USING OF PWM TECHNIQUE TO CONTROL SPEED OF A MOTOR ,WHEN LPC2148 MICROCONTROLLER IS USED

[ANJUSHA]

sir,  i gone many forums,many sites….concept of pwm in lpc2148 is not understanding properly.can any one tell me,,how pwm technique is used to control speed of a motor?

[AJISH ALFRED]

Hi Abiram,
Suppose you have a 12V DC motor, and you are supplying 12V DC consistantly for the motro. In that case the motor will run at its maximum rated speed, say 3000rpm.
If you apply this 12V at every one millisecond with a delay of 1ms where there is no supply, the motor will run at half the speed, 1500rpm. Here the ON TIME of the supply is 1ms and OFF TIME is also 1ms. If you increase OFF TIME to 1.5ms and reduce the ON TIME to 0.5ms the speed of the motor will again reduce to 750rpm.

Hence if voltage is applied in pulses the speed depends on the ON TIME and OFF TIME. This is the concept of PWM or PULSE WIDTH MODULATION.

PWM is a wave with constant pulse PERIOD which means (ON TIME + OFF TIME) is a constant. To increase the speed simply increase the ON TIME and decrease the OFF time. To decrease the speed, reduce the ON time and increase the OFF time.

[ANJUSHA]

thank you so  much.i understand well…. sir i tried with 8051,its ok..but in ARM7 LPC2148 controller, there are two frequencies.one is crystal frequency =12MHz   and  PCLK(pheripheral clock)=30MHz, where (VPBDIV=0X02 i.e    1/4 of the default PLL output=60MHz )….now my doubt is which frequency we have to consider in  performing PWM  in this controller and let me know how? how frequency and Ton and Toff are related and with which frequency?

[AJISH ALFRED]

Hi Abiram,

That’s a very good question !

Frequency is nothing but 1/ PERIOD or 1/(Ton + Toff) which will be a constant value.

The frequency or the time period (inverse of frequency) for a PWM is decided based on where we are going to apply it. For example very high frequency PWM are used in audio output circuits, medium frequency are used in light intensity control circuits and low frequency are used for motor control circuits.

A motor has a property called inertia of rotation and due to that the motor continue rotating even at the Toff period of a PWM wave. If the Toff is too large then the motor will come to a stop at Toff period. Similarly the Ton should be long enough for the motor to escape from the inertia of non rotation initially and start rotating. All these depends on the motor that you use.

I suggest to keep the Ton and Toff in milliseonds range for the motor. For that you have to use the low frequency oscillator in the controller only.

How to configure the controller, please go through the datasheet or you can find lot of sample codes from the internet.

[Prasad Bangar]

Hello sir,
I am doing project in motor control I want the steps to do the programming of stepper motor in lpc2148 using pwm technique.
can you help me sir??

[Ashutosh Bhatt]

stepper motor does not run with PWM

rather it will run on PRF – pulse repeatition frequency

high PRF means more RPM (speed) and vice versa

[Prasad Bangar]

Hello 
I just want control the speed DC motor will also help me guide me accordingly.
I am working on LPC2148 using L293D driver.

[Prasad Bangar]

I just wanted to ask this that can I use for loop to intereupt the signal which just similar to PWM??

[ANJUSHA]

i dont exactly about the concept ur asking…..just refer "ocfreaks lpc2148  " it might give an idea for you.

[ANJUSHA]

once see the l293d driver internal circuit, its a h-bridge….

one driver can drive 2 motors

and for PWM technique refer "OcFreaks LPC2148 PWM TECHNIQUE". It will be very helpful….

[Author]

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