by: val tocitu jason kulpe alexandre mariuzza me 4447/6405 october 29 th, 2009

By: Val Tocitu

Jason KulpeAlexandre Mariuzza

ME 4447/6405October 29th, 2009

10/29/2009Pulse Width Modulation - Val Tocitu, Jason Kulpe, Alex Mariuzza 2

Introduction and definitions Types of PWM Methods of generation Characteristics of PWM Applications and examples Implementation on the HCS12

Presenter: Val Tocitu

What is it?Output signal alternates between on and off within

specified periodControls power received by a deviceThe voltage seen by the load is directly proportional

to the source voltage


DefinitionsDuty Cycle: on-time / periodVlow is often zero


LOWHIAVG VDDVV )1(

Types of Pulse WidthPulse center fixed, edges modulatedLeading edge fixed, tailing edge modulatedTailing edge fixed, leading edge modulatedPulse Width constant, period modulated


Types of Pulse Width


Analog PWM signals can be made by combining a saw- tooth waveform and a sinusoid


PWM output is formed by the intersection of the saw-tooth wave and sinusoid

Analog Generation of PWM

Digital Methods of Generating PWMDigital: Counter used to handle transitionDelta : used to find the PWM at a certain limitDelta Sigma: used to find the PWM but has advantage of

reducing optimization noise


Applications to DC Motors


• The voltage supplied to a DC motor is proportional to the duty cycle

• Both brushed and brushless motors can be used with PWM

• Both analog and digital control techniques and components are available

Three Phase AC motors with PWM

3 different AC currents at different phasesPhase: 120 degrees apart

Creates constant power transfer

Rotating magnetic fieldPulses substitute for AC

current


Space Vector ModulationUsed for three-phase AC motorsConvert DC current to AC currentGates turned on/off at different intervals3 PWM created


Motor Control Diagrams


• average value proportional to duty cycle, D• low power used in transistors used to switch

the signal• fast switching possible due to MOSFETS

and power transistors at speeds in excess of 100 kHz

• digital signal is resistant to noise • less heat dissipated versus using resistors

for intermediate voltage values


Advantages of PWM


CostComplexity of circuitRadio Frequency InterferenceVoltage spikes Electromagnetic noise

Disadvantages of PWM



Presenter: Jason Kulpe

• In the past, motors were controlled at intermediate speed by using resistors to lower delivered power

• Electric stove heater• Lamp dimmers• Voltage regulation – convert 12 volts to 5 volts by

having a 41.7% duty cycle• Sound production: PWM controlled signals give

sound effects similar to a chorus• Power transfer: PWM used to reduce the total

power given to a load without relying on resistive losses


Applications of PWM

• commonly used in toys• lowpass filter smooths out transients from harmonic effects• frequency values of harmonics doesn’t change, but the

amplitude does, which adjusts the analog output signal


PWM used with D/A conversion


• clock signal is found “inside” PWM signal• more resistant to noise effects than binary data alone• effective at data transmission over long distance transmission lines

PWM used to transmit data in telecommunications

1. Must be at least 10 times higher than the control system frequency

2. Higher than 20kHz – audible frequency of sounds to avoid annoying sound disturbances, caused by magnetostriction

3. If too low the motor is pulsed, not continuous, because the motor’s inductance can not maintain the current

4. Inverse of frequency should be much less than the motor/load time constant

5. Higher error from ripple voltages

Frequency of the PWM Signal Upper LimitsLower Limits

1. If too high the inductance of the motor causes the current drawn to be unstable

2. MOSFET transistor generates heat during switching

3. Limited by resolution of controller4. Eddy currents generated in

electromagnetic coils which lead to adverse heating

5. Heat losses in electromagnetic materials is proportional to frequency squared


Potentiometer is used to adjust the duty cycle


Example: PWM with 555 Timer

1. Maxon EC-16 brushless motor, Time constant = 8.75 ms

2. Want to avoid audible frequenciesf ≥ 20 kHz

3. PID control loop running at 150 Hzf ≥ 10 ∙ 150 Hz

Requirements

Hz1175.81

fmsf


Example: Specifying circuit elements

11

44.1

CRf

Set f to 25 kHz to add in a factor of safety

Choosing C1 to be 100 nF, R1 is 576 Ω ~ 500 Ω

Recalculating with these values f = 28.8 kHz

This circuit has a PWM frequency according to:

≥ 117 Hz≥ 20 kHz≥ 1.5 kHz

f

Check constraints


Example: Specifying circuit elements

Matlab can do PWM!


The procedure works similar to the generation of analog PWM using a sinusoid and saw-tooth wave

18 kHz frequencyContinuous 28 amps$55.95

Where can I buy a PWM controller?Texas InstrumentsDigikeyMouser ElectronicsCritical Velocity Motor Control

Texas Instruments TAS5508B8-Channel Digital Audio PWM Processor64 pin chip, max 192 kHz frequency$7.25

120 amps, used for hybrid vehicles$469.00

SMALL

BIGGER HUGE




Presenter: Alex Mariuzza

PWM ImplementationThe signal is outputted

through Port PSix ChannelsDedicated Chip


PWM Module Each channel has a

dedicated counter Programmable duty

and periodIndependently

adjustable clock, polarity, and alignment


PWM Module- Other Features

8-bit and 16-bit resolution supportedTwo PWM channels can be concatenated together

Four source clocks (A, SA, B, SB)Emergency ShutdownSome changes take a complete cycle to be implemented Modes of Operation:

Normal: everything is availableWait: Low-power consumption and clock disabledFreeze: Option to disable clock is available


PWM ConfigurationConfigured through

specific registersRegisters are located from

$00E0 to $00FEThere are repeated

registers (ex. 0013-0017)


PWM Enable Register

Located at $00E0Code Warrior variable: PWMESet PWMEx to 0 to disable the channelSet PWMEx to 1 to enable itChannel is activated when bit is set If 16-bit resolution used, then PWME4/2/0 are deactivated


PWM Polarity Register

Located at $00E1Code Warrior variable : PWMPOLSet PPOLx to 0, signal goes from low to highSet PPOLx to 1, signal goes from high to low


PWM Clock Select Register

Located at $00E2Code Warrior variable : PWMCLKSet PCLK5/4/1/0 to 0 to use clock ASet PCLK5/4/1/0 to 1 to use clock SASet PCLK3/2 to 0 to use clock BSet PCLK3/2 to 1 to use clock SB


PWM Prescaler Register

Located at $00E3Variable: PWMPRCLKUsed to prescale clocks A and B

Frequency PWM)12(

FrequencyClock Bus

Frequency PWMResolution

FrequencyClock BusPresclarer

N


PWM Scale A Register

Located at $00E8Code Warrior variable:

PWMSCLAStore a hexadecimal value

in order to change the clock frequency of SA

Note: if set to $00, PWMSCLA is set to 256

PWMSCLA2

FrequencyA Clock FrequencySA Clock


PWM Scale B Register

Located at $00E9Code Warrior variable:

PWMSCLBStore a hexadecimal value

in order to change the clock frequency of SA

Note: if set to $00, PWMSCLB is set to 256

PWMSCLB2

Frequency BClock Frequency SBClock


PWM Center Align Register

Located at $00E4Code Warrior variable: PWMCAESet CAEx to 0 for left align signalSet CAEx to 1 for center align signalNote: can only be set when channel is disabled


Signal AlignmentSignal changes when counter

is equal to period register In the center aligned mode,

the PWM counter goes from a down-count to a up-count to down-count, etc.

In the left aligned mode, the PWM counter is a up-counter and rests to zero when it overflows


PWM Control Register

Located at $00E5Code Warrior variable: PWMCTLSet CONxy to 0 to keep PWM channels separate (8-bit)Set CONxy to 1 to concatenate PWM channels x and y together (16-

bit).Channel y determines the configurationx becomes the high byte and y becomes the low byteBits PSWAI and PFRZ set either wait or freeze modeChanges only occur when channels are disabled


PWM Counter Register

Located at $00EC through $00F1Code Warrior variable: PWMCNTxOne per channelIt tracks the cycle countsIt can be readIf written to, the count is reset to $00 and a up-count starts


PWM Period Register

Located at $00F2 through $00F7 Variable: PWMPERxStore a hexadecimal value to limit

maximum value of counterChanges occur when:

Current period endsCounter is written toChannel is disabled

Frequency Signal PWM

Frequency Source PWMPWMPERx

Left-Aligned:

Frequency Signal PWM2

Frequency Source PWMPWMPERx

Center-Aligned:


PWM Duty Register

100

PWMPERxCycleDuty -PWMPERx PWMDTYx

• Located at $00F8 through $00FD• Code Warrior variable: PWMDTYx• Store a hexadecimal value to control when signal changes • Changes occur when:

• Current period ends• Counter written to• Channel is disabled

100

PWMPERx CycleDuty PWMDTYx

Polarity = 0:

10/29/2009 44Polarity = 1:

PWM Shutdown Register

• Located at $00FE

• Code Warrior variable: PWMSDN

• PWMENA: Enables and disables emergency shut down

• PWMIF (Interrupt flag): Set when an input is detected in pin 5

• PWMIE (Interrupt Enable): Enables and disables CPU interrupts

• PWMRSTRT: Resets the counters

• PWMLVL (Shutdown Output Level): Determines if output is high or low when shutdown

• PWM5IN (Input Status): Reflects status of pin 5

• PWM5INL: Determines active level of pin 5 45

How it all worksClock A, SA, B, or SB


ExampleDesired Signal:

8 kHz PWM signalDuty Cycle of 30%Left AlignedChannel 1Low → High8-bit channel

$02PWMPRCLK491.3108)12(

108Prescaler

38

6

$00PWMCLK

FA$250108

102

108

4108

PWMPER13

6

3

6

AF$175100

25030-250 PWMDTY1

$00PWMCAE

$02PWME

$00PWMPOL

$00PWMCTL


Assembly CodePWME EQU $00E0 PWMPOL EQU $00E1 PWMCLK EQU $00E2 PWMPRCLK EQU $00E3 PWMCAE EQU $00E4 PWMCTL EQU $00E5 PWMPER1 EQU $00F3PWMDTY1 EQU $00F9

ORG $1000LDAA #$00STAA PWMCLK ; Sets source clocks to clock ASTAA PWMPOL ; The signal goes from low to highSTAA PWMCTL ; Makes all channels 8-bitSTAA PWMCAE ; Signals are left alignedLDAA #$FASTAA PWMPER1 ; Sets the period to 250 clock cycles LDAA #$AFSTAA PWMDTY1 ; Makes the duty cycle equal to 30%LDAA #$02STAA PWMPRCLK ; Sets the prescaler to 4STAA PMWE ; Enables and starts channel 1……10/29/2009 48

C Code#include <hidef.h> /* common defines and macros */

#include <mc9s12c32.h> /* derivative information */

#pragma LINK_INFO DERIVATIVE “mc9s12c32”

// Set up chip in expanded mode

MISC = 0x03;

PEAR = 0x0C;

MODE = 0xE2;

//Set up PWM Registrer

PWMCLK = 0; // Sets source clocks to clock A

PWMPOL = 0; // The signal goes from low to high

PWMCTL = 0; // Makes all channels 8-bit

PWMCAE = 0; // Signals are left aligned

PWMPER1 = 250; // Sets the period of the signal to 250 clock

PWMDTY1 = 175; // Makes the duty cycle equal to 30%

PWMPRCLK = 2; //Sets the prescaler to 4

PMWE = 2; //Enables and starts channel 1

….


http://en.wikipedia.org/wiki/Pulse-width_modulationhttp://www.netrino.com/Embedded-Systems/How-To/PWM-Pulse-Width-ModulationCetinkunt, Sabri. Mechatronics. Hoboken, NJ: Wiley, 2006. Print.http://www.jimfranklin.info/microchipdatasheets/00538c.pdfhttp://www.allaboutcircuits.com/vol_6/chpt_6/9.htmlhttp://www.dprg.org/tutorials/2005-11a/index.htmlhttp://www.4qdtec.com/pwm-01.htmlhttp://skywalker.cochise.edu/rgill/ch02elec.ppthttp://pcbheaven.com/wikipages/PWM_Modulation/Matlab 2009 online documentationhttp://www.epanorama.net/links/motorcontrol.html#acMC9S12C Family, MC9S12GC Family Reference Manual, (pp. 347-382)

REFERENCES


Questions?


by: val tocitu jason kulpe alexandre mariuzza me 4447/6405 october 29 th, 2009

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