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Real-time control of nugget formation in spot welds
EUROMICRO EUROMICROJournal6 (1980)296-303
Real-Time Control of Nugget Formation in Spot Welds Alan A. Richard, Alan C. Traub and Riccardo Vanzetti
VanzettiInfrared& ComputerSystems, Inc., NeponsetStreet,Canton,MA02021,USA
In the mass production of resistance spot welds for commercial use, several uncontrollabZe variables tend to produce welds of varying quality. We describe a microprocessing technique which is used with an infrared thermalsensing method to ensure weld uniformity. An optical fiber bundle '~ipes" infrared radiation from a heated zone near the weld. A detection system provides a thermal signature of this zone, which reflects the progress of the heating inside the weld. A "standard" weld is made by trial and error, its thermal signature to serve as a model for all later welds of the same type. By controlling the welding-machine power in real time, the midroprocessor ensure that all such welds will have the same thermal signature and, t~us, the same quality.
I.
INTRODUCTION
We begin this paper with a background discussion, in Section 2, on how a resistance spot welding machine operates. The substance of our work is then described in the ensuing sections. An appendix to the paper describes the infrared temperature measurement process which is a v i t a l ingredient in our control method.
This paper describes a microprocessor temperature-feedback control system f o r use with e l e c t r i c a l resistance spot welders. I ts purpose is to improve the u n i f o r m i t y of weld-nugget size and strength in spite of normal factors which tend to cause non-uniform weld q u a l i t y . Such factors include l i n e - v o l t a g e v a r i a t i o n s , currentshunting by nearby welds, and v a r i a t i o n s in worksheet condition. Pre-heating of the worksheets by previously made welds is another f a c t o r , as are differences in the amount of heatsinking at the center and at the edges or corners of the worksheets. With conventional, open loop welding machines, such v a r i a t i o n s lead e i t h e r to weak welds, which were i n s u f f i c i e n t l y heated, or to overheated ones which are wasteful of energy and which age the welding t i p s prematurely.
2.
TYPICAL SPOT WELDER OPERATION
A t y pic a l resistance spot welding machine consists e s s e n t i a l l y of a supporting frame which holds together a number of separate systems. A lower welding t i p is securely mounted to the frame and an upper welding t i p is held in an air-actuated piston so that the t i p s may be brought together during welding. Both t i p s are connected to a low-voltage, high current source which is switchoperated and whose current and duration are c o n t r o l l e d by the operator. These controls and others are a v a i l a b l e at a b u i l t - i n or an adjacent control panel. A water-cooling system removes excess heat from the t i p s and from certain i n t e r n a l e l e c t r i c a l parts ("contactors"). and a two-stage foot-operated switch is provided f o r ( I ) lowering the piston and (2) applying welding power at the t i p s . A view of a t y pic a l welding machine appears as Figure I.
Various methods seek to overcome the weld q u a l i t y v a r i a t i o n s , such as metering the t o t a l applied energy per weld or by the use of acoustic emissions which occur during nugget formation, but these methods have not been widely adopted. In this paper, we describe a method which controls weld formation by c o n t r o l l i n g the welding power, in real time, so that the weld thermal h i s t o r y always follows a prescribed course. That course is the stored thermal h i s t o r y of a previously made t r i a l weld which was judged to be of acceptable qOality.
When two sheets of metal are to be spot welded together, they are pressed between the tips under c o n t r o l l e d pressure, an e l e c t r i c a l current is passed through them, and melting and fusion occur.
During weld formation, the temperature of a nearby zone is monitored continuously, i t s own thermal h i s t o r y being a scaled-down and s l i g h t l y delayed reproduction of the one which occurs inside the weld zone. Because the weld thermal h i s t o r y is one of the most important influences upon weld q u a l i t y , we believe that the control of this parameter is more important than any other.
In a d d i t i o n to heating and timing controls, most welding machines o f f e r such features as: Squeeze: This is an automatically controlled time period, s t a r t i n g when the t i p s close, during which the workpieces are held together under pressure before the welding current is applied. 296
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of the 50 or 60 Hertz l i n e , usually with a maximum duration of 99 cycles on small machines, or j u s t under two seconds. The time duration is often called the "weld count." When adjusting the weld count, the operator also adjusts the amount of current, using a control called "percent current" or "heat" or s i m i l a r terminology. In most modern machines, the actual means by which the welding current is c o n t r o l l e d is contrary to what one might expect. That is, i t is carried out by i n t e r r u p t i n g the welding voltage f o r a c o n t r o l l e d f r a c t i o n of each h a l f cycle of the l i n e frequency, instead of resist i v e l y or i n d u c t i v e l y varying the current amplitude. This method of control is called "phase-angle c o n t r o l . " Thus, f or any setting of the heat control below 100%, the welding power is switched on and o f f at a precise moment during each h a l f cycle. The switching is accomplished by what are called "contactors" in the primary c i r c u i t of the welding transformer, there being one each f o r the p o s i t i v e and the negative h a l f cycles. These are f a s t - a c t i n g switches capable of handling high amperage and t h e i r switching action is c o n t r o l l e d by timing c i r c u i t s in the control system. They take the form of e i t h e r s o l i d - s t a t e SCR's or gaseous discharge tubes called ignitrons Figure I. machine.
A t y p i c a l resistance spot welding
I t allows the t i p s to s e t t l e s l i g h t l y into the upper and lower workpiece surfaces by v i r t u e of cold deformation, thereby making better e l e c t r i c a l contact. Hold: At the completion of welding, the t i p s may again be a u t o ma ti c a l l y held together f o r a timed i n t e r v a l during cooling of the workpieces. Single/Repeat Selector Switch: I f the f o o t pedal is held down while t h i s switch is in the "Single" p o s i t i o n , only one weld w i l l occur u n t i l the pedal is raised and is lowered again. In the "Repeat" p o s i t i o n , this switch allows the welding cycle to be repeated as long as the f o o t pedal is down. Off: This is a timer control which inserts, between repeated weld cycles, a time i n t e r v a l during which no welding current flows. Its use is to allow the t i p s to cool s l i g h t l y between repeated welds so that they w i l l not suffer damage due to cumulative heating. This time i n t e r v a l is inserted between any "hold" and "squeeze" i n t e r v a l s which are in use. An important part of a spot welding machine is the e l e c t r i c a l current supply. Spot welders are usually operated from a 220 or 440 VAC l i n e . A massive transformer w i t h i n the welder reduces the supply voltage to a low voltage with a highcurrent c a p a b i l i t y , usually thousands of amperes. For each type of weld to be made, the duration of the welding current is previously selected by the operator, by use of a knob on the control panel. The units on this control are in cycles
Figure 2. Fiber optic cable conveys infrared thermal signal from w i t h i n the upper welding t i p to a nearby sensor. The mode of switching is as follows. Let us say that the operator c a l l s f o r "60-percent current" on his heat control. At the s t a r t of each v o l t age h a l f - c y c l e , a timing system in the e l e c t r o n i c control ensures that the contactors w i l l stay o f f u n t i l 40% of the h a l f cycle period has elapsed. The voltage into the transformer is then switched on, abruptly, f o r the remaining 60% of the h a l f - c y c l e period. Whichever contactor is conducting during t h i s time w i l l then return to the non-conducting state at the end of
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Real-Time Control of Nugget Formation in Spot Welds
the h a l f cycle, that i s , at the "zero crossing." The zero crossing is an important timing marker in an e l e c t r o n i c welding control because a l l i n t e r v a l s w i t h i n any h a l f cycle are measured from that moment.
optimized the controls f o r the best weld, he returns the switch to a "Run" mode. At t h i s time, the thermal signature of the last-made weld becomes locked into a memory, serving as a model f o r the automatic control system. Thereafter, the system makes the necessary moment-by-moment power adjustments during each l a t e r weld, thus steering a l l new thermal h i s t o r i e s on t h e i r proper courses so that a l l welds are formed a l i k e .
A 60-percent current setting does not necess a r i l y d e l i v e r 60% of the heat to a weld compared to a lO0-percent current setting. This is p a r t l y because the amount of heating varies with the square of the current. Moreover, because the current is changing during the conductive portion of a h a l f - c y c l e , a time i n t e g r a l is involved which is not l i n e a r with the units on the heat control d i a l . These units are, therefore, a r b i t r a r y values which allow the operator to return to a previous value or to vary the heating by a reproducible amount.
The operator may select his "standard" weld f o r a given run e i t h e r on the basis of a visual judg ment or by performing standard destructive tests on the weld nugget. 3.
Most welding transformers are provided with manually selectable taps on the primary winding so that current-values outside the range of the heat-control knob may be used. The heat-control knob is not normally set below 40% on 220-volt i g n i t r o n - t y p e welders or 20% on 440-volt welders because the f i r i n g of the i g n i t r o n s is r e l a t i v e l y unstable below a c e rta i n voltage.
The d r i v i n g element of the system is an eight b i t microprocessor which handles a l l of the control functions. These include the standard functions which are provided with conventional c o n t r o l l e r s f o r spot welding machines. Convent i o n a l c o n t r o l l e r s customarily make use of discrete l o g i c . The primary welding parameters are c o n t r o l l e d by the use of counters which generate delays in units of cycles of the AC power source. Time-intervals shorter than a h a l f cycle are managed by free-running clocks. These are used to measure pulse-width modulated
In the development to be described here, the operator sets a selector switch to a " C a l i b r a t e " mode and proceeds to make t r i a l welds in the normal manner, by operating his standard panel controls. When he is s a t i s f i e d that he has
NCS,OHAL S
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THE MICROPROCESSORCONTROLLER
A block diagram of the system is presented in Fig. 3.
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Fig. 3. Block Diagram of Automatic Control System f o r Spot Welds.
Alan A. Richard, Alan C. Traub and Riccardo Vanzetti signals f o r phase modulating the f i r i n g angles of the contactors (SCR's or i g n i t r o n s ) in order to control the time-averaged power into the workpieces. In designing the temperature-control function of our system, i t was obvious to us that a microprocessor was the most e f f i c i e n t means f o r storing and managing the temperature information involved in welding. I t also became clear that the same u n i t could handle the conventional control functions as w e l l , without the need f o r a d d i t i o n a l hardware. The I n t e l 8085 f a m i l y was used, o f f e r i n g a large selection of peripheral chips with combined functions. The CPU hardware is implemented in the conventional manner. The 8155 RAM I / 0 chip was chosen f o r two reasons. I t not only stores temporary constants and the temperature p r o f i l e but i t contains programmable input-output ports f o r accessing data. Nine bits of the 8155 are used fo r the a n a l o g - t o - d i g i t a l section of the system. The thermal data are quantized to one part in 256 using an e i g h t - b i t d i g i t a l - t o - a n a l o g converter connected to one port of the 8155. The output of the D/A feeds the comparator. The other comparator input is fed with the l i n e a r i z e d (see i n f r a ) temperature data. The comparator output enters the second 8 - b i t port as a single b i t . The software conversion routine performs successive approximations on the data. I t outputs a t r i a l data word and checks the comparator output to see i f the signal is above or below the t r i a l value. This t r i a l and error method starts with the most s i g n i f i c a n t b i t and ends with the l e a s t s i g n i f i c a n t . A ft e r eight attempts, the t r i a l value is equivalent to the input signal with a r e s o l u t i o n of eight b i t s .
is the value I00 minus the integer representing the desired percent current. Thus, f o r 60% current, the integer is 60 and the value entered is 40. At zero crossing, a countdown is started, and when i t reaches zero, a pulse is produced. This pulse is used to i n t e r r u p t the CPU which then f i r e s the contactor f or the remainder of the h a l f cycle. Because the RAM chip contained s u f f i c i e n t timing and input/output c a p a b i l i t i e s , the choice was made not to use a m u l t i - f u n c t i o n ROM I / 0 u n i t , which might otherwise have been done. The main memory containing the program is implemented by
The amount of r a d i a t i o n reaching the detector is a highly nonlinear function of t a r g e t temperature, as stated by Planck's Law and, thus, so is the thermal signal voltage. Therefore, a one-degree temperature change at a r e l a t i v e l y low temperature produces a much smaller signal change than a onedegree change does at a higher temperature. For t h i s reason, an analog l i n e a r i z e r is used before the A/D converter. Were t h i s not done, an A/D converter with an equivalent r e s o l u t i o n of 14 b i t s would be needed to d i s t i n g u i s h low temperature values at the s t a r t of weld formation. ( A l t e r n a t i v e l y , one might use an analog m u l t i plexer with a m p l i f i e r s and auto-range the A/D over three decades.) As an added advantage, the 8155 combination RAM I / 0 chip also performs a timing function which provides an i n t e r r u p t . In order to control the energy-per-half-cycle in the weld, one must convert a binary number in memory into a pulsewidth modulated signal. The CPU clock runs at 6.144 MHz. This is divided down to 83.333 ~Sec clock pulses, each of which is 0.01 h a l f cycle at the 60-Hz l i n e frequency used in the USA. At the s t a r t of each h a l f cycle ( t h a t i s , at each zero crossing), a number is loaded i n t o the counter which w i l l determine when the contactor is to f i r e during that h a l f cycle. This number
299
Fig. 4. Flow Chart of the System.
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Real-Time Control of Nugget Formation in Spot Welds
a conventional UV-erasable read only memory. The t o t a l program, including conventional weldingcontrol functions and temperature-control s o f t are, comprises s l i g h t l y less than 1,000 bytes when w r i t t e n in assembly language. Figure 4 is a s i m p l i f i e d flow chart of the software. A second m u l t i f u n c t i o n peripheral chip is used to con~nunicate with the welding operator. The 8279 is a keyboard-input~and-display d r i v e r . It contains an i n t e r n a l memory which automatically scans and debounces the thumbwheel switches which are used f o r entering the welding parameters. Such switches are used in our present system f o r entering values corresponding to weld count, percent current, squeeze, hold and o f f . The d r i v e r section of the 8279 is used to d r i v e LEDs which indicate when power is applied to the solenoid valve which controls the welding piston and also when power is applied to the welding t i p s . These functions are c o n t r o l l e d by the operator's twostage foot switch. In a d d i t i o n , other LEDs a l e r t the operator to various welding-machine malfunctions which may occur, such as tip-overheating due to the possible loss of cooling water, or a contactor malfunction. A group of ten LEDs is arrayed as a bar graph which, a f t e r each weld, displays the r e l a t i v e peak temperature f o r that weld, the displayed values being quantized into ten steps. In t h i s way, the operator may be reassured that a l l intended a l i k e welds are reaching the same peak temperature w i t h i n a few percent when a l l parts of the system are performing as they should. Otherwise, the need f o r a system correction would be indicated. This might be the case, f o r example, i f the operator were using 80% current f o r a normal weld and i f the reserve 20% of a v a i l a b l e welding current were not s u f f i c i e n t to correct an e s p e c i a l l y troublesome weld. In t h i s case, a lower peak temperature would be indicated, and the operator could correct t h i s by making a new standard weld at, say, 60% current, leaving a more ample reserve of power. The bar-graph display covers a range of peak temperatures from 10% below the value f o r the standard weld to 10% above i t . A peak temperature deviation greater than +10% causes a l l LEDs to l i g h t and an audible alarm to sound. Peak temperatures which are low by more than 10% r e s u l t in no LEDs l i g h t i n g and, again, an audible alarm. For each standard weld the 8279 adjusts the display so that the lower f i v e LEDs w i l l be activated when the correct peak temperature is reached. In t h i s mode o f display operation, which we w i l l c a l l the "peak-picker" mode, the 8279 f i n d s the maximum temperature f o r each weld and displays a corresponding value on the bar graph u n t i l the next weld is begun. In an a l t e r n a t i v e "tracking mode," which is a v a i l a b l e to the operator via a selector switch, the bar graph is used to display instantaneous r e l a t i v e temperatures while heating and cooling are occurring. These are displayed by a r i s i n g and f a l l i n g pattern of activated LEDs which simulate the temperature s i t u a t i o n in ten quantized steps. The tracking mode provides
CONTACTOR MALFUNCTION
RESISTANCE WELDING
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CONTROLLER SQUEEZE
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COOLING MALFUNCTION
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PEAKTEMP
WELD COUNT
THIS SPACE RESERVED FOR OPTIONAL FEATURES
RUN
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WELD
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%CURRENT SINGLE
VALVE
REPEAT
Fig. 5. Front Panel Layout of Production Prototype. visual temperature-rate information, as could be done by an oscilloscope, and may be used f o r diagnostic purposes. Fig. 5 shows a control panel layout which is intended f o r a production model f o r the near future. 4.
SOFTWAREAND WELDER OPERATION
The two manually selectable modes of operation which are a v a i l a b l e to the operator are "Calibrate" and "Run." (In Fig. 4 they appear as "Set Up" and "Automatic.") In the c a l i b r a t i o n mode, the system behaves as a conventional welding c o n t r o l l e r , allowing the operator to adjust his welding parameters by use of the control panel, but with one exception. Each time that a weld is made, i t s thermal h i s t o r y is entered into a memory, replacing any previous one which is stored. In the Run mode, the last-recorded thermal signature remains in memory f o r use on l a t e r welds u n t i l a new c a l i b r a t i o n is made. The thermal signature is stored in the form of a discrete voltage value f o r each h a l f cycle of the l i n e frequency. These are stored at successive locations in the RAM. At the end of each " C a l i brate" weld, the integer corresponding to the peak temperature is scaled up to a value of 256, with the m u l t i p l i c a t i v e constant being computed and stored. These values are cleared during each new "Calibrate" weld, but the last-acquired ones are retained when the selector switch is placed
Alan A. Richard, Alan C. Traub and Riccardo Vanzetti
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in "Run." Thereafter, a l l new thermal signatures are scaled up by the m u l t i p l i c a t i v e constant. The purpose of scaling is to ensure that a given thermal error signal at any moment during welding w i l l bring about a given amount of power correct i o n regardless of whether moderate-temperature or high-temperature welds are being made on a given day. A -10% signal error f o r the former, as an example, would have a d i f f e r e n t absolute value than a -10% e r r o r f o r the l a t t e r . The c o n t r o l l e r would thus c a l l f o r a d i f f e r e n t amount of correction in the two cases. The scaling method is the equivalent of converting absolute signal differences to r e l a t i v e percentage errors. I t offers the a d d i t i o n a l feature of u t i l i z i n g the f u l l eight bits of information in order to preserve r e s o l u t i o n of the error signal. In the "Run" mode, j u s t as the operator starts to make a new weld, the system performs j u s t as i t does in the " C a l i b r a t e " mode. The CPU scans the panel controls, picking up counts from the various switches (squeeze, percent current, weld count, etc.) and monitoring the position of the two-stage f o o t switch. When i t receives the command to supply welding current, welding power is i n i t i a l l y applied at the level which is called f o r on the percent-current d i a l . As welding begins, no computer action is taken u n t i l the signal voltage rises above a predetermined threshold. The reason f o r t h i s is to avoid a~ over-correction in the u n l i k e l y event that no thermal signal is sensed. This could happen because of a thermal-sensing system f a i l u r e such as due to an e l e c t r o n i c malfunction, mechanical damage to the o p t i c a l f i b e r s , and so f o r t h . In such an event, a weld which was progressing normally would be f a l s e l y sensed as underheated, and the correction system would c a l l f o r a large increase in welding power. Thus, i f the preset threshold is not reached, no correction is made and the weld proceeds normally. We c a l l t h i s threshold the " F a i l s a f e " threshold. I f i t is not crossed during a weld, an alarm l i g h t and buzzer a l e r t the operator to a possible malfunction. The correction function comes i n t o play a f t e r the f a i l s a f e threshold is crossed. From then on, once per h a l f - c y c l e , the thermal signal due to the weld is sampled and is compared with the corresponding one in the memory, that i s , with the one f o r the same h a l f cycle in the sequence. I f a p o s i t i v e or negative correction is c a l l e d f o r , the CPU adjusts the phase angle of the contactor f i r i n g f o r the next h a l f cycle. In t h i s way, i f an e r r o r is sensed i n i t i a l l y , j u s t a f t e r the f a i l s a f e threshold is crossed, the system "steers" the thermal signature back on course so that i t tends to duplicate the standard one in the memory. The same is true i f a thermal error should arise elsewhere in the heating cycle, such as due to the occurrence of a spark, which extracts power and i n t e r r u p t s the heating cycle. Likewise, a l i n e - v o l t a g e drop during welding, due to the start-up of other equipment on the same l i n e , w i l l be corrected by the system.
Fig. 6. Three "uncontrolled" welds made at d i f ferent transformer tap settings (Thermal signal versus time).
Fig. 7. Same three welds under automatic thermal control. Fig. 6 is an oscillogram showing the thermal signatures of three welds formed in the " C a l i brate" or uncontrolled mode of operation. The three were i n t e n t i o n a l l y made at d i f f e r e n t transformer tap settings so as to simulate a condition which would cause noticeably d i f f e r e n t thermal h i s t o r i e s . The welding conditions were otherwise i d e n t i c a l in the three cases. Approximately half-second welding durations were used. In Fig. 7, the lowest thermal signature of Fig. 6 was duplicated in the " C a l i b r a t e " mode, and the "Run" mode was then set i n t o operation. The two higher-current welds of Fig. 6 were then repeated, but the automatic control system held t h e i r heating cycles w i t h i n closer range of that f o r the "standard" weld. The closeness with which the system can duplicate a standard thermal h i s t o r y via power corrections depends upon several factors. These include the amount of correction which is called f o r in response to a given e r r o r signal, as well as the temperature and welding duration which are called f o r . In a d d i t i o n , there w i l l be a given amount of thermal time-lag because of the separation of the infrared-monitored zone from the weld s i t e , and the lag w i l l be greater when t hic k er metals are being welded. Because of
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Real-Time Control
o f Nugget F o r m a t i o n i n Spot Welds
this lag, a corrected curve will differ t o some e x t e n t f r o m t h e s t a n d a r d , as i s seen i n F i g . 7. Besides their other uses, software techniques a r e used f o r e l e c t r o n i c noise suppression. In a production welding environment, the supply v o l t a g e and t h e e l e c t r o m a g n e t i c e n v i r o n m e n t a r e o f t e n " n o i s y " due t o t h e i n t e r a c t i o n from other e q u i p m e n t , e s p e c i a l l y because o f t h e f a s t r i s e t i m e s o f o t h e r SCR's which a r e s w i t c h i n g h i g h current levels. I f l e f t u n c h e c k e d , such n o i s e would i n t e r f e r e w i t h normal m i c r o p r o c e s s o r operation, C o n s e q u e n t l y , each i n p u t s i g n a l i s checked s u c c e s s i v e l y a number o f t i m e s , a t i n t e r v a l s o f I0 ~Sec, t y p i c a l l y . Certain of the s i g n a l s need be examined o n l y a f e w t i m e s w h i l e o t h e r s a r e checked as many as 256 t i m e s . Until t h e CPU i s s a t i s f i e d t h a t t h e s i g n a l has s e t t l e d down t o a c o n s t a n t v a l u e , no a c t i o n i s t a k e n . Repeated n o i s e s p i k e s o r s w i t c h - c o n t a c t bounce would go u n d e t e c t e d o n l y i f t h e i r f r e q u e n c y and duration coincided with those of the checking schedule. T h i s method o f n o i s e s u p p r e s s i o n has p r o v e d more e f f e c t i v e t h a n t h e use o f h a r d w a r e filtering. As a n o t h e r s o f t w a r e n o i s e - p r e v e n t i o n m e a s u r e , a l l programmable i n p u t / o u t p u t ports are reprogrammed on each m a j o r l o o p , t h a t i s , a t t h e s t a r t o f each w e l d . I t had e a r l i e r been f o u n d that noise could reset the I/0 without the knowledge of the computer, thus turning input p o r t s i n t o o u t p u t ones and v i c e v e r s a . Such f u n c t i o n s would t h e n be i n o p e r a t i v e u n t i l re-powering or re-setting o f t h e CPU. 5.
DEVELOPMENT CYCLE
The system was d e s i g n e d by use o f s e v e r a l d e v e l o p m e n t t o o l s t o ease and t o s h o r t e n t h e development cycle. The S e r i e s I I Development System w i t h t h r e e f l o p p y d i s c s and w i t h a 64K memory was h e l p f u l in a l l o w i n g t h e program t o
F i g . 8. SDK-85 System D e s i g n e r ' s prototype development.
Kit
used i n
be d e v e l o p e d i n a t o p - d o w n m a n n e r . T h i s p r o v i d e d s h o r t t u r n - a r o u n d t i m e s d u r i n g program changes and a l l o w e d c o m p l e t e d o c u m e n t a t i o n c o n t r o l . Most o f t h e s u p p o r t r o u t i n e s were debugged i n t h e d e v e l o p m e n t system i n RAM. Simple I / 0 t e s t programs were d e v e l o p e d f o r each f u n c t i o n . An SDK-85 system d e s i g n e r ' s k i t was used i n b u i l d i n g the first p r o t o t y p e , shown in F i g u r e 8. This a l l o w e d us t o c o n c e n t r a t e o u r d e s i g n e f f o r t on problems r e l a t e d o n l y t o o u r s p e c i f i c a p p l i c a t i o n , i n s t e a d o f h a v i n g t o debug a CPU system as w e l l . When 80% o f t h e s o f t w a r e bugs had been f o u n d and a l l o f t h e I / 0 f u n c t i o n s had p r o v e n o u t , t h e d e s i g n e r ' s k i t b r e a d b o a r d was t h e n used as a v e h i c l e f o r u p g r a d i n g t h e s o f t w a r e and f o r testing various hardware improvements. A l l in a l l , t h e d e v e l o p m e n t p r o c e d u r e worked out w e l l , a l l o w i n g us t o t r y and t o t e s t one change a t a time without the risk of interaction o f new elements. Appendix INFRARED SENSING OF WELD TEMPERATURES
The heat from the sun reaches the earth after traveling through 93 million miles of cold vacuum in the form of electromagnetic waves or radiation. These waves include both visible radiation, or light, and invisible radiation including ultraviolet and infrared. When any of these radiations reach a surface and are absorbed by i t , they are converted to heat. The sun emits its radiation by virtue of being "white hot". On earth, other hot surfaces are familiar to us as being "yellow hot", "orange hot" or "red hot", in the order of descending temperature, As these incandescent surfaces become cooler, more of their radiation consists of the longer-wave infrared rays. Below incandescence, all of their radiation lies in the infrared spectrum. The cooler the surface, the longer are the emitted infrared waves. The intensity and spectral character of radiation from a hot surface vary in a definite way with the surface temperature. In 1847, F,W. Draper described this relationship and suggested that incandescent temperatures could be measured remotely by optical devices. Further developments led to the non-contact measurementof sub-incandescent temperatures by use of infrared, whose discovery had been announced in 1800 by the British astronomer, William Herschel. As more sensitive detection methods evolved, i t became possible to measure surface temperatures, using emitted infrared, as low as room temperature and below. It is equally possible to measure extremely cold surfaces by the amount of radiation which they absorb from the infrared detector. In another development in the 1920's, the concept was introduced that light can be conducted along transparent, flexible f i l a ments, such as of glass. By using a bundle of such filaments, i t became possible to convey radiation in curved paths to otherwise inaccessible places. In properly arranged bundles, one could even transmit visible images "around corners". The concept of the flexible filament was an extension of ideas which had evolved in the previous century. Even then, i t was known that smooth-walled glass rods, and even a stream of water passing through the air, could conduct light from end to end by multiple internal reflections, To the contained light-rays, the conductor appears to have mirror-like walls, and so the rays "bounce" from wall to wall in their course from one end to the other. Between reflections, the rays travel in straight lines, as they are expected to. These early developments have led to commercial uses of flexible optical-fiber bundles for various purposes. Presently they are coming into use for communications purposes. Light-transmitting flexible fibers also have the a b i l i t y to convey certain infrared wavelengths from place to place. This makes possible their use, with infrared detectors, in remote temperature measurement. The fibers add the convenience that radiation from inaccessible targets can be "piped" to the detector. Most fibers, however, are poor transmitters of the longer infrared waves, and so temperatures below certain values are not easily measured through them. However, temperatures above IOO°C can be measured through certain types of available fibers,
Alan A. Richard, Alan C. Traub and Riccardo Vanzetti In the spot welding a p p l i c a t i o n , radiation from a region near the weld is piped to an infrared detector. As the weld is being formed, i t carries an individual temperature h i s t o r y or "thermal signature". Two welds which were formed with the same thermal signature can be presumed to be of the same mechanical q u a l i t y , and tests have borne t h i s out. Because the actual weld nugget is formed between the worksheets and is inaccessible, the thermal h i s t o r y at the weld center cannot be measured d i r e c t l y . Instead, i t is possible to measure the thermal h i s t o r i e s of various nearby regions which ~z~ accessible, such as nearby parts of the worksheet or of the welding t i p . Each of these locations w i l l have i t s own thermal h i s t o r y , each being lower than, and s l i g h t l y delayed from, that at the center of the weld. However, they accurately r e f l e c t the nature of the true thermal h i s t o r y , being a l i k e when the welds are a l i k e , and being d i f f e r e n t otherwise.
303
Real-Time Control of Nugget Formation in Spot Welds Alan A. Richard, Alan C. Traub and Riccardo Vanzetti
VanzettiInfrared& ComputerSystems, Inc., NeponsetStreet,Canton,MA02021,USA
In the mass production of resistance spot welds for commercial use, several uncontrollabZe variables tend to produce welds of varying quality. We describe a microprocessing technique which is used with an infrared thermalsensing method to ensure weld uniformity. An optical fiber bundle '~ipes" infrared radiation from a heated zone near the weld. A detection system provides a thermal signature of this zone, which reflects the progress of the heating inside the weld. A "standard" weld is made by trial and error, its thermal signature to serve as a model for all later welds of the same type. By controlling the welding-machine power in real time, the midroprocessor ensure that all such welds will have the same thermal signature and, t~us, the same quality.
I.
INTRODUCTION
We begin this paper with a background discussion, in Section 2, on how a resistance spot welding machine operates. The substance of our work is then described in the ensuing sections. An appendix to the paper describes the infrared temperature measurement process which is a v i t a l ingredient in our control method.
This paper describes a microprocessor temperature-feedback control system f o r use with e l e c t r i c a l resistance spot welders. I ts purpose is to improve the u n i f o r m i t y of weld-nugget size and strength in spite of normal factors which tend to cause non-uniform weld q u a l i t y . Such factors include l i n e - v o l t a g e v a r i a t i o n s , currentshunting by nearby welds, and v a r i a t i o n s in worksheet condition. Pre-heating of the worksheets by previously made welds is another f a c t o r , as are differences in the amount of heatsinking at the center and at the edges or corners of the worksheets. With conventional, open loop welding machines, such v a r i a t i o n s lead e i t h e r to weak welds, which were i n s u f f i c i e n t l y heated, or to overheated ones which are wasteful of energy and which age the welding t i p s prematurely.
2.
TYPICAL SPOT WELDER OPERATION
A t y pic a l resistance spot welding machine consists e s s e n t i a l l y of a supporting frame which holds together a number of separate systems. A lower welding t i p is securely mounted to the frame and an upper welding t i p is held in an air-actuated piston so that the t i p s may be brought together during welding. Both t i p s are connected to a low-voltage, high current source which is switchoperated and whose current and duration are c o n t r o l l e d by the operator. These controls and others are a v a i l a b l e at a b u i l t - i n or an adjacent control panel. A water-cooling system removes excess heat from the t i p s and from certain i n t e r n a l e l e c t r i c a l parts ("contactors"). and a two-stage foot-operated switch is provided f o r ( I ) lowering the piston and (2) applying welding power at the t i p s . A view of a t y pic a l welding machine appears as Figure I.
Various methods seek to overcome the weld q u a l i t y v a r i a t i o n s , such as metering the t o t a l applied energy per weld or by the use of acoustic emissions which occur during nugget formation, but these methods have not been widely adopted. In this paper, we describe a method which controls weld formation by c o n t r o l l i n g the welding power, in real time, so that the weld thermal h i s t o r y always follows a prescribed course. That course is the stored thermal h i s t o r y of a previously made t r i a l weld which was judged to be of acceptable qOality.
When two sheets of metal are to be spot welded together, they are pressed between the tips under c o n t r o l l e d pressure, an e l e c t r i c a l current is passed through them, and melting and fusion occur.
During weld formation, the temperature of a nearby zone is monitored continuously, i t s own thermal h i s t o r y being a scaled-down and s l i g h t l y delayed reproduction of the one which occurs inside the weld zone. Because the weld thermal h i s t o r y is one of the most important influences upon weld q u a l i t y , we believe that the control of this parameter is more important than any other.
In a d d i t i o n to heating and timing controls, most welding machines o f f e r such features as: Squeeze: This is an automatically controlled time period, s t a r t i n g when the t i p s close, during which the workpieces are held together under pressure before the welding current is applied. 296
Alan A. Richard, Alan C. Traub and Riccardo Vanzetti
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of the 50 or 60 Hertz l i n e , usually with a maximum duration of 99 cycles on small machines, or j u s t under two seconds. The time duration is often called the "weld count." When adjusting the weld count, the operator also adjusts the amount of current, using a control called "percent current" or "heat" or s i m i l a r terminology. In most modern machines, the actual means by which the welding current is c o n t r o l l e d is contrary to what one might expect. That is, i t is carried out by i n t e r r u p t i n g the welding voltage f o r a c o n t r o l l e d f r a c t i o n of each h a l f cycle of the l i n e frequency, instead of resist i v e l y or i n d u c t i v e l y varying the current amplitude. This method of control is called "phase-angle c o n t r o l . " Thus, f or any setting of the heat control below 100%, the welding power is switched on and o f f at a precise moment during each h a l f cycle. The switching is accomplished by what are called "contactors" in the primary c i r c u i t of the welding transformer, there being one each f o r the p o s i t i v e and the negative h a l f cycles. These are f a s t - a c t i n g switches capable of handling high amperage and t h e i r switching action is c o n t r o l l e d by timing c i r c u i t s in the control system. They take the form of e i t h e r s o l i d - s t a t e SCR's or gaseous discharge tubes called ignitrons Figure I. machine.
A t y p i c a l resistance spot welding
I t allows the t i p s to s e t t l e s l i g h t l y into the upper and lower workpiece surfaces by v i r t u e of cold deformation, thereby making better e l e c t r i c a l contact. Hold: At the completion of welding, the t i p s may again be a u t o ma ti c a l l y held together f o r a timed i n t e r v a l during cooling of the workpieces. Single/Repeat Selector Switch: I f the f o o t pedal is held down while t h i s switch is in the "Single" p o s i t i o n , only one weld w i l l occur u n t i l the pedal is raised and is lowered again. In the "Repeat" p o s i t i o n , this switch allows the welding cycle to be repeated as long as the f o o t pedal is down. Off: This is a timer control which inserts, between repeated weld cycles, a time i n t e r v a l during which no welding current flows. Its use is to allow the t i p s to cool s l i g h t l y between repeated welds so that they w i l l not suffer damage due to cumulative heating. This time i n t e r v a l is inserted between any "hold" and "squeeze" i n t e r v a l s which are in use. An important part of a spot welding machine is the e l e c t r i c a l current supply. Spot welders are usually operated from a 220 or 440 VAC l i n e . A massive transformer w i t h i n the welder reduces the supply voltage to a low voltage with a highcurrent c a p a b i l i t y , usually thousands of amperes. For each type of weld to be made, the duration of the welding current is previously selected by the operator, by use of a knob on the control panel. The units on this control are in cycles
Figure 2. Fiber optic cable conveys infrared thermal signal from w i t h i n the upper welding t i p to a nearby sensor. The mode of switching is as follows. Let us say that the operator c a l l s f o r "60-percent current" on his heat control. At the s t a r t of each v o l t age h a l f - c y c l e , a timing system in the e l e c t r o n i c control ensures that the contactors w i l l stay o f f u n t i l 40% of the h a l f cycle period has elapsed. The voltage into the transformer is then switched on, abruptly, f o r the remaining 60% of the h a l f - c y c l e period. Whichever contactor is conducting during t h i s time w i l l then return to the non-conducting state at the end of
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Real-Time Control of Nugget Formation in Spot Welds
the h a l f cycle, that i s , at the "zero crossing." The zero crossing is an important timing marker in an e l e c t r o n i c welding control because a l l i n t e r v a l s w i t h i n any h a l f cycle are measured from that moment.
optimized the controls f o r the best weld, he returns the switch to a "Run" mode. At t h i s time, the thermal signature of the last-made weld becomes locked into a memory, serving as a model f o r the automatic control system. Thereafter, the system makes the necessary moment-by-moment power adjustments during each l a t e r weld, thus steering a l l new thermal h i s t o r i e s on t h e i r proper courses so that a l l welds are formed a l i k e .
A 60-percent current setting does not necess a r i l y d e l i v e r 60% of the heat to a weld compared to a lO0-percent current setting. This is p a r t l y because the amount of heating varies with the square of the current. Moreover, because the current is changing during the conductive portion of a h a l f - c y c l e , a time i n t e g r a l is involved which is not l i n e a r with the units on the heat control d i a l . These units are, therefore, a r b i t r a r y values which allow the operator to return to a previous value or to vary the heating by a reproducible amount.
The operator may select his "standard" weld f o r a given run e i t h e r on the basis of a visual judg ment or by performing standard destructive tests on the weld nugget. 3.
Most welding transformers are provided with manually selectable taps on the primary winding so that current-values outside the range of the heat-control knob may be used. The heat-control knob is not normally set below 40% on 220-volt i g n i t r o n - t y p e welders or 20% on 440-volt welders because the f i r i n g of the i g n i t r o n s is r e l a t i v e l y unstable below a c e rta i n voltage.
The d r i v i n g element of the system is an eight b i t microprocessor which handles a l l of the control functions. These include the standard functions which are provided with conventional c o n t r o l l e r s f o r spot welding machines. Convent i o n a l c o n t r o l l e r s customarily make use of discrete l o g i c . The primary welding parameters are c o n t r o l l e d by the use of counters which generate delays in units of cycles of the AC power source. Time-intervals shorter than a h a l f cycle are managed by free-running clocks. These are used to measure pulse-width modulated
In the development to be described here, the operator sets a selector switch to a " C a l i b r a t e " mode and proceeds to make t r i a l welds in the normal manner, by operating his standard panel controls. When he is s a t i s f i e d that he has
NCS,OHAL S
'
THE MICROPROCESSORCONTROLLER
A block diagram of the system is presented in Fig. 3.
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Alan A. Richard, Alan C. Traub and Riccardo Vanzetti signals f o r phase modulating the f i r i n g angles of the contactors (SCR's or i g n i t r o n s ) in order to control the time-averaged power into the workpieces. In designing the temperature-control function of our system, i t was obvious to us that a microprocessor was the most e f f i c i e n t means f o r storing and managing the temperature information involved in welding. I t also became clear that the same u n i t could handle the conventional control functions as w e l l , without the need f o r a d d i t i o n a l hardware. The I n t e l 8085 f a m i l y was used, o f f e r i n g a large selection of peripheral chips with combined functions. The CPU hardware is implemented in the conventional manner. The 8155 RAM I / 0 chip was chosen f o r two reasons. I t not only stores temporary constants and the temperature p r o f i l e but i t contains programmable input-output ports f o r accessing data. Nine bits of the 8155 are used fo r the a n a l o g - t o - d i g i t a l section of the system. The thermal data are quantized to one part in 256 using an e i g h t - b i t d i g i t a l - t o - a n a l o g converter connected to one port of the 8155. The output of the D/A feeds the comparator. The other comparator input is fed with the l i n e a r i z e d (see i n f r a ) temperature data. The comparator output enters the second 8 - b i t port as a single b i t . The software conversion routine performs successive approximations on the data. I t outputs a t r i a l data word and checks the comparator output to see i f the signal is above or below the t r i a l value. This t r i a l and error method starts with the most s i g n i f i c a n t b i t and ends with the l e a s t s i g n i f i c a n t . A ft e r eight attempts, the t r i a l value is equivalent to the input signal with a r e s o l u t i o n of eight b i t s .
is the value I00 minus the integer representing the desired percent current. Thus, f o r 60% current, the integer is 60 and the value entered is 40. At zero crossing, a countdown is started, and when i t reaches zero, a pulse is produced. This pulse is used to i n t e r r u p t the CPU which then f i r e s the contactor f or the remainder of the h a l f cycle. Because the RAM chip contained s u f f i c i e n t timing and input/output c a p a b i l i t i e s , the choice was made not to use a m u l t i - f u n c t i o n ROM I / 0 u n i t , which might otherwise have been done. The main memory containing the program is implemented by
The amount of r a d i a t i o n reaching the detector is a highly nonlinear function of t a r g e t temperature, as stated by Planck's Law and, thus, so is the thermal signal voltage. Therefore, a one-degree temperature change at a r e l a t i v e l y low temperature produces a much smaller signal change than a onedegree change does at a higher temperature. For t h i s reason, an analog l i n e a r i z e r is used before the A/D converter. Were t h i s not done, an A/D converter with an equivalent r e s o l u t i o n of 14 b i t s would be needed to d i s t i n g u i s h low temperature values at the s t a r t of weld formation. ( A l t e r n a t i v e l y , one might use an analog m u l t i plexer with a m p l i f i e r s and auto-range the A/D over three decades.) As an added advantage, the 8155 combination RAM I / 0 chip also performs a timing function which provides an i n t e r r u p t . In order to control the energy-per-half-cycle in the weld, one must convert a binary number in memory into a pulsewidth modulated signal. The CPU clock runs at 6.144 MHz. This is divided down to 83.333 ~Sec clock pulses, each of which is 0.01 h a l f cycle at the 60-Hz l i n e frequency used in the USA. At the s t a r t of each h a l f cycle ( t h a t i s , at each zero crossing), a number is loaded i n t o the counter which w i l l determine when the contactor is to f i r e during that h a l f cycle. This number
299
Fig. 4. Flow Chart of the System.
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Real-Time Control of Nugget Formation in Spot Welds
a conventional UV-erasable read only memory. The t o t a l program, including conventional weldingcontrol functions and temperature-control s o f t are, comprises s l i g h t l y less than 1,000 bytes when w r i t t e n in assembly language. Figure 4 is a s i m p l i f i e d flow chart of the software. A second m u l t i f u n c t i o n peripheral chip is used to con~nunicate with the welding operator. The 8279 is a keyboard-input~and-display d r i v e r . It contains an i n t e r n a l memory which automatically scans and debounces the thumbwheel switches which are used f o r entering the welding parameters. Such switches are used in our present system f o r entering values corresponding to weld count, percent current, squeeze, hold and o f f . The d r i v e r section of the 8279 is used to d r i v e LEDs which indicate when power is applied to the solenoid valve which controls the welding piston and also when power is applied to the welding t i p s . These functions are c o n t r o l l e d by the operator's twostage foot switch. In a d d i t i o n , other LEDs a l e r t the operator to various welding-machine malfunctions which may occur, such as tip-overheating due to the possible loss of cooling water, or a contactor malfunction. A group of ten LEDs is arrayed as a bar graph which, a f t e r each weld, displays the r e l a t i v e peak temperature f o r that weld, the displayed values being quantized into ten steps. In t h i s way, the operator may be reassured that a l l intended a l i k e welds are reaching the same peak temperature w i t h i n a few percent when a l l parts of the system are performing as they should. Otherwise, the need f o r a system correction would be indicated. This might be the case, f o r example, i f the operator were using 80% current f o r a normal weld and i f the reserve 20% of a v a i l a b l e welding current were not s u f f i c i e n t to correct an e s p e c i a l l y troublesome weld. In t h i s case, a lower peak temperature would be indicated, and the operator could correct t h i s by making a new standard weld at, say, 60% current, leaving a more ample reserve of power. The bar-graph display covers a range of peak temperatures from 10% below the value f o r the standard weld to 10% above i t . A peak temperature deviation greater than +10% causes a l l LEDs to l i g h t and an audible alarm to sound. Peak temperatures which are low by more than 10% r e s u l t in no LEDs l i g h t i n g and, again, an audible alarm. For each standard weld the 8279 adjusts the display so that the lower f i v e LEDs w i l l be activated when the correct peak temperature is reached. In t h i s mode o f display operation, which we w i l l c a l l the "peak-picker" mode, the 8279 f i n d s the maximum temperature f o r each weld and displays a corresponding value on the bar graph u n t i l the next weld is begun. In an a l t e r n a t i v e "tracking mode," which is a v a i l a b l e to the operator via a selector switch, the bar graph is used to display instantaneous r e l a t i v e temperatures while heating and cooling are occurring. These are displayed by a r i s i n g and f a l l i n g pattern of activated LEDs which simulate the temperature s i t u a t i o n in ten quantized steps. The tracking mode provides
CONTACTOR MALFUNCTION
RESISTANCE WELDING
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CONTROLLER SQUEEZE
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COOLING MALFUNCTION
ALARM RESET
FAILSAFE
PEAKTEMP
WELD COUNT
THIS SPACE RESERVED FOR OPTIONAL FEATURES
RUN
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OFF WELD
WELD
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NO WELD
%CURRENT SINGLE
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Fig. 5. Front Panel Layout of Production Prototype. visual temperature-rate information, as could be done by an oscilloscope, and may be used f o r diagnostic purposes. Fig. 5 shows a control panel layout which is intended f o r a production model f o r the near future. 4.
SOFTWAREAND WELDER OPERATION
The two manually selectable modes of operation which are a v a i l a b l e to the operator are "Calibrate" and "Run." (In Fig. 4 they appear as "Set Up" and "Automatic.") In the c a l i b r a t i o n mode, the system behaves as a conventional welding c o n t r o l l e r , allowing the operator to adjust his welding parameters by use of the control panel, but with one exception. Each time that a weld is made, i t s thermal h i s t o r y is entered into a memory, replacing any previous one which is stored. In the Run mode, the last-recorded thermal signature remains in memory f o r use on l a t e r welds u n t i l a new c a l i b r a t i o n is made. The thermal signature is stored in the form of a discrete voltage value f o r each h a l f cycle of the l i n e frequency. These are stored at successive locations in the RAM. At the end of each " C a l i brate" weld, the integer corresponding to the peak temperature is scaled up to a value of 256, with the m u l t i p l i c a t i v e constant being computed and stored. These values are cleared during each new "Calibrate" weld, but the last-acquired ones are retained when the selector switch is placed
Alan A. Richard, Alan C. Traub and Riccardo Vanzetti
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in "Run." Thereafter, a l l new thermal signatures are scaled up by the m u l t i p l i c a t i v e constant. The purpose of scaling is to ensure that a given thermal error signal at any moment during welding w i l l bring about a given amount of power correct i o n regardless of whether moderate-temperature or high-temperature welds are being made on a given day. A -10% signal error f o r the former, as an example, would have a d i f f e r e n t absolute value than a -10% e r r o r f o r the l a t t e r . The c o n t r o l l e r would thus c a l l f o r a d i f f e r e n t amount of correction in the two cases. The scaling method is the equivalent of converting absolute signal differences to r e l a t i v e percentage errors. I t offers the a d d i t i o n a l feature of u t i l i z i n g the f u l l eight bits of information in order to preserve r e s o l u t i o n of the error signal. In the "Run" mode, j u s t as the operator starts to make a new weld, the system performs j u s t as i t does in the " C a l i b r a t e " mode. The CPU scans the panel controls, picking up counts from the various switches (squeeze, percent current, weld count, etc.) and monitoring the position of the two-stage f o o t switch. When i t receives the command to supply welding current, welding power is i n i t i a l l y applied at the level which is called f o r on the percent-current d i a l . As welding begins, no computer action is taken u n t i l the signal voltage rises above a predetermined threshold. The reason f o r t h i s is to avoid a~ over-correction in the u n l i k e l y event that no thermal signal is sensed. This could happen because of a thermal-sensing system f a i l u r e such as due to an e l e c t r o n i c malfunction, mechanical damage to the o p t i c a l f i b e r s , and so f o r t h . In such an event, a weld which was progressing normally would be f a l s e l y sensed as underheated, and the correction system would c a l l f o r a large increase in welding power. Thus, i f the preset threshold is not reached, no correction is made and the weld proceeds normally. We c a l l t h i s threshold the " F a i l s a f e " threshold. I f i t is not crossed during a weld, an alarm l i g h t and buzzer a l e r t the operator to a possible malfunction. The correction function comes i n t o play a f t e r the f a i l s a f e threshold is crossed. From then on, once per h a l f - c y c l e , the thermal signal due to the weld is sampled and is compared with the corresponding one in the memory, that i s , with the one f o r the same h a l f cycle in the sequence. I f a p o s i t i v e or negative correction is c a l l e d f o r , the CPU adjusts the phase angle of the contactor f i r i n g f o r the next h a l f cycle. In t h i s way, i f an e r r o r is sensed i n i t i a l l y , j u s t a f t e r the f a i l s a f e threshold is crossed, the system "steers" the thermal signature back on course so that i t tends to duplicate the standard one in the memory. The same is true i f a thermal error should arise elsewhere in the heating cycle, such as due to the occurrence of a spark, which extracts power and i n t e r r u p t s the heating cycle. Likewise, a l i n e - v o l t a g e drop during welding, due to the start-up of other equipment on the same l i n e , w i l l be corrected by the system.
Fig. 6. Three "uncontrolled" welds made at d i f ferent transformer tap settings (Thermal signal versus time).
Fig. 7. Same three welds under automatic thermal control. Fig. 6 is an oscillogram showing the thermal signatures of three welds formed in the " C a l i brate" or uncontrolled mode of operation. The three were i n t e n t i o n a l l y made at d i f f e r e n t transformer tap settings so as to simulate a condition which would cause noticeably d i f f e r e n t thermal h i s t o r i e s . The welding conditions were otherwise i d e n t i c a l in the three cases. Approximately half-second welding durations were used. In Fig. 7, the lowest thermal signature of Fig. 6 was duplicated in the " C a l i b r a t e " mode, and the "Run" mode was then set i n t o operation. The two higher-current welds of Fig. 6 were then repeated, but the automatic control system held t h e i r heating cycles w i t h i n closer range of that f o r the "standard" weld. The closeness with which the system can duplicate a standard thermal h i s t o r y via power corrections depends upon several factors. These include the amount of correction which is called f o r in response to a given e r r o r signal, as well as the temperature and welding duration which are called f o r . In a d d i t i o n , there w i l l be a given amount of thermal time-lag because of the separation of the infrared-monitored zone from the weld s i t e , and the lag w i l l be greater when t hic k er metals are being welded. Because of
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Real-Time Control
o f Nugget F o r m a t i o n i n Spot Welds
this lag, a corrected curve will differ t o some e x t e n t f r o m t h e s t a n d a r d , as i s seen i n F i g . 7. Besides their other uses, software techniques a r e used f o r e l e c t r o n i c noise suppression. In a production welding environment, the supply v o l t a g e and t h e e l e c t r o m a g n e t i c e n v i r o n m e n t a r e o f t e n " n o i s y " due t o t h e i n t e r a c t i o n from other e q u i p m e n t , e s p e c i a l l y because o f t h e f a s t r i s e t i m e s o f o t h e r SCR's which a r e s w i t c h i n g h i g h current levels. I f l e f t u n c h e c k e d , such n o i s e would i n t e r f e r e w i t h normal m i c r o p r o c e s s o r operation, C o n s e q u e n t l y , each i n p u t s i g n a l i s checked s u c c e s s i v e l y a number o f t i m e s , a t i n t e r v a l s o f I0 ~Sec, t y p i c a l l y . Certain of the s i g n a l s need be examined o n l y a f e w t i m e s w h i l e o t h e r s a r e checked as many as 256 t i m e s . Until t h e CPU i s s a t i s f i e d t h a t t h e s i g n a l has s e t t l e d down t o a c o n s t a n t v a l u e , no a c t i o n i s t a k e n . Repeated n o i s e s p i k e s o r s w i t c h - c o n t a c t bounce would go u n d e t e c t e d o n l y i f t h e i r f r e q u e n c y and duration coincided with those of the checking schedule. T h i s method o f n o i s e s u p p r e s s i o n has p r o v e d more e f f e c t i v e t h a n t h e use o f h a r d w a r e filtering. As a n o t h e r s o f t w a r e n o i s e - p r e v e n t i o n m e a s u r e , a l l programmable i n p u t / o u t p u t ports are reprogrammed on each m a j o r l o o p , t h a t i s , a t t h e s t a r t o f each w e l d . I t had e a r l i e r been f o u n d that noise could reset the I/0 without the knowledge of the computer, thus turning input p o r t s i n t o o u t p u t ones and v i c e v e r s a . Such f u n c t i o n s would t h e n be i n o p e r a t i v e u n t i l re-powering or re-setting o f t h e CPU. 5.
DEVELOPMENT CYCLE
The system was d e s i g n e d by use o f s e v e r a l d e v e l o p m e n t t o o l s t o ease and t o s h o r t e n t h e development cycle. The S e r i e s I I Development System w i t h t h r e e f l o p p y d i s c s and w i t h a 64K memory was h e l p f u l in a l l o w i n g t h e program t o
F i g . 8. SDK-85 System D e s i g n e r ' s prototype development.
Kit
used i n
be d e v e l o p e d i n a t o p - d o w n m a n n e r . T h i s p r o v i d e d s h o r t t u r n - a r o u n d t i m e s d u r i n g program changes and a l l o w e d c o m p l e t e d o c u m e n t a t i o n c o n t r o l . Most o f t h e s u p p o r t r o u t i n e s were debugged i n t h e d e v e l o p m e n t system i n RAM. Simple I / 0 t e s t programs were d e v e l o p e d f o r each f u n c t i o n . An SDK-85 system d e s i g n e r ' s k i t was used i n b u i l d i n g the first p r o t o t y p e , shown in F i g u r e 8. This a l l o w e d us t o c o n c e n t r a t e o u r d e s i g n e f f o r t on problems r e l a t e d o n l y t o o u r s p e c i f i c a p p l i c a t i o n , i n s t e a d o f h a v i n g t o debug a CPU system as w e l l . When 80% o f t h e s o f t w a r e bugs had been f o u n d and a l l o f t h e I / 0 f u n c t i o n s had p r o v e n o u t , t h e d e s i g n e r ' s k i t b r e a d b o a r d was t h e n used as a v e h i c l e f o r u p g r a d i n g t h e s o f t w a r e and f o r testing various hardware improvements. A l l in a l l , t h e d e v e l o p m e n t p r o c e d u r e worked out w e l l , a l l o w i n g us t o t r y and t o t e s t one change a t a time without the risk of interaction o f new elements. Appendix INFRARED SENSING OF WELD TEMPERATURES
The heat from the sun reaches the earth after traveling through 93 million miles of cold vacuum in the form of electromagnetic waves or radiation. These waves include both visible radiation, or light, and invisible radiation including ultraviolet and infrared. When any of these radiations reach a surface and are absorbed by i t , they are converted to heat. The sun emits its radiation by virtue of being "white hot". On earth, other hot surfaces are familiar to us as being "yellow hot", "orange hot" or "red hot", in the order of descending temperature, As these incandescent surfaces become cooler, more of their radiation consists of the longer-wave infrared rays. Below incandescence, all of their radiation lies in the infrared spectrum. The cooler the surface, the longer are the emitted infrared waves. The intensity and spectral character of radiation from a hot surface vary in a definite way with the surface temperature. In 1847, F,W. Draper described this relationship and suggested that incandescent temperatures could be measured remotely by optical devices. Further developments led to the non-contact measurementof sub-incandescent temperatures by use of infrared, whose discovery had been announced in 1800 by the British astronomer, William Herschel. As more sensitive detection methods evolved, i t became possible to measure surface temperatures, using emitted infrared, as low as room temperature and below. It is equally possible to measure extremely cold surfaces by the amount of radiation which they absorb from the infrared detector. In another development in the 1920's, the concept was introduced that light can be conducted along transparent, flexible f i l a ments, such as of glass. By using a bundle of such filaments, i t became possible to convey radiation in curved paths to otherwise inaccessible places. In properly arranged bundles, one could even transmit visible images "around corners". The concept of the flexible filament was an extension of ideas which had evolved in the previous century. Even then, i t was known that smooth-walled glass rods, and even a stream of water passing through the air, could conduct light from end to end by multiple internal reflections, To the contained light-rays, the conductor appears to have mirror-like walls, and so the rays "bounce" from wall to wall in their course from one end to the other. Between reflections, the rays travel in straight lines, as they are expected to. These early developments have led to commercial uses of flexible optical-fiber bundles for various purposes. Presently they are coming into use for communications purposes. Light-transmitting flexible fibers also have the a b i l i t y to convey certain infrared wavelengths from place to place. This makes possible their use, with infrared detectors, in remote temperature measurement. The fibers add the convenience that radiation from inaccessible targets can be "piped" to the detector. Most fibers, however, are poor transmitters of the longer infrared waves, and so temperatures below certain values are not easily measured through them. However, temperatures above IOO°C can be measured through certain types of available fibers,
Alan A. Richard, Alan C. Traub and Riccardo Vanzetti In the spot welding a p p l i c a t i o n , radiation from a region near the weld is piped to an infrared detector. As the weld is being formed, i t carries an individual temperature h i s t o r y or "thermal signature". Two welds which were formed with the same thermal signature can be presumed to be of the same mechanical q u a l i t y , and tests have borne t h i s out. Because the actual weld nugget is formed between the worksheets and is inaccessible, the thermal h i s t o r y at the weld center cannot be measured d i r e c t l y . Instead, i t is possible to measure the thermal h i s t o r i e s of various nearby regions which ~z~ accessible, such as nearby parts of the worksheet or of the welding t i p . Each of these locations w i l l have i t s own thermal h i s t o r y , each being lower than, and s l i g h t l y delayed from, that at the center of the weld. However, they accurately r e f l e c t the nature of the true thermal h i s t o r y , being a l i k e when the welds are a l i k e , and being d i f f e r e n t otherwise.
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