# Stepmotor vs brushless DC servo

Hi All,
After solving my optimal control problem, I am facing one more problem. I need to control a rotation acceleration using an electric
motor. I need to make decision between stepmotor or brushless DC servo. The goal is to have control an angular acceleration. (Due to the fact that my dynamic equation includes second order derivative of a rotational angle.). What are the pros and cons for the two options? How to control the angular acceleration?
The motor will be used to drive a inertial load of about 0.15 kg* m^2. The rotation speed is from 0 to +- 33 rpm with peak torque of about 10 Nm.
Thanks a lot in advance,
Everett X. Wang
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On Fri, 17 Aug 2007 23:11:13 +0000, Everett X. Wang wrote:

I prefer to use commutated (regular DC, or brushless with appropriate sensors) motors for highly inertial loads. With steppers you either have low enough inertia that you can just start stepping, or you have to ramp up you acceleration. The problem is that this ramp has to be completely open-loop -- and I'm never happy with that.
So if it were me, I'd go with the brushless motors.
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Tim Wescott
Control systems and communications consulting
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Tim Wescott wrote:

A stepper tightly coupled to an encoder with as many steps as the motor (or a multiple of that) can be driven just like a brushless motor with Hall-effect switches. the dividing line is a bit arbitrary. Bypass the hall effects, driving the coils directly from logic and a typical brushless motor can be driven like a three-phase stepper.
Jerry
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Engineering is the art of making what you want from things you can get.
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On Fri, 17 Aug 2007 23:24:28 -0400, Jerry Avins wrote:

Granted. But then you're back to something commutated. It solves the problem at the cost of more complexity, which is exactly what I was recommending.
Out of curiosity: have you done this? I can see how it would work, but you'd pretty much need a microprocessor with some clever start-up code.
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Tim Wescott
Control systems and communications consulting