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Question on designing close loop DC brushless motor control

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swear_swear

New Member
I am designing a PWM close loop DC brushless motor control circuit.
For open loop speed control, there are no problem.
But now I would like to design a close loop automatically speed adjustment circuit. E.g: for a range of loading applied, the control circuit can auto adjust to a constant speed.
I am thinking on sensing the output current of the coil and feedback to the PWM control to adjust the duty cycle of each pulse.
But still don't know how to design the current sensing circuit.
Can any professional guy help me to finish the design??
Really thx. :shock: :shock: :shock:
 

mechie

New Member
Motor speed control

Is it impossible to have some other speed measurement system ?
A pulse-per-rev generator would simplify things for the feedback.

What kind of size is this motor (physical and voltage/current) and what speed does it do?
 

Roff

Well-Known Member
If you intend to force constant current through the motor via feedback, you will get the poorest speed regulation possible. In order to maintain constant speed, a motor's current has to increase as load increases. Constant voltage has better regulation, but as mechie said, you really need tachometer feedback. If you can add a tachometer wheel or some other device that will give you multiple pulses (the more the better) per revolution, and make this the sensor for your feedback loop, you will get better response time to load changes, especially if you are running at low RPMs.

Ron
 

swear_swear

New Member
What is tachometer to sensing?

Thank you for your reply but what is tachometer feedback?
Actually my DC brushless motor is 24V dc input and with about 2000rpm.
Can you give me a detail on how to make such design?
As i am using PWM to control speed. But for constant speed, seem very difficult as what you want. (Load increase , current increase)
Can any professional give me a design or a circuit about this?
THX
 

mechie

New Member
Motor speed control

Tacho - some form of signal that is proportional to the motor's speed.

If you have a sensor to measure the speed (ideally a voltage proportional to the speed) then it is easy to check this against a reference using a simple op-amp circuit. This will give an error signal proportional to the magnitude of the speed error, this could be used directly as a signal to your PWM circuit.
NOTE This is simple proportional only control and so will run with a constant error. This error MUST exist or there would be no output from the op amp! A more complex system (proportional PLUS integral) would remove this error but also complicate building and set-up.
 

swear_swear

New Member
Thanks for your reply.
But it is seem not easy to find a point of voltage which is proportional to the speed. Can you give me a more detail circuit about the complex system which you have mentioned? As I am not a professional on circuit design but need to finish this design as soon as i can.
Hope any professional can give me more advise.
THX
 

swear_swear

New Member
I think that I will use the coil back emf to maintain the constant speed.
But how can I got the signal of the back EMF?
How can I design the detecting circuit and feed to the PWM controller input? Any professional can help me?
THX
 

Sebi

Active Member
The brushless DC motors always contains some device for commutation.In small DC-fan is a Hall-effect sensor for this function.
Just apply a third wire from Hall-output (here is pulse train and the frequency depend from RPM)
 
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