ENGR 6806 – Motor Control

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ENGR 6806 Motor Control

• Prepared By Rob Collett
• September 15, 2004
• Email robert_at_engr.mun.ca
• Office EN2074

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Presentation Outline

• Introduction
• Motor Basics
• H-Bridges
• Using The PIC for Motor Control
• Motor Encoders
• Grounding
• Conclusions and Recommendations

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1.0 Introduction What Not to Think

• Our team already has a motor guy this should be
  good time to take a nap.
• Some of this stuff is theory why is this guy
  wasting my time with that?
• I dont have a clue what hes talking about.

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2.0 Motor Basics

• Pop Quiz A motor is like a(n)
• A) Resistor
• B) Capacitor
• C) Inductor
• D) Crazy space-aged device we arent really
  meant to understand

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The Answer Is(Not D)

• C) An Inductor!! sort of

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The Problem

• Whats wrong with the circuit below?

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Well, think about it

• An inductor is a short circuit at DC!
• This means well have an infinite current!
• Infinite current Infinite Speed!!

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Get to the Point

• A motor is like a REAL inductor not an IDEAL
  inductor.
• It has resistance!

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Remember this Waveform!

• Note how the current levels off.
• This will provide a steady speed.

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3.0 H-Bridge Basics

• H-Bridges are used to control the speed and
  direction of a motor.
• They control the motor using Power Electronics
  transistors to be precise.
• Remember transistors for Term 4?

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For 1,000,000Whats a Transistor?

• Transistors are electronic devices that can act
  as either
• Amplifiers
• Switches
• Well be using them as switches that control the
  flow of power to the motor.

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A Closer Look at Transistors

• Note how Digital Logic at the Base controls Power
  Flow in the other two ports

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Controlling Motor Speed

• By turning our transistors (switches) ON and OFF
  really fast, we change the average voltage seen
  by the motor.
• This technique is called
• Pulse-Width Modulation (PWM).

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PWM Basics

• The higher the voltage seen by the motor, the
  higher the speed.
• Well manipulate the PWM
• Duty Cycle.

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The Problem with PWM

• Remember our little talk about motors?
• Remember that motors are like inductors?
• Remember this waveform?

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Whats the Problem?

• If we switch our transistors too quickly, the
  current wont have enough time to increase.

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The Solution

• The period (not to be confused with duty cycle)
  of our PWM needs to be long enough for the
  current to reach an acceptable level

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Direction Control using the H-Bridge

• The H-Bridge Chip has a Direction Pin that can
  be set using digital logic High/Low
• This pin enables/disables flow through the
  transistors

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The H-Bridge Chip

• The H-Bridge were using (the LMD18200) has 11
  pins
• Some pins involve logic signals, others involve
  power signals, others wont be connected
• Power signals No breadboard
• No breadboard Soldering

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H-Bridge Pins

• Pin 1 Bootstrap 1 (10nF cap to Pin 2)
• Pin 11 Bootstrap 2 (10nF cap to Pin 10)
• Pin 2 Output to Motor (M)
• Pin 3 Direction Input (From PIC)
• Pin 5 PWM Input (From PIC)
• Pin 6 Power Supply (Vs)
• Pin 7 Ground
• Pin 10 Output to Motor (M-)
• Pin 4 Brake (Not Used Connect to GND)
• Pin 8 Current Sense (Not connected)
• Pin 9 Thermal Flag (Not connected)

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H-Bridge Wiring (From the Lab Handout)

• But wait
• Theres something missing!

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Another Problem

• Were dealing with a high voltages and currents
  that are being switched at high frequencies.
• This is going to cause spiking in our power
  supply not to mention a whack of noise.
• Surely there must be some kind of component that
  prevents instantaneous changes in voltage.

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Of Course! Capacitors!

• Capacitors across the H-Bridge power supply will
  prevent spiking.
• Two parallel capacitors are recommended
• 200uF
• 1uf
• (Be sure to check voltage ratings)
• Why two capacitors?

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4.0 Using The PIC for Motor Control

• Well use the PIC to generate digital logic
  signals to control our
• H-Bridge transistors
• So well need
• A digital high/low for direction
• output_high(PIN_A0)
• A PWM for speed control

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Setting the PWM Signal

• This can be tough because we need to use a timer
  to set the PWM frequency.
• We also need to figure out how to control the PWM
  duty cycle.
• This is going to take some programming!

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Setting up a PWM Signal

• Step 1
• Tell the PIC we want a PWM signal
• setup_ccp1(CCP_PWM)
• Step 2
• The PIC uses a timer called Timer2 to control
  the PWM frequency. We need to set this frequency
• setup_timer_2(T2_DIV_BY_X, Y, Z)
• But what are X, Y, and Z?
• - See handout for example.

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Setting up a PWM Signal

• Step 3
• We said before that setting the PWM Duty Cycle
  will set the speed of the motor.
• So, to start the motor, we could say
• set_pwm1_duty() (0 lt lt 100)
• To stop the motor, we could say
• set_pwm1_duty(0)

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5.0 Motor Encoders

• Motor Encoders allow for us to track how far our
  robot has travelled.
• The encoders count wheel revolutions using
  optical sensors.
• These sensors count notches on the Drive Shaft of
  the motor.

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Some Encoder Details

• There are 512 notches on the drive shaft.
• There is a 5.91 gear ratio. (This means the
  drive shaft spins 5.9x faster than the wheel.)
• The top wheel speed is around 800rpm (using a 30V
  supply).

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Some Electrical Details

• The encoders well be using have 4 wires
• 5V Power Supply (Red)
• GND (Black)
• Channel A a.k.a. CHA (Blue)
• Channel B a.k.a. CHB (Yellow)
• Channels AB will give us the signals to count
  wheel revolutions.

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How Encoders Work

• CHA and CHB are actually square waves separated
  by 900.

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Counting Encoder Cycles

• So, if we know the current encoder state and the
  last encoder state, we can tell which direction
  were going.
• By counting the number of times weve changed
  states, we can tell how far weve gone.
• Just remember that there are 4 encoder states per
  notch!

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6.0 Grounding Advice

• What is Ground?
• What is Ground on a Robot?
• Power Supply Grounds
• Batteries and Grounding
• Use a Grounding Panel!
• Attach your Panel to your Robot!!

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Conclusions and Recommendations

• Help is here if you need it.
• robert_at_engr.mun.ca
• EN 2074
• My robot isnt working perfectly.
• Dont let your robot take years off your life!
• Good Luck!