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Transcript
ROBOTICS ELECTROMECHANICAL ACTUATORS BY Rajesh Kakumanu Assistant Professor, Mechanical Engineering. Dept., V.J.I.T, Hyd. ---------------------------------------------------------------------------------------------------------------INTRODUCTION MOTORS Motors are rotary actuators which make rotational movement when external energy is applied to it. If the external energy given is electrical energy then it is called electric motor. Electric motors are electromechanical devices which converts electrical energy into mechanical energy. Electric motors are the most common actuators used in majority of the mechanical system where motion and force are involved. Electric motors can be classified based on, a. its functionality such as torque motor, gear motor, servomotor etc. b. the type of current used i.e., d.c motors and a.c motors. Fig. 1.1 : Force on a conductor carrying current The basic principles involved in the action of a motor are, a. A force F is exerted on a conductor of length L carrying a current /, placed in a magnetic field of flux density B at right angles to the conductor. The force so exerted is given by F = B1L. DC MOTORS Principle: A loop of coil carrying current free to rotate, when placed in a field of permanent magnet; is acted upon by forces on its sides at right angles to the field, rotates by 90°. If the rotation were to be continued the direction of current flowing through the coil is to be reversed. In conventional d.c motors, coils of wire are mounted in slots provided on a cylinder of magnetic material called the armature. The armature is mounted on bearings in a magnetic field produced by field poles. In the fig. 1.1, the magnetic field is produced by the current carried by the field coil. The end of each coil is connected to the next segment of the segmented ring called commutator which delivers current and controls its direction into the armature coil. Solid brushes provide stationary electrical contact to the moving commutator conducting segments. (Brushes in early motors consisted by bristles of copper wire flexed against the commutator and hence the term brush). Brushes are usually made out of conducting solid graphite which provides large contact area, spring loaded for ensuring continual contact and self lubricating. In the air gap between the rotor and the stator the magnetic fields interact. As the armature rotates, the commutator reverses the direction of current in each coil as it moves between the field poles. Fig. 1.2 : Principle of dc motor Fig. 1.3. : dc motor This will take care of the forces acting on the coil to remain acting in the same direction and continue rotation. The direction of rotation of the rotor can also be reversed, by reversing either the field current or armature current depending on the configuration of the field coil and armature coil. A.C MOTORS Electric motors using A.C. supply are called A.C. motors. Classification of a.c motors Alternating current motors are broadly classified into two groups I. a. Single-phase a.c motors and b. Poly-phase a.c motors II a. Induction motors b. Synchronous motors. That is a.c motors can be single phase induction or synchronous motors or polyphase induction or synchronous motors. Single phase motors are used for low-power requirement and poly phase for higher power requirement. Induction motors are inexpensive than synchronous motors and are widely used. Single phase induction motor This consists of a single phase stator winding with a cage rotor represented schematically in fig. 5.58. The rotors are made up of either copper or aluminum bars that fit into slots in the end Fig. : 1.4. : Single phase squired cage indication motor rings to form a complete electrical circuit. Instead of being concentrated coil, the actual stator winding is distributed in slots in order to give an approximately sinusoidal distributed m.m.f (magneto motive force) in space. When an alternating current is passed through the stator windings, an alternating magnetic field is produced. As a result of electromagnetic induction, emfs are induced in the conductors of the rotor and current flows in the rotor. Such a motor inherently has no starting torque. Drawbacks of single phase induction motor Single phase induction motors suffers from several drawbacks. They are, a. Low over load capacity b. Low efficiency c. Low power factor d. No self starting The frequency of the a.c supply determines the speed of the motor. For a two-pole single phase motor supplied with constant frequency supply will alternate the magnetic field with this frequency. This speed of rotation of magnetic field is called synchronous speed. The rotor rotates at slower speed than the rotating stator fields (this is called slip) making the induction impossible. Hence the term asynchronous. Because of this asynchronous motors are sometimes referred to as induction motors. Generally the slip is around 1 to 3 percent. Three-phase induction motor Poly-phase induction motor is, by very considerable margin the most widely used a.c motor, Advantages of poly-phase motor a. Low cost b. Simple and extremely rugged construction c. High efficiency d. Reasonably good power factor e. Low maintenance cost f. Simple starting arrangement. Three-phase induction motor: This has a stator with three windings mounted 120o art, each winding connected to one of the three lines of the supply, Since the three phases reach their maximum current at different times, it can be considered that the magnetic field rotate the stator poles complete one rotation in one full cycle of the current. Fig. 1.5 : Three Phase Induction moses The magnetic field is much smoother than with single phase induction motor and has the advantage of self-starting. The direction of rotation of the motor can be changed by changing the direction of rotation of magnetic field by interchanging any two of the line connections. The speed of revolving of the magnetic field produced by primary currents is called the synchronous speed of the motor, and is given by N= 120f/p where f is the supply frequency and p is the number of poles. This revolving sweeps across the rotor conductors and thereby induces an emf in these conductors. Synchronous motors: Synchronous motors have a rotor of permanent magnet or can Demagnetized by supplying d.c supply separately. The magnetic field of the stator due to a.c supply rotates, and so the magnets of the rotor fig. 5.60. The rotor has two poles and the stator has two poles per phase. The magnetic field rotates through 360O in one cycle of supply and the frequency of rotation will be equal to frequency of supply current. They are used when a precise speed is desired. They are also not of self starting type and a separate system has to be used for starting them. This gives constant speed from no load to full load. Electromagnetic power varies linearly with the voltage. They operate at higher efficiencies, especially in the low speed but it may fall out of synchronous and stop when over loaded. Advantages of a.c motor over d.c motors Fig. 1.6 : Three phase two poles synchronous motor a. Cost is less b. More rugged c. Reliable d. Maintenance free. Variable speed a.c motor The disadvantage in a.c motor is the speed control being more complex than d.c motors and hence speed controlled d.c motors are much cheaper than speed controlled a.c motors. Speed of a.c motor depends on the frequency of the a.c supply and one method of controlling the speed is by controlling the frequency of the a.c supply. But the torque developed by a.c motor remains constant when the ratio of stator voltage to frequency is constant. When the frequency is varied for controlling the speed, the torque developed will also vary. To overcome this problem a.c supply is first rectified to d.c using a converter and then using an inverter the d.c again converted back to a.c, this inversion being at selected frequency of a.c. The other method is to convert a.c into a.c at the desired frequency using a cycloconverter without converting a.c to dc. Fig shows the basic concept of variable speed a.c motor using a converter and an inverter. Concept of variable speed a.c. motor Stepper motors A stepper motor is a special type of d.c motor that produces rotation at equal angles called step's for each digital pulse supplied to its input. For example if a pulse can produce a rotation of 10o then 36 pulses will produce one rotation or 360°. Number and rate of the pulse control the position and speed of the motor shaft. Generally stepper motors are manufactured with steps per revolution of 12, 24, 72, 144, 180 and 200 resulting in shaft increments of 30°, 15°, 5°, 2.5° 2° and 1.8° per step. Special microstepping circuitry is sometime provided to allow many more steps per revolution, offer 10,000 steps/revolution or even more. Performance characteristics The following are the performance characteristics of a stepper motor. a. Rotation in both directions b. Precision angular incremental changes c. Holding torque at zero speed d. Capability of digital control Classification of stepper motor a. Permanent magnet b. Variable reluctance and c. Hybrid type The other classifications are a. Bipolar stepper motor and b. Unipolar stepper motor Permanent magnet stepper motor: In this type the stator consists of wound poles and the rotor poles are permanent magnets. The permanent magnet motor has the advantage of a small residual holding torque called the detent torque even when the stator is not energized. Basic concept of step rotation of stepper motor Figure shows the basic principle of how a rotor moves in steps in the case of a permanent magnet rotor. Consider a four stator poles and permanent magnet rotor, as shown .To start with in step 0, the rotor is in equilibrium since opposite poles are adjacent to each other and hence attracts each other. The rotor can remain in this position and can with stand the opposing torque called holding torque until the magnetization of the stator poles are changed. Once the magnetization of the stator poles are changed (step 0 to step 1) a torque is induced to the rotor causing it to move by 90° in the CW direction and the next equilibrium position is achieved as shown in step 1. When the magnetization is again changed (step 1 to step 2) again a torque is induced in the rotor and it rotates by another 90° and another equilibrium position is obtained as shown in step 2. Successive change of magnetization of stator poles thus rotates the poles in steps of 90°. The direction of rotation of the rotor depends on the direction of sequencing of magnetization of poles, i.e., counter clockwise sequence of magnetization of stator poles, rotates the rotor in CCW direction. Variable reluctance stepper motor The variable reluctance stepper motor has a Ferro- magnetic rotor rather than a permanent magnet rotor. Motion and holding are results of minimization of the magnetic reluctance between the stator and the rotor poles. The number of poles on the rotor will always be smaller in number than that on the stator. When current is passed through a pair of stator poles having maximum reluctance path, magnetic field is produced with lines offered trying to shorten themselves, rotates the rotor until the stator and rotor poles lines up which will be the shortest or minimum reluctance path. The steps of angles that are generally obtained with this type of motor is 7.5° or 15°. A variable reluctance motor has the advantage of a low rotor inertia and hence faster dynamic response. Variable reluctance stepper motor Hybrid Stepper motor: This combines the feature of both permanent magnet and variable reluctance stepper motors. It consists of a permanent magnet mounted inside iron caps. These caps have teeth cut on it as shown. This unit forms the rotor of the hybrid stepper motor. The rotor unit itself has minimum reluctance position in response to a pair of energized stator coils. Such motors find its application extensively in high accuracy positioning (e.g., computer hard disc), with a typical step angle of 0.9° and 1.8°. Hybrid stepper rotor ------------------------------------------------