Gloss sensor resistant to tilting and shifting paper and with improved calibration

A gloss sensor for determining the gloss of a surface, for example, of moving paper being produced by a paper making machine. The gloss sensor normally is installed in a scanner which scans the moving paper in a crosswise direction. It includes a standard TAPPI light source which impinges upon the paper at a specified angle and the intensity of the detected reflected light is related to gloss. Calibration is provided by an oscillating angle light source which with the same lamp has a direct reference path to the detector. Compensation for tilting or shifting paper is provided by the same oscillating light source which when the paper tilts or shifts still allows a reflected light beam to reach the detector. Change of angle from the standard TAPPI angle due to parallel paper shifts is compensated.

The present invention is directed to a gloss sensor resistant to tilting 
and shifting paper and with improved calibration and more specifically to 
a sensor for use with a paper making machine. 
BACKGROUND OF THE INVENTION 
In the paper making industry where paper is being produced at a high rate 
from a paper making machine, for quality and feedback control the paper is 
scanned crosswise by a moving head containing a number of sensors to 
determine parameters such as basis weight, moisture and gloss. The final 
value of gloss is a rather arbitrary number determined by standards in the 
paper making industry; namely, TAPPI standard T 480 om-90 which involves 
projecting onto the paper surface an incident beam of light at a 
particular angle (for example, 15.degree.), detecting the reflected beam 
and measuring its intensity. To calibrate the above TAPPI standard a 
polished black glass standard is used. Then an intermediate standard which 
is calibrated against that may be a polished ceramic tile. Some gloss 
sensors actually mechanically carry such a tile in a moving measuring head 
and lower the tile into the light beam to calibrate the instrument. This, 
of course, is mechanically complicated and there are some problems of 
environmental conditions such as heat, dirt and also accurate positioning. 
Another gloss technique of Valmet Automation of Canada provides a separate 
reference beam apart from the incident measuring beam. Here there are two 
separate light sources and detectors; moreover, the light source is a 
different type than the standard source defined by the above TAPPI 
standards. Thus, the correlation to the industry standard is suspect. 
Finally the moving paper sheet inherently tilts or produces waves so that 
the surface moves to, in some cases, during the measuring process cause 
the reflected beam to miss the detector entirely. Also there are parallel 
shifts. 
All of the foregoing implies a reliability of gloss measurement much less 
than desired. 
OBJECT AND SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
gloss sensor. 
In accordance with the above object there is provided a gloss sensor for 
determining the gloss of a surface comprising a light source for emitting 
an incident light beam onto the surface at an angle causing the light beam 
to be reflected from the surface. A light detector is positioned to detect 
the reflected light. In one aspect of the invention the light source 
includes a single lamp and means for aiming both the incident light beam 
onto the surface and also a reference light beam aimed directly at the 
detector without reflection. The light source provides an emission of 
light at oscillating angles substantially near the incident angle whereby 
any tilt or shift of the moving surface which would otherwise cause the 
reflected light to miss the detector is compensated for. The magnitude of 
the detected reflected light is sensed to determine the gloss and the 
reference beam is used for calibration purposes. Any error in the gloss 
value due to a parallel shift of the paper is compensated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a scanning sensor head 10 as it would scan a moving 
paper sheet 11, moving in the machine direction 12, crosswise to that 
direction (into or out of the drawing as illustrated in FIG. 1). The 
various light beams 13 illustrated incident on the paper surface of paper 
11 and reflected from it represent one slice of the paper 11. Eventually 
as the sensor unit 10 moves across the paper, hundreds of samples would be 
taken of the gloss parameter for a single crosswise scan Measuring head 10 
as discussed above might include other measuring devices such as for basic 
weight and moisture. However only the gloss sensor is shown and includes 
an overall light source 14 which has a lamp 16 which is a standard lamp 
which under TAPPI standards has a color temperature of 2850.degree. K. 
which is juxtaposed with a diffuser sheet 17 and a rectangular vertical 
window 18. Sheet 17 has a thickness of 20-30 microns and is composed of a 
tetrafluoroethylene resin; e.g. Teflon.TM.. 
Referring to FIG. 3 the diffuser sheet is shown at 17 and then behind it is 
a metal sheet or coating 19 having the vertical window 18. And the rays of 
the lamp 16 shining on the face of the diffuser sheet are shown as the 
rays 16'. 
Finally to provide an oscillating light source there is a rotating disk 21 
which is rotated by motor 22. Referring briefly to FIG. 2, the rotating 
disk 21 has an inner circular concentric track 23 and outer track 24 
including the spiral segments which are in the form of six apertures 26a 
through 26f. The inner track 23 has staggered apertures 27a through 27f. 
These are staggered radially to allow only one light beam (reference or 
measurement) at a time to be received by detector 26. The illustration of 
how these apertures line up with the effective vertical line light source 
or slit provided by aperture 18 is illustrated by the apertures 26' and 
27'. Since these apertures are spiral segments, they in effect oscillate 
between +1.degree. and -1.degree. as illustrated by the arrows in FIG. 3 
so marked. Thus when the light passes through these crosswise moving 
apertures an oscillating light beam is formed both by the inner ring 23 
and the outer ring 24. 
Referring back to FIG. 1 these two separate light beams are indicated 
generally by the light rays 13 and they pass through a planoconvex lens 31 
and through a source window 30 into the cavity 32 to provide an incident 
measuring beam or set of light rays 33 which is incident upon the surface 
of paper 11 and is reflected therefrom as shown by reflected beam 34 to 
impinge upon the detector 36. Then a direct beam 37, which is a reference 
light beam, which goes directly between the light source 14 and the 
detector 36. And there are of course an intermediate detector window 38, 
another planoconvex lens 39, angle reflector 41, a planoconvex lens 42 and 
finally the detector 36. Such detector is again in accordance with the 
above TAPPI standards to provide a CIE luminous efficiency function which 
has an effective wavelength of 572 nm. 
The output signal of the detector 36 is illustrated in FIG. 4A. It has a 
first part shown at 46 which is a measurement signal produced by the 
incident light beam 33 and reflected light beam 34 and then both earlier 
and later in time a reference signal 47 which is produced by the direct 
beam 37 between the light source 14 and the detector 36. Details of the 
processing of this signal will be discussed below. But one of the 
advantages of the present invention is that because of the oscillating 
light source produced by the spiral segments 27a through 27f (see FIG. 2), 
compensation or immunity to movement of the paper sheet by tilting or 
waves is provided. 
FIG. 5 illustrates a nominally flat sheet 11 and then a tilted sheet 11' 
where the incident light beam is generally shown at 33. In the normal case 
light ray 51 impinges on the paper sheet 11 at, for example, a 15.degree. 
angle in relation to the sheet, and then the reflected ray 52 goes toward 
the detector at the same angle. However, if the sheet tilts, as shown at 
11', that beam now becomes the light ray 53 and misses the detector. 
However, because of the oscillation of the beam by oscillating aperture 
27' (see FIG. 3) which changes the effective angle at which it hits the 
sheet, and also the location, the beam 50 will impinge upon the tilted 
sheet 11' at perhaps a relatively earlier or later point in time but will 
produce the reflected ray 54 which will be detected by the detector window 
55. A vertical shift of the sheet (see FIG. 7) is compensated for in the 
same manner to ensure the light ray hits the detector. 
Thus, in partial summary, the light source provides a crosswise moving slit 
across a fixed line source varying the angle of the beam to provide both a 
straight through path to provide both a reference and measurement beam 
with the same light source and detector beam. Secondly, the same crosswise 
moving slit provides for the measurement beam due to the oscillation 
provided by the spiral as indicated in FIG. 3, immunity or compensation 
for tilting or shifting paper. Of course, in the case of the reference 
beam, an oscillation is not absolutely necessary but if used, the required 
tolerances for the reference beam are relaxed. 
Referring to FIG. 2 it is believed that the segments or fragmental spirals 
are the most efficient for producing both the measurement and reference 
beam paths and providing an oscillating movement. But alternatives such as 
oscillating slits 26, 27, driven, for example, by a separate driving unit 
or a tuning fork could also be used. And, of course, rather than the 
segmented slits as in FIG. 2, a continuous spiral would work but reduce 
the sampling rate by perhaps an order of magnitude. 
The processing of the electrical signals from detector 36 are illustrated 
both in FIG. 6 and by the waveforms of FIG. 4. From a broad standpoint, 
all that needs to be done is that the peaks of the measurement and 
reference signals must be sensed, background noise subtracted out 
(indicated as signal minimum in FIG. 4A) and then the ratio taken to 
provide a signal directly proportional to gloss. 
Referring now to the circuit of FIG. 6 in conjunction with FIG. 4, the 
output of detector 36 is connected to the preamplifier 61 which drives the 
low pass filter 62 which has a signal output on line 63 which is that 
illustrated in FIG. 4A. A derivative of the signal output is taken at 64 
(FIG. 4B) and then fed through a comparator 66 to provide on the line 67 
the A output indicated in FIG. 4D. Thus the differentiation finds the 
peaks of the signals 46, 47 and these are the trailing edges of the 
waveforms A of FIG. 4D. The leading edges are generated by the signal 
minimum. Another comparator 68 compares with a voltage reference, Vref, 
(FIG. 4A) to provide the waveform B illustrated in FIG. 4C. In other 
words, the reference signal is distinguished from the measured signal 
because of its much higher amplitude which, of course, is true because of 
the direct line between light source and detector. .A "D" type flip-flop 
69 receives both the A and B signals as indicated and produces on its Q 
and Q bar outputs the signals shown in FIGS. 4E and 4F. 
To provide an analog measurement of signal minimum, as indicated in FIG. 
4A, there is an analog sample and hold (S/H) unit 71 with the A output on 
line 67 sampling the signal line 63. The two other S/H units 76 and 77 are 
driven respectively by the Q and Q bar signals (see FIGS. 4E and 4F) to 
provide the reference max signal and a measured max signal which are also 
indicated in FIG. 4A. 
Signal minimum, which is background noise, is subtracted via the units 81 
and 82 to provide inputs to the gloss computing unit 83 (see FIG. 9) of 
gloss-reference and gloss-measurement signals. The magnitude of 
gloss-meas. is related of course to gloss and the gloss-ref. signal is a 
standardization or calibration signal. In the computing unit 83 (FIG. 9) 
gloss (without being corrected for parallel paper shift to be discussed 
below) is a ratio of gloss-measurement and gloss-reference times a 
calibration factor A; namely, A(Meas./Ref.). The calibration is partially 
determined in the factory setup of the present unit by utilizing the black 
glass standard used by TAPPI. Then, of course, it depends on the various 
optical characteristics of the specific sensor unit dirt build up, drift, 
etc. 
However, in addition to compensating for the tilting of the moving paper 
sheet, a parallel or vertical shift of a sheet should be accommodated in 
the paper making process. When high gloss paper is being produced, the air 
bearing supports for the paper cannot control the level of the paper too 
severely or otherwise undesirable marking of the high gloss paper results. 
Referring to FIG. 7 this parallel shift is illustrated where the desired 
level or location of the paper is shown at level 1l' but actually the 
paper is at the level 11. This causes a change of the angle of reflection, 
as well as incidence of the light beam. But without any additional 
correction the oscillating angle light source of the present invention 
still accommodates this change of angle so that a reflected beam is still 
received at the intermediate detector window 38. But, as illustrated in 
FIG. 8, the received measured detected light waveform 86 is shifted in 
time designated the time interval T from the reference signal 47 compared 
to the nominal standardized 15.degree. signal 87 which has been designated 
as having a time interval T'. This is a theoretical time interval for the 
reflected light detected at the standardized 15.degree. TAPPI angle. The 
parallel shift shown in FIG. 7 from 11' to 11 causes this change of time 
which is directly proportional to angle of incidence and reflection. Thus 
in accordance with the present invention by comparing the actual measured 
time interval T to the theoretical time interval T' for the standardized 
angle of 15.degree., a corrected gloss measurement (corrected for parallel 
shift) may be obtained. Still referring to the comparison between the 
measured waveform 86 and a theoretical waveform 87, because of the 
shallower angle produced by the paper sheet being shifted to the position 
shown at 11 in FIG. 7, according to Fresnel's law, such shallower angle 
produces a greater reflection. Thus, the amplitude of waveform 86 is 
higher than the standard waveform 87 and the resulting measurement error 
is illustrated. This error has been found to be about eight percent per 
degree of angle change. 
To summarize, the deviation of the effective incident angle of the beam to 
the moving surface from a theoretical TAPPI angle is proportional to the 
change in time interval. As will be discussed below, because of Fresnel's 
law, this is non-linear. 
As discussed above, a suitable gloss measurement is provided even if no 
correction for a parallel shift is made by taking a ratio as illustrated 
in FIGS. 6 and 9 of the gloss-ref. and gloss-meas. outputs. However to 
correct for parallel shifts, as illustrated in FIG. 6 a duty cycle to 
voltage unit 72 is driven by the Q and Q-bar inputs and also a 10 volt 
reference unit 73 (10 volts is merely chosen for convenience; this could 
be any arbitrary voltage). The output is in a measurement termed gloss-pos 
which is also connected as shown in FIG. 9 to the computational unit 83. 
It is clear from the equation or algorithm shown there that "pos" is a 
correction factor which if not needed drops out and then the gloss 
measurement is dependent only on the ratio of the remaining measurements. 
FIG. 6A illustrates the circuitry for the duty cycle to voltage conversion 
unit 72. Here the 10 volt reference unit 73 is connected to one terminal 
of a switch 74 which is driven by the Q bar output between the reference 
unit and ground. An operational amplifier 75 with an associated capacitor 
C1 is charged toward the 10 volt reference voltage as long as switch 74 is 
in the position illustrated. Thus the charge on capacitor C1 is directly 
proportional to the duty cycle having a time interval T of the wave form 
86 as illustrated in FIG. 8. This voltage on C1 is captured by the 
switching capacitor filter 78 whose switch is driven by Q. Thus the output 
of the duty cycle to voltage unit 72 is 10 volts times the ratio of T and 
T.sub.O. This voltage is then coupled to the computational unit 83 in FIG. 
9 and the correction made as shown by the algorithm to provide a corrected 
gloss output. 
Specifically as illustrated the gloss sensor algorithm calculates the gloss 
from the following three sensor outputs: 
GLOSS-MEAS 
GLOSS-REF 
GLOSS-POS 
EQU Gloss=A*(I1/I2)*{1-C*(13/E-1)-D*(I3/E-1).sup.2 }+B, 
where: 
I1=GLOSS-MEAS 
I2=GLOSS-REF 
I3=GLOSS-POS 
A=aplus.sensor.gloss.coeA 
B=aplus.sensor.gloss.coeB 
C=aplus.sensor.gloss.coeC 
D=aplus.sensor.gloss.coeD 
E=aplus.sensor.gloss.coeE 
Default value for A is 100, B, C, and D will default to zero and E is a 
scaling factor with a default value of 100,000. I3/E is made equal to one 
when the TAPPI angle of 15.degree. is present. B is a calibration constant 
which varies the final gloss output by less than .+-.2%. 
The foregoing is accomplished in a standard computer language such as C++. 
The above equation is, of course, exactly equivalent to that shown in FIG. 
9. The squared term provides the non-linearity of Fresnel's law. 
With regard to the relative change in time illustrated in FIG. 8 between 
the actually measured signal 86 and the theoretical signal 87, whether 
this is a "leading" or "lagging" function is dependent on the direction of 
rotation of the rotating spirals illustrated in FIG. 2. 
Where the erroneous angle is due to tilt of the paper only the above 
correction is not believed to be as effective. But most flutter is mainly 
a parallel shift. 
Thus in summary an improved gloss sensor has been provided where because of 
the separate reference beam which uses the same light source detector as 
the measuring beam, standardization and calibration is easily 
accomplished. Then this same technique for producing the reference beam, 
that is the oscillating slit on the light source also compensates for a 
tilted or wavy paper surface and also parallel shift.