PROGRAMMABLE AUTOMATION Chapter 2 PROGRAMMABLE AUTOMATION NC/CNC/DNC

Conventional Manufacturing Manufacturing Support System Facilities Automated Manufacturing Fixed Automation Programmable Automation Flexible Automation

Conventional Manufacturing Manufacturing Support System Facilities Automated Manufacturing Fixed Automation Programmable Automation Flexible Automation

Programmable Automation CAD CAE CAPP CAM CAQC: Computer Aided Quality Check

Automated Manufacturing Programmable Automation CAD CAE CAPP CAM CAQC: Computer Aided Quality Check

CAM CAM CAM NC Machines CNC Machines DNC Machines

CAM NC Machines CNC Machines DNC Machines Hardware Machine Control Unit (MCU) The Method of Program execution CNC Machines DNC Machines

CAM NC Machines CNC Machines DNC Machines Hardware Machine Control Unit (MCU) The Method of Program execution CNC Machines Functions of MCU in CNC CNC Programming with Fanuc Controller (G-Code & M-Code) DNC Machines

CAM NC Machines CNC Machines DNC Machines Hardware Machine Control Unit (MCU) The Method of Program execution CNC Machines Functions of MCU in CNC CNC Programming with Fanuc Controller (G-Code & M-Code) DNC Machines Working Principle

Comparison Comparison of Conventional Machines with CNC Comparison of NC with CNC Machine

1st CNC by MIT

CNC Programming Axis Designation Tool Movements Reference Points Coding system CNC Syntax Programming Mode Tool Offsets G-Codes & M-Codes in Detail (With Program)

At 1970-1972 first Computer Numeric Control machines were developed. HISTORY US Air Force commissioned MIT to develop the first "numerically controlled" machine in 1949. It was demonstrated in 1952. At 1970-1972 first Computer Numeric Control machines were developed. Today, computer numerical control (CNC) machines are found almost everywhere, from small job shops in rural communities to companies in large urban areas. 2004

NC Machines Numerical Control (NC)is a form of programmable automation in which the mechanical actions of a machine tool or other equipment are controlled by a program containing coded alphanumeric data. The alphanumeric data represent relative positions between a workhead & a workpart. NC in Machine tool applications: drilling, milling NC in non Machining application: assembly, drafting, inspection

NC SYSTEM ELEMENTS A typical NC system consists of the following elements Program of Instruction (part program) Machine control unit Processing equipment (Machine tool & cutting tool) 2004

Program of Instruction Machine Control Unit (MCU) Basic Components of NC System Program of Instruction Machine Control Unit (MCU) Processing Equipment

NC Machine System

PART PROGRAM A part program is a series of coded instructions required to produce a part. It controls the movement of the machine tool and the on/off control of auxiliary functions such as spindle rotation and coolant. The coded instructions are composed of letters, numbers and symbols and are arranged in a format of functional blocks as in the following example N10 G01 X5.0 Y2.5 F15.0    |       |       |         |        |    |       |       |         |       Feed rate (15 in/min)    |       |       |        Y-coordinate (2.5")    |       |      X-coordinate (5.0")    |      Linear interpolation mode   Sequence number 2004

Program is coded on a suitable medium In case of NC machines program was coded on 1-inch wide punched tape. Tape may be paper, plastic or magnetic tape. Program is read by a suitable input device.

PROGRAM INPUT DEVICE The program input device is the mechanism for part programs to be entered into the NC control. The most commonly used program input device is punched tape reader. 2004

Punch Card

Punched Tape Sequence of processing steps is called a part program. The medium to write a part program in NC machine was a 1-inch punched tape. NC Machine coding system uses alphabetical characters ,numerical and other special symbols in part program.

There are 2 standard coding system used in NC. The Electronic Industry Association (EIA) EIA RS-244 B International Standard Organization (ISO) ASCII

Program Writing Program Writing Parity checking

N10 G01 X5.0 Y2.5 F15.0 N

N10 G01 X5.0 Y2.5 F15.0 N 1

N10 G01 X5.0 Y2.5 F15.0 N 1

N10 G01 X5.0 Y2.5 F15.0 N 1 Space

N10 G01 X5.0 Y2.5 F15.0 N 1 Space G

N10 G01 X5.0 Y2.5 F15.0 N 1 Space G

N10 G01 X5.0 Y2.5 F15.0 N 1 Space G 1

N10 G01 X5.0 Y2.5 F15.0 N 1 Space G 1 Space

Parity 5

Parity 5

Punched Tape Reader

Machine Tool In NC Machine tools each axis of motion is equipped with a separate driving device which replaces the handwheel of the conventional machine. The driving device may be a dc servo motor, a hydraulic actuator or a stepper motor. Selection depends on accuracy & power requirement. Motion to tool or worktable is transmitted by gears & ball screws. Machine tool drives are also fitted with feedback devices (Closed loop control).

MACHINE CONTROL UNIT The machine control unit (MCU) is the heart of a NC system. It is used to perform the following functions: Read coded instructions, one by one Decode coded instructions Provide decoded information to the control loops of the machine axes of motion. Control machine tool operation. Advance tape each time the previous instructions were fulfilled. Axis of a machine tool is defined as a path along which relative motion between the cutting tool and the workpiece occurs and a machine can have more than one axis.

The MCU includes the Data Processing Unit (DPU) and the Control Loop Unit (CLU)

MCU DPU reads the part program, decodes it, processes the information and passes data to the CLU Such data contains the new required position of each axis, its direction of motion and velocity, and auxiliary control signals (like coolant on/off) CLU convert the information to control signals and drives the mechanism (Motor & Ball screw), receives feedback (about position and velocity) for each axis and instructs DPU to read new instructions

DPU includes at least Input device, Punched tape reader Reading circuits & parity checking logic Decoding circuits for distributing data among the controlled axes. Interpolators, which supplies velocity commands between the successive points taken from the drawing.

CLU consists of the following circuits: Position control loops for all the axes of motion Velocity control loops Deceleration & backlash take-up circuits Auxiliary function control, such as coolant on & off, spindle on & off

DRIVE SYSTEM A drive system consists of amplifier circuits, stepping motors or servomotors and ball lead-screws. The MCU feeds control signals (position and speed) of each axis to the amplifier circuits. The control signals are augmented to actuate stepping motors which in turn rotate the ball lead-screws to position the machine table. 2004

Each Slide is also fitted with feedback transducer, which continuously monitors the slide position and compares with the programmed position as well as feedrate. 2 types of feedback are taken, one for position & one for speed.

Drive Motors Servo Motors Stepper Motors Hydraulic Motors

Servo Motors A servo motor can be either DC or AC, and is usually comprised of the drive section and the resolver/encoder. Servo Motors are variable speed motors that rotates in response to the applied voltage. DC Servo motors are controlled by varying the voltage magnitude. AC Servo Motors are Controlled by varying the Voltage frequency.

A servo motor is much smoother in motion than a comparable stepper, and will have a much higher resolution for position control. The servo family is further divided into AC and DC types. An AC servo had the advantage of being able to handle much higher current surges than a DC, as the DC has brushes, which are the limiting factor in this case.

AC Servo is Stronger than DC servo The trend for manufacturers of “serious” CNC machinery is to use AC servos. “Entry level” machines may have DC servos, or even steppers.

STEPPING MOTORS A stepping motor provides open-loop, digital control of the position of a workpiece in a numerical control machine. The drive unit receives a direction input (cw or ccw) and pulse inputs. For each pulse it receives, the drive unit manipulates the motor voltage and current, causing the motor shaft to rotate by fixed angle (one step). The lead screw converts the rotary motion of the motor shaft into linear motion of the workpiece . 2004

STEPPING MOTORS 2004

A stepper motor is wound in such a way that the rotation has a certain number of discrete "steps". The number of steps per revolution is rather high, around two hundred or so, and varies by model and manufacturer. What this means is that the motor has effectively a resolution (smallest controlled movement) equal to the number of steps for that motor.

Stepper systems are often “open loop” which means that the controller only tells the motors how many steps to move and how fast to move, but does not have any way of knowing where they actually are. This can lead to errors, should a situation arise where the motors are unable to comply with the commanded move.

This can be very obvious, where the motion stops and it sounds like you stripped a gear, or subtle, where the motor only misses a “few” steps. The result is the same - the controller thinks you are at X25.5, Y15.5 and in reality you might be at X25.3, Y15.4 . This can lead to a cumulative error, which may in turn lead to crashes, not to mention out of spec parts.

Hydraulic Motor Variable Speed Motors

RECIRCULATING BALL SCREWS Transform rotational motion of the motor into translational motion of the nut attached to the machine table. 2004

RECIRCULATING BALL SCREWS Accuracy of NC/CNC machines depends on their rigid construction, care in manufacturing, and the use of ball screws to almost eliminate slop in the screws used to move portions of the machine. 2004

2004

TYPES of NC CONTROL SYSTEMS Open-loop control Closed-loop control 2004

Open Loop Control System In open-loop control system step motors are used Step motors are driven by electric pulses Every pulse rotates the motor spindle through a certain amount By counting the pulses, the amount of motion can be controlled No feedback signal for error correction Lower positioning accuracy Less costly

CLOSED-LOOP CONTROL SYSTEMS In closed-loop control systems DC or AC motors are used Position transducers are used to generate position feedback signals for error correction Better accuracy can be achieved More expensive Suitable for large size machine tools 2004

Feed back An integral feedback device (encoder and tachometer) are either incorporated within the servo motor or are remotely mounted, often on the load itself. These provide the servo motor's position and velocity feedback that the controller compares to its programmed motion profile and uses to alter its velocity signal.

Optical Encoder Stepper Motors are provided with encoders.

Closes loop system Optical Encoder Lead screw Gear box (rge) MCU DAC Servo Motor Feed back signal

Precision in NC Positioning Precision in NC Machine defined as Control resolution Accuracy Repeatability

Control Resolution Is the distance between 2 adjacent addressable points in the axis movement. Depends on (a) The electromechanical components of the positioning system (b) Number of bits used by the controller to define the axis coordinate location CR=Max{CR1, CR2}

Addressable Points Linear Axis Control Resolution = CR

Accuracy Accuracy is effected by mechanical errors. Accuracy means how exactly the position specified in the part program is achieved . Accuracy in the worst case: desired target point lies in the middle of 2 addressable points. This gives maximum possible positioning error. Accuracy= CR/2 + 3σ

Repeatability Repeatability= ± 3σ

Addressable Points Distribution of Mechanical Errors Repeatability =±3 σ Linear Axis Control Resolution = CR

Addressable Points Distribution of Mechanical Errors Desired Position Accuracy = CR/2 + 3σ Repeatability =±3 σ Linear Axis Control Resolution = CR