Servo Motor Control

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Servo Motor Control
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Design Project

• You are to design a mechanical device that can
  tilt a given labyrinth board in two directions.

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One Concept

• Could use two motors.
• Must be able to control the rotation angle of
  each.

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Another Example

• Control the steering angle of an autonomous
  ground vehicle.
• Must be able to control the rotation angle.

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Design Choices

• What power source will be used?
• How much power is needed?
• Is a speed reducer needed?
• Open-loop or closed-loop control?

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Design Choices

• What power source will be used?
• electric servo motor
• electric stepper motor
• hydraulic actuator
• pneumatics

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Design Choices

• How much power is needed?
• determine the maximum product of motor torque
  times angular velocity

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Design Choices

• Is a speed reducer needed?
• a typical DC motor can rotate at a top speed of
  2600 rpm (43? rev/sec)
• gear box
• epicyclic gear train
• ball screw drive

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gear box
worm gear drive
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epicyclic gear train
ball screw drive
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Design Choices

• Open-loop or closed-loop control?
• open-loop
• well suited for remote control using a joystick
• computer control can be implemented using a
  stepper motor
• closed-loop
• requires a sensor to measure the motors rotation
  angle

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Design Choices

• What power source will be used?
• electric servo motor
• How much power is needed?
• experiment or perform analysis(very little
  torque will be needed to rotate the labyrinth
  board further the rotational speed will be very
  low)
• Is a speed reducer needed?
• yes
• Open-loop or closed-loop control?
• closed-loop

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How does this work?
motor
gear reducer
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?
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Motor and Gear Box
how is the motor connected to the steering shaft?
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Motor and Gear Box
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motor with gear reducer
flex coupling
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Servo Motor Control

• select means of angle measurement
• optical encoder
• incremental
• absolute
• potentiometer
• resolver

how does the computer know the current steering
angle?
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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)

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Incremental Optical Encoder
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Incremental Optical Encoder
you can count pulses, but which way is it
rotating?
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Incremental Optical Encoder
an encoder will typically also have anindex that
will blink once each revolution
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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
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Incremental Optical Encoder
note many encoders will have an index that will
pulse once per revolution
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Honeywell / Clarostat 600128CN1 Allied
Electronics part 753-0059
cost 41.67
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Incremental Optical Encoder
problem - you start up the device - where are
you when you start?
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Absolute Optical Encoder
BEI brand encoder 455
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How does this work?
motor
gear reducer
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)
how does the computer know the current steering
angle?
computer controls the current going to the motor
incremental optical encoder with homing switch to
initialize position
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How does the computer control the motor?
control the current (via the voltage)
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motor with gear reducer
flex coupling
optical encoder
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Closed Loop Control
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Homing and Limit Protection

• we need to add a switch to set the homing
  position
• we need to have overshoot switches

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motor with gear reducer
flex coupling
optical encoder
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Summary of Closed-Loop Control
computer controls the current going to the motor
motor
gear reducer
desired steering angle compared to current
steering angle
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