Micromouse Lecture 1Encoders, Motors, Power, Mini Project #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!

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.

Power Regulation: Capacitors Use capacitors to smooth out noise in your

signal

Motors

Brushed and Brushless

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?

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

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

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.

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

Motor Driver: The H-bridge

Simplified diagram

Turn Left Turn Right

Motor Driver: The H-bridge

void turnRight(){ digitalWrite(1A, LOW); digitalWrite(2A, HIGH); digitalWrite(3A, HIGH); digitalWrite(4A, LOW); analogWrite(motorPwrR, 100); analogWrite(motorPwrL, 100);}

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);}

Actual implementation for two motors

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

Encoders

Helps you determine how far you have travelled in the maze.

Rotary Encoders attached to wheels Optical Magnetic with Hall Effect Sensor

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.

Rotary Encoder: Magnetic Attach magnets to a disc Use Hall effect sensors to detect the

changing magnetic field

Now How Does the Code Work? Count the ticks to see how far the wheels

have turned

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

Interrupts to the rescue!

Interrupts allow you to process data and then go back to what you were doing

Interrupts to the rescue!

Interrupts allow you to process data and then go back to what you were doing

“Yo imma let you finish, but this is some of the most important data of ALL TIME”

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!

Using interrupts

Types of interrupts: RISING FALLING CHANGE LOW HIGH

Arduino boards: only two interrupt pins

Teensy: all pins!

Sample Interrupt program

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!

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!

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!

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”

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

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!