NUMERICAL CONTROL OF MACHINE TOOLS (DE) ME453
UNIT 2
CNC machine tools
Accuracy, precision, resolution and structure Drives
Feedback devices
Actuators reciprocating ball screws linear motion systems
rotary and linear transducers DNC
Adaptive control systems (AC)
Unit 2
PROBLEM WITH NC MACHINE
• Part programming mistakes:
– In preparing the punched tape, part programming mistakes are common.
– The mistakes can either be syntax or numerical errors.
• Non optimal speeds and feeds:
– In NC machines the control system does not provide the opportunity to make changes in speeds and feeds during the cutting process.
• Punched tape:
– Another problem related to programming is the tape itself.
• Tape reader:
– The tape reader that interprets the punched tape is the least reliable hardware component of the machine.
• Controller:
– The NC controller unit is hard wired. This means that its control features cannot be easily alerted to incorporate improvements into the unit.
INTRODUCTION: CNC MACHINES
• What is a CNC Machine?
• CNC : Computer and Numeric Control
•
Conventionally, an operator decides and adjusts various machines parameters like feed , depth of cut etc. depending on type of job , and controls the slide movements by hand. In a CNC machine functions and slide movements are controlled by motors using computer programs.• 4 Axis VMC
7Vertical knee milling machine
Drilling machines
Vertical boring machines
• Computer numerical control is an NC system that utilizes a dedicated, stored program computer to perform some or all of the basic numerical control functions.
• By integrating a computer processor, computer numerical control, or “CNC” as it is now known, allows part machining programs to be edited and stored in the computer memory as well as permitting diagnostics and quality control functions during the actual machining.
• All CNC machining begins with a part program, which is a sequential instructions or coded commands that direct the specific machine functions.
• The part program may be manually generated or, more commonly, generated by computer aided part programming systems.
INTRODUCTION: CNC MACHINES
• CNC can control the motions of the work-piece or tool, the input parameters such as feed, depth of cut, speed, and the functions such as turning spindle on/off, turning coolant on/off.
• Dedicated microprocessors are built into the control to perform basic and advanced NC functions.
• Control signals in CNC systems are in the form of binary words, where each word contains fixed number of bits.
• 32 bits or 64 bits are commonly used, representing different axial positions.
INTRODUCTION: CNC MACHINES
CNC machines offer the following advantages in manufacturing.
• Higher flexibility: This is essentially because of programmability, programmed control and facilities for multiple operations in one machining center.
• Increased productivity: Due to low cycle time achieved through higher material removal rates and low set up times achieved by faster tool positioning, changing, automated material handling etc.
• Improved quality: Due to accurate part dimensions and excellent surface finish that can be achieved due to precision motion control and improved thermal control by automatic control of coolant flow.
• Reduced scrap rate: Use of Part programs that are developed using optimization procedures
• Reliable and Safe operation: Advanced engineering practices for design and manufacturing, automated monitoring, improved maintenance and low human interaction
INTRODUCTION: CNC MACHINES
On the other hand, the main disadvantages of CNC systems are
• Relatively higher cost compared to manual versions
• More complicated maintenance due to the complex nature of the technologies
• Need for skilled part programmers.
• The above disadvantages indicate that CNC machines can be gainfully deployed only when the required product quality and average volume of production demand it.
INTRODUCTION: CNC MACHINES
COMPONENTS OF CNC SYSTEMS
Any CNC system consists of following components:
1. Part program
2. Program input device
3. Machine control unit
4. Drive system 5. Machine tool
6. Feedback system
CNC MACHINES
MACHINECONTROLLERUNIT MCU
The machine control unit (MCU) is the backbone of CNC systems.
Consists of some electronic hardware that reads the NC program, interprets it and conversely translates it for mechanical actions of the machine tool.
Following six functions are being done by MCU:
• Read coded instructions
• Decode coded instructions
• Implement interpolations to generate axis motion commands
• Feed axis motion commands to amplifier circuits to drive axis mechanisms
• Receive the feed back signals of position and speed for each drive axis
MACHINE CONTROLLER UNIT MCU
Dell 2135CN & 2155CN Machine Control Unit Board (MCU)
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MACHINE CONTROLLER UNIT MCU
The MCU may be of three types :
• Housed MCU Machine Control Unit
May be mounted on the machine tool or may be built in the casing of the
Machine.
• Swing Around MCU Machine Control Unit 107
It is directly mounted on the machine, which can swing
• Machine Control Unit
It is enclosed in a separate cabinet which is installed at some remote or same place near to the machine.
105
Housed MCU Machine Control Unit
106
Machine Control Unit
Swing Around MCU Machine Control Unit
(G&M CodeProgram)
ComputerNumericalControl Master ControlUnit
The Axis amplifier
increases the electrical current from the CNC
control unit
MCU
•The CNC program contains all the G&M code of that is interpreted by the motion control unit.
•When the machine is instructed to move to a position in X, Y (lathe), or X,Y and Z (for a 3 axis machine) then a designated number of pulses are sent to the axis drive amplifier.
•This in turn pulses the drive motor which moves the machine table or axis.
•There is axis feedback from the drive motor which is fed back into the axis drive amplifier to ensure that the motor has moved the
correct amount of divisions.
•As the machine table or tool moves there is a linear encoder which reads the actual position of the table/tool and feeds this back to the motion control unit.
•This will be compared to the original value and the control unit will adjust the amount of pulses needed to move the tool/table to the correct position.
MACHINE CONTROLLERUNIT MCU
A TYPICAL CNC LATHE/MILLING MACHINE PROGRAM BEING INTERPRETED BY THE MACHINE CONTROL UNIT
CNC Program G and M codes
G00 X20 z30
T01
M04 S200
G01
Send signals to servo interface for
X and Z axis
Servo interface to amplify signals
Servo motors move axis
Encoder sends
Positional data To control unit
Are axis at
positi ye on?
s Request
machine to have T01 fitted
Is T01 Fitted ?
ye s
Electrical pulse to
Tool change Is T01
Fitted? yes no
Send signals to Spindle
servo interface
Servo interface to amplify signals
Servo motors to rotate spindle
Encoder sends
Positional data To control unit
Is
spindle at speed?
ye no s
Send signals to
servo interface for Servo interface Servo motors Encoder
sends Are axis
TYPES OF CNC MACHINE CONTROL UNITS
FANUC
CONTROLL
SIEMENS
GSK
MECH 3 etc
CNC MACHINES
FUNCTIONS:
• The principles function of CNC are:
– Machine Tool Control
– In-process compensation
– Improved programming and operating features
– Diagnostics
– Straight CNC 27
CNC MACHINES
FUNCTIONS: Machine Tool Control
• The primary function of the CNC system is control of the machine tool.
• This involves conversion of the part program instructions into machine tool motions through the computer interface and servo- system.
• The capability to conveniently incorporate a variety of control features into the soft-wired controller unit is the main advantage of CNC.
• Some of the control function, such as circular interpolation can be accomplished more efficiently on the hard-wired circuits from the computer.
• This fact has lead to the development of two alternative controller designs to CNC .
– Hybrid CNC
CNC MACHINES
FUNCTIONS: Machine Tool Control: Hybrid CNC
• In Hybrid CNC system, the controller consists of soft-wired computer plus hard wired logic circuits.
• In hard-wired components perform the function which they do best such as feed rate generation and circular interpolation.
• Use of these hard-wired circuits saves the computer from performing less calculation and hence less expensive computer is required in hybrid CNC controller.
CNC MACHINES
FUNCTIONS: Machine Tool Control: Straight CNC
• It uses a computer to perform all the CNC functions.
• The only hard-wired elements are those required to interface the computer with machine tool and the operator’s console.
• Interpolation, tool position feedbacks and all other functions are performed by computer software.
• Hence the computer required in a straight CNC system must be more powerful than that needed for a hybrid system.
• The advantage gained in the straight CNC configurations is additional flexibility.
CNC MACHINES
FUNCTIONS: In-process compensation
• This involves the dynamic correction of the machine tool motions for changes or errors which occur during processing.
• Some of these in-process compensation are:
– Adjustments for errors sensed by in-process inspection probe and gauges.
– Re-computation of axis positions when an inspection probe is used to locate a datum reference on a work piece.
– Offsets adjustments for tool radius and length.
– Adaptive control adjustments to speed and/or feed.
CNC MACHINES
FUNCTIONS: Improved programming and operating features
• The flexibility of the soft-wired has permitted the introduction of many convenient programming and operating features.
• Some of these are:
– Editing of part program on the machine.
– Graphic display of the of the tool path to verify the tape.
– Use of especially written subroutine.
– Manual data input (MDI).
• CNC machine tools are complex and expensive system.
• The complexity increases the risk of component features which lead to system down time.
• CNC machines are often equipped with a diagnostics capability to assist in maintaining and repairing the system.
CNC MACHINES
FUNCTIONS: Diagnostics
PARTS SUITABLE FOR CNC MACHINES
• Aerospace equipments.
• Automobile Parts.
• Complex shapes.
• Electronic industry uses CNC e.g.
Printed circuit board.
• Electrical industry uses CNC e.g. Coil winding.
• For small to medium batch quantity.
• Where the set-ups are very large.
• Where the tool storage is a problem.
• Where much metal needs to be removed.
• When the part geometry is so complex.
• The operations are very complex. 33
is a measure of the spread of different Precision:
• Precision readings.
• Precision and accuracy are unrelated to each other, meaning that you can be very PRECISE but not ACCURATE.
• Precision is also as a synonym for the resolution of the measurement.
• Measurement that can distinguish the difference between, 0.01 and 0.02 is more precise (has a greater resolution) than one that can only tell the difference between 0.1 and 0.2 even though they may be equally
PRECISION IN NC POSITIONING
The Green is more precise than the Red.
Three measures of precision:
• Accuracy
• Control resolution
• Repeatability
• Accuracy:
The “accuracy” of a measurement refers to how close a measurement is to a “true” (actual) value.
Maximum possible error that can occur between the desired target point and the actual position taken by the system.
This is the Actual value. Accuracy is the distance of the
measurement from the Actual value. The Red is more accurate than the Green.
Resolution:
Resolution refers to the smallest change that a sensor
can detect in the quantity it is measuring.
Repeatability:
The ability of an operator to consistently repeat the same measurement of the same part, using the same gauge, under the same conditions.
Operator 1 measures a single part with Gauge A 20 times, and then measures the same part with Gauge B.
The solid line is the measurements from Gauge A. The dashed line is the measurements from Gauge B.
Gauge A has less variation, so it is more repeatable than Gauge B.
Considerations
• Sustain loads over long periods of time.
• Vibration problem in machining process.
• Large quantity of swarf should be handled which requires handling of hot chips along with thermal distortions.
• Higher magnitudes of multidirectional forces.
STRUCTURE
Structure of an NC machine tool links all it’s components
and is the basic platform on which all these components
are laid upon.
STRUCTURE
Consequences
• NC machines tend to be heavier structures as they have to provide adequate strength and rigidity.
• To avoid vibration problems the structure is properly damped by means of spring, mass and dampers.
• Overhangs of cutting forces, motor forces and frictional forces are to be reduced.
• Implementation of proper heat management arising within the machine tool due to drives and pumps.
• Designs involving better swarf removal, like Slant bed
are employed.
STRUCTURE
Consequences
• Air conditioning is often maintained to eliminate the distortion of the structure due to variations of environmental temperatures.
• Alloyed Iron and fabricated steel structures are used the structure.
• They are made of cast iron.
DRIVES
Drives are the units which provide the desired cutting speed at the main spindle.
• Basic function of a NC/CNC machine is to provide automatic and precise motion control to its elements such work table, tool spindle etc.
• Drives are used to provide such kinds of controlled motion to the elements of a NC/CNC machine tool.
• A drive system consists of drive motors and ball lead- screws.
• The MCU sends the amplified control signals to
actuate drive motors which in turn rotate the ball lead-
screws to position the machine table or cause rotation
of the spindle.
TYPES OF DRIVES
• Electrical drives
Small in size and easy to control. They have the following types
• Direct current (DC)
• Alternating current (AC)
• Hydraulic drives
Have large power to size ratio and provide step-less motion with great accuracy.
- Difficult to maintain and are bulky.
- Petroleum based hydraulic oil is employed which may have fire hazards at upper level of working temperatures.
- Hydraulic elements need special treatment to protect them
• Pneumatic drives
- This drives use air as working medium which is available in abundant and is fire proof.
- Although simple in construction and quite cheap,
these drives generate low power having less
positioning accuracy and are noisy.
DRIVES IN CNC MACHINE TOOLS
Schematic of a spindle drive
The various drives used in CNC machines can be classified as:
a) Spindle drives to provide the main spindle power for cutting action b) Feed drives to drive the axis
Spindle drives
Provide angular motion to the workpiece or a cutting tool.
Requirements
• To maintain the speed accurately within a power band which will enable machining of a variety of materials with variations in material hardness.
• The speed ranges from 10 to 20,000 rpm.
• High overload capacity.
• Compact drive with highly smooth operation.
•
DRIVES IN CNC MACHINE TOOLS
Schematic of a Feed drive
Feed drives
These are used to drive the slide or a table.
Requirements
• The feed motor needs to operate with constant torque characteristics to overcome friction and working forces.
• Extremely variable drive speed and small positioning resolution..
• The feed motor must run smoothly.
• Low rotor inertia and quick response in case of contouring operation where to work several feed drives have
simultaneously.
• High torque to weight ratio.
ELECTRICAL DRIVES
Electric drives are mostly used in position and speed control systems.
DRIVES IN CNC MACHINE TOOLS
DC MOTORS
• A DC motor is a device that converts direct current (electrical energy) into rotation of an element (mechanical energy).
DC MOTORS
BRUSHED DC MOTORS
BRUSHLESS
DC MOTORS
DRIVES IN CNC MACHINE TOOLS
DRIVES IN CNC MACHINE TOOLS
Brushed Motor
BRUSHED SERVOMOTORS
• This type of motor produces a magnetic field in a wound rotor (the part that rotates) by passing an electrical current through a commutator and carbon brush assembly.
• This is achieved by constructing the armature as a series of small sections connected in sequence to the
• To produce a constant torque from the motor, electric & magnetic fields must remain constant in magnitude and in relative motion.
• DC servomotors are high performance motors and are useful as prime movers in numerically controlled machine tools where starts and stops must be made quickly and accurately.
• The light weight and low inertia armatures of DC servomotors respond quickly to the excitation voltage changes.
• Brushes are used to connect the rotor winding.
• Brush wear occurs, and it increases dramatically in low‐pressure environment.
• Sparks from the brushes may cause explosion if the environment contains explosive materials.
DRIVES IN CNC MACHINE TOOLS
Brushless Motor
BRUSHLESS SERVOMOTORS
• The rotor becomes a permanent magnet and the stator becomes a wound iron core.
• Construction speeds up heat dissipation and reduces rotor inertia.
• The major difference is that the brush less motor maintains position by electrical commutation, rather than mechanical commutation.
• Typical components of AC motors are stator and a rotor.
• The stator is the stationary electrical component.
• It consists of a group of individual electro-magnets arranged in such a way that they form a hollow cylinder, with one pole of each magnet facing toward the center of the group.
• The rotor is located inside the stator and is mounted on the AC motor's shaft. The rotor then is the rotating part of the AC motor.
DRIVES IN CNC MACHINE TOOLS
AC MOTORS
•The objective of these motor components is to make the rotor rotate which in turn will rotate the motor shaft.
•If you progressively change the polarity of the stator poles in such a way that their combined magnetic field rotates, then the rotor will follow and rotate with the
As shown in the figure, the stator has six magnetic poles and the rotor has two poles.
At time 1, stator poles A-1 and C-2 are north poles and the opposite poles, A-2 and C-1, are south poles. The S-pole of the rotor is attracted by the two N-poles of the stator and the two south poles of the stator attract the N-pole of the rotor.
At time 2, the polarity of the stator poles is changed so that now C- 2 and B-1 are N-poles and C-1 and B-2 are S-poles. The rotor then is forced to rotate 60 degrees to line up with the stator poles as shown.
At time 3, B-1 and A-2 are N- pole. At time 4, A-2 and C-1 are N. As each change is made, the opposite poles on the stator attract the poles of the rotor. Thus, as the magnetic field of the stator rotates, the rotor is forced to rotate with it.
SYNCHRONOUS MOTORS
• A synchronous motor is an AC motor which runs at constant speed fixed by frequency of the system and irrespective of load acting on them.
• Synchronous motor are have high efficiency and are mainly used in high precision application.
• 3 phase AC supply is given to stator to create rotating magnetic field in stator.
• The rotor winding is fed with dc supply which magnetizes the rotor.
• The stator poles are rotating with synchronous speed, and they rotate around very fast and interchange their position.
DRIVES IN CNC MACHINE TOOLS
•The constant speed characteristics is achieved by interaction between a constant and rotating magnetic field.
•Rotor of a synchronous motor produces constant magnetic field and stator produces revolving magnetic field.
•The field coil stator is excited by a three phase AC supply. This will produce a revolving magnetic field which rotates at synchronous speed.
N
S= 120 f /P
NS = synchronous speed
f = frequency of electricity P = No. of poles
• The main advantage of this motor is the low rotor inertia, high power and low weight.
• So called because voltage is induced in the rotor, but for this to happen, the rotor must rotate at a lower speed than the magnetic field to allow for the existence of an induced voltage.
INDUCTION MOTOR
• However, most AC motors used today are not synchronous motors.
Instead, so-called "induction" motors.
• Induction motors are quite commonly used in industrial automation.
• So how is an induction motor different? The big difference is the manner in which current is supplied to the rotor. There is no external power supply.
DRIVES IN CNC MACHINE TOOLS
• When a conductor (aluminum bars in the case of a rotor, see Figure) is moved through an existing magnetic field or when a magnetic field is moved past a conductor, the relative motion of the two causes an electric current to flow in the conductor.
• This is referred to as "induced" current flow. In an induction motor the current flow in the rotor is not caused by any direct connection of the conductors to a voltage source, but rather the influence of
65
• The induced current that is produced in the rotor results in a magnetic field around the rotor conductors as shown in Figure.
• This magnetic field around each rotor conductor causes each rotor conductor to act like the permanent magnet. As the magnetic field of the stator rotates, due to the effect of the three- phase AC power supply, the induced magnetic field of the rotor is attracted and will follow the rotation. The rotor is connected to the motor shaft, so the shaft rotates and drives the connection load.
• The main limitation of AC motors over DC motors is that speed is more difficult to control in AC motors.
• To overcome this limitation, AC motors are equipped with variable frequency drives but the improved speed control comes together with a reduced power quality.
SERVOMOTOR
Electric Motor + Servomechanism
Servomotor
DC Motor + Servomechanism
DC Servomotor AC Motor + Servomechanism
AC Servomotor
64
65
angular
• The main reason for using servomotor is to obtain precision.
• Normal electric motor starts rotating as when power is supplied and rotation continues unless power is switched off.
• With servomotor the desired angular position is obtained.
Suppose one wants to rotate the shaft from 0 to 35 degree. The shaft will stop at 35 degree and there will be no further movement.
Servomechanism
2 1 Control Device
Output signal of the system
feedback Output sensor
Reference Input Signal
Error detector amplifier
• Servomechanism is an automatic closed loop feedback control system.
Motor
Servo-system consists of the following components:
1.Control device 2.Output sensor 3.Feedback system
3
1
2 The signal 1st and 2nd
are compared and the 3rd signal from the error detecting amplifier is fed to the control device which is the motor.
• Suppose we want to rotate the shaft from 0 to 5 degree and then from 5 to 45 degree.
• Once it reaches 45 degree the output signal from the output sensor will be automatically equal to the Reference electrical Input .
• When they become equal the difference between signal 1st and 2nd is zero and there will be no signal to be amplified hence there will be no signal to power the control device. Therefore motor stops rotating.
• And to produce a further action by the control dvice it requires a 3rd signal.
• As long as there is a logical difference between 1st and 2nd the 3rd signal will be generated and once the motor achieves i6t7s desired output the difference will get zero.
• This is SERVOMECHANISM.
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• Servo control consists of all the activities, which allow several axes to effectively maintain the trajectory calculated by the interpolator.
• In CNC systems the position and velocity of the machine tool axes must be controlled closely and in a coordinated manner.
• Each axis is separately driven and follows the command signal produced by the interpolator.
• The control system can be either open-loop (as in PTP systems) or closed-loop (as in contouring systems).
DC SERVOMOTORS
Working Principle of DC 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.
SPECIAL PURPOSE COMPONENTS
POTENTIOMETER
MAKES THE SERVO 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.
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.
GEAR ARRANGEMENT
INTELLIGENT CIRCUITRY
Potentiometer acts as transducer or output sensor which takes position of the input of shaft of the motor and generates voltage signal as output.
Working Principle of DC Servo Motor
• 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 reached 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.
DCServoMotor
DC Servo Motor
• The motor which is used as a DC servo motor generally have a separate DC source in the field of winding & armature winding.
• The control can be archived either by controlling the armature current or field current. Based on the applications the control should be applied to the DC servo motor.
• DC servo motor provides very accurate and also fast respond to start or stop command signals due to the low armature inductive reactance.
• DC servo motors are used in CNC machines and similar equipments.
AC Servo Motor
• AC servo motor is an AC motor that includes encoder used with controllers for giving closed loop control and feedback.
• This motor can be placed to high accuracy and also controlled precisely as compulsory for the applications.
• Frequently these motors have higher designs of tolerance or better bearings and some simple designs also use higher voltages in order to accomplish greater torque.
• Applications of an AC motor mainly involve in automation, robotics, CNC machinery, and other applications a high level of precision and needful versatility.
ACServo Moto 33
34
Servo Motor
35
36
Cutaway view through stator of induction motor.
STEPPER MOTORS
• A stepper motor is a pulse-driven motor that changes the angular position of the rotor in steps.
• A stepper motor is a device that converts the electrical pulses into discrete mechanical rotational motions of the motor shaft.
• Due to this nature of a stepper motor, it is widely used in low cost, open loop position control systems.
• Typical step resolution is 1.8 degrees.
However, micro-step motors are capable of 0.0144 degree steps.
• Stepper motors are usually used in open loop control systems, though an encoder may be used to confirm positional
DRIVES IN CNC MACHINE TOOLS
ACTUATION SYSTEMS IN NC MACHINE TOOLS
Hardware devices that convert a controller command signal into a change in a physical parameter usually mechanical (e.g., position or velocity)
• A transducer because it changes one type of physical quantity into some alternative form.
• Is usually activated by a low-level command signal, so an amplifier is often required to provide sufficient power to drive the actuator.
• Actuation Systems are provided corresponding to each of the automatic control functions.
• Receive commands from the MCU.
• Desirables
⁻ Efficient
⁻ Stiff
⁻ Quick response
⁻ Virtually free from backlash influences
ACTUATION SYSTEMS IN NC MACHINE TOOLS
Actuators used in an automated system in general are of different types such as electrical, hydraulic or pneumatic.
• Electrical actuators – Electric motors
• DC servomotors
• AC motors
• Stepper motors – Solenoids
• Hydraulic actuators
– Uses hydraulic fluid to amplify the controller command signal
• Pneumatic actuators
– Uses compressed air as the driving force
OPEN LOOP SERVO SYSTEM
No feedback to verify that the actual position achieved is the desired position
• Basic Length Unit (BLU): Position resolution of the axis of motion.
For example,
• 1 BLU = 0.0001" means that the axis will move 0.0001" for every one electrical pulse received by the motor. The BLU is also referred to as Bit (binary digit).
• Pulse = BLU = Bit
Example 1
• The XY table of a drilling machine has to be moved from the point (1,1) to the point (6,3). Each axis can move at a velocity of 0.5"/sec, and the BLU is 0.0001", find the travel time and resolution.
– Travel time in X-axis is (6-1)/0 .5 = 10 sec, in Y-axis is (3-1)/0 .5= 4 sec .
– Travel time = 10 sec
– Resolution = BLU = 0.0001
Example 2
A CNC milling machine has to cut a slot located between the points (0,0) and (4,3) on the XY-plane where the dimensions are in inches.
If the speed along the slot is to be 0.1 in/sec, find the cutting time and axial velocities.
Distance traveled along the slot = (16+9)1/2 = 5"
Cutting time = 5/0.1 = 50 sec
Vx = xV/(x 2+y 2)1/2 = 4(0.1)/5 = 0.08 in/sec Vy =yV/ (x 2+y 2)1/2 = 3(0.1)/5 = 0.06 in/sec
If the velocity in Y-axis is off by 10%, what would be the new position ?
New velocity in y is 0.9 x 0 .06 = 0.054 in/sec
In 50 sec, the y- will move a distance [50(0.054)] = 2.7 in.
The stepping motor is driven by a series of electrical pulses generated by the MCU.
Each pulse causes the motor to rotate a fraction of one revolution.
The fraction is expressed in terms of the step angle, α given by α = 360/N, degrees
where N = number of pulses required for one revolution
– If the motor receives "n" number of pulses then the total angle,
A = n (360/N), degrees
In terms of the number of revolutions, it would be (n/N)
If there is a 1 :1 gear ratio between the motor and the leadscrew, then the leadscrew has (n/N) revolutions. If the pitch of leadscrew is p (in/rev), then the distance traveled axially, say x, can be used to
The pulse frequency, f, in pulses/sec determines the travel speed of the tool or the workpiece .
60 f = N (RPM) where N = number of pulses per revolution, RPM = RPM of the lead screw
The travel speed, V, is then given by V = p (RPM) where p pitch in in/rev
Example 3
A stepping motor has N = 150, p = 0.2"/rev; If n = 2250 pulses, what is the distance traveled in x-direction ? What should be the pulse frequency for a travel speed of 16 in./min ?
x = p (n/N)
x = (0.2) (2250)/150 = 3“
V= p (RPM) 16'= 0 .2 (RPM), from which, RPM = 80
60 f = N (RPM)
f = (150) (80)/60 = 200 Hz
Example 4
A stepping motor of 200 steps per revolution is mounted on the leadscrew of a drilling machine. If the pitch is 0.1 in/rev.,
a. What is the BLU ?
b.If the motor receives a pulse frequency of 2000 Hz, what is the speed of the table ?
BLU = p/step per revolution BLU = 0 .1/200 = 0.0005“
Table speed = (p) (RPM)
= (p) (60f)/N
Table speed = (p) (RPM) = (0 .1) (60) (2000)/200 = 60 in/min
CLOSED LOOP SERVO SYSTEM
Uses feedback measurements to confirm that the final position is the specified position
• Closed -loop NC systems are appropriate when there is a force resisting the movement of the tool/workpiece .
• DC/AC Servo motors are generally employed as the driving component to provide the machine slide motion.
• Requires external encoders/sensors for observing the position of the work table and sending the feedback signal to the feedback controller.
• Both the input to the control loop and the feedback signals are a sequence of pulses, each pulse representing a BLU unit.
• The two sequences are correlated by a comparator and gives a signal, by means of a digital-to-analog converter, (a signal representing the position error), to operate the drive motor (DC servomotor) .
Example 5
Consider a CNC worktable driven by a closed-loop control system consisting of a servomotor, leadscrew, and optical encoder. The leadscrew has a pitch, p = 0.2" and is coupled to the motor shaft with a screw to motor gear ratio of 1 : 4. The encoder generates 150 pulses per revolution of the leadscrew. If the number of pulses and the pulse rate received by the control system are 2250 and 200 Hz respectively, calculate:
a. Table speed
b. Motor speed in RPM
c. Distance traveled by the table
b.
c.
a. V= p(RPM) = 0.2(RPM) = (0 .2) (60 f)/N = (0 .2) (60) (200)/150
= 16 in/min
RPM of the leadscrew = (60) (200)/(150) = 80 RPM of the motor = 4X80=320
x = p(n/N) = (0.2) (2250)/150 = 3"
Example 6
A dc servomotor is coupled to a leadscrew which drives the table of a CNC machine tool. A digital encoder, mounted at the end of the screw, emits 500 pulses per revolution. If the pitch is 5 mm per rev, and the motor rotates 600 rpm (1 :1 gear ratio), calculate the
a. Table speed b. BLU
c. Frequency of pulses transmitted by the encoder a. V= p(RPM) = 5 (600) = 3000 mm/min = 3 m/min b. BLU = p/N (pulses per revolution)
5/500 = 0.01 mm
c. RPM = 600 = (60f)/N = 60 f/500 from which f = 5000 Hz
BALL SCREW MECHANISM
• A ball screw is a mechanical linear actuator that translates rotational motion to linear motion with little friction.
• Sliding friction is reduced to rolling friction.
• A threaded shaft provides a helical raceway for ball bearings which act as a precision screw.
• As well as being able to apply or withstand high thrust loads, they can do so with minimum internal friction.
• They are made to close tolerances and are therefore suitable for use in situations in which high precision is necessary.
• The ball assembly acts as the nut while the threaded shaft is the screw.
• In contrast to conventional lead screws, ball screws tend to be rather bulky, due to the need to have a mechanism to re-circulate the balls.
• Efficiency is generally > 90% whereas sliding friction type lead screws is rarely above 25%.
BALL SCREW MECHANISM
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FEEDBACK DEVICES IN CNC MACHINE TOOLS
• Feedback system is actually a measuring device.
• The feedback allows the control system to compare the machine’s actual position with the programmed command.
• The result of this comparison provides the ability of velocity and position control.
• The standard feedback devices are either digital transducers or analogue transducers whose output is converted to digital form.
• Transducers may be linear or rotary devices, where the rotary type is more commonly used in the CNC machine tools.
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Positioning Feedback Devices
These are basically feedback devices for the positioning of the table, and are classified as:
• Rotary Transducers
Generally connected directly to the screw or by means of precision gearing.
• Linear Transducers
One part of the device is fixed to the machine tool structure, whereas the other part is attached to a slide which moves over the stationary part.
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ROTARY TRANSDUCERS
Positioning Feedback Devices which are rotary in nature There are following two basic types of rotary transducers.
• Resolvers
‒ Analog device.
‒ Require conversion of analog to digital form
• Encoders
‒ Numerical device.
‒ Outputs digital data directly
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•Resolvers and Encoders are vastly different though they serve the same purpose in many applications.
•In rotary applications, both of them are used to sense the speed, direction and position of a rotating shaft.
•They function as transducers by transforming mechanical motion into electronic information.
•This electronic information is feedback to electronic devices controlling the mechanical motion.
•Feedback is the vital link that closes the control system loop to improve the system performance.
SERVOMOTOR with RESOLVER
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ROTARY TRANSDUCERS
Resolver
Construction
• Typically small AC motors in construction.
• Consists basically of a stator and a rotor.
• Resolvers are made of tough materials that withstand harsh environments over extended periods of time like copper wire, iron laminations, steel housings, and high temperature coatings.
• With few exceptions, resolvers can operate at higher temperatures.
• Electronic circuitry known as converter is used to transform cyclic output signals into digital representation of shaft angle.
Resolver:
•Resolvers use magnetic flux between windings on iron cores much like transformers.
•Resolvers are rotary transformers with one primary winding and two secondary windings (sine (SIN) and the cosine (COS) windings.
•The primary winding is generally on the rotor and the two secondary windings are on the stator.
•The reference winding is the primary winding that, through a transformer known as the rotary transformer, is excited by an AC voltage applied to the primary side of the transformer.
•The rotary transformer then passes the voltage off to the secondary side of the transformer.
•As the motor spins, the voltage output from the SIN and COS windings change according to the shaft position.
•The SIN and COS windings are mounted 90 degree from each other in relation to the shaft.
• An AC voltage (6 V to 60 V, 400 Hz to 10,000 Hz) applied to the rotating coil (rotor) induces a voltage across the gap in the stationary coil (stator).
•As a result, transformer stator coil induces the input voltage across the gap to the transformer rotor on the rotating member of the resolver.
•The current in the transformer rotor flows through the resolver rotor creating the magnetic field of the rotor coil.
•Again the voltage is induced back across the gap to the resolver stator coils.
•The amplitude of the induced stator voltage is proportional to the relative angular positions of the rotor and stator coils.
•As the reference winding spins, the angle of difference between the reference and SIN/COS windings change, represented as the theta rotation angle.
•The voltages induced upon the SIN and COS windings are equivalent to the reference voltage multiplied by the SIN winding and COS
Working of Resolver
Since there are two stator windings at 900, the output voltages are at 900 electrical phase. These voltages are called sine and cosine as given by the equations:
V
s= V
REFX sin θ V
c= V
REFX cos
θWhere
VREF = Reference voltage
θ = rotor angle with respect to the stator
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The output of a resolver is therefore an analog AC voltage equivalent to the absolute angle between the rotor and the stator. The absolute position is calculated by:
θ = ARCTAN (sin θ / cos θ) = ARCTAN (Vs / Vc)
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ENCODER
• An encoder is a sensor of mechanical motion that generates digital signals in response to motion.
• As an electro-mechanical device, an encoder is able to provide motion control system users with information concerning position, velocity and direction.
• There are two different types of encoders:
– linear and rotary
• A linear encoder responds to motion along a path, while a rotary encoder responds to rotational motion.
• An encoder is generally categorized by the means of its output.
• An incremental encoder generates a train of pulses which can be used to determine position and speed.
• An absolute encoder generates unique bit configurations to track positions directly.
Encoders
Advantages
• low cost
• high accuracy
(e. g. by gear ratio) Disadvantages
• elastic effects and back lash
OPTICAL ENCODER
• Device for measuring rotational position and speed.
• Common feedback sensor for closed-loop CNC control.
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OPTICAL ENCODER
Working of Rotary Encoders
• Optical encoders rely on opto electronic components to detect rotary motion.
• A light source emits a beam across a gap onto a photodetector.
• A code-wheel with graduated patterns of opaque and clear spaces rotates between the source and detector.
• The clear areas allow light to pass through the code-wheel to the photodetector.
• The photodetector produces an electrical current proportional to the intensity of the illumination reaching it.
• Opaque areas in the code-wheel interrupt the light beam allowing little or no light to reach the photo detector.
• The optical encoder's disc is made of glass or plastic with transparent and opaque areas.
• A light source and photo detector array reads the optical pattern that results from the disc's position at any one time.
This code can be read by a controlling device, such as a microprocessor or microcontroller to determine the angle of the
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Light Source Sensor Reed
Head
Solar Cell produces voltage
Oscilloscope
Moved up and down
Metal scale
Time (sec) Voltage
• The encoder is
composed off REED HEAD and the SCALE.
• The REED HEAD travels with the axis, while the scale is stationary.
• The scale can be made either of glass or metal.
• In the encoder metal scale has reflective markings.
• The light from the source in the REED HEAD reflects off markings and are in turn picked by the sensor.
LINEAR ENCODER
Magnitude of movement
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• Each time the light touches the tick mark voltage is produced.
• If the tick mark is 1mm apart and if one gets voltage spikes, it is known that it has travelled one tick mark i.e., 1 mm.
• So suppose one gets 5 voltage spikes then the movement is 5 mm.
• If oscilloscope is connected at the Reed Head voltage peaks are generated on the oscilloscope.
• By counting these peaks the movement is known.
Light Source Sensor Reed
Head
Solar Cell produces voltage Oscilloscope
Moved downward
Metal scale
Voltage
Direction of movement
Signal 1 Signal 2 Signal 1
119 Signal 2
Time (sec)
120
• For this second light source and sensor are added to determine direction.
• So we will have two bits of light emitted and after striking the reflective tick will be received by the sensor.
• If moving downward first signal will on the top of the tick mark first and its square/sine wave peak will be first as compared to second signal.
• Signal 1 is in front of signal 2 hence we are travelling downward.
Light Source Sensor Reed
Head
Solar Cell produces voltage Oscilloscope
Metal scale
Voltage
Reference Point
Signal 1 Signal 2 Signal 1
Signal 2
Time (sec)
Home
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• This is accomplished by proximity sensors.
• When the axis is moved in the specific direction, they recognize the home switch and call it (0,0) point.
• Hence it will be determine where the things are kept on table.
DNC
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Direct Numerical Control (DNC)
• Direct Numerical Control Software (DNC) solution is a multi- threaded communications and file management system that allows for simultaneous upload and download of multiple CNC controls.
• It remotely uploads and downloads directly from the CNC control, and files are automatically named, saved, and stored in the correct folder.
• The remote sends back directory listing, and uses “long” file names.
• DNC Software is easily connected to CAD/CAM systems over industry standard networks and is available in a Client-Server configuration.
DNC
Components of a DNC system Central computer
Bulk memory, which stores the part program Telecommunication lines
Machine tools
The computer calls the part program instructions from bulk storage an d sends them to the individual machines as the need arises.
It also receives data back from the machines.
The two way information flow occurs in real time, which means that each machine’s requests for instructions must be satisfied almost instantaneously.
Similarly computer must always be ready to receive information from the machine and to respond accordingly.
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DNC
Depending on the number of machines and the computational requirements that are imposed on the computer, it is sometimes necessary to make use of satellite computers.
These satellites are minicomputers, and they serve to take some of the burden off the central computer. Each satellite computer controls several machines.
Group of of part program instructions are received from the central computer and stored in buffers.
They are then dispensed to the individual machines as required.
Feedback data from the machines are also stored in the satellite's buffer before being collected at the central computer.
