Servo Motors

This is nothing but a simple electric motor, controlled with the help of servomechanism. If the motor as a controlled device, associated with servomechanism is DC motor, then it is commonly known DC Servo Motor. If AC operates the controlled motor, it is called AC Servo Motor.

Servo Motor Theory

There are some special types of application of electric motor where rotation of the motor is required for just a certain angle. For these applications, we require some special types of motor 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.

The servo motor is usually a simple DC motor controlled for specific angular rotation with the help of additional servomechanism (a typical closed-loop feedback control system). Now day’s servo system has large 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 widely 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. The servo motor is unlike a standard electric motor which starts turning as when we apply power 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 to the question “what is servo motor?”

Servo motor is a special type of motor which is automatically operated up to a certain limit for a given command with the help of error-sensing feedback to correct the performance.

Servo Motor System

Servomechanism | Theory and Working Principle of Servo Motor

Servo Motor Working Principle

Before understanding the working principle of servo motor we should understand first the basic of servomechanism.

Servomechanism

A servo system mainly consists of three basic components – a controlled device, a output sensor, a feedback system. This is an automatic closed loop control system. Here instead of controlling a device by applying the variable input signal, the device is controlled by a feedback signal generated by comparing output signal and reference input signal.

When reference input signal or command signal is applied to the system, it is compared with output reference signal of the system produced by output sensor, and a third signal produced by a feedback system. This third signal acts as an input signal of controlled device. This input signal to the device presents as long as there is a logical difference between reference input signal and the output signal of the system. After the device achieves its desired output, there will be no longer the logical difference between reference input signal and reference output signal of the system. Then, the third signal produced by comparing theses above said signals will not remain enough to operate the device further and to produce a further output of the system until the next reference input signal or command signal is applied to the system. Hence, the primary task of a servomechanism is to maintain the output of a system at the desired value in the presence of disturbances.

Working Principle of Servo Motor

A servo motor is basically a DC motor (in some special cases it is AC motor) along with some other special purpose components that make a DC motor a servo. In a servo unit, you will find a small DC motor, a potentiometer, gear arrangement and an intelligent circuitry. The intelligent circuitry along with the potentiometer makes the servo to rotate according to our wishes. As we know, a small DC motor will rotate with high speed but the torque generated by its rotation will not be enough to move even a light load.

This is where the gear system inside a servomechanism comes into the picture. The gear mechanism will take high input speed of the motor (fast) and at the output, we will get an output speed which is slower than original input speed but more practical and widely applicable. Say at initial position of servo motor shaft, the position of the potentiometer knob is such that there is no electrical signal generated at the output port of the potentiometer. This output port of the potentiometer is connected with one of the input terminals of the error detector amplifier. Now an electrical signal is given to another input terminal of the error detector amplifier. Now difference between these two signals, one comes from potentiometer and another comes from external source, will be amplified in the error detector amplifier and feeds the DC motor.

This amplified error signal acts as the input power of the DC motor and the motor starts rotating in desired direction. As the motor shaft progresses the potentiometer knob also rotates as it is coupled with motor shaft with help of gear arrangement. As the position of the potentiometer knob changes there will be an electrical signal produced at the potentiometer port. As the angular position of the potentiometer knob progresses the output or feedback signal increases. After desired angular position of motor shaft the potentiometer knob is reaches at such position the electrical signal generated in the potentiometer becomes same as of external electrical signal given to amplifier. At this condition, there will be no output signal from the amplifier to the motor input as there is no difference between external applied signal and the signal generated at potentiometer. As the input signal to the motor is nil at that position, the motor stops rotating. This is how a simple conceptual servo motor works.

Servo Motor Control

Servo Motor Control

For understanding servo motor control let us consider an example of servomotor that we have given a signal to rotate by an angle of 45o and then stop and wait for further instruction. The shaft of the DC motor is coupled with another shaft called output shaft, with the help of gear assembly. This gear assembly is used to step down the high rpm of the motor’s shaft to low rpm at the output shaft of the servo system. servo motor

The voltage adjusting knob of a potentiometer is so arranged with the output shaft by means of another gear assembly, that during rotation of the shaft, the knob also rotates and creates an varying electrical potential according to the potentiometer. This signal i.e. electrical potential is increased with angular movement of potentiometer knob along with the system shaft from 0o to 45o. This electrical potential or voltage is taken to the error detector feedback amplifier along with the input reference commends i.e. input signal voltage. servo motor

As the angle of rotation of the shaft increases from 0o to 45o the voltage from potentiometer increases. At 45o this voltage reaches to a value which is equal to the given input command voltage to the system. As at this position of the shaft, there is no difference between the signal voltage coming from the potentiometer and reference input voltage (command signal) to the system, the output voltage of the amplifier becomes zero.

 

servo motor t

As per the picture given above the output electrical voltage signal of the amplifier, acts as input voltage of the DC motor. Hence, the motor will stop rotating after the shaft rotates by 45o. The motor will be at this rest position until another command is given to the system for further movement of the shaft in the desired direction. From this example we can understand the most basic servo motor theory and how servo motor control is achieved. NB: Although in practical servo motor control system, instead of using simple https://www.electrical4u.com/potentiometer-working-principle-of-potentiometer/ we use digital or analog position sensor encoder.

From this basic working principle of servo motor it can be concluded. The shaft of the servo is connected to a potentiometer. The circuitry inside the servo, to which the potentiometer is connected, knows the position of the servo. The current position will be compared with the desired position continuously with the help of an Error Detection Amplifier. If a mismatch is found, then an error signal is provided at the output of the error amplifier and the shaft will rotate to go the exact location required. Once the desired location is reached, it stops and waits.

Continuous Rotation Servo Motors

Continuous rotation servo motors are actually a modified version of what the servos are actually meant to do, that is, control the shaft position. The 360o rotation servos are actually made by changing certain mechanical connections inside the servo. However, a certain manufacturer like parallax sells these servos as well. With the continuous rotation servo you can only control the direction and speed of the servo, but not the position. Two of the most popular servo motor manufacturers are FUTABA and HITEC.

Servo Motor Controller or Servo Motor Driver

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 servo motor controller

Controlling Servo Motor

The servo motor has three terminals.

  1. Position signal(PWM Pulses)
  2. Vcc (From Power Supply)
  3. Ground

servo motor connector 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. PWM pulses for Servo Motor

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.

Servo Motor Applications in Robotics Solar Tracking System etc

Servo motor mechanism is used in a large number of applications which are critical in position control. We cannot use DC motor to control a tail of aircraft because controlling a DC motor for accurate positioning is not only difficult but almost impossible.

Servo Motor in Robotics

servo mechanism

One of the most popular servo motor applications is robotic. Consider a simple pick and place robot. Pick and place robot is such a robotic machine which is used to pick an object from one position and place the object at different position. Now, in order to pick an object from position A and place it in position B the motors which are used to actuate the joints are servo motors. This is because; we have to plan the angular movement of each and every joint to complete this task of pick and place. Once this data is fed to the robot controller, the robot will continuously do its job. The controller will send PWM data to the individual motors of the robot. This gives precise angular control of the arm which is not possible with a regular DC motor.

Types of DC Servo Motor

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.

Servo Motor in Conveyors

Conveyors are used in Industrial manufacturing and assembling units to pass an object from one assembly station to another. Let’s consider an example of bottle filling process, in the process the bottle needs to be filled with the liquid and moved to the next stage which is mainly the packaging stage. So in order to achieve this conveyor belts are used with servo motors so that the bottle moves precisely to the desired location and stops so that the liquid can be poured into it and then it is guided to the next stage. This process continues until stopped. Hence the precise position control ability of the servo shaft comes in handy.

Servo Motor Applications AS Camera Auto Focus

auto focus servo Today’s modern digital cameras are very advanced. One of the advanced features is its ability to auto focus on the object to be captured. When the image of the object is created within the digital signal processor of the camera, it is checked for sharpness. Basically, if the focal length (measured from camera lens) is not proper, the image appears to be blurred. The corrective action to position the lens precisely so that the sharpest image is captured is done using a highly precise servo motor fitted within the camera. This is another important example of servo motor applications.

Servo Motor in Robotic Vehicle

robotic vehicle The robotic vehicles today which are used for highly complex military as well as industrial applications use servo motors for wheels. Here, the angular movement is not of importance since the servo used is a continuous rotation servo. The ability of the servo to generate enough torque so as to quickly move the vehicle from halt and then to quickly stop it as well is what is important. Also the velocity with which the vehicle should move can be controlled.

Servo Motor in Solar Tracking System

Solar power generation and usage is gaining importance as people move towards clean and renewable energy regime. Earlier, Solar panels that were installed were static and remained in one position for the entire duration of the day. General Science dictates that the Sun is not always facing in one direction and that its position relative to the Solar panel will change. This implies that we are not fully utilizing the power of the sun to extract maximum energy out of it. But, if we attach servo motors to the solar panel s in such a way that we are able to precisely control its angle of movement so that it closely follows the Sun, then the overall efficiency of the system vastly increases. This is another application of servo where angle control is critical and achievable by a servo motor. Solar Tracking System

These are some of many Servo Motor Applications.

DC Servo Motors | Theory of DC Servo Motor

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.

Types of DC Servo Motor

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. dc servo motor

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. magnetizing 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. armature controlled dc servo motor 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. magnetizing-curve-2 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.

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