DC

SERVO-MOTOR

Guided by: Prof. J.I.PATEL

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 operation.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.

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Each elements serves a functional purpose in matching the regulated or controlled quantity to the reference quantity.

The error-detecting device determines when the regulated quantity is different from the reference quantity.

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 input.

The closed loop at feedback control system is shown in figure.

Servo-motor:The motors which respond to the error signal abruptly and accelerate the load quickly are called the servo-motors. 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 signal. The fundamental characteristics to be sought in any servo-motors 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.

ContinuedThere 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

DC Servo-motor:Unlike large industrial motors, dc servomotors are not used for continuous energy conversion. The basic operating principle is same as other electromagnetic motors.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 source. 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.

Construction: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.

Construction:

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 components, namely:1. DC motor2. Gear assembly3. Position-sensing device4. 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 control. Which type of control should be applied to the DC servo motor, is being decided depending upon its specific applications.

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ContinuedIn 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 saturation level. 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 motor.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 φI∝ a. 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.

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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.

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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

φI∝ a. 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.

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Working: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

degrees.6. If the pulse is longer than 1.5ms, the shaft turns closer to 180 degrees.

Torque-Speed Characteristics: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 by servomotors.The variation in torque-speed characteristics due to variation in temperature is shown in graph.

Some other Characteristics:

Some controlling circuits used:

Feedback sensor used:

Performance Specifications: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.

Shaft Speed: •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

Terminal Voltage: •Less than 20 VDC •20 to 50 VDC •50 to 100 VDC •100 to 180 VDC •180 VDC and up

Continuous Current: •Less than 1 amps •1 to 4 amps •4 to 8 amps •8 to 17 amps •17 amps and up

Continuous Torque: •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:

Some name-plates 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 loads.Has "reserve" power. 2-3 times continuous power for short periods.Has "reserve" torque. 5-10 times rated torque for short periods.Motor stays cool. Current draw proportional to load.Usable high speed torque. Maintains rated torque to 90% of NL RPMAudibly quiet at high speeds.Resonance and vibration free operation.

Advantages:

Requires "tuning" to stabilize feedback loop.Motor "runs away" when something breaks. Safety circuits are required.Complex. Requires encoder.Brush wear out limits life to 2,000 hrs. Service is then required.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 often required.Poor motor cooling. Ventilated motors are easily contaminated.

Disadvantages:

Applications: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 helicopters.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.

Some animations showing Applications:

Thank

You

Prepared by:Solanki Jeegnesh (120230109048) Pandya Uday (120230109049) Dr. S. & S.S. Ghandhy Government Engineering College, Surat