Applied Control Systems Robotics

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Applied Control SystemsRoboticsRobotic
Control
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DC Motors

• Most common and cheapest
• Powered with two wires from source
• Draws large amounts of current
• Cannot be wired straight from a PIC
• Does not offer accuracy or speed control

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Stepper Motors

• Stepper has many electromagnets
• Stepper controlled by sequential turning on and
  off of
• magnets
• Each pulse moves another step, providing a step
  angle
• Example shows a step angle of 90
• Poor control with a large angle
• Better step angle achieved with the toothed disc

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Stepper motor operation
Step1
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Stepper motor operation
Step 2
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Stepper motor operation
Step 3
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Stepper motor operation
Step 4
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Stepper Motors

• 3.6 degree step angle gt 100 steps per
  revolution
• 25 teeth, 4 step 1 tooth gt 100 steps for
  25teeth
• Controlled using output Blocks on a PIC
• Correct sequence essential
• Reverse sequence - reverse motor

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Servo motors

• Servo offers smoothest control
• Rotate to a specific point
• Offer good torque and control
• Ideal for powering robot arms etc.
• However
• Degree of revolution is limited
• Not suitable for applications which require
• continuous rotation

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Servo motors

• Contain motor, gearbox, driver controller and
  potentiometer
• Three wires - 0v, 5v and PIC signal
• Potentiometer connected to gearbox - monitors
  movement
• Provides feedback
• If position is distorted - automatic correction

5V
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Servo motors Operation

• Pulse Width Modulation (0.75ms to 2.25ms)
• Pulse Width takes servo from 0 to 150 rotation
• Continuous stream every 20ms
• On programming block, pulse width and output pin
• must be set.
• Pulse width can also be expressed as a variable

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Open and Closed Loop Control

• All control systems contain three elements
• (i) The control
• (ii) Current Amplifiers
• (iii) Servo Motors

• The control is the Brain - reads instruction
• Current amplifier receives orders from brain and
  sends
• required signal to the motor
• Signal sent depends on the whether Open or
  Closed loop
• control is used.

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Open Loop Control

• For Open Loop Control
• The controller is told where the output device
  needs to be
• Once the controller sends the signal to motor it
  does not
• receive feedback to known if it has reached
  desired position
• Open loop much cheaper than closed loop but less
  accurate

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Open Loop Control
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Closed Loop Control

• Provided feedback to the control unit telling it
  the actual
• position of the motor.
• This actual position is found using an encoder.
• The actual position is compared to the desired.
• Position is changed if necessary

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The Encoder

• Encoders give the control unit information as to
  the actual
• position of the motor.
• Light shines through a slotted disc, the light
  sensor counts
• the speed and number of breaks in the light.
• Allows for the calculation of speed, direction
  and distance
• travelled.

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Closed Loop Control

• The desired value is compared to the actual
  value.
• Comparator subtracts actual from desired.
• The difference is the error which is fed to the
  controller
• which generates a control action to eliminate
  the error.