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Author Topic: linear analog servo motor drive  (Read 1557 times)
ammar87
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« on: January 07, 2013, 03:03:48 PM »



hello my friends
i am trying to build a linear analog servo drive for a servo motor and YES it is analog not a PWM for many reasons one of them :is that this drive is a part of control system i am working on and i want to reduce the noise to minimum.
so i dig the web and i found several power op amp: like L165,L2720, LM675,LM12
i am kid of confused because all i want is :
Class-AB linear power Amplifier with no distortion around zero
i am trying to  use the power op-amp rather than power transistors to reduce the number of components and the issues related to them .
here is the motor specifications:
http://www.specamotor.com/en/Faulhaber/motors/2338-006S/datasheet_motor.html
i suppose to control the position in bi-direction.
the questions:
1-which power op-amp should i use?
2- is there a ready drive that i can buy ?
3-is this circuit correct to control the motor position in  bi-direction?

any help will be more than appreciated
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KevinIV
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« Reply #1 on: January 07, 2013, 07:57:45 PM »

An analog motor requires an AC signal to change motor rotation direction. Their used more in appliances and their rotation direction doesn't change. Servo motors are normally digital motors. But if it's an analog motor, the frequency output of the DAC and the motors inductive reactance will require more than a power opamp.
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TH0R
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« Reply #2 on: January 19, 2013, 04:21:16 PM »

Hi there ammar87!

I second KevinIV.

You will need to compensate for the DC motor reactance and also depending on the maximum current at stall I am not so sure the power opamp will be capable of working out for you. At quick glance L165 has max output current of 3A which can be easily exceeded. The heat dissipated by the regulating element can be quite a lot, as well. For example, powering the system at 12VDC and operating the motor at low speeds (say 3V) with some load on it (say using 1A) causes the linear amplifier to dissipate 9W continuously. That means you will also need a decent heat sink.

Also, the DC motor brushes / commutator will output a lot of noise due to brush bouncing and mechanical switching from commutator plates (EMI). Some of that EMI is conducted some is radiated.

Long story short, to answer your questions:

1. I've used OPA549 by Texas Instruments for different applications, but non the less it is quite capable of delivering high current into a load (up to 8A continuous and 10A peak). You will still need a heatsink. I could not open the motor link you provided.

2. I am not aware of one. Most people use PWM due to the high power efficiency. Noise can be reduced by reducing the switching frequency and reducing the slew rate of the rectangular waveform. That sacrifices efficiency but reduces noise due to high speed switching.

3. It depends if the amplifier is connected to bipolar power supply. If it is and the DAC can output bidirectional voltage then yes. At the very least you will need to connect a snubber across the motor (resistor and capacitor in series) to compensate for some of the noise and the motors inductive reactance.

Hopefully that was not too much and helps you  Cheesy
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ammar87
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« Reply #3 on: January 24, 2013, 04:00:42 PM »

thank you very much that's  very helpful
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