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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
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