Ultrasonic web edge detection method and apparatus

The edge of a web of material is inserted into a gap in a detector head between an ultrasonic blockage transmitter and an ultrasonic blockage receiver such that the magnitude of the pulse of sound from the transmitter that is received by the receiver is related to the portion of the web that blocks the beam of sound and thereby the position of the edge of the web. The ultrasonic frequency of each pulse is preferably very high to provide a narrow beam. The second half of the electronic drive pulse to the transmitter is preferably 180.degree. out of phase with the first half of the pulse to reduce excessive ringing of the transmitter. A compensation transmitter and compensation receiver, which are mounted proximate to the blockage transmitter and the blockage receiver, transmit similar sound signals across the gap but are unoccluded by the web. The apparatus includes a controller with a microprocessor which: PA0 (A) receives the electrical pulses from the two receivers, PA0 (B) determines the peak values of the pulses, PA0 (C) averages pulse peak values to provide averaged values which reduce the effect of spurious signals variations, and PA0 (D) normalizes the value of the blockage receiver signal with the compensation receiver signal to compensate for transient changes in ambient conditions, to provide an error correcting output signal which can be used to bring the position of the web back to a desired position.

FIELD OF THE INVENTION 
This invention pertains generally to machines for the handling of web and 
sheet materials and particularly to apparatus for monitoring the position 
of the edge of a moving web to allow the position of the moving web to be 
controlled. 
BACKGROUND OF THE INVENTION 
In the handling of various types of web and sheet materials, it is 
important to be able to accurately position the moving material to ensure 
that the material remains on track and precisely aligned for various 
subsequent operations, such as cutting, slicing, printing and the like. 
Edge detectors which detect the lateral position of the edge of the moving 
web are utilized in such industries as paper making and converting, where 
the moving material is paper or nonwoven fibrous webs, in the printing 
industry, for photographic film manufacturing, and in the plastic 
packaging and forming industry. 
A variety of techniques have been utilized to sense the position of the 
moving web, including photoelectric sensors in which the amount of 
interruption of a beam of light by the web is detected, air sensors in 
which a moving stream of air is directed across the edge of the web and 
the occlusion of the air is detected, and ultrasonic sensors which direct 
a beam of ultra-high frequency sound across the edge of the web and detect 
the amount of occlusion of the beam by the web. These transducers provide 
an electrical signal which is related to the lateral position of the web, 
with this signal being utilized to control positioning mechanisms to bring 
the moving web back to its desired edge position. Ultrasonic edge position 
detectors have a number of advantages over photoelectric and air 
transducers, particularly with transparent or translucent web material 
such as thin paper sheets or transparent plastic, where photoelectric 
sensors may be difficult or impossible to use. 
In an ultrasonic web edge detector, a sound emitting transducer 
(transmitter) projects a beam of high frequency sound across a gap where 
it is either received directly by a microphone (receiver) on the other 
side of the gap or is reflected back to a microphone. As the edge of a web 
enters the gap, it partially blocks the sound beam, with the sound energy 
received by the microphone being roughly inversely related to the 
percentage of occlusion of the sound beam by the web. The relationship 
between the degree of occlusion and the signal provided by the microphone 
can be determined for a particular web material and the processing 
electronics which receives the signal can be adjusted accordingly so that 
the final control signal is truly proportional to the lateral position of 
the web edge. 
While ultrasonic web detectors enjoy several advantages over other types of 
edge sensors, various ambient operating conditions can affect the accuracy 
of the control signals produced by the sensing system. For example, 
changes in the relative humidity of the ambient air can affect the 
propagation of the ultrasonic signal and thereby affect calibration, so 
that a sensor which is properly calibrated on one day may be somewhat off 
in its readings the next day when the ambient atmosphere has a different 
relative humidity. Preferably, the edge detector should be relatively 
insensitive to the elevational position of the web in the gap so that as 
the web moves toward or away from the receiving transducers because of 
transient undulations in the traveling web, the sensor does not interpret 
these motions as changes in the lateral position of the web. Conventional 
non-pulsed ultrasonic sensors have problems due to the continuous nature 
of the sensing beam of energy. Reflections of this energy will cause 
interference from the reflective energy to be sensed in addition to the 
desired portion of the unblocked beam. These reflections are portions of 
the ultrasonic energy that have been returned to the detector sensor after 
bouncing off of objects not in the immediate area of the transducers and 
can interfere with and greatly reduce the accuracy of the sensors. This 
reflected energy problem can be reduced by pulsing the ultrasonic signal 
from the transducer. A particular problem that has been experienced under 
certain conditions with pulsed ultrasonic transducers is the phenomenon of 
"ringing", in which the transmitter continues to oscillate after it has 
received a burst of signal energy near the resonant frequency of the 
transmitter. Other conditions which can affect the accuracy of the reading 
from the edge sensor include the temperature of the air, which also 
affects the sound conduction of the air in the gap, the temperature of the 
ultrasonic transducers which affects their sensitivity, and air currents 
in the gap which can cause transient variations in the signal produced by 
the sensor and which effectively add a "noise" component to the signal of 
interest. 
SUMMARY OF THE INVENTION 
The present invention provides ultrasonic web edge detection which is 
relatively invariant to changes in ambient conditions, such as 
temperature, humidity, air or adding nitrogen or other cases to the 
ambient air currents and the elevational position of the web, to produce 
an output control signal which is a highly reliable estimate of the web 
edge position. The apparatus of the invention utilizes a detector head 
with a gap into which the web can pass. A blockage transmitter transmits a 
beam of ultrasound across the gap to a blockage receiver with the edge of 
the web partly occluding this beam. The detector head further includes a 
compensation transmitter and compensation receiver mounted in close 
proximity to the blockage transmitter and blockage receiver to transmit a 
second beam of ultrasound across the gap at a position which will not be 
occluded by the web. Any transient ambient conditions which will affect 
the transmission of sound across the gap, such as changes in air 
temperature or humidity, or transient air currents, will affect the beam 
between the compensation transmitter and compensation receiver in 
substantially the same way as the beam between the blockage transmitter 
and the blockage receiver. A signal from the compensation receiver may 
then be utilized to compensate or normalize the signal from the blockage 
receiver so that the effects of changes in the aforesaid transient 
conditions can be cancelled out. The analysis of the signals from the two 
receivers is preferably carried out in a controller employing a 
microprocessor which receives a digitized version of the signals from the 
two receivers and utilizes software programming to provide the proper 
compensation or normalization. The microprocessor may also be programmed 
to properly accommodate the particular material of the web to provide an 
accurate reading of web position. 
The apparatus of the invention also preferably utilizes a pulsed sound 
output operation in a manner which reduces the ringing that may otherwise 
occur. Each of the transmitters is controlled by the microprocessor to 
provide an output pulse comprised of a properly shaped high frequency 
sound signal, preferably at a frequency of approximately 200 kHz. Such 
high frequency signals result in a particularly narrow and well defined 
beam of sound across the gap in the detector head, enhancing the accuracy 
of the measurement of web position since the sound which passes the edge 
of the web will spread less than conventional lower frequency sound 
signals, which are usually in the range of 40 kHz or less. In addition, 
the pulse is preferably composed of two half portions at the desired 
frequency, with the second half portion being preferably 180.degree. out 
of phase with the first half portion. The change in phase of the sound 
signal has the effect of reducing the ringing of the transmitter 
transducer since the energy in the second half of the input signal to the 
transducer is out of phase with any resonance that has built up in the 
transducer during the first half of the input signal. Generally, the 
optimal frequency to obtain the minimum length of required pulse width is 
the resonant frequency of the transducer. By properly forming the driving 
pulse to the transducer, particularly with the phase reversal, a driving 
pulse can be used which is at the resonant frequency of the transducer 
without producing excessive ringing. The result is a short pulse burst 
having an envelope with a well defined peak. The electrical output signal 
from the receiver can be evaluated to measure the peak of the envelope of 
the received signal, with the value of the peak being roughly inversely 
related to the portion of the beam which is occluded by the web. 
The output of the blockage receiver or the compensation receiver is a 
series of pulses which are analyzed to provide a series of pulse peak 
magnitude values; these are utilized by the microprocessor controller of 
the system to determine the relative web edge position. The series of 
numerical values which are received by the microprocessor corresponding to 
these peak measurements will contain information on the actual position of 
the web edge corrupted by non-systematic time varying signals which are 
unrelated to web position, i.e., "noise". This noise may be due to such 
transient phenomena as localized air currents, dust, dirt, spurious sound 
signals which are picked up by the receiver, rapidly varying changes in 
the elevational position of the web, and so forth. Generally, these noise 
components will change at a rate faster than the rate at which web 
position would ordinarily change. To minimize the effects of these higher 
frequency noise components, the pulse height data is preferably smoothed 
by the microprocessor controller by performing a weighted averaging of the 
input data, with each new pulse sample value being added in a properly 
weighted manner with an average of a desired number of previous values. In 
this manner, the control signal provided by the apparatus is relatively 
stable and nonsusceptible to transient disturbances. 
The detector head of the present invention may be carried out in 
alternative embodiments, including a structure in which the transmitters 
and receivers are located at positions remote from the position of the web 
itself. For example, where a web is to be measured in a high temperature 
environment, such as in a dryer oven for photographic film, a web head may 
be utilized which is comprised of ultrasonic wave guides, formed as tubes, 
which extend from the transmitters and receivers located outside the dryer 
oven, through a wall of the oven, to positions inside the oven wherein the 
tips of the tubes define the sensing gap through which the web edge will 
pass. The tips of the tubes which extend to the compensating transmitter 
and compensating receiver are positioned closely adjacent to the ends of 
the tubes for the blockage transmitter and blockage receiver so that the 
conditions across the tips of the two sets of tubes will be substantially 
similar. 
Further objects, features, and advantages of the invention will be apparent 
from the following detailed description when taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
A detector head in accordance with the present invention is shown generally 
at 20 in FIG. 1, comprising a metal frame having a central base section 
21, an upper arm 22, and a lower arm 23. The upper and lower arms 22 and 
23 extend from the base and define a gap between them into which a web 25 
of material such as paper or film can pass. A blockage transmitter 27 
mounted to the arm 23 transmits a narrowly defined beam 28 of ultrasound 
across the gap to a blockage receiver 29, mounted to the other arm with 
the edge 30 of the web 25 shown blocking a part of the beam 28 for 
illustrative purposes in FIG. 1. Generally, the magnitude of energy in the 
ultrasound that will be received by the blockage receiver 29 will be 
roughly inversely related to the percentage of the beam 28 that is being 
occluded by the web 25, thus defining a relationship between the edge 30 
of the web and the energy received by the receiver 29. 
The detector head 20 also includes a compensation transmitter 32 mounted to 
one of the arms 23 and a compensation receiver 33 mounted across the gap 
to receive a beam of ultrasound 34 from the transmitter 32. The 
compensation transmitter 32 is mounted in close proximity to the blockage 
transmitter 27 and, similarly, the compensation receiver 33 is mounted in 
close proximity to the blockage receiver 29. The exact positioning of the 
transmitters and receivers is not critical, although the respective 
transmitters and receivers should be close enough together so that each of 
the beams 28 and 34 encounter substantially the same ambient air 
conditions. Generally, the transmitters and receivers may be positioned 
approximately an inch or two apart to yield satisfactory performance. It 
is preferred, although not necessary, that each of the transmitters be 
substantially identical in characteristics and similarly that each of the 
receivers be substantially identical. Under such conditions, the outputs 
of the receivers 29 and 33 should be substantially the same under similar 
ambient air conditions. However, the apparatus of the invention can be 
programmed to accommodate differences in the characteristics of the 
respective transmitters and receivers so that the output signal from the 
compensation receiver can be utilized to normalize or compensate the 
output signal from the blockage receiver in a satisfactory manner. The 
arms 22 and 23 preferably have beveled inwardly facing surfaces 36 and 37, 
respectively, as best shown in FIG. 10 to minimize reflections of sound 
energy off of the arms back toward the receivers. 
The transducers 27, 29, 32 and 33 may comprise, for example, conventional 
piezoelectric transducers consisting of a crystal disk with metal films on 
its two flat parallel faces to which alternating electrical potential is 
applied to cause the disk to vibrate. A preferred transducer is a 
Murata-Erie model MA200A1. The transducer may be in an "open" design in 
which the piezo element is mounted behind a protective screen or a closed 
design in which the piezo element is mounted directly on the underside of 
the top of the case which is formed to resonate at the desired frequency. 
Another structure for the detector head of the present invention is shown 
generally at 40 in FIG. 2. This head is especially adapted for sensing the 
position of a web in a hostile environment, such as within a dryer oven 
through which a plastic web or film web is passed. The detector head 40 
has a base section 41 which contains a blockage transmitter 42 and a 
blockage receiver 43. The signal from the blockage transmitter 42 is 
transmitted through a heat insulating coupler 44 to a hollow tube 45, 
which serves as an ultrasound wave guide, which has a tip 46 which is 
positioned at one end of a gap through which the web 47 passes. On the 
other end of the gap is the tip 49 of an ultrasound wave guide 50 which 
transmits the ultrasound energy received at the tip 49 back to the 
receiver 43, with an insulating coupler 51 connecting the tube-like wave 
guide tube 50 to the base 41. Similarly, the compensation transmitter (not 
shown in FIGS. 2 and 3) is mounted to the base 41 and transmits an 
ultrasound signal through a hollow tube wave guide 54 to a tip 55 at one 
side of the gap. A tip 56 of a hollow tube wave guide 57 receives the 
sound and directs it back to a receiver 59, with the tube wave guide 57 
being connected to the base 41 through an insulating coupler 60. The wave 
guides 45, 50, 54 and 57 extend through the walls 61 of the oven, with the 
base 41 containing the transducers mounted at a position remote from the 
wall of the oven so that the sensitive transducers are not exposed to the 
heat from the oven. The heat transmitted through the metal of the 
tube-like wave guides is insulated from the transducers by the insulating 
couplers. In this manner, the compensation transmitter and compensation 
receiver can accurately sense the ultrasound transmission conditions 
inside the oven, and the apparatus of the present invention can utilize 
the information from the compensation receiver to compensate accurately 
the signal received from the blockage receiver. 
It should be understood that the detector head of the present invention may 
also utilize reflection of the ultrasound signal across the gap. In such a 
case, the blockage transmitter and blockage receiver would be mounted on 
one side of the gap adjacent to one another and the compensation 
transmitter and compensation receiver would similarly be mounted on the 
same side of the gap (which may or may not be the same side as the 
blockage transmitter and receiver). To minimize cross signal interference 
between the two transmitters and receivers, it is preferred that each pair 
of transmitters and receivers be mounted laterally spaced from one 
another, in a manner analogous to the way in which the tips 55 and 56 of 
the wave guides for the compensation transmitter and receiver are spaced 
away from the tips 46 and 49 of the wave guides of the blockage 
transmitter and receiver. 
The transducer drive and signal processing components of the ultrasonic 
edge detection apparatus of the present invention are shown in simplified 
block diagram form in FIG. 4. An oscillator 70 generates continuous timing 
pulses at a proper frequency and provides these pulses on a line 73 to a 
pulse generator 71 which is controlled via control lines and data bus 95 
by the computer control unit 74 of the system to provide a desired output 
drive pulse, on a line 75, of a form and in a manner which is described 
further below. The output on the line 75 is provided through a multiplexer 
77 to either a first amplifier 78 or a second amplifier 79. The output on 
a line 80 from the first amplifier 78 leads to the compensation 
transmitter 32 and the output from the amplifier 79 on a line 81 leads to 
the blockage transmitter 27. The multiplexer 77, controlled by the 
computer controller 74 by a control signal on a line 82, allows the pulses 
from the pulse generator 71 to be directed to either the compensation 
transmitter or to the blockage transmitter, in a desired fashion, which 
may be alternating pulses, or, if desired, some other sequence. For 
example, the blockage transmitter may receive more pulses than the 
compensation transmitter since the conditions that the compensation 
transducers detect change relatively slowly compared to the web movement. 
The output signal from the blockage receiver 29 on a line 86 is provided to 
an amplifier 87 and then to a multiplexer 83 which is controlled by a line 
84 from the computer controller 74. Similarly, the electrical output 
signal from the compensation receiver 33 is provided on an output line 88 
through an amplifier 89 to the multiplexer 83. The multiplexer 83 is set 
up to connect its output line 90 to the proper one of the amplifiers 87 or 
89 so that the signal on the output line 90 will be from the blockage 
receiver 29 when it is desired to measure the pulses from the blockage 
transmitter 27 and will be from the compensation receiver 33 when it is 
desired to measure pulses from the compensation transmitter 32. 
The signal on the output line 90 from the multiplexer 83 is a continuous 
time varying or analog electrical signal which corresponds to the sound 
signal detected by one of the receivers 29 or 33. This analog signal is 
converted to digital data by an analog to digital converter 91. The 
converter 91 has a sample rate which is fast enough to obtain all the 
information in the signal on the line 90. For example, as explained 
further below, it is preferred that the frequency of the ultrasonic pulses 
from the transmitters 27 and 32 be at approximately 200 kHz. To properly 
sample this signal, the converter 91 thus must sample at least the Nyquist 
rate of 400 kHz, and preferably at a somewhat higher rate. 
The output data from the A to D converter 91 is provided to a greatest 
value latch 93 and to a comparator 94. Both the latch and the comparator 
94 are in communication with the computer controller 74 by a 
communications bus 95. The state of the comparator is also provided on a 
line 96 to the latch 93 and the latch also receives the reset/start signal 
on the line 72 from the computer controller 74. The comparator also 
provides its state on a line 97 to a timer latch 98, the output of which 
is provided on a bus 99 to the computer controller 74. The computer 74 is 
also in communication with a timer 100 by a communications bus 101 and by 
providing the reset/start signal to the timer on the line 72. The output 
of the timer 100 is provided on a line 104 to the timer latch 98. The 
computer controller 74 processes the signals that it receives and provides 
an output data signal on lines 105 to a digital to analog converter 106, 
the analog output of which is provided through an amplifier 107 to a motor 
driver 108 which drives a motor or valve controller for controlling a 
positioning roller or other Web Guide Device (not shown) to laterally 
position the moving web to correct the position of the web. 
The amplifier circuits 87 and 89 also preferably include band-pass filters 
centered at 200 kHz and the amplifiers may be of variable gain to allow 
gain control of the signals from the receivers. The computer controller 
74, under control of its software, selects one of the channels from the 
amplifier 87 or 89 to be further conditioned and read by the analog to 
digital converter 91. Typically, a converter can be used which requires 
signals to be in the range of 0 to 5 volts so that all negative signals or 
excursions must be converted to that range by conditioning circuits (not 
shown). Several options are available for conditioning and can be selected 
by another analog multiplexer (not shown). The signal may be passed on as 
is, or inverted, and the magnitude of the signal brought to within the 
desired voltage range. The signals from the ultrasonic receivers will be 
pulse bursts at 200 kHz. As explained further below, since the control 
program is evaluating the burst for a maximum or pulse peak, the signal 
must be converted to a rectified output to allow sampling in the range of 
0 to 5 volts. The circuits 87 and 89 provide this rectification and 
selective filtering. 
The analog to digital converter is preferably a high speed microprocessor 
compatible device, e.g., with an 8 bit output, which has a conversion 
rate high enough to adequately sample the 200 kHz pulse signal. For 
example, the conversion sample period may be 1.95 microseconds to sample 
the signal. 
Referring again to FIG. 4, when the computer controller 74 provides a reset 
signal on a line 72, the greatest value latch 93 is reset to its initial 
value and the timer 100 is reset. The computer controller 74 then puts out 
a start signal on a line 72 which starts the timer 100 and enables the 
greatest value latch 93. The start signal on line 72 also enables the 
pulse generator 71 which puts out a pulse to either the blockage 
transmitter 27 or the compensation transmitter 32. When the pulse from the 
transmitter reaches the proper receiver, output data will be fed from the 
analog to digital converter 91 to the comparator 94 and the greatest value 
latch 93. The comparator 94 continuously compares the value stored in the 
greatest value latch 93 with the new incoming data on the output line 92 
from the converter 91. When the comparator determines that the new value 
on the line 92 is greater than the value in the latch 93, the comparator 
provides an output signal on its output line 96 to enable the latch to 
accept the new value that is on the line 92 at that time. Simultaneously, 
the comparator provides an output signal on a line 97 to the timer latch 
98 to enable the timer latch to accept and store the new time value from 
the timer 100 at the time when the comparator enabled the greatest value 
latch to accept the new value. In this manner, the greatest value latch 93 
will ultimately contain the peak value of the pulse signal from the 
receiver and the timer latch 98 will contain the time at which this peak 
value occurred. If a transducer is placed such that a signal can be read 
that is a component of a reflected pulse, then once the acquisition is 
complete, the computer controller 74 can read the timer latch 98 to derive 
the time position of the peak relative to the start of the pulse and 
thereby determine the physical distance of the web material from the 
transducer. Such a reflected pulse may be obtained by utilizing a third 
receiver (not shown) that may be mounted closely adjacent the blockage 
transmitter 27 and whose output signal would be transmitted through 
another channel passing through the multiplexer 83 to the A to D converter 
91. 
The pulse generation is designed to control the pulse burst frequency to 
allow the minimum possible pulse width. The oscillator 70 may comprise, 
for example, a 20 mHz clock source and a programmable frequency divider so 
that the output of the oscillator 70 is at the desired frequency, which is 
preferably the resonant frequency of the transmitters 27 and 32. The pulse 
generator 71 acts to gate the output from the oscillator 70 to provide a 
particular pulse sequence when the start signal is provided from the 
computer controller on the line 72. The pulse generator 71 processes the 
output signal from the oscillator preferably to provide a series of pulses 
of the form illustrated in FIG. 6. The period of the oscillation is 5 
microseconds for a 200 kHz frequency, with each half pulse being 2.5 
microseconds in width. However, the oscillating signal undergoes a phase 
reversal halfway through the third pulse at a position indicated at 109 in 
FIG. 6, dividing the pulse signal provided on the line 75 into a first 
half portion and a second half portion, with the second half portion being 
180.degree. out of phase with the first half portion. When the waveform 
of FIG. 6 is provided to either of the transducers 27 or 32, where the 
carrier frequency of the pulse oscillation is at or close to the resonant 
frequency of the transducers, the output ultrasound pulse has the waveform 
of FIG. 7, building up to a maximum at the end of the drive pulse of FIG. 
6 and then decaying back to zero. 
A further block diagram of the computer controller with its input/output 
communications, comprising the block 74 in FIG. 4, is shown in FIG. 5. The 
computer controller includes a microprocessor 110 (e.g., a 64180 processor 
running at 6 MHz) with associated read only memory 111, random access 
memory 112, and erasable read only memory 113. A voltage monitor circuit 
115 and a watchdog circuit 116 are utilized to ensure relatively fault 
free operation. A pair of serial interfaces through an RS-232 
drive/receive interface 118 provide communication options, while a dual 
digit LED display 119 can provide basic diagnostic indications. A four 
channel counter/timer 121 can be configured as desired to be used in 
several ways under the control of software. Digital inputs are received by 
the microprocessor through optical couplers 123, which are connected to 
signals or switches located at a distance from the microprocessor, and 
digital switch inputs 124 and hexadecimal switch inputs 125 from front 
panel switches and push buttons provide direct communication by the user 
with the microprocessor. Digital outputs are provided from the 
microprocessor through high current digital output drivers 127. 
A basic flow diagram of the operation of this system as controlled by the 
computer controller 74 is set forth in the FIGS. 8-9. With reference to 
FIG. 8, after the program start, the system carries out initialization of 
all process parameters (block 130) and then proceeds to cause a pulse to 
be sent to the blockage transmitter (block 131). The program then receives 
the blockage receiver signal, finding and storing the peak value (132). 
The timer is then checked to see whether one second has elapsed from the 
time that the command to pulse the blockage transmitter was sent (block 
134); if it has, the compensation transmitter is then pulsed (block 135), 
the signal from the compensation receiver is received, its peak value is 
found (block 136), and the new compensation data is then averaged into the 
existing compensation average value (block 137). The new compensation 
average value is then used in block 140 to normalize the blockage data. If 
at the decision block 134 it was found that the one second timer had not 
yet run, the program jumps blocks 135, 136, and 137 and immediately 
proceeds to normalize the blockage data with the compensation average 
(block 140). This normalization may be carried out in various ways as most 
appropriate for the data being analyzed. For example, the normalization 
may be accomplished by dividing the blockage value by the compensation 
average value. The normalization may also be carried out by subtracting 
the compensation average value from the blockage value, or by appropriate 
weighted subtractions or divisions. Any such modification is referred to 
herein as "normalization" or "normalizing". The compensated blockage value 
determined at 140 is then averaged into the existing average blockage 
value at 141. This averaging may be carried out in various desired ways to 
optimize to the particular process being controlled. For example, simple 
arithmetic averaging of the existing blockage value with the new blockage 
value can be utilized, or there may be a weighted average which weights 
the new value differently than the existing average value, or the average 
value may consist of an average taken over a previous set of values. 
Different averaging techniques can be used which vary the weighting of the 
new sample vs. the old samples (existing average). As one example, the 
compensation data is averaged by weighting the new sample by 1/4 and the 
existing average by 3/4: 
##EQU1## 
Where A(N)=N.sup.th average 
S=New sample 
As a further example, the blockage data is averaged by weighting all of the 
samples in the average equally. The number (N) of samples in the average 
can be selected to be from 1 to 127. The most recent N samples are stored 
in memory. The averaging is done by calculating their sum and dividing by 
N. This can be called a "sliding" or "boxcar" average since all the 
samples used are given equal weight. 
The new blockage sample (data) is preferably normalized by multiplying it 
by the ratio of the value of compensation data at "standard" conditions 
(temperature and humidity) divided by the current value of the 
compensation data. The current compensation data used is the averaged 
compensation value. 
At "standard" conditions, the normalizing factor is 1, making no change to 
the blockage data. 
##EQU2## 
Where Vstd=Value of compensation data at standard conditions 
Vavg=Current averaged compensation value 
S=New blockage sample 
The averaged and normalized value is then utilized to calculate the error 
or deviation from the set value which corresponds to the desired position 
of the edge of the web (block 142, FIG. 9). The error is the difference 
between the "null" value and the averaged normalized blockage value. The 
absolute value of this difference determines the magnitude of the 
correction signal output to the motor (blocks 143-150), and the sign of 
the difference determines the polarity (in or out). 
The "null" value can be described as the "preselected value indicating the 
desired position of the web edge". Thus, 
EQU ERROR=NULL-AVERAGED NORMALIZED BLOCKAGE SIGNAL 
The amount of the error is then checked to see whether or not it is within 
a dead band value (143). If not, the error is then checked to see whether 
it is within a single pulse range (144). The single pulse range is the 
amount of error which can be corrected by a single output pulse. If the 
error is within this range, the program outputs a single width pulse to 
the motor for fine correction (145). If the error is not within the single 
pulse range, it is then checked to see whether it is within the double 
pulse range (block 147). If the error is within the double pulse range, 
the system outputs a double width pulse to the motor (block 148) to 
accomplish moderate correction of the web position. If the error as 
checked at 147 is not within the double pulse range, the system outputs a 
fixed level signal to the motor to achieve maximum correction (block 150). 
Exits from the blocks 143 (if the error is within the dead band value), 
145, 148 and 150 proceed to block 151 to display the data on the terminal 
to the operator in accordance with a selected display option. 
After completion of display of data at block 151, the program then proceeds 
to check for keyboard input (block 160, FIG. 9), and if there is no input, 
then the program proceeds to loop back (161) to again pulse the blockage 
transmitter at 131. If there is keyboard input, the system then inputs the 
process command (bIock 162) from the keyboard to change the process 
parameters and proceeds to return back through the loop to begin the 
process again. 
The operation of the pulse generator 71 may be illustrated with reference 
to the more detailed block diagram of FIG. 11. The major element in the 
control of the output pulse train from the generator 71 is a 10 bit (plus 
sign) Digital-to-Analog converter (DAC) 180. By controlling the amplitude, 
sign and reference inputs digitally, complete control of the output pulse 
train can be accomplished. The amplitudes or pulse height is determined by 
an 8 bit data value from the computer controller 74 through data bus 95. 
Sign control on the line 189 determines whether the pulse is positive or 
negative while the Reference Input on the line 190 gates the pulse On or 
Off. Proper time sequencing as well as pulse width is controlled by an 8 
stage shift register 182 whose clock frequency is determined by a variable 
count divider 184 under control of the computer controller. Pulse width 
control allows for the fine tuning of the resonant frequency of the 
transducers 27 and 32. Control logic in the form of flip-flops 186, 187, 
and 188 insures proper sequencing for start-up and for the gating of the 
sign and enabling (referencing) inputs. The output of the DAC is connected 
to the amplifier 78 for power gain prior to driving the transducer 27 (or 
transducer 32). 
Operation proceeds as follows: 1) the computer controller 74 determines the 
proper values for the DAC amplitude and the variable divider data and 
places that data at the respective points in the circuit; 2) the CPU 
initiates a Start command which sets the Start Flip-Flop 186; 3) Control 
logic allows the Variable Divider 184 and Shift Register 182 to generate 
clock pulses; 4) Logic connection to the Shift Register sets the Sign and 
Enable flip-flops 187 and 188; 5) For each additional pulse generated by 
the Divider 184, the sign level will change states until five pulses have 
been generated; 6) At this time the Sign flip-flop 187 is inhibited and 
the Enable flip-flop 188 is cleared causing the DAC 180 output to go to 
zero for one cycle; 7) Additional clock pulses will now generate four more 
output pulses; 8) the last pulse results in a reset of all the logic until 
the computer controller 74 generates another sequence. 
It is understood that the invention is not confined to the particular 
embodiments set forth herein as illustrative, but embraces all such 
modified forms thereof as come within the scope of the following claims.