7. control

Department of Mechatronics Engineering CHUNGNAM NATIONAL UNIVERSITY

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©2008-2015 Young-Woo Park

ControlControl

Professor Young-Woo Park, Ph.D.Lecture 7


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Trend

totally manual totally computer-based

Evolution Manual Control

external wheels to move rack-and-pinion linkage in turn move the worktable & cutting tool relative each other.

mechanical stops and systems of gears and cams allow repetitive and sequential action.

with an operator still in total control

hydraulic powers with piston actuators chiefly for repetitive operations with human monitoring

IntroductionIntroduction


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Limited Switches

provide some basic automation to either shut off the machine OR

control simple repetitive motions

Hard-Wired Automation

switches and relays: earliest electronic controls send a fixed sequence of commands to the MT’s drive motors

changes in operation required manually reconnecting wires & repositioning contact points

ladder logic manually reprogrammed

potentiometers generate analog signals that carries commands to the drives



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Ladder diagrams are specialized schematics commonly used to document industrial control logic systems.

they resemble a ladder.

with 2 vertical rails (supply power) & as many "rungs" (horizontal lines) as there are control circuits to represent.

ladder diagram for a lamp that is controlled by a hand switch:

"L1" & "L2" = the two poles of a 120 VAC supply

= the "hot" & grounded (“neutral”) conductors



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soft-wired automation

programmable logic controller (PLC) still handles many machining and manufacturing operations.

uses soft-wired electronic circuits.

coded tape or cards a reader senses the holes & generates digital commands.

reader speed in characters per second determines the MT’s operating speed.

Today, data formerly punched into a tape are recorded in computer memory.

CNC

CAD/CAM



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Composition

INPUT OUTPUT Sensing Devices Valves

Switches and Pushbuttons Solenoids

Proximity Sensors Motor

Limit Switches Actuators

Pressure Switches Pumps

PLCPLC


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PLC Operations (four steps) Input Scan

Scans the state of the Inputs

Program Scan

Executes the program logic

Output Scan

Energize/de-energize the outputs

Housekeeping

PLCPLC


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PLCPLC


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PLCPLC


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Relayed Video http://www.youtube.com/watch?v=WCAs1KYyuuw&feature=related

PLCPLC


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Two key elements of the computers computing power

calculate the interpolation and generation of tool path commands.

memory

holds machine operating commands and tool path program.

CAD/CAM procedure

design a part using a CAD program.

transfer the CAD file to a CAM program.

send NC codes as data signal to the MT’s drives.

CAD/CAMCAD/CAM


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trend

off-line programming on-line programming

due to today’s advanced microprocessor and chip technology

the user can enter data at any time.

another way to create CAM files: digitizing

a probe or a laser is moved over the model’s surface.

these motions are converted to dimensions.

is commonly used where a sample of a part exists but no engineering print.

CAD/CAMCAD/CAM


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Open-Loop Control Stepper motor system

Current pulses sent from control unit to motor

Each pulse results in a finite amount of revolution of the motor

The user is free to modify the program.

Accepts a wide range of software, not just a limited number of proprietary programs.

Control SystemsControl Systems


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Open-Loop Limitations Control unit “assumes” desired position is achieved

No positioning compensation

Typically, a lower torque motor

Open-Loop Advantages Less complex, Less costly, and lower maintenance costs



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Closed-Loop Control Variable DC motors - Servos

Positioning sensors -Resolvers

Feedback to control unit

Position information compared to target location

Location errors corrected



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Closed-Loop Advantages DC motors have the ability to reverse instantly to adjust for position error

Error compensation allows for greater positional accuracy (.0001”)

DC motors have higher torque ranges vs. stepper motors

Closed-Loop Limitations Cost



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Servo System CNC, servomotors, amplifiers, and feedback devices

Motion Control for High Speed MachiningMotion Control for High Speed Machining


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What is a Servo? Servo control, which is also referred to as "motion control" is used in industrial processes to move a specific load in a controlled fashion

A graphical representation of a electromechanical servo system

typically used in high precision, low to medium power, & high-speed applications

flexible, efficient, and cost-effective



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Controller Motion controllers (MC) are built specifically for the motion control

Commands & I/O are specific to the needs of those in the servo industry.

Unlike the others, MCs are PC based, allowing for a GUI

Usually, there are advanced features that allow ease of tuning, commutation sensing, and other functions.

A MC, in general, makes your life easier than a PLC or controller

Because of the added features, they are typically more expensive.



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Drive the link between the controller and motor

Also referred to as servo amplifiers

translate the low energy reference signals from the MC into high energy power signals to the motor

Trend

Higher bandwidth to increase production throughput.

Increased velocity &position control to allow for more intricate & miniaturized manufacturing.

Increased network capability to closely coordinate axes within a machine & coordinate machines within a factory.

Simplified, user-friendly and universal operation



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Feedback Feedback devices are used to ensure that the motor or load reaches the commanded position or velocity.

Servo amplifiers & controllers use this feedback to determine how much current to deliver to the motor at any time, based on its present position and velocity versus where it needs to be.

Types of feedback

absolute

incremental



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Feedback (Cont’d) Encoders

The most prevalent position feedback device in motion control

Linear encoders can go to sub-micron resolutions

Rotary encoders with resolutions over 100k counts per revolution

Quadrature and Sinusoidal encoder



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Feedback (Cont’d) Hall Sensors

A low resolution feedback often necessary for commutation control

can also be used for velocity feedback at higher velocities.

Resolver

A rotary transformer

Feedback of choice for high temperature & high vibration environment



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Proportional Servo System velocity of each axis distance between the actual & ∝ command position during each sample period = servolag

this error signal is used to determine acceleration/deceleration & steady-state velocities.

L = feedrate/servogain



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Servo Following Error difference in commanded & actual position at any moment in time

a major cause of path error

use DSP for reducing this error to near zero.



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Maximum Block Processing Time the processing speed of the CNC, with each stroke generated for every axis which must be read, interpreted and activated.

Tb = maximum stroke length/feedrate



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Servo Cycle Time the amount of time on a CNC control takes for each measuring & command cycle

the faster the desired speed, the faster servo cycle time must be.



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Look Ahead is necessitated by our desire to travel at feedrates faster than a machine can stop in the smallest distance between programmed points.



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Imagine traveling along this curve from left to right at 300 IPM. Your machine will need more than 0.100" of travel to stop the X axis to create the nearly sharp corner at point B.

With only about 0.010" between the points, a gouge like the one shown will occur unless look ahead intervenes to start a slowdown way back at point A.


Date post:	20-Jan-2017
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7. control

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