ENGR 6806 – Motor Control

ENGR 6806 – Motor Control Prepared By: Rob Collett September 15, 2004 Email: robert@engr.mun.ca Office: EN2074

Presentation Outline • Introduction • Motor Basics • H-Bridges • Using The PIC for Motor Control • Motor Encoders • Grounding • Conclusions and Recommendations

1.0 Introduction What Not to Think… • “Our team already has a motor guy… this should be good time to take a nap.” • “Some of this stuff is theory… why is this guy wasting my time with that?” • “I don’t have a clue what he’s talking about.”

2.0 Motor Basics Pop Quiz: A motor is like a(n)… A) Resistor B) Capacitor C) Inductor D) Crazy space-aged device we aren’t really meant to understand

The Answer Is…(Not D) • C) An Inductor!!… sort of…

The Problem: • What’s wrong with the circuit below?

Well, think about it… • An inductor is a short circuit at DC! • This means we’ll have an infinite current! • Infinite current = Infinite Speed!!

Get to the Point… • A motor is like a REAL inductor… not an IDEAL inductor. • It has resistance!

Remember this Waveform! • Note how the current levels off. • This will provide a steady speed.

3.0 H-Bridge Basics • H-Bridges are used to control the speed and direction of a motor. • They control the motor using Power Electronics… transistors to be precise. • Remember transistors for Term 4?

For $1,000,000:What’s a Transistor? • Transistors are electronic devices that can act as either: • Amplifiers • Switches • We’ll be using them as switches that control the flow of power to the motor.

A Closer Look at Transistors • Note how Digital Logic at the Base controls Power Flow in the other two ports

Controlling Motor Speed • By turning our transistors (switches) ON and OFF really fast, we change the average voltage seen by the motor. • This technique is called Pulse-Width Modulation (PWM).

PWM Basics • The higher the voltage seen by the motor, the higher the speed. • We’ll manipulate the PWM Duty Cycle.

The Problem with PWM… • Remember our little talk about motors? • Remember that motors are like inductors? • Remember this waveform?

What’s the Problem? • If we switch our transistors too quickly, the current won’t have enough time to increase.

The Solution: • The period (not to be confused with duty cycle) of our PWM needs to be long enough for the current to reach an acceptable level:

Direction Control using the H-Bridge • The H-Bridge Chip has a “Direction Pin” that can be set using digital logic High/Low • This pin enables/disables flow through the transistors

The H-Bridge Chip • The H-Bridge we’re using (the LMD18200) has 11 pins • Some pins involve logic signals, others involve power signals, others won’t be connected • Power signals = No breadboard • No breadboard = Soldering

H-Bridge Pins • Pin 1: Bootstrap 1 (10nF cap to Pin 2) • Pin 11: Bootstrap 2 (10nF cap to Pin 10) • Pin 2: Output to Motor (M+) • Pin 3: Direction Input (From PIC) • Pin 5: PWM Input (From PIC) • Pin 6: Power Supply (Vs) • Pin 7: Ground • Pin 10: Output to Motor (M-) • Pin 4: Brake (Not Used – Connect to GND) • Pin 8: Current Sense (Not connected) • Pin 9: Thermal Flag (Not connected)

H-Bridge Wiring (From the Lab Handout) But wait… There’s something missing!

Another Problem: • We’re dealing with a high voltages and currents that are being switched at high frequencies. • This is going to cause spiking in our power supply… not to mention a whack of noise. • Surely there must be some kind of component that prevents instantaneous changes in voltage.

Of Course! Capacitors! • Capacitors across the H-Bridge power supply will prevent spiking. • Two parallel capacitors are recommended: • 200uF • 1uf (Be sure to check voltage ratings) • Why two capacitors?

4.0 Using The PIC for Motor Control • We’ll use the PIC to generate digital logic signals to control our H-Bridge transistors • So we’ll need • A digital high/low for direction output_high(PIN_A0); • A PWM for speed control

Setting the PWM Signal • This can be tough because we need to use a timer to set the PWM frequency. • We also need to figure out how to control the PWM duty cycle. • This is going to take some programming!

Setting up a PWM Signal • Step 1: Tell the PIC we want a PWM signal: • setup_ccp1(CCP_PWM); • Step 2: The PIC uses a timer called “Timer2” to control the PWM frequency. We need to set this frequency: • setup_timer_2(T2_DIV_BY_X, Y, Z); But what are X, Y, and Z? - See handout for example.

Setting up a PWM Signal • Step 3: • We said before that setting the PWM Duty Cycle will set the speed of the motor. • So, to start the motor, we could say: • set_pwm1_duty(#); (0 < # < 100) • To stop the motor, we could say: • set_pwm1_duty(0);

5.0 Motor Encoders • Motor Encoders allow for us to track how far our robot has travelled. • The encoders count wheel revolutions using optical sensors. • These sensors count notches on the Drive Shaft of the motor.

Some Encoder Details… • There are 512 notches on the drive shaft. • There is a 5.9:1 gear ratio. (This means the drive shaft spins 5.9x faster than the wheel.) • The top wheel speed is around 800rpm (using a 30V supply).

Some Electrical Details… • The encoders we’ll be using have 4 wires: • 5V Power Supply (Red) • GND (Black) • Channel A a.k.a. CHA (Blue) • Channel B a.k.a. CHB (Yellow) • Channels A&B will give us the signals to count wheel revolutions.

How Encoders Work • CHA and CHB are actually square waves separated by 900.

Counting Encoder Cycles • So, if we know the currentencoder state and the last encoder state, we can tell which direction we’re going. • By counting the number of times we’ve changed states, we can tell how far we’ve gone. • Just remember that there are 4 encoder states per notch!

6.0 Grounding Advice • What is “Ground”? • What is “Ground” on a Robot? • Power Supply Grounds • Batteries and Grounding • Use a Grounding Panel! • Attach your Panel to your Robot!!

Conclusions and Recommendations • Help is here if you need it. • robert@engr.mun.ca • EN 2074 • “My robot isn’t working perfectly.” • Don’t let your robot take years off your life! • Good Luck!

ENGR 6806 – Motor Control