Micromouse Meeting #3 Lecture #2 Power Motors Encoders

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

Next  Sensors!  Meet your team if you haven’t already