dc motor control without mc

8/12/2019 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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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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dc motor control without mc

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