cnc control system and interface

Compiled by-

Bhanu PS Tomar

M.Tech 2nd Sem(PE)

CNC Control System And Interface

Saturday 15 April 20232

Overview of presentation

1. CNC control system- An overview

2. Open loop and closed loop control system system

3. Feedback subsystem in CNC

4. Feedback for CNC

5. Block diagram for CNC

6. CNC interpolation

7. Hardware and software interpolation

8. Features of CNC

9. Advantages and disadvantages of CNC

Bhanu PS Tomar

Saturday 15 April 2023Bhanu PS Tomar3

CNC control system Fundamental aspects of control-

In general CNC machines are programmed and controlled to accomplish the major three actions-

A. Positioning of tool in 2 or 3 or more axes.

B. Motion-relative velocity of tool wrt work piece

C. Switching function i.e. direction of rotation of spindle, coolant ON/OFF etc. are to be controlled.

D. CNC systems require motor drives to control both the position and the velocity of the machine axes. Each axis must be driven separately and follow the command signal generated by the NC control. There are two ways to activate the servo drives: the open-loop system and the closed-loop system.


Based upon the nature of the positioning and motion,CNC control is classified into two groups,

1. Point to Point control-it is purely for positioning of the tool at programmed points.e.g drilling,tapping,boring operations

2. Continuous path control- there is a functional relationship between individual cordinate axes. i.e 3 axes,5 axes machining.

5 axes machining- 3 translational and 2 rotational axes.

best suited for doubly curved surface maching.i.e in spacecraft,aircraft and fluid handling devices.


Open loop and closed loop cnc system

Open loop-

Programmed instructions are fed into

the controller through an input device.

These instructions are then converted

to

electrical pulses (signals) by the

controller and sent to the servo

amplifier to energize the servo

motors.


Drawbacks of open loop

The primary drawback of the open-loop system is that there is no feedback system to check whether the program position and velocity has been achieved. If the system performance is affected by load, temperature, humidity, or lubrication then the actual output could deviate from the desired output.

For these reasons, the open-loop system is generally used in point-to-point systems where the accuracy requirements are not critical. Very few, if any, continuous-path systems utilize open-loop control.


Closed loop system The closed-loop system has a feedback subsystem to monitor the actual

output and correct any discrepancy from the programmed input.

Closed-loop systems are very powerful and accurate because they are capable of monitoring operating conditions through feedback subsystems and automatically compensating for any variations in real-time.

Most modern closed-loop CNC systems are able to provide very close resolution of 0.0001 of an inch. Closed-looped systems would, naturally, require more control devices and circuitry in order for them to implement both position and velocity control. This, obviously, makes them more complex and more expensive than the open-loop system


Comparison b/w open and closed loop system


Feedback subsystems in closed loop system

Feedback subsystems are of two types-

Analogue feedback system: Variations in physical systems such as position and velocity are being measured as voltage levels in analogue feedback system. Tachometers are typically used to measure the velocity, whereas the resolvers are used to measure position. There are two feedback loops in CNC drive systems; position loop and velocity loop.

NOTE-A resolver is a type of rotary electrical transformer used for measuring degrees of rotation. It is considered an analogue device, and has a digital counterpart, the rotary (or pulse) encoder.


Digital feedback system: A digital feedback system uses a digital position transducer to measure the position. Encoders are popular digital position transducers. Comparator and amplifier are replaced by up-down counter and digital-to-analogue converter.

It monitor output variations by electrical pulses.


Feedback for closed loop system

The feedback in a closed loop system is the information delivered from the CNC machine to the controller.

Feedback data is data that either confirms or denies that the motors have moved the machining table to the correct position and at the correct speed.

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Feedback can be accomplished in one of two ways

Linear Scales

EncodersNote- SERVOMOTOR-it is an automatic device that uses error sensing negative

feedback to correct the performance of a mechanism.


Bhanu PS Tomar


Linear scale-

It measures the absolute/exact position of the slide directly.

It is a graduated scale made of either glass or stainless steel along with a measuring surface and scanning unit.


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.

They convert rotary movement into digital signals.


Types of encoders

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.

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Difference between incremental and absolute encoder-

Incremental rotary encoder outputs the pulse corresponding to the rotation angle only while rotating, and is the counting measurement method that adds up the pulse from the measurement beginning point.

Absolute rotary encoder outputs the signal of position corresponding to the rotation angle by code. The Absolute position from the starting point is measured by the output code data. The position is memorized though the power supply is cut off.


Bhanu PS Tomar


CNC- conceptual arrangements

Typical block diagram of cnc system


Cnc interpolation interpolation is a method of constructing new data points

within the range of a discrete set of known data points.

Estimation of an unknown quantity between two known quantities .It is a mathematical process to determine new points on curved surface within two end points.

It produces a series of intermediate data points between given coordinate positions and computes the axial velocity of an individual axis along the contour path.


Types of interpolation-

To coordinate the axis movements in CNC control these types of interpolation are used

Linear interpolation: This moves tool from start point to the target point along a straight line. It can be implemented in a 2-D plane or 3-D space. the programming command should indicate X, Y, Z coordinates of target point, and feed rate.

Circular interpolation: It is programmed to cut circular arcs in three principal planes; namely XY, YZ, ZX. Direction, target position, arc radius, cutting plane, and feed rate must be specified in the program.

Helical interpolation: Helical interpolation combines the two-axis circular interpolation with a linear interpolation in third axis. i.e. machining of helical pockets and threads


Parabolic interpolation: It uses three non-collinear points to approximate curves that are of free forms. It reduces the number of programmed points by as much as 50 times the number required by the linear interpolation mode. It is mainly used in mold and die making and curves of high order and high degree.

Cubic interpolation: Cubic interpolation approximates the surfaces defined by third-order geometry. It involves the motion of three axes to machine complex shapes such as automobile sheet metal dies.


Modern approaches to interpolation-

1.hardware interpolation- It consists of a pair of DDA integrators. It is capable of doing linear and circular operations

according to instruction from the punched tapes. The main feature of hardware interpolation is that it

controls simultaneously two axes which can be X and Y,X and Z or Y and Z.

Hardware circuits generate the points that keep the tool path within tolerance.

It is used for fine interpolation.


2.Software interpolation- This is the computer based approach of hardware

interpolation.

Basically it is a computer program which simulates a single cycle of hardware interpolator and the feed rate control.

The computer analysis program divides the tool path into segments.

Used for coarse(rough) interpolation.

It is microprocessor based interpolation system,samples about 100 times/second are to be taken in this system.


Requirements of interpolation system-

1. The amount of data required for interpolation should be as small as possible.

2. The velocity along the interpolated path should be constant.

3. The curve must pass accurately through the numerically specified reference points to prevent any cumulative error.

4. The curve produced by interpolation must approximate as closely as possible to the required work piece contour.

5. Data required for the servomotor should be made available at a faster rate as demanded by the process.


Features of CNC For a CNC machine control unit (MCU) decides cutting speed, feed,

depth of cut, tool selection , coolant on off and tool paths.

The MCU issues commands in form of numeric data to motors that position slides and tool accordingly.

The tool or material moves.

Tools can operate in 1-5 axes.[multi axis machining]

Larger machines have a machine control unit (MCU)which manages operations

Movement is controlled by a motors .

Feedback is provided by sensors.Bhanu PS Tomar


besides previous features of CNC system we can go for the following features also-

1. Construction of the control

2. Operation

3. Operating aids

4. Colour graphics

5. Graphic simulation

6. Programming aids

A. cutter radius compensation.

B. blueprint programming

C. polar coordinates

D. cylindrical interpolation

E. Subroutines

F. Turning/milling operations- programmed contour in Cartesian coordinates while working in polar coordinates

G. Machining cycles- subroutines for often needed machining operations.Bhanu PS Tomar


7. program archiving

8. Integrated interface control- use of PLCs

9. Computer link


CNC machines- Advantages

Reduces product cost Improved product quality Facilitate production planning and control Increased productivity Greater flexibility

Limitations High initial investment High maintenance For low production it is costlier process


Any Questions…..????

Thank you.

cnc control system and interface

Engineering