Question on PWM

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28 Aug 2012 23:37 #23728 by brianm
Question on PWM was created by brianm
I am a little confused on using PWM. I am using a mesa 5i20 to generate a PWM signal for a Pico system brushless servo amplifier.
I have a 48V power supply for the motors and they are rated for 48V

From reading the documentation, (I understand that), duty cycle = pwm value/scale
If I set max_output to 1 (the PID will never generate a value greater than 1)
If I set my scale to 1
Then the max duty cycle would be 1 - which would translate to 48V to the motor if my PID was at 1?
Then I would have to play with the PID values to get a stable system.

Am I understanding this correctly?

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28 Aug 2012 23:58 #23729 by PCW
Replied by PCW on topic Re:Question on PWM
Yes, but there are two types of PWM commonly used, PWM/DIR
(where PWM is 0 to 100% and DIR changes the sign of the output voltage)

and Locked-antiphase PWM where a 50% duty cycle is 0 voltage,
0% duty cycle is full negative and 100% is full positive voltage

Maybe the drives manual or Jon Elson can determine which type you need for his drive

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29 Aug 2012 00:12 #23730 by brianm
Replied by brianm on topic Re:Question on PWM
I am pretty sure I need the DIR type. The amplifier needs a dir signal.

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29 Aug 2012 00:17 #23731 by PCW
Replied by PCW on topic Re:Question on PWM
OK so your understanding is correct
Also some drives cannot accept 100% PWM
so you should check this also (and set PID max_output accordingly)

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01 Sep 2012 04:04 #23823 by jmelson
Replied by jmelson on topic Re:Question on PWM
PCW wrote:

OK so your understanding is correct
Also some drives cannot accept 100% PWM
so you should check this also (and set PID max_output accordingly)

Right, it is a sign/magnitude drive.
0% duty cycle is OK, but you should not exceed 95% to allow the bootstrap
capacitors to recharge. It is designed for a PWM frequency of 50 KHZ,
but should work OK at lower frequencies.

You need to provide +12 V to the enable pin to enable the amp. This
should be driven by a DC solid state relay controlled by the out-of-estop
circuit.

Note there is only one ground connection on the servo amp, so you need
to have the minus of the 12 V power supply for the servo amp connected
to the minus of the motor power supply.

Jon

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01 Sep 2012 16:50 - 01 Sep 2012 16:51 #23833 by PCW
Replied by PCW on topic Re:Question on PWM
The 5I20s max PWM rate is 100MHz/512 = ~195 Khz but lower rates
will have higher resolution:

0 to 24.4 KHz = 12 bits
24.5 KHz to 48.8 KHz = 11 bits
48.9 KHz to 97.6 KHz = 10 bits
97.7 KHz to 195.3 KHz = 9 bits

So if the drive is not picky about PWM rate, I'd run it at 48 KHz so you get 11 bit PWM
resolution and still be close to the drives suggested PWM rate
Last edit: 01 Sep 2012 16:51 by PCW.

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