Virtual Mechanisms 1

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Virtual Mechanisms 1

• Introduction to Engineering Science
• Wellesley Intersession
• January 2006

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DC Motor Model
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Motor Driver Circuit
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Computer Output Controls Motor Current

• Computer Outputs a voltage Imtr_cmd 24
• Op-amp gets voltage to appear across a known
  resistor Rckt 24 Ohms
• Voltage V across Rckt generates current
• I (Imtr_cmd 24) / 24
• All current I going through Rckt also goes
  through motor.
• How should we command desired torque?
• Imtr_cmd tdesired / Kmtr

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Actual Motor Voltage is read into computerActual
Motor Current is read into computer

• Actual Motor Voltage reading is straight
• Actual 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 2 3