was not fully understood. In order to achieve successful cross machine weight control, the following pre conditions are necessary:
This measurement and control has reduced the machine direction product variability by 50 % or more, which in turn has produced improved paper runnability, improved quality and better utilisation of raw materials. The last 18 months have seen very rapid progess in the implementation of cross machine direction
175
A.J. Wilkinson and A. Herin g
176
Pre Condition 1 Accurate stable sensors. An accurate sensor is fundamental to any process control because the control can be no better than the measurement on which it is based. The cross machine direction control of basis weight imposes additional demands on the sensors being used. Sensors which are adequate for machine direction control may be inadequate for cross machine direction control. F or satisfactory machine direction control, sensors need only provide accurate information on the average value of the basis weight. Cross machine direction demands accurate cross direction resolution as well. Providing the appropriate resolution requires careful sensor design from both the sensor physics and signal processing standpoints. A sensor suitable for cross machine weight control must be insensitive to outside influences such as sheet pass line changes and be adquately compensated for any thermal or mechanical misalignment of their scanning mechanisms. The sensor must also be very stable and provide a high signal to noise ratio measurement.
Fig. 1 shows the stock flow distribution pattern which results when the position of slice screw is changed. This distribution pattern causes interaction between slice screw positions in the cross machine direction as illustrated in Fig. 2. This coupling between adjacent control positions can change with both the position across the machine and with time. A successful control strategy must adapt to the response changes. Pre Condition 3 Precise information on the position of the slice in the cross machine direction. As already indicated the head box slice lip is the most appropriate means of achieving cross machine weight control. If the slice lip is to be used successfully, however, care must be taken to avoid causing damage by forcing the slice into a position where it becomes permanently deformed and to prevent continuous cycling of the slice position which can eventually lead to fatigue failure. actuator n
Pre Condition 2 Any strategy An appropriate control strategy. adopted to perform cross machine control must take into account the individual characteristics of both the process and the control method being used. The traditional method of performing cross direction basis weight control is to change the position of the slice lip of the paper machine head box in the cross machine direction. Adjusting the shape of the slice lip has been by manual repositioning of the slice adjusting screws.
actuator n - 1
actuator n + 1
Fig. 2 Weight profile response to a single actuator It is, therefore, essential that an accurate and continuous measurement of the slice position be fed back to the control system at each slice screw position and that the control system only makes infrequent adjustments to the slice. We shall return to the choice of actuator for mov ing the slice la ter in the paper.
Fig. 1 Headbox, slice and wire of paper machine Despite being the most suitable method of cross direction control the slice lip has a number of characteristics which make it difficult to use and any successful control strategy must be designed to overcome these characteristics.
OPERA TING PRINCIPLES USED IN A SUCCESSFUL CROSS MACHINE BASIS WEIGHT CONTROL SYSTEM Fig. 3 illustrates the block diagram of a cross machine direction weight control loop. If is the vector representing the basis weight profile across the machine and is the vector representing the head box slice position across the machine. The fundamental problem which must to solved is the interaction which occurs between adjacent actuators. In other words a change in one element
Control Technique for Cross Machine Control adjustments.
Disturbance
Basis Weight Profile
Fig. 3 CD weight control loop of the u vector. Fig. 2 illustrates the typical response. To obtain satisfactory control, decoupling of the response of adjacent actuators must be achieved. The decoupling problem may be solved by describing a model of the process using matrices. A detailed treatment of the model and the control theory applied in producing this system has already been published in the work by Richards and Wilhelm Ref 1 and 2. A brief summary follows.
A complication in producing CD weight control is that the profile response changes as a function of machine speed, wire conditions, stock drainage and head box conditions. This means that the elements of matrix A are not constants but change with operating conditions. As a result self tuning becomes an essential ingredient of any control system which is to provide satisfactory control without the need for frequent retuning. PROFILE MANAGER The commercial embodiment of the principles outlined briefly above is the AccuRay Profile Manager, the system shown in block diagram form in Fig. 4. It consists of: a)
A scanning basis weight measurement which provides information on the cross direction basis weight profile
b)
Slice position feedback from each slice screw on the paper machine headbox. This information is essential both to prevent slice damage from excessive bending and to make precise slice positioning possible.
c)
A process identification which identifies the response of the process to a slice screw position change. The process identifier estimates the parameters in Matrix A which was described previously. It is the ability to identify the changing process response which provides this system with its self tuning ability.
d)
A process model which models the paper machine. This is in essence the model described in equation 0). The model is continually updated by inputs from the process identifier as the process response changes. The model permits the determination of the position of each slice screw to obtain the desired basis weight profile.
e)
A means of moving the slice to its new position. The slice screw can be moved manually or automatically. The AccuRay Profile Manager as the self tuning cross machine software is known, is available in two forms:
The model may be described by a matrix equation,
A /::, u
{Hj
where the elements /::, u represent changes in slice position at each actuator position and the elements /::, tj represent changes in the weight profile of corresponding positions. If the responses to all actuators are similar and symetric about the actuator positions, the matrix A will be band diagonal and symetric. For example :-
A
=
(a b cOO 0 ••• 0 ) (b a b cOO ••• 0 ) (c b a b ca ••• 0 )
(0 c b ab c ••. 0 ) (. (. (. (0 • • •
)
) ) 0 c b a
)
(2)
Where a, band c represent the profile response to a unit change in one slice screw centered at that slice screw, the adjacent screws and the screws two positions away respectively. The inverse of this matrix may be used to define the control action required. This control action will completely level the cross direction profile provided the matrix A accurately describes the process response.
/::, u
=
tjA -1 /::' tj *
177
(3)
where /::, tj* represents the desired change in profile and tj is a scaler gain factor generally less than unity provided to allow more conservative
i)
Guidance Where a digital display on the paper machine headbox informs the machine operator of the position to which he has to move each slice screw. See Fig. 5. In this way it is possible for the operator to level the profile with one set of slice screw adjustments. This represents a great time saving with much better results than present trial and error methods.
A.J. Wilkinson and A. Hering
178
ii)
Full automatic control The control drives the slice actuator to the precise position required. The slice is driven to a new position and then no further adjustment is made until required. This avoids continuous slice movement which was the major disadvantage inherent in many of the early attempts at cross machine weight control.
Protection of Slice The Profile Manager system ensures that no change in slice position is ever requested which would damage the slice. Attention is paid to the absolute position of each slice screw to ensure that it is not out of range, and to the relative position of each slice screw with respect to adjacent screws to avoid excessive slice bending.
Motors Many attempts have been made to use motors for cross direction basis weight control in the past and most attempts failed. The result is that there is a widespread but mistaken impression by many in the paper industry that motors should not be considered for cross direction control. In fact, the reason for failure was not the motors, but the control software or the basic measurement on which control was based. Motors are excellent slice actuators when correctly installed and many recent installations using the AccuRay Profile Manager software have been outstanding successes. Systems are commercially available from the following manufacturers: Devron, Beloit, Voith, Duff Norton and many others. Fig. 6 shows a photograph of a recent installation of the Beloit BeJcomp System and Fig. 7 shows a photograph of the Devron Autoslice Unit.
Control Objective The Profile Manager control objective is normally to reduce the cross direction weight variation to a minimum. It is, however, possible to modify this objective to take the moisture profile into account. In this case the control objective could be minimisation of moisture profile variation rather than weight profile variation, or some compromise between the two. The equation describing this optimisation is,
J
+
. . . ..
(4)
Where the function J the least squares error is minimised and a + b = 1. To minimise only weight variations a = 1, b = o. To minimise only moisture variations a = 0, b = 1 or a compromise of the two is possible.
Thermal and Hydraulic Actuators Two different types of Thermal actuators are available on the market. One is the Equilex system produced by Chleque Frote, which relies on the thermal expansion of a metal rod to move the slice. A more recent development is the Thermal Hydraulic unit produced by Devron which relies on the thermal expansion of a fluid to move the slice. This unit has a greater range of movement and is somewhat more compact in size than units relying on the thermal expansion of a metal rod. Fig. 8 shows a photograph of the Devron Thermal Hydraulic Actuator. It includes integral position feedback and a locking mechanism which holds the unit in its last position when power is removed. Motorised Robots
A REVIEW OF ACTUA TORS CURRENTLY IN USE OR UNDER DEVELOPMENT FOR MOVING THE SLICE LIP The recent developments in control technology which have made cross direction basis weight control possible have provided additional impetus to the development of actuators for moving the slice of the paper machine headbox. Many manufacturers are developing and introducing actuators, and at such a rapid pace that any review is likely to need revision within a matter of months. No one actuator can be expected to be the ideal solution in every situation. The variety of actuators now available on the market make it possible to select an appropriate actuator for most headboxes in use today. The actuators available fall into three main categories, namely Motors, Thermal and Hydraulic Actuators, Motorised Robots.
Great success has been achieved recently using a motorised robot. In this system a single motor on a traversing rail is directed to the slice screw to be adjusted by the process control system; it engages with the existing slice screw and drives the screwto the position desired. This system holds out the prospect of being one of the least expensive slice control actuators. Fig. 9 provides a photograph of the robot in position on a headbox. In summary, a wide variety of actuators are available and the prospect is that many more will become available in the near future. RESUL TS FROM OPERATING SYSTEMS At the time of described in this paper machines. have been taken
writing, May 1983, the system paper was operational on over 10 Over 20 additional system orders and will be installed in the near
Control Technique for Cross Machine Control
179
, MlN.JT(.5 ArT(R 1ST CON TRa.. ACTJ()f'oI CClfo4'l.ETE
future. Initial results have been very encouraging. The following is a summary of some of the results which have been obtained from the use of this system.
1)
Wiggins Teape, Fort William Mill, UK
PROOLIC T
. .... oRONT
BASIS W£IChT Hlqn
lU
h'V£t
2'.2
An • •
lid
Low
IS.'
•
1
Consolidated Papers PM15, Wisconsin Rapids, Wisconsin, USA See Fig. 5. Initial results with Profile Manager guidance. Base sheet weight range 40 - 60 / 3,300 ft 2 - Valley Airpadded Headbox. Cross direction variability reduction (2 sigma CD) of 55% obtained by operator using system. Nymolla PM1, Sweden
3)
AccuRay Actuator Robot - see Fig. 9. Initial results from this machine - Voith Headbox 80 gsm copying grade 54om/min. Reduction of CD 2 sigma variation by 54%. Accompanying moisture reduction of 40%. 4)
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Initial results with Profile Manager guidance. Base sheet weight range 45 - 225 gsm - Beloit Walmsley Headbox. Cross direction variability reduction (2 sigma CO) of 46% obtained in a single set of adjustments by the operator. 2)
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Proprietary Customer AB Initial results from site using Beloit Belcomp system - see Fig. 6. 40% reduction in basis weight peak to peak variation. 30.4% reduction in moisture peak to peak variation.
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Fig. 4 (b) These figures demonstrate the reduction in cross direction weight variations when the automatic control is switched on. Note the total elapsed time between the two projections is only 12 minutes. CONCLUSIONS A combination of accurate sensors, the computing power of microprocessors, self tuning control software and well designed reliable actuators have made the automatic cross direction control of the basis weight profile on a paper machine a reality. The improvements in paper machine performance which this control produces will ensure its rapid application to very many paper machines in the next few years. This is only one example of the many exciting developments under way in the field of process control computers applied to paper production. A number of equally spectacular advances can be expected in the next few years using similar techniques. REFERENCES
Proprietary Customer XY
5)
Richards, G.A. (November 1982). Cross direction weight control. Japan Pulp and Paper
Initial results from a Oevron Autoslice - See Fig. 7. Basis weight 243Ibs/3,000ft 2. 44% reduction in cross direction 2 sigma weight variations. BEFORE CXlNTRCL
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Wilhelm, R.G. (1982). Self tuning control strategies. Multifaceted solutions to paper machine control problems. I S A Wilkinson, A.J. (July, August 1982). The role of automatic cross machine control in improving runnability. Paper Technology and Industry
180
A. J . Wi l k in son a nd A. He rin g
ER Guidan ce System to level the Fig. 5 A machin e operato r using the AccuRa y PROFIL E MANAG The operato r adjusts each slice Mill. slice on the machin e at Wiggins Teape, Fort William above the headbox which he is display digital system the by d provide ions followin g the instruct watchin g.
under the cover at right drive shafts Fig 6 Photog raph of the Beloit Belcom p System . Motors indicati on is provide d on each slice position coupled to individu al slice screws. Dial microm eter screw.
Control Technique for Cross Machine Control
181
Fig. 7 The Devron Autoslice. A machine operator lifting the cover over the electric motors which position the slice screws as instructed by the fully automatic AccuRay PROFILE MANAGER. Provision for manual adjustment exists. Dial micrmeters indicate the position of each slice screw.
Fig. 8 Devron Thermo Hydraulic Actuator with integral dial micrometer.
182
A.J. Wilkinson and A. Hering
Fig. 9 AccuRay Robot installed at Nymolla in Sweden with the covers removed. The unit is driven to the appropriate slice screw fitted with a knurled top where the motor driven "hand" engages to adjust the slice.