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PWM Pulse Width Modulation

PWM Pulse Width Modulation. ME 4447/6405 November 3 rd , 2011. Gregory Bonisteel Bryan Oneal Jieun Yoo. Outline. Introduction and Definition Duty Cycle Types of PWM Method of Generation Implementation on the HCS12 Applications of PWM Choosing PWM Frequency. Presenter: Jieun Yoo.

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PWM Pulse Width Modulation

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

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

  3. Presenter: JieunYoo • Introduction and Definition • 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. On Off VH Duty Cycle (D) VL Period (T)

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

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

  13. 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

  14. 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

  15. PWM Register Memory Map

  16. 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

  17. 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

  18. 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

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

  20. 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

  21. 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.

  22. 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

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

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

  25. 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

  26. 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

  27. 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

  28. PWM Shutdown Register $00FE

  29. 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

  30. 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

  31. Assembly Code PWME EQU $00E0 PWMCAE EQU $00E4 PWMDTY0 EQU $00F8PWMPER0 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 ...

  32. 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

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

  34. PWM Video Into to Applications • You Tube search: PWM Tutorial OR Click Link

  35. 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.

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

  37. 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

  38. 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.

  39. 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).

  40. 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

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

  42. 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.

  43. 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.

  44. 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

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

  46. Brightness controlled with a PWM circuit.

  47. Frequency of the PWM Signal Upper Limits Lower 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

  48. Choosing your PWM frequency Input signal (PWM) Output signal (actuator response)

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

  50. Where can I buy a PWM controller? - 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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