Transcript ME 534: Computer Control of Machines

ME 440: Numerically
Controlled Machine Tools
Electric Motors and Drive Systems
Assistant Prof. Melik Dölen
Department of Mechanical Engineering
Middle East Technical University
Outline – Motors
• Motors in CNC Technology
• Classification of Motors
–
–
–
–
Stepper Motors
DC Motors
Brushless DC Motors
Induction Motors
• Fundamentals of Motor Drives
– DC Motor
– Stepper Motor
Chapter 11
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Electrical Motors
• In most CNC machine tool applications, electrical motors
are extensively used as actuators:
– Axis motion
– Spindle motion
• Four motor systems are common alternatives in machine
tool designs:
– Stepper motors: Simple applications (e.g. desktop manufacturing
tools)
– DC motors: Earlier CNC machine tools and specialized machine
tools
– Brushless DC motors: Principle axis drives for contemporary CNC
machine tools
– AC (Induction) motors: High-power spindle drives.
Chapter 11
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Stepper Motors
• Stepper Motors
– Permanent Magnet
• Relies on rotor magnets
– Variable Reluctance
• Relies on rotor saliency
– Hybrid Motors
• Relies on both rotor saliency and magnets
• Each pulse moves rotor by a discrete angle
(i.e. “step angle”).
• Counting pulses tells how far motor has turned
without actually measuring (no feedback!).
Chapter 11
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Advantages / Disadvantages
 Low cost
 Simple and rugged
 Very reliable
 Maintenance free
 No sensors needed
 Widely accepted in
industry
Chapter 11
 Resonance effects
are dominant
 Rough performance
at low speed
 Open-loop operation
 Consume power even
at no load
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(Simplified) Full-Step Operation
• Rotor of a PM stepper
motor consists of a
permanent magnet:
– Stator has a number of
windings.
• Just as the rotor aligns
with one of the stator
poles, the second
phase is energized.
• The two phases
alternate on and off to
create motion.
• There are four steps.
Chapter 11
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(Simplified) Half-Step Operation
Chapter 11
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Half-Step Operation (Cont’d)
• Commutation sequence has eight steps instead
of four.
• The main difference is that the second phase is
turned on before the first one is turned off.
• Sometimes, both phases are energized at the
same time.
• During the half-steps, the rotor is held in
between the two full-step positions.
• A half-step motor has twice the resolution of a
full-step motor.
– Very popular due to this reason.
Chapter 11
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Winding Connections
Bipolar (4-wire):
Unipolar (5-wire):
1
A
C
3
B
D
1
A
C
2
4
3
B
D
2
4
5
• Unipolar motor:
Unipolar (6-wire):
1
A
4
B
3
C
D
– Current flows through a coil
only in one direction.
2
• Bipolar motor:
– Current flowing through a
winding changes direction
during the operation.
5
6
Chapter 11
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Actual Stepper Motor*
• The stator of a real motor
constitutes more coils
(typically 8).
• These individual coils are
interconnected to form
only two windings:
– one connects coils A, C, E,
and G:
• A and C have S-polarity
• E and G have N-polarity
– one connects coils B, D, F,
and H:
• B and D have S-polarity
• F and H have N-polarity
Chapter 11
ME 440
[*] Courtesy of Microchip.
10
PM Stepper-Motor Animations*
Full-step:
Chapter 11
Half-step:
ME 440
[*] Courtesy of Motorola, Inc.
11
VR Full-Step Motor
• Rotor and stator saliency
• Unequal number of poles
• Stator current effectively
pulls rotor pole in line with
stator pole.
3600
Step angle 
Nr Ns
Chapter 11
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VR Half-Step Motor
• Possible to move rotor
by half steps by exciting
two windings equally.
• Finer steps (a.k.a.
“micro-steps”) are
possible by exciting two
windings unequally.
Chapter 11
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Conventional DC Motor
Courtesy of Motorola, Inc.
Chapter 11
• The stator of a DC motor is composed of
two or more permanent magnet pole
pieces.
• The rotor is composed of windings which
are connected to a mechanical
commutator. In this case the rotor has
three pole pairs.
• The opposite polarities of the energized
winding and the stator magnet attract and
the rotor will rotate until it is aligned with
the stator.
• Just as the rotor reaches alignment, the
brushes move across the commutator
contacts and energize the next winding.
• A spark shows when the brushes switch
to the next winding.
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Brushless DC Motor
• A brushless DC motor (BLDC) has a
rotor with permanent magnets and a
stator with windings.
• It is essentially a DC motor turned
inside out. The brushes and
commutator have been eliminated
and the windings are connected to
the control electronics.
• The control electronics replace the
function of the commutator and
energize the proper winding.
• he energized stator winding leads
the rotor magnet, and switches just
as the rotor aligns with the stator.
• BLDC motors are potentially cleaner,
faster, more efficient, less noisy and
more reliable.
Chapter 11
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AC (Induction) Motor
• Motor is essentially driven like an
AC synchronous motor by
applying sinusoidal current to
motor windings.
• The drive needs to generate 3
currents that are in the correct
spatial relationship to each other
at every rotor position.
• High-resolution optical encoder is
needed to control the
commutation accurately.
• Very smooth low speed rotation.
• Negligible torque ripple.
Chapter 11
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Operating Modes of DC Motor
Tm
Reverse Generator
Forward Motor
ia
_
+
M
Va
M
Va
_
+
ia
m
ia
_
+
M
Va
Va
M
_
+
• In motor mode, the
machine drives the
“load” and needs
energy from the
supply.
• In generator mode,
the “load-side” drives
the machine and it
generates power.
ia
Reverse Motor
Chapter 11
Forward Generator
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“Forward Motor” Control
• Electronically-controlled
(unidirectional) switch is
turned on/off rapidly.
Electronically controlled
power switch
Va
+
+
– Pulse width modulation
VDC
VDC
Va
M
Td
_
Va
t
Tp
• Desired (average) voltage
at the terminals of DC
motor is obtained via
controlling switching times:
S1
Ra
La
+
+
VDC
Td
Va  VDC
 VDC  d
Tp
ia
Back e
a
E.M.F.
_
D1
where Tp is PWM period
(constant) and Td/Tp = d
is called duty cycle.
DC Motor
Chapter 11
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Forward Motor Control (Cont’d)
Mode 1:
S1 :on
La
Ra
ia
+
D1 :off
VDC
ea
• When S1 is turned off, ia
flowing through the motor
cannot be cut off
immediately.
_
• The “clamp” diode allows
current flow in Mode 2:
Mode 2:
S1 :off
D1 :on
– La drives a decaying
current.
La
Ra
ia
VDC
– It must flow somewhere!
+
ea
_
Chapter 11
• If D1 isn’t in place, a very
large voltage will build up
across S1 and blow it up.
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Four-Quadrant Motor Control
D2
D4
S2
S4
M
+
VDC
S3
S1
D1
Half-Bridge
Chapter 11
D3
Half-Bridge
ME 440
• “H” bridge is used to
operate the motor in four
quadrants.
• Driver is composed of two
half-bridges.
• Switches in a half-bridge
cannot turned at the
same time.
– causes short-circuit.
– If one of the switches is
turned, the other must be
off.
20
Forward Motor
Mode 2:
Mode 1:
S2
S4
D2
D4
S2
D2
S1
D1
S3
D3
M
VDC
D4
ia
M
ia
S4
VDC
S3
D3
S1
D1
• To go forward,
– S3 is fully turned on;
– PWM and ~PWM (inverted PWM) signals are applied to S2 and
S1 respectively.
• Unidirectional switch S1 can carry current only in the
indicated direction.
Chapter 11
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Reverse Motor
Mode 2:
Mode 1:
S2
S4
D2
S2
D4
D2
S4
D4
S3
D3
VDC
VDC
ia
S1
D1
M
M
ia
S3
D3
S1
D1
• To go backward,
– S1 is fully turned on;
– PWM and ~PWM signals are applied to S4 and S3
respectively.
Chapter 11
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DC Motor Drivers
• Commercial Motor Drivers
– Include all bells and whistles!
• Custom Solutions (high-power)
– Switches: Power MOSFETs, IGBT
– Needs gate drivers and signal isolation barriers.
• Bridge ICs (upto a few-hundred Watts)
– LMD 18200
– L298
• For driving small DC motors,
– L293D
– ULN 2003A
Chapter 11
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Electromagnetic Relays
• Relays are electromagnets connected
to mechanical switches.
– When the electromagnets are energized,
the switches are pulled into contact.
– Hence, the corresponding circuit is powered
up.
• Relays allow the control of high-power
devices.
– Small power is sufficient to energize
electromagnets in relays.
– Suitable for on/off control of slow devices:
• Pump (AC/DC) motors, solenoids
• Heaters, lamps, etc.
– If compared to solid-state switches, relays
are more susceptible to malfunction.
Chapter 11
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Simple On/Off Control
12V 24V
12V 24V
Relay
Relay
1N4148
ULN2003A
9
1 k
2N3904
RX#
Chapter 11
1N4004
M
RX#
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16
M
1N4004
25
Drives for VR Stepper Motors
• Currents do not need to reverse.
• Circuit uses incomplete switch poles that can pass
current only one direction through motor phase.
Chapter 11
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