Journal Publications (click here for publications ordered by project, with student theses)

N. Neville, M. Buehler and I. Sharf, “Bipedal RHex running with two motors,” Int. J. Robotics Research, in preparation.

M. Ahmadi, H. Michalska, and M. Buehler, "Control and Stability Analysis of Limit Cycles in a Hopping Robot,” IEEE Trans. on Robotics, 23(3):553-563, June 2007.

F. Aghili, J. M. Hollerbach, and M. Buehler, "A Modular and High-Precision Motion Control System with an Integrated Motor," IEEE/ASME Trans. on Mechatronics, 12(3):317-329, June 2007.

G. Dudek, M. Jenkin, E. Milios, M. Buehler et al., “AQUA: An Amphibious Autonomous Robot,” Cover Feature, IEEE Computer, 40(1):46-53, Jan. 2007.

M. Ahmadi and M. Buehler, “Controlled Passive Dynamic Running Experiments with ARL Monopod II,” IEEE Transactions on Robotics, 22(5):974-986, Oct. 2006.

M. Armada and M. Buehler, Editorial for the Special Issue on the 2004 Conference on Climbing and Walking Robots, Int. J. Robotics Research, 25(9):821, Sept. 2006.

K. Iagnemma and M. Buehler, Editorial for the Special Issue on the DARPA Grand Challenge, J. Field Robotics, 23(8) and 23(9), Aug./Sept. 2006.

M. Buehler, “Summary of DGC 2005 Results,” J. Field Robotics, 23(8) and 23(9), Aug./Sept. 2006.

I. Poulakakis, E. G. Papadopoulos, and M. Buehler, “On the Stability of the Passive Dynamics of Quadrupedal Running with a Bounding Gait,” Int. J. Robotics Research, 25(7):669-687, July 2006.

C. Prahacs, A. Saunders, M. K. Smith, D. McMordie, and M. Buehler, “Towards Legged Amphibious Mobile Robotics,” J.  Engineering Design and Innovation, Volume 1P, 2005.

I. Poulakakis, J. A. Smith, and M. Buehler, “Modeling and Experiments of Untethered Quadrupedal Running with a Bounding Gait: The Scout II Robot,” Int. J. Robotics Research, 24(4):239-256, April 2005.

D. E. Koditschek, R. J. Full, and M. Buehler, “Mechanical Aspects of Legged Locomotion Control,” J. Arthropod Structure & Development, 33(3):251-272, July 2004.

F. Aghili, M. Buehler, and J. M. Hollerbach, "Experimental Characterization and Quadratic Programming-Based Control of Brushless-Motors," IEEE Trans. Control Systems Technology, 11(1):139-146, Jan. 2003.

M. Buehler, "Dynamic Locomotion and Energetics of RHex, a Six-Legged Robot," (Abstract), The Physiologist, 45(4):340, Aug. 2002.

F. Aghili, M. Buehler, and J. M. Hollerbach, "Development of a High Performance Direct-Drive Joint," Advanced Robotics, 16(3):233-250, 2002.

M. Buehler, "Dynamic Locomotion with One, Four and Six-Legged Robots," Invited Paper, Journal of the Robotics Society of Japan, 20(3):15-20, April 2002.

F. Aghili, M. Buehler, and J. M. Hollerbach, "Design of a Hollow Hexaform Torque Sensor for Robot Joints," Int. J. Robotics Research, 20(12):967-976, Dec. 2001.

R. Altendorfer, E. Z. Moore, H. Komsuoglu, M. Buehler, H. B. Brown Jr., D. McMordie, U. Saranli, R. Full, and D. E. Koditschek, "RHex: A Biologically Inspired Hexapod Runner," Autonomous Robots 11:207-213, 2001.

F. Aghili, M. Buehler, and J. M. Hollerbach, "Motion Control Systems with H-Infinity Positive Joint Torque Feedback," IEEE Trans. Control Systems Technology , 9(5):685 - 695, Sept. 2001.

U. Saranli, M. Buehler, and D. E. Koditschek, "RHex: A Simple and Highly Mobile Hexapod Robot," Int. J. Robotics Research, 20(7):616-631, July 2001.

F. Aghili, M. Buehler, and J. M. Hollerbach, "Optimal Commutation Laws in the Frequency Domain for PM Synchronous Direct-Drive Motors," IEEE Trans. Power Electronics, 15(6):1056-1064, Nov. 2000.

F. Aghili, M. Buehler, and J. M. Hollerbach, "Sensing the torque in a robot's joint," Feature Focus Article, Mechanical Engineering, 120(9):66-69, Sep. 1998.

P. Gregorio, M. Ahmadi, and M. Buehler, "Design, Control, and Energetics of an Electrically Actuated Legged Robot," IEEE Transactions on Systems, Man, and Cybernetics, 27B(4):626-634, Aug. 1997.

G. Mennitto and M. Buehler, " LADD Transmissions: Design, Manufacture, and New Compliance Models," ASME Journal of Mechanical Design, 119(2):197-203, June 1997.

M. Ahmadi and M. Buehler, "Stable Control of a Simulated One-Legged Running Robot with Hip and Leg Compliance," IEEE Transactions on Robotics and Automation, 13(1):96-104, Feb. 1997.

G. Mennitto and M. Buehler, "CARL: A Compliant Articulated Robotic Leg for Dynamic Locomotion," Robotics and Autonomous Systems, 18:337-344, 1996.

M. Buehler, D. E. Koditschek, and P. J. Kindlmann, "Planning and Control of Robotic Juggling and Catching Tasks," Int. J. Robotics Research, 13(2):101-118, April 1994.

D. E. Koditschek and M. Buehler, "Analysis of a Simplified Hopping Robot," Int. J. Robotics Research, 10(6):587-605, Dec 1991.

M. Buehler, D. E. Koditschek, and P. J. Kindlmann, "A Family of Robot Control Strategies for Intermittent Dynamical Environments," IEEE Control Systems Magazine, 10(2):16-22, Feb. 1990.

 

Book Chapters

I. Poulakakis, J.A. Smith, and M. Buehler, "On the Dynamics of Bounding and Extensions Towards the Half-Bound and the Gallop Gaits," In H. Kimura, K. Tsuchiya, A. Ishiguro, and H. Witte, editors, “Adaptive Motion of Animals and Machines,” p. 79-88, Springer-Verlag, 2006.

D. Campbell and M. Buehler, "Preliminary Bounding Experiments in a Dynamic Hexapod," In B. Siciliano and P. Dario, editors, “Experimental Robotics VIII,” p. 612-621, Springer-Verlag, 2003.

R. Altendorfer, U. Saranli, H. Komsuoglu, D.E. Koditschek, H.B. Brown Jr., M. Buehler , N. Moore, D. McMordie, and R. Full, " Evidence for Spring Loaded Inverted Pendulum Running in a Hexapod Robot," in D. Rus and S. Singh, editors, "Experimental Robotics VII," p. 291-302, Springer-Verlag, 2001.

S. Talebi, I. Poulakakis, E. Papadopoulos, and M. Buehler, "Quadruped Robot Running with a Bounding Gait," In D. Rus and S. Singh, editors, “Experimental Robotics VII,” p. 281-289, Springer-Verlag, 2001.

M. Ahmadi and M. Buehler, "Preliminary Experiments with an Actively Tuned Passive Dynamic Running Robot," In A. Casals and A.T. de Almeida, editors, "Experimental Robotics V," p. 249-260, Springer-Verlag, 1998.

P. Gregorio, M. Ahmadi, and M. Buehler, "Experiments with an Electrically Actuated Planar Hopping Robot," In T. Yoshikawa and F. Miyazaki, editors, “Experimental Robotics III,” p. 269-281, Springer-Verlag, 1994.

M. Buehler and D. E. Koditschek, "From Stable to Chaotic Juggling," In F. L. Lewis, M. W. Spong and C. T. Abdallah, editors, "Robot Control," p. 525-530, IEEE Press, 1993.

M. Buehler, D. E. Koditschek, and P. J. Kindlmann, "Planning and Control of Robotic Juggling Tasks," In H. Miura and S. Arimoto, editors, “Robotics Research,” p. 270-281, MIT Press, 1990.

M. Buehler, D. E. Koditschek, and P. J. Kindlmann, "A Simple Juggling Robot: Theory and Experimentation," In V. Hayward and O. Khatib, editors, “Experimental Robotics I,” p. 35-73, Springer-Verlag, 1990.

 

Edited Volumes

2005 DGC Book Cover M. Buehler, K. Iagnemma, and S. Singh, The 2005 DARPA Grand Challenge – The Great Robot Race, Springer Verlag, 2007.

K. Iagnemma and M. Buehler, Special Issue on the DARPA Grand Challenge, Part 2, Journal of Field Robotics, Volume 23, Issue 9, Sept. 2006.(APEX Grand Award 2007)

K. Iagnemma and M. Buehler, Special Issue on the DARPA Grand Challenge, Part 1, Journal of Field Robotics, Volume 23, Issue 8, Aug. 2006.(APEX Grand Award 2007)

M. Armada and M. Buehler, Special Issue on the 2004 Conference on Climbing and Walking Robots, The International Journal of Robotics Research, Volume 25, Issue 9, Sept. 2006.

 

Conference Papers

N. Neville, M. Buehler, and I. Sharf, “A Bipedal Running Robot with One Actuator per Leg,” IEEE Int. Conf. Robotics and Automation (ICRA), Orlando, FL, USA, May 2006.

R. Playter, M. Buehler, and M. Raibert, “Bigdog,” in Proc. SPIE Defense&Security Symposium, Unmanned Systems Technology, Orlando, FL, April 2006.

A. Saunders, D. I. Goldman, R. J. Full, and M. Buehler, “The RiSE climbing robot: Body and Leg Design,” Proc. SPIE Defense&Security Symposium, Unmanned Systems Technology, Orlando, FL, April 2006.

M. Buehler, F. Grimminger, D. Campbell, and M. Raibert, “Biologically Inspired Robots at Boston Dynamics,” bionic 2006 IndustrieKongress, Berlin, Germany, March 2006.

M. Buehler, R. Playter, and M. Raibert, “Robots Step Outside,” Int. Symp. Adaptive Motion of Animals and Machines (AMAM), Ilmenau, Germany, Sept 2005.

C. Georgiadis, M. Buehler, and M. Nahon, “Simulation of an Underwater Hexapod Robot,” Int. Symp. Unmanned Untethered Submersible Technology (UUST), Durham, NH, Aug. 2005.

K. Autumn, M. Buehler, et al, "Robotics in Scansorial Environments," Proc. SPIE Defense&Security Symposium, Orlando, FL, March 2005.

C. Georgiadis, A. German, A. Hogue, H. Liu, C. Prahacs, A. Ripsman, R. Sim, L.-A. Torres, P. Zhang, M. Buehler, G. Dudek, M. Jenkin, and E. Milios, "AQUA: An Aquatic Walking Robot," Proc. UUVS, Southampton, UK, 2004.

C. Georgiadis, A. German, A. Hogue, H. Liu, C. Prahacs, A. Ripsman, R. Sim, L.-A. Torres, P. Zhang, M. Buehler, G. Dudek, M. Jenkin, and E. Milios, "AQUA: An Aquatic Walking Robot," IEEE/RSJ Int. Conf. Intelligent Robots and Systems (IROS), Sendai, Japan, Sept. 2004.

A. Sato and M. Buehler, “A Planar Hopping Robot with One Actuator: Design, Simulation, and Experimental Results,” IEEE/RSJ Int. Conf. Intelligent Robots and Systems (IROS), Sendai, Japan, Sept/Oct. 2004.

M. Jenkin, G. Dudek, E. Milios, M. Buehler, and C. Prahacs, “AQUA: An amphibious walking and swimming robot,” RoboNexus, Santa Clara, CA, Oct. 2004.

D. E. Koditschek, R. J. Full, and M. Buehler, “A Principled Approach to Bioinspired Design of Legged Locomotion Systems,” Proc. SPIE Unmanned Ground Vehicle Technology VI, Orlando, FL, April 2004.

I. Poulakakis, J. A. Smith, and M. Buehler, "Experimentally Validated Bounding Models for Scout II Quadrupedal Robot," IEEE Int. Conf. Robotics and Automation (ICRA), New Orleans, LA, April 2004.

J. D. Weingarten, G. Lopes, R. E. Groff, M. Buehler, and D. E. Koditschek, “Automated gait adaptation for legged robots,” IEEE Int. Conf. Robotics and Automation (ICRA), New Orleans, LA, April 2004.

C. Prahacs, A. Saunders, M. K. Smith, D. McMordie, and M. Buehler, "Towards Legged Amphibious Mobile Robotics," Canadian Design Engineering Network (CDEN/RCCI) Design Conference, Montreal, Canada, July 2004.

N. Neville and M. Buehler, "Towards Bipedal Running of a Six Legged Robot," 12th Yale Workshop on Adaptive and Learning Systems, Yale University, New Haven, CT, May 2003.

D. McMordie, C. Prahacs, and M. Buehler, "Towards a Dynamic Actuator Model for a Hexapod Robot," IEEE Int. Conf. on Robotics and Automation (ICRA), Taipei, Taiwan, Sept. 2003.

I. Poulakakis, E. Papadopoulos, and M. Buehler, "On the Stable Passive Dynamics of Quadrupedal Running," IEEE Int. Conf. on Robotics and Automation (ICRA), Taipei, Taiwan, Sept. 2003.

D. Campbell and M. Buehler, "Stair Descent in the Simple Hexapod 'RHex'," IEEE Int. Conf. on Robotics and Automation (ICRA), Taipei, Taiwan, Sept. 2003.

C. Steeves, M. Buehler, and S. G. Penzes, "Dynamic Behaviors for a hybrid leg-wheel mobile platform," Proc. SPIE, Vol. 4715, Orlando, FL, April 2002.

E. Z. Moore, D. Campbell, F. Grimminger, and M. Buehler, "Reliable Stair Climbing in the Simple Hexapod 'RHex'," IEEE Int. Conf. Robotics and Automation (ICRA), Washington DC, USA, May 2002.

M. Buehler, "RePaC design and control: Cheap and fast autonomous runners", Int. Conf. Climbing and Walking Robots (CLAWAR), Karlsruhe, Germany, Sept. 2001.

D. McMordie and M. Buehler, "Towards Pronking with a Hexapod Robot", Int. Conf. Climbing and Walking Robots (CLAWAR), Karlsruhe, Germany, Sept. 2001.

E.Z. Moore and M. Buehler, "Stable Stair Climbing in a Simple Hexapod", Int. Conf. Climbing and Walking Robots (CLAWAR), Karlsruhe, Germany, Sept. 2001.

I. Poulakakis, J. Smith, E. Papadopoulos, and M. Buehler, "Opportunities for Adaptation and Learning in Dynamically Stable Legged Robots", 11. Yale Workshop on Adaptive and Learning Systems, Center for Systems Science, Yale University, New Haven, CT, June 2001.

C. A. Brosinsky, S. G. Penzes, M. Buehler, and C. Steeves, "Integrating intrinsic mobility into unmanned ground vehicle systems", the SPIE 15th Annual International Symposium on Aerospace/Defense Sensing, Simulation and Controls, Vol. 4364-06, Orlando, FL, April 2001.

M. de Lasa and M. Buehler, "Dynamic Compliant Quadruped Walking," IEEE Int. Conf. Robotics and Automation (ICRA), Seoul, Korea, May 2001.

F. Aghili, M. Buehler, and J. M. Hollerbach, "Quadratic Programming in Control of Brushless Motors," IEEE Int. Conf. Robotics and Automation (ICRA), Seoul, Korea, May 2001.

H. Komsuoglu, D. McMordie, U. Saranli, N. Moore, M. Buehler, and D. E. Koditschek, "Proprioception Based Behavioral Advances in Hexapod Robot," IEEE Int. Conf. Robotics and Automation (ICRA), Seoul, Korea, May 2001.

H. Komsuoglu, R. Altendorfer, R. Full, U. Saranli, B. Brown, D. McMordie, E. Moore, M. Buehler, and D. Koditschek, "Evidence for Spring Loaded Inverted Pendulum Running in a Hexapod Robot," Int. Symp. Experimental Robotics, Honolulu, HI, Dec. 2000.

S. Talebi, I. Poulakakis, E. Papadopoulos, and M. Buehler, "Quadruped Robot Running with a Bounding Gait," Int. Symp. Experimental Robotics, Honolulu, HI, Dec. 2000.

F. Aghili, M. Buehler, and J. M. Hollerbach, "Development of a High Performance Direct-Drive Joint," IEEE/RSJ Int. Conf. Intelligent Robots and Systems, (IROS), Takamatsu, Japan, Nov. 2000.

M. de Lasa and M. Buehler, "Dynamic Compliant Walking of a Quadruped Robot : Preliminary Experiments," 3rd Int. Conf. on Climbing and Walking Robots, Madrid, Spain, Oct. 2000.

S. Talebi, M. Buehler, and E. Papadopoulos, "Towards Dynamic Step Climbing for a Quadruped Robot with Compliant Legs," 3rd Int. Conf. on Climbing and Walking Robots, Madrid, Spain, Oct. 2000.

M. Buehler, U. Saranli, D. Papadopoulos, and D. E. Koditschek "Dynamic Locomotion with four and six-legged robots," Int. Symp. Adaptive Motion of Animals and Machines (AMAM), Montreal, Canada, Aug. 2000.

G. Hawker and M. Buehler, "Quadruped Trotting with Passive Knees - Design, Control, and Experiments," IEEE Int. Conf. Robotics and Automation (ICRA), p. 3046-3051, San Francisco, CA, April 2000.

D. Papadopoulos and M. Buehler, "Stable Running in a Quadruped Robot with Compliant Legs," IEEE Int. Conf. Robotics and Automation (ICRA), San Francisco, CA, April 2000.

U. Saranli, M. Buehler, and D. E. Koditschek, "Design, Modeling and Preliminary Control of a Compliant Hexapod Robot," IEEE Int. Conf. Robotics and Automation (ICRA), San Francisco, CA, April 2000.

M. Buehler, A. Cocosco, K. Yamazaki, and R. Battaglia, "Stable Open Loop Walking in Quadruped Robots with Stick Legs," IEEE Int. Conf. Robotics and Automation (ICRA), Detroit, MI, May 1999.

M. Ahmadi and M. Buehler, "The ARL Monopod II Running Robot: Control and Energetics," IEEE Int. Conf. Robotics and Automation (ICRA), Detroit, MI, May 1999.

F. Aghili, M. Buehler, and J. M. Hollerbach, "Torque Ripple Minimization in Direct-Drive Systems," IEEE/RSJ Int. Conf. Intelligent Robots and Systems (IROS), Victoria, BC, Canada, Oct. 1998.

M. Buehler, R. Battaglia, A. Cocosco, G. Hawker, J. Sarkis, and K. Yamazaki, " SCOUT: A Simple Quadruped that Walks, Climbs, and Runs," IEEE Int. Conf. Robotics and Automation (ICRA), Leuven, Belgium, May 1998.

F. Aghili, M. Buehler, and J. M. Hollerbach, " Disturbance Attenuation and Load Decoupling with H Infinity Positive Joint Torque Feedback," IEEE Int. Conf. Robotics and Automation (ICRA), Leuven, Belgium, May 1998.

F. Aghili, M. Buehler, and J. M. Hollerbach, "A New Joint Torque Sensor for Robots," ASME Int. Mech. Eng. Congress and Exhibition (IMECE), Dallas, Texas, Nov. 1997.

F. Aghili, M. Buehler, and J. M. Hollerbach, "Model Matching Solution for Optimal Positive Joint Torque Feedback," IEEE/RSJ Int. Conf. Intelligent Robots and Systems (IROS), Grenoble, France, Sept. 1997.

G. Petryk and M. Buehler, "Robust Estimation of Pre-Contact Object Trajectories," Fifth Symposium on Robot Control (SYROCO), Nantes, France, Sept. 1997.

F. Aghili, M. Buehler, and J. M. Hollerbach, "Optimal Commutation Laws for Torque Control of Synchronous Motors," American Control Conference (ACC), Albuquerque, NM, June 1997.

M. Ahmadi and M. Buehler, "Preliminary Experiments with an Actively Tuned Passive Dynamic Running Robot," Int. Symp. Experimental Robotics (ISER), Barcelona, Spain, June 1997.

F. Aghili, M. Buehler, and J. M. Hollerbach, "Dynamics and Control of Direct-Drive Robots with Positive Joint Torque Feedback," IEEE Int. Conf. Robotics and Automation (ICRA), Albuquerque, NM, April 1997.

G. Petryk and M. Buehler, "Dynamic Object Localization via a Proximity Sensor Network," IEEE/SICE/RSJ Int. Conf. Multisensor Fusion and Integration for Intelligent Systems, Washington, DC, Dec. 1996.

F. Aghili, M. Buehler, and J. M. Hollerbach, "A New Indirect Adaptive Control Strategy for a Synchronous Direct Drive Motor," IEEE Int. Conf. Robotics and Automation (ICRA), Minneapolis, MN, April 1996.

H. Michalska, M. Ahmadi, and M. Buehler, "Vertical Motion Control of a Hopping Robot," IEEE Int. Conf. Robotics and Automation (ICRA), Minneapolis, MN, April 1996.

M. Ahmadi and M. Buehler, "A Control Strategy for Stable Passive Running," IEEE Int. Conf. Intelligent Robots and Systems (IROS), Pittsburgh, PA, Aug. 1995.

G. Mennitto and M. Buehler, "Experimental Validation of Compliance Models for LADD Transmission Kinematics," IEEE Int. Conf. Intelligent Robots and Systems (IROS), Pittsburgh, PA, Aug. 1995.

G. Mennitto, P. Gregoire, and M. Buehler, "CARL: A Compliant Articulated Robot Leg for Dynamic Locomotion," Int. Conf. Intelligent Autonomous Systems (IAS4), Karlsruhe, Germany, March 1995.

M. Zhang, J. Damianakis, and M. Buehler, "Sensing and Control for Autonomous Grasping in Dynamic Environments," 1. World Congress on Intelligent Manufacturing Processes and Systems, Mayagüez, Puerto Rico, Feb. 1995.

M. Zhang and M. Buehler, "Sensor-Based Online Trajectory Generation for Smoothly Grasping Moving Objects," IEEE Int. Symp. Intelligent Control, Columbus, OH, Aug. 1994.

J. M. Hollerbach, L. Guigovaz, M. Buehler, and Y. Xu, "Screw Axis Measurement for Kinematic Calibration of the Sarcos Dextrous Arm," IEEE/RSJ Int. Conf. Intelligent Robots and Systems (IROS), Yokohama, Japan, July 1993.

M. Moghaddam and M. Buehler, "Control of Virtual Motion Systems," IEEE/RSJ Int. Conf. Intelligent Robots and Systems (IROS), Yokohama, Japan, July 1993.

H. Rad, P. Gregorio, and M. Buehler, "Design, Modeling and Control of a Hopping Robot," IEEE/RSJ Int. Conf. Intelligent Robots and Systems (IROS), Yokohama, Japan, July 1993.

M. Buehler and D. E. Koditschek, "From Stable to Chaotic Juggling: Theory, Simulation, and Experiments," IEEE Int. Conf. Robotics and Automation (ICRA), Cincinnati, OH, May 1990.

M. Buehler, L. Whitcomb, F. Levin, and D. E. Koditschek, "A Distributed Message Passing Computational and I/O Engine for Real-Time Motion Control," American Control Conf. (ACC), Pittsburgh, PA, June 1989.

M. Buehler, D. E. Koditschek, and P. J. Kindlmann, "A Family of Robot Control Strategies for Intermittent Dynamical Environments," IEEE Int. Conf. Robotics and Automation (ICRA), Scottsdale, AZ, May 1989.

M. Buehler, N. Vlamis, C. J. Taylor, and A. Ganz, "The Cyclops Vision System," North American Transputer Users Group, Salt Lake City, UT, April 1989.

M. Buehler, L. L. Whitcomb, F. Levin, and D. E. Koditschek, "A New Distributed Real-Time Controller for Robotics Applications," IEEE COMPCON Conf., San Francisco, CA, Feb/March 1989.

M. Buehler, D. E. Koditschek, and P. J. Kindlmann, "A One Degree of Freedom Juggler in a Two Degree of Freedom Environment," IEEE Conf. Intelligent Robots and Systems (IROS), Tokyo, Japan, Oct. 1988.

L. L. Whitcomb, M. Buehler, and D. E. Koditschek, "Preliminary Experiments in Real-Time Distributed Motion Control," North American Transputer Users Group (NATUG), NY, Oct. 1988.

D. E. Koditschek and M. Buehler, "A Distributed Control System for a Juggling Robot," 2. Annual Workshop on Parallel Computing, Portland, OR, April 1988.

M. Buehler and D. E. Koditschek, "Analysis of a Simplified Hopping Robot," IEEE Int. Conf. Robotics and Automation (ICRA), Philadelphia, PA, April 1988.