Robotics - unit-2-- Drive Systems

Types of Drive Systems

1. --- Robotics UNIT-2

2. ROBOT DRIVE SYSTEMS AND END EFFECTORS Pneumatic Drives-Hydraulic Drives-Mechanical Drives-Electrical Drives-D.C. Servo Motors, Stepper Motors , A.C. Servo Motors-Salient Features, Applications and Comparison of all these Drives , End Effectors-Grippers-Mechanical Grippers, Pneumatic and Hydraulic- Grippers, Magnetic Grippers, Vacuum Grippers; Two Fingered and Three Fingered Grippers; Internal Grippers and External Grippers ; Selection and Design Considerations.

3. ROBOT DRIVE SYSTEMS Drive means operate the robot. The drive system is to provide a means to control the speed and also torque (or) power. Drive system is used for converting hydraulic, pneumatic, and electrical energy into useful mechanical energy. It is used to motion transfer and drive the robot.

4. Types of drive system Hydraulic drive systems Pneumatic drive systems Electric drive systems

5. Hydraulic drive systems Pressurised fluid is used to transmit and control power. The hydraulic drive is mostly suitable for heavy load robot applications The term hydraulic refers to the transfer of energy from pressure difference not from the kinetic energy of flow

6. Features of Hydraulic drive system High force capability High dynamic response Assisted braking torque Good mechanical stiffness High power per unit weight and volume

7. Consists of 1.Actuators 2.Control valve 3.Power supply

8. Advantages No reduction gear is needed High accuracy and better response Excellent for heavy duty and large robots It provide more power than electric drives

9. Disadvantages High maintenance More expensive system Not suitable for clean air environment Requires pump, reservoir, motor and hoses More floor space is required

10. Pneumatic drive systems Pressurised air is used to control power. With pneumatic valves control the flow of energy from pressurized gas Features of Pneumatic drive system Compressed air has most of the desired properties and characteristics of a gas for pneumatic system. It is not poisonous and non flammable.

11. Consists of 1.Actuators 2.Control valve 3.Power supply

12. Pneumatic System in Packaging

13. Advantages Good accuracy Excellent speed Easy installation Small leakage can be tolerated Economical than hydraulic drives

14. Disadvantages Difficult to control Air needs preconditioning Precision is less than electric drives More vibration is generated More noise is produced

15. Electric drive systems An electric drive system is defines as a form of machine equipment designed to convert electric energy into mechanical energy and provide electric control of the process. Features of Pneumatic drive system Electric drive offers energy transformation The control components are used to limit the amplitude(armature current of DC motor)

16. There are three major types of electric drives 1. AC servo motor 2. DC servo motor 3. Stepper motor Parts of electric drive 1. Power modulator 2. Motor 3. Control unit 4. Sensing unit

17. Advantages Simple construction Clean air environment Requires less floor space Electric drive robot is quiet operation Disadvantages Poor dynamic response A large and heavier motor must be used. It is most costly Electric drive system do not provide as much speed and power compared to hydraulic system.

18. Actuators Actuators are the device used for converting hydraulic, pneumatic and electrical energy into mechanical energy. The mechanical energy used to get the work done. Types of Actuators 1. Hydraulic actuators 2. Pneumatic actuators 3. Electrical actuators a) Servomotor b) Stepper motor c) AC (or) DC motor

19. Hydraulic Actuators Hydraulic actuators transform the hydraulic energy stored in a reservoir into mechanical energy by means of suitable pumps. Hydraulic actuators are also fluid power device for industrial robots which utilise high pressure fluid such as oil to transmit forces to the point of application desired

20. Characteristics of hydraulic actuating systems The designer should know the basics of Operating cycle Operating pressures Types pf pumps Maximum and minimum operating and ambient temperatures Loads encountered by various components Features of the hydraulic actuating systems High force capability Good mechanical stiffness High power per unit weight and volume

21. Elements of hydraulic actuation system Hydraulic reservoir Filters Hydraulic pump Cylinders Motors Hydraulic valve Accumulators Hydraulic hose Hydraulic seal piping

22. Types of Hydraulic Actuators Single acting spring return type Double acting cylinder Ram type

23. Advantages Self lubrication No mechanical linkage is required It can also provide precise control at low speeds Capable of with standing shock loads Greater load carrying capacity Disadvantages Expensive Noisy operation High maintenance Not energy sufficient Not suitable for clean environment Leakage can occur causing a loss in performance

24. Pneumatic Actuators Pneumatic actuators utilise pneumatic energy provided by a compressor and transforms it into mechanical energy by means of pistons (or) turbines. Pressurised air is used to transmit and control power. Features of pneumatic actuators Limit cycling Tend to have performance inconsistency Pneumatic actuators are capable of modulating process control It can handle high torque loads

25. Common parts of a pneumatic system Compressor Check valves Regulators Gauges Accumulator Feed lines

26. Advantages Control is simple It is cheapest form of all actuators No mechanical transmission is usually required Individual components can be easily interconnected Very quick response time and faster cycles Disadvantages More noise and vibration Not suitable for heavy loads Air compressor is required

27. Electric Actuators An actuator obtaining electrical energy from mechanical system is called electric actuator Features of the electric actuators High band with provide accurate and fast control High maximum force allows high acceleration

28. Electrical actuators comprise the following Drive system 1. AC motor 2. DC motor 3. Stepper motor Switching device a) Mechanical switch 1. Solenoids 2. Relays b) Solid state switch 1. Diodes 2. Thyristor 3. Transistors

29. Advantages High power conversion efficiency They are easily maintained and repaired Structural components can be light weight No pollution of working environment Disadvantages Compliance and wear problems are causing inaccuracies A larger and heavier motor must be used which is costly They cannot be used in explosive atmospheres Applications Wide range of industries where positioning is needed Automation applications

30. Various kind of Motors 1. Stepper motor 2. Servo motor i ) AC Servo motor ii) DC Servo motor

31. Stepper Motor A stepper motor is an electromechanical device . Stepper motor is a device which transforms electrical pulses into equal increments of rotary shaft motion called steps. It converts electrical power into mechanical power.

32. Working of Stepper Motor

33. Permanent Magnet type stepper Motor The permanent magnet type stepper motor has a stator. That is of electromagnet. A rotor that is of permanent magnet. Therefore this motor is called permanent magnet type stepper motor.

34. Permanent Magnet type stepper Motor

35. Advantages A simpler and more reliable Field flux is less affected by temperature rise Higher efficiency due to the absence of field losses Less heating making it is possible to totally enclose the motor Disadvantages Permanent magnets stepper motor cannot produce a high flex density Extra ampere-turns cannot be added to reduce the armature reaction Applications Automobiles and air conditioners Computer drives Electric tooth brushes, portable vacuum cleaners and food mixers

36. Variable reluctance type stepper motor The principle of variable reluctance stepper motor is based on the principle of the flux lines which capture the low reluctance path. The stator and the rotor of the motor are aligned in such as way that the magnetic reluctance is minimum.

37. Variable reluctance type stepper motor

38. Advantages High rate of acceleration High torque to inertia ratio Simple and low cost machine Disadvantages Generally noisy There is no winding on rotor Lower torque capability

39. Hybrid type stepper motor The hybrid type stepper motor as the name recommends is a blending of both permanent magnet type stepper motor (PM) and variable reluctance motor (VR)

40. Hybrid type stepper motor

41. Advantages It has more torque Lower stepping rate More efficient at low speeds The length of step is smaller Disadvantages Higher inertia If the magnetic strength is varied, the performance of the motor is affected. The weight of the motor is more because of the presence of the rotor magnet. Cost is more

42. Torque vs speed characteristics

43. Salient features of stepper motor Stepper motor is small step angle They are high stepping rate and accuracy Stepper motor has high positioning accuracy A stepper motor is used to achieve precise positioning is a digital control They are ideal for applications requiring quick positioning over a shot distance Capabilities of stepper motor Precise positioning control Finer positioning capabilities Low speed with high precision Easy control with pulse signals Generating high torque with a compact size

44. Advantages Low cost High reliability High torque at low speeds Operates in almost any environment Stepper motor is simple to operate Disadvantages lower efficiency Low torque to inertia ratio Lower power output for their weight and size Smooth movement often requires micro stepping Applications Gaming machines Textile machinery Welding equipment

45. Servo Motor The motors that are used in automatic control systems are called servo motor. The servo motors are used to convert electrical signal applied to them into an angular displacement of the shaft. Features of the servo motor Fast response Steady state stability Wide range of speed control Low mechanical and electrical inertia

46. Controlling of Servomotors

47. Advantages Servo motor is small and efficient High speed operation is possible Disadvantages Higher cost It is not suitable for precision control of rotation It is not suitable if we need to prevent vibration Uses of the servo motor The servo motor is built into the camera to correct a lens of the camera to improve out of focus image The servo motor is used in textile to control spinning and weaving machines Used in metal forming and cutting machines to provide specific motion control for milling machines.

48. Applications of the servo motor Toys Cars Robotics Aeroplanes Computers Home electronics CD/DVD player

49. AC Servo Motor Servo motor is basically consists of stator and rotor

50. Rotor The rotor is generally of two types. 1. squirrel cage rotor 2. Drag cup type rotor

51. Types of AC Servo motor

52. 2 phase AC Servomotor The stator of the two phase AC servo motor has the two distributed winding which are displaced from each other by 90 degrees electrical. One winding is known as a reference phase and other one is known as control phase

53. 3 phase AC Servomotor Three sets of winding are placed 120 electrical degrees apart with each set connected to one phase of the three phase power supply. When three phase current passes through the stator windings, a rotating magnetic field effect is produced that travels around the inside of the stator core.

54. Advantages Less maintenance High efficiency High speed operation is possible Resonance and vibration free operation Disadvantages Complex Poor motor cooling Motor can be damaged by sustained over load Most difficult to control in position application Application Robotics Machine tools Suited for lower power application

55. DC Servo Motor DC servo motor is more (or) less similarly to the normal DC motor. DC motors are separately excited DC motor (or) permanent magnet DC motor. They are controlled by armature voltage. The armature is designed to have large resistance, so that the torque speed characteristics are linear. There fore a step change in the armature voltage results in quick change in position (or) speed of the motor.

56. Types of the DC servo Motor

57. 1. Series Motor The series motor have a high starting torque and draws large current The speed regulation of this kind of motor is poor

58. 2. Split series Motor Split series motor with split field rate with some fractional kilowatts Split series motor has a typical torque speed curve

59. 3. Shunt Control Motor It has two separate winding 1. Field winding is on the stator 2. Armature winding is on the rotor of the machine

60. 4. Permanent Magnet shunt motor Permanent magnet shunt motor is a fixed excitation motor where the field is actually supply by a permanent magnet

61. Brushless DC servo motor

62. Pulse coded modulation

63. Torque speed characteristics

64. Advantages Free of vibration and resonance High torque to inertia ratio High output than from a 50Hz motor of same size High efficiency Easier speed control from zero speed to full speed in both direction Disadvantages Overload can damage motor Has complex architecture and requires encode The brush turnout in limited life of 2000 hrs, then service is required Motor does not work when something breaks, hence safety circuits are needed