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Manuscript received September 9, 2005. This work was supported by Siemens Automation and Drivers, who provided all of the equipment necessary in implementing the crane control. Furthermore, they provided financial support for graduate students to install and program the controller. K. A. Hekman was with the American University in Cairo, Cairo 11511 Egypt. He is now with the Engineering Department, Calvin College, Grand Rapids, MI 49546 USA. (phone 616-526-7095, fax: 616-526-6501, e-mail hekmancalvin.edu) W. E. Singhose is with the George W. Woodruff school of Mechanical Engineering, The Georgia Institute of Technology, Atlanta GA 30332 USA (e-mail:William.Singhoseme.gatech.edu) Abstract When cranes move objects in a workspace, the payload frequently swings with large amplitude motion. Open loop methods have addressed this problem, but are not effective for disturbances. Closed loop methods have also been used, but require variable speed driving motors. This paper develops a feedback based method for controlling single speed motors to cancel the measured payload oscillations by intelligently timing the ensuing on and off motor commands. The oscillation suppression scheme is experimentally verified on a bridge crane. I. INTRODUCTION Cranes are frequently used to transport objects in a cluttered workspace. One inherent problem with cranes is that the payload can swing freely. These oscillations pose safety hazards and can damage the payload or other objects in the workplace. Traditionally, an experienced crane operator has been required to keep the oscillations under control. More recently, various control approaches have been applied to augment the operators skill. These approaches fall into open and closed loop categories. One open loop approach used is input shaping, which has proven effective on cranes for reducing sway during and after the move 1,2,3, including during hosting 4. Shapers can be designed with increased robustness to modeling inaccuracies 5 (i.e. cable length changing the frequency). Another open loop approach is optimal control, which calculates a motion trajectory off line based on the mathematical model of the system 6,7. However, if the model is inaccurate, the performance will suffer. This is also the case with input shaping, but to a lesser degree. In addition, optimal control has not been used with current crane operator interfaces, as the path is not known beforehand. System model uncertainties and external disturbances provide the motivation for feedback control. Controllers have used the position and velocity of the trolley and the cable swing angle 8,9,10,11 or the spreader inclination 12 to generate trolley commands that reduce payload oscillations. Wave absorption control adjusts the trolley velocity to absorb any waves that are being returned by the payload, thereby canceling the oscillation 13. Feeding a delayed angle measurement back to the desired position has also been shown effective in reducing payload oscillations 14. Sorenson et al. 15 developed a control system that combined input shaping and PD feedback control. The feedback control used measurements from an overhead camera and compared the crane response to the modeled shaped response. In another method to reduce the effect of a disturbance, Park and Chang 16 proposed a “commandless” input shaping method for a telescopic handler. To compensate for the vibrations from unloading the handler, they introduce a pulse that induces vibration equal in magnitude but opposite in direction of the vibration from unloading. They show the methods potential by using it to reduce vibration by about 75%. However, issues of properly timing the impulse and ease of calibration remain. All of the feedback methods require the velocity or acceleration of the trolley to be precisely controlled. The research here is based on using measurement of payload swing to generate commands for simple on-off motors to cancel the payload swing, making it applicable to a broader range of cranes. II. VECTOR BASED INPUT SHAPER CALCULATION Booker 17 provides a framework for analyzing oscillations with vectors. Singhose et al. 18 provide insight into how vibration cancellation can be achieved in a vector-based analysis of input shapers. An impulse of magnitude A 1 applied to an undamped second-order system of unit mass will induce a response of () tAtx sin 1 = . (1) This has a magnitude A 1 and phase angle of zero. Similarly, if a second impulse of magnitude A 2 was applied at time T 2 , then it would result in an output of () ( ) ( ) 2222 sinsin TtATtAtx = , tT 2 . (2) This has a magnitude A 2 and phase angle =T 2 . The magnitudes and angles can be transformed into vector notation as seen in Fig. 1. Summing these vectors gives the total vibration response, as seen in Fig. 2. The corresponding time response of these impulses is seen in Fig. 3. After the second impulse, the total response matches the amplitude and phase of A R . If the system has damping, then this method needs to be modified. First, the angle changes to T 2 1= . (3) Feedback Control for Suppression of Crane Payload Oscillation Using On-Off Commands Keith A. Hekman, and William E. Singhose Proceedings of the 2006 American Control Conference Minneapolis, Minnesota, USA, June 14-16, 2006 WeC11.4 1-4244-0210-7/06/$20.00 2006 IEEE 1784 Second, damping causes the amplitude to decay over time. To account for the decay, calculations use the effective amplitude at t=0 that results in the required amplitude at T 2 of 2 1 22 = eAA eff . (4) A shaper can be designed such that the sum of all the effective impulses results in zero vibration, as seen in Fig. 4. To do this, the A 3eff is chosen to be the negative of A R from Fig. 2. To get the magnitude of this canceling impulse, it must be converted to the time it will occur using (3) and 2 1 33 = eAA eff . (5) In reality, systems are not moved with impulses. To create a practical command, the impulse sequence is convolved with the desired command. For example, Fig. 5 shows a step command convolved with two impulses produces a stair step command. The resulting command will not produce any residual vibrations. III. PAYLOAD OSCILLATION CANCELLATION. The goal of this research is not to create commands that result in no residual oscillation for point-to-point motion. Rather, the measured payload swing is used to create commands for simple on-off motors that cancel any oscillation once it occurs. When creating such commands, the magnitude of the actuator force vector cannot be arbitrarily chosen, as the motor can only be turned on and off. However, turning the motor on or off will cause payload oscillations, which can be represented as vectors. Unlike a pure impulse, these vectors will not have zero phase angles, as the motor does not instantly stop or accelerate to full speed. Therefore, by the time the command is completed, the payload will have some displacement and some velocity, giving a vector representation similar to Fig. 6. The vector for turning the motor off should have a similar magnitude, but in the opposite direction, assuming that the acceleration and deceleration dynamics are similar. If not, it can be represented by its own unique amplitude and phase. The controller developed here will use two command switches (on-off) to eliminate the position and velocity components of the vibration. The controller needs to calculate the appropriate times for these commands in real time. To make this calculation, a vector triangle is used, as seen in Fig. 7. The three sides of the triangle are the current vibration level (A vib ), and the vibration amplitudes of “on” and “off” commands. If the triangle can be created, then the oscillations can be forced back to zero (the origin of the vector diagram.) Assuming that the operator wants the crane to be moving, then the command sequence would be “off”, wait, then “on” again. Certain components of the triangle are known: the magnitude of the current vibration and the effect of turning the crane on (A on ) and off (A off ). The A 2eff A 1 A 3eff Fig. 4. Summing three vectors to get zero vibration * Shaped CommandInput ShaperInitial Command 0 0 0 Time Time Fig. 5. Creating a stair step command by convolution A on on / . Fig. 6. Vector representation for turning the motor on. A on eff on =T A vib A off vib A on eff on =T A vib A off vib (a) (b) off off Fig. 7. Vector diagram for calculating time to turn motor off. A 1 A 2 Time A m pl itu d e T 2 A 2 A 1 Impulse Sequence Vector Diagram 2 =T 1 =0 Fig. 1. Impulse sequence and corresponding vector diagram A 2 A 1 A R R Fig. 2. Summing two vectors to get the total response time response to A 1 response to A 2 time total response response to A R A 1 A 2 A 1 A R A 2 Fig. 3. Time response of impulses (adapted from 18) 1785 unknowns are the time until the crane is turned back “on” again (T), and at what existing vibration phase angle the crane should be turned “off” ( vib ). Since it is a triangle, there are two possible solutions as shown in Fig. 7. The time response of these solutions is given in Fig. 8. The solution in Fig. 7a is preferable as it has a smaller angle on =T, so the time until the vibration is canceled is shorter. Also, the swing angle is less. To find vib , the intermediate angles seen in Fig. 9 are used. From the law of cosines, cos2 222 viboffviboffeffon AAAAA += (6) oneffonoffeffonoffvib AAAAA cos2 222 += . (7) From (7) + = effonoff vibeffonoff on AA AAA 2 cos 222 1 (8) If there is no damping, then the solution can be solved directly since A on eff =A on . Note that most cranes have near zero damping, but if the damping is significant, then the same equation can be used to solve for on , but it must be solved iteratively, with 2 1 = on eAA oneffon . (9) Equation (8) is initially calculated using =0. After on is found, from Fig. 9 can be calculated using + = viboff effonviboff AA AAA 2 cos 222 1 (10) Once is known, vib can be calculated using += offvb (11) Once the controller has turned off the crane, it then waits until the angle of the vibration is opposite in direction to on . At this point, the controller turns the motor back on. If the calibration is perfect, oscillations will be eliminated. If the operator desires the crane to be stopped, then vibrations can be canceled by moving the overhead support either forward or backward. This results in two different phase angles of vibration that can be used for the controller, as seen in Fig. 10. In part (a), the reverse direction, the diagram is basically the same as Fig. 7a, except the on and the off are exchanged. Based on this, += + = + = ronvb vibon eff offvibon eff offon vib eff offon off AA AAA AA AAA 1 222 1 222 1 2 cos 2 cos (12) For Fig. 10b, the vector diagram has the same geometry for as (a), only rotated by radians. Therefore += fonvb 2 . (13) The controller compares the existing vibration phase angle to (12) and (13) and uses whichever angle occurs first. Once the crane is stopped, then the controller waits until the oscillation phase angle is opposite to that of the “on” command. Then, the motor is turned back on. The maximum oscillation magnitude that can be canceled using an on-off command is approximately twice the oscillation induced by an “on” command. If the current oscillation magnitude is larger than this, then the controller calculations are based on the maximum cancellation level. As a precaution, this maximum level can be reduced to limit the distance moved in canceling the oscillations, thus limiting the angle on and T. A limitation of this oscillation cancellation method is that it assumes that superposition can be applied for the vector representations of induced vibration. This is only true if the motor has time to reach its steady state velocity between the vectors, so small payload oscillations cannot be eliminated. Therefore, an oscillation magnitude threshold is used. IV. CONTROLLER IMPLEMENTATION The proposed controller using the oscillation cancellation techniques from Section III was implemented on a large bridge crane. The crane has a camera mounted on the trolley to measure the payload swing in the horizontal plane. The camera can also measure the height of the payload. All of the control actions were based on a single payload height. A off r eff off =T A vib A on r vib 1 A vib vib 2 (a, reverse) (b, forward) on f on r off A off f eff A on f Fig. 10. Vector diagram for when to turn on motor. off on m o t o r i npu t 0 time p a y lo a d s w in g from (a) from (b) Fig. 8. Time Response from vector diagram of Fig. 7. A on eff on =T A vib A off Fig. 9. Angles used to calculate command initiation time. (b ) Pa yloa d Sw in g (a) Mot o r In put 1786 The system could be calibrated at different heights and the timings would be based on the camera measured height. A. System Calibration The controller calculations (8)-(13) require the magnitude and phase angle of the oscillations caused by turning the motor “on” and “off”. These can be calculated by plotting the crane input and response on the same graph, as seen in Fig. 11. Fig. 11a shows the motor being turned off at about 5.5 seconds while the crane is moving forward. The motor takes about a second to come to rest after the command is issued. Fig. 11b shows the payload swing angle and the oscillation level m given by () 2 2 & +=m (14) where is the natural frequency of the system. The times of the zero crossing of the swing angle before (t b ) and after (t a ) the input change (t i ) were recorded. The phase angles of the oscillation before ( b ) and after ( a ) the input can be calculated using ( ) ( ) pipaapipbb tttttttt =+= 22 (15) where t p is the time of one oscillation period. The complex vector of the input transition is given by f off ba i f off i b i a eAememA = . (16) where m b and m a are the amplitudes of the oscillation before and after the input the subscript f denotes forward motion. A similar procedure can be done for the remaining vectors. Graphically, the s can be plotted for starting and stopping, for both forward and reverse, as seen in Fig. 12. On average the oscillation had an amplitude of a=0.052 radians at an angle of =1.11(+) radians (63.6 (+180). B. Controller Response for User Motion Fig. 13 shows the response of the crane to a user request to move backward. In Fig. 13a, the user input is shown using a line with circles. The resulting trolley speed is seen with a dashed line. As shown in Fig. 13b, the crane motion excites oscillations in the payload. Fig. 13c shows the phase angle of the oscillations given by = & 1 tan . (17) It also shows the switch angle calculated from (11)-(13). At the initial crossing (at about 2 seconds), the oscillation level is not large enough (1.7) to trigger a control action. It is one period later (at about 6 seconds) that the oscillation canceling control action takes place. At this point, the controller briefly turns off the crane motor, as seen in Fig. 13a by the solid line. At this point the switch angle jumps to the angle to turn back on the motor. At 7 seconds, this angle occurs, and the crane motor is turned back on. When the crane reaches full speed (at about 8 seconds), the oscillation level is quite small (about a half degree). The operator stops pressing the reverse button at about 11 seconds, as seen by the sold line in Fig. 13a. When the crane stops, oscillations are again induced. When the desired command is at rest, there are two switch angles -0.04 -0.02 0 0.02 0.04 -0.04 -0.02 0 0.02 0.04 / back start back stop forward start forward stop . Fig. 12. Oscillation vectors for different commands 0 1 us e r i nput ( % f u l l s peed) t i control motor speed 0 2 4 6 8 10 -5 0 5 t b t a m b m a s w i ng ang l e ( o ) time (s) payload angle vibration level Fig. 11. Measured bridge crane response to an “off” command (a) U s e r I nput ( % f u ll s p e e d ) (b ) s w ing a n gle ( ) -1 -0.5 0 0.5 1 u s e r i n p u t ( % fu l l s p e e d ) user input control scaled motor speed -4 -2 0 2 4 pay l o ad s w i n g ( o ) payload ang. oscillation amp. 0 2 4 6 8 10 12 14 16 -90 0 90 180 270 time (s) ph as e an gl e ( o ) oscillation ang. switch ang. Fig. 13. Response to an operator input of reverse (a) us er i nput (% f u l l s p eed) (b) payl oad s w i ng ( ) (c) phas e angl e ( ) 1787 -1 -0.5 0 0.5 1 u s e r i n p u t ( % fu l l s p e e d ) user input control sc. motor speed -5 0 5 pay l o ad s w i n g ( o ) payload ang. oscillation amp. 0 1 2 3 4 5 6 7 8 9 10 11 12 -90 0 90 180 270 time (s) ph as e an gl e ( o ) oscillation ang. switch ang. Fig. 15. Response to a large disturbance (a) us er i nput (% f u l l s p eed) (b) payl oad s w i ng ( ) (c) phas e angl e ( ) given by (12) and (13), as the crane can cancel the oscillation by going both forwards and backwards. The first angle occurs at a little after 13 seconds, and the crane moves forward slightly to cancel the oscillations. Once the crane is back at rest very little oscillations remain. C. Controller Response for Disturbance Rejection Fig. 14 shows the response of the crane to a disturbance when the bridge is at rest. At a little after 1 second, the payload was disturbed. At about 3 seconds, the oscillation phase angle matched the switch angle condition, and the controller commanded the trolley to move forward. At a little past 4 seconds, the phase angle matched again, and the trolley was commanded to stop. When the trolley came to rest at about 5.5 seconds, the payload swing was small. Fig. 15 shows the response to a very large disturbance. At about 1 second, the payload was pushed with a disturbance that is larger than a single on-off command can suppress. In this case, the first on-off control action suppressed the maximum amount, and then the second control action canceled the remaining oscillations by going in the opposite direction. This resulted in little oscillation after the crane had completed the second command at 6 seconds. The controller can also eliminate disturbances while the crane is moving, as seen in Fig. 16. While the crane was moving forward with little oscillation, the payload was disturbed at about 1 second. The controller initially gives a stop command at about 1.5 seconds, which ceases shortly afterward (before the oscillation phase angle matched the switch angle, not eliminated. The controller then waited until the appropriate phase angle, and then stopped the trolley at a time of about 6 seconds. At the appropriate time (7.5 seconds), the crane began accelerating but based on the time safety factor from off .) However, the payload was still being disturbed so the oscillations were, resulting in little oscillation when the crane had returned to full speed. V. CONCLUSION A control strategy has been developed for on-off motors to eliminate bridge crane payload oscillations. The control -1 -0.5 0 0.5 1 u s e r i n p u t ( % fu l l s p e e d ) user input control scaled motor speed -5 0 5 pay l o ad s w i n g ( o ) payload ang. oscillation amp. 0 1 2 3 4 5 6 7 8 9 10 -90 0 90 180 270 time (s) ph as e an gl e ( o ) oscillation ang. switch ang. Fig.14. Response to a disturbance while crane is at rest (a) us er i nput (% f u l l s p eed) (b) payl oad s w i ng ( ) (c) phas e angl e ( ) -1 -0.5 0 0.5 1 u s e r i n p u t ( % fu l l s p e e d ) user input control scaled motor speed -5 0 5 pay l o ad s w i n g ( o ) payload ang. oscillation level 0 1 2 3 4 5 6 7 8 9 10 -90 0 90 180 270 time (s) ph as e an gl e ( o ) oscillation ang. switch ang. Fig. 16. Response to a disturbance when moving (a) us er i nput (% f u l l s p eed) (b) payl oad s w i ng ( ) (c) phas e angl e ( ) 1788 uses the swing angle of the payload, and its derivative, to decide when to turn the crane on and off. The method can reduce the oscillations when the crane is moving or at rest. The strategy was implemented on a large bridge crane. Numerous experiments demonstrated the control systems effectiveness. VI. REFERENCES 1 Starr, G. P., “Swin