Monitoring weld quality via forging assembly dynamics

To measure the relative power during a welding process, a position sensitive transducer is attached to a welding electrode to submit a signal in response to electrode motion. An accelerometer, carried on an axle of an electrode roll for a Soudronics pulse type resistance welder, will emit a signal indicative of the quality of the weld. The accelerometer measures the forging taking place during welding by means of its position sensitivity and the amount of forging has been found to be a function of the characteristics of the weld.

BACKGROUND OF THE INVENTION 
This invention relates to an apparatus for measuring the relative power 
consumed during a welding process and, in particular, covers an apparatus 
to be used as a transducer in connection with a Soudronic welder adapted 
to weld the longitudinal side seam of a thin metal can body. 
Soudronic.RTM., a Trademark of Soudronic A. G. of Switzerland, 8962 
Bergdietikon 2H, Schwerz, Suisse, welders for this type of application 
have a secondary transformer rating of 4 to 8 volts and 5000 amps. The 
welding is AC resistance type in the frequency range of about 50 to 500 Hz 
with each alternating waveform producing a power pulse. A traveling 
electrode being a copper wire is positioned between the surfaces to be 
welded and the electrode rolls connected to the output terminals of the 
secondary winding of the welding transformer. The copper wire is used 
between each of the electrodes and the metal surface to be welded and is 
moved continuously in order to prevent deterioration of the welding 
electrodes. 
Can bodies are generally hollow cylindrical constructions which are formed 
along a longitudinal edge into a closed cylinder leaving both ends open. 
The meeting edges of the cylinder thus formed from a flat blank of 
material are overlapped for purposes of welding. The blanks are preferably 
fashioned from preprinted (lithography), tinplate or tin free steel 
chrome-type such as MRT3 Such material presently ranges from 65 to 112 
pound plate weight per base box which represents a range of 0.007" to 
0.0123" in thickness depending upon the application of the container to be 
formed from the tinplate and/or tin free steel chrome-type. A welded side 
seam is preferable to other forms to side seams such as a soldered can 
seam or a glued together joint. More particularly, in aerosol containers 
which must be capable of withstanding up to 200 pounds per square inch of 
internal pressure, a welded longitudinal side seam has a great many 
advantages. Similarly, in containers which are of a particular 
configuration which is too large to be drawn (as, for example, a two-piece 
container is), a welded side seam gives the requisite strength and 
simplifies the manufacture of such containers as they are too long or too 
large for drawing. In other applications it is important to have 
lithograph information on the exterior surface of the containers. Quality 
lithography cannot be applied at high-speed to a preformed drawn container 
so a container with a manufactured side seam is required. 
Hall effect devices have been used in connection with a number of 
transducer applications some of which have been applied to welding 
machines see, for example, Noth U.S. Pat. No. 3,240,961; Hill U.S. Pat. 
No. 3,194,939; Barnhart et al U.S. Pat. No. 3,335,258 and Treppa et al 
U.S. Pat. No. 3,389,239. Each of the foregoing is designed to use a Hall 
device in combination with a welder for purposes of current determination. 
Similarly, the Hood U.S. Pat. No. 3,365,665 shows a Hall transducer which 
has been used in a system for measuring current flow in high voltage 
conductors e.g. power lines. Assignee of the present invention has a 
co-pending application on a Hall effect transducer, U.S. Ser. No. 093,855. 
These arrangements are not entirely responsive to the condition of the 
metal to be welded in that they primarily sense current flow and do not 
take into account the relative position of the welding electrodes. In the 
past welding monitors using voltage, current or Hall effect measuring 
transducers have been used to determine the condition of the power flow 
during welding. These techniques have been deficient in that they measure 
only one parameter which makes up the power available between the welding 
electrodes. 
Other techniques that have been used as a means of monitoring weld quality 
do not possess the desired lack of sensitivity to outside effects and in 
most cases measure a parameter that does not totally characterize the 
quality of a weld. For example, monitoring welding electrode voltage only 
insures that a voltage is present that is sufficient to produce the 
necessary heating if all other factors are constant such as surface 
resistance and plate integrity. If either of these factors vary there will 
be no indication of it by monitoring the welding voltage. 
As another example, monitoring welding current will yield information that 
insures that each attempt at weld nugget formation has sufficient current 
available to produce the required heating. However, should the plate 
weight vary, for example a 10% increase, there is no indication that 
welding current will change significantly since a 10% increase in 
thickness will result in an insignificant change in bulk material 
resistance. However, a 10% increase in thickness can have a significant 
effect on the rate of heat dissipation and the amount of metal which must 
be heated to an acceptable temperature. Without a corresponding change in 
welding current for a material thickness increase no detectable 
information is available on which to act. 
Neither voltage nor current monitoring or the combination of the two will 
accurately account for the insidious effects of intermittent. variable and 
unpredicted shunt resistance paths. These can momentarily alter the 
current flowing through the desired weld zone and thereby effect the weld 
nugget quality without leaving a measurable trace. As a single measurable 
parameter the weld forging roll dynamic motion offers a method of 
singularly monitoring the effect of any or all weld parameter variations 
and to provide an indicator value with which to adjust the easily altered 
welding current parameter. Electrode force or voltage are other control 
parameters which could be adjusted. In short, monitoring the weld forging 
roll dynamics appears to be a good measurement tool for the purpose. 
In a high-speed operation such as welding thin metal can bodies at several 
hundred per minute with an alternating current welder, the influences of 
input current and voltage as well as ambient temperature becomes 
significant when one is trying to measure small changes in the welder 
operating conditions. It is, therefore, the function of the circuit herein 
to completely compensate for the aforesaid conditions by providing an 
electrode motion responsive transducer which will be useful in monitoring 
the electrode forging action used to weld the side seam of a thin metal 
container and same will be set forth in the following summary. 
SUMMARY OF THE INVENTION 
A physical measuring device to determine rate of change in the electrode 
velocity normal to the direction of motion of the shell during welding 
whereby such a measurement can be used in a system for adjusting the 
welding power and/or rejecting defective welds. The concept appreciates 
that welding is a combination of heat and forging and same can be 
monitored by variations in the forging under constant force due to changes 
in the heat generated during welding. That is to say that, in areas of 
high resistance to the flow of welding power the heat generated will be 
greater thus permitting a greater amount of forging with the same amount 
of force. Consequently, the force on the welding electrodes, if constant, 
will vary the position of the electrodes relative to the weld as a 
function of the power flow. It becomes possible to measure the weld 
quality then by application of a position, velocity, or acceleration 
transducer mounted to an electrode. 
Such a transducer will give a dynamic signal which can be looped or fed 
back to control any of the parameters which will change the welding power 
input. The overall simplicity of this system is appealing in that 
conventional transducers can be easily applied to existing equipment and 
will give measurable signals that are usable for monitoring and control. 
A preferred arrangement of the present invention includes a single axis 
accelerometer to monitor the acceleration characteristic of the welding 
roll assembly in a Soudronic seam welding system. The system in its 
simplest form has an accelerometer attached to the spring loaded spindle 
for a welding roll that provides an output signal proportional to the 
second derivative of the vertical displacement versus time curve of the 
welding roll assembly or (d.sup.2 x/dt.sup.2), where x is an unknown 
displacement dependent on the weld forging roll spring mass dynamics, 
material plasticity characteristics and forging roll speed. Consequently, 
as the spacing between the rolls varies as a function of the heating and 
forging process, an electrical signal proportional to acceleration is 
generated. 
The advantage of this system for monitoring the weld operation is that in 
theory the welding roll dynamics should faithfully represent the result of 
applying heating and forging force to form a weld nugget. Stated another 
way, the follow-up motion of the weld forging rolls which have a fixed 
dynamic spring mass system will accurately and repeatably indicate whether 
the combination of welding parameters have achieved a successful weld 
nugget formation. 
The accelerometer transducer is electrically isolated with a ceramic 
standoff riding upon the end of an outer electrode spring spindle which 
applies the forging force. The accelerometer is a single axis instrument 
for generating vertical acceleration time waveform curves as the electrode 
rolls are displaced during welding. Changes in the adjustment of the heat 
control for the welder are measurable by changes in those curves. To 
calibrate, the welder is run without current flowing. This establishes 
background vibration not beneficial to the ideal formation of individual 
welding nuggets, for example, vibrations caused by the feed and gauging 
fingers preceding the electrodes and the lap thickness transition between 
can bodies. The low or no current tests also indicated the effect low heat 
has on the dynamic, vertical motion characteristics of the outer 
electrode. 
Recognizable and significant difference between the waveforms for 
acceptable production welds and incomplete welds are measurable. Reduced 
heating produced less plastic deformation of the steel joint between the 
electrodes, thus changing the slopes and the amplitudes of the 
accelerationtime waveforms. The repeatability of multiple waveforms made 
with the same welding schedule is best toward the middle of the welded 
seam of a given can. The transducer is sensitive to a very slight change 
in the heat control. 
The Soudronics heat control is a precise timing device which regulates the 
portion of a half cycle during which welding current is flowing through 
the welding transformer. One hundred percent heat control means that 
welding current flows for the maximum possible time during each half 
cycle. Delaying the gate signal which triggers conduction through a 
control SCR would result in a shorter welding current pulse in the 
transformer secondary, and less heat in the weld nugget. The Soudronics 
heat control varies time t in equation: 
EQU Welding heat=current squared, times the resistance, times the time t. 
The effects of a given heat control setting varies depending upon the 
characteristics of the switching of the SCR and the frequency of the 
alternating current flowing in the primary of the welding transformer. For 
example, a difference between 92% and 93% heat would produce a pulse 
duration change of the order of a fraction of a millisecond, and the 
accelerometer can sense the difference in the motion of the electrode. 
The accelerometer is a non-intrusive, non-destructive sensor capable of 
providing real time, dynamic information and an electrical signal which is 
a function of the formation of very nugget in the seam. The seismic mass 
in the accelerometer responds to the forging phase of nugget formation, 
and consequently it responds to all parameters affecting heating of the 
weld nugget. 
Considering the millions of seamweld nuggets which have been made, one must 
conclude that the average performance of the welder and the process are 
acceptable and that the vast majority of nuggets are properly formed. In 
order to improve process efficiency or welded seam quality the challenge 
is in developing a waveform pattern recognition system and discrimination 
strategy which will ignore the good nuggets and seek out the bad nuggets, 
i.e., welds. To correlate waveforms of the type generated fast enough or 
long enough to guarantee all cans produced, requires electronic circuitry 
which considers peak voltage, slope of the voltage curve, or the area 
under the voltage curve. An acceptable criterion for a two millisecond 
decision is required. 
Damping of the accelerometer of filtering the electrical output will 
attenuate irrelevant vibration in the welding roll assembly. A single 
parameter measurement is used to provide feedback information to a 
negative feedback servo loop control system. The function of the system is 
to compare the measured characteristics of weld forging roll dynamics to a 
predetermined set point and continuously adjust a welding variable such as 
current and time (I & t) or heat to maintain the preferred dynamic 
performance. 
Closed loop control in this system is extremely desirable since it will 
reduce process performance variation due to changing welding parameters, 
lack of objectivity on the part of operators and their inability to 
effectively follow the process because of its high speed nature. A 
Soudronic W.I.M.A. welding system generates many spot welds per second and 
this makes it impossible to exercise judgment as to the quality of each of 
these welds and effectively react to make corrections on a spot-to-spot 
basis. 
In a control system, a command or standard signal can be compared to the 
feedback signal from the accelerometer. Any deviation or error between 
these inputs can be used to implement adjustment of the welding heat 
control setting and/or forging force. The feedback signal is from the weld 
forging roll accelerometer and indicates the level of heating, weld joint 
condition and/or the level of forging action. 
Once the required level of forging associated with a good weld has been 
established as an input command standard level, the process can be 
controlled by a closed loop system which will continuously compare the 
input command with the feedback information. The parameters which vary 
under normal operation include, plate thickness, tin coating weight, 
forging force, electrode temperature, temper, surface roughness and 
surface resistance. The variations due to abnormal conditions include, 
joint fit up characteristics (overlap), contamination in the welding 
margin, burrs, abnormal shunt current paths and missing weld current 
pulses. Each of these parameters will result in a specific response of the 
forging roll accelerometer. Analysis of each will indicate that feedback 
information from the forging roll will be of the correct polarity or 
direction to provide an unambiguous control signal. 
OBJECTS OF THE INVENTION 
It is, therefore, an object of the present invention to provide equipment 
which responds to the forging process during welding of thin plate at high 
speeds. 
It is a further object of the present invention to provide a system which 
is instantaneously responsive to the level of resistance between the 
electrodes of a welder. 
It is still a further object of this invention to provide a monitor which 
is responsive to variations in metal plate thickness. 
It is yet another object of this invention to provide a technique which is 
simple, reliable, low cost and has the capabilities of detecting small 
differences in the metals to be welded.

DETAILED DESCRIPTION OF THE DRAWINGS 
Fusion processes such as electric arc, gas flame, or laser welding depend 
upon the flow of molten metal to achieve the bond and to overcome 
deficiencies in joint fitup. Resistance seamwelding of steel is like a 
blacksmith's forge weld. In that process, the object is to minimize 
overheating the metal until molten and to achieve bonding by the plastic 
deformation of red to white hot metal. To achieve this deformation, the 
blacksmith forged with his hammer. The spring loaded, welding rolls of a 
seam welder do multiple duty. As electrical and thermal conductors they 
perform vital heat control functions. At the same time, or only slightly 
out of phase, the electrode rolls produce the plastic deformation required 
for proper forging. 
The schematic sketch in FIG. 1 shows the preferred location of an 
accelerometer 10 at the end of the spring spindle 11. Transverse 
constraints 12 control the action of the spring 13 and keep the spring 
spindle 11 moving vertically. The spring 13 has a force which passes 
through the nip of the electrode rolls 15 and 16 for the outer and inner 
rolls, respectively. The weld nugget 17 being formed at any given instant 
of the welding is a function of the load from the spring and the welder 
power pulse. More particularly, a Soudronic ABM 270 (270 Hz frequency 
converter) produces 540 half cycle current pulses or weld nuggets per 
second. 
A vertical force vector free body diagram would consist of the spring 13 
preload acting downward, the mass of the electrode assembly accelerated by 
gravity, and the upward reaction force at the outer surface of the lap 
joint. The motion of the outer electrode roll 15 and the accelerometer 10 
is analogous to that of a car's hub cap as the wheel rolls down a highway. 
If the road is smooth and level (i.e., no current, no heat), there is no 
vertical displacement. When sinusoidal welding pulses (see FIG. 1) vary 
the heat developed between the electrode rolls 15 and 16, the displacement 
of the roll is like the car wheel traversing a series of uniformly spaced 
potholes filled with mud. The softer the mud and the deeper the hole, the 
more violent is the bouncing of the wheel and the larger are the vertical 
acceleration vectors. 
Oscilloscope photographs of acceleration-time curves have been made under 
various welding conditions. Without current flowing, the trace produced 
resembled a smooth highway, no potholes. The slight ripples in 
acceleration curves were due to variations in wire thickness, roll bearing 
eccentricities, or low level background vibrations. The differences in 
waveshape attributable to no heat (no current), proper heat and 
insufficient heat are obvious in such traces. 
A moving copper wire 18 upper and 19 lower is respectively on the outside 
and the inside of the can seam overlap. The wire is then used at 18 and 19 
as a traveling intermediate electrode. To eliminate problems of electrode 
contamination and for economy, both sides of the wire are used in the 
welding process. This is done by means of returning the wire to be used 
again but to use the other side thereof. The upper weld electrode roll 15 
applies pressure and the current passes through both the upper and lower 
electrode welding rolls 15 and 16 into sections of wire 18 and 19 which 
are the electrodes. The overlap seam is thus fused by pulsed electric 
resistance welding and one spot weld is made after another. These pulses 
generate the weld nuggets 17. The Soudronic welder uses a round continuous 
length of copper wire which is flattened by profiling rolls (not shown). 
The power supply for the welder is a motor operating off of standard line 
current which turns an alternator that generates single phase power at 380 
volts and 270 cycles per second. The alternator power is the input for the 
primary windings for a transformer coil. This input is not constant as the 
Soudronics' power control circuit (not shown) includes an SCR which turns 
the power on for a portion of each half cycle and turns the power off when 
the half cycle crosses zero voltage. Consequently, the input power is on 
for a percentage of the total cycle and provides 540 pulses per second. 
Each pulse should generate sufficient heat for a proper weld i.e., weld 
nugget 17. 
Turning now to FIG. 2 which is a microphotograph enlarged 150 times taken 
longitudinally along a lap joint, it will be noted that the grain size 
through the center of the lap joint is much greater. This evidences proper 
welding heat and forging pressure as is necessary to produce a continuous 
seamless bond. The grain growth is schematically represented in FIG. 1 as 
a weld nugget and is a crystalline area located primarily at the joint 
where a weld current pulse occurs. The pulsing is frequent enough to 
produce a continuous seamless joint but there are areas of greater grain 
growth which are consequence of the varying heat. The depiction in FIG. 1 
is exaggerated for purposes of understanding and the microphotograph shown 
in FIG. 2, is representative of the actual joint at the lap seam. This 
microphotograph represents a longitudinal section taken through the center 
of the lap joint and as such the ripples caused by the weld pulses are not 
immediately apparent along the outer surface of the joint. That is to say 
that, with the magnification and the process used to generate this 
cross-section, it is difficult to show in such a small portion of a 
longitudinal side seam weld the actual rippled surface as exaggerated and 
depicted in FIG. 1. 
In FIG. 3, an enlarged (150 times) microphotograph of a welded seam which 
is unsatisfactory is shown. The weld in this microphotograph is poor 
because insufficient heat was available to join the overlapped metals. As 
will be noted, there is a line at the middle of the joint which shows the 
complete failure to bond or weld. There is also a notable lack of 
crystalline growth in the central portion where the layers are juxtaposed. 
This depicts clearly a difference which without destructive analysis 
cannot be readily determined. 
It has been found that the waveform generated by the accelerometer 10 of 
FIG. 1 varies depending upon the nature of the weld e.g., that shown in 
FIG. 2 or FIG. 3. More particularly, as the amount of energy put into the 
weld increases the slope of the accelerometer spike, indicative of 
vertical acceleration will become more steep. In the event there is too 
much energy applied and/or force at the upper electrode 15, the spike will 
become so steep that longitudinal microphotographs of welds produced will 
show almost entirely crystalline structure having large grain areas and 
rough outer surfaces in that the overlapped metal is severely worked and 
spattered. Conversely, in the situation where the pressure and/or energy 
is inadequate the slope of the spike will be more horizontal indicating 
little or no forging during welding. When checked by microphotographs, the 
result will be like FIG. 3 with perhaps even greater spacing between the 
lapped metal which indicates complete failure to form a seam. 
In FIG. 1, the accelerometer 10 is shown schematically. However, in the 
preferred embodiment a Model EGC-500DS-50 Miniature Heavy Duty 
Accelerometer made by Entran Devices, Inc., of New Jersey was used. This 
transducer has a compensated operating range of 80.degree. to 180.degree. 
and is linear to .+-., less than 1%. The particular unit used in the 
preferred embodiment has a range of .+-.50 units of acceleration relative 
to the motion of the device with a sensitivity norm of 4 mV per unit of 
acceleration and a useful frequency of up to 600 Hz. 
The accelerometer was installed atop the spindle 11 inside the spring 13. 
In order to electrically isolate it from the power transmitted to the 
outer electrode 15, an insulator 20 was included between the spindle 11 
and the lever arm 21 which is used to carry the outer electrode roll 15. 
More particularly, electrode roll 15 rotates about a central axis 22 which 
is disposed at one end of arm 21 and the other end of arm 21 an axis 23 
carries it for swinging motion relative to the main support for the 
welder. Similarly, inner electrode roll 16 is carried for rotation on its 
axis 24 on the main support of the welder. The difference between 
electrode outer roll 15 and the inner electrode roll 16 is that the outer 
15 is permitted to swing with arm 21 relative to the welder. 
The lap joint 25 in FIG. 1 consists of the outer overlap portion 26 of the 
can body and the inner underlap portion 27 of the can body. These are 
brought together by conventional means (not shown), which rolls the flat 
precut body blank into an individual can tube arranged to have the desired 
amount of overlap and positioned to travel between electrode rolls 15 and 
16. Two feed fingers 28 (only one is shown) push and square the can body 
with respect to the electrode rolls 15 and 16. Can bodies may be fed at 30 
to 35 meters per minute which will make a 211.times.604 can at a rate of 
180 per minute. Such a can is made out of 75 pound plate being the common 
can maker's designation of pounds of steel per base box. The latter being 
a fixed area of 31,360 square inches per side of plate in a base box. The 
Soudronic's welder will put out between 20 to 25 pulses per inch, i.e., 
spot welds and the speed of the seam welding is a function of the weight 
of the plate from which the body is fashioned. At a given seam welding 
speed, production rate will increase as can height decreases. Thinner 
materials will permit higher seam welding rates. Between each container 
there is a space whereby the next adjacent can is approximately 1 to 2 
m.m. from the preceding can. The correct distance between adjacent can 
bodies must be maintained uniform at all times. If the containers are too 
closely spaced they will hit resulting in either a bad weld at the end or 
even welding together. Alternatively, if the containers are too far apart 
there will be a weld buildup at the longitudinal leading and trailing ends 
of the side seam. The distance between cans is adjustable by changing the 
electrode wire speed and can be easily determined from the weld wire after 
it has passed through the electrode rolls. Starting at the left side of 
FIG. 4, the accelerometer 10 is shown in block diagram form. Above 
accelerometer 10 is shown the type of trace being seriatum instantaneous 
transducer output which might appear on an oscilloscope were it to be used 
as herein described. The output of the accelerometer 10 is connected to a 
calculator circuit which could be programmed to calculate the area under a 
portion of the accelerometer trace or to approximate the slope of a 
portion of the accelerometer trace. Such curves are shown adjacent to the 
calculator circuit block. A differentiator or integrator can be used in 
the calculator circuit. 
It is important to appreciate that the trace taken from the accelerometer 
should be representative of an average welding pulse. For this purpose 
circuitry below the accelerometer block and calculator circuit block are 
included. More specifically, a trace of the welder power waveform is used 
to determine the portion of each pulse to which the calculator circuit 
reacts. More particularly, the output of the Soudronic's welder power 
supply alternator should be a pure and repeatable waveform which is 
synchronized but phase shifted with respect to the pulse depicted by the 
accelerometer output trace. The power waveform is input to a circuit which 
isolates the preferred portion of each cycle for analysis. That is to say 
that, the timing of when the calculator circuit operates is controlled by 
the isolation circuit such that an output of the isolation circuit will 
operate as an on/off control for the calculator circuit. 
To be certain that the accelerometer trace is taken during a representative 
portion of the welding of an individual can body an optically actuated 
switch is used to signal when the can body is disposed under the 
accelerometer 10. It is preferred that readings be taken at a more central 
portion of the can body such that transients at the ends of the container 
are not included. The optical switch is positioned to signal a pulse 
squaring circuit when the timing for accelerometer readings is proper. The 
squared pulse thus issuing sets a shift register set-reset circuit which 
controls a shift register designed to take four readings seriatum. 
Triggering of the shift register is also accomplished by a signal from the 
isolation circuit. Consequently, the shift register reacts in accordance 
with signals from the isolation circuit and the optical switch whereby 
four readings are taken during a prescribed portion of a can cycle. 
The four readings are sent from the shift register to four circuits for 
pulse sampling. The four samples obtained are then sent to a sample and 
hold circuit for each. The output from the calculator circuit is thus 
controlled and evaluated as data obtained in accordance with sample and 
hold. The four signals once analyzed are independently sent to a summing 
amplifier which averages the signals and gives a common overall output. 
The output is thus available for control of a Soudronic welder by means of 
an adjustment as described. The output is also available for reading on a 
meter, or an oscilloscope or as input for a computer and printer. 
The signals from the accelerometer can be used to provide a continuous 
signal suitable for use in a closed loop control system. In FIG. 5, a 
block diagram is shown for an arrangement which can be used to constantly 
monitor the welding process in contradistinction to the monitoring 
technique already disclosed. Examination of the entire side seam weld for 
a container is considered advantageous, but the ability to disregard 
information from unwanted inputs is difficult to overcome. Inputs such as 
vibrations from the various machine mechanisms or welding discontinuities 
because of the gap between containers have been major stumbling blocks. 
The circuit disclosed in FIG. 5 recognizes the problems of such inputs by 
analysis of the entire waveshape from the accelerometer output. This 
technique is different from the previously discussed technique in that 
more than a single side of a waveform or polarity of the accelerometer 
output is used. 
Computing techniques such as the use of RMS (Root Mean Square) calculations 
for analysis of the total accelerometer waveform output are meaningful. 
The results of such a calculation are a single polarity DC output which 
with filtering yield a visual presentation of the welder performance. 
Similarly, such an output can be used in a closed loop control system to 
adjust welder operation parameters on a slowly changing basis. The degree 
of filtering can be tailored to adjust the response time for the control 
loop or visual display. In addition, a filtered output signal can be used 
as a tracking reference to set the limits imposed for individual welding 
pulse analysis. 
The use of RMS conversion of the accelerometer signal appears to meet three 
primary requirements considered important to weld quality. That is to say 
that, the signal necessary for visual monitoring of welding performance is 
available. Moreover, a signal necessary for closed loop control of the 
process is provided. Finally, the reference value necessary for tracking 
an individual weld pulse and thereby detecting questionable weld nuggests 
is obtainable. The circuit design blocked out in FIG. 5 allows for easy 
compensation of the effects of gravity (for welder operating in a vertical 
plane) on the weld monitoring system so that the final output signal is of 
the form: 
##EQU1## 
where e in.sub.s =f(d.sup.2 x/dt.sup.2) 
where 
x=displacement of weld forging mechanism 
t=time 
e in.sub.g =f(gravational effects) 
The output of the computational conversion can be filtered through a simple 
integrating function such as: 
##EQU2## 
where t=the time constant of the filter circuit that sets the break point 
frequency of the low pass filters. Other types of filtering can be used to 
enhance specific characteristics of the signal. 
FIG. 5 shows the accelerometer output above the block for the 
accelerometer. The two leads of the accelerometer are connected to a 
differential amplifier which amplifies their output and rejects the common 
noise in both leads. The output from the differential amplifier is 
connected to a device which accounts for the effects of gravity. The 
device is based on an AC coupled amplifier with unity gain which centers 
the signal relative to the horizontal axis of the waveform. The corrected 
signal is then sent to a Root Mean Square converter. Such devices are 
useful for measuring electrical signals derived from mechanical phenomena, 
such as strain, stress, vibration, shock, bearing noise and acoustical 
noise. The electrical signals produced by these mechanical actions are 
often noisy, non-sinusoidal and superimposed on DC levels. The requirement 
for true RMS to provide a constant, valid and accurate measurement is 
satisfied by the converter. The waveform signal below the horizontal axis 
is shifted above the horizontal axis by squaring, and the output is 
changed from a waveform to a DC level or value for each welding pulse. 
Those individual values are a function of the shape and amplitude of the 
particular wave. The output from the Root Mean Square converter consists 
of a series of individual DC values which are representative of some 
parameter consistent with each welding pulse. It has been found that this 
parameter can be used as a measure of the successful or unsuccessful 
operation of the welder. 
For comparison and evaluation purposes, the unfiltered output of the RMS 
converter is provided to one side of a comparator differential amplifier 
and a filtered output is provided to the other input of the comparator 
differential amplifier. The filter is basically a low pass unit which is 
in the nature of an integrating amplifier. A resistance capacitance 
circuit sets the time constant for the integration. The comparator uses 
the filtered input as one reference against which the unfiltered input is 
analyzed. Should an individual DC level of the unfiltered input be 
substantially different from the datum established by the waveform of the 
filtered input a signal is transmitted from the comparator to a can 
rejection mechanism at the right time which automatically winnows the 
defective can from the production stream. 
Varying the amount of filter changes the waveform and the reference in FIG. 
5 to 1 (time constant) or 2 (time constant) is merely illustrative. Other 
connections to the output of the Root Mean Square converter lead to a 
welder control filter where the time constant is greater than that of the 
can rejection filter. Such a waveform establishes a slower and smoother 
rate of fluctuation as a function of the unfiltered signal. This filter 
may have twice the time constant of the can rejection filter and as such 
would provide a signal capable of adjusting welder parameters as mentioned 
herein frequently enough to keep the welder operating at peak performance. 
Likewise, a final filter with a still greater time constant called "N" can 
be used to amend the unfiltered signal sufficiently so that a visual weld 
quality display can be provided which will fluctuate with an appropriate 
frequency to exhibit the general trend of operation, thus permitting an 
operator to oversee the ultimate function of the machine. 
Consequently, the process of welding can be monitored, recorded and/or 
controlled by means of a simple device which measures and analyzes the 
actual process of forging during resistance welding. It is, therefore, 
desired that the invention in its broadest context include all circuits 
and transducer devices which operate to measure and evaluate the forging 
action which occurs during automatic welding. The claims which follow are 
intended to include all such arrangements and approaches which will 
achieve the concept hereinbefore stated.