Servo Motor Control
Design Problem
• You are to design an automated goalie for an air
hockey table. The goalie can be 1 inch wide.
• Tasks
– determine puck location
– position blocker
EML 2023
Department of Mechanical and Aerospace Engineering
2
Concepts
• You are to design an automated goalie for an air
hockey table. The goalie can be 1 inch wide.
• Tasks
– determine puck location
• camera placed above
table
– position blocker
EML 2023
Department of Mechanical and Aerospace Engineering
3
Concepts
• You are to design an automated goalie for an air
hockey table. The goalie can be 1 inch wide.
• Tasks
– determine puck location
• camera placed above
table
– position blocker
• linear motion of blocker
piece using belt and
pulleys
• linear motion of blocker
using screw
• linear motion of blocker
using slider crank mechanism
• rotational motion of blocker piece
EML 2023
Department of Mechanical and Aerospace Engineering
4
Concepts
• You are to design an automated goalie for an air
hockey table. The goalie can be 1 inch wide.
• Tasks
– determine puck location
• camera placed above
table
– position blocker
• linear motion of blocker
piece using belt and
pulleys
• linear motion of blocker
using screw
• linear motion of blocker
using slider crank mechanism
• rotational motion of blocker piece
EML 2023
Department of Mechanical and Aerospace Engineering
5
Concepts
• You are to design an automated goalie for an air
hockey table. The goalie can be 1 inch wide.
• Tasks
– determine puck location
• camera placed above
table
– position blocker
• linear motion of blocker
piece using belt and
pulleys
• linear motion of blocker
using screw
• linear motion of blocker
using slider crank mechanism
• rotational motion of blocker piece
EML 2023
Department of Mechanical and Aerospace Engineering
6
Concepts
• You are to design an automated goalie for an air
hockey table. The goalie can be 1 inch wide.
• Tasks
– determine puck location
• camera placed above
table
– position blocker
• linear motion of blocker
piece using belt and
pulleys
• linear motion of blocker
using screw
• linear motion of blocker
using slider crank mechanism
• rotational motion of blocker piece
EML 2023
Department of Mechanical and Aerospace Engineering
7
Concept Selection
• list the pro’s and con’s of each design
• formulate concept evaluation table
EML 2023
Department of Mechanical and Aerospace Engineering
8
How does this work?
gear reducer
motor
desired steering angle
how is the motor
connected to the
steering shaft?
how does the
computer cause
the motor to turn?
how does the
computer know
the current
steering angle?
EML 2023
Department of Mechanical and Aerospace Engineering
9
DC Motor
• a typical DC motor can rotate
at a top speed of 2600 rpm
(43⅓ rev/sec)
• gearing is used which slows
the speed of the output shaft,
but increases the torque
• gear train
• worm gear drive
• epicyclic gear train
EML 2023
Department of Mechanical and Aerospace Engineering
10
gear train
epicyclic gear train
11
worm gear drive
How does this work?
gear reducer
motor
desired steering angle
how is the motor
connected to the
steering shaft?
how does the
computer cause
the motor to turn?
how does the
computer know
the current
steering angle?
EML 2023
Department of Mechanical and Aerospace Engineering
12
Motor and Gear Box
how is the motor
connected to
ground?
how is the motor
connected to the
steering shaft?
EML 2023
Department of Mechanical and Aerospace Engineering
13
Motor and Gear Box
EML 2023
Department of Mechanical and Aerospace Engineering
14
motor with
gear reducer
flex coupling
EML 2023
Department of Mechanical and Aerospace Engineering
15
How does this work?
gear reducer
motor
desired steering angle
how is the motor
connected to the
steering shaft?
how does the
computer cause
the motor to turn?
how does the
computer know
the current
steering angle?
EML 2023
Department of Mechanical and Aerospace Engineering
16
Servo Motor Control
select means of angle measurement
– optical encoder
• incremental
• absolute
– potentiometer
– resolver
how does the
computer know
the current
steering angle?
EML 2023
Department of Mechanical and Aerospace Engineering
17
Incremental Optical Encoder
• this disk has 12 slots (30º resolution)
• typical incremental encoders
have up to 2540 counts per
revolution (0.142º resolution)
($488.41 each at McMaster-Carr)
EML 2023
Department of Mechanical and Aerospace Engineering
18
Incremental Optical Encoder
EML 2023
Department of Mechanical and Aerospace Engineering
19
Incremental Optical Encoder
you can count pulses, but which
way is it rotating?
EML 2023
Department of Mechanical and Aerospace Engineering
20
Incremental Optical Encoder
an encoder will typically also have an
index that will ‘blink’ once each revolution
EML 2023
Department of Mechanical and Aerospace Engineering
21
Incremental Optical Encoder
quadrature:
- there are 4n ‘events’ per one revolution of the disk
- the resolution can in effect be four times the number of slots
EML 2023
Department of Mechanical and Aerospace Engineering
22
Incremental Optical Encoder
note:
many encoders will have an index that will pulse once per revolution
EML 2023
Department of Mechanical and Aerospace Engineering
23
Honeywell / Clarostat 600128CN1
Allied Electronics part # 753-0059
cost:
$41.67
EML 2023
Department of Mechanical and Aerospace Engineering
24
Incremental Optical Encoder
problem: - you start up the device
- where are you when you start?
EML 2023
Department of Mechanical and Aerospace Engineering
25
Absolute Optical Encoder
BEI brand encoder $455
EML 2023
Department of Mechanical and Aerospace Engineering
26
Homing and Limit Protection
• for an incremental optical encoder, we need to add a
switch to set the homing position
• we also need to have limit switches
EML 2023
Department of Mechanical and Aerospace Engineering
27
motor with
gear reducer
flex coupling
optical encoder
homing switch
limit switch
EML 2023
Department of Mechanical and Aerospace Engineering
28
How does this work?
gear reducer
motor
desired steering angle
how is the motor
connected to the
steering shaft?
how does the
computer cause
the motor to turn?
gear reducer and flex
couplings (if needed)
computer controls the current
going to the motor
how does the
computer know
the current
steering angle?
incremental optical encoder
with homing switch to initialize
position
EML 2023
Department of Mechanical and Aerospace Engineering
29
How does the computer control the motor?
control the current (via the voltage)
EML 2023
Department of Mechanical and Aerospace Engineering
30
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Department of Mechanical and Aerospace Engineering
31
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Department of Mechanical and Aerospace Engineering
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Department of Mechanical and Aerospace Engineering
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Summary of Closed-Loop Control
computer controls the
current going to the
motor
desired steering angle
compared to current steering angle
gear reducer
motor
gear reducer and flex
couplings (if needed)
connect motor to
steering shaft
incremental optical
encoder (with homing
switch to initialize
position) feeds back
current motor position
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EML 2023
Department of Mechanical and Aerospace Engineering
38
EML 2023
Department of Mechanical and Aerospace Engineering
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EML 2023
Department of Mechanical and Aerospace Engineering
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