Transcript Micromouse Lecture #2 Power Motors Encoders

Micromouse Meeting #3
Lecture #2
Power
Motors
Encoders
 Microcontroller – pick one yet?
Previous Stuff
 Meet your team
 Some teams were changed
High Level
Diagram
 Everything needs power
 Batteries
Power
 Supply a constant voltage
 Supply as much current as needed
 (Ideally)
=
(almost)
 Different components require different supply voltages
Power
Regulation
 MCU: ~5V
 Gyro: 5V or 3.3V
 Supply too little, components don’t work properly
 Supply too much, components tend to light on fire
 Common voltage divider circuit
Power
Regulation
 Does not work for Micromouse!
 Battery voltage decreases as it discharges
 If input voltage decreases, output voltage also decreases
Solution:
Voltage Regulator
Power
Regulation
 These will output a constant voltage even if the
input voltage changes
 Inside is a complicated mess of transistors and other
components
 Check datasheet for input voltage range
 Convert electrical energy to mechanical energy
Motors
 Two types:
 Brushed
 Brushless
 Brushed motors take a DC signal
 So they are also known as DC motors
Motors:
Brushed
 Power an inductor to rotate a magnet
 Increase the voltage and/or current -> Increase the rotation speed
 Reverse the polarity of the input voltage -> Reverse the rotation
 Most digital microcontrollers do not have an analog signal output
 MCU’s output digital signals – either high or low
 How do we control brushed motors?
 “Fake” analog voltage signal
 Square wave with a certain frequency
 This can be used to control the speed of a motor
Pulse Width
Modulation
(PWM)
• Speed is controlled by rapidly turning the
motor on and off
• Turn the motor on for a greater fraction of
the time to make it rotate faster
• The percent of time the PWM signal is on
is the duty cycle
• 0% duty cycle is same as off all the time;
100% duty is same as on all the time
Microcontrollers have libraries/functions that
make generating PWM signals really easy
 PWM signals can control the speed of the motors easily – cool
 Problem: Connect a pin on a MCU to a motor and output a PWM
Motors: Signal
Power and
turning
 The motor barely moves
 MCUs cannot provide enough current to turn motors at fast
enough speeds
 Another problem: Microcontrollers cannot invert the PWM
signal to rotate the motor in the other direction
Solution: Motor Driver
 Use the PWM signal to control a transistor
Motors: Driver
 The transistor acts as a two-state switch
that can handle lots of current
 The transistor switches on and off
according to the PWM
 The motor can be directly powered by the
battery, but now its speed can be
controlled too
Motors:
Rotation
Control
 Motor driver circuit can pour all the current the battery can supply
to the motor – nice
 Problem: How can the motor change direction?
 Previous circuit allows current to flow in only one direction
Solution: Use H-Bridges
Motor Driver:
H-Bridge
 These use several driver circuits
 All contained in an IC
H-Bridge:
Simplified
diagram
 Turn selected switches on/off to control the current
path
H-Bridge
States
Close these switches:
Motor turns in one direction
Close these switches:
Motor turns in other direction
 Datasheet of H bridge describes which pins does what
 Goal is the same as brushed motors:
rotate something
 Mechanics is different
Motors:
Brushless
 Multiple inductors attract and
repel the magnet
 Has more control over DC motors
 Controlling brushless motors are
more complicated
 But fairly easy to do with IC
chips/software libraries
 While the mouse is moving around the maze, it needs to memorize it
 It needs some way to tell how many cells it has transversed
 So we need some kind of cell counter
Cell Counter
How does the
mouse know
going this far is
four cells long?
Solution: Rotary Encoder
Rotary
Encoder
 Attach something to the wheels to count how many times the
wheels have turned to get distance
 Two major flavors
 Optical
 Magnets with Hall effect sensor
 LED shines light through holes in a disc
Rotary
Encoder:
Optical
 A detector on the other side counts how many times the disc
turns
 Attach magnets to a disc
 Use Hall effect sensors to detect the changing magnetic field
Rotary
Encoder:
Magnetic
Next
 Sensors!
 Meet your team if you haven’t already