Lecture 4-ELECTRIC MOTORS-ACTUATION SYSTEMS2.ppt
- 1. Robot actuation and feedback components: Course Code: ME 6320 Course Credit 6 Prepared by; Masoud Kamoleka Mlela BSc, Electromechanical Eng. (UDSM) MSc, Renewable Energy (UDSM) PhD, Mechanical Engineering (HEU, CN). Automation and Robotics Technology
- 2. Electric motors Drive systems are frequently used as the final control element in speed-control systems. Drive systems are classified into two major categories: a) D.C. motors b) A.C. motors 2
- 3. Basic Principles A force is exerted on a conductor in a magnetic field when a current passes through it. For a conductor of length ‘L’ carrying a current ‘I’ in a magnetic field of flux density ‘b’ at right angles to the conductor, the force ‘F’ is equal to B * I * L. 3
- 4. When a conductor moves in a magnetic field, an e.m.f. is induced across it. The induced e.m.f. ‘e’ is equal to the rate at which the magnetic flux Ø (flux equals the product of the flux density and the area) swept through the conductor changes (Faraday’s law) i.e. The minus sign indicates the induced e.m.f is in the direction as to oppose the change producing it. 4
- 5. The DC Motor A motor is an electromechanical component that yields a displacement output for a voltage input, that is, a mechanical output generated by an electrical input. The mechanical output is used to drive an external load 5
- 6. A DC motor consists of a rotating cylinder called the armature with a current carrying conductor wrapped around it. The armature is placed in a magnetic field between two stationary poles as shown below. 6
- 7. The principles of operation are as follows The brushes (spring-loaded carbon contacts) contact the rotating commutator causing current ia to flow in the armature conductors. The current carrying armature conductor, in the presence of the magnetic field Bf produced by a stationary permanent or electro-magnet, experiences a force F given by: whose direction is given by the left-hand rule, and where l is the length of the armature conductor cutting the magnetic field. The commutator ensures that the current changes direction in the armature coil every half cycle to maintain the continuous motion of the rotor. 7
- 8. The force F develops a turning torque proportional to the armature current, i.e. Tm α ia, that rotates the armature and shaft where the load is normally connected. The movement of the armature conductor in the magnetic field induces a voltage at the conductor terminals according to Faraday's law. This voltage is called the back electromotive force or emf and is proportional to the angular velocity of the armature, 8
- 9. • Most motors work on the electrical principle of induction. When an electric current flows through a wire, it generates a magnetic field around the wire. • Conversely, by placing a charged coil of wire in an existing magnetic field (say, between two magnets), the coil will be attracted to one magnet and repelled by the other, or vice versa, depending on the current flow. • When current flows through a wire, the higher the current, the greater the magnetic field generated; therefore, the greater the attraction or repulsion. The coil is mounted on a spinning shaft in the middle of the motor. As one magnet alternately attracts the coil and the other repulses the coil, it spins from one magnet to the other resulting in circular motion. These concepts are captured in Figures 9
- 10. Types of D.C. motor 1. Series wound motor (fig. a) • With the series wound motor the armature and fields coils are in series. Such a motor exerts the highest starting toque and has the greatest no-load speed. With light loads there is a danger that a series wound motor might run at too high a speed. • Reversing the polarity of the supply to the coils has no effect on the direction of rotation of the motor. It will continue rotating in the same direction since both the field and armature currents have been reversed. 10
- 11. 2. Shunt wound motor (fig.b) • With the shunt wound motor the armature and field coils are in parallel. It provides the lowest starting toque, a much lower no-load speed and has good speed regulation. • Because of this almost constant speed regardless of load, shunt wound motors are very widely used. • To reversed the direction of rotation, either the armature or field supplied must be reversed. 11 • For this reason, the separately excited windings are preferable for such a situation.
- 12. 3. Compound motor (fig. c) • The compound motor has two field windings. One in series with the armature and one in parallel. • Compound wound motors aim to get the best features of the series and shunt wound motors, namely a high starting torque and good speed regulation. 12
- 13. 4. Separately excited motor (fig. d) • The separately excited motor has separate control of the armature and field currents and can be considered to be a special case of the shunt bound motor. 13
- 14. DC Motors The stator is the stationary outside part of a motor. The rotor is the inner part which rotates. In the motor animations, red represents a magnet or winding with a north polarization, while green represents a magnet or winding with a south polariztion. Opposite, red and green, polarities attract. 14
- 15. DC Motors Just as the rotor reaches alignment, the brushes move across the commutator contacts and energize the next winding. In the animation the commutator contacts are brown and the brushes are dark grey. A yellow spark shows when the brushes switch to the next winding. 15
- 16. Brushless DC Motors • A brushless dc motor has a rotor with permanent magnets and a stator with windings. It is essentially a dc motor turned inside out. The control electronics replace the function of the commutator and energize the proper winding. 16
- 17. DC Motor Applications • Automobiles – Windshield Wipers – Door locks – Window lifts – Antenna retractor – Seat adjust – Mirror adjust – Anti-lock Braking System 17 •Cordless hand drill •Electric lawnmower •Fans •Toys •Electric toothbrush •Servo Motor
- 18. STEPPER MOTOR • Motors convert electrical energy into mechanical energy. • A stepper motor converts electrical pulses into specific rotational movements. The movement created by each pulse is precise and repeatable. • Stepper motor is the device that produces rotation through equal angles, so called steps, for each digital pulse supplied to its input • Example, if with such motor 1 pulse produces a rotation of 6° then 60 pulses will produce a rotation through 360°. 18
- 19. Stepper Motor Specifications • Phase This term refers to the number of independent windings on the stator, e.g. a four-phase motor. The current required per phase and its resistance and inductance will be specified so that the controller switching output is specified 19 The following are some of the terms commonly used in specifying stepper motors:
- 20. • Step angle This is the angle through which the rotor rotates for one switching change for the stator coils. • Holding torque This is the maximum torque that can be applied to a powered motor without moving it from its rest position and causing spindle rotation. 20 Stepper Motor Specifications
- 21. • Pull-in torque This is the maximum torque against which a motor will start, for a given pulse rate, and reach synchronism without losing step. • Pull-out torque This is the maximum torque that can be applied to a motor, running at a given stepping rate, without losing synchronism. 21 Stepper Motor Specifications
- 22. • Pull-in rate This is the maximum switching rate at which a loaded motor can start without losing a step • Pull-out rate This is the switching rate at which a loaded motor will remain in synchronism as the switching rate is reduced. • Slew range This is the range of switching rates between pull-in and pull-out within which the motor runs in synchronism but cannot start up or reversed 22 Stepper Motor Specifications
- 23. Full Stepper Motor • This animation demonstrates the principle for a stepper motor using full step commutation. The rotor of a permanent magnet stepper motor consists of permanent magnets and the stator has two pairs of windings. Just as the rotor aligns with one of the stator poles, the second phase is energized. The two phases alternate on and off and also reverse polarity. There are four steps. One phase lags the other phase by one step. This is equivalent to one forth of an electrical cycle or 90°. 23
- 24. Half Stepper Motor • This animation shows the stepping pattern for a half-step stepper motor. The commutation sequence for a half-step stepper motor has eight steps instead of four. The main difference is that the second phase is turned on before the first phase is turned off. Thus, sometimes both phases are energized at the same time. During the half-steps the rotor is held in between the two full-step positions. A half-step motor has twice the resolution of a full step motor. It is very popular for this reason. 24
- 25. More on Stepper Motors • Note how the phases are driven so that the rotor takes half steps 25
- 26. More on Stepper Motors • Animation shows how coils are energized for full steps 26
- 27. More on Stepper Motors • Full step sequence showing how binary numbers can control the motor 27 • Half step sequence of binary control numbers
- 28. Stepper Motor Applications • Robots • Film Drive • Optical Scanner • Printers • ATM Machines 28 • Blood Analyzer • FAX Machines
- 29. Servo Motor • A servo motor is a type of motor that can rotate with great precision. • Normally this type of motor consists of a control circuit that provides feedback on the current position of the motor shaft, this feedback allows the servo motors to rotate with great precision. • If you want to rotate an object at some specific angles or distance, then you use a servo motor • It is just made up of a simple motor which runs through a servo mechanism. • If motor is powered by a DC power supply then it is called DC servo motor, and if it is AC-powered motor then it is called AC servo motor. 29
- 30. Servo Motor Working Mechanism • It consists of three parts: – Controlled device – Output sensor – Feedback system • It is a closed-loop system where it uses a positive feedback system to control motion and the final position of the shaft. • Here the device is controlled by a feedback signal generated by comparing output signal and reference input signal. • A servo consists of a Motor (DC or AC), a potentiometer, gear assembly, and a controlling circuit. • We use gear assembly to reduce RPM and to increase torque of the motor. 30
- 31. • at initial position of servo motor shaft, the position of the potentiometer knob is such that there is no electrical signal generated at the output port of the potentiometer. • Now an electrical signal is given to another input terminal of the error detector amplifier. • Now the difference between these two signals, one comes from the potentiometer and another comes from other sources, will be processed in a feedback mechanism and output will be provided in terms of error signal. • This error signal acts as the input for motor and motor starts rotating. 31 Servo Motor Working Mechanism
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