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
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
Motors Convert electrical energy to mechanical energy 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
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
Solution: Motor Driver 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
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
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
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
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