Mitsubishi Electric MELSEC iQ-F Series User manual

MELSEC iQ-F
FX5 User's Manual (Positioning Control)

7
FX Series Programmable Controllers
Introduction to FX Positioning Control Systems
1 The Basics of Positioning Control
1.1 What is positioning control?
1
The Basics of Positioning Control
2
Positioning by AC Servo System
3
Components of Positioning Control
4
Learning to Use FX Positioning Control
1. The Basics of Positioning Control
1.1 What is positioning control?
The positioning controller, together with the programmable logic controller, personal computer and operator
interface, is one of the four main units of FA (factory automation).
Among these units, the positioning controller plays an important role and is regarded as the center of the
mechatronics field in which many senior engineers have been playing active roles.
Positioning is all about motion, and motion often involves speed and precision. And since speed can be
directly related to productivity, positioning is an area of much development. When the speed of a machine
increases, a problem with the stop precision is often generated. In order to solve this problem, diversified
grades of positioning controllers have been required and developed.
Improving machine efficiency generates immeasurable added value, including reduced labor costs and
improved conservation of machine floor space for the same quantity of production. If there are no problems
related to the positioning aspect of a machine, it may mean that the machine is not running as efficiently as it
could be. This is where the science of developing and retrofitting an optimum positioning control system
comes in.
1.2 Actuators for positioning
The options available for positioning control depend on the type of actuator driving the system. An actuator is
a mechanical device that moves or controls a specific element or a series of elements within a system.
In a mechanical system, an actuator is often used with a sensor to detect the motion or position of a
workpiece. The following illustrations provide examples of diversified actuators, their features and their weak
points.
Actuator(s) Features and Drawbacks Schematic drawing
Pneumatic
• Air source and high grade piping
are required.
• High torque is not available.
• Multi-point positioning is complex
and very difficult to achieve.
• Change in positioning is difficult.
Brake motor
• Positioning mechanism is simple.
• Repeatability is poor.
• Change in positioning is difficult.
(When optical sensors or limit
switches are used for stop)
Compressor
Workpiece
Air cylinder
Piping
Motor with brake
Limit switch

8
FX Series Programmable Controllers
Introduction to FX Positioning Control Systems
1 The Basics of Positioning Control
1.2 Actuators for positioning
Clutch brake
• Frequent positioning is possible.
• Life of friction plate is limited.
• Change in positioning is difficult.
(When optical sensors or limit
switches are used for stop)
Stepping motor
• Simple positioning mechanism.
• If load is heavy, motor may step
out and displacement can occur.
• Motor capacity is small.
• Precision is poor at high speed.
DC servo system
• Positioning precision is accurate.
• Maintenance is required for motor
brushes.
• It is not suitable for rotation at
high speed.
General purpose
inverter and
general purpose
motor
• Multi-speed positioning is
available using a high-speed
counter.
• High precision positioning is not
available.
• Large torque is not available at
start.
(Specialized inverter is required)
AC servo system
• Positioning precision is good.
• Maintenance is not required.
• Positioning address can be easily
changed.
• It is compact, and offers high
power.
Actuator(s) Features and Drawbacks Schematic drawing
Clutch
brake
unit
Constant
quantity
feed hoppe
r
Optical
sensor
Can feed
mechanism
Speed reducer
Motor
Controller
Stepping
motor
DC servo amplifier
DC servo motor
General-purpose
inverter
Motor with brake
Lifter
Sheet material
AC servo amplifier
Cutte
r
AC servo
motor

9
FX Series Programmable Controllers
Introduction to FX Positioning Control Systems
1 The Basics of Positioning Control
1.3 Positioning method type
1
The Basics of Positioning Control
2
Positioning by AC Servo System
3
Components of Positioning Control
4
Learning to Use FX Positioning Control
1.3 Positioning method type
In general, there are two methods to control the movement of a workpiece: speed control and position control.
For basic, more rudimentary positioning, speed control can be used with an inverter and general purpose
motor. For systems where precision is a must, servo systems are required for the advanced handling of pulse
commands.
*1. The stop precision shows a value in a case where the low speed is 10 to 100 mm/s.
Control method Description Schematic drawing
Speed
control
Limit switch
method
Two limit switches are provided in
places where a system’s moving
part passes. At the first limit
switch, the motor speed is
reduced. At the second limit
switch, the motor turns off and the
brake turns on to stop the moving
part.
In this method, because position
controllers are not required, the
system configuration can be
realized at reasonable cost.
(Guideline of stopping precision:
Approximately ±1.0 to 5.0 mm)*1
Pulse count
method
A position detector (such as a
pulse encoder) is set up in a
motor or rotation axis. The pulse
number generated from the
position detector is counted by a
high-speed counter. When the
pulse number reaches the preset
value, the moving part stops.
In this method, because limit
switches are not used, the stop
position can be easily changed.
(Guideline of stopping precision:
Approximately ±0.1 to 0.5 mm)*1
Position
control
Pulse
command
method
An AC servo motor which rotates
in proportion to the input pulse
number is used as the drive
motor.
When the pulse number
corresponding to the movement
distance is input to the servo
amplifier of the AC servo motor,
positioning can be performed at
high speed in proportion to the
pulse frequency.
(Guideline of stopping precision:
Approximately ±0.01 to 0.05
mm)*1
BIM
INV
DC0 to 10V
High speed
Low speed
Limit switch fo
r
changeover to
low speed
Limit switch
for stop
Ball screw
IM: Inductive motor
B: Brake
INV: Inverter
Moving part
Movement
distance
PLG
High speed
Low speed
Ball screw
IM: Inductive motor
PLG: Pulse generator
INV: Inverter
PLC: Programmable controlle
r
Moving part
Movement distance
PLC
High-speed
counter unit
DC0 to
10V
Pulses are
fed back.
IM
INV
PLG SM
Servo
amplifier
Ball screw
SM: Servo motor
PLG: Pulse generator
PLC: Programmable
controller
Moving part
Movement distance
PLC
Position controller
Pulses are
fed back.
Command
pulse

12
FX Series Programmable Controllers
Introduction to FX Positioning Control Systems
2 Positioning by AC Servo System
2.1 Advantages for using an AC servo system
2. Positioning by AC Servo System
2.1 Advantages for using an AC servo system
With an AC servo system, positioning can be performed by many diversified methods. Typically, a position
controller, servo amplifier and servo motor are required for positioning with an AC servo system. The
representative servo system configuration is shown below.
In the latest AC servo systems, conventional weak points have been improved as follows:
• Although the latest systems are completely digital, they are equipped with parameters in conformance to
diversified mechanical specifications and electrical specifications so that simple set-up is possible.
• As frequent operation is enabled by a low inertia motor, the maximum torque is increased and the system
can be applied to a wide variety of machines.
• The latest systems are equipped with an auto tuning function, with which the servo amplifier automatically
detects the load inertia moment and adjusts the gain. This is possible even if the load inertia moment is
unknown.
• The command communication cycle from the controller to the servo amplifier is improved for
synchronization accuracy and better speed/positioning accuracy.
• The latest systems also allow for long-distance wiring, reduced noise resistance, and simplified wiring.
The top advantages to using an AC servo system are described below.
Compact and light servo
system Robust servo system Easy servo system Good cost performance servo system
In the FA workplace, a
downsized AC servo
system occupying less
space is beneficial.
In accordance with severe
operation conditions, a
tougher AC servo system is
often required.
AC servo systems are
easier to handle than
hydraulic equipment. Easy
systems are also flexible for
new staff.
An AC servo system with
good cost performance
saves a company in overall
engineering costs.
Positioning
controller
Deviation
counter
DC ACDCAC DC
Converter
Smoothing
circuit Inverter
PWM (pulse width
modulation) control
Current
control
Speed
command Feedback
current
PLG
SM
Feedback
pulse
Encode
r
Servo
motor
Command
pulse
Servo amplifier
The positioning controller
generates a specified quantity
of forward rotation (or reverse
rotation) pulses at a specified
frequency.
The command pulse number
is subtracted by the feedback
pulse number, and the speed
command to drive the servo
motor is made from the
deviation (accumulated pulse
number).
When the accumulated pulse
number becomes 0, the servo
motor stops.
The servo motor is equipped
with a built-in encoder (pulse
generator), dedicated to high
speed response, and suitable
for positioning control.
Commercial
power supply


Positioning
command
control
Deviation
counter
Smoothing
circuit Inverter
PWM (pulse width
modulation) control
Current
control
Speed
command Feedback
current
Feedback pulse
R
Regenerative
brake
Pulse
magnication
(Electronic gear)
Dynamic
brake
Counter clear
Zero point
return
control
Parameter
Command
pulse
Position controller
Servo
ready
Zero point signal (PG0)
Converter
AC DC DC DC AC
Near point DOG signal
AC power
supply
PLG
SM
Near pointLimit switch
(LS) Limit switch
Drive mechanism
Servo motor
Encoder (pulse generator)
Speed
reducer
Moving element
Ball screw
dog switch (LS)
Auxiliary device such as chuck, drill and cylinder
Servo amplier

13
FX Series Programmable Controllers
Introduction to FX Positioning Control Systems
2 Positioning by AC Servo System
2.2 Examples of AC servo systems
1
The Basics of Positioning Control
2
Positioning by AC Servo System
3
Components of Positioning Control
4
Learning to Use FX Positioning Control
2.2 Examples of AC servo systems
Positioning indicates the operation to move an object, such as a workpiece or tool (drill or cutter), from one
point to another point and to stop it with efficiency and precision.
In other words, the principle of positioning is the control of speed in accordance with the position, performed
to promptly eliminate the remaining distance to the target position. The flexibility to change the target position
electrically and easily is an important requirement.
Several cases of positioning using an AC servo motor are systematically shown below.
Type of machine Description Schematic drawing
Constant feed
In the press/shear process for
cutting, punching, etc., the
processed material is positioned
with high precision to produce a
constant sized product.
Tapping
In order to tap a workpiece, “1. Quick
feed”, “2. Cutting feed” and “3. Quick
return” are performed repeatedly.
Drilling in steel
sheet
In order to perform processing on a
flat face, positioning with high
precision is performed by two motors
(X axis feed motor and Y axis feed
motor).
Index table
The position of the circular table is
indexed. The index position is set on
the outside (digital switch) or the
inside (program). Shortcut drive is
performed depending on the index
position.
Uncoiler Roll feeder
Servo motor
Press main unit
Slide
Drill
M
M
Workpiece
Timing
belt
Pulley Feed
motor
Quick
feed
Cutting
feed
Ball
screw
Quick
return
Drilling
X axis
Workpiece
X-Y table
X axis feed motor
Y axis feed moto
r
Y axis
Drill unit
M
M
Worm
wheel
Servo
motor
Index table

14
FX Series Programmable Controllers
Introduction to FX Positioning Control Systems
2 Positioning by AC Servo System
2.2 Examples of AC servo systems
Lifter moving-
up/down
As negative load is applied on the
servo motor in positioning of the lifter
in the vertical direction, a
regenerative option is also used.
In order to hold the lifter stationary
and prevent drop of the lifter by
power interruption, a servo motor
with an electromagnetic brake is
used.
Cart travel
control
A servo motor is mounted in the
travel cart as the drive source.
A mechanism such as rack and
pinion is adopted to prevent slippage
between the wheels and rails.
Carrier robot
After the conveyor stops, the 2-axis
servo system and the arm lifting
mechanism transfer workpieces to a
palette. The workpiece input
positions on the palette can be set to
many points so that setup change
can be easily performed, even if the
palette position and the palette
shape change.
Type of machine Description Schematic drawing
Lifter
Servo
motor
Servo
amplifier
Regenerative
option
Drive wheel
(on each of left
and right sides)
Cart
Servo moto
r
to drive
slide arm
Servo motor
to drive
travel head
X direction
Workpiece
Y direction
Travel head
Slide arm
A
rm vertical axis
(driven by air cylinder)
Pallet
Conveyor

18
FX Series Programmable Controllers
Introduction to FX Positioning Control Systems
3 Components of Positioning Control and Their Roles
3.1 Positioning controller
3.1 Positioning controller
Positioning controllers use programs and parameters to send positioning commands to the servo amplifier.
Contents related to programs and parameters are described below.
3.1.1 Command pulse control method
There are two types of control formats used for outputting command pulses from an FX Series positioning
controller:
• PLS/DIR (Pulse/Direction) method
• FP/RP (Forward Pulse/Reverse Pulse) method
Each method requires two outputs from the controller to control specific signals for direction and pulse
control. A third method, known as the A phase/B phase method, uses overlapping pulse signals to specify
direction.
1. PLS/DIR method
In the PLS/DIR method, one output sends pulses to the drive unit while the other output specifies the direction
of travel.
*1. “ON” and “OFF” represent the status of the controller’s output.
“H” and “L” respectively represent the HIGH status and the LOW status of the waveform. The
command pulse pattern in the figure assumes negative logic.
2. FP/RP method
In the FP/RP method, each output has a different direction and operates individually to send pulses to the
drive unit.
*2. “ON” and “OFF” represent the status of the controller’s output.
“H” and “L” respectively represent the HIGH status and the LOW status of the waveform. The
command pulse pattern in the figure assumes negative logic.
OFF
Output #1 Pulse train
Output #2 Direction
Forward rotation Reverse rotation
H
L
H
L
1
ON*1*
OFF
Output #1 Forward rotation
pulse train (FP)
Output #2 Reverse rotation
pulse train (RP)
Forward rotation Reverse rotation
*2
OFF
*2
H
L
H
L

21
FX Series Programmable Controllers
Introduction to FX Positioning Control Systems
3 Components of Positioning Control and Their Roles
3.2 Servo amplifier and servo motor
1
The Basics of Positioning Control
2
Positioning by AC Servo System
3
Components of Positioning Control
4
Learning to Use FX Positioning Control
3.2 Servo amplifier and servo motor
The servo amplifier controls the movement quantity and the speed according to the commands given by the
positioning controller. The servo motor then transmits rotation to the drive mechanism after receiving signals
from the servo amplifier.
3.2.1 Positioning control in accordance with command pulse
In accordance with speed and position command pulses from the positioning controller, PWM (pulse width
modulation) control is performed by the main circuit of the servo amplifier in order to drive the motor. The
rotation speed and the rotation quantity are fed back to the amplifier from the encoder attached to the servo
motor.
3.2.2 Deviation counter function
The difference between the command pulses and the feedback pulses counted by the deviation counter in the
servo amplifier is called accumulated pulses.
While the machine is operating at a constant speed, the accumulated pulse quantity is almost constant.
During acceleration and deceleration, the accumulated pulse quantity changes more dramatically.
When the accumulated pulse quantity becomes equivalent to or less than a specified quantity (in-position set
value) after command pulses have stopped, the servo amplifier outputs the positioning complete signal.
The servo motor continues operation even after that. Then, when the accumulated pulse quantity becomes 0,
the servo motor stops.
The time after the servo motor outputs the positioning complete signal until it stops is called the stop settling
time.
3.2.3 Servo lock function
The servo motor is controlled so that the accumulated pulse quantity counted in the deviation counter
becomes 0.
For example, if an external force for forward rotation is applied on the servo motor, the servo motor performs
the reverse rotation operation to eliminate the accumulated pulses.
3.2.4 Regenerative brake function
During deceleration, because the servo motor rotates by the load inertia of the drive mechanism, it functions
as a generator and electric power returns to the servo amplifier.
The regenerative resistor absorbs this electric power and functions as a brake (called a regenerative brake.)
A regenerative brake is required to prevent regenerative over voltage in the servo amplifier when the load
inertia is large and operations are frequently performed.
The regenerative resistor is required when the regenerative power generation quantity during deceleration
exceeds the allowable regenerative electric power of the servo amplifier.
Accumulated pulses in deviation counter Servo motor
Minus pulses Reverse rotation operation
Plus pulses Forward rotation operation
0 (zero) Stop
Stop settling time
Motor speed
Speed
Accumulated
pulses
Command speed
Time
The accumulated pulse quantity is 0, and
positioning is completed.