Introduction to Motors

SensorBot

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• What is a motor?

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• Where can you find motors?

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What is a Motor?

• Our bots have two motors to move around.

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• A motor is a rotary machine, driven by an electrical current.

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• Rotary machine = machine with rotating part

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• Converts electrical energy into mechanical energy.

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Electricity

Electromagnet

Spinning Axel

Fan blades

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How does a motor work?

• Inside the motor is an electromagnet.

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• An electromagnet is a device that turns into a magnet when you run electricity through it.

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• You can make an electromagnet just by making a coil of wire and putting electricity through it.

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• You can also put a piece of metal inside of the coil to make the magnetic field stronger.

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How do Electric Motors Work

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• Motor Control

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Changing the speed of a motor

• The speed of a motor can be changed by increasing or decreasing the voltage into the motor.

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• Voltage is the amount of electrical power available to push electricity through a circuit and is measured in units of Volts.

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Controlling a motor

• A battery puts out a continuous level of voltage.

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• Given a circuit with only a battery and a motor there is no way to control the speed of the motor.

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• But in order to get a robot to navigate in a controlled way, we need to be able to speed up or slow down each of its motors independently.

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Change the battery

• The first way to control motor speed is to change the battery that can supply more power:
• AA batteries supply 1.5 volts
• 9V batteries supply 9 volts
• LiPo (Lithium Polymer) batteries supply 3.7 volts

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• Stitch batteries together back-to-back to increase the voltage.
• How much voltage if you put two AA batteries back-to-back? How about 5?

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Change Resistance

• Another way to slow down a motor is to add a resistor to the circuit.

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• A resistor will reduce the electricity flowing through it but not block it entirely.

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• Resistance is measured in a unit called Ohms.

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A sample resistor

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How does this apply to motor speed?

• What if we instead used a variable resistor?

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• Variable resistor = you can change the amount of resistance

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• Potentiometers have a knob that you can turn to change the resistance.

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Knob

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But we can’t swap batteries or change resistors

There are problems with using a variable resistor or changing batteries to control motors:

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• We would need a way to turn the knob.

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• Manually swapping batteries is a bit cumbersome.

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• If we aren’t careful, there is a potential to damage electronic components.

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We will control our motors a different way…

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Pulse width modulation

What would happen if we switch the battery on and off very quickly over and over?

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1. When the switch is closed, electricity can flow through. The motor will start to turn.
2. When the switch is open, electricity can’t flow through. The motor will “coast” until it is turned on again.
3. Then the sequence repeats over and over.

How long the switch is closed during each cycle determines the average voltage applied to the motor over time – this controls its speed.

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This method is called Pulse Width Modulation or PWM!

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Duty Cycle

• How long switch stays closed – called the duty cycle – determines how much voltage is delivered to the motor over time.

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• Your project can open and close the switch about 1000 times per second.

When closed, your project supplies 5 Volts to the motors.

When open, your project supplies 0 Volts to the motors.

This waveform shows the open and closing of the switch that sends power to the motors.

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Some math…

• Your project generates 5-volt PWM signals.

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• If the duty cycle is 50%, what is the average voltage?

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• If the duty cycle is 75%, what is the average voltage?

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• What about 25%?

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Advantages of PWM

Using pulse width modulation eliminates the problems encountered when using a variable resistor to control motor speed.

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• When the switch is open (“off”), no electricity is used.

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• When the switch is closed (“on”), all the electricity is used to drive the motor.

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• The battery will last a lot longer since it’s not on all the time.

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• The motor speed can be fine-tuned much more easily with PWM.

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Recap of Intro

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Take a few minutes and answer questions 1-3 under the

“Motors Assessment”

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TinkRworks, Inc.

501 W Lake Street, Suite 106

Elmhurst, IL 60126

www.TinkRworks.com

© TinkRworks, Inc. All rights reserved

Sparking CREATIVITY

Fostering INNOVATION

Sharpening PROBLEM-SOLVING skills

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