servomotors (Coursework Sample)

Servomotors are kind of rotary actuators that can have a precise control of velocity, acceleration and angular position. It comprises of a suitable motor and a sensor who both coupled to sense the position feedback. In most of the cases sophisticated controller is also used which is a dedicated module specifically designed for servo motors. Servomotors term is mostly used for a motor dedicated for usage in closed loop system as they don't fall in specific class of motors. Servomotors applications include process/industrial control, robots, CNC etc. Mechanism Servomotors are a close loop servomechanism which utilizes position feedback to control the motion and final position. It has an input of analogue or a digital signal according to which the commanding output is given to motor shaft. A type of Encoder is paired with the motor which give speed and position feedback. Position is measured in the simplest case. Measured position of the external input is compared to the controller, and output to the command position. If the position of the output is different from the required, error signal is produced inducing motor to rotate in either direction, which is required for appropriate positioning of the output shaft. The error signal approaches to zero as the position approach to required and motor finally stops. The simplest of the servomotors use only position sensing through bang-bang control and potentiometer of their motor; the motor rotation is always at full speed or stopped. Servomotors of such type are not widely used in industrial motion control, but they are the basis of cheap and simple servos used in radio-controlled models. Servomotors of sophisticated kind measure both the speed and the position of the output shaft. Not only this they can also control the motor speed, instead of always being at full speed. Both of these improvements, usually along with a PID controlalogorithm, bring the servomotor to its commanded position precisely and more quickly, with less overshooting. Stepper motors vs servomotors Stepper continuously consumes power to reach the commanded position and then to hold that position. Whereas, a servomotor consumes power for reaching the commanded position but after it they rests. Servomotors are regarded as high performance substitute of stepper motor. The stepper motor has built-in output steps, which makes it intrinsic ability to control position. Therefore, they are often used as an open-loop control without having a feedback encoder, because the drive signals given to it defines the number of steps for movement to rotate, but this requires the stepper motor on power up to make the controller ‘know' the position. Hence, the stepper motor will be activated by the controller, on first power up, and turn it to a known position, e.g. until the end limit switch is activated. When the inkjet printer is switch on, it can be observed that the controller will move ink jet carrier to extreme left and right for establishing the end positions. Now in case of servomotor, it will immediately turn to the angle instructed by the controller, irrespective of the initial position at power up. Performance of stepper motor is limited due absence of feedback feature, which make it to only drive the load which is well within its capacity, otherwise under load missed steps may generate positioning errors which may lead to system restart or recalibration. The controller and encoder makes servomotor costly, but it is compensated by the optimize performance of the overall system (for all of power, accuracy and speed) in relation to the capacity of the basic motor. Servomotors have the advantage in larger systems, where a powerful motor shares an increasing proportion cost of the system. In recent years, increasing popularity has been observed in used of closed loop stepper motors. They are similar to servomotor but have a difference in achieving the smooth motion through software control. Magnetic encoders as the feedback device have been employed by the top 3 manufactures of the closed loop system due to resistance to vibration and low cost. The closed loop stepper motor has the cost to performance ratio benefit. Also PID controller is not needed to be tune on a closed loop stepper system which saves time. Laser cutting machines can be offered in two ranges, the high-performance range using servomotors and low-priced range using stepper motors, along with many other applications. Encoders Encoders were made along with the first servomotors. During world war II, intensive work was done on these systems in development of anti-aircraft artillery and radar. Resistive potentiometers are used as position encoder in simple servomotors. They have a close competition with stepper motors and are mostly used at the cheapest and simplest level. During potentiometer track they have electrical noise and wear. Although the speed signal can be generated by electrically differentiating the position signal, PID controllers are a more precise encoder which utilizes the same speed signal. Rotary encoders, either incremental or absolute are used in current servomotors. At power on, absolute encoder can determine their position but these are expensive and complicated. However incremental encoders are cheap, work at faster speeds and simple in use. Stepped motors which is an incremental system, mostly combine simple zero sensor with their inherent ability of measuring rotation intervals to set position at start-up. In some cases a motor with a separate and external linear encoder is used instead of servomotors. Though these linear encoder systems with the addition of motors avoid erroneousness in the drive train between linear carriage and motor, their design is complicated as they are not a factory-made compact system. Motors In servomotor any type of motor may be used. Permanent magnet DC motors with brushes are the simplest to be used as they are simple and cheap. Servomotors used in small industry are typically electronically commutated motors without brushes. AC induction motors are used in large industrial servomotors, often allowing control of their speed with the help of variable frequency drives. Brushless AC motors having permanent magnetic field are used as a compact package for ultimate performance and large versions having DC motors. Standard industrial component is a drive module for servomotors. These modules are designed on the basis of three-phase MOSFET H Bridge. Their input is pulse count (rotation distance) and single direction. They may also have over torque, over temperature and stall detection features. As the Overall System dynamics and gear head ratio depends on the application, which makes the encoder type more difficult to produce the overall controller as an off-the-shelf module and hence these are mostly implemented as part of the main controller. DC motor Motor is a device which converts electrical energy into mechanical energy. Motor can be divided into two categories: 1 Alternating current (AC) motor 2 Direct current (DC) motor In a simple DC motor torque for rotation is produce by the interaction between the electricity and magnetic field. Permanent magnet DC motor is better than AC motor because it provide better control on speed with high torque and is used widely in industry. DC motors can be used with portable sources like batteries and solar cells which makes them more usable and provide cost effective solution because AC power supply is not available everywhere. DC motor shows response at both current and voltage. Current in the armature winding gives the torque while the voltage shows the speed of the motor. The load and current has a proportional relation if the load is increased the drawing current from supply will increase to maintain the speed constant and if supply does not provide the enough current the speed will reduce. In other words applied voltage effect speed and torque is controlled by current. If the chopping circuit is used the results drawn from DC motor become more effective. In lifting and transportation we mostly use low power DC motors as low power AC motors torque capability is not as good. DC motor are used in the application where torque cannot be compromised like electric cars, railway engines, car windows, robotic applications, elevators, complex industrial mixing process and wide variety of small appliances. There are several types of DC motor but most common are brushless DC motor, servo motor, stepper motor, brushed DC motor. DC motors have three winding techniques such as series DC motor, shunt DC motor and compound DC motor Fig 1.1 Parts DC motor Types of DC motor 1 Brushless DC motor: It is a DC current synchronous electric motor with electronic communication system. This motor does not have brushes or commentator like mechanical parts. There is a linear relationship between current to torque and voltage to rpm. In this type there is no permanent magnet on stator while motor have a rotating permanent magnet. Controller converts the DC current given to motor into AC current. Such kind of design is useful in removing difficulties like transferring power to rotor. It advantages include little maintenance, long life and high efficiency while more complicated controller and high capital cost are its disadvantages. AC motor and brushless motor are very similar to each other and both required an external communication for generating torque. These are futhrer divided into two subtype motors, 1 Reluctance motor 2 Stepper motor Reluctance motors are kind of motors which do not have a permanent magnet and its pole are pulled for alignment by moving stator drive. Whereas stepper motors are normally run by controllers and have many poles on stator. They are used mostly in highly precise CNC machines. 2.Brushed DC motor....