cnc machine

EFFICIENT CNC MACHINE MONITORING &

CONTROL USING FPGA

SYED TAHIR HUSSAIN RIZVI 2009-MS-CE-10

Milling Machine

• Conventionally, an operator decides and adjusts various machines parameters like design, depth of cut etc depending on type of job

• Controls the slide movements by hand.

C N C

COMPUTER NUMERIC CONTROL

Controlling a Machine by giving some numerical values through Computer

NUMERIC CONTROL

Automation of machine tools that are operated by programmed commands encoded on a storage medium

No need to manually controlled via handwheels or levers

In a CNC Machine functions and slide movements are controlled by motors using computer programs.

In the first NC machines , that were motors that moved the controls to follow points fed into the system on punched tape.

In modern CNC systems, design is automated using computer-aided design (CAD) and computer-aided manufacturing (CAM) programs.

Basic Parts of CNC

CNC system consists of three basic components:

1. Part program: a file/code (set of commands) which describes the sequence of operations to be done.

2. Processing equipment: the unit which performs the manufacturing operations according to the part program.

3. Machine control unit (MCU): stores the program and executes it by converting each command into actions by the processing unit.

If the MCU is a computer, then the NC is called CNC (Computer Numerical Control)

CNC MACHINE

CAD/CAM produces a computer file that is used to extract the commands needed to operate a particular machine via a postprocessor, and then loaded into the CNC machines for production.

A computer aided design, or CAD, system uses computers to graphically create product designs and models. These designs can be reviewed, revised, and refined for use and application. Once finalized, the CAD design is then exported to a computer aided manufacturing, or CAM, system.

CAM systems assist in all phases of manufacturing a product, including machining, scheduling, management and quality control.

CNC Machine Control Unit (MCU)

Decides Parameters Cutting speed Depth of cut Tool selection Tool paths

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

Programmed instructions are converted into output signals which in turn control machine operations such as spindle speeds, tool selection, tool movement, and cutting fluid flow.

By using a computer processor (CNC), it allows programs to be edited and stored in the computer memory as well as allow to monitor quality control functions during the actual machining.

Basic CNC Principle

The requirement of any CNC machine is automatic, precise, and consistent motion control.

All computer controlled machines are able to control motion in various directions. Each of these directions of motion is called an axis.

Additionally, a CNC axis may be either a linear axis in which movement is in a straight line, or a rotary axis with motion following a circular path.

Axis to be Controlled

In CNC machining, more than one axis can be controlled simultaneously. These axis are:

- x, y, z axes (linear axes)

- a, b, c axes (rotational axes around x, y, z axes respectively

Axis to be Controlled

In 2-axis milling machines, the x and y axes can be controlled simultaneously.

In 3-axis milling machines, the x, y, and z axes can be controlled simultaneously.

In 2 ½ milling machines, the z-axis is fixed at a certain value and then the x and y axes are controlled simultaneously.

Basic CNC Principle

Each axis consists of a mechanical component, such as a slide that moves, a motor that powers the mechanical movement, and a ball screw to transfer the power from the motor to the mechanical component.

These components, along with the computer controls are referred to as an axis drive system.

Motors and loop control

Two types of motor are commonly used: Stepper Motors Servo motors

Stepper motors typically use open loop motion control

Servo motors use closed loop motion control

Stepper Motor

Stepper motors can be seen as motors controlled by electrical pulses.

Common stepper motors require 200 pulses per rotation, so by sending 600 pulses, the motor should make exactly 3 turns. This is called "open loop motion control" because there is no mechanism that checks that the motor turns as expected.

The main drawback of stepper motors is the possibility of "missing steps" if driven too fast, so stepper motors are always used at conservative speeds (i.e. with enough margin to avoid missing steps).

Servo Motors

Servo motors don't use pulses, but continuous current, which is simpler than pulses, but getting exact motion is more complex.

To get 3 turns, the controller needs to send some current to the motor, and needs a way to monitor the rotation, so that it stops sending current once the 3 turns are achieved. This is called "closed loop control" because of the mechanism that monitors the motor rotation.

This allows for faster operation and efficiency, but is more complex and slightly less precise than stepper motors.

Open & Closed Loop Controls

http://iesu5.ieem.ust.hk/dfaculty/ajay/courses/ieem215/lecs/cnc_f7.gif

Advantages of CNC Machine

Easier to program;

Easy storage of existing programs;

Easy to change a program

Avoids human errors

NC machines are safer to operate

Complex geometry is produced as cheaply as

simple ones

Usually generates closer tolerances than manual

machines

How to increase efficiency of motion controller & monitoring system?

The industry of machine tools keeps a constant progress that responds to

the most demanding market requirements, such as

production-time decreasing and

zero-defect manufacturing

CNC Position Controller using FPGA


A motion controller for high-speed CNC machines has higher demands on the electronics design compared with a conventional speed CNC

Requires higher sampling rates of Position Controller (servo loop update time) that reduce the available time for on-line processing.

In CNC, the position controller need different modules to implement the movement control of machine tools.

The velocity profile generator module can generate velocity profiles with the desired acceleration and deceleration characteristics.

The interpolation calculation module move to the desired location without any sharp jerks.

The PID control module controls the position and velocity of the motors.

The feedback counter module calculates the current motor position.

The data converter module converts the output of the circuit into the

driving signals for servo drivers of several types.

The clock generator module provides various clock signals for several modules in the system.

The external interface module communicates with the host computer


Most motion control systems use a motion controller that is based on a digital signal processor (DSP) and a microprocessor,. A multiple axis motion control system needs many functions in order to quickly and accurately perform the complex tasks required for industrial robots and automation systems.

Complicated multiplicative, divisional, and trigonometric function calls are needed to perform repetitive calculations. This requirement leads to a complex hardware design, expensive computation, increased system size, high cost, and high power consumption .


The field-programmable gate array (FPGA) can be used to implement digital systems.

In contrast, DSPs and personal computers (PCs) have pipelined architectures. The computing time of an FPGA-based controller can be relatively short regardless of the complexity of the control algorithm because of its parallel processing architecture.


FPGA is an array of basic logic blocks where the user can define its interconnectivity, making them programmable in a fully open architecture.

FPGA provides the advantages of a general purpose processor and a specialized circuit that can be reconfigured as many times as it is necessary until the required functionality is achieved.

FPGA is more flexible and its design cycle is much shorter because of its reconfigurability.

FPGA has become more popular for hardware designers to provide high-performance digital signal processing, providing solutions that are often 10X to 100X faster than can be accomplished with PC or Single Board Computer (SBC) processors. FPGA has a natural parallel architecture for high-speed computation.

The FPGA-based implementation of control algorithm offers advantages such as high-speed computation, complex functionality and real-time processing capabilities.


High-speed machining and servo loop update cycle Feed rate is the relative velocity at which the cutter is advanced

along the workpiece. Feed rate units depend on the motion of the tool and workpiece; In linear systems (e.g., milling), the units are typically distance per time (inches per minute [in/min or ipm] or millimeters per minute [mm/min]).

The control resolution of CNC is the distance that the axis moves

in position control cycle (a servo control cycle).

In the high-speed machining process, CNC must to maintain

control resolution, while ensuring high-speed feed rate. When the feed rate is 30m/min and position control cycle is 1ms (frequency is 1 KHz), the control resolution of CNC can only attain 0.5 mm.

Feed Rate=(30m/min)=(30/60 m/sec)=(.5 m/sec)

Control Resolution =(.5 m/sec)*(1msec)=.5 mm


If keeping feed rate unchanged and improving the frequency of servo loop update to 10 KHz (servo loop update cycle is 0.1 ms), the control resolution would attain 0.05mm.

CNC Position Controller receives the position command calculated by processor and controls accurately servos revolving which drive the axis to move to the target position.

In order to reduce the response time for CNC position controller for all 3 axis, fpga can be used for calculation of position of all axis at the same time.


Monitoring of CNC Machine using FPGA


The vibrations are one of the most significant variables to be monitored on machine tools since they directly affect the end piece finishing.

Time domain, Fast Fourier transform (FFT), and Discrete Wavelet Transform (DWT) are techniques often used for vibration analysis

It is difficult to find an instrument for vibration analysis that allows individually applying the time, FFT, and DWT techniques.


Data processing of the instrument should performs the calculation in a lower time than that required for the next module for further processing or decision making tasks; for instance, in a typical industrial controller, the processing parameters are required every time the control loop is updated.

Continuous online monitoring, parallel processing of parameters and controlling parameters are different operations to be served.

A continuous-online-monitoring instrument can be implemented. This instrument can provides the parameters for improving the control and decision-making processes in CNC machine tools


Time-domain analysis is utilized in measuring the vibration levels, based on the amplitude. The vibration-signal waveform can give, by itself, relevant information regarding the analyzed process such as dc level and rate of change of the vibration signal.

FFT analysis is applied to detect failures on gearboxes and mechanical unbalance. These defects are reflected on the signal spectrum with a significant increase in the fundamental frequency

DWT analysis can be applied for detecting cutting-tool breakage on machine tools and broken bars in induction motors

Conclusion

The control and monitoring of CNC machine are two major parts. So, on working them an efficient CNC machine can be achieved. For this, a lot of calculations and tasks are required to be performed in fastest time.

So, field-programmable gate array can be used to achieve real-time and continuous online analysis and a fast motion controller.

References

Efficient Implementation of CNC Position Controller using FPGA

(Yaodong Tao, Hu Lin, Yi Hu, Xiaohui Zhang, Zhicheng Wang)

The Design of a High Performance Modular CNC System

Architecture

(Claudio Ambran, Kevin Oldknowb, Giuseppe Migliorini", and Ian Yellowleyb)

FPGA-Based Vibration Analyzer for Continuous CNC Machinery Monitoring With Fused FFT-DWT Signal Processing

(Jose de Jesus Rangel-Magdaleno, Rene de Jesus Romero-Troncoso)

An FPGA-Based Multiple-Axis Motion Control Chip

(Jung Uk Cho, Quy Ngoc Le, and Jae Wook Jeon)

Design of Reconfigurable CNC System Based on FPGA

(LI Dongdong, Wu Liming, Zhu Gaofeng, Lai Nanhui)

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