Virtual Mechanisms

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Virtual Mechanisms

• ECS
• Fall 2005

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DC Motor Model
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Motor Driver Circuit
Vpcout
Imtr
IRckt
Rckt
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Computer Output Controls Motor Current

• Computer Outputs a voltage Vpcout
• Op-amp gets voltage to appear across a known
  resistor Rckt
• Voltage V across R generates current
• IRckt Vpcout / Rckt
• All current I going through R also goes through
  motor.
• Thus, motor current Imtr Vpcout / Rckt
• How should we command desired torque?
• Vpcout Rckt tdesired / Kmtr

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Motor Driver Circuit
Vpcout
Imtr
IRckt
Rckt
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New Motor Drive Circuit
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Caveats for New Circuit

• You MUST rewire the terminal board, and connect
  screw terminals 2, 4, 6, and 8 together, and
  separately terminals 3, 5, 7, and 9 together.
• Finally, you MUST turn OFF differential mode on
  your Simulink analog inputs, and set up Simulink
  blocks to average together A/D input channels 1,
  3, 5, and 7, and separately, channels 2, 4, 6,
  and 8.

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Motor Voltage is read into computerMotor Current
is read into computer

• Motor Voltage reading is straight
• Motor Current reading is multiplied by Rckt to
  create a voltage for the A/D
• How do we figure out w from motor voltage?
• Vemf Vmtr Imtr Rmtr
• w Vemf / Kmtr

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What we can now do

• Command Motor Torque
• by commanding desired motor current
• Check if we got torque we asked for
• by checking actual motor current
• Measure motor speed
• by measuring motor voltage and
• subtracting IR drop of motor resistance

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How do we measure motor angle?

• Integrate w dt !
• Is this reliable over long periods?
• No!
• What goes wrong?
• Drift!
• What can we do to get decent short-term readings?
• Leak!
• Insert a high-pass filter
• Dont let low frequencies through (especially DC)
• Let high frequencies through

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Huygens Eureka Notebook entry, January 20,
1675 Showing the invention of the hair spring
balance wheel clock mechanism
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Thuret Clock, 1675Built to demonstrate the
invention
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Lab 8
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Todays Lab 8 SystemA mixture of simulated lab
set-upActual Control System
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Simulated Motor
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Simulated Motor
W
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Simulated Physical World
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Simulated Physical World
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Simulated Electronics
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Simulated Electronics
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Inverse Model of Electronics(Part of Actual
Control System)
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Inverse Model of Electronics(Part of Actual
Control System)
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Inverse Model of Motor(Part of Actual Control
System)
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Inverse Model of Motor(Part of Actual Control
System)
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Inverse Model PhysicalMotor Electronics
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Inverse Model of Friction Added Virtual
Damping(Part of Actual Control System)
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Inverse Model of Friction Added Virtual
Damping(Part of Actual Control System)
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Added Spring(Part of Actual Control System)
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Added Spring(Part of Actual Control System)
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High-Pass Filterto Prevent Integrator Drift
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SIMULINKDEMOS

• Next Time
• Modeling the
• ESCAPEMENT