Servo Motors And Dc Motors Engineering Essay

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This section focuses on the review of literature of servo motors and DC motors, in general as well as in context of the current research project. The chapter begins with introduction of servo and DC motors, followed by their usage in the current study. Lastly, the chapter ends with a conclusion of the use of Servo and DC motors in the current work.

Introduction:

Servo motors and DC motors:

Motors are by far one of the largest consumers of energy on the factory front, and the accuracy in motor control is the easiest measure of minimizing energy consumption. Electric motors are generally categorized in terms of their functions as servo meters, gear motors, and so on, as well as by their electrical specifications as Alternating current (AC) and Direct current (DC) motors. While DC motors are given more preference mainly in the variable seed applications, increasing use of AC motors are seen prior to enhancements in solid state elements. In this context, servo motor is a motor particularly used for positioning or controlling speed in closed loop control systems. The current project makes use of a servo meter to turn over a wide range of speed instruction obtained from the computer. In general, DC and AC servo meters are primarily found in applications depending on their machine structure. DC servo motors have been applied in computers, robotics, numerical control machines, industrial components, speed control of vehicles and alternators, control mechanism purposes, and so forth. Further, the field of control of mechanical linkages as well as robots sees the most potential use and research works of DC motors [1].

On the other hand, servo motors are extensively equipped for applications relating to radio-controlled models, such as cars, planes, robotics, test equipments, industrial automation, etc. Although a servo is not easily defined nor is self-explanatory as its mechanism does not apply to any particular device or machine. Typically, it is a term that applies to a task or a function.

Any electric motor works on the principle of electromagnetism, as aforesaid. A current-carrying conductor produces a uniform magnetic field which when placed in an external magnetic field experiences a force proportional to the current flowing within the conductor, as well as to the strength of the external magnetic field. The internal composition of a DC motor is designed to facilitate harness of the magnetic interaction taking place between a current-carrying conductor and an external magnetic field for producing rotational motion [2] [5].

DC motors are broadly utilized in robotics due to their smaller size and greater energy output. Experts suggest that they are very good at generating power for drive wheels of a mobile robotic car as well as for other mechanical assemblies.

DC motor speeds are easily varied hence they are commonly used in applications where there are speed control, servo control, as well as for positioning needs. A servo motor can be either AC or DC, and typically constitutes the drive section and the encoder. Moreover, a servo motor is much smoother in operation and motion relative to a stepper motor, and has a much greater resolution for position control [4].

Essentially, servo motors are employed in closed loop control systems wherein work is considered to be the control variable. As shown in the figure below, the digital servo motor controller dictates working of the servo motor by transmitting velocity command pulses or signals to the amplifier, which in turn drives the servo motor.

Servo Motors

With this, an essential feedback device, i.e. resolver, or devices, i.e. tachometer and encoder are either integrated in the servo motor or are mounted in isolation, mainly on the load itself. This setting offers the servo motor's position as well as velocity feedback that the controller equates with its coded motion profile and uses for altering its velocity signal. Furthermore, servo motors are characterized by a motion profile, which is nothing but a set of instruction coded within the controller which defines the servo motor working with respect to time, position and velocity. Indeed, the ability of the servo motor to become compatible with the differences between the motion profile and feedback signals greatly relies on the kind of controls and servo motors utilized [2] [5].

Working of Servo motors:

Servo motors fall under a special class of motors mainly designed for applications that involve position control, torque and velocity control. These motors specialize in techniques such as lowering mechanical time constant, lowering electrical time constant, generate permanent magnetic force of high flux density for generating the field, and support of fail-safe electro-mechanical brakes. Furthermore, for application where the load often needs to be speedily accelerated or decelerated, the motor's mechanical and electrical time constants plays a pivotal role. In such cases, the mechanical time constants are decreased by reducing the rotor inertia. Therefore, the rotor of such motors generally has an elongated body. Moreover, servos are controlled my transmitting them a pulse having variable width, as shown in the following schematic. As a servo is equipped with an output shaft, a coded signal is used to position it to certain angular position [1]. As long as the coded pulse is present on the input line, the servo motor will continue to maintain the angular position of the output shaft. As and when the coded pulse changes, there is a change in the angular position of the shaft. Practically, servo motors are employed in radio controlled aircrafts in order to position control surfaces such as elevators and rudders. Similarly, servos are also seen in radio controlled cars, and robots in particular. The motors are of very small size, usually built in control circuitry, and are tremendously powerful for their size. Hence, a lightly loaded servo motor doesn't not use up much energy [8].

The following schematic presents the control circuitry, the motor, set of gears and the holding case. Also, 3 wires can be seen that are used for connecting to the outside world, each for power, ground and control.

Picture of servo guts

Typically, the parameters of the coded signal include a minimum pulse, a maximum pulse, and a repetition rate. For a given rotation constraint of the servo, a neutral position is one at which the servo has equal quantity of potential rotation in the clockwise as well as in the anticlockwise direction. Here, it crucial to note that different servos are associated with different constraints on their rotation; however, they all consist of a neutral position which is always approximately 1.5 milliseconds [6] [9]. The servo motor is made up of some control circuit and a variable resistor or potentiometer which is connected to the output shaft. This potentiometer enables the control circuitry to monitor and control the current angle of the servo meter. When the shaft is positioned at the accurate angle the motors stops operating. In case the circuit detects that the angle is incorrect, it will turn the motor in the correct direction. Furthermore, the output shaft of the servo motor can travel approximately 180 degrees [5]. Commonly, its range is around 210 degree but it can vary by manufacturer. A normal servo motor can be used to control an angular motion from 0 to 180 degrees. In addition, it is mechanically cannot turn any further because of a mechanical stop that is built on top of the main output gear. In essence, the amount of power given to the motor is directly dependent on the distance it needs to move. Hence, if the shaft tries to turn a larger distance the motor will operate at its maximum speed. However, if it needs to turn just a small amount, the motor will operate at a slower speed. This phenomenon is known as proportional control.

Furthermore, the control wire is used for communicating the angle at which the servo meter must turn. This angle depends on pulse code modulation or pulse width modulation (PWD), which is the duration or modulation of a pulse which is applied to the control wire. As the servo anticipates seeing a pulse for every .02 seconds, the duration of the pulse determines how far the motor turns. For instance, a 1.5 milliseconds pulse in likely to make the motor turn to a complete 90 degree position, termed as the neutral position. However, in case the pulse is lesser than 1.5 milliseconds, the motor is likely to turn the shaft to near 0 degrees. And if the pulse is longer than 1.5 ms, then the shaft turns nearer to 180 degrees [5].

As depicted in the diagram below, one can see the duration of the pulse dictating the angle of the output shaft, indicated by the green circle with the arrow.

Pulse Coded Modulation Picture

http://www.servocity.com/assets/images/Untitled-165.jpg

Working of a DC motor:

A DC motor is comprised of 6 fundamental components, namely, axle, rotor, stator, field magnets, brushes and commutator. Most commonly, the external magnetic field in generated in DC motors by high intensity permanent magnets. As shown in the schematic, the stator forms the stationary part of the motor which holds the motor causing and two or more permanent magnetic poles. The stator is responsible for determining the rotation of the rotor along with the axle and the connected commutator. In addition, the rotor is made up of windings or a core, which are electrically attached to the commutator [9].

Image

Furthermore, the geometrical arrangement of the brushes, commutator points, and rotor core is such that on application of power, the polarities of the energized core and the stator magnets misalign, and the rotor will rotate only when it is closely aligned to the stator's magnets. Once the rotor is accurately aligned, the brushes will move to the next commutator points and charge the next windings, and so on.

In a two-pole motor, the rotation reverses the direction of the current across the rotor winding, resulting into a "flip" of the rotor's magnetic field, leading it to continuous rotations. However, in real life, DC motors always have more than two poles. This is so in order to eliminate "dead spots" in the commutator [3] [7].

There is no better way to view how a simple DC motor is integrated by various components, than by simply opening it up. However, this is a tedious job and requires the destruction of a good motor. The following diagram represents the interior components of a disassembled DC motor. This motor is a basic 3-pole Dc motor having 2 brushes and 3 commutator points. It shows the use of iron core windings which have several benefits. Firstly, the iron core offers a powerful, rigid support for the armature and is particularly a crucial consideration for higher-torque motors. Also, the core pushes heat away from the rotor windings thereby enabling the motor to be driven even harder. Iron construction is also inexpensive in comparison to other construction types. However, iron core construction is also associated with several drawbacks. The iron winding has a comparatively greater inertia which tends to restrict motor acceleration. In addition, this construction leads to high winding inductances that limit brush as well as commutator life.

http://www.solarbotics.net/starting/200111_dcmotor/pix/mabuchi_exploded.jpg

In smaller DC motors, an alternative design is used which is characterized by a "coreless" winding. But this design is highly reliant on the coil wire itself in terms of structural integrity. As a result, the winding is hollow and the permanent field magnet can be placed within the rotor coil. Additionally, coreless DC motors have relatively lower armature inductance than iron core DC motors of more or less same size, extending brush and commutator life span [4].

In essence, the coreless design enables manufacturers to produce smaller motors; because of the shortage of iron in their rotors, coreless motors are sensitive to overheating. Therefore, this design is usually used only smaller, low-power DC motors. Again, the inner components of a coreless motor can be said to be instructive, as shown in the figure below [6].

http://www.solarbotics.net/starting/200111_dcmotor/pix/pagermotor_exploded.jpg

Summary:

A DC motor uses a commutator built upon the shaft which automatically changes the polarity of the armature winding when the shaft rotates. This switching keeps the magnetic fields between the armature and stator in such a state that allows for continuous rotation or the armature. Without commutation the motor shaft would rotate only until the magnetic fields lined up North to South at which time the motor would stop turning. On the other hand, a Servo motor does not make use of a commutator. Instead multiple sets of field windings are located about the stator. A single pair of these field windings is energized at a time. The shaft rotates into alignment with the energized field and stops movement. In order to make a servo motor turn the fields are energized in turn (STEPS) to make a rotating field. The armature then follows this rotating field. In essence the "Commutation" or switching On/Off of the field coils is done electronically. This can be done with a driver circuit of a micro controller. The advantage of a servo motor is precise rotational control which is based upon the number of steps/rev and any associated gearing. Very precise movements can be performed as attested to by their use in hard drives and automation [6] [7].

Likewise, DC motors provide torque. A high torque DC motor like a car starter is series wound. Other DC motors like shunt wound are used in automation. The advantage with the use of a DC motor is easy speed control by easily varying the voltage applied to the motor. Rotational control of either may be done with some form of feedback mechanism such as a potentiometer connected to the shaft. The feedback is used as a signal to a motor controller which will stop the rotation of the shaft with the help of a control of the particular motor in use. One may also consider using the same motor you currently have and consider providing some means to control the motor controller which drives the motor, such as 4-20 ma input to a motor controller which is designed to drive either a DC motor or Servo motor. Only the existing motor and the mechanical setup need to be retained while the existing manual motor controller can be changed to a type that will interface to your application [9].

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