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Micromouse Lecture 1

Encoders, Motors, Power, Mini Project #1. Micromouse Lecture 1. 10/24/2014. Power Regulation: A Problem. We cannot use a simple voltage divider to provide a consistent voltage source. As the voltage from the battery decreases during discharge, the output voltage will decrease!.

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Micromouse Lecture 1

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  1. Encoders, Motors, Power, Mini Project #1 Micromouse Lecture 1 10/24/2014

  2. Power Regulation: A Problem • We cannot use a simple voltage divider to provide a consistent voltage source. • As the voltage from the battery decreases during discharge, the output voltage will decrease!

  3. Power Regulation: The Solution • In order to provide a consistent source of potential difference, we need to use voltage regulators. • We will provide you with a 5V regulator to connect to your LiPo battery pack for Mini Project #1.

  4. Power Regulation: Capacitors • Use capacitors to smooth out noise in your signal

  5. Motors • Brushed and Brushless

  6. Motors: Brushed • Brushed motors take a DC signal. • Powers 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. • So how do we control brushed motors?

  7. Pulse Width Modulation (PWM) • Mimics an analog voltage signal • Square wave with a certain frequency • This can be used to control the speed of a motor

  8. 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

  9. Motor Driver: A Problem • We cannot connect the MCU to the motor. • MCUs don’t provide enough current to power motors. • Microcontrollers cannot invert the PWM signal to rotate the motor in the other direction.

  10. Motor Driver: A Solution • Have the PWM control a H-bridge • PWM controls transistors (think of them as switches) that allows the battery to pour all its current to the motor • Easy to control direction

  11. Motor Driver: The H-bridge • Simplified diagram Turn Left Turn Right

  12. Motor Driver: The H-bridge • Actual implementation for two motors void turnLeft() { digitalWrite(1A, HIGH); //Right wheel digitalWrite(2A, LOW); //Right wheel digitalWrite(3A, LOW); //Left wheel digitalWrite(4A, HIGH); //Left wheel analogWrite(motorPwrR, 100); analogWrite(motorPwrL, 100); } void turnRight() { digitalWrite(1A, LOW); digitalWrite(2A, HIGH); digitalWrite(3A, HIGH); digitalWrite(4A, LOW); analogWrite(motorPwrR, 100); analogWrite(motorPwrL, 100); }

  13. Motors: Brushless • Goal is the same as brushed motors: rotate something • Mechanics is different • Multiple inductors attract and repel the magnet • Has more control than DC motors • Controlling brushless motors are more complicated • But fairly easy to do with IC chips/software libraries

  14. Encoders • Helps you determine how far you have travelled in the maze. • Rotary Encoders attached to wheels • Optical • Magnetic with Hall Effect Sensor

  15. Rotary Encoder: Optical • Light reflects off alternating bright or dark areas. The detector determines when the light was shone on it. (Mini Project #1 uses this). • Another method: LED shines through a teeth in a disc to detector on other side.

  16. Rotary Encoder: Magnetic • Attach magnets to a disc • Use Hall effect sensors to detect the changing magnetic field

  17. Now How Does the Code Work? • Count the ticks to see how far the wheels have turned

  18. How do we put an encoder to use? • Encoders are constantly outputting data • Your MCU needs to read the values all the time! • If your MCU is reading values 100% of the time it can’t do anything useful! • Otherwise you are losing data, or must litter your code with checks every few lines! • Not a good solution 

  19. Interrupts to the rescue! • Interrupts allow you to process data and then go back to what you were doing

  20. Interrupts to the rescue! • Interrupts allow you to process data and then go back to what you were doing • “Yoimma let you finish, but this is some of the most important data of ALL TIME”

  21. Interrupts • An interrupt handler is a short function that runs when an external event happens • Rest of the program pauses, and continues after interrupt is done • From the perspective of each, the other doesn’t exist* • *If your interrupt handler runs for too long (and too often) it can choke your entire program!

  22. Using interrupts • Types of interrupts: • RISING • FALLING • CHANGE • LOW • HIGH • Arduino boards: only two interrupt pins • Teensy: all pins!

  23. Sample Interrupt program

  24. Programming with interrupts • The volatile keyword: • Tells the compiler “this value can change at any time!” • MCU will look up value in memory each time and not an old value in a register • Anything your interrupt handler modifies should be volatile, or you may get bugs!

  25. Programming with interrupts • Increment a counter and return (be fast)! • Don’t want to use this counter directly • If you accidentally overwrite it, you might not be able to know how far you went! • Using good coding style you can prevent mistakes! • But I’m too good to make such silly mistakes! • Nonsense, we are all human, mistakes happen!

  26. Programming with interrupts • The static keyword: • Means “this variable/function can only be used in this file only!” • Return value of counter with a function! • Now nobody from the outside can mess with it directly!

  27. How to choose interrupt type • Depends on how fast your encoders are • On super high resolution encoders, it may be sufficient to track a single pin per encoder • On lower resolution encoders its better to track both pins changing • Tracking a pin change gives you even “more” resolution • If a wheel is on the edge between ticks, its possible to get “false positives”

  28. Mini Project #1: Encoders • Demonstrate basic understanding of motor control and encoders. • DUE DATE: 11/7/2014 SIGN UP EARLY • Parts (abridged) • Teensy 3.1 • Motor • H-bridge • Encoders

  29. So What Do I Need to Do? • Keep your eyes peeled for an email for when the spec sheet for Mini Project #1 is up. • Do not hesitate joining a group you don’t know to complete the Mini Projects. Make friends! • Start early!

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