Date post: | 22-Dec-2015 |
Category: |
Documents |
Upload: | anabel-lamb |
View: | 223 times |
Download: | 3 times |
Micromouse Meeting #3Lecture #2
PowerMotorsEncoders
Previous Stuff
Microcontroller – pick one yet?
Meet your team Some teams were changed
High Level Diagram
Power
Everything needs power
Batteries Supply a constant voltage Supply as much current as
needed (Ideally)
=(almost)
Power Regulation
Different components require different supply voltages
MCU: ~5V Gyro: 5V or 3.3V
Supply too little, components don’t work properly
Supply too much, components tend to light on fire
Power Regulation
Common voltage divider circuit
Does not work for Micromouse!
Battery voltage decreases as it discharges
If input voltage decreases, output voltage also decreases
Power Regulation
Solution:Voltage Regulator
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
Motors
Convert electrical energy to mechanical energy
Two types: Brushed Brushless
Motors:Brushed
Brushed motors take a DC signal So they are also known as DC motors
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?
Pulse Width Modulation (PWM)
“Fake” analog voltage signal
Square wave with a certain frequency
This can be used to control the speed of a motor
• 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
Motors: Signal Power and turning
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
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
Motors: Driver
Use the PWM signal to control a transistor
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
Solution: Motor Driver
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
Motor Driver: H-Bridge These use several driver
circuits
All contained in an IC
Solution: Use H-Bridges
H-Bridge: Simplified diagram
Turn selected switches on/off to control the current path
H-Bridge States
Datasheet of H bridge describes which pins does what
Close these switches:Motor turns in one direction
Close these switches:Motor turns in other direction
Motors: Brushless
Goal is the same as brushed motors: rotate something
Mechanics is different 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
Cell Counter
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
How does the mouse know going this far is four cells long?
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
Solution: Rotary Encoder
Rotary Encoder: Optical
LED shines light through holes in a disc
A detector on the other side counts how many times the disc turns
Rotary Encoder: Magnetic
Attach magnets to a disc
Use Hall effect sensors to detect the changing magnetic field
NextSensors!Meet your team if you haven’t already