Application of ultrasonic sensors to robotic seam tracking

IEEE TRANSACTIONS ON ROBOTICS AND AUTOMATION, VOL. 5 , NO. 3, JUNE 1989 3 3 1

Application of Ultrasonic Sensors to Robotic Seam Tracking

BENIGN0 MAQUEIRA, CHARLES I. UMEAGUKWU, AND JACEK JARZYNSKI

Abstract-An automatic seam tracking system that uses an ultrasonic- based sensor has been developed and interfaced to a P-50 Process Robot in an effort to achieve on-line seam tracking of joints, without the use of workpiece geometrical models or subsequent “teaching” routines [l]. The transducer is used to inspect the workpiece ahead of the torch and to measure joint orientation as well as lateral deviation caused by curvature or discontinuities in the joint path. Measurements of echo pulse amplitude and transit time are combined with a particular torch-sensor arrangement and transducer positioning strategy in order to eliminate ambiguities resulting from symmetry in the acoustic signal and from environmental factors such as joint geometry and roughness. Data pertaining to the joint orientation and lateral deviation are acquired periodically by sampling equi-spaced points along the joint as the torch advances. With each sample, data are transferred to the microcomputer, where a trajectory-generating algorithm calculates the x , y , 0 coordinates of the torch tip trajectory needed to meet the tracking requirements. The actual tracking task is accomplished on-line by executing a series of shifts in these coordinates.

INTRODUCTION

HE ARC-WELDING process relies heavily on accurate T tracking of joint position and orientation for the production of high-quality weldments [2], [3]. Thermal expansions experienced by the workpiece during the welding process and inaccuracies associated with workpiece positioning render off- line programming of welding robots inadequate due to deviations of the joint from the “taught” path [4]. Current empha- sis is therefore on real-time seam tracking to compensate for these deviations and eliminate all “teaching” requirements.

The objective of this paper is to demonstrate how a single ultrasonic transducer can be used in real time to obtain the geometrical information required to track joints in two- dimensional space. The technique described in this paper as- sumes that the pieces to be welded are flat, and the plane which contains the seam is called the horizontal plane. The investigators have worked to develop a data acquisition system independent of joint geometry and size. The system requires only that the joint possess geometrical features that reflect sound waves at 45” from the horizontal. This is the case both for V-groove joints (where a strong echo is received from the sloping surfaces of the grove) and lap joints (where an echo

Manuscript received November 15, 1988. This paper was presented at the International Workshop on Industrial Applications of Machine Vision and Machine Intelligence, Tokyo, Japan. Feb. 1987. This work was supported by the Computer Integrated Manufacturing Systems Program at the Georgia Institute of Technology.

B. Maqueira is with Texas Instruments, Dallas, TX 75266. C. I. Umeagukwu and J. Jarzynski are with the George W. Woodruff

School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332.

IEEE Log Number 8927073.

+--! FIELD OF VIEW

FIELD OF V E W

(b) The incident sound beam and the reflected sound wavefronts from (a) a V-groove joint, and (b) a lap joint insonified at 45’.

is received from the 90” corner). In addition, the investigators wished to eliminate the need for scanning routines that consume time by periodically examining large portions of the workpiece. They achieved this by using an unfocused sensor mounted on a positioning mechanism that receives input from the echo pulse amplitude and then adjusts the sensor position as the joint orientation changes during the tracking task.

A recent study of the application of acoustic sensors to robotic seam tracking was reported by Estochen et al. [5]. These authors used a focused ultrasonic beam to perform a two-dimensional surface height sampling of the workpiece. At each position along the seam the ultrasonic transducer performed a linear scan across the seam. At each sampling point the distance from the surface to the transducer was computed from the measured time of flight of the ultrasonic signal. The transducer was initially pointed along the vertical, but the an- gular orientation of the transducer could be changed at each sampling point to align the ultrasonic beam so as to be normal to the workpiece surface.

In contrast to the method used by Estochen et al., the investigators in the present study insonified the seam with a sound beam at 45” to the workpiece surface, as shown in Fig. 1. The measurements were made a a frequency of 100 kHz, where the wavelength in air is 0.13 in (0.33 cm). Strong echoes were observed both from V-groove joints (Fig. l(a))

Fig. 1.

1042-296X/89/0600-0337$01 .00 0 1989 IEEE

338 IEEE TRANSACTIONS ON ROBOTICS AND AUTOMATION, VOL. 5, NO. 3, JUNE 1989

SENSOR AXIS OF ROTATION\

EDGE OF TORCH R A M ALSO T4NGENT OF

SEAM 4 T TORCH TIP)

- T4NGENT TO T K SEAM

POINT ON SEAM BEING INSPECTED

Fig. 2. Geometrical constructions for joint orientation and deviation measurements.

and from lap joints (Fig. l(b)). The paths of the signals and returning acoustic waves for different sensor positions on a V-groove in particular are shown in Fig. 2. The advantages of the 45" transducer configuration used in the present study are as follows:

a) A simple one-dimensional (rotation about vertical axis) scan is sufficient for tracking the seam. The data sampling and computations required by this scan are simple and can be rapidly executed. This is because the only requirement of the system using 45' configuration is to measure the orientation of the transducer relative to the torch and the range from the transducer to the seam at the instant that the maximum amplitude occurs. And the maximum amplitude occurs when the transducer axis is normal to the face of the groove. It is important to point out that the scan is along the face of the groove, not across the seam.

b) Both the time of flight and the amplitude of the ultrasonic echo are measured and utilized in the tracking algorithm. Using standard electronic circuits, these are the simplest measurements that can be made on the echo, and the resulting data lead to a relatively simple algorithm that can be rapidly executed by the controlling computer.

c) The performance of the system is relatively insensitive to the alignment of the transducer with respect to the workpiece. The echo is approximately omnidirectional in the vertical plane since it contains strong contributions from the edges of the seam. In a horizontal plane the transducer continuously scans over a range of angles, which includes the angle at which the echo amplitude is maximum.

Acoustic sensors provide information on both joint orientation and range distance [5]. This information can be derived from the reflected echo pulse amplitude and transit time, re- spectively. However, when the reflecting feature (i.e., joint) has dimensions comparable to the diameter of the transmit- ting element, the reflected echo amplitude is affected by the feature's orientation with respect to the sensor, by the perpendicular distance from the feature to the sensor's line of sight (sensor axis), and by the joint geometry, size, and roughness. Moreover, the variations in pulse amplitude are symmetrical about the normal to the feature, making it difficult to determine on which side of the normal the sensor is located.

Despite these ambiguities, it is known that the sensor axis lies in a plane normal to the joint when the echo pulse is a local maximum (as opposed to a global maximum). This fact is illustrated by the middle sensor position in Fig. 2. If a global maximum is observed, the sensor axis not only lies in this plane but also intersects the feature if it is symmetrical. These observations have been implemented with a transducer positioning strategy to determine the joint orientation from the echo pulse amplitude. The transit time is then used to determine the location of the joint with respect to the sensor. These measurements of the acoustic signal are reliable and readily available from a peak detector and clock timer.

The data acquisition, which consists of a Krautkramer LS 3- 20 air-coupled transducer, a LAM80/8 sound distance meter, and a positioning mechanism, was implemented on a P-50 Process Robot. The system was used to obtain all the nec- essary information required by a trajectory-generating algorithm. The algorithm uses information on joint orientation and its deviation from the torch tip to generate the robot path commands required to track the joint. The system is capable of tracking joints that reflect sound waves at 45" from the horizontal, such as lap and V-groove joints. No assumptions about the joint path are required except for those pertaining to the initial 2 in encountered. This portion of the joint must be straight but can be at an arbitrary horizontal orientation and location with respect to the torch. The system can handle singularities in the form of abrupt horizontal changes in the joint path of up to 15" and maximum curvatures determined by the intensity of reflected waves, which must remain above a threshold value.

PROCEDURES

Transducer Arrangement and Positioning Strategy The torch, transducer, and workpiece arrangement used in

this investigation are shown in Fig. 3. The transducer is oriented at 45" from the horizontal and directed toward a small region directly ahead of the torch. The lever arm rotates the sensor about a vertical axis located 2 in (5.08 cm) ahead of the torch tip. The sensor axis and vertical axis intersect at the workpiece surface such that rotations of the sensor about its axis of rotation allow for the reception of pressure waves from a fixed point on the workpiece at various orientations.

The mechanism and torch bracket are mounted to the P-50 wrist. Both remain fixed with respect to each other as the wrist rotates. The mechanism oscillates the sensor about its axis of rotation by means of a permanent magnet dc motor geared to the lever arm pivot shaft in a 500: 1 ratio.

With this arrangement, a small region located ahead of the torch can be inspected to determine the seam deviation (SD) and its orientation with respect to the torch $. Fig. 2 shows the top view and geometrical constructions used to define these and other variables. The inspected region on the workpiece (defined by the intersection of the incident ultrasonic beam and the workpiece surface) will be referred to as the exposed surface. Its boundaries represent the locus of points from which the strength of reflected echoes falls below a certain threshold value. As long as the joint remains inside the exposed surface,

MAQUEIRA et al.: ULTRASONIC SENSORS FOR ROBOTIC SEAM TRACKING 339

P - 5 0 ROBOT ARM

I SENSOR AXIS OF

MOTOR VOLTAGE INPUT

TORCH WORKING TIP INSPECTION

P - 5 0 ROBOT

P - 5 0 ROBOT RANGE OUTPUT lanolog1

VOLTAGE PULSE

UTA PEAK DETECTOR

AMPLITUDE

COMPUTER CONTROLLER RANGE A N 0 POTENTIOMETER

A / D CONVERSIONS

Fig. 3. Seam tracking of weld joints. Equipment setup and information flow.

detectable sound waves will be reflected off the joint edges and received by the transducer. For lap and V-groove joints, the reflections appear as a single voltage pulse of varying amplitude and transit time depending on the seam deviation from the sensor axis of rotation and its orientation with respect to the sensor.

The sensor orientation with respect to the torch is measured by the angle 9. Measurements of this angle are provided by a potentiometer mounted on the mechanism and geared to the lever arm shaft in a 4:l ratio. With reference to Fig. 2, as the sensor rotates clockwise the received echo pulse will in- crease in magnitude and reach a maximum when the sensor overshoots the joint normal. Thus the pulse amplitude provides information on the joint orientation with respect to the transducer. The seam deviation measured horizontally along the lever arm axis from the sensor axis of rotation to the joint can be determined from range measurements. Fig. 4 shows the front view with the sensor and its axis of rotation both in the plane of the figure. For the two-dimensional case, the height h is a constant. The seam deviation can be calculated from

SD = r - d(R, - d)2 - h2 (1) where r is the constant radius of rotation, R, is the range from the sensor to the joint, and d is a parameter characteristic of the particular joint. For a V-groove, d is the perpendicular distance from the groove face to the groove center line at the level of the workpiece surface, and for a lap joint d is zero.

The original intention was to use the pulse amplitude as feedback to control the mechanism such that the sensor would

JOINT CENTER

Fig. 4. Geometrical constructions used to derive seam deviation from range measurements.

always be maintained perpendicular to the joint. This is the optimal sensor position from which the strongest reflections are obtained. Also, independent tracking of the joint orientation with the sensor itself provides a means of maintaining the sensor within its operating range even if a corner or excessive curvatures are encountered in the joint path.

The range measurements are independent of the pulse amplitude. However, as the seam deviation increases, the pulse amplitude will decrease due to the reception of weaker sound wave echoes that emanate from regions close to the exposed surface boundaries. These variations in pulse amplitude cannot be decoupled from variations caused by changes in the joint orientation. This coupling effect, along with the symmetrical variations of the pulse amplitude about the joint normal, makes it difficult to quantify the joint orientation with respect to the sensor from measurements of the pulse amplitude.

To overcome this difficulty, the investigators controlled the mechanism to oscillate the sensor about the joint normal with an amplitude of approximately 5 '. Feedback is provided by the pulse amplitude. However, instead of using the magnitude of the pulse amplitude, the mechanism controller is designed to detect the instant at which a maximum pulse amplitude occurs. Once detected, the sensor is known to be aligned with the joint normal and the angle 4 can be sampled to determine 0, which provides a measure of the joint orientation with respect to the torch. Since the sensor oscillates about a vertical axis, the point of inspection on the joint is defined by the joint normal that intersects the sensor axis of rotation. The seam deviation corresponding to this point is obtained by also sampling the range measurements when a peak pulse amplitude is detected.

Continuous oscillations of the sensor about the joint normal provide a means of maintaining the sensor within its operating range while a series of points along the joint is sampled for measurements of 9 and R, as the torch advances. The data sampling rate is equal to the frequency of sensor oscillations since two samples must first be averaged in order to compensate for errors introduced by time delays associated with the detection of the peak pulse amplitude.

Trajectory-Generating Algorithm As the torch advances, the required shift in translation and

orientation (x , y, 0 shift) from one sampled point to the other

340 IEEE TRANSACTIONS ON ROBOTICS AND AUTOMATION, VOL. 5 , NO. 3, JUNE 1989

JOINT CENTER LINE-

PROJECTED POINT OF I N S P E C T I O N 7 SEAM TANGENT

SENSOR “VIEW” -SENSOR AXIS

Fig. 5. Trajectory-generating algorithm parameters. Point-by-point tracking of position and orientation.

is determined with respect to the robot base frame. Fig. 5 shows the top view with the z-axis pointing out of the page. The x-y-z reference frame coincides at all times with the torch tip, but maintains the same orientation as the robot frame of reference x ’ - y ’ 4 . Both of these frames are left-handed in accordance with the robot manufacturer’s convention. The torch tip is currently at a particular point on the joint PO where capital “p” is used to denote position vectors with respect to the robot reference frame. The orientation of the joint (and torch) at this point has been defined by 00 as shown.

The sensor axis of rotation is parallel to the z-axis and is located a projection distance L ahead of the torch tip. The sensor is shown perpendicular to the joint thus defining P, , the point currently being sampled. In this position, the sensor angle (and projected joint orientation) with respect to the torch is 4, (= 0,). At the instant when the sensor axis is normal to the workpiece surface, 4, becomes 0,. The projected joint orientation with respect to the robot frame is 6, as defined by the joint tangent at P, . Between PO and P, are n - 1 points that have been sampled previously just as P, is about to be sampled.

Data acquisition consists of obtaining measurements of 4, and R , with the sensor in the position shown. Equation (1) can be used to determine SD, (a negative value in Fig. 4) while 6, is determined from

( 2 )

When the torch tip reaches point P , - l , the robot arm must shift by a vector amount P, - P,- l = dP, in order to ad- vance to position P, in a straight path. Assuming that the components of PO (Pxo, Pyo, 0,) are known, the components of P, can be calculated from

(3)

e, = eo + 4, - 90 o .

Px, = Pxo + L sin eo - SD, cos e,, Py, = Pyo + L cos 00 + SD, sin 8,. (4)

The shift from P,-1 to P, has an x component given by

dPx, = P x , - ~ ( 5 )

where

Px,-, = P X - ~ + L sin - SD,-I cos (6)

Substituting (3) and (6) into (5)

dPx, = Pxo - P X - ~ + L(sin Bo - sin e- , )

But Pxo - Px-1 = dPxo; therefore

dPx, = dPxo + L(sin Bo - sin e - , )

The corresponding equations for they and 8 shift components are

dPy, = dPyo + L(cos 60 - cos 0 - 1)

+SD, sin 8, - SD,-1 sin 8,-1. (9)

and de, = e, - e,-1. (10)

Note that the equations for dPx and dPy are independent of the torch tip position with respect to the robot base frame. However, position feedback is required to monitor the torch tip position and determine when the torch reaches a particular sampled point on the joint.

Tracking of orientation and position has been accomplished by executing a series of shifts in torch position and orientation. These shifts are calculated by @)-(lo) and stored in memory until they are required for actual execution. Calculations of dP, can be made just after the robot executes the dPo shift and point P , is sampled. This timing allows for calculations of figure shifts (dP,) during the execution of a shift already calculated (dp I ) .

A trajectory-generating algorithm was developed to perform data acquisition of point Pi , process the data, calculate future shifts (dPi ) , and store them until the torch has advanced to point Pi-1, at which time the data for shift dPj are transferred to the robot controller. Fig. 6 shows the overall system flow chart diagram. All three loops operate in parallel; however, they are synchronized through data transfer such that one loop does not get ahead of the other.

To perform the tracking task, the torch is first brought to its initial tracking position, which consists of a taught location. Due to workpiece positioning errors, the torch tip may or may not coincide with the point; therefore, a seam search routine is executed to locate the point on the joint closest to the torch tip and command the robot to correct for any errors associated with workpiece positioning. Once these initial errors are cor- rected, the first n - 1 shifts are calculated assuming the first 2 in (5.08 cm) of the joint are straight. This assumption is made because the first n - 1 points on the joint are never sampled by the data acquisition subsystem. Since the sensor is located ahead of the torch, data sampling begins with point P,. Once P, is sampled, data pertaining to shift d p l are transferred to the robot controller. At this instant the path control loop and trajectory-generating algorithm perform their functions in parallel. While the shift is being executed, data from P, are used to calculate shift dP, . The shift size is chosen such that calculations of dP, are completed before the torch reaches

34 1 MAQUEIRA el al.: ULTRASONIC SENSORS FOR ROBOTIC SEAM TRACKING

MAIN ALGORITHM I Calculate F i rs t n - l S h l f l s I

Dolo Transfer

DATA ACQUISITION LOOP

Woit for Maximum

PATH CONTROL LOOP

Shift Doto

6 Execute Shift

Fig. 6 . Overall system flow chart diagram

point P I . In this way, shift dP2 can be transferred to the robot controller as soon as point P I is reached.

The data acquisition loop continuously transfers data on 4 and R, to the trajectory-generating algorithm. These data are ignored until the calculations of each shift are completed and the torch reaches P I . Once P I is reached, the most recent data acquired are assigned to point P, , and shift dP2 is immediately transferred to the robot controller to update the torch position. The procedure is repeated until the end of the joint is reached.

Experimental Setup The equipment setup is shown in Fig. 3. The LAM80/8

drives the transducer at 100 kHz. Voltage pulses of this frequency are applied to the transducer with a repetition rate of 400 Hz. Each voltage pulse gives rise to pressure waves that propagate through air and are reflected by the joint edges back to the transducer. These reflected pressure waves in turn produce other voltage pulses that are amplified and timed by the LAM8018 outputs, the aniplified voltage pulses, and an analog voltage proportional to the distance from the sensor to the joint. A peak detector (KB-Aerotech model UTA-4) is used to obtain an analog voltage proportional to the voltage pulse amplitude. This peak detector output is monitored by the PM controller to determine the instant at which a maximum amplitude occurs. The PM controller consists of a differen- tiating circuit to detect the maximum amplitude, and an A/D converter to sample the LAM80/8 output of range and the mechanism potentiometer at the instances of maximum amplitude. An MC6801P1 microprocessor is used to perform the data acquisition and transfer the results to the computer through RS232 communications. Also, the 6801 receives input from the differentiator to determine the dc motor voltage polarity required to oscillate the sensor about the joint normal. The algorithm employed for this purpose is shown in Fig. 7. Data acquisition is performed and transferred to the computer as soon as the pulse amplitude it observed to decrease.

Apply -Vm w dA/dl > 0

I I "Settling" Time Delay

I "Overshoot" Time Delay I I I

Settling" Time Delay U i

Fig. 7. Flow chart of PM control algorithm.

The computer executes the trajectory-generating algorithm while obtaining data on & and Rgn from the PM controller and position feedback from the robot controller. The computer repeatedly transfers the x , y , t9 components of each shift to the robot controller, which then advances the torch along the joint.

RESULTS

The mechanism oscillates the sensor at 2.1 Hz with an amplitude of 5" to each side of the joint normal. To demonstrate the ability of the mechanism and PM controller to maintain the sensor within its operating range, the robot wrist was rotated at 18"/s with respect to the plate. Fig. 8 shows the potentiometer output ( a measure of 4) versus time.

Despite the exaggerated relative motion between the joint normal and positioning mechanism, the results show that the pulse amplitude can be used as feedback to maintain the sensor within its operating range while data are being acquired.

A 0.25-in (0.635-cm) aluminum plate containing a V- groove was used to evaluate the overall system response. The V-groove was milled at a depth of 0.125 in (0.3175 cm) with its sides cut at 45" from the vertical. Fig. 9 shows the plate and joint path geometry used.

Fig. 10 shows the robot path trajectory commanded by the sensory subsystem as the joint in Fig. 9 is approached from 165" corner. During the actual tracking process, two points along the path were observed to compare the actual position of the torch tip to that commanded by the sensory subsystem.

342 IEEE TRANSACTIONS ON ROBOTICS AND AUTOMATION, VOL. 5 , NO. 3, JUNE 1989

10-

9 -

8-

7-

w 6- (3

The results of that observation are shown below:

Calculated Actual Torch Tip Position Position

Point X Y e X Y e V-GROOVE SECTION

1 4.53 3.53 66.9 4.521 3.523 67.1 N A - A 2 8.11 4.33 87.1 8.10 4.339 87.4

I

1' 7 155

h-- 17" -d Fig. 9. Workpiece and joint path geometry: V-groove specimen.

Y-COORDINATE (inches)

Fig. 10. Robot path trajectory for the V-groove specimen. Actual seam tracking results when the seam is approached from 165" comer.

The precision of the tracking system relies on the paramet- ric measurements made for each motion of the torch tip. These parameters are range (R,) and orientation (9 = 0) measurements. The accuracy of the computed R, is determined by the accuracy in the measurement of the time of the ultrasonic echo, and the accuracy with which the A/D converter samples the output voltage (proportional to the echo time of flight) from the ultrasonic receiving unit (LAM80/8). The accuracy of the orientation measurement is determined by the signal-to- noise ratio in the output from the potentiometer mounted on the sensor rotation mechanism and the accuracy with which the A/D converter samples the potentiometer output. The overall accuracy for R, was determined directly to be k0.02 in (kO.051 cm), by comparing the R, calculated from echo time of flight with a direct precise measurement of R, using a ruler. The k0.02 in (k0.051 cm) should be compared with the accuracy of kO.013 in (k0.033 cm) (approximately 0.1 ultrasonic wavelength) projected by the transducer manufacturer for the time-of-flight measurement [6]. The overall accuracy of the orientation measurement was also determined directly to be k0.5", by changing the seam orientation by a known angle (measured precisely with a protractor) and comparing it with the corresponding angle determined from the potentiometer output. The accuracy with which the torch tip can be centered over the seam varied from k0.0625 in (0.1588 cm) to k0.1875 in (0.4763 cm). This accuracy is influenced by the accuracy of the robot, the resolution of the robot, and the measurement accuracy of the sensor. The sensor accuracy, including the response time of the sensor, was determined experimentally as described above.

CONCLUSIONS AND RECOMMENDATIONS

The results of this investigation demonstrate the technical feasibility of using a single ultrasonic sensor to track joints

MAQUEIRA et al.: ULTRASONIC SENSORS FOR ROBOTIC SEAM TRACKING 343

in two-dimensional space without physical contact with the workpiece. With the sensor oriented at 45” from the horizontal, sensor oscillations about a vertical axis can be employed to determine the joint lateral position as well as its orientation with respect to the torch. The errors associated with the 015- entation and lateral measurements are accurate to within 1” and are less than the diameter of typical welding wires.

Only measurements of pulse amplitude and transit time of the ultrasonic signal are required. Although both seam deviation and orientation with respect to the sensor affect the pulse amplitude, the sensor oscillations provide a means of decoupling these effects by monitoring the variations in pulse amplitude instead of relying on its absolute magnitude. Imple- mentation of sensor oscillations also eliminates the ambiguities associated with symmetrical variations of the pulse amplitude about the joint normal. Since the PM controller is designed to oscillate the sensor about this normal, the algorithm employed automatically keeps track of the side of the normal on which the sensor is located. Additional advantages of the positioning strategy concern the filtering out of environmental factors. These factors include joint geometry, roughness, and equipment gain settings, all of which affect the magnitude of the pulse amplitude. Since the technique employed does not rely on the magnitude of the pulse amplitude, the effects of these environmental factors are eliminated. Such factors do not affect the instant at which a maximum amplitude occurs.

The system described here can be implemented with any joint geometry that provides reflections at 45”. The results shown were obtained with a V-groove joint of 0.125-in (0.3175-cm) depth; however, the system has also been implemented on lap joints formed by 0.125-in (0.3175-cm) thick plates and V-grooves of 0.0625-in (0.1588-cm) depth.

The joint is inspected ahead of the torch tip primarily to avoid excessive interference from the welding activity. This technique allows for the implementation of shielding devices placed between the torch and the sensor. However, with the trajectory-generating scheme described, inspection of the joint ahead of the torch introduces a time delay associated with the time required to travel the projection distance L. As future shifts are calculated, they must be stored in memory until the torch advances to the appropriate location. The result is open-loop control within the projection distance. That is, any deviations in joint path which occur between the torch tip and sensor axis of rotation will not be detected by this scheme. However, changes which occur ahead of the exposed surface will be detected and compensated for (e.g., those due to thermal expansions).

Several modifications can be introduced to minimize the time delay and achieve closed-loop control. First of all, the distance L should be minimized and the parameter n allowed to take on the value 1. This will eliminate the need to store shifts; as soon as a shift is calculated, it can be transferred to the robot controller for execution. A second modification is to use the sampled data for P I , for example, and project this information backwards in order to approximate the location of Po. To improve such approximations, the joint curvature can be estimated from data on previously sampled points. The control task would then consist of driving the torch tip to point Po so as to eliminate tracking errors as the torch advances.

WORK IN PROGRESS

The system is being modified to eliminate the requirement that the first portion of any seam path be straight. In the new system an attempt is being made to reduce the distance between the sensor axis and the torch tip to its optimum level. The new system requires that the sensor axis be positioned at the very beginning of the workpiece to be welded. With this new arrangement, the algorithm is being modified, so that for any distance less than the distance between the sensor axis and the torch tip, the arc will not be turned on. This will make it possible for the sensor to have time to gather enough data before the torch tip reaches the workpiece.

The work reported here was a case of pure seam tracking without the arc on. However, the authors have taken this re: search a step further, by investigating the effects of electrical noise, welding noise, spatter, arc intensity, temperature gra- dient, and shielding gas on the seam tracking process. It was found that these environmental conditions affect the ultrasonic signal a great deal. However, these effects were eliminated through proper shielding and the use of a horn concentrator. The result of this research is being prepared for publication.

REFERENCES

B. Maqueira, “Robotic seam tracking of weld joints through the use of an ultrasonic sensor: System development and implementation,” M.S. thesis, Georgia Inst. Technol., 195 pp., 1986. V. M. Mazurov, V. S. Karpov, V. M. Panarin, A. A. Malyutin, V. N. Shestakov, and P. I. Chinaev, “System for automatic tracking of the joint using the arc as the sensing element,” Weld Production, vol. 31, pp. 42-43, k b . 1984. C. G. Morgan, J. S. E. Bromley, P. G. Davey, and A. R. Vidler, “Visual guidance techniques for robot arc-welding,” Proc. SPIE- Int. Soc. Optical Eng., vol. 449, pt. 2, pp. 390-399, Nov. 1983. J. E. Agapakis, K. Masubuchi, and N. Wittels, “General visual sensing techniques for automated welding fabrication,” in Proc. 15th

E. L. Estochen, C. P. Neuman, and F. B. Prim, “Application of acoustic sensors to robotic seam tracking,” IEEE nuns. Ind. Elec- tron., vol. IE-31, no. 3, Aug. 1984. Krautkramer, LAM 80 Operating Manual. KB-Aerotech, Ultrasonic Transducer Analyzer Manual. C. I. Umeagukwu, W. H. Peters, J. R. Dickerson, and W. F. Ranson, “Automated ultrasonic measuring system,” Exper. Tech., 1987. - , “Microcomputer controlled ultrasonic testing system,” in Proc. Southeastern C o d . on Theoretical and Applied Mechan- ics (Columbia, SC 1986), vol. XIII. Electro-Craft Corp. DC Motors Speed Controls Servo Systems. Motorola, Inc., MC 6801 &Bit Single-Chip Microcomputer (Ref- erence Manual), 2nd ed., 1983.

ISIR, p ~ . 103-114, NOV. 1985.

Benigno Mnqueirn was born in Artemisa, Cuba, on kbruary 15, 1961. In 1969 he moved to the United States and is currently a U.S. citizen. He received the B.S.M.E. degree from the University of Miami, Miami, FL, in 1985 graduating Magna Cum Laude. In 1986, he received the M.S.M.E. degree from the Georgia Institute of Technology, Atlanta, and also earned a Multidisciplinary Certificate in Computer Integrated Manufacturing Systems.

Currently, he holds a U.S. Patent on a wall-bed lifting mechanism he designed for Comfort Wall

Beds, Inc. Since March 1987 he has been employed with Texas Instruments, Inc. in the Control Systems Technology Center, Dallas, TX, working with the design and simulation of line-of-sight stabilization systems. His interests include utilization of digital signal processors to implement advanced control techniques.

Mr. Benigno is a member of Tau Beta Pi and Pi Tau Sigma.

344 IEEE TRANSACTIONS ON ROBOTICS AND AUTOMATION, VOL. 5, NO. 3, JUNE 1989

Charles I. Umengukwu received the B.S. degree in mechanical engineering from the University of North Carolina, Charlotte, in 1978, and the M.S. and Ph.D. degrees from the University of South Carolina, Columbia, in 1981 and 1985, respec- tively.

In 1985 he joined the faculty of the School of Mechanical Engineering at the Georgia Institute of Technology, Atlanta. His current research interests center on the application of microprocessors, ultrasonic and fiber-optic sensors to robotics, and unat-

tended manufacturing systems: welding processes, and printed circuit board analysis and inspection.

Dr. Umeagukwu is a member of the American Society of Mechanical, Manufacturing Engineers, Society of Experimental Mechanics, Nondestruc- tive Testing, and American Welding Society, and Sigma Xi.

Jacek Jarzynski was born in Warsaw, Poland. He received the B.S. degree in physics from Imperial College, London University, London, England, in 1957 and the Ph.D. degree in molecular ultrasonics in 1961.

From 1963 to 1971 he was with the Department of Physics, American University, Washington, DC, where he was engaged in teaching and research on the equation of state and transport properties of liq- uids. From 1971 to 1985 he worked at the Naval Research Laboratory, Washington, DC, where he

was involved in various areas of underwater acoustics, including sound prop- agation in composite materials, and fiber optic hydrophones. In 1986 he joined the faculty of the School of Mechanical Engineering at the Georgia Institute of Technology, Atlanta. His present research activities include work on scattering of underwater sound, development of fiber-optic sensors, and ultrasonic nondestructive testing of materials.

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Application of ultrasonic sensors to robotic seam tracking