Note: Descriptions are shown in the official language in which they were submitted.
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~8~6
BAC~GROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention deals with a method of controlling the
:~ pigmentation of thermoplastic film. More specifically, the method
involves measuring the apparent thickness of the pigmented film
with a nucleonic thickness gauge and controlling pigment addition
in response to a comparison between the thickness measured with
the nucleonic gauge and the thickness determined from the weight
of the film.
`) DESCRIPTIOM OF THE PRIOR A~T
- ` ~
Thermoplastic film may be produced by feeding a thermo- ;
plastic resin, often in the form o~ solid pellets, to an extruder
which mechanically works and heats the resin, transforming it in-
to a plastic state. The molten resin is then extruded through an
extrusion die to form a thin, thermoplastic film.
Pigment may be added to the extruder so that it is
mixed with the thermoplastic material in order to vary the color
of the extruded ~ilm. Such pigments generally represent less
than 10~ and typically about 1%, of the total pigmented film
weight.
As the pigment is quite expensive, normally costing
several times as much as thermopl~stic materialg close control
of the amount of pigment in each portion of the film is essential
for economy o~ operation. Previously, the amount of pigment added
~5 was determined by visually observing the color of the product,
thus allowing for a rather wide margin of error.
....
Another way of determining the amount of pigment in the
pigmented plastic film is to weigh the film in order to ascertain
if the film weight equals the total of the required amounts of film
.
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and pigment. However, this method is also inaccurate because
-' it assumes that the thickness of' the ~ilm is constant. This is
....
' not necessarily so and a very slight decrease or increase in film''` thickness, as might be caused by extrusion die wear or film build-
up at the die exit, ~or example, will produce substantial varia-
' tions in the amount of thermoplastic film extruded. A change in
die setting of about 10 microns will alter the thickness of 500
- micron rilm by 2.5~. As the entire amount of ~dded pigment typi-cally represents abou~ 1~ of the total weight o~ the pigmented
~'-' 10 film, a simple weighing of the pigmented ~ilm would not allow'one
to distinguish a decrease in pigment ~rom a detrease in film thick-
ness. Indeed, an excess of pigment and a def'icîency of film could
well cancel each other out, producing a total weight equalling
'- that for the correct amounts of pigment and ~ilm but with an in- correctly pigmented film.
; The thickness of the pigmented ~ilm could be measured
mechanically, in conjunction with a measurement of the weight, in
order to determine the variations in film thicknessO A problem
- associated with the mechanical measurement of film thickness is
that the thickness varies across the surface o~ the ~'ilm sheet.
.i:
This is obviously true of film produced from a rotating die which
~' relocates the variations in film thickness produced by any irre
;'' gularities in the die opening across the entire film as the die
'' rotates. ~hus, a large number of thickness readings would be re-
'; 25 quired. Again, the errors produced by an excess of pigment and a
,1:
'`' deficiency of thermoplastic film could well cancel each other out
~ resulting in a thickness which would appear to indicate a proper
'~' proportion.
The measurement of thickness by nucleonic or radiation
"`' 30 gauges is known. U. S. Patent No. 2988641 to ~ough, for example7
- discloses a backscatter radiation gauge ~or measuring the thickness
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of calendered sheet material. The variations in the measured
thickness can be monitored and used to control the setting of
the calender rolls by a feedback loop. The thickness gauge
makes a comparison with a set point controller which itself
comprises a radiation guage set to minotor changes in the
composition of the sheet material. This patent does not,
however, deal with the problem of regulating the amount of
pigment in the material.
Continuous weight m ~,itors for sheet materials are also
known. For example, U.S. Patent No. 2726922 to Merrill et
al. discloses a continuous weighing scale which can be set
to monitor variations from a predetermined thickness.
SUMMARY OF THE INVENTION
., .
~ We have now devised a method for controlling the
- pigmentation of an extruded thermoplastic resin film. The
method involves measuring the thickness of the film with a
nucleonic or radiation thickness gauge and making a comparison
' of this thickness with the thickness determined by measuring
the weight of the pigmented film. The amount of pigment
added to the resin to make the film is then adjusted, if
necessary, in response to the comparison.
The method is well adapted to the continuous production
of thermoplastic film and it is capable of making rapid cor-
rections in response to deviations from the desired ratio of
pigment to resin.
The present invention, therefore, in one aspect,
resides in a method for producing pigmented thermoplastic
, film with a specific predetermined pigment concentration,
.
comprising: measuring the thickness of said pigmented film
by means of a radiation thickness gauge-; measuring the
:'
j weight of said pigmented film; calculating the thickness of
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: said pigmented Eilm from said measured weight; and adjusting
:~ the amount of said pigment mixed with said thermoplastic
~.` film in response to a comparison of said thickness as mea-
-". sured using said radiation gauye with said thickness calcu-
., .
,: lated from the weight of said pigmented thermoplastic film
.~t ;'.
.iU~ to achieve said preaetermined pigment concentration.
: In another aspect, this invention resides in an
.i~
: apparatus for producing pigmented thermoplastic film at a
: .
-~ specific predetermined pigment concentration comprising: a
.~ 10 radiation thickness gauge including a circuit producing a
signal responsive to a measur~ d film thickness; means for
.
, weighing said pigmented thermoplastic film including a circuit
` producing a signal in response to a measured film weight ;
.~
. first logic circuit means for computing the film thickness
~,~` from said measured film weight; second logic circuit means
.,~ ., ... ': for comparing said radiation signal and said second logic
:,'. circuit means signal; and means responsive to said second
Y. logic circuit for adjusting the amount of pigment mixed with
. said thermoplastic film.
:: .
.. 20 BRIEF DESCRIPTION OF THE DRAWINGS
.
. FIGURE 1 is a flow sheet illustrating the steps used
~ in the control of film pigmentation,
-~, FIGURE 2 is a schematic representation of one form of
apparatus
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26
- used to control film pigmentation,
FIGURF 3 is a schematic representation of another form o~
apparatus used to control film pigmentation,
FIGURE 1~ is a schematic representation of another form of ap-
paratus used to control film pigmentation,
- FIGURE 5 is a flow diagram o~ a digital computer subroutine used
to control ~ilm pigmentation.
_ESCRIPTION OF SPECIFIC EMBODIMENTS
The thermoplastic film will normally be a polyolefin,
1 O polyvinyl chloride, a polyester or a polyamide, of which the
polyolefins are the most common. The most common materials are
;- polyethylene, polypropylene~ polystyrene~ polyvinyl chloride or
copolymers thereof such as ethylene-~inyl chloride, ethylene-
vinyl acetate or ethylene-propylene. Blends of different polymers
1 5 may also be used. Typical types of pigments include the following:
Calcium carbonate, CaC03; Calcium sulfate, CaS04; Barium sulfate,
; BaSO4; Silicon dioxide (silica), SiO2; Calcium silicate, CaSiO3;
-~ Aluminum hydrate, Al(OH)3; Aluminum oxide, A1203; Chromium oxide,
Cr203; Lead sulfate, PbSO4; Lead carbonate, PbCO3; Lead chromate,
PbCr203, Lead oxide, PbO; Lead sulfide, PbS; Lead acetate,
- Pb(C2H302)2; Zinc oxide, ZnO; Zinc sulfide, ZnS; Clay, A1203.2SiO2;
s Mica, K20.3A1203.6SiO2; Zinc yellow, ~ZnO.4Cr203.K20, ZnCrO.~Zn(OH)2;
Manganese dioxide MnO2; potassium, sodium or ammonium ferri ferro-
cyanide, cobalt aluminate and, more typically, tit~nium dioxide,
.
7 25 TiO2, Iron oxides Fe203; Fe304; Fe203.H20; Iron sul~ates,
Fe2(SO4)3.7H20; Iron chromate Fe2(Cr203)3.
The preferred type of nucleonic or radiation thickness
gauge is the "backscatter" radiation gauge. These operate by
detecting the backscatter of ionizing radiation from the film in
L8~ 6
an ionization cha~her. T~e i~nizing xadiatiQn i~ normally
alpha, heta or qamma radiation, mos~t co~monly heta radiation as
.,
; this poses e~er health hazard~ tllen the use of the more
~ energetic gamma radiation. The radiation i5 normally supplied
- from a radioactive source such a~ a Ra-266 for alpha rays; Sr-90,
C-14, Th-204, Kr-85 or Pm-147 for ~eta rays or Co-60 for gamma
rays.
The backscatte~ thickne~s gaugas direct the radiation
onto the material from a radiation source and most of the rays
lG pass through t~e material as it is normally quite thin and
;,
- fairly transparent to ionizing radiati~nO A portion of the rays
: . ,
are, however, scattered back in the direction ~rom which they
came by elastic collisions within the atomic structure of the
material. These ~ackscattered electrons are detected, toge~her
~ith electrons reflected from the metal supports below the film,
,.:
in an ionization cham~er ~hich givas an output proportional to
the thickness of the material. From this the thickness of the
:
material can be determined after suitable calibration of the
gauge has been made.
2Q As an alternati~e to a backscatter thickness gauge, a
transmission gauge may be used. Gauges of this type rely upon
the absorption of the radiation within the material and therefore
the type and energy of the radiation require to be matched to the
absorption characteristics of the material. l'ypical types of
radiation used in transmission gauges are alpha, beta, gamma,
X-ray and infra red~ The detectors used in these gauges provide
:
~ an output proportional to the thickness of the material and, after
:~ .
calibration, give a direct reading of the thickness.
W~ile radiation gauges, especially the backscatter
,.
3Q gauges, are quite satisfactory for detQrmining the thickness of
homogeneou~ materials, they are somewhat inaccurate when used to
.
measure the thickness of hetero~eneous materials, that i~,materials
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-- - r-- ~ --~ ~ r-- --.- --- - -__. . _ _ . _ . _ .. _ . _ . . . . . .
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made up of more than one type of molecule. The reason ~or
this is that the sca-ttering or absorption of radiation is
an atomic phenomenon and therefore depends upon the atoms
and, consequently, the molecules in the material. With a
backscatter thickness gauge, the scattering depends on the
atomic number of the material through which the radiation
passes. For a typical pigment, about 1~ increase in the
amount o~ pigment added to a thermoplastic ~ilm produces an
:
apparent increase of about 10~ in the pigmented film density
... .
and thickness when measured by a backscatter gauge calibrated
for film alone. This is because the pigment has a higher electron
... .
weight density ratio than the thermoplastic ~ilm.
This phenomenon is used to control the pigmentation of
;; the film. By comparing the film density or thickness as measured
with the radiation thickness gauge (preferably a beta backscatter
gauge) ~ith the thickness as determined by weighing the film,
f`' the amount of pigment in the film can be determined to very close
; tolerances. To take an example using a beta ray backscatter
gauge, an increase in beta ray reading without an increase in
~,i
weight of similar magnitude indicates that the amount of pigment
has increased. Taking the example previously described~ if the
beta reading increased 10~ while the weight increased 1~, it
would be clear that there had been approximately a 1~ increase
in the pigment added. I~ the beta ray reading decreased 10~
-- 25 and the weight also decreased 10~ such a comparison would in-
dicate that the amount of ~ilm decreased approximately 9~. As
,.:
a final illustration, if the beta ray reading increased 20~
while the weight increased 11~, the pigment addition would have
been increased by about 1~ and the rilm by about 10~. Thus, the
comparison gives a measure of the amount of pigment in the film.
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The amount of pigment added to the ~ilm can the~n ~e cont~olled
in order -to coxrect any deviations ~ro~ the desired amount.
The relationship between the thickness as determined
~y the radiation gauge and the t~lickness computed from the we~ght
measurement can readily ~e determined by calibration of the in-
strum~nts using known standards. The relationship will, of
course, vary according to t~e materials Cresin, pigment) used
and the type o~ radiat~on gauge.
.:,.
The conti~uous wei~ht meas~re~ent can ber-~de~by a
lQ roller scale. Scales of this type can be used ~or continuously
monitoxing ~he ~eights of films and other sheet materials~ They
comprise a pair of rollers over which the film passes, the
....
rollers beiny attached to a device for determining the weight
of the film resting on the rollers, e.g., a balance beam or a
. .
sensitive spring. The scale may be equipped with an electrical
j`~ sensing device to monitor the ~eight electrically and produce
an output signal proportional to the weight of the film. Calibr-
~; ation will enable this signal to he used to give a direct read-
ing of film weight. Also, since the rollers are not the only
2~ members which support the film (other rollers for advancing the
film also support itl, calibration is necessary if a direct read-
ing of a~solute film weight is desired. However, the present
method does not: require an absolute indication of the film weight
~ (actually, we.ight per unit area~ but rather requires only a
- reLative indication. Thus, the scale can be adjusted to a set
point and deviations from the set point monitored. Continuous
scales o~ this type are described, for example, in U.S. Patent
No. 2,726,922 to Merrill et al.
~s an alternative to the continuous monitoring scale,
3Q a semi-continuous scale can ~e used. This comprises a scale
.,
~ ~hich measures the weight of a particular amoLmt of film after
'
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- it has been cut off the continuous web advancing from the ex-
: truder. This type of scale does not give the close, continuous
control of the continuous scale (and is~ therefore3 less pre-
ferred from a theoretical point of view) but if the line speeds
-`5 are high enough and the sequential weighings performed suf-
. .
ficiently quickly, satisfactory control is, in fact, obtained.
The weighings may be performed on one or more cut-off lengths
-
of the film, either for purposes of practical convenience or to
gather a suf~icient weight of film material to prevent inaccuracies
,
in weighing. This type of weighing is particularly useful when
~ articles comprising a cut-off length of film are being producedg
!', ~or example~ bags from a continuous tubular film. The bags can
- be wei~hed singly after being cut off the tube or, for example,
in batches of five. This gives su~ficiently close control of
~15 the pigmentation.
- While the comparison of density and thickness produced
- through weighing and a radiation gauge could be performed manual-
ly, such a process would be extremely slow, as would the changes
-~ in pigment addition in response to the thickness comparison. As
a correct'Lon to the pigment addition is desired as soon as a dif-
- ference is noted between the radiation gauge reading and the weight
reading, ~he performance of the comparison by a computer is
advantageous due to the computer's speed and accuracy. The ap-
:.
plication of a computer to the method of this lnvent-lon greatly
increases its utllity. Either a special purpose analog computer
` or an appropriately programmed general purpose digital computer
can be used to effect the control responses. The invention will
- be described below with reference to the use of a general purpose
digital computer but it should be understood that analog control
may also be used.
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In Figure 1 of the accompanying drawings~ the steps
of the method are shown. The resin is fed into an extruder 10
... .
through resin inlet 11 and pigment is fed in through pigment
. ~
`~ inlet 12. In the extruder the res-Ln is heated, melted and in-
-~ timately blended with the pigment and the pigmented blend ex-
truded through the extruder dle in the normal way. The pigmented
extruded film 13 then passes to a radiation thlckness gauge 14,
preferably a beta ray backscatter gauge, which produces an output
- signal partly representative of the thickness of the film. How-
ever~ as mentioned previously, this determination i8 also depen-
.:
; dent upon the amount of pigment in the film. The output signal
~,; from gauge 14 is fed through line 15 to a comparator 16 which
~ also receives an output signal through line 17 from scale 18.
.... . .
The comparator 16 makes a comparison between the
thickness as determined by the radia~ion gauge and the thickness
- as determined by the weight measurement. From this comparison
an indication of the amount of pigment in the film can be ob-
tained. This indication can be fed by line 19 to control mecha-
nism 20 which determines whether there is a deviation from the
desired pigment content and, if there is, develops an appropriate
control response. The control response, in the form of an out-
put signal is fed through line 21 to pigment control valve 22
which regulates the amount of pigment admitted to the extruder
10 through inlet 12.
~5 The schematic shown in Figure 1 is,of course, in the
;'.
~ simplest form for purposes of illustration. Normally, the con-
:~ trol mechanism will have inputs indicating the rate of resin
,
~ feed, extruder output and so on, in order to maintain full con-
,~
trol over the process. If the comparator and control mechanism
~0 are digital devices, analog-to-digital converters will be incor-
,. i
porated in lines 15 and 17 and a digital-to-analog converter in
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line 21 to cont~ol yalve 22. Such mea~s~u~e~ are, of cour~e,
: well kno~n in th~ art.
. .,
- Referring no~ to Pigure 2, p~gment and thermoplastic
film resin are mixed in a ~opper 25 ~hich ~eeds an extr~der 26
with a d;e head 27. The pi~ment admitted to ~e hopper 25 from
a pigment aupply 28 is controlled ~y a pigment control valve 29.
~` The pigmented film emerges from the die head 27 in the form of a
~ble 30 ~hich i~ collapsed by rollers 31 and 32 to form a flat-
ten~d tube. The extruded film passes under a ~eta ray back-
. . lQ scatter thickness gauge having a source 33 which discharges the
~eta rays, source 33 heing surrounded ~y a radioactive shield
34. The backscatter detector 35 ~hich measures the amount of
beta rays reflected, is above the source 33 and shielded from it
by shield 34. The detector transmits a corresponding digital
and or analog ~ignal to a comparator ~6 ~y means of line 37, an
analog/~igital converter 38 and line 39.
The extruded iilm is ~eighed by a continuous scale or
,: .
weight sensor 4Q here illustrated schematically. This ~cale is
preferably of the roller type as will be discussed further below.
2Q The scale produces an output signal which is fed to comparator 36
by means of line 41, analog to digital converter 42 and line 43.
- The comparator 36 compares the output signal from the
radiation thickness gauge;with the output signal from the scale
~ :.
and, according to a predetermined schedule, dete~ines whether
. .................................................... .
there are any deviations from the desired pigment content. If
there are, it sends a control response to servo ~ontrol 44 through
~ line 45, digital-to-analog converter 46 and line 47. The servo
i,';
control operates pigment control valve 29 through oonnection 48,
so as to make the appropriate control response.
3U Figure 3 sho~s, again in schematic form, the~axtru~er,
.:
thickness gauge and co~tinuous scale for controlli~g the pigment
!~
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addition via a suitable control circuit. The extruder 60
`~- melts the resin and blends it with the added pigment. The
molten resin blend is extruded through a rotating annular die
61 to form a tubular film 62. The film may be oriented by
, pressurizing the interior of the t;ube, as is well known in the
art. Pinch rollers 63 and 64 trap the air in the blown up tube and
squash the tube flat to form a flattened tube 65 which advances
in the direction shown by the arrows.
A beta ~ay backscatter thickness gauge 68 is supported
0 over the film as it emerges from the die. The gauge 68 obtains
thickness readings around the whole periphery of the blown film
~s the die rotates. The output from the gauge is taken to the
~ processing circuits by cable 69.
- This type of thickness gauge obtains an average or
~-5 integrated representation of the film thickness. Thus, any
minor local deviations of pigment concentration which may arise
do not cause a control response to be taken unnecessarily. Simi~
: .;:. .
` larly, a time delay response may be built into -the control mecha-
nism so that short-lived deviations do not cause an unnecessary
0 correction. Thusg area and time integration can be used to pre-
vent unnecessary control responses which could, in fact, disturb
the overall standard of pigmentation.
; The film is weighed by means of a continuous scale or
weight sensor comprising a pair of rollers 70 and 71 in a sup-
porting frame 72 arr~ged so that the flattened film passes over
- the rollers and rests on them. The supporting frame 72 is sus-
.:
pended by a rod 73 from a balance beam 74 pivoted about axis 75
by a suitable beam suspension. At the end of beam 74 remote
from rod 73 is a spring 76 whose loading can be ad~usted by means
0 of screw 77 fitted with graduated ~nob 78. A rod 79 is also
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attached to this end of beam 74 and it extends down ~nto
balancing or differential transformer 80 in which it supports
.
- an iron core which~ at the null point, rests evenly between
the two transformer windings. Changes in the weight of the
i film will cause the core to move in the transformer and this
will change the electrical balance which can then be detected
as an output analog signal. The null position can be set by
control knob 78 to allow for different film weights.
This type of weight sensor gives only a relative in-
.0 dication but this is sufficient for the present purposes as it
- can be set to the desired weight and if an indication of absolute
weight is required it can be calibrated.
- Figure 4 shows, again schematically, another radiation
th~ckness gauge and weight scale. The extruder 90 extrudes the
!5 pigmented resin blend through die 91 to form a tubular film 92
which is collapsed by pinch rollers 93 and 94 to form a flattened
tubular film 95 which moves in the dîrection shown by the arrows.
An integrating or averaging beta ray backscatter thickness gauge
.:
with travelling head 96 on twin transver-se rails 97 is positioned
'0 over the film. Head 96 which contains the beta ray source and the
` ionization chamber detector can be traversed along the rails by
means such as a threaded rod or a piston and cylinder (not shown)
- 60 that a particular portion of the film can be examined or it
can be continuou~ly traversed back-and-forth to give a reading
-25 averaged over the whole width of the film. The output signal
~rom the detector is passed to the comparator and control mecha-
nism by cable 98.
;:
` The ~attened film then passes over guillotine bed
~` 99 and it can then be cut off by guillotine blade 100. The
~30 portions of the film which have been cut off fall onto scale pan
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~48~;~6
lQl ~hich has suitahl~ ~ight ~en~QX$ underneath it (not ~ho~n2.
The output signal fr~m t~e ~e~rs can ~e ~ed out ~y cable il-
lustrated diagrammatically hy dashed line la2. The cut portions
of the ~ilm can ~e removed eit~er manually or ~y suitable auto-
matic devices a~ter one or more portion~-have accumul~ted on the
scale pan. The control mechanism can ~e calibrated to allow for
di~ferent numbers o~ cut off portions on the pan~as desired~
As prevîou~ly m~ntioned, the comparison between the
-- weights as determined by the radiation gauge and the film weight
lQ can be determined either by a spQcial purpose analog computer or,
with suitable digitisation of t~e input data, by a general pur-
pose digital computer suitable programmed. A typical subroutine
for a general purpose digital computer (~oneywell 316) is ill-
ustrated in schematic form in Figure 5 and described briefly
below, as follows:
Color control pulses/mil Deviation IDUM(l)
:::
Bag Weight Grams x lQ IDUM(3)
Bag ~idt~ IDUM(5)
MIN Pulses for Color Ctrl ID~M(6)
2~ MAX Pulses for Color Ctrl IDUM(7)
LENGTH of two bags ASUM~3)
.: .
AVG Backscatter gauge reading GSUM(10)
Film Density DENS
LINE NO
1 C .... Color Control (CT~L 1
2 W = IDUM (3~
3 W = W/4535.9
4 F = IDUM ~5
~ 5 F = F/1200
- 6 C .... Calculate the Div from Avg Fab
;~i Gauge Reading
7 G ~ ~/(.5 x ASUM (3~ x Dens x F) - GSUM (10)
8 C .... Check ~/- Di~
9 IF~G~ 1,7,2
3Q lQ 1 I ~ 1
11 Go to 3
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12 2 I = 2
13 C Calculate Color Ctrl Pulse B
14 3 F = IDUM (1)
ICTRL = ABS(G)*F
16 C Check Min and Mac Ctrl Pulse
17 IF (ICTRL - IDUM (6))7, 7, 4
18 4 IF ~ICTRL - IDUM ~7))6, 6, 5
19 5 ICTRL = IDUM (7)
C ~... Set Control Table
21 5 NCTRL (I) = ICTRL
22 7 RETURN
- 23 END
.
(1) Line 1 is a comment titling the color control
subroutine. -
~ (2) Line 2 equates W with the bag weight in Decagrams.
: (3) Line 3 converts W into pounds.
- ~4) Line 4 equates F with the bag width.
~ . .
(5) Line 5 converts F into feet.
- (6) Line 6 is a comment titling the subcategory of
. ,,
~0 calculating the dif~erence in beta ray backscatter
gauge and weight thickness reading.
~: (7? In line 7 G is set equal to the bag weight W,
divided by the bag length (0.5 times the length
of two bags) times the film d~ sity times the bag
~.
width (thus determining the bag thickness as de-
kermined by bag weight) rninus the average beta
. "j,
~`- ray gauge reading,
(8) Llne 8 is a comment stating that the value of G
will be evaluated to ascertain i~ itis negative,
~-~) zero or positive.
,;~ (9) Line 9 states that if the difference in thickness
?' of the pigmented fllm G (as determined by weight
le 68 the thickness determined by beta ray gauge)
.
;; is negative~ the computer is directed to statement
.
1 (line 10) if G is zero, the computer is directed
to statement 7 (line 22) which in turn directs the
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computer to return to the main program, no
color control correction being necessary; if
G ls positive, the computer is directed to
statement 2 (line 12);
(10) Line 10 equates I with 1.
- (11) Line 11 directs the computer to Statement 3.
(12) Line 12 equates I to 2!
(13) Line 13 is a comment titling the subcategory of
the subroutine as "calculate control pulses".
i (14) Line 14 equates F with the color control pulses/
mil deviation.
(15) Line 15 equates ICTRL with the absolute value of
G (difference between weight thickness and beta
ray thickness in mils) times the color control
i pulses/mil deviation.
(16) Line 16 is a comment titling a subcategory of the
subroutine as a check for minimum and maximum
control pulses.
(17) Line 17 subtracts the minimum control pulses from
.... .
l ICTRL. I~ the result is negative or zero the
computer is directed to return to the main purpose
via statement 7; if the result if positive, the
~ computer is directed to statement 4 (line 18).
; (18) Line 18 subtracts t~e maximum pulses for color
..
~j control from ICTRL. If the result is negative
or zero the computer is directed to statement 6
(line 21); if the result is positive~ the com-
puter is directed to statement 5 (line 19).
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