Transcript ENGR 6806 – Motor Control

ENGR 6806 – Motor Control
Prepared By: Rob Collett
September 15, 2004
Email: [email protected]
Office: EN2074
Presentation Outline
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Introduction
Motor Basics
H-Bridges
Using The PIC for Motor Control
Motor Encoders
Grounding
Conclusions and Recommendations
1.0 Introduction
What Not to Think…
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“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 don’t have a clue what he’s talking
about.”
2.0 Motor Basics
Pop Quiz: A motor is like a(n)…
A) Resistor
B) Capacitor
C) Inductor
D) Crazy space-aged device we
aren’t really meant to
understand
The Answer Is…(Not D)
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C) An Inductor!!… sort of…
The Problem:
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What’s wrong with the circuit
below?
Well, think about it…
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An inductor is a short circuit at DC!
This means we’ll have an infinite current!
Infinite current = Infinite Speed!!
Get to the Point…
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A motor is like a REAL inductor…
not an IDEAL inductor.
It has resistance!
Remember this Waveform!
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Note how the current levels off.
This will provide a steady speed.
Vs Vs  t 
i (t )   e
R R
  LR
3.0 H-Bridge Basics
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H-Bridges are used to control the
speed and direction of a motor.
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They control the motor using
Power Electronics… transistors to
be precise.
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Remember transistors for Term 4?
For $1,000,000:
What’s a Transistor?
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Transistors are electronic devices
that can act as either:
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Amplifiers
Switches
We’ll be using them as switches
that control the flow of power to the
motor.
A Closer Look at Transistors
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Note how Digital Logic at the Base
controls Power Flow in the other two
ports
Controlling Motor Speed
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By turning our transistors
(switches) ON and OFF really
fast, we change the average
voltage seen by the motor.
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This technique is called
Pulse-Width Modulation
(PWM).
PWM Basics
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The higher the voltage seen by the
motor, the higher the speed.
We’ll manipulate the PWM
Duty Cycle.
The Problem with PWM…
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Remember our little talk about
motors?
Remember that motors are like
inductors?
Remember this waveform?
What’s the Problem?
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If we switch our transistors too
quickly, the current won’t have
enough time to increase.
The Solution:
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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:
Direction Control using
the H-Bridge
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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
The H-Bridge Chip
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The H-Bridge we’re using (the
LMD18200) has 11 pins
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Some pins involve logic signals,
others involve power signals,
others won’t be connected
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Power signals = No breadboard
No breadboard = Soldering
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H-Bridge Pins
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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)
H-Bridge Wiring
(From the Lab Handout)
But wait…
There’s something missing!
Another Problem:
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We’re dealing with a high voltages
and currents that are being
switched at high frequencies.
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This is going to cause spiking in
our power supply… not to mention
a whack of noise.
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Surely there must be some kind of
component that prevents
instantaneous changes in voltage.
Of Course! Capacitors!
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Capacitors across the H-Bridge
power supply will prevent spiking.
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Two parallel capacitors are
recommended:
200uF
 1uf
(Be sure to check voltage ratings)
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Why two capacitors?
4.0 Using The PIC for
Motor Control
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We’ll use the PIC to generate
digital logic signals to control our
H-Bridge transistors
So we’ll need
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A digital high/low for direction
output_high(PIN_A0);
A PWM for speed control
Setting the PWM Signal
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This can be tough because we need
to use a timer to set the PWM
frequency.
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We also need to figure out how to
control the PWM duty cycle.
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This is going to take some
programming!
Setting up a PWM Signal
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Step 1:
Tell the PIC we want a PWM signal:
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setup_ccp1(CCP_PWM);
Step 2:
The PIC uses a timer called “Timer2”
to control the PWM frequency. We
need to set this frequency:
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setup_timer_2(T2_DIV_BY_X, Y, Z);
But what are X, Y, and Z?
- See handout for example.
Setting up a PWM Signal
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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:
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set_pwm1_duty(#); (0 < # < 100)
To stop the motor, we could say:
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set_pwm1_duty(0);
5.0 Motor Encoders
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Motor Encoders allow for us to
track how far our robot has
travelled.
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The encoders count wheel
revolutions using optical sensors.
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These sensors count notches on
the Drive Shaft of the motor.
Some Encoder Details…
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There are 512 notches on the drive
shaft.
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There is a 5.9:1 gear ratio. (This
means the drive shaft spins 5.9x
faster than the wheel.)
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The top wheel speed is around
800rpm (using a 30V supply).
Some Electrical Details…
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The encoders we’ll be using have
4 wires:
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5V Power Supply (Red)
GND (Black)
Channel A a.k.a. CHA (Blue)
Channel B a.k.a. CHB (Yellow)
Channels A&B will give us the
signals to count wheel revolutions.
How Encoders Work
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CHA and CHB are actually square
waves separated by 900.
Counting Encoder Cycles
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So, if we know the current encoder
state and the last encoder state,
we can tell which direction we’re
going.
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By counting the number of times
we’ve changed states, we can tell
how far we’ve gone.
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Just remember that there are 4
encoder states per notch!
6.0 Grounding Advice
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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!!
Conclusions and
Recommendations
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Help is here if you need it.
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[email protected]
EN 2074
“My robot isn’t working perfectly.”
Don’t let your robot take years off
your life!
Good Luck!