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Servo Motor and Stepper Motor C. Archana (12bec112)

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types and working of servo motors

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Servo Motor and Stepper Motor

C. Archana(12bec112)

SERVOMOTOR

What is SERVO MOTOR??

• It is 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.

Why SERVO MOTOR?

• 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 help of additional servomechanism.

• 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.

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 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 feedback system. This third signal acts as 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 output signal of the system. After the device achieves its desired output, there will be no longer logical difference between reference input signal and reference output signal of the system.

• The primary task of a servomechanism is to maintain the output of a system at the desired value in the presence of disturbances.

• 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.

• The 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.

• The angle is determined by the duration of a pulse that is applied to the control wire. This is called Pulse width Modulation. The servo expects to see a pulse every 20 ms. The length of the pulse will determine how far the motor turns. For example, a 1.5 ms pulse will make the motor turn to the 90 degree position (neutral position).

• When these servos are commanded to move they will move to the position and hold that position. If an external force pushes against the servo while the servo is holding a position, the servo will resist from moving out of that position. The maximum amount of force the servo can exert is the torque rating of the servo. Servos will not hold their position forever though; the position pulse must be repeated to instruct the servo to stay in position.

• When a pulse is sent to a servo that is less than 1.5 ms the servo rotates to a position and holds its output shaft some number of degrees counterclockwise from the neutral point.

• When the pulse is wider than 1.5 ms the opposite occurs. • The minimal width and the maximum width of pulse that will

command the servo to turn to a valid position are functions of each servo. Different brands, and even different servos of the same brand, will have different maximum and minimums. Generally the minimum pulse will be about 1 ms wide and the maximum pulse will be 2 ms wide.

Applications of SERVO MOTOR

• In ROBOTICS

• We can 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.

• In CONVEYOR BELTS

• 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.

• In Solar Tracking System

We attach servo motors to the solar panels 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.

• As Camera Auto Focus

• In modern digital cameras are very advanced. One of the advanced features is its ability to auto focus on the object to be captured 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.

Stepper Motor

It is an electromagnetic actuator. It is an incremental drive (digital) actuator and is driven in

fixed angular steps.This mean that a digital signal is used to drive the motor and every time it receives a digital pulse it rotates a specific number of degrees in rotation.

• Each step of rotation is the response of the motor to an input pulse (or digital command).

• •Step-wise rotation of the rotor can be synchronized with pulses in a command-pulse train, assuming that no steps are missed, thereby making the motor respond faithfully to the pulse signal in an open-loop manner.

• Stepper motors have emerged as cost-effective alternatives for DC servomotors in high-speed, motion-control applications (except the high torque-speed range) with the improvements in permanent magnets and the incorporation of solid-state circuitry and logic devices in their drive systems.

• •Today stepper motors can be found in computer peripherals, machine tools, medical equipment, automotive devices, and small business machines, to name a few applications.

TYPES OF MOTORS AVAILABALE

Stepper Motor Basics

STEP ANGLE

• The step angle, the number of degrees a rotor will turn per step, is calculated as

• For this motor:

ADVANTAGES OF STEPPER MOTORS

• Position error is noncumulative. A high accuracy of motion is possible, even under open-loop control.

• Large savings in sensor (measurement system) and controller costs are possible when the open-loop mode is used.

• Because of the incremental nature of command and motion, stepper motors are easily adaptable to digital control applications.

• No serious stability problems exist, even under open-loop control.

• Torque capacity and power requirements can be optimized and the response can be controlled by electronic switching.

• Brushless construction has obvious advantages.

DISADVANTAGES OF STEPPER MOTORS• They have low torque capacity (typically less

than 2,000 oz-in) compared to DC motors.• They have limited speed (limited by torque

capacity and by pulse-missing problems due to faulty switching systems and drive circuits).

• They have high vibration levels due to stepwise motion.

• Large errors and oscillations can result when a pulse is missed under open-loop control.

Stepping Sequence

• There is sequential excitation of the adjacent phases. Such a sequence is called as a stepping sequence.

• Half Step Sequence• Full Step Sequence• Microstepping

Half Step Sequence

Full Step Sequence

Half Step Sequence Full Step Sequence

Microstepping

• Both in full step mode and in half step mode complete windings are energised based on a certain pulse pattern. The motor rotates suddenly by a fixed angle. In microstepping mode the control electronics generate a PWM with very fine resolution, so that the windings are supplied with a constant current flow.

Applications of Stepper Motors

• Industrial Machines – Stepper motors are used in automotive gauges and machine tooling automated production equipments.

• Security - new surveillance products for the security industry.

• Medical – Stepper motors are used inside medical scanners, samplers, and also found inside digital dental photography, fluid pumps, respirators and blood analysis machinery.

• Consumer Electronics – Stepper motors in cameras for automatic digital camera focus and zoom functions.

• And also have business machines applications, computer peripherals applications.

Thank You!!