SIGNIFICANT REFERENCES

Mining applications

 

1.      Commissioning of excavator SChRs800 after flooding in 2014.
 Open pit mine Drmno, Kostolac.

Complete electrical equipment has been replaced. Project completed in 2006. and adjusted to new conditions. Commissioning in November 2014. PLC AC800F, ABB, distributed modules on PROFIBUS network, 7 nodes, optical and wire network with total length of 250m. Completely new 6kV facility has been designed with multifunctional SIPROTEC protection relay, type 7SJ631, on PROFIBUS network. Three operator panels, including one graphical, two absolute encoders on PROFIBUS network, two (DCS800 - ABB) thyristor rectifiers with external units for excitation for slewing drive on PROFIBUS network. Programmed excavator operation in horizontal and vertical cut. New system for longitudinally and transversely measurement of excavator incline.

2.      Electrical project, design of new control software and commissioning of new belt conveyor station B2000 with frequency controlled drives

Belt conveyor station B2000 4x1000kW with controlled drives and transport on open pit mine (OPM)  Drmno. Distributed modules on PROFIBUS network, 6 nodes, optical communication, operator panels. Measuring of the belt tension force, automatic slippage prevention, controlled starting, load equalization. Frequency converter ACS800 and PLC AC800, ABB. Etherenet communication on the machine and integration into central system for control and supervision. Contractor Goša FOM Smederevska Palanka and GAT Novi Sad, 2014. 

3.      Electrical project, design of new control software and commissioning of new belt conveyor station B2000 with frequency controlled drives

Belt conveyor station B2000 4x1000kW with controlled drives and transport on open pit mine (OPM)  Drmno. Distributed modules on PROFIBUS network, 6 nodes, optical communication, operator panels. Measuring of the belt tension force, automatic slippage prevention, controlled starting, load equalization. Frequency converter ACS800 and PLC AC800, ABB. Etherenet communication on the machine and integration into central system for control and supervision. Contractor Goša FOM Smederevska Palanka, 2013.

4.      Reconstruction of the electrical part of the spreader ARs 7200/2,
Open pit mine Drmno, Kostolac.

Supervision and commissioning during the reconstruction in 2012. Two PLC AC800F - ABB, two SCADAs DigiVis - ABB, distributed I/O modules on PROFIBUS network, optical and wire network 1000 m long. Two graphical panels, three absolute encoders on PROFIBUS network, two (DCS800 - ABB) thyristor rectifiers for slewing drive on PROFIBUS network. Semiautomatic spreader operation. Project completed in 2011.

5.      Reconstruction of electrical part of excavator SRs 2000/2,
Open pit mine Drmno, Kostolac.

Supervision and commissioning during the reconstruction in 2011. Two PLC AC800F - ABB, three SCADAs DigiVis - ABB, distributed I/O modules on PROFIBUS network, optical and wire network 1200 m long. Two graphical panels, five absolute encoders on PROFIBUS network,one incremental encoder, three (DCS800 - ABB) thyristor rectifiers for slewing drive of upper part of the excavator, six on the transport drive and two on the hoist drive of the boom, all on PROFIBUS network. Programmed excavator operation. Project completed in 2010.

6.      Electrical project, design of new control software and commissioning of new belt conveyor station B2000 with frequency controlled drives

Two belt conveyor stations B2000 4x1000kW with controlled drives and transport on open pit mine (OPM)  Drmno. Distributed modules on PROFIBUS network, 6 nodes, optical communication, operator panels. Measuring of the belt tension force, automatic slippage prevention, controlled starting, load equalization. Frequency converter ACS800 and PLC AC800, ABB. Etherenet communication on the machine and integration into central system for control and supervision. Cooperation between Goša FOM Smederevska Palanka and Faculty of Electrical Engineering, Belgrade, 2010.

7.      Reconstruction of the electrical part of the spreader ARs 3500,
Open pit mine Drmno, Kostolac.

Supervision and commissioning during the reconstruction in 2009. PLC AC800F - ABB, distributed I/O modules on PROFIBUS network, optical and wire network 450 m long. Two graphical panels and two alphanumerical panels, three absolute encoders on PROFIBUS network, two (ACS800 - ABB) frequency converters of the slewing drive of the discharge part on PROFIBUS network. Project completed in 2008.

8.      Project and commissioning of spreader ARs 2000 on the OPM „Drmno” - Electrical part

Project, software, commissioning. Distributed modules aon PROFIBUS network, 12 nodes, optical communication, operator panel. Frequency converter ACS800 and PLC AC800, ABB. Etherenet communication on the machine and integration into central system for control and supervision. Cooperation between  TKF Germany,  EXOR ESI Belgrade and Faculty of Electrical Engineering, Belgrade, 2007.-2008.

9.      Project of new electrical equipment and control software for belt conveyor with frequency controlled drives

Five belt conveyors B2000 4x1000kW with controlled drives and transport on open pit mine (OPM)  Drmno. Distributed modules on PROFIBUS network, 6 nodes, optical communication, operator panel. Measuring of the belt tension force, automatic slippage prevention, controlled starting, load equalization. Frequency converter ACS800 and PLC AC800, ABB. Etherenet communication on the machine and integration into central system for control and supervision. Cooperation between Goša FOM Smederevska Palanka and  Faculty of Electrical Engineering, Belgrade, 2007.-2008.

10.  Reconstruction of the electrical system for the excavator SRs 470,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning, 2008. PLC AC800F, ABB, distributed I/O modules on PROFIBUS network, 16 PROFIBUS nodes, optical and wire network 350 m long – in ring structure. Frequency controlled drives for Slewing drives. Frequency controlled drive with active front end on Operator Cabine Winch drive. Four operator panels, including one graphical panel. Semiautomatic operation of excavator.

11.  Reconstruction of the electrical system for the excavator SRs 400,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning, 2008. PLC AC800F, ABB, distributed I/O modules on PROFIBUS network, 16 PROFIBUS nodes, optical and wire network 350 m long – in ring structure. Frequency controlled drives for Slewing and Crawler drives. Four operator panels, including one graphical panel. Semiautomatic operation of excavator.

12.  Reconstruction of the electrical system for the Mobile transfer conveyor (BandWagen) BW 2400,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning, 2008. PLC AC800F, ABB, distributed I/O modules on PROFIBUS network, 16 PROFIBUS nodes, optical and wire network 350 m long – in ring structure. Frequency controlled drives for Slewing and Crawler drives. Four operator panels, including one graphical panel.

13.  Reconstruction of the electrical system for the Mobile transfer conveyor (BandWagen) BRs 1400,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning, 2008. PLC AC800F, ABB, distributed I/O modules on PROFIBUS network, 8 PROFIBUS nodes, network 100 m long. Two operator panels, one alphanumerical, and one graphical panel.

14.  Project of new electrical equipment and control software for belt conveyor with frequency controlled drives

Four belt conveyors  B1800 4x630kW with controlled drives and transport on OPM  Drmno. Distributed modules on PROFIBUS network, 6 nodes, optical communication, operator panel. Measuring of the belt tension force, automatic slippage prevention, controlled starting, load equalization. Frequency converter ACS800 and PLC AC800, ABB. Etherenet communication on the machine and integration into central system for control and supervision. Cooperation between EXOR ESI Beograd, Sever ATB Subotica and  Faculty of Electrical Engineering, Belgrade, 2007.-2008.

15.  Main mining project on OPM „Buvač”

Project of electrical energy supplying, including project of electrical energy supplying for the dewatering system, 2007.

16.  Project of new electrical equipment and control software for belt conveyor with frequency controlled drives

Four belt conveyor B1400 4x315kW with controlled drives and transport on OPM  Drmno. Distributed modules on PROFIBUS network, 6 nodes, optical communication, operator panel. Measuring of the belt tension force, automatic slippage prevention, controlled starting, load equalization. Frequency converter ACS800 and PLC AC800, ABB. Etherenet communication on the machine and integration into central system for control and supervision. Cooperation between EXOR ESI Beograd and  Faculty of Electrical Engineering, Belgrade, 2006.

17.  Study „The choice of the optimal maintenance system in PE Open pit mine „Kostolac””
PE Open pit mine Kostolac.

Study was completed for the Public Enterprise ”Electric power industry of Serbia”, in cooperation between Faculty of Electrical Engineering and Faculty of Mining and Geology in Belgrade, July 2006.

18.  Optimization of drives and construction elements of belt conveyor systems on open pit mines during the reconstruction and modernization

Project  funded by Ministry of science and environmental protection of Republic Serbia, cooperation between Faculty of Mining and Geology and Faculty of Electrical Engineering in Belgrade, 2005. - 2008.god.

19.  Reconstruction of the control system for the excavator SCH 800
Open pit mine Drmno, Kostolac.

Project, 2006, supervision and commissioning 2007. PLC AC800F, ABB, distributed I/O modules on PROFIBUS network, 7 nodes, optical and wire network 250 m long. Three operator panels, including one graphical panel. Program operation of excavator.

20.  Reconstruction of the control system for the excavator ERs 710 u, Drmno.
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning, 2005. PLC AC800F, ABB, distributed I/O modules on PROFIBUS network, 12 nodes, optical and wire network 350 m long. Four operator panels, including one graphical panel. Semiautomatic operation of excavator.

21.  Reconstruction of the control system for the excavator SRs 470
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning, 2005. PLC AC800F, ABB, distributed I/O modules on PROFIBUS network, 14 nodes, wire network 400 m long. Four operator panels, including two graphical panels. Cable drum drive realized with frequency converter, ABB ACS-800. Program operation of excavator.

22.  Reconstruction of the control system for the excavator ERs710j,
Open pit mine Drmno, Kostolac.

Realization (supervision and commissioning), 2005. Project completed in 2004. PLC AC800F, ABB, distributed I/O modules on PROFIBUS network, 11 nodes, optical and wire network 350 m long. Three operator panels, including one graphical panel. Semiautomatic operation of excavator. Photos.

23.  Reconstruction of the electrical part for the excavator SRs 1300,
Open pit mine Drmno, Kostolac.

Supervision and commissioning during the reconstruction in 2004. PLC AC800F - ABB, distributed I/O modules on PROFIBUS network, 33 nodes in total, optical and wire network 900 m long. Six operator panels, including two graphical panels, four position encoders on PROFIBUS network, two incremental encoders, one frequency converter for bucket wheel drive, three frequency converters for slewing drive and six frequency converters on the transport drive, all on PROFIBUS network. Program operation of excavator. Project completed in 2003. Photos.

24.  Reconstruction of the control system for the bucket wheel excavator SRs 2000,
Open pit mine Drmno, Kostolac.

Realization (project, supervision and commissioning), 2003. PLC AC800F, ABB, distributed I/O modules on PROFIBUS network, 20 nodes, optical and wire network 800 m long. Six operator panels, including two graphical panels. Program operation of excavator. Photos.

25.  Reconstruction of the control system for the spreader A2Rs 5500,
Open pit mine Drmno, Kostolac.

Project completed in 2003. Supervision and commissioning during the reconstruction in 2004. PLC AC800F, ABB, distributed I/O modules on PROFIBUS network, 11 nodes, optical and wire network 330 m long. Four operator panels, including one graphical panel. Photos.

26.  Spreader control system A2 Rs-B 7200,
Open pit mine Drmno, Kostolac.

Realization (project, supervision and commissioning), 2002. The old control system is replaced with the modern generation PLC, remote control and the control of the complete BTO-II system with the wireless ethernet; in the software special attention is paid on safety functions, equipment used: PLC ABB AC800F, distributed modules of ABB type S800 on PROFIBUS protocol, optical communication between nearby cabinet-clusters, network 300 m long, 11 nodes, 3 operator panels; for the slewing drive frequency converters DANFOSS, of the VLT5000 type, with PROFIBUS communication, too. Photos.

27.  New bucket wheel drive with frequency converters on the excavator SRs1300,
Open pit mine Drmno, Kostolac. Commissioned during reconstruction in 2004.

Bucket wheel drive with induction motor and rotor resistive starters is replaced with 1000kW motor fed from controlled drive. Motor supplying voltage is 690 V. ABB drive has diode supplying unit with two modules and inverter part with three modules in one compact electrical cabinet. Control of the drive is performed from distributed control system with the use of PROFIBUS protocol. Photos.

28.  New slewing drive with frequency converter on the excavator SRs1300,
Open pit mine Drmno, Kostolac.

Commissioned during reconstruction in 2004. Three drives with frequency converters controlled from distributed control system with the use of PROFIBUS protocol. Drives are operating as one multi motor drive with the average speed control and dynamic torque distribution, without speed sensor. In the control system is also integrated the gear tooth protection from mechanical tension. Photos.

29.  New transport drive with frequency converters on the excavator SRs1300,
Open pit mine Drmno, Kostolac. Commissioned during reconstruction in 2004.

Six drives with induction motors fed from frequency converters (one drive per caterpillar tread). Control of the drive is performed from distributed control system with the use of PROFIBUS protocol. Drives are operating as one speed sensor-less multi-motor drive with average speed control and dynamic torque distribution. Gear tooth protection and caterpillar star protection from mechanical stress is included in the control algorithm. Photos.

30.  Integration of the new slewing drive with frequency converter into the control system of the spreader A2Rs 5500, Open pit mine Drmno, Kostolac.

Commissioned in 2004. Two drives with frequency converters are integrated into the existing PROFIBUS network on the spreader distributed control system. Drives are operating as one speed sensor-less multi-motor drive with the average speed control and dynamic torque distribution. Motor power is 7,5kW. Mechanical tension protection of mechanical assemblies is integrated in the control system. Photos.

31.  New drive for the winch boom of the bucket wheel with four quadrant frequency converters for the excavator SRs 2000,
 Open pit mine Drmno, Kostolac.

Commissioned in 2004. Winch boom drive of the bucket wheel with induction motors and rotor resistive starters is replaced with the drive, which consists of two four quadrant converters (active rectifier and inverter), which supply two 160kW motors. Drives are operating as one multi motor drive in the „master-follower” order, mutually connected with the use of optical – fiber connection. Control of the drive is performed from the existing distributed control system with the use of PROFIBUS network. Photos.

32.  Integration of the new slewing drive with frequency converters into the control system for the excavator SRs 2000,
 Open pit mine Drmno, Kostolac.

Commissioned in 2004. Three drives with frequency converters are integrated into the existing PROFIBUS network, on the distributed control system for the Excavator. Drives are operating as one multi motor drive with the average speed control and dynamic torque distribution. Mechanical tension protection of mechanical assemblies is also integrated in the control system. Photos.

33.  Integration of the new transport drive with frequency converters into the control system for the excavator SRs 2000,
 Open pit mine Drmno, Kostolac.

Commissioned in 2004. Six drives with frequency converters (one drive per caterpillar tread) are integrated into the existing PROFIBUS network, on the distributed control system for the Excavator. Drives are operating as one multi motor drive with the average speed control and dynamic torque distribution. Gear tooth protection and caterpillar star protection from mechanical stress is included in the control algorithm. Photos.

34.  Integration of the new transport drive with frequency converters into the control system for the spreader A2RsB-7200,
 Open pit mine Drmno, Kostolac.

Commissioned in 2004. Six drives with frequency converters (one drive per caterpillar tread) are integrated into the existing PROFIBUS network, on the distributed control system for the Spreader. Drives are operating as one multi motor drive with the average speed control and dynamic torque distribution. Gear tooth protection and caterpillar star protection from mechanical stress is included in the control algorithm. Photos.

35.  Slewing electrical drive with induction motors and frequency converters for the spreader A2 Rs-B 7200, Open pit mine Drmno, Kostolac.

Realization (project, supervision and commissioning), 2002. Two drives with induction motors fed from frequency converters, speed sensor-less, are integrated into PROFIBUS network of the control system. Drives are operating with dynamic torque distribution. DANFOSS frequency converters of the VLT5000 type are used, with PROFIBUS communication.

36.  New slewing drive with frequency controlled drive at PKM1 and PKM2, Stockyard Drmno,
 Open pit mine Drmno, Kostolac.

Project, supervision and commissioning in 2005. In the existing control system of the machine, new slewing drive with the power dependant regulation of the slewing drive and the bucket wheel drive, is integrated. Drives are realized with induction motors fed from frequency converters, without speed sensors.

37.  “Development and application of the new concept for starting and control of drives for mining machine movement at open pit mines along main operating axes”,

National program of technological development of Ministry of science and environmental protection, project manager B. Jeftenić, Faculty of Electrical Engineering, Belgrade 2005 2007.

38.  Validation of belt conveyer application with variable speed control at open pit mines of EPS (Public Enterprise „Electric power industry of Serbia”)

Study completed for the Public Enterprise „Electric power industry of Serbia” Belgrade, 2005. All advantages of belt conveyer application with variable speed control are exactly presented in the study.

39.  „Operational lifetime extension of the main equipment at the open pit coal mines of Public Enterprise „Electric power industry of Serbia”, I phase – bucket wheel excavators”,

Study completed for the Public Enterprise „Electric power industry of Serbia”, Belgrade 2003. In the study, there are clearly indicated criteria of temporary condition assessment of bucket wheel excavators at the open mines in Serbia. The detailed analyses is completed for the each excavator separately, which has given investors the recommendations for the future priorities.

40.  Reconstruction of the control system for the excavator ERs 710j,
Open pit mine Drmno, Kostolac.

Project, 2004. PLC AC800F - ABB, distributed I/O modules on PROFIBUS network, 9 nodes, optical and wire network 350 m long. Three operator panels, including one graphical panel.

41.  Reconstruction of the control system for the excavator SRs 1300,
Open pit mine Drmno, Kostolac.

Project 2003. PLC AC800F, ABB, distributed I/O modules on PROFIBUS network, 17 nodes, optical and wire network 600 m long. Six operator panels, including two graphical panels. Photos.

42.  Reconstruction of the control system for the spreader A2Rs 5500,
Open pit mine Drmno, Kostolac.

Project 2003. PLC AC800F, ABB, distributed I/O modules on PROFIBUS network, 11 nodes, optical and wire network 330 m long. Four operator panels, including one graphical panel. Photos.

43.  Project for revitalization of II ECS system at the Open pit mine Drmno, Kostolac.

Analyses of electrical equipment condition of II BTO system, 2002.

44.  Project of reconstruction and modernization of III ECS system on PO „Ćirikovac”.
Kostolac, 2003

Includes six subsystems: bucket wheel excavator SRs 1300, spreader A2RsB 5500x50, 4 belt conveyers B=1600mm; project of electrical part incorporates project of complete installations, HV and LV distribution, all electrical drives, new control system with PLC.

45.  Reconstruction of the control system for the belt conveyor U-Z-1,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning in 2005. Distributed modules on PROFIBUS protocol, 5 nodes; optical communication, operator panel. Belt tension measurement, measurement of all motors currents, as well as all characteristic temperatures.

46.  Reconstruction of the control system for the belt conveyor U-Z-2,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning in 2005. Distributed modules on PROFIBUS protocol, 5 nodes; optical communication, operator panel. Belt tension measurement, measurement of all motors currents, as well as all characteristic temperatures.

47.  Reconstruction of the control system for the belt conveyor U-Z-3,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning in 2005. Distributed modules on PROFIBUS protocol, 5 nodes; optical communication, operator panel. Belt tension measurement, measurement of all motors currents, as well as all characteristic temperatures.

48.  Reconstruction of the control system for the belt conveyor U-Z-4,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning in 2005. Distributed modules on PROFIBUS protocol, 5 nodes; optical communication, operator panel. Belt tension measurement, measurement of all motors currents, as well as all characteristic temperatures.

49.  Reconstruction of the control system for the belt conveyor U-I-1,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning in 2005. Distributed modules on PROFIBUS protocol, 5 nodes; optical communication, operator panel. Belt tension measurement, measurement of all motors currents, as well as all characteristic temperatures.

50.  Reconstruction of the control system for the belt conveyor U-I-2,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning in 2005. Distributed modules on PROFIBUS protocol, 5 nodes; optical communication, operator panel. Belt tension measurement, measurement of all motors currents, as well as all characteristic temperatures.

51.  Reconstruction of the electrical part of belt conveyor without its own transport drive,
Open pit mine Drmno, Kostolac.

Project and completing of the bid documentation for the job realization. Distributed modules on PROFIBUS protocol, 5 nodes, optical communication, operator panel.  Belt tension measurement, measurement of all motors currents, as well as all characteristic temperatures, 2005.

52.  Reconstruction of the electrical part of belt conveyor with its own transport drive,
Open pit mine Drmno, Kostolac.

Project and completing of the bid documentation for the job realization. Distributed modules on PROFIBUS protocol, 5 nodes, optical communication, operator panel.  Belt tension measurement, measurement of all motors currents, as well as all characteristic temperatures, 2005.

53.  Reconstruction of the control system for the belt conveyor J-IV-4,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning in 2005. Distributed modules on PROFIBUS protocol, 5 nodes; optical communication, operator panel. Belt tension measurement, measurement of all motors currents, as well as all characteristic temperatures.

54.  Reconstruction of the control system for the belt conveyor J-I-1,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning in 2004. Distributed modules on PROFIBUS protocol, 5 nodes; optical communication, operator panel. Belt tension measurement, measurement of all motors currents, as well as all characteristic temperatures. Photos.

55.  Reconstruction of the control system for the belt conveyor J-I-3,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning in 2004. Distributed modules on PROFIBUS protocol, 5 nodes; optical communication, operator panel. Belt tension measurement, measurement of all motors currents, as well as all characteristic temperatures. Photos.

56.  Reconstruction of the control system for the belt conveyor J-I-5,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning in 2004. Distributed modules on PROFIBUS protocol, 5 nodes; optical communication, operator panel. Belt tension measurement, measurement of all motors currents, as well as all characteristic temperatures. Photos.

57.  Reconstruction of the control system for the belt conveyor (V-1) J-IV-1,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning in 2003. Distributed modules on PROFIBUS protocol, 5 nodes; optical communication, two operator panels. Belt tension measurement, measurement of all motors currents, as well as all characteristic temperatures. Photos.

58.  Reconstruction of the control system for the belt conveyor (ZT 4) J-IV-2,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning in 2003. Distributed modules on PROFIBUS protocol, 5 nodes; optical communication, two operator panels. Belt tension measurement, measurement of all motors currents, as well as all characteristic temperatures. Photos.

59.  Reconstruction of the control system for the belt conveyor (OT-3) J-IV-3,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning in 2003. Distributed modules on PROFIBUS protocol, 5 nodes; optical communication, two operator panels. Belt tension measurement, measurement of all motors currents, as well as all characteristic temperatures. Photos.

60.  Control system for the belt conveyor J-II-4,
Open pit mine Drmno, Kostolac.

Project, supervision and commissioning in 2003. Distributed modules on PROFIBUS protocol, 4 nodes; optical communication, operator panel. Measurement of all motors currents, as well as all characteristic temperatures. Photos.

61.  Control system for the belt conveyor J-II-1,
Open pit mine Drmno, Kostolac.

Realization (project, supervision and commissioning) in 2003. Distributed modules on PROFIBUS protocol, 4 nodes; optical communication, operator panel. Measurement of all motors currents, as well as all characteristic temperatures. Photos.

62.  Control system for the belt conveyor J-II-5,
Open pit mine Drmno, Kostolac.

Realization (project, supervision and commissioning) in 2002. The old control system is replaced with PLC of the newest generation, remote supervision and control of complete ECS-II system through wireless ethernet-a; thermal protection of drive motors windings and bearings, and possibility for automatic tension-relaxion of conveyer belt are additionally included in software; used equipment: PLC ABB AC800F, distributed modules ABB type S800 with PROFI-bus protocol, optical communication between clusters, network length 20m, network speed 12MBita/s. Photos.