7. control

Department of Mechatronics Engineering
CHUNGNAM NATIONAL UNIVERSITY
CAD/CAM
©2008-2015 Young-Woo Park
ControlControl
Professor Young-Woo Park, Ph.D.
Lecture 7

CAD/CAM
 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

CAD/CAM
 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

CAD/CAM
 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

CAD/CAM
 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

CAD/CAM

CAD/CAM
 Composition
 INPUT OUTPUT
Sensing Devices Valves
Switches and Pushbuttons Solenoids
Proximity Sensors Motor
Limit Switches Actuators
Pressure Switches Pumps
PLCPLC

CAD/CAM
 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

CAD/CAM
PLCPLC

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

CAD/CAM
 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

CAD/CAM
 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

CAD/CAM
 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

CAD/CAM

CAD/CAM
 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

CAD/CAM
 Closed-Loop Control
 Variable DC motors - Servos
 Positioning sensors -Resolvers
 Feedback to control unit
 Position information compared to target location
 Location errors corrected

CAD/CAM
 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

CAD/CAM
 Servo System
 CNC, servomotors, amplifiers, and feedback devices
Motion Control for High Speed MachiningMotion Control for High Speed Machining

CAD/CAM
 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

CAD/CAM
 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 µcontroller
 Because of the added features, they are typically more expensive.

CAD/CAM
 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

CAD/CAM
 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

CAD/CAM
 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

CAD/CAM
 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

CAD/CAM
 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

CAD/CAM
 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.

CAD/CAM
 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

CAD/CAM
 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.

CAD/CAM

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

CAD/CAM
 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.

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