Dc Motor Control Without Mc
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Transcript of Dc Motor Control Without Mc
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Introduction and definitions
Types of PWM
Methods of generationCharacteristics of PWM
Applications and examples
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What is it? Output signal alternates between on and off within
specified period
Controls power received by a device The voltage seen by the load is directly proportional
to the source voltage
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Definitions Duty Cycle: on-time / period
Vlow is often zero
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LOWHIAVG VDDVV )1(
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Types of Pulse Width
Pulse center fixed, edges modulated
Leading edge fixed, tailing edge modulated
Tailing edge fixed, leading edge modulated Pulse Width constant, period modulated
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Types of Pulse Width
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Analog PWM signals can be made by combining a
saw- tooth waveform and a sinusoid
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PWM output isformed by the
intersection of
the saw-tooth
wave andsinusoid
Analog Generation of PWM
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Digital Methods of Generating PWM
Digital: Counter used to handle transition
Delta : used to find the PWM at a certain limit
Delta Sigma: used to find the PWM but has advantage of
reducing optimization noise
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Applications to DC Motors
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The voltage supplied to a DC motor is proportional
to the duty cycle
Both brushed and brushless motors can be used
with PWM Both analog and digital control techniques and
components are available
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Motor Control Diagrams
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average value proportional to duty cycle, D
low power used in transistors used to switch
the signal
fast switching possible due to MOSFETS
and power transistors at speeds in excess
of 100 kHz
digital signal is resistant to noise less heat dissipated versus using resistors
for intermediate voltage values
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Advantages of PWM
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Cost
Complexity of circuit
Radio Frequency Interference
Voltage spikes
Electromagnetic noise
Disadvantages of PWM
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In the past, motors were controlled at intermediate
speed by using resistors to lower delivered power
Electric stove heater
Lamp dimmers Voltage regulationconvert 12 volts to 5 volts by
having a 41.7% duty cycle
Sound production: PWM controlled signals give
sound effects similar to a chorus Power transfer: PWM used to reduce the total
power given to a load without relying on resistive
losses
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Applications of PWM
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commonly used in toys
lowpass filter smooths out transients from harmonic effects
frequency values of harmonics doesnt change, but theamplitude does, which adjusts the analog output signal
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PWM used with D/A conversion
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1. Must be at least 10 times higher
than the control system frequency
2. Higher than 20kHzaudible
frequency of sounds to avoidannoying sound disturbances,
caused by magnetostriction
3. If too low the motor is pulsed, not
continuous, because the motors
inductance can not maintain the
current4. Inverse of frequency should be
much less than the motor/load time
constant
5. Higher error from ripple voltages
Frequency of the PWM SignalUpper LimitsLower Limits
1. If too high the inductance of the
motor causes the current drawn to
be unstable
2. MOSFET transistor generates heatduring switching
3. Limited by resolution of controller
4. Eddy currents generated in
electromagnetic coils which lead to
adverse heating
5. Heat losses in electromagneticmaterials is proportional to
frequency squared
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Matlab can do PWM!
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The procedure works similarto the generation of analog
PWM using a sinusoid and
saw-tooth wave
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PWM Module Each channel has a
dedicated counter
Programmable duty
and period
Independentlyadjustable clock,
polarity, and
alignment
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PWM Module- Other Features
8-bit and 16-bit resolution supported
Two PWM channels can be concatenated together
Four source clocks (A, SA, B, SB)
Emergency Shutdown
Some changes take a complete cycle to be implemented
Modes of Operation:
Normal: everything is available
Wait: Low-power consumption and clock disabled
Freeze: Option to disable clock is available
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http://en.wikipedia.org/wiki/Pulse-width_modulation
http://www.netrino.com/Embedded-Systems/How-To/PWM-Pulse-Width-Modulation
Cetinkunt, Sabri. Mechatronics. Hoboken, NJ: Wiley, 2006. Print.
http://www.jimfranklin.info/microchipdatasheets/00538c.pdf
http://www.allaboutcircuits.com/vol_6/chpt_6/9.html
http://www.dprg.org/tutorials/2005-11a/index.html
http://www.4qdtec.com/pwm-01.html
http://skywalker.cochise.edu/rgill/ch02elec.ppt
http://pcbheaven.com/wikipages/PWM_Modulation/
Matlab 2009 online documentation
http://www.epanorama.net/links/motorcontrol.html#acMC9S12C Family, MC9S12GC Family Reference Manual, (pp. 347-382)
REFERENCES
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Questions?
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