CNC FEED DRIVES

contents 1.Introduction 2.Requirements of CNC feed drives 3.Servo motor 3.1 Servo drive control 3.2 Components of servo controlled cnc. 3.3 servo-drive control 4.DC Servo Motor 5. AC Servo Motor 6.Comparison between D.C. And A.C. Servomotor 7. Stepping Motor 7.1 Construction 7.2 Working 7.3 Advantages 7.4 Disadvantages 8.Reference

1.INTRODUCTION The driving system is an important component of a CNC machine as the accuracy and repeatability depend very much on the characteristics and performance of the driving system. The driving system is classified as spindle drives and feed drives. The spindle drives are used to provide angular motion to the work piece or a cutting tool. The feed drives are used to drive the slide or a table. The feed drive consists of an electromotor and mechanical transmission elements.

Fig 1.working principle of spindle drives Fig 2.Working principle of CNC feed drive

This system usually uses electric motors although hydraulic motors are sometimes used for large machine tools. Electric Motors An electric motor is an electric machine that converts electrical energy into mechanical energy Most electric motors operate through the interaction between an electric motor's magnetic field and winding currents to generate force within the motor. Hydraulic motor A hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement (rotation)

The main purpose is to move the working parts of machine tool (working table, tool unit, spindle unit etc.) The requirement is that the driving system has to response accurately according to the programmed instructions. The motor is coupled either directly or through a gear box to the machine lead screw to moves the machine slide or the spindle. Two types of electrical motors are commonly used. Servomotors. DC Servomotors. AC Servomotors. Stepper motors.

2.Requirements of cnc feed drives. High torque-to-weight ratio Low armature or rotor inertia Low electrical and mechanical time constants Permanent magnet construction Maximum speed up to 3000rpm High peak torque for quick response The required constant torque for overcoming frictional and working force must be provided.

3.Servo motor Positions mechanical elements within a given time and with a given precision. Operates at low power ranges. Factors to be considered in servo action are : Dynamics Position exactness Peak torque Speed regulation range etc

3.1 servo drive control

3.2 Components of servo controlled cnc Motor lead screw rotation table moves Motor speed control feedback position sensed by encoder

3.3 servo-drive control Working of a servo motor : The desired position is given as input to the system. The actual position is measured by the encoder is given to position controller as feedback. This results in the desired speed which is compared with actual speed feedback by the Tachogenerator. This comparison gives out the output of the desired current to be compared with the feedback of actual current. The current controller gives the output of control voltage to the motor. A six pulse-bridge supplied with a 3-phase system gives the direct voltage depending on the control voltage.

4.DC Servo Motor This is the most common type of feed motors used in CNC machines The principle of operation is based on the rotation of an armature winding in a permanently energized magnetic field. The armature winding is connected to a commutator, which is a cylinder of insulated copper segments mounted on the shaft DC current is passed to the commutator through carbon brushes, which are connected to the machine terminals. The change of the motor speed is by varying the armature voltage and the control of motor torque is achieved by controlling the motor's armature current. In order to achieve the necessary dynamic behavior it is operated in a closed loop system equipped with sensors to obtain the velocity and position feedback signals.

4.1 Features of dc servomotor Smooth rotation at speed less than one rpm. Bush life of more than 4000 rpm. Sensor less switching. Transistor controller and diode rectifier. Resolver for speed control and position control and switching control. Resolver ® performs the task of both tachogenerator and pulse-generator (p-encoder).

Fig 3.Working of DC Servomotor

Fig4. Control of DC Servomotor

5. AC Servo Motor In an AC servomotor, the rotor is a permanent magnet while the stator is equipped with 3-phase windings The speed of the rotor is equal to the rotational frequency of the magnetic field of the stator, which is regulated by the frequency converter. AC motors are gradually replacing DC servomotors The main reason is that there is no commutator or brushes in AC servomotor so that maintenance is virtually not required Furthermore, AC servos have a smaller power-to-weight ratio and faster response.

Fig 5.Working principle of AC Servo motor

6.comparison between d.c. and a.c. servomotor

7. Stepping Motor A stepping motor is a device that converts the electrical pulses into discrete mechanical rotational motions of the motor shaft This is the simplest device that can be applied to CNC machines since it can convert digital data into actual mechanical displacement

7.1 construction Stator has a winding made of concentrated coils on distinct poles. Rotor has a permanent magnet cylinder. The stator windings are inserted on the periphery of the yoke and this is slide on the permanent magnet pole cylinder. The rotor shaft is located at the center of the yoke, mounted on bearings Examples of stepping motor application are the magnetic head of floppy-disc drive and hard disc drive of computer, daisy-wheel type printer, X-Y tape control, and CNC EDM Wire-cut machine

Fig 6 STEPPER MOTOR.

Fig 7 working of stepper motor The rotation of the magnet inside the yoke will energize the poles Fig 7 working of stepper motor

7.3 advantages It is not necessary to have any analog-to-digital converter nor feedback device for the control system. They are ideally suited to open loop systems Low cost Ruggedness Simplicity in construction High reliability No maintenance

7.4 disadvantages Slow speed Low torque Low resolution Easy to slip in case of overload

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