Study and Implementation of a Force Stepper and a

Study and Implementation of a Force Stepper and a Part Fit-Up Solver Algorithm for a Servo Controlled MFDC Spot Welder Dhanasekaran Venugopal, Manohar Das

Vernon Fernandez Department of Mechanical Engineering Lawrence Technological University, Southfield, MI 48075

Department of Electrical and Computer Engineering Oakland University, Rochester, MI 48309 Abstract - The goal of this research is to study the effect of a timevarying electrode force on the weld nugget size for mid frequency direct current (MFDC) resistance spot welding (RSW) systems. Using SORPASTM, a software for simulation and optimization of resistance welding process, simulations were performed for MFDC RSW with changing force profiles and weld times. Additionally, an algorithm was developed to automatically detect a bad stack fit-up and correct it by applying an additional compensating force. This was implemented on a resistance spot welding gun fitted with a 4.5 KW servo motor along with a built-in resolver, a UnidriveTM motor controller and a real time embedded program. The simulation and experimental results are presented and compared with results obtained using a constant current (CC) welding process with constant force profile.

A. Principle of RSW The heat required by the RSW process is produced by the resistance offered, to the passage of welding current, by a sheet metal stack utilizing the principle of Joule heating. Typically a current of the order of 6-20 Kilo Amperes is passed through the stack for a short time. The resulting Joule heating causes localized melting of the stack and formation of a weld nugget. Also, the voltage is regulated to a value sufficient enough to pass the desired current through the sheet metal stack. The heat generated through this process is given by: T3

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INTRODUCTION

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Resistance spot welding (RSW) is extensively used, in the automotive industry, to weld two or more over-lapping metal sheets. RSW has three major process parameters, namely, weld current, weld time and electrode force, to control the weld nugget formation for a given stack of metals and electrode configuration [1]. Figure 1 shows the welding current and force profiles of a constant current RSW. At time T1, the electrode force increases to a pre-determined fixed value and subsequently drops to zero at time T4. The current follows a similar profile, rising at time T2 and dropping to zero at time T3, where T1 < T2 < T3 < T4. 10

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The resistance R(t) is a summation of the contact resistance between each electrode and sheet metal, the resistance of each sheet and the contact resistance at each sheet metal joint interface [2]. Figure 2 shows the components of this resistance, which is usually referred to as dynamic resistance since its value changes during the welding cycle as a function of temperature. Figure 3 shows the nature of the dynamic resistance during the course of MFDC RSW. R(t) is considered in the literature to be one of the important parameters for estimation of the quality of the weld nugget [2], [3]. MFDC RSW is predominantly used in industry rather than alternating current (AC) RSW due to the following reasons as demonstrated by Li [4]:

Weld Time (msec) Weld Current(KA)

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Figure 1. Single pulse CC welding current and force profiles

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MFDC consumes approximately 10% less energy MFDC produces larger nugget MFDC has a wider weld lobe

there is a variation in the magnitude of dynamic resistance for different force levels. Hence controlling the electrode force along with welding current and welding time should give better control over the highly dynamic and complex RSW process. Also, by varying the force, bigger nuggets can be realized for a given weld current and weld time. Energy efficiency of a RSW process may also improve by using a variable force profile. Initial simulation studies show a change in nugget diameter for different constant electrode force values, as depicted in Table 1 and Figure 4. Table 1. Effect of constant electrode force on nugget size for a 3T stack (4.8 mm B Cap,_Shank 5/8 inch) 0.71 mm MildGal+ 1.2mmTRIP780. + 2.0mmUSIBOR1500 Weld Current – 7 KA Nugget Size (mm) 500 600 800 900 (LB) (LB) (LB) (LB) 3.204 exp 4.433 4.019 3.733

Figure 2. Components of Dynamic Resistance. Courtesy Garza et al.

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Figure 3. Dynamic Resistance during MFDC RSW

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II. SIMULATION STUDY OF THE EFFECT OF A MULTI-LEVEL FORCE PROFILE

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It is well known that the electrode force is one of the important factors that determines the size and strength of a weld nugget. It is assumed that by keeping the electrode force low at the start of a RSW process, the stack resistance, R(t), increases, thus generating more heat at the start of the cycle and producing a bigger weld nugget for a given weld current and weld time [5], [7]. In such cases, the electrode force is increased during the later part of the cycle to contain the molten metal within the stack. Such an electrode force profile is referred to as a force stepper in this study. Tang et al [6] have studied the benefits of such a force stepper using similar force profiles for alternating current (AC) RSW on bare steel sheet metal stacks. Also, their study was restricted to twolevel force steppers only. This paper presents results of a similar study for MFDC RSW on zinc coated steel sheets at relatively higher levels of weld current. The contact resistances vary with the magnitude of the electrode force, however the bulk sheet metal resistance stays almost constant for typical electrode forces used in the automotive industry. SORPAS simulations demonstrate that

Figure 4. Effect of constant Electrode force on nugget size for a 3T stack (circled spot indicates expulsion) Simulation studies also demonstrate that the nugget size increases for a 1.2mm-1.2mm stack of TRIP 780 steel using a 4.8 mm B-cap and a force profile that starts at 450 lb and is stepped up to 670 lb for varying percentages of the cycle time. The results of these simulations are presented in Table 2 and Figure 5. Similar studies were conducted for a 3T stack by varying the force from 600 lb to 700 lb, 600 lb to 800 lb and 600 lb to 900 lb. The nugget size was largest for the step from 600 lb to 700 lb. The results shown in Figure 6 and Table 3 demonstrate that a lower force increases the nugget size.

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Table 2. Effect of change of force from 450 lb to 670 lb at varying percentages of cycle time on Nugget Size Change of Time of Force in % of Weld Cycle

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(4.8 mm B Cap,_Shank 5/8 inch) 1.2 mm HSLA340 + 1.2 mm TRIP780 stack Weld Current – 7 KA, weld force – 450 to 670 LB

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Figure 6. Comparison of nugget sizes as a function of percentage of cycle time for various force steppers and step sizes. Table 4 and Figure 7 show the effect of varying the force from 400 lb to 550 lb to 670 lb applied at varying percentages of cycle time for a 3T stack.

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Table 4. Effect of change of force from 400 lb to 550 lb to 670 lb at varying percentages of cycle time (Three steps)

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(4.8 mm B Cap,_Shank 5/8 inch) 2mmTRIP780 + 1.2mmTRIP780 stack Weld Current – 5.5 KA Percentage Percentage Percentage Nugget of weld of weld of weld Size time for time for time for (mm) 400 LB 550 LB 670 LB 0 0 100 4.119 20 20 60 4.55 20 30 50 4.557 20 40 40 4.562 20 50 30 4.564

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Figure 5. Effect of change of force from 450 lb to 670 lb at varying percentages of cycle time on Nugget Size Table 3. Comparison of nugget size as a function of percentage of cycle time for various force steppers and step sizes Change of Time of (4.8 mm B Cap,_Shank 5/8 Force in % of Weld inch) Cycle 0.71 mm MildGal+ 1.2mmTRIP780 + 2.0mmUSIBOR1500 stack Weld Current – 7 KA Nugget Size (mm) 600 – 600 – 600 – 700 800 900 (LB) (LB) (LB) 20 4.274 4.072 3.887 30 4.299 3.939 3.928 40 4.251 3.894 3.742 50 4.24 3.96 3.823 60 4.219 4.046 3.98 70 4.24 3.96 3.823

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Figure 7. Effect of three levels of force at varying percentages of cycle time on nugget size

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Based on the above simulations, the following observations can be drawn. • •

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With a two-level force profile, a difference in force of more than 200 LB causes expulsion during welding. The increase in the nugget size resulting from a two level force is approximately 1.5% for the same settings of weld current and weld time. However, it helps prevent expulsion. With a three-level force profile, the first level of force can be brought down to about 55 to 60 % of final force without causing considerable expulsion. A three-level force profile produces about 10% increase in nugget size for the same settings of weld current and weld time.

It may be noted that in the case of a three-level force stepper, the first force step has to be maintained at a very low level, which causes expulsion in situations where part fit-up issues exist. This can be corrected by providing a compensating force that eliminates part fit-up issues. This is discussed in the following section. III. DEFINITION AND SOLUTION TO FIT-UP ISSUES A fit-up issue concerns the presence of an air gap between two adjacent metal sheets being welded. In a typical production environment, the air gap ranges from a fraction of a millimeter to a few millimeters. Under such conditions, a part of the applied electrode force is used to eliminate the existing air gap, thus reducing the inter-joint force. Due to the loss of a fraction of the inter-joint force, the stack is unable to contain the molten metal, thus producing expulsion during the welding process. Figures 8 and 9 show two stacks, one without fit-up and the other with fit-up, respectively.

Figure 8. Fit-up free sheet metal stack

Air gap - Part Fit-Up issue

Figure 9. Fit-up affected sheet metal stack A fit-up condition demands a real time solution to determine the size of the air gap (fit-up) in a sheet metal stack. Once the size of the air-gap is determined, the extent of loss of the inter-joint force can be estimated. Then a compensating electrode force can be added in real time to the electrode force specified by the welding standards. This compensating 289

force is used to compress the metal sheets, thus eliminating the air gap and maintaining the required inter-joint force. The resulting dynamic resistance of the sheet metal stack is thus maintained within acceptable limits, resulting in a stable RSW process. The study conducted by Zhang et al [8] used welding current to indirectly compensate for the effect of part fit-up. The weld current was varied throughout a weld cycle to get a required nugget size. In contrast, this paper uses a compensating electrode force, which has a direct control over part fit-up, to nullify the effects of part fit-up issues [9]. The algorithm developed here does not require the use of neurofuzzy or other complex processing schemes. It uses a real time embedded solution, which is very cost effective and can be easily implemented in practice. Also, it is fast and simpler to implement. IV. IMPLEMENTATION OF A FORCE STEPPER AND FIT-UP SOLVER ALGORTIHM A. Implementation of a force stepper The experimental setup consists of an Emerson UnidriveTM SP servo controller configured to control a 4.5 KW servo motor that applies force to the electrode of a PICOTM pinch type spot welding gun. The gun is fitted with a 50 KVA RomanTM transformer that is powered by a BoschTM MFDC weld controller. Strain gages mounted on the spot welding gun, monitor the force at the electrodes [9]. The strain gage output along with the output from the servo motor controller were monitored by a data acqusition card that was interfaced with Matlab. The electrode force is controlled by controlling the motor torque. In order to step the force at a given instant during the welding cycle, the electrode position is continuously monitored by means of a resolver that is integral with the motor. Figure 10 shows the weld active pulse and strain voltage signal that corresponds to electrode force as a function of weld cycle time for a two-level force profile. The weld active pulse indicates the duration for which electrode force and weld current are active. Figure 11 shows the profile for a three-level force stepper. The signals in Figures 10 and 11 demonstrate that the force levels can be changed during a weld cycle. In this application, the force change occurs within about 30 msec, which is well within a typical weld cycle time.

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without air-gap and with air-gap between the sheets, respectively. The difference in the width is proportional to the magnitude of the air-gap. For a stack with air-gap, the rate of increase of motor current depends on the stiffness of the distorted parts. Also, for a metal stack with thicker sheets, the rate of increase of motor current would be higher than one with thinner sheets. The plots in Figures 13 and 14 show that it is possible to detect the extent of fit-up and apply a compensating electrode force to stabilize the spot welding process.

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As addressed earlier, sheet metal stacks (parts to be welded) in assembly plants get bent and bulged out, due to welds made earlier at some other locations of the part. These conditions are referred to as fit-up. In this study the motor current and electrode position are used as parameters to detect part fit-up [9]. Figure 12 depicts the simplified flow diagram of a fit-up solver algorithm. A part fit-up issue is identified if the motor current crosses a certain predefined value before reaching a certain position. The magnitude of the part fit-up issue is estimated by measuring the instant the motor current crosses a predefined value and by the rate of increase of motor current. Once the air-gap between the parts is determined, a compensating electrode force is estimated and added to the electrode force as specified in the standards. Thus the inter-joint force is maintained within allowable limits for each spot weld during production. Strain voltages obtained for stacks with and without fit-up are shown in Figure 13. The width of the fitup-detect pulse_1 and fit-up-detect pulse_2 correspond to

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Figure 12. Flow chart for fit-up solver algorithm. The weld start pulse is denoted by “wsp”, the measured stack thickness is denoted by MST and the reference stack thickness is denoted by RST.

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ACKONWLEDGEMENT

REFERENCES

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The simulation studies described above have been conducted using SORPAS, which is a dedicated professional software for simulation and optimization of resistance welding processes. SORPAS is developed and marketed by SWANTEC Software and Engineering ApS, Denmark. This research has been supported in part by SWANTEC and Huys Industries. The authors gratefully acknowledge their support of this work.

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Figure 14. Increase in electrode force as a function of weld time for stack with fit-up.

[1] H. Zhang and J. Senkara, “Resistance Welding Fundamentals and Applications”, CRC Press, Taylor & Francis Group, Florida (FL), 2006. [2] F.J. Garza and M. Das, "Identification of time-varying resistance during welding", Instrumentation and Measurement Technology Conference, 2000. IMTC 2000, Proceedings of the 17th IEEE, Volume 3, 1-4 May 2000 Page(s):1534 – 1539 [3] J.H. Han, P. Shan, Sh.S. Hu, “Contact analysis of fractal surfaces in earlier stage of resistance spot welding”, Materials Science and Engineering, A 435–436, pp. 204–211, 2006 [4] W. Li, E. Feng, D Cerjanec, G. A. Grzadzinski, “Energy Consumption in AC and MFDC resistance spot welding”, Sheet Metal Welding Conference XI, Sterling Heights, MI May 1114, 2004. [5] H. Tang, W. Hou, S.J. Hu, H. Zhang, “Force Characteristics of resistance spot welding of steels”, Supplement to The Welding Journal, pp. 175-183, July 2000 [6] H. Tang, W. Hou, S.J. Hu, “Forging force in resistance spot welding”, Proc Instn Mech Engrs Vol 216 Part B: J Engineering Manufacture, pp.957-968, 2002 [7] A. Kirchheim, A. Lehmann,G. Schaffner, R. Deuerling,N. Jeck, “Force monitoring optimizes resistance welding and related joining processes”, DVS Annual Welding Conference, Essen, Germany, 13.-14.9.2005 [8] Y.S. Zhang and G.L Chen, “A Neuro-fuzzy approach to part fitup fault control during resistance spot welding using servo gun”, School of Mechanical and Power Engineering, Shanghai Jiao Tong University, Shanghai, 200030, China

[9] Dhanasekaran Venugopal, “Development of force stepper and fit-up solver techniques for a servo controlled resistance spot welding gun”, Thesis Report 2008, Oakland University, Rochester, MI.

V. CONCLUSION This study focuses on the benefits of using a multi-level force profile during a MFDC resistance spot welding process. Both simulation studies and actual implementation are undertaken. The simulation results show that a three-level force stepper can increase the nugget size by 10% for the same settings of weld current and weld time. The force stepper takes only about 30 msec to change force levels, which is approximately 40% faster than the method used by Tung at el. The fit-up solver algorithm is very fast and takes only about 10 msec to detect fit-up and apply a compensating force.

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