Electrical Actuation System

Electrical Actuation System

Lecture 9

(Chapter 9)

SME 3252: Mechatronics Lecture 7

Introduction

• The electrical systems used as actuators are:

1. Switching devices – mechanical switches and solid-state switch

2. Solenoid type device

3. Drive system – motors (AC, DC, Stepper)


7.5: DC Motor • Has six basic parts -- axle, rotor (a.k.a., armature),

stator, commutator, field magnet(s), and brushes • The stator is the stationary part of the motor -- this

includes the motor casing, as well as two or more permanent magnet pole pieces

• The rotor (together with the axle and attached commutator) rotates with respect to the stator. The rotor consists of windings (generally on a core), the windings being electrically connected to the commutator


http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motdc.html


7.5.1: DC Motor Operation



a) Current in DC Motor



b) Magnetic Field in DC Motor



c) Force in DC Motor



d) Torque in DC Motor



Voltage• DC motors are non-polarized - can reverse

voltage without any bad things happening• Typical DC motors are rated from about 6V-12V• Motors operate at different voltages because

voltage is directly related to motor torque• More voltage, higher the torque• A DC motor is rated at the voltage it is most

efficient at running• Apply too few volts, it just won’t work. If apply

too much, it will overheat and the coils will melt• General rule - try to apply as close to the rated

voltage of the motor• Recommendation - do not surpass 12V


Torque• two torque value ratings given: operating

torque - the torque the motor was designed to give and stall torque - the torque required to stop the motor from rotating

• Torque ratings can change depending on the voltage applied

http://www.societyofrobots.com/mechanics_statics.shtml


3 ways to brake DC motors • Controls Method

Requires an encoder placed onto a rotating part of DC motor. Write an algorithm that determines the current velocity of motor, and sends a reverse command to H-bridge until the final velocity equals zero

• Mechanical Method The mechanical method is what is used on cars. Something with very high friction and wear resistance is needed, e.g. a servo actuated brake l.

• Electronic Method The least reliable, but the easiest to implement. The basic concept of this is that if the power is shorted and ground leads motor, the inductance created by motor in one direction will power motor in the opposite direction. Although motor will still rotate, it will greatly resist the rotation.

http://www.societyofrobots.com/actuators_dcmotors.shtml

http://www.societyofrobots.com/sensors_encoder.shtml

http://www.societyofrobots.com/schematics_h-bridgedes.shtml

http://www.societyofrobots.com/actuators_servos.shtml


• Connect a MOSFET (transistor) and a relay as shown

• The MOSFET turns on the relay, which creates a short between the motor leads

• Turn the MOSFET on (set C high) with microcontroller to brake the motor

• Basically the motor will still have an H-bridge for normal control

• When braking, the H-bridge is turn off and use the braking circuit


7.5.4: Control of d.c. motor

• Control d.c. motor speed from microprocessor – pulse width modulation or PWM

• Taking a constant dc supply voltage and chopping it so that average voltage is varied

• PWM can be obtained using basic transistor circuit

• Transistor is switch on and off based on signal applied to its base (for one direction movement)

• H circuit (four transistors) – enable motor to be in forward and reverse direction


Closed-loop control

• Closed-loop control – use feedback to control motor speed if conditions change

a) Feedback signal by tachogenerator - which gives analog signal

b) Feedback signal by encoder – digital signal

c) System is digital with PWM is used to control average voltage applied to armature


FINAL PROTOTYPE

Analog Distance Sensor

PIC Microcontroller

DC Geared Motor

Motor Driver

IR Sensor


Advantage and disadvantage of d.c. motor


7.6: AC Motor

• AC motors can be classified into 2 – single phase and three phase AC motors

• To have 3 phase power on a robot, use either big and expensive DC to AC converter, or tether it to a wall socket

• AC motors is used if the robot is stationary, such as a robot arm


Voltage

• Polarized (current cannot be reversed)

• Typically from 120-240V AC, usually to match mains power

• Higher voltages generally mean more torque, but also require more power

• Rarely used on mobile robots due to power requirements


Current• 2 types of current: stall and operating current (max and

minimum) 1. Stall Current - The current a motor requires when

powered but held so that it does not rotate 2. Operating Current - The current draw when a motor

experiences zero resistance torque • It is best to determine current curves relating voltage,

current, and required torque for optimization • When a motor experiences a change in torque (such as

motor reversal) expect short lived current spikes • Current spikes can be up to 2x the stall current, and

can fry control circuitry if unprotected • Use diodes to prevent reverse current to your circuitry • Check power ratings of your circuitry and use heat

sinks if needed


Torque

• When buying a motor, consider stall and operating torque (max and minimum)

• Stall Torque - The torque a motor requires when powered but held so that it does not rotate

• Operating Torque - The torque a motor can apply when experiencing zero resistance torque


Control method• Modifying the AC frequency can alter speed and

torque • Encoder - device which counts rotations of wheel

or motorshaft to determine velocity for a control feedback loop

• Tachometer - device which measures current draw of motor to control output torque

• To maintain constant torque at diff speed when freq. varied, a.c. in converted to d.c. by a converter and converted back to a.c. by inverter


Magnetic Brake

Flexible Coupling

Inductive Proximity Sensor

Extension Shaft

Timing Plate

inverter is to vary the input voltage (stator voltage) into the induction motor

PSM student’s project – Wong Kien Fatt


Advantage and disadvantage of AC motor

• Advantage – more rugged, reliable and maintenance free

• Disadvantage – more complicated compared to d.c. motor,


7.7: Stepper Motor• Stepper motors come in two main types –

Variable Reluctance and Permanent Magnet.

Permanent Magnet (PM or tin-can)


How it works (VR)

1. The upper electromagnet is activated and the teeth of the central cog line up accordingly.

2. The upper electromagnet is deactivated and the right one turned on. The closest cog teeth then jump to line up with this. This causes a step (e.g. 1.8° turn).

3. The right electromagnet is deactivated and the lower one is turned on. The cog teeth then jump to line up with the bottom electromagnet. This causes another step.

4. The bottom electromagnet is deactivated and the left-most one turned on. The cog teeth then jump to line up with this. This causes another step. On a motor which has a step angle of 1.8°, 200 steps are required for a full rotation.

http://www.societyofrobots.com/member_tutorials/node/28

Advantages and disadvantages of stepper motor



Servo Motor• DC motors with built in gearing and feedback control

loop circuitry, require no driver• Extremely popular with robot• Most servo motors can rotate about 90 to 180 degrees or

a full 360 degrees or more• unable to continually rotate – e.g. driving wheel• precision positioning makes them ideal for robot arms and

legs, rack and pinion steering• To use a servo, simply connect the black wire to ground,

the red to a 4.8-6V source, and the yellow/white wire to a signal generator (e.g. from microcontroller)

• Vary the square wave pulse width from 1-2ms and your servo is now position/velocity controlled.


Servo wiring• All servos have three wires - Black or Brown is for

ground, Red is for power (~4.8-6V), Yellow, Orange, or White is the signal wire (3-5V).

• Servo Voltage (Red and Black/Brown wires)• Typical operation is from 4.8V to 6V• Recommended is 6V, higher torque

• Signal Wire (Yellow/Orange/White wire) • Signal wire is what you use to command the servo• Simply send an ordinary logic square wave to servo at a

specific wave length, and the servo goes to a particular angle (or velocity if your servo is modified)

• The wavelength directly maps to servo angle.


7805

Diode

(IN4001)

Battery Source

9V330 uF 10 uF GND

IN OUT 5V

0.1 uF

Connect to Microcontroller

Voltage Regulator

Microcontroller power source circuit schematic

18F452

5V

10kΩ

MCLR

VDD

OSC1

OSC2

RB7 Servo 1

RD0

RD1

Switch RB6 Servo 2

RB5 Servo 3

RB4 Servo 4

RB3 Servo 5

RB2 Servo 6

RB1 Servo 7

RB0 Servo 8

VssLimit Switch

From Voltage Regulator

(7805)

RA0Start Button

Crystal 20MHz

30pF

30pF

RD7

RD6

RD5

RD4

RX

TX

RC5

RD2

RC4

RD3

Led 1

Led 2

Led 3

Led 4

Led 5

Led 6

Led 7

Led 8

Connect to Boatloader, 9

Connect to Boatloader, 10

Left Sensor

Right Sensor

RC2

RC3Led 9

Led 10


Square wave signal

• The standard time vs. angle is represented in this chart:

Advantages and disadvantages of servo motor



Sensor system

Servo motor 9V battery for microcontroller

6V / 4.5AH battery for all

outputs

End of Lecture 9

Electrical Actuation System

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