electronic problem generating a PWM signal with crio

[mariomoskis]

Hello, i am development the next circuit to control a dc motor(Vmax=3v Imax=200mA), crio generates a pwm signal 5V with a  module ni9401 

i have the next problem:

when i did it with a 555 instead of crio all work well

but when i use crio, the supply voltage which i am giving to the AO is limited to 3V because the output current of the AO is the limit which my dc motor can work  200mA

and i need at least 4V of supply voltage to get it works well, because 4V is the minimus voltage which need my AO

i think that the problem isn´t about crio because if i put a resistor instead of my motor//diode all is ok,but when i put the motor with the diode i have the problem

why am i having this problem when i am using crio if the circuit is the same? am i forgetting some characteristic of crio ?

i hope all is explained good,could somebody help me with this?

thanks

[johnsold]

Some of your comments do not make sense.

1. What was the output voltage of the 555 circuit?

2. What is your pwm frequency? Is the 555 frequency and the cRIO frequency the same?

3. You do not show any supply voltage connections anywhere in your drawing.  What points are you referring to when you talk about supply voltages?

4. What do you mean by AO?  Typically AO refers to Analog Output (usually via a Digital to Analog converter) from a DAQ device.  I suspect that you mean something else.

5. Where is the other end of the motor and the anode of diode connected?

6. What duty cycle range do you expect to use?

Lynn

[mariomoskis]

ok sorry ,i will try to explain it better, could you delete my first message because it was explained bad?

i generate the pwm signal with the same duty cicle and frequency in crio and 555 as you said me, and i saw that the problem is exactly the same

so i will explain it again,but i think that it is a electronic problem, not about crio

i have the next circuit to control my dc motor, the values which i want to work, are drawing on the image

so the problem is the next:

both cases are the same,the generation of the pwm signal is for frequencies lower than 500Hz and i will change the duty cycle to turn the motor faster or slower, but i can´t have a current or voltage in the output of the operational amplifier higher than the values which the motor can have.

So the problem is that i need a supply voltage for my operational amplifier higher than 5V

but when give more than 3V, the current in the output of the operational amplifier is higher than 200mA(the maximum current which can enter to my dc motor)

how could i get a supply voltage higher than 5V for my operational amplifier and don´t give more than 200ma to my motor?

[johnsold]

That is much clearer.  Thank you.  Generally the moderators do not remove posts unless they are highly offensive.  No one will be upset that someone who obviously does not use English as native langauge had some difficulty communicating.  I am sure you are doing much better than I would in your first language or any other language.

Presuming that the left end of the motor (as drawn in your diagram) is connected to ground, then the voltage across the motor should never exceed 3 V, regardless of the power supply connected to the amplifier.  The data sheet I looked at did not specify the performance at low power supply voltages.  For testing purposes I would run it with +/-12 or 15 V power supplies.  When everything else is working then see if it will work from a 5 V supply, if that is what you want to use in the final version.

Most motors draw much more than the rated current when they are starting up.  The motor is not rotating so it is not generating back EMF.  Thus the current is limited by the DC resistance of the motor which is typically much lower than what you would calculate from Rated Voltage/Rated Current.  So is expected that a motor will need to draw more current for a short time while it is starting up.  If you do not allow this to happen the motor will start very sluggishly or not at all.

While the motor is running you could apply higher voltages (especially in a PWM system) for short periods of time without damage as long as the voltage is not so high that the insulation breaks down or as long as the average current stays within the motor rating.

You could probably operate that motor from a 5 V or even a 15 V supply with a switching transistor in PWM mode, provided that you kept the average current within the rated limit and the maximum pulse width was short enough that the instantaneous current did not get too high.  That pulse width may depend on the motor inductance and resistance.

Some sophisticated PWM systems monitor the motor current and turn off the power to the motor if the instantaneous current exceeds some limit.

Lynn

[mariomoskis]

yes, the left end of the motor in connected to ground and i know that the voltage across the motor will never exceed 3V, that is what i want because my motor is limied to work in a range from 1.5V to 3V

when i am generating the pwm signal with crio, i do it with a module ni9401(digital output)

i have the date sheet of my operational amplifier (i attache it) and in the graphic (Supply Current versus Supply Voltage with No Load) we can see the minimum voltage which it need (more or less vcc=+2V vee=-2V) or the same (Vcc=4 and Vee to ground). So i take 5V to be more sure that it will works in its range.

i already did the same circuit,but i put a resistor instead of my motor//diode and i was able to supply my operational amplifer with more than 5V

so the problem is that when i put the motor//diode instead of the resistor, i am not able to supply this amplifier to 5V as i did first with the resistor, because when this supply voltage is higher than 3V the motor is consumed more than its maximum current alllowed 200mA,

and that´s why the amplifier isn´t working like voltage follower (i don´t have the 3V which i should have in the output of my operational amplifier as i want.

so the question is: i need to supply my amplifier with al least 5V,but i can´t have more than 200mA in the output of the amplifier as i am having when i supply it with more than 3V!!! how could i get this???

do you understand more or less what i need?

thanks

[johnsold]

Yes, I understand what you are asking.  That is the same datasheet I found.

What I do not understand is what is happening with your motor and amplifier. How much current does the motor draw if you connect it directly to a 3 V power source?  Is it possible that your particular may have an internal short circuit causing it to draw more than rated current?  Does the motor run at the normal speed?

If you change the positions of the 10 kohm and 15 kohm resistors you should get 2 V at the output of the amplifier, rather than 3 V.  If you run it that way with the digital output turned on continuously (5 V), does the motor run at 1/2 to 2/3 of maximum speed? How much current does it draw? Does the motor speed or current change if you change the amplifier power supply voltage but nothing else?

Lynn

[mariomoskis]

if i conect directly my motor to a power sourse, when the power source=1V my motor consumes 0.2A(the limit) and when the power source=0.4V the motor is stopped and consumes 14mA

the circuit which i have connected is the same that i draw in the image

If i change the positions of the 10 kohm and 15 kohm resistors, i have 2V at the output of the amplifier and the motor consumes 0.3A, if i run it that way with the digital output turned on continuously =5 V

how could i know the speed which is running the motor?

yes, if  i change the supply voltage , the current and speed change too

[mariomoskis]

and i saw that the motor can consume 0.3A, so it is the limit,not 0.2A at i said . Sorry

so i can give a supply voltage of 5V to the amplifier as i need,but this is the minimus value which it can have, and the same for CI NE555, so at least i need 6V!

but if i try to supply the amplifier with more than 5V i have the same problem again, the current which consumed the motor is higher than 0.3A(the limit)

[johnsold]

Perhaps you are running the PWM too slow.  Then the circuit settles to the DC or continuous values before the control signal changes state.

What happens if you set the power supply to 6 V and run the PWM cclock at 10 kHz and 50% duty cycle?

Lynn

[mariomoskis]

yes.that is the problem!

with CINE 555

when i generate the signal at 100Hz i have the problem

at 1khz it works almost perfect,and at 10khz it works perfect

so when i increase the frequency, the motor consumes lees current! but  i don´t understand why!

with ni9401

i just generate pwm signal with frequency lower than 500hz,because i am using my only way 'scan mode'

then in this case i have the problem which i had at the beginnig about the supply voltage for my amplifier

the solution for this is reduce the current which my motor comsumes,but i don´t know how to get it! is it possible??