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Pulse-width modulation
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==Applications== ===Servos=== PWM is used to control servomechanisms; see [[servo control]]. ===Telecommunications=== In [[telecommunications]], PWM is a form of signal [[modulation#Pulse modulation methods|modulation]] where 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). _ _ _ _ _ _ _ _ | | | | | | | | | | | | | | | | Clock | | | | | | | | | | | | | | | | __| |____| |____| |____| |____| |____| |____| |____| |____ _ __ ____ ____ _ PWM signal | | | | | | | | | | | | | | | | | | | | _________| |____| |___| |________| |_| |___________ Data 0 1 2 4 0 4 1 0 The inclusion of a [[clock signal]] is not necessary, as the leading edge of the data signal can be used as the clock if a small offset is added to each data value in order to avoid a data value with a zero length pulse. _ __ ___ _____ _ _____ __ _ | | | | | | | | | | | | | | | | PWM signal | | | | | | | | | | | | | | | | __| |____| |___| |__| |_| |____| |_| |___| |_____ Data 0 1 2 4 0 4 1 0 ===Power delivery=== PWM can be used to control the amount of power delivered to a load without incurring the losses that would result from linear power delivery by resistive means. Drawbacks to this technique are that the power drawn by the load is not constant but rather discontinuous (see [[Buck converter]]), and energy delivered to the load is not continuous either. However, the load may be inductive, and with a sufficiently high frequency and when necessary using additional passive [[electronic filter]]s, the pulse train can be smoothed and average analog waveform recovered. Power flow into the load can be continuous. Power flow from the supply is not constant and will require energy storage on the supply side in most cases. (In the case of an electrical circuit, a capacitor to absorb energy stored in (often parasitic) supply side inductance.) High [[frequency]] PWM power control systems are easily realisable with semiconductor switches. As explained above, almost no power is dissipated by the switch in either on or off state. However, during the transitions between on and off states, both voltage and current are nonzero and thus power is dissipated in the switches. By quickly changing the state between fully on and fully off (typically less than 100 nanoseconds), the power dissipation in the switches can be quite low compared to the power being delivered to the load. Modern semiconductor switches such as [[MOSFET]]s or [[insulated-gate bipolar transistor]]s (IGBTs) are well suited components for high-efficiency controllers. Frequency converters used to control AC motors may have efficiencies exceeding 98%. Switching power supplies have lower efficiency due to low output voltage levels (often even less than 2 V for microprocessors are needed) but still more than 70–80% efficiency can be achieved. Variable-speed [[computer fan control]]lers usually use PWM, as it is far more efficient when compared to a [[potentiometer]] or rheostat. (Neither of the latter is practical to operate electronically; they would require a small drive motor.) Light dimmers for home use employ a specific type of PWM control. Home-use light dimmers typically include electronic circuitry that suppresses current flow during defined portions of each cycle of the AC line voltage. Adjusting the brightness of light emitted by a light source is then merely a matter of setting at what voltage (or phase) in the AC half-cycle the dimmer begins to provide electric current to the light source (e.g. by using an electronic switch such as a [[TRIAC|triac]]). In this case the PWM duty cycle is the ratio of the conduction time to the duration of the half AC cycle defined by the frequency of the AC line voltage (50 Hz or 60 Hz depending on the country). These rather simple types of dimmers can be effectively used with inert (or relatively slow reacting) light sources such as incandescent lamps, for example, for which the additional modulation in supplied electrical energy which is caused by the dimmer causes only negligible additional fluctuations in the emitted light. Some other types of light sources such as light-emitting diodes (LEDs), however, turn on and off extremely rapidly and would perceivably flicker if supplied with low-frequency drive voltages. Perceivable flicker effects from such rapid response light sources can be reduced by increasing the PWM frequency. If the light fluctuations are sufficiently rapid (faster than the [[flicker fusion threshold]]), the human visual system can no longer resolve them and the eye perceives the time average intensity without flicker. In electric cookers, continuously variable power is applied to the heating elements such as the hob or the grill using a device known as a [[simmerstat]]. This consists of a thermal oscillator running at approximately two cycles per minute and the mechanism varies the duty cycle according to the knob setting. The thermal time constant of the heating elements is several minutes so that the temperature fluctuations are too small to matter in practice. ===Voltage regulation=== {{Main|Switched-mode power supply}} PWM is also used in efficient [[voltage regulator]]s. By switching the voltage to the load with the appropriate duty cycle, the output will approximate a voltage at the desired level. The switching noise is usually filtered with an [[inductor]] and a [[capacitor]]. One method measures the output voltage. When it is lower than the desired voltage, it turns on the switch. When the output voltage is above the desired voltage, it turns off the switch. ===Audio effects and amplification=== Varying the duty cycle of a pulse waveform in a synthesis instrument creates useful timbral variations. Some synthesizers have a duty-cycle trimmer for their square-wave outputs, and that trimmer can be set by ear; the 50% point (true square wave) is distinctive because even-numbered harmonics essentially disappear at 50%. Pulse waves, usually 50%, 25%, and 12.5%, make up the [[Video game music|soundtracks of classic video games]]. The term PWM as used in sound (music) synthesis refers to the ratio between the high and low level being secondarily modulated with a [[low-frequency oscillator]]. This gives a sound effect similar to [[Chorus (audio effect)|chorus]] or slightly detuned oscillators played together. (In fact, PWM is equivalent to the sum of two [[sawtooth wave]]s with one of them inverted.)<ref>{{Cite web|url=https://www.soundonsound.com/techniques/synthesizing-strings-pwm-string-sounds|title=Synthesizing Strings: PWM & String Sounds|website=www.soundonsound.com}}</ref> [[Class-D amplifier]]s produce a PWM equivalent of a lower frequency input signal that can be sent to a [[loudspeaker]] via a suitable filter network to block the carrier and recover the original lower frequency signal. Since they switch power directly from the high supply rail and low supply rail, these amplifiers have efficiency above 90% and can be relatively compact and light, even for large power outputs. For a few decades, industrial and military PWM amplifiers have been in common use, often for driving [[servomotor]]s. Field-gradient coils in [[MRI]] machines are driven by relatively high-power PWM amplifiers. Historically, a crude form of PWM has been used to play back [[PCM]] digital sound on the [[PC speaker]], which is driven by only two voltage levels, typically 0 V and 5 V. By carefully timing the duration of the pulses, and by relying on the speaker's physical filtering properties (limited frequency response, self-inductance, etc.) it was possible to obtain an approximate playback of mono PCM samples, although at a very low quality, and with greatly varying results between implementations. The [[Sega 32X]] uses PWM to play sample-based sound in its games. In more recent times, the [[Direct Stream Digital]] sound encoding method was introduced, which uses a generalized form of pulse-width modulation called [[pulse-density modulation]], at a high enough sampling rate (typically in the order of MHz) to cover the whole [[Acoustics|acoustic]] frequencies range with sufficient fidelity. This method is used in the [[Super Audio CD|SACD]] format, and reproduction of the encoded audio signal is essentially similar to the method used in class-D amplifiers. ===Electrical=== SPWM (sine–triangle pulse-width modulation) signals are used in [[solar inverter]] design. These switching signals are fed to the [[FET]]s that are used in the device. The device's efficiency depends on the harmonic content of the PWM signal. There is much research on eliminating unwanted harmonics and improving the fundamental strength, some of which involves using a modified carrier signal instead of a classic sawtooth signal<ref>Hirak Patangia, Sri Nikhil Gupta Gourisetti, "A Harmonically Superior Modulator with Wide Baseband and Real-Time Tunability", IEEE International Symposium on Electronic Design (ISED), India, Dec.11.</ref><ref>Hirak Patangia, Sri Nikhil Gupta Gourisetti, “Real Time Harmonic Elimination Using a Modified Carrier”, CONIELECOMP, Mexico, Feb 2012.</ref><ref>Hirak Patangia, Sri Nikhil Gupta Gourisetti, “A Novel Strategy for Selective Harmonic Elimination Based on a Sine-Sine PWM Model”, MWSCAS, U.S.A, Aug 2012.</ref> in order to decrease power losses and improve efficiency. Another common application is in robotics where PWM signals are used to control the speed of the robot by controlling the motors. ===Soft-blinking LED indicator=== PWM techniques would typically be used to make some indicator (like an [[LED]]) ''soft blink''. The light will slowly go from dark to full intensity, and slowly dimmed to dark again. Then it repeats. The period would be several soft blinks per second up to several seconds for one blink. An indicator of this type would not disturb as much as a ''hard-blinking'' on/off indicator. The indicator lamp on the [[IBook|Apple iBook G4, PowerBook 6,7 (2005)]] was of this type. This kind of indicator is also called ''pulsing glow'', as opposed to calling it ''flashing''.
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