First, What is the meaning of servo?
In modern usage the term servo or servo-mechanism is
restricted to a feedback control system in which the
controlled variable is mechanical position or time
derivatives of position such as velocity and acceleration.
A servo is a device, electrical, mechanical or electro
mechanical, that upon receipt of a stimulus or input, will
employ feedback for velocity and/or position control,
creating a closed loop.
A feedback system is a control system which tends to
maintain a prescribed relationship between a controlled
quantity and a reference quantity by comparing their
functions and using the difference as a means of control.
Electrical feedback control systems rely mainly on
electrical energy for operation. Important characteristics
usually required for such a control are 1)fast response,
2)high accuracy, 3)unattended control and 4)remote
The essential for such a control are 1)an error detecting
device, 2)an amplifier and 3)error correcting device, as
illustrated in following figure.
Each elements serves a functional purpose in matching
the regulated or controlled quantity to the reference
The error-detecting device determines when the
regulated quantity is different from the reference
It then sends out an error signal to the amplifier which
in turn supplies power to the error correcting device.
With this power the error correcting device changes
the regulated quantity so that it matches the reference
The closed loop at feedback control system is shown
The motors which respond to the error signal abruptly
and accelerate the load quickly are called the servo-
A servo-motor is motor which is a part of
servomechanism. It is typically paired with some type of
encoder to provide positioning and speed feedback and
some error correcting device which actuates the supply
The fundamental characteristics to be sought in any
1)The motor output torque should be proportional the
voltage applied to it.
2) The direction of the torque developed by the servo-
motor should be depend upon the instantaneous
polarity of the control voltage.
There are mainly two types of servo-motors,
1)AC Servo-motor 2)DC Servo-motor
AC servo-motors are generally preferred for low-
power use. And for high-power use DC servo-
motors are preferred because they operate more
efficiently than comparable to AC servo-motors
Unlike large industrial motors, dc servomotors are
not used for continuous energy conversion. The
basic operating principle is same as other
Design, construction and mode of operation are
different. The rotors of this kind of motor are
designed with long rotor lengths and smaller
diameters. They have large size than that of
conventional motors of same power ratings.
There are various types of dc servomotors which are
1)Series motors, 2)Split series motors, 3)Shunt
control motor & 4)Permanent magnet shunt motor.
Series motors: The series motor has a high starting
torque and draws large current .Speed regulation of this
kind of motor is poor . Reversal can be obtained by
reversing the polarity of field voltage with split series
field winding (i.e. one winding for direction of rotation).
This method reduces motor efficiency to some extent .
Split series motors: Split series motor are the dc
series motor with split-field rated with some fractional
kilowatt . This type of motor can operate as a separately
excited field-controlled motor. The armature is supplied
with a constant current source. Split series motor has a
typical torque-speed curve . This curve denotes high
stall torque and a rapid reduction in torque with
increase in speed. This results in good damping.
Shunt control motor: DC shunt type servomotor is
not different from any other dc shunt motor. It has two
separate windings: field windings placed on stator and
armature winding placed on the rotor of the machine.
Both windings are connected to a dc supply source. In a
conventional dc shunt motor, the two windings are
connected in parallel across the dc supply. In case of a
servomotor, the windings are supplied with separate dc
Permanent magnet shunt motor: Permanent
magnet shunt motor is a fixed excitation motor where
the field is actually supplied by a permanent magnet.
Performance is similar to armature controlled fixed field
motor that we are going to know in the next section.
It has construction as same as dc motor. It is consist
of stator and rotor and controlling parts.
It has feedback generator for generating feedback
for controlling the speed & torque.
It has two ports one for dc supply and other for
controlled dc supply.
In DC operation, servomotors are usually responds to
error signal abruptly and accelerate the load quickly. A DC
servo motor is actually an assembly of four separate
1. DC motor
2. Gear assembly
3. Position-sensing device
4. Control circuit.
Working principle of DC servomotor:
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
Which type of control should be applied to the
DC servo motor, is being decided depending upon
its specific applications.
In case of field controlled dc motor, the field is excited by
the amplified error signal mentioned earlier. The armature
winding is energized from a constant current source.
Torque developed is proportional to field current (Ifl) up to
This method is applied in small servomotors. It has longer
time constant owing to highly inductive field circuit so
dynamic response is slower than armature controlled dc
But in armature controlled dc motor, the motor armature
is energized by amplified error signal and field is supplied
from a constant current source. High field flux density also
increases torque sensitivity of motor (torque proportional
to φ Ia). Here dynamic response is faster because it has
shorter time constant of the resistive circuit.
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
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
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.
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
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
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.
1. The servo motor has some control circuits and a potentiometer (a
variable resistor, aka pot) that is connected to the output shaft.
2. The potentiometer allows the control circuitry to monitor the current
angle of the servo motor. If the shaft is at the correct angle, then the
motor shuts off.
3. If the circuit finds that the angle is not correct, it will turn the motor the
correct direction until the angle is correct.
4. The output shaft of the servo is capable of travelling somewhere around
180 degrees. Usually, its somewhere in the 210 degree range, but it
varies by manufacturer.
5. A normal servo is used to control an angular motion of between 0 and
180 degrees. A normal servo is mechanically not capable of turning any
farther due to a mechanical stop built on to the main output gear.
6. The amount of power applied to the motor is proportional to the
distance it needs to travel.
7. So, if the shaft needs to turn a large distance, the motor will run at full
speed. If it needs to turn only a small amount, the motor will run at a
slower speed. This is called proportional control.
Working using PCM:
1. The control wire is used to communicate the angle.
2. The angle is determined by the duration of a pulse that is applied to the control wire.
This is called Pulse Coded Modulation.
3. The servo expects to see a pulse every 20 milliseconds (.02 seconds). The length of
the pulse will determine how far the motor turns.
4. A 1.5 millisecond pulse, for example, will make the motor turn to the 90 degree
position (often called the neutral position).
5. If the pulse is shorter than 1.5 ms, then the motor will turn the shaft to closer to 0
6. If the pulse is longer than 1.5ms, the shaft turns closer to 180 degrees.
The torque-speed characteristics of separately excited dc servo-motor
is shown in bellow mentioned figure.
Their armature is deliberately designed
to have large resistance so that torque
speed characteristics are linear and
have large negative slope as shown in
figure. The negative slop serves the
purpose of providing the viscous
damping for the servo drive system
As shown in figure the armature mmf and excitation field mmf are in
quadrature this fact provides the fast torque response because
torque and flux become decoupled accordingly a step change in
armature voltage or current a quick change in the position or speed
of the rotor.
The motor output torque is
proportional to the voltage
applied to it (i.e. controlled
voltage developed by amplifier
in response to an error signal).
The instantaneous polarity of
control voltage governs the
direction of torque developed
The variation in torque-speed
characteristics due to variation
in temperature is shown in
Some other Characteristics:
DC servomotors share many performance specifications that are applicable to
all types of DC motors. To properly size a motor, these specifications must be
matched according to the load requirements of the application.
Shaft speed (RPM) defines the speed at which the shaft rotates,
expressed in rotations per minute (RPM). Typically, the speed provided by
the manufacturer is the no-load speed of the output shaft, or the speed at
which the motor's output torque is zero.
Terminal voltage refers to the design voltage of the DC motor. Essentially
the voltage determines the speed of a DC motor, and speed is controlled by
raising or lowering the voltage supplied to the motor.
Torque is the rotational force generated by the motor shaft. The torque
required for the motor is determined by the speed-torque characteristics of
the various loads experienced in the target application.
Starting torque - The torque required when starting the motor, which
is typically higher than the continuous torque.
Continuous torque - The output torque capability of the motor under
constant running conditions.
•Less than 1,610 rpm
•1,610 to 3,187 rpm
•3,187 to 4,700 rpm
•4,700 to 7,090 rpm
•7,090 rpm and up
•Less than 20 VDC
•20 to 50 VDC
•50 to 100 VDC
•100 to 180 VDC
•180 VDC and up
•Less than 1 amps
•1 to 4 amps
•4 to 8 amps
•8 to 17 amps
•17 amps and up
•Less than 0.45 Nm
•0.45 Nm to 1.70 Nm
•1.70 Nm to 5 Nm
•5 Nm to 17 Nm
•17 Nm and up
Continuous Output Power:
•Less than 0.4 HP
•0.4 to 1 HP
•1 to 2 HP
•2 to 6 HP
•6 HP and up
Some ratings of dc servo-motor available:
High output power relative to motor size and weight.
Encoder determines accuracy and resolution.
High efficiency. It can approach 90% at light loads.
High torque to inertia ratio. It can rapidly accelerate
Has "reserve" power. 2-3 times continuous power for
Has "reserve" torque. 5-10 times rated torque for
Motor stays cool. Current draw proportional to load.
Usable high speed torque. Maintains rated torque to
90% of NL RPM
Audibly quiet at high speeds.
Resonance and vibration free operation.
Requires "tuning" to stabilize feedback loop.
Motor "runs away" when something breaks. Safety circuits
Complex. Requires encoder.
Brush wear out limits life to 2,000 hrs. Service is then
Peak torque is limited to a 1% duty cycle.
Motor can be damaged by sustained overload.
Bewildering choice of motors, encoders, and servo-drives.
Power supply current 10 times average to use peak torque.
Motor develops peak power at higher speeds. Gearing
Poor motor cooling. Ventilated motors are easily
DC servomotors finds its applications in various domain. Some
of them are given below:
For very high voltage power systems, dc motors are
preferred because they operate more efficiently than
comparable ac servomotor.
It has also find its application in inkjet printers and RC
To drive conveyors used in Industrial manufacturing and
assembling units to pass an object from one assembly
station to another.
It is also used in solar tracking system.
DC servomotors are widely used in robots, toy cars and
other position controlled devices.
Widely used in radars, computers, robots, machine tools
tracking system, process controllers etc.