Introduction to MotorsIntroduction to MotorsUnderstanding the CEENBot’s muscles

Presented by: Alisa N. Gilmore, P.E.

Senior Lecturer, UNL Computer and Electronics Engineering Dept.NSF ITEST SPIRIT Workshop Summer 2008

The Peter Kiewit Institute Omaha, NE

Robot DefinedRobot Defined

A robot is “an autonomous system which exists in the physical world, can sense its environment, and can act on it to achieve some goals.” – The Robotics Primer by Maja Mataric

Autonomous – programmableExists in physical world – has a bodyCan sense its world– has sensorsCan act on it – possesses effectors &

actuators, i.e. legs, arms (wheels) & muscles (motors)

OverviewOverview

Motors in context of robotics, different types of robots have different types of motors

Overview of motor types / characteristics◦All motors convert electric energy to mechanical

motion◦Motor characteristics: AC or DC power source,

torque, speed performance◦Industrial robotics: AC servo motor ◦Mobile robotics & Hobby robots: dc motor, dc servo

motor, and stepper motorsPrinciple of operation of a DC motor

◦Inside a DC motorPrinciple of operation of stepper motors

◦Performance advantages of stepper motor over DC motor and DC servo motor

CEENBot stepper motor operation/control

Motor BasicsMotor Basics

Either an AC or DC electrical energy source serves as the input to the motor.

The result is mechanical motion of the output shaft, most often a rotation about the shaft, provided the load carried by the shaft does not exceed the maximum load the motor is designed to carry.

Choosing a MotorChoosing a Motor

There are numerous ways to design a motor, thus there are many different types of motors.

The type of motor chosen for an application depends on the characteristics needed in that application.

These include:◦How fast you want the object to move, ◦The weight, size of the object to be moved,◦The cost and size of the motor,◦The accuracy of position or speed control needed.

Motor ParametersMotor Parameters

The level of performance a motor can provide is described by its parameters. These include:

Rated Speed ◦Speed measured in shaft revolutions per minute

(RPM)Torque

◦Rotational force produced around a given point, due to a force applied at a radius from that point, measured in lb-ft (or, oz-in).

HorsePower = Speed x Torque / 5252.11... ◦A measure of work expended: 1 HP = 33,000 foot-

pounds per minute. Torque-Speed performance of a motor

Types of MotorsTypes of Motors

The different types of motors possess different operating characteristics.

A brief overview of some operation characteristics of:◦AC motors ◦DC motors ◦DC servo motors ◦Stepper motors

AC Motor CharacteristicsAC Motor Characteristics

When power is applied, AC motors turn in one direction at a fixed speed.

Both reversable and non-reversable models available

Usually high voltage (110V AC and up)Inexpensive and commonly availableOptimized to run at a fixed, usually high

speed. If the applied load is greater than the

capacity of the motor, the motor will stall and possibly burn out.

DC Motor CharacteristicsDC Motor Characteristics

When power is applied, DC motors turn in one direction at a fixed speed.

They are optimized to run at a fixed, usually high speed.

Most common found in toys, hobby planes, inexpensive robots, blender, toothbrush, screwdriver, etc.

Speed can be varied if a (pulse width modulation) PWM controller is added.

Almost all can be reversed.Inexpensive and commonly available.Not suitable for positioning unless some kind of

position feedback is added.If the applied load is greater than the capacity of

the motor, the motor will stall and possibly burn out.

DC Servo MotorsDC Servo Motors

Applications that require Servo motors involve control of acceleration, velocity, and/or position to very close tolerances. These motors allow for fast starts, stops and reversals, and very accurate control.

DC servo motors consist of a DC motor combined with feedback for either position or speed.

A servo system is closed loop with a motor, feedback signal, desired input signal, and a controller which constantly adjusts the position or speed in reaction to the feedback.

Servo motor controllers are complex.

Stepper MotorsStepper Motors

A stepper motor will not automatically turn when power is applied.

It requires a separate controller circuit to cause the motor to move.

Controllers for stepper motors are easier to implement than closed loop servo systems.

Precise positioning is possible by keeping count of steps, no feedback is required. It is open loop.

They are inexpensive and commonly available, especially in salvaged computer equipment.

Note: If the applied load is greater than the capacity of the motor, the motor may not step, thereby making precise positioning no longer possible.

DC Electric MotorsDC Electric Motors

DC Electric Motors use Direct Current (DC) sources of electricity:◦Batteries◦DC Power supply

Principle of How Motors Work: Electrical current flowing in a loop of wire will

produce a magnetic field across the loop.

When this loop is surrounded by the field of another magnet, the loop will turn, producing a force (called torque) that results in mechanical motion.

Motor BasicsMotor Basics

Motors are powered by electricity, but rely on principles of magnetism to produce mechanical motion.

Inside a motor we find: ◦Permanent magnets, ◦Electro-magnets, ◦Or a combination of the two.

MagnetsMagnets◦A magnet is an object that possesses a

magnetic field, characterized by a North and South pole pair.

◦A permanent magnet (such as this bar magnet) stays magnetized for a long time.

◦An electromagnet is a magnet that is created when electricity flows through a coil of wire. It requires a power source (such as a battery) to set up a magnetic field.

Current in a coil creates a Current in a coil creates a MagnetMagnet

Current Flowing through a coil or wireLEFT: Current Enters A North Pole on TopRIGHT: Current Enters B (Reversed) North Pole on Bottom

A Simple ElectromagnetA Simple ElectromagnetA Nail with a Coil of Wire

Q – How do we set up a magnet?A – The battery feeds current through the coil of

wire. Current in the coil of wire produces a magnetic field (as long as the battery is connected).

A Simple ElectromagnetA Simple ElectromagnetA Nail with a Coil of Wire

Q - How do we reverse the poles of this electromagnet?

A – By reversing the polarity of the battery!

+ -

S N

The Electromagnet in a Stationary The Electromagnet in a Stationary Magnetic FieldMagnetic Field

If we surround the electromagnet with a stationary magnetic field, the poles of the electromagnet will attempt to line up with the poles of the stationary magnet.

The rotating motion is transmitted to the shaft, providing useful mechanical work. This is how DC motors work!

OPPOSITE POLES ATTRACT!

DC Motor Operation PrinciplesDC Motor Operation Principles Once the poles align, the nail (and shaft) stops rotating. How do we make the rotation continue?

By switching the poles of the electromagnet. When they line up again, switch the poles the other way, and so on.

This way, the shaft will rotate in one direction continuously!

Brushed DC Motor Brushed DC Motor ComponentsComponents

How the Commutator WorksHow the Commutator WorksAs the rotor turns, the commutator terminals

also turn and continuously reverse polarity of the current it gets from the stationary brushes attached to the battery.

Inside a Toy MotorInside a Toy Motor(Similar to TekBot Motor)(Similar to TekBot Motor)

Inside the Motor, cont.Inside the Motor, cont.

Advantages of Stepper MotorAdvantages of Stepper Motor

The DC motors on the TekBot offer limited speed control and low torque.

The CEENBot uses a stepper motor for each

wheel.

The stepper motors on the CEENBot enables accurate wheel positioning with high holding torque and allows for open-loop speed control (wheel position feedback is option).

Stepper Motor OperationStepper Motor Operation

A stepper motor consists of:◦ A permanent magnet rotating shaft (or rotor)◦ Electromagnets on the stator – the stationary portion that

surrounds the motor

The stepper motor moves as the permanent rotor magnet attempts to line up with the poles of the electromagnets on the stator.

The electromagnets are digitally switched to change their pole orientation, which when done in a sequence produces continuous rotation of the rotor.

http://www.interq.or.jp/japan/se-inoue/e_step1.htm

The smallest step of angular rotation a stepper motor can make is called its resolution.

Unlike the example, which had 90 degrees per step resolution, real motors employ a series of mini-poles on the stator and rotor to increase resolution.

The same sequence of 4 stepping phases is

used to control this scenario. There is no increase in control complexity.

http://www.interq.or.jp/japan/se-inoue/e_step1.htm

CEENBot stepper motors have a resolution of 1.8 degrees per step.

◦Q: How many steps are needed to make 1 complete wheel revolution?

Because the rotor is fixed by magnetism in the stationary condition, the stationary torque is large. It allows one to make a precise stop at some angle and hold it there.◦The CEENBot can better hold its position on a ramp.

Speed control is achieved by digitally cycling through the phases at a desired speed of rotation.

A microcontroller is used to reverse the current after each step, which changes the poles of the corresponding electromagnets.

Unipolar & Bipolar SteppersUnipolar & Bipolar Steppers

The stepper motor example is similar to the CEENBot motor, except that it is unipolar.

It has 6 wires to connect, verses the 4 wires of the bipolar stepper motors you will install on the CEENBot.

The difference is the bipolar provides greater torque since an entire coil is energized instead of a half coil for each state of the electromagnet.

The unipolar is simplier to control since the two coils that make up the stepper are centertapped, a wire is connected midway on each coil and is tied to power. To reverse power, simply alternate the grounding of one of the two terminals connected to a coil. This reverses current flow, and thus reverses the poles of the electromagnet. However, only one half of each coil is energized at a time.

Bipolar motors require a slightly more involved controller that must reverse the current flow through the coils by alternating the polarity of the terminals.

This is done simply with the aid of a microcontroller.

ReferencesReferences

“The Difference Between Stepper Motors, Servos, and RC Servos” by Roger Arrick http://www.arrickrobotics.com/motors.html

Making Things – “General Information on Motors” http://www.makingthings.com/teleo/products/documentation/app_notes/motors_general.htm

“How Stepper Motors Work” by Images Scientific Instruments http://www.imagesco.com/articles/picstepper/02.html

CEENBot Stepper Motor & PM DC Motor Testing Unit Operations Manual by Ben Barenz, CEEN Student

Hansen Corp. “Servo motors” http://www.hansen-motor.com/servo-motors.htm

Animated operation of a Unipolar stepper motor: http://www.interq.or.jp/japan/se-inoue/e_step1.htm

Basic Motor Theory by Reliance Electric: http://www.reliance.com/mtr/mtrthrmn.htm