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PWM Pulse Width Modulation

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Presentation on theme: "PWM Pulse Width Modulation"— Presentation transcript:

1 PWM Pulse Width Modulation
ME 4447/6405 November 3rd, 2011 Gregory Bonisteel Bryan Oneal Jieun Yoo

2 Outline Introduction and Definition Types of PWM Method of Generation
Duty Cycle Types of PWM Method of Generation Implementation on the HCS12 Applications of PWM Choosing PWM Frequency

3 Presenter: Jieun Yoo Introduction and Definition Types of PWM
Duty Cycle Types of PWM Method of Generation Implementation on the HCS12 Applications of PWM Choosing PWM Frequency

4 Definition Pulse Width Modulation (PWM) is the way of controlling a digital signal simulating an analog signal. The on-off behavior changes the average power of signal. Output signal alternates between on and off with in specified period.

5 Duty Cycle A percentage measurement of how long the signal stays on.
Period (T) Duty Cycle (D) VL VH On Off

6 Duty Cycle Duty Cycle: Average signal :
(Usually, VL is taken as zero volts for simplicity.) Period (T) Duty Cycle (D) VL VH On Off

7 Duty Cycle Characteristic
Presented by: Bryan O’Neal Duty Cycle Characteristic The average value of a PWM signal increases linearly with the duty cycle

8 Types of PWM – Lead Edge Modulation
The lead edge is fixed at the lead edge of the window and the trailing edge is modulated.

9 Types of PWM – Trail Edge Modulation
The trail edge is fixed and the lead edge is modulated.

10 Types of PWM – Centered Pulses
The pulse center is fixed in the center of the time window and both edges of pulse are modulated

11 Analog Generation of PWM
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

12 Digital Generation - Delta Method
Output signal compared with limits Every time limits reached, changes state

13 Digital Generation - Delta Sigma Method
PWM signal generated by Delta method Error = output – reference Error integrated State changes when integration reaches limits

14 Presenter: Gregory Bonisteel
Introduction and Definition Duty Cycle Types of PWM Method of Generation Implementation on the HCS12 Applications of PWM Choosing PWM Frequency

15 Pulse Width Modulator: PWM8B6CV1
Similar to output compare Port P Six 8-bit channels or three 16-bit channels for greater resolution Four clock sources (A, B, SA and SB) provide for a wide range of frequencies Emergency shutdown Modes of operation Wait mode Freeze mode

16 PWM Block Diagram Each Channel has; Enable/disable switch
Dedicated counter Programmable period and duty cycle Programmable center or left aligned Programmable clock select logic Software selectable duty pulse polarity

17 PWM Register Memory Map

18 PWM Enable Register Located at $00E0
Set PWMEx to 0 to disable the channel Set PWMEx to 1 to enable it Note: Channel is activated when bit is set If 16-bit resolution used, then PWME4/2/0 are deactivated

19 PWM Polarity Register Located at $00E1
Set PPOLx to 0, signal goes from low to high Set PPOLx to 1, signal goes from high to low

20 PWM Clock Select Register
Located at $00E2 Set PCLK5/4/1/0 to 0 to use clock A Set PCLK5/4/1/0 to 1 to use clock SA Set PCLK3/2 to 0 to use clock B Set PCLK3/2 to 1 to use clock SB

21 PWM Prescale Clock Select Register
Located at $00E3 Used to prescale clocks A and B

22 PWM Center Align Enable Register
Located at $00E4 Set CAEx to 0 for left align signal Set CAEx to 1 for center align signal Note: Can only be set when channel is disabled Signal changes when counter is equal to period register

23 Left vs. Center Aligned In the left aligned mode, the PWM counter is a up-counter and rests to zero when it overflows In the center aligned mode, the PWM counter goes from a down-count to a up-count to down-count, etc.

24 PWM Control Register Located at $00E5
Set CONxy to 0 to keep PWM channels separate (8-bit) Set CONxy to 1 to concatenate PWM channels x and y together (16-bit). x becomes the high byte and y becomes the low byte Channel y determines the configuration Bits PSWAI and PFRZ set either wait or freeze mode Note Changes only occur when channels are disabled

25 PWM Scale A Register Located at $00E8
Programmable scaling of clock A to generate clock SA Note

26 PWM Scale B Register Located at $00E9
Programmable scaling of clock B to generate clock SB Note

27 PWM Channel Counter Register
Located at $00EC through $00F1 One per channel It tracks the cycle counts When channel is enabled up-count starts Note Writing to counter while a channel is enable can cause irregular PWM cycles

28 PWM Channel Period Register
Located at $00F2 through $00F7 PWMPERx Store a hexadecimal value to limit maximum value of counter Changes occur when one of following happen Current period ends Counter is written to Channel is disabled

29 PWM Channel Duty Register
Located at $00F8 through $00FD Store a hexadecimal value to control when signal changes Changes occur when: Current period ends Counter written to Channel is disabled

30 PWM Shutdown Register $00FE

31 Example: Configuring PWM Channel 0
Frequency: 40 kHz Period = 1/Frequency = 25μs Duty Cycle = 50% Positive polarity Left aligned output To choose clock source, consider resolution of PWM Number of distinct duty cycle values is equal to the PWM period in clock cycles Bus clock period is 125 ns  200*125ns = 25μs Since 200 < 255, we can use clock A with a prescaler=1

32 PWMCLK = #$00 - PWM0 uses clock A PWMPRCLK = #$00 - Prescaler = 1
Example: Configuring PWM Channel 0 PWMCLK = #$00 - PWM0 uses clock A PWMPRCLK = #$00 - Prescaler = 1 PWMPOL = #$01 - Positive polarity PWMCAE = #$00 - Left aligned PWMPER0 = #$C8 - Period = 200 PWMDTY0 = #$64 - Duty cycle = 100/200 = 50% PWME = #$01 - Enable PWM channel 0

33 Assembly Code PWME EQU $00E0 PWMCAE EQU $00E4
PWMDTY0 EQU $00F8 PWMPER0 EQU $00F2 PWMPOL EQU $00E1 PWMCLK EQU $00E2 PWMPRCLK EQU $00E3 ORG $1000 LDAA #$00 STAA PWMCLK ;Use Clock A STAA PWMPRCLK ;Clock A prescaler = 1 STAA PWMCAE ;Left aligned output LDAA #$01 STAA PWMPOL ;Positive polarity (starts high) LDAA #$C8 STAA PWMPER0 ;Period = 200 (25μs) LDAA #$64 ;100 decimal STAA PWMDTY0 ;Duty cycle = 50% (100/200) LDAA #$01 STAA PWME ;Enable PWM Channel 0 ...

34 Configuring Channel 0 in C Code
// Setup chip in expanded mode MISC = 0x03; PEAR = 0x0C; MODE = 0xE2; TERMIO_Init(); // Init SCI Subsystem EnableInterrupts; PWMPER0 = 200; // set PWM period (125 ns * 200 = 25 us = 40 kHz) PWMDTY0 = 100; // set initial duty cycle (100/200 = 50%) // setup PWM system PWMCLK_PCLK0 = 0; // set source to clock A PWMPRCLK_PCKA0 = 0; // set prescaler for clock A = 1, so clock A = bus clock PWMPRCLK_PCKA1 = 0; PWMPRCLK_PCKA2 = 0; PWMCAE_CAE0 = 0; // "left aligned" output PWMPOL_PPOL0 = 1; // set duty cycle to indicate % of high time PWMCNT0 = 0; // write to counter to make changes take effect PWME_PWME0 = 1; // enable PWM 0

35 Presenter: Bryan Oneal
Introduction and Definition Duty Cycle Types of PWM Method of Generation Implementation on the HCS12 Applications of PWM Choosing PWM Frequency

36 PWM Video Into to Applications
You Tube search: PWM Tutorial OR Click Link

37 Motivation for PWM In the past, motors were controlled at intermediate speeds by using variable resistors to lower delivered power For example, a variable resister located in the foot pedal and connected in series with the motor of a sewing machine was used to control its speed. This method was inefficient PWM provided a great way to have compact and low cost means for applying adjustable power for many devices.

38 PWM Applications Use as ADC DC Motors Telecommunications
Voltage regulation RC devices Audio/Video effects Power delivery Amplification

39 PWM used with D/A conversion
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

40 Application to DC Motors
Voltage supplied is directly proportional to the duty cycle Ability to control the speed of the motor via the duty cycle Example Can be used in regulating room temperature. A PC can sense the current temperature (using an analog-to-digital converter) and then automatically increase/decrease the fan's speed accordingly.

41 PWM used to transmit data in telecommunications
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 The widths of the pulses correspond to specific data values encoded at one end and decoded at the other. Pulses of various lengths (the information itself) will be sent at regular intervals (the carrier frequency of the modulation).

42 Using PWM to generate an analog voltage level
Any shape waveform can be created PWM frequency should be much higher than the frequency of waveform generated

43 RC Devices Transmitters send PWM signals to the receivers on board of Radio controlled devices for specific control.

44 Audio devices Used in audio amplifiers to generate output signals for cellphone speakers to high-power stereo systems Produce less heat than traditional analog amplifiers Saving energy. Critical for hand held electronics. Gives a sound effect similar to chorus when used in audio circuit.

45 Video devices PWM dimming provides superior color quality in LED video display With a 12 bits resolution the TLC5940 PWM dimming can provide up to 68.7 million colors to a pixel.

46 Power delivery effective at data transmission over long distance transmission line Power transfer: PWM used to reduce the total power given to a load without relying on resistive losses

47 Example: PWM with 555 Timer
Potentiometer is used to adjust the duty cycle

48 Brightness controlled with a PWM circuit.

49 Frequency of the PWM Signal
Lower Limits Upper Limits Must be at least 10 times higher than the control system frequency Higher than 20kHz – audible frequency of sounds to avoid annoying sound disturbances. If too low the motor is pulsed, not continuous, because the motor’s inductance can not maintain the current Inverse of frequency should be much less than the motor/load time constant Higher error from ripple voltages If too high the inductance of the motor causes the current drawn to be unstable MOSFET transistor generates heat during switching Limited by resolution of controller Eddy currents generated in electromagnetic coils which lead to adverse heating Heat losses in electromagnetic materials is proportional to frequency squared

50 Choosing your PWM frequency
Input signal (PWM) Output signal (actuator response)

51 Matlab can do PWM! The procedure works similar to the generation of analog PWM using a sinusoid and saw-tooth wave

52 HUGE Where can I buy a PWM controller? Large - Texas Instruments
- Digikey - Mouser Electronics - Critical Velocity Motor Control HUGE Large SMALL Texas Instruments TAS5508B 8-Channel Digital Audio PWM Processor 64 pin chip, max 192 kHz frequency $7.25 18 kHz frequency Continuous 28 amps $55.95 120 amps, used for hybrid vehicles $469.00

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