Pulse-width modulation
The pulse width modulation ( PWM, is, and pulse width modulation ( PWM ), pulse length modulation (PLM ), subharmonic method or pulse width modulation (PWM ) in English pulse- width modulation ( PWM) ) is a form of modulation in which a technical size (eg electric current ) alternates between two values. Here, at constant frequency, the duty cycle of a rectangular pulse is modulated, ie the width of the pulses forming it.
- 4.1 from analog signals
- 4.2 For digital signals
Modulation
A pure pulse width modulated signal is generated for example by a linearly or descending signal ( triangular or sawtooth ) is compared with the analog input signal, depending on its value, a short or a long time on this. At the intersections of the output signal between the two logic levels is switched. It has functions like a digital signal has the advantage that it only - in this case two - can take on discrete values (see below under Areas ), but is infinitely variable in its duty cycle, that is, not in discrete time.
A simpler way is to influence the time constant of the astable flip-flop to the input signal. Since the other time constant will not change in proportion, to obtain a mixture of pulse width and frequency modulation, which depending on the use case, a meaning.
To generate a PWM signal from existing digital data (eg, Motor Control ) appropriate counter / comparator circuits are used. Many microcontroller included directly PWM modules or supported by appropriate timer functions whose implementation.
Demodulation
A PWM signal is generally demodulated by a low-pass. The resulting demodulated technical size corresponds to the DC value and so that the average height of the surface of the size modulated, mathematically determined from the integral over an integer number of periods is divided by the duration of the integration.
An illustrative example of this type of modulation is a switch that allows you heating constantly on and off. The longer the on-time relative to the period, the higher the average heating power. The temperature of the heated building can only comparatively slowly follow -up and shutdown; by its thermal inertia results in the necessary low-pass behavior for demodulation.
Areas of application
The pulse width modulation is used to transmit information and also often used to control the energy conversion in a technical system.
Measurement
Pulse width modulation is often used to transmit analog measured values of the sensors over long lines or radio. Since a voltage drop occurs on long lines, would create a distortion in the form of a voltage level in transmission of information. In transmission with pulse width modulation, it is sufficient if the receiver still can distinguish level 1 and 0. The same applies to a transfer by radio, where the reception intensity is influenced by many environmental factors.
Control technology
In order to transmit analog signals by a digital line, taking advantage of the smoothing low-pass effect of a capacitance or inductance, such as a motor or a coil, to control these by means of digital pulses. Thus, with digital circuits (eg microcontrollers ), which can process only digital signals, analog devices ( motors, etc.) drive.
The control unit need not be a digital device itself necessarily. For example, to control the servos ( transfer of the set value ), an analog value is modulated by a rotary potentiometer and demodulated again in the servo. This is used if both advantages of analog signals ( high-resolution, simple, robust and störungssicherere technology) as well as advantages of digital signals (Konstanz, simple, efficient amplification) are generally required. Similarly, the speed of which is controlled in newer PC or mainly CPU fan with PWM.
Another example is the dimming by PWM control. This technique is particularly used in light emitting diodes ( LEDs), as they are often used as a backlight in cell phones or even in cockpit view or brake light in newer vehicles. At sufficiently short, on and off the human eye perceives only the average luminosity, so that it can be controlled linearly with the duty cycle. This is to ensure that the clock frequency is set sufficiently high (eg, 70 kHz ), so that the eye, even with rapid motion, no brightness variations can perceive ( fibrillation). At high switching frequencies, the power of an individual pulse is short with respect to the heat capacity of the emitting chip. As a result, this heated only according to the average performance and not according to the peak performance. This is especially for temperature sensitive loads such as high power light emitting diodes is of great advantage. A low-pass filtering is not desired for driving LEDs as color and efficiency are current dependent and the luminosity strongly non- linear function of the operating current.
Power Electronics
The mean of a voltage can be reduced by means of PWM stepless proportional to the duty cycle. The setting of the reduced voltage is possible at relatively low power dissipation, because the circuit breakers are operated (except in the moments of switching ) only in two states: full blocking (only leakage current at full voltage) or fully switched (only forward voltage at full current). Thus, the PWM has importance in the power electronics.
Applications are DC-DC converter, inverter and electric motors, heating elements, dimmers, switching power supplies, class D amplifier and electronically commutated fans.
Analog- to-digital converter
A pulse width modulated signal can be directly processed by digital electronics, eg by means of a binary counter and a higher frequency count clock. For detecting the pulse width of the counter is set to 0 at the rising edge, wherein the negative edge of it is read ( value N1). The pulse period has to be constant, best derived from the same count clock by counting to N2.
A similar method is, for example, on the joystick port ( joystick ) is applied to the sound card of the PC (the beginning of the pulse here proceeds by the program).
Digital- to-analog converter
Another application can be found in digital -to-analog converters in measurement technology and sound production, for example in synthesizers or CD players.
Communications Engineering
The pulse width modulation is applied to the wireless transmission of physical quantities ( telemetry ) and the energy-saving production of an amplitude modulation transmitter in bulk.
Generation
For analog signals
A PWM signal may also be by means of an analogue comparator, by comparing the analog signal are generated with a suitable carrier signal, as shown in the adjacent circuit diagram, in which are used as modulation signal, in particular saw-tooth and triangle waves are used:
- Soaring sawtooth signal ( reset edge- modulated): The leading edge (rising edge ) of the switching function is fixed and the position of the trailing edge (falling edge ) is modulated.
- Declining sawtooth signal ( front edge- modulated): The position of the leading edge of the switching function is modulated and the trailing edge remains fixed.
- Triangular signal for symmetrical modulation: In this type of modulation, the positions of both sides of the switching function can be modulated. A change in the setpoint within a launch period only slightly, the two switching edges are approximately symmetrical with respect to the vertices of the triangular signal.
Another way of generating the pulse width modulation is a multi-vibrator, wherein the duty cycle by a variable resistor or capacitor can be changed.
To determine the pulse duration is made to approach that of the arithmetic mean of the signal to be modulated in a pulse period should correspond exactly to the equivalent value of the PWM pulse train. Both function curves (analog signal and a PWM signal) that is the same voltage-time area in a pulse interval to a voltage signal.
For digital signals
To generate a PWM signal from existing digital data (eg, Motor Control ) appropriate counter / comparator circuits are used. Many microcontroller included directly PWM modules or supported by appropriate timer functions whose implementation. The duration of a single pulse is split with an 8 -bit resolution in 256 steps can be switched of which depending on the desired output level 0 to 255.
Problems in practice
A significant problem when using the method of the PWM is in practice the formation of harmonics ( sl harmonics). These are formed as multiples of the modulation frequency and can be used in the controlled by PWM inductors cause unwanted side effects such as noise, heat and problems with electromagnetic compatibility. This can be remedied here by compensation by means of a switched- capacitor or by changing the modulation frequency of the PWM. Typical applications that handle also these same problems are frequency or the chopper control.
Advantages of the pulse-width modulation
The advantage of the PWM signal is that it is formed of two voltage levels (low and high). In the circuit implementation of a PWM generator by the bipolar or MOS transistors or IGBTs, can - in contrast to a generator with a continuous ( analog) variable voltage - operate in the low-loss circuit operation. Although the two voltage levels of the rectangular signal correspond to two logic levels, but these numbers do not represent a binary code represents the information is contained in the analog pulse-width ratio. It can be realized even up to the top kilowatt range in signal amplifier with PWM frequencies in the lower kilohertz range. In the electronic amplifier according to the PWM principle, under the designation D-class amplifier ( Class D digital amplifiers ) are known.