Yayınlar & Eserler

Yayın Ağı
Makaleler 29
Tümü (29)
SCI-E, SSCI, AHCI (28)
SCI-E, SSCI, AHCI, ESCI (28)
Scopus (26)

1. From Walking to Running: A Comprehensive Clock Torque Actuated SLIP Framework for Trotting in Quadrupedal Locomotion

Akbaba M., SARANLI U., TURGUT A. E., ANKARALI M. M.
JOURNAL OF BIONIC ENGINEERING , 2025 (SCI-Expanded, Scopus) identifier identifier

2. Design and verification of a parallel elastic robotic leg

TANFENER E., KARAGÖZ O. K., CANDAN S. Ş., TURGUT A. E., YAZICIOĞLU Y., ANKARALI M. M., et al.
Bioinspiration and Biomimetics , cilt.19, sa.2, 2024 (SCI-Expanded, Scopus) identifier identifier identifier

3. Analysis and control of a running spring-mass model with a trunk based on virtual pendulum concept

Karagoz O. K., Secer G., Ankarali M. M., Saranli U.
Bioinspiration and Biomimetics , cilt.17, sa.4, 2022 (SCI-Expanded) identifier identifier identifier

4. Efficient bipedal locomotion on rough terrain via compliant ankle actuation with energy regulation

Kerimoglu D., Karkoub M., Ismail U., MORGÜL Ö., SARANLI U.
BIOINSPIRATION & BIOMIMETICS , cilt.16, sa.5, 2021 (SCI-Expanded, Scopus) Sürdürülebilir Kalkınma identifier identifier identifier

5. The Effects of Clinical and Home-based Physiotherapy Programs in Secondary Head and Neck Lymphedema

Ozdemir K., KESER İ., DÜZLÜ M., ERPOLAT Ö. P., SARANLI U., TUTAR H.
LARYNGOSCOPE , cilt.131, sa.5, 2021 (SCI-Expanded, Scopus) identifier identifier identifier

6. Robotic Task Planning Using a Backchaining Theorem Prover for Multiplicative Exponential First-Order Linear Logic

Kortik S., SARANLI U.
JOURNAL OF INTELLIGENT & ROBOTIC SYSTEMS , cilt.96, sa.2, ss.179-191, 2019 (SCI-Expanded, Scopus) identifier identifier

7. Frequency-Domain Subspace Identification of Linear Time-Periodic (LTP) Systems

Uyanık İ., Saranlı U., Ankaralı M. M., Cowan N., Morgül Ö.
IEEE TRANSACTIONS ON AUTOMATIC CONTROL , cilt.64, ss.2529-2536, 2019 (SCI-Expanded, Scopus) Creative Commons License identifier identifier

8. Control of Planar Spring-Mass Running Through Virtual Tuning of Radial Leg Damping

Secer G., SARANLI U.
IEEE TRANSACTIONS ON ROBOTICS , cilt.34, sa.5, ss.1370-1383, 2018 (SCI-Expanded, Scopus) Sürdürülebilir Kalkınma identifier identifier

9. Estimation of Ground Reaction Forces Using Low-Cost Instrumented Forearm Crutches

Seylan C., SARANLI U.
IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT , cilt.67, sa.6, ss.1308-1316, 2018 (SCI-Expanded, Scopus) identifier identifier

10. LinGraph: a graph-based automated planner for concurrent task planning based on linear logic

Kortik S., SARANLI U.
APPLIED INTELLIGENCE , cilt.47, sa.3, ss.914-934, 2017 (SCI-Expanded, Scopus) Creative Commons License identifier identifier

11. Stability and control of planar compass gait walking with series-elastic ankle actuation

KERIMOGLU D., MORGUL O., SARANLI U.
TRANSACTIONS OF THE INSTITUTE OF MEASUREMENT AND CONTROL , cilt.39, sa.3, ss.312-323, 2017 (SCI-Expanded, Scopus) Creative Commons License identifier identifier

12. Approximate analytical solutions to the double-stance dynamics of the lossy spring-loaded inverted pendulum

SHAHBAZI M., SARANLI U., BABUSKA R., LOPES G. A. D.
BIOINSPIRATION & BIOMIMETICS , cilt.12, sa.1, 2017 (SCI-Expanded, Scopus) Creative Commons License identifier identifier identifier

13. Identification of a vertical hopping robot model via harmonic transfer functions

Uyanik I., Ankarali M. M., Cowan N. J., SARANLI U., Morgul O.
TRANSACTIONS OF THE INSTITUTE OF MEASUREMENT AND CONTROL , cilt.38, sa.5, ss.501-511, 2016 (SCI-Expanded, Scopus) Creative Commons License identifier identifier

14. Experimental Validation of a Feed-Forward Predictor for the Spring-Loaded Inverted Pendulum Template

Uyanik I., Morgul O., SARANLI U.
IEEE TRANSACTIONS ON ROBOTICS , cilt.31, sa.1, ss.208-216, 2015 (SCI-Expanded, Scopus) Creative Commons License identifier identifier

15. Optimal control of a half-circular compliant legged monopod

AYDIN Y. O., SARANLI A., YAZICIOĞLU Y., SARANLI U., LEBLEBICIOGLU K.
CONTROL ENGINEERING PRACTICE , cilt.33, ss.10-21, 2014 (SCI-Expanded, Scopus) identifier identifier

16. Reactive Planning and Control of Planar Spring-Mass Running on Rough Terrain

Arslan O., Saranli U.
IEEE TRANSACTIONS ON ROBOTICS , cilt.28, sa.3, ss.567-579, 2012 (SCI-Expanded, Scopus) Creative Commons License identifier identifier

17. Model-Based Proprioceptive State Estimation for Spring-Mass Running

Gur O., Saranli U.
ROBOTICS: SCIENCE AND SYSTEMS VII , ss.105-112, 2012 (SCI-Expanded) identifier

18. A Modular Real-Time Fieldbus Architecture for Mobile Robotic Platforms

Saranli U., Avci A., Oeztuerk M. C.
IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT , cilt.60, sa.3, ss.916-927, 2011 (SCI-Expanded, Scopus) Creative Commons License identifier identifier

19. Control of underactuated planar pronking through an embedded spring-mass Hopper template

Ankarali M. M., Saranli U.
AUTONOMOUS ROBOTS , cilt.30, ss.217-231, 2011 (SCI-Expanded, Scopus) Creative Commons License identifier identifier

20. Analysis and Control of a Dissipative Spring-Mass Hopper with Torque Actuation

ANKARALI M. M., Saranli U.
ROBOTICS: SCIENCE AND SYSTEMS VI , ss.41-48, 2011 (SCI-Expanded) identifier

21. Approximate analytic solutions to non-symmetric stance trajectories of the passive Spring-Loaded Inverted Pendulum with damping

Saranli U., Arslan O., Ankarali M. M., Morgul O.
NONLINEAR DYNAMICS , cilt.62, ss.729-742, 2010 (SCI-Expanded, Scopus) Creative Commons License identifier identifier

22. Stride-to-stride energy regulation for robust self-stability of a torque-actuated dissipative spring-mass hopper

Ankarali M. M., Saranli U.
CHAOS , cilt.20, 2010 (SCI-Expanded, Scopus) Creative Commons License identifier identifier identifier

23. Solving models of controlled dynamic planar rigid-body systems with frictional contact

GREENFIELD A., Saranli U., RIZZI A.
INTERNATIONAL JOURNAL OF ROBOTICS RESEARCH , cilt.24, sa.11, ss.911-931, 2005 (SCI-Expanded, Scopus) Creative Commons License identifier identifier

24. Multi-point contact models for dynamic self-righting of a Hexapod

Saranli U., Rizzi A. A., Koditschek D. E.
Springer Tracts in Advanced Robotics , cilt.17, ss.409-424, 2005 (SCI-Expanded, Scopus) identifier

25. Model-based dynamic self-righting maneuvers for a hexapedal robot

Saranli U., RIZZI A., KODITSCHEK D.
INTERNATIONAL JOURNAL OF ROBOTICS RESEARCH , cilt.23, sa.9, ss.903-918, 2004 (SCI-Expanded, Scopus) Creative Commons License identifier identifier

26. RHex: A biologically inspired hexapod runner

ALTENDORFER R., MOORE N., Komsuolu H., BUEHLER M., BROWN H., MCMORDIE D., et al.
AUTONOMOUS ROBOTS , cilt.11, sa.3, ss.207-213, 2001 (SCI-Expanded, Scopus) Creative Commons License identifier identifier

27. RHex: A simple and highly mobile hexapod robot

Saranli U., Buehler M., Kodıtschek D.
INTERNATIONAL JOURNAL OF ROBOTICS RESEARCH , cilt.20, ss.616-631, 2001 (SCI-Expanded, Scopus) Creative Commons License identifier identifier

28. Evidence for spring loaded inverted pendulum running in a hexapod robot

ALTENDORFER R., Saranli U., KOMSUOGLU H., KODITSCHEK D., BROWN H., BUEHLER M., et al.
EXPERIMENTAL ROBOTICS VII , cilt.271, ss.291-302, 2001 (SCI-Expanded) identifier

29. Design, modeling and preliminary control of a compliant hexapod robot

Saranli U., Buehler M., Koditschek D. E.
Proceedings-IEEE International Conference on Robotics and Automation , cilt.3, ss.2589-2596, 2000 (Scopus) identifier
Hakemli Bilimsel Toplantılarda Yayımlanmış Bildiriler 43

1. Contact Consistent Disturbance Estimation for Quadruped Robots

Ozden H. C., Tanfener E., Orhon H. E., Banus O. T., Saranlı U., Ankaralı M. M., et al.
IEEE 20th International Conference on Automation Science and Engineering (CASE), Bari, İtalya, 28 Ağustos - 01 Eylül 2024, ss.1081-1087, (Tam Metin Bildiri) identifier identifier

2. Swarm Robotics and Archaeology: A Concepts Paper

Kalayci T., TURGUT A. E., Branting S., SARANLI U., Cuneyitoglu M.
15th European Robotics Forum, Rimini, İtalya, 13 - 15 Mart 2024, cilt.33, ss.357-361, (Tam Metin Bildiri) Sürdürülebilir Kalkınma identifier identifier

3. Characterization of Fixed Points of Spring-Mass Model with a Body Govde Eklenmis Yay-Kutle Modelinin Sabit Noktalarinin Karakterizasyonu

Karagoz O. K., Sever I., Dilsad Er G., SARANLI U., ANKARALI M. M.
28th Signal Processing and Communications Applications Conference, SIU 2020, Gaziantep, Türkiye, 5 - 07 Ekim 2020, (Tam Metin Bildiri) identifier

4. Modelling, control and design of a clutched parallel elastically actuated articulated robotic leg through virtual tunable damping

TANFENER E., CANDAN S. Ş., TURGUT A. E., SARANLI U.
ASME 2020 International Mechanical Engineering Congress and Exposition, IMECE 2020, Virtual, Online, 16 - 19 Kasım 2020, cilt.7A-2020, (Tam Metin Bildiri) identifier

5. Characterization of Fixed Points of Spring-Mass Model With a Body

Karagoz O. K., Sever I., Er G. D., SARANLI U., ANKARALI M. M.
28th Signal Processing and Communications Applications Conference (SIU), ELECTR NETWORK, 5 - 07 Ekim 2020, (Tam Metin Bildiri) identifier identifier

6. Analysis and Control of a Body-Attached Spring-Mass Runner Based on Central Pivot Point Approach

Karagoz O. K., Sever I., Saranlı U., Ankaralı M. M.
IEEE/ASME International Conference on Advanced Intelligent Mechatronics (AIM), ELECTR NETWORK, 6 - 09 Temmuz 2020, ss.495-500, (Tam Metin Bildiri) Creative Commons License identifier identifier

7. Deadbeat Control of Running with the ATRIAS Biped Based on Spring-Mass Model with Trunk and Tunable Leg Damping

SEÇER G., SARANLI U.
Dynamic Walking, Pensacola, Fl, Amerika Birleşik Devletleri, 21 - 24 Mayıs 2018, ss.1-5, (Tam Metin Bildiri)

8. Energy efficient control of a 1D hopper through tunable damping

SEÇER G., SARANLI U.
Dynamic Walking, Michigan, Amerika Birleşik Devletleri, 1 - 05 Haziran 2017, ss.1-5, (Tam Metin Bildiri)

9. Energy efficient control of planar monopedal running through tunable damping

Seçer G., SARANLI U.
Dynamic Walking Conference, 4 - 07 Temmuz 2016, (Tam Metin Bildiri)

10. Parametric Identification of Hybrid Linear-Time-Periodic Systems

Uyanik I., SARANLI U., Morgul O., Aukarah M. M.
6th IFAC Symposium on System Structure and Control (SSSC), İstanbul, Türkiye, 22 - 24 Haziran 2016, cilt.49, ss.7-12, (Tam Metin Bildiri) identifier identifier

11. Koltuk Değneklerinden Basınç ve İvme Ölçümleri ile Yeryüzü Tepki Kuvveti Tahmini

SEYLAN Ç., SARANLI U.
Türkiye Robotbilim Konferansı, Türkiye, 26 - 27 Ekim 2015, (Özet Bildiri)

12. Extending The Lossy Spring-Loaded Inverted Pendulum Model with a Slider-Crank Mechanism

Orhon H. E., Odabas C., Uyanik I., Morgul O., SARANLI U.
International Conference on Advanced Robotics (ICAR), İstanbul, Türkiye, 27 - 31 Temmuz 2015, ss.99-104, (Tam Metin Bildiri) Creative Commons License identifier identifier

13. Toward Data-Driven Models of Legged Locomotion using Harmonic Transfer Functions

Uyanik I., Ankarali M. M., Cowan N. J., Morgul O., SARANLI U.
International Conference on Advanced Robotics (ICAR), İstanbul, Türkiye, 27 - 31 Temmuz 2015, ss.357-362, (Tam Metin Bildiri) Creative Commons License identifier identifier

14. Stability of a Compass Gait Walking Model with Series Elastic Ankle Actuation

Kerimoglu D., Morgul O., SARANLI U.
International Conference on Advanced Robotics (ICAR), İstanbul, Türkiye, 27 - 31 Temmuz 2015, ss.351-356, (Tam Metin Bildiri) identifier identifier

15. Path Following with An Underactuated Self-Balancing Spherical-Wheel Mobile Robot

Inal A. N., Morgul O., SARANLI U.
International Conference on Advanced Robotics (ICAR), İstanbul, Türkiye, 27 - 31 Temmuz 2015, ss.194-199, (Tam Metin Bildiri) identifier identifier

16. Stability of Planar Compass Gait Walking with Series Elastic Ankle Actuation

Kerimoğlu D., MORGÜL Ö., SARANLI U.
Dynamic Walking Conference, 20 - 24 Temmuz 2015, (Özet Bildiri)

17. Identifying Stability Properties of a Hybrid Spring–Mass–Damper via Piecewise LTI Approximation and Harmonic Transfer Functions

Uyanık İ., Ankaralı M. M., Cowan N., Morgül Ö., Saranlı U.
Dynamic Walking 2015, Zürich, İsviçre, 21 - 24 Temmuz 2015, (Özet Bildiri) Creative Commons License

18. Independent estimation of input and measurement delays for a hybrid vertical spring-mass-damper via harmonic transfer functions

Uyamk I., Ankarali M. M., Cowan N. J., Saranli U., Morgül Ö., Özbay H.
12th IFAC Workshop on Time Delay Systems, TDS 2015, Michigan, Amerika Birleşik Devletleri, 28 - 30 Haziran 2015, cilt.28, ss.298-303, (Tam Metin Bildiri) identifier

19. System Identification of Legged Locomotion via Harmonic Transfer Functions and Piecewise LTI Approximation

Uyanık İ., Ankaralı M. M., Cowan N., Morgül Ö., Saranlı U.
Dynamic Walking 2014, Zürich, İsviçre, 10 - 13 Haziran 2014, (Özet Bildiri) Creative Commons License

20. Linear Planning Logic: An Efficient Language and Theorem Prover for Robotic Task Planning

Kortik S., SARANLI U.
IEEE International Conference on Robotics and Automation (ICRA), Hong Kong, PEOPLES R CHINA, 31 Mayıs - 07 Haziran 2014, ss.3764-3770, (Tam Metin Bildiri) Creative Commons License identifier identifier

21. A Real-Time Inertial Motion Blur Metric: Application to Frame Triggering Based Motion Blur Minimization

Mutlu M., SARANLI A., SARANLI U.
IEEE International Conference on Robotics and Automation (ICRA), Hong Kong, PEOPLES R CHINA, 31 Mayıs - 07 Haziran 2014, ss.671-676, (Tam Metin Bildiri) Creative Commons License Sürdürülebilir Kalkınma identifier identifier

22. Control of Hopping Through Active Virtual Tuning of Leg Damping for Serially Actuated Legged Robots

SEÇER G., SARANLI U.
IEEE International Conference on Robotics and Automation (ICRA), Hong Kong, PEOPLES R CHINA, 31 Mayıs - 07 Haziran 2014, ss.4556-4561, (Tam Metin Bildiri) identifier identifier

23. A Real-Time Inertial Motion Blur Metric

Mutlu M., SARANLI A., SARANLI U.
22nd IEEE Signal Processing and Communications Applications Conference (SIU), Trabzon, Türkiye, 23 - 25 Nisan 2014, ss.2225-2228, (Tam Metin Bildiri) Creative Commons License Sürdürülebilir Kalkınma identifier identifier

24. Control of Monopedal Running Through Tunable Damping

SEÇER G., SARANLI U.
21st Signal Processing and Communications Applications Conference (SIU), CYPRUS, 24 - 26 Nisan 2013, (Tam Metin Bildiri) identifier identifier

25. A 3D Dynamic Model of a Spherical Wheeled Self-Balancing Robot

Inal A. N., Morgul O., SARANLI U.
25th IEEE\RSJ International Conference on Intelligent Robots and Systems (IROS), Algarve, Portekiz, 7 - 12 Ekim 2012, ss.5381-5386, (Tam Metin Bildiri) Creative Commons License identifier identifier

26. A DYNAMIC MODEL OF RUNNING WITH A HALF-CIRCULAR COMPLIANT LEG

Ankarali M. M., SAYGINER E., YAZICIOĞLU Y., SARANLI A., SARANLI U.
15th International Conference on Climbing and Walking Robots and the Support Technologies for Mobile Machines, Maryland, Amerika Birleşik Devletleri, 23 - 26 Temmuz 2012, ss.425-432, (Tam Metin Bildiri) identifier identifier

27. TD-SLIP: A better predictive model for human running

Ankaralı M. M., Cowan N., Saranlı U.
Dynamic Walking 2012, Florida, Amerika Birleşik Devletleri, 21 - 24 Mayıs 2012, (Özet Bildiri) Creative Commons License

28. Model-based proprioceptive state estimation for spring-mass running

Gür Ö., Saranli U.
International Conference on Robotics Science and Systems, RSS 2011, California, Amerika Birleşik Devletleri, 27 Haziran - 01 Temmuz 2011, cilt.7, ss.105-112, (Tam Metin Bildiri) identifier

29. Quadrupedal Bounding with an Actuated Spinal Joint

Culha U., Saranli U.
IEEE International Conference on Robotics and Automation (ICRA), Shanghai, Çin, 9 - 13 Mayıs 2011, ss.1392-1397, (Tam Metin Bildiri) Creative Commons License identifier identifier

30. Adaptive Control of a Spring-Mass Hopper

Uyanik I., Saranli U., Morgul O.
IEEE International Conference on Robotics and Automation (ICRA), Shanghai, Çin, 9 - 13 Mayıs 2011, ss.2138-2143, (Tam Metin Bildiri) Creative Commons License identifier identifier

31. Analysis And Control Of A Dissipative Spring-mass Hopper With Torque Actuation

ANKARALI M. M., SARANLI U.
Robotics: Science And Systems, Zaragoza, İspanya, 27 - 30 Haziran 2010, ss.41-48, (Tam Metin Bildiri) identifier

32. Control of Underactuated Planar Hexapedal Pronking Through a Dynamically Embedded SLIP Monopod

Ankaralı M. M., Saranli U., SARANLI A.
IEEE International Conference on Robotics and Automation (ICRA), Alaska, Amerika Birleşik Devletleri, 3 - 08 Mayıs 2010, ss.4721-4727, (Tam Metin Bildiri) Creative Commons License identifier identifier

33. Reactive Footstep Planning for a Planar Spring Mass Hopper

Arslan O., Saranli U., Morgul O.
IEEE RSJ International Conference on Intelligent Robots and Systems, Missouri, Amerika Birleşik Devletleri, 10 - 15 Ekim 2009, ss.160-166, (Tam Metin Bildiri) Creative Commons License identifier identifier

34. An Analytical Solution to the Stance Dynamics of Passive Spring-Loaded Inverted Pendulum with Damping

ANKARALI M. M., Arslan O., Saranli U.
12th International Conference on Climbing and Walking Robots the Support Technologies for Mobile Machines (CLAWAR), İstanbul, Türkiye, 9 - 11 Eylül 2009, ss.693-700, (Tam Metin Bildiri) Creative Commons License identifier

35. An Approximate Stance Map of The Spring Mass Hopper with Gravity Correction For Nonsymmetric Locomotions

Arslan O., Saranli U., Morgul O.
IEEE International Conference on Robotics and Automation, Kobe, Japonya, 12 - 17 Mayıs 2009, ss.1829-1830, (Tam Metin Bildiri) Creative Commons License identifier identifier

36. Using constrained intuitionistic linear logic for hybrid robotic planning problems

Saranli U., Pfenning F.
IEEE International Conference on Robotics and Automation, Rome, İtalya, 10 - 14 Nisan 2007, ss.3705-3706, (Tam Metin Bildiri) Creative Commons License identifier identifier

37. Robotics in scansorial environments

AUTUMN K., BUEHLER M., CUTKOSKY M., FEARING R., FULL R., GOLDMAN D., et al.
Conference on Unmanned Ground Vehicle Technology VII, Florida, Amerika Birleşik Devletleri, 29 - 31 Mart 2005, cilt.5804, ss.291-302, (Tam Metin Bildiri) identifier identifier identifier

38. Multi-point contact models for dynamic self-righting of a hexapod

Saranli U., RIZZI A., KODITSCHEK D.
6th International Workshop on Algorithmic-Foundations-of-Robotics, Zeist, Hollanda, 11 - 13 Temmuz 2004, cilt.17, ss.409-424, (Tam Metin Bildiri) identifier

39. Template based control of hexapedal running

Saranli U., KODITSCHEK D.
20th IEEE International Conference on Robotics and Automation (ICRA), Taipei, Tayvan, 14 - 19 Eylül 2003, ss.1374-1379, (Tam Metin Bildiri) identifier identifier

40. Back flips with a hexapedal robot

Saranli U., KODITSCHEK D.
19th IEEE International Conference on Robotics and Automation (ICRA), Washington, Kiribati, 11 - 15 Mayıs 2002, ss.2209-2215, (Tam Metin Bildiri) identifier identifier

41. Evidence for Spring Loaded Inverted Pendulum Running in a Hexapod Robot

Altendorfer R., SARANLI U., Komsuoğlu H., Koditschek D. E., Brown H. B., Buehler M., et al.
International Symposium on Experimental Robotics, 11 - 13 Aralık 2000, (Tam Metin Bildiri)

42. Proprioception based behavioral advances in a hexapod robot

Komsuoglu H., McMordie D., Saranli S., Moore N., Buehler M., Koditschek D.
IEEE International Conference on Robotics and Automation, Seoul, Güney Kore, 21 - 26 Mayıs 2001, ss.3650-3655, (Tam Metin Bildiri) identifier

43. Toward the control of a multi-jointed, monoped runner

Saranli U., SCHWIND W., KODITSCHEK D.
IEEE International Conference on Robotics and Automation, Leuven, Belçika, 16 - 20 Mayıs 1998, ss.2676-2682, (Tam Metin Bildiri) Creative Commons License identifier identifier
Kitaplar 1

1. Analysis and control of a dissipative spring-mass hopper with torque actuation

Ankaralı M. M., Saranlı U.
Robotics: Science and Systems VI, Yoky Matsuoka,Hugh Durrant-Whyte,José Neira, Editör, The MIT Press, Cambridge (MA), USA , Massachusetts, ss.41-48, 2011
Metrikler

Yayın

75

Yayın (WoS)

57

Yayın (Scopus)

57

Atıf (WoS)

1927

H-İndeks (WoS)

16

Atıf (Scopus)

2546

H-İndeks (Scopus)

18

Atıf (Scholar)

419

H-İndeks (Scholar)

9

Proje

15

Fikri Mülkiyet

1

Tez Danışmanlığı

29

Açık Erişim

36
BM Sürdürülebilir Kalkınma Amaçları