08-06-2012, 02:18 PM
SPEED CONTROL OF DC MOTOR USING PWM TECHNIQUE
26768482-Speed-Control-of-Dc-Motor-pwm.pdf (Size: 895.75 KB / Downloads: 38)
Pulse Width Modulation (PWM) Basics
There are many forms of modulation used for communicating
information. When a high frequency signal has amplitude varied in response to a
lower frequency signal we have AM (amplitude modulation). When the signal
frequency is varied in response to the modulating signal we have FM (frequency
modulation. These signals are used for radio modulation because the high
frequency carrier signal is needs for efficient radiation of the signal. When
communication by pulses was introduced, the amplitude, frequency and pulse
width become possible modulation options. In many power electronic converters
where the output voltage can be one of two values the only option is modulation of
average conduction time.
Linear Modulation
The simplest modulation to interpret is where the average ON time
of the pulses varies proportionally with the modulating signal. The advantage of
linear processing for this application lies in the ease of de-modulation. The
modulating signal can be recovered from the PWM by low pass filtering. For a
single low frequency sine wave as modulating signal modulating the width of a
fixed frequency (fs) pulse train the spectra is as shown in Fig 1.2. Clearly a low
pass filter can extract the modulating component fm.
Sawtooth PWM
The simplest analog form of generating fixed frequency PWM is by
comparison with a linear slope waveform such as a saw tooth. As seen in Fig 1.2
the output signal goes high when the sine wave is higher than the saw tooth. This
is implemented using a comparitor whose output voltage goes to logic HIGH when
ne input is greater than the other. Other signals with straight edges can be used
for modulation a rising ramp carrier will generate PWM with Trailing Edge
Modulation.
Regular Sampled PWM
The scheme illustrated above generates a switching edge at the
instant of crossing of the sine wave and the triangle. This is an easy scheme to
implement using analog electronics but suffers the imprecision and drift of all
analog computation as well as having difficulties of generating multiple edges
when the signal has even a small added noise. Many modulators are now
implemented digitally but there is difficulty is computing the precise intercept of the
modulating wave and the carrier. Regular sampled PWM makes the width of the
pulse proportional to the value of the modulating signal at the beginning of the
carrier period. In Fig 1.5 the intercept of the sample values with the triangle
determine the edges of the Pulses. For a saw tooth wave of frequency fs the
samples are at 2fs.
