What is Servo Motor

Servo Motor Theory There are some special types of application of electrical motor where rotation of the motor is required for just a certain angle not continuously for long period of t

What is Servo Motor?

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What is Servo Motor?

This is nothing but a simple electrical motor , controlled with the help of

servomechanism If the motor as controlled device, associated with

servomechanism is DC motor, then it is commonly known DC Servo Motor

If the controlled motor is operated by AC, it is called AC Servo Motor

Servo Motor Theory

There are some special types of application of electrical motor where

rotation of the motor is required for just a certain angle not continuously for long period of time For these applications, some special types of motor are required with some special arrangement which makes the motor to rotate a

certain angle for a given electrical input (signal) For this purpose servo motor comes into picture This is normally a simple DC motor which is

controlled for specific angular rotation with the help of additional

servomechanism (a typical closed loop feedback control system) Now day’s servo system has huge industrial applications.

Servo motor applications are also commonly seen in remote controlled toy cars for controlling the direction of motion and it is also very commonly used

as the motor which moves the tray of a CD or DVD player Besides these, there are other hundreds of servo motor applications we see in our daily life The main reason behind using a servo is that it provides angular precision, i.e it will only rotate as much we want and then stop and wait for next

signal to take further action This is unlike a normal electrical motor which starts rotating as and when power is applied to it and the rotation continues until we switch off the power We cannot control the rotational progress of electrical motor, but we can only control the speed of rotation and can turn it

ON and OFF.

Now we come to the specific answer of the question "what is servo

motor?" Servo motor is a special type of motor which is automatically

operated up to certain limit for a given command with help of error-sensing

feedback to correct the performance

Servo Motor Controller or Servo Motor Driver

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A servo motor controller is a circuit that is used to control the

position of a servo motor It is also called as a servo motor driver A servo motor controller consists of a controller, the servo motor and the power supply unit.Servo motor driver may be used to control a single servo or even a group of servo motors In many projects where servo motor controlling is the mainstay of the task to be accomplished, the controller must drive more than one servo An example of this is an RC airplane, which uses many servos

Essential Components

1. A micro-controller

2. A power supply unit

Miscellaneous Components

1. A potentiometer

2. Connectors, wires etc

Micro-controller

A servo motor is driven by applying the voltage signal to it regular intervals The servo is sensitive to timing variations A pulse of

specific width has to be applied at specific intervals of time

Typically, the duration of pulse varies from 0ms to 2.2ms and the repetition rate is 50Hz to 60Hz For precise position control, the controller that is chosen must have timers that have the required resolution Also, if more than one motor has to be controlled

simultaneously, the processor clock must be fast enough For a

single motor control, an 8051 can be used like a AT89s51 or a

P89v51RD2 But for more than one motor, we must use a PIC, like a PIC18F or a ATMEGA, so that it’s internal PWM can be utilized

However, the selection of micro-controller depends totally on the designer and the project requirements

Power Supply

The design of the power supply unit servo motor controller depends

on the number of servo motors that are interfaced to the board Servo motors operate from 4.8V to a 6V supply voltage The typical value is 5v Applying voltages greater than the supply voltage is not advisable as it may render the motor permanently useless The

current draw of the motor is variable and depends on the torque that it generates Also it will draw less current when in idle mode and more current when it is running A servo motors maximum

current draw is given as its stall current This is the maximum

current it will draw when running with the maximum torque before

it stops due to overload This current value can be as high as 1 A for

some motors For a single motor control a voltage regulator like a LM317 can be used along with a suitable heat sink But when

multiple motors need to be interfaced, a high quality supply with higher current rating must be used A SMPS (Switched mode power supply) can be a good option Block Diagram below showing

interconnections in a Servo Motor Driver

Controlling Servo Motor

The servo motor has three terminals

1.

Position signal(PWM Pulses)

2. Vcc (From Power Supply)

3. Ground

The servo motor angular position is controlled by applying PWM pulses

of specific width The duration of pulse varies from about 0.5 ms for

0 degree rotation to 2.2 ms for 180 degree rotation The pulses

need to be given at frequencies of about 50Hz to 60Hz In order to generate the PWM (Pulse Width Modulation) waveform, as shown in figure below, one can use either the internal PWM module of the micro-controller or the timers can be used Using the PWM block is more flexible as most micro-controller families design the blocks to suit the needs of application like Servo motor For different widths of PWM pulses, we need to program the internal registers accordingly Now, we also need to tell the microcontroller how much it has to rotate For this purpose, we can use a simple potentiometer and use

an ADC to get the rotation angle or for more complex applications

an accelerometer can be used

Program Algorithm

Let us design the Program to control a single servo and the position input is given via the potentiometer connected to a pin of controller

1. nitialize the port pins for input/output

2. Read the ADC for desired servo position

3. Program the PWM registers for the desired value As soon as you trigger the PWM module, the selected PWM channel pin goes high (logic 1) and after the required width is reached, it will

again go low (logic 0) So after triggering the PWM, you must start a timer with a delay of about 19 ms and wait until the timer overflows

4. Go to step 2

There are various modes of PWM available which you can use

depending on the microcontroller you choose Some degree of

optimization should be done in the code to control the servo If you plan to use more than one servo than you will require as many PWM channels Each servo can be given the PWM signal sequentially But

you must take care that the pulse repetition rate for each servo is maintained Otherwise the servo will run out of synchronization Note If you plan on making your own board for the servo motor controller, give proper thickness for tracks carrying the current to the servo Proper ERC and DRC rule check must be followed The PWM signals for a

continuous rotation servo are not same as that of a 180 degree servo The servo datasheet should be consulted for such motors A servo motor is critical to voltage fluctuations and too high voltage may damage the

internal feedback control circuit So the power supply must be thoroughly designed to the servo specifications and checked before deployment Heat sink must be used if necessary.

DC Servo Motors | Theory of DC

Servo Motor

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As we know that any electrical motor can be utilized as servo motor

if it is controlled by servomechanism Likewise, if we control a DC motor by means of servomechanism, it would be referred as DC servo motor There are different types of DC motor, such shunt

wound DC motor, series DC motor, Separately excited DC motor, permanent magnet DC motor, Brushless DC motor etc Among all mainly separately excited DC motor, permanent magnet DC motor

and brush less DC motor are used as servo.

Separately Excited DC Servo Motor

DC Servo Motor Theory

The motors which are utilized as DC servo motors, generally have separate DC source for field winding and armature winding

The control can be archived either by controlling the field current or armature current Field control has some specific advantages over armature control and on the other hand armature control has also some specific advantages over field control Which type of control should be applied to the DC servo motor, is being decided depending upon its specific applications Let's discus DC servo motor working principle for field control and armature control one by one

Field Controlled DC Servo Motor Theory

The figure below illustrates the schematic diagram for a field

controlled DC servo motor In this arrangement the field of DC

motor is excited be the amplified error signal and armature winding

is energized by a constant current source

The field is controlled below the knee point of magnetizing saturation curve At that portion of the curve the mmf linearly varies with

excitation current That means torque developed in the DC motor is directly proportional to the field current below the knee point of magnetizing saturation curve

From general torque equation

of DC motor it is found that, torque T ∝ φIa Where, φ is field flux and Ia is armature current But in field controlled DC servo motor, the armature is excited by constant current source, hence Ia is

constant here Hence, T ∝ φ

As field of this DC servo motor is excited by amplified error signal, the torque of the motor i.e rotation of the motor can be controlled

by amplified error signal If the constant armature current is large

enough then, every little change in field current causes

corresponding change in torque on the motor shaft The direction of rotation can be changed by changing polarity of the field The

direction of rotation can also be altered by using split field DC

motor, where the field winding is divided into two parts, one half of the winding is wound in clockwise direction and other half in wound

in anticlockwise direction The amplified error signal is fed to the junction point of these two halves of the field as shown below The magnetic field of both halves of the field winding opposes each

other During operation of the motor, magnetic field strength of one half dominates other depending upon the value of amplified error signal fed between these halves Due to this, the DC servo motor rotates in a particular direction according to the amplified error

signal voltage

The main disadvantage of field control DC servo motors, is that the dynamic response to the error is slower because of longer time

constant of inductive field circuit The field is an electromagnet so it

is basically a highly inductive circuit hence due to sudden change in error signal voltage, the current through the field will reach to its steady state value after certain period depending upon the time constant of the field circuit That is why field control DC servo motor arrangement is mainly used in small servo motor applications The main advantage of using field control scheme is that, as the motor

is controlled by field - the controlling power requirement is much lower than rated power of the motor

Armature Controlled DC Servo Motor Theory

The figure below shows the schematic diagram for an armature

controlled DC servo motor Here the armature is energized by

amplified error signal and field is excited by a constant current

source

The field is operated at well beyond the knee point of magnetizing saturation curve In this portion of the curve, for huge change in magnetizing current, there is very small change in mmf in the motor field This makes the servo motor is less sensitive to change in field current Actually for armature controlled DC servo motor, we do not want that, the motor should response to any change of field current

Again, at saturation the field flux is maximum As we said earlier, the general torque equation of

DC motor is, torque T ∝ φIa Now if φ is large enough, for every little change in armature current Ia there will be a prominent

changer in motor torque That means servo motor becomes much sensitive to the armature current

As the armature of DC motor is less inductive and more resistive, time constant of armature winding is small enough This causes

quick change of armature current due to sudden change in armature voltage That is why dynamic response of armature controlled DC servo motor is much faster than that of field controlled DC servo motor

The direction of rotation of the motor can easily be changed by

reversing the polarity of the error signal

Permanent Magnet DC Servo Motor

Field control is not possible in the case of permanent magnet DC motor as the field is a permanent magnet here DC servo motor working principle in that case is similar to that of armature

controlled motor