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Old 2nd December 2010, 07:10 AM   #11
kenneth is offline kenneth  Sweden
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Thanks! this was a lot of help.
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Old 2nd December 2010, 05:59 PM   #12
kenneth is offline kenneth  Sweden
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The math looks difficult. My DC100 motor runs at 3,9v to achive 33,33 and 5v for 45 evolutions per minut. The winding impedance is 30 ohm. Are the suggested 9v supply for the opamaps or the input (control) voltage. Thanks
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Old 2nd December 2010, 07:11 PM   #13
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by kenneth View Post
The math looks difficult. My DC100 motor runs at 3,9v to achive 33,33 and 5v for 45 evolutions per minut. The winding impedance is 30 ohm. Are the suggested 9v supply for the opamaps or the input (control) voltage. Thanks

The voltages I quoted are for the supplies.. You will need a separate reference which could very easily be derived from one of the supplies, I'd recommend an LM385-1.2V reference, a 10T pot for each speed, and a switch to select speed. The LM385 is a shunt reference and can be run off of the 9V supply using a series resistor..
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Last edited by kevinkr; 2nd December 2010 at 07:38 PM.
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Old 2nd December 2010, 08:32 PM   #14
kevinkr is offline kevinkr  United States
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I've been thinking about your application and based on the application notes I recommend the following values for the basic controller circuit:

R1: 1.0K 1% 100ppm 1/4W
R2: 5.11K 1% 100ppm 1/4W
R3: Calculated value 851 ohms either use 845 1% 100ppm 1/4W or my preference would be 250 ohm 10T low TC pot, and 681 1% 100ppm 1/4W in series. This controls the speed regulation (see app note) set pot to ~170 ohms - decrease value slightly if motor does not maintain speed when load changes.
RS: 2.7 1% 100PPM 1/2W or greater..
RCL: 56.1K 1% (for OPA547) IL ~250mA
Power Op-amp: OPA547 in TO-220 package.
Buffer: OP27C
Reference: LM385-1.2V

Motor current under load is going to be modest, stalled 130mA - 170mA at 33rpm and 45rpm respectively.

The power supply needs to be able to furnish 250mA without sagging significantly. It does not necessarily need to be regulated as the reference will provide a stable reference voltage for the controller.

The reference should be run at ~10mA allowing for a couple of mA for the resistor dividers/pots. I'd restrict the adjustment range on the top and bottom end of each pot making it the center resistor in a 3 resistor divider.

The reference voltage fed to R1 should be buffered so that controller performance is not compromised by changes in closed loop gain as a function of speed setting. Strikes me that the OP27C would be a good choice for this application. The pot(s) should be 1K.

For 33.33rpm based on the numbers you provided the control voltage will be -763mV, and for 45rpm -978mV. The buffer at its simplest is a non-inverting unity gain configuration, given the high open loop gain and the moderate precision CM errors in non-inverting mode are not significant saving a couple of resistors.

Be sure to install 0.1uF decoupling capacitors at each supply pin of both the OPA547 and OP27.. The supply should be well filtered, but does not really need to be regulated - if you wish to carry this a step further LM7809 and LM7909 will get the job done cheaply and well.

http://focus.ti.com/lit/ds/symlink/opa547.pdf

http://focus.ti.com/docs/prod/folders/print/op27.html

http://www.national.com/ds/LM/LM185-1.2.pdf


Please download the datasheets for the recommended parts. Any questions just post.

The LM7809 and LM7909 are made by Fairchild and others, and are available from Digikey. (Under $1 each)

The OPA547 is stocked by Digikey and is $11.

The OP27GPZ (made by AD) is in stock at Digikey for $2.70
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Old 2nd December 2010, 10:02 PM   #15
kevinkr is offline kevinkr  United States
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Default Reference, Voltage Divider Strings, Buffer

Hopefully this will help, and again the value of the resistors was based on the voltages you posted. Worst case you may be tweaking a few resistors to get enough adjustment range.

The 33 and 45 rpm pots would go to a switch, and you should probably add a 1M resistor at the input of the buffer op-amp to gnd to prevent bad behavior when switching from one speed to the other.. (Op-amp going to one rail or the other.)

Pots are 500 ohm, and should be low TC and relatively tight tolerance say 5%. All other resistors 1% 100ppm 1/4W
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Last edited by kevinkr; 2nd December 2010 at 10:09 PM.
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Old 3rd December 2010, 08:23 AM   #16
kenneth is offline kenneth  Sweden
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Thanks,
I will for sure build this circuit.
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Old 3rd December 2010, 02:20 PM   #17
kevinkr is offline kevinkr  United States
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Let me know how it turns out. This is the "quick and dirty" approach since I don't have the parts and motor on hand I figured it would just be simplest to stick closely to the application note which I think under the circumstances is the best route to success..

I expect it will be a very great deal better than running without any sort of control loop.
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Old 22nd June 2012, 12:42 PM   #18
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Hi Kenneth,
Any update on how the circuit turned out? I am building a new turntable and I am looking into DC motor along with a DC power supply and this thread is one of the most interesting i've found so far but I'd love to hear how the circuit turned out. And thanks Kevinkr for the explanations.
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Old 22nd June 2012, 04:56 PM   #19
kevinkr is offline kevinkr  United States
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I'm actually curious to know how it turned out as well..
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Old 22nd June 2012, 11:23 PM   #20
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These are the problems it will have:

Firstly, being a pfb loop, it will amplify any current noise (such as commutation spikes) back into the circuit. The higher the loop gain (needed to get good accuracy) the worse this gets. To limit this, you can interpolate a low pass filter in the loop but slowing the loop slows the transient response. No free lunch.

The control loop tuning will not stay stable with temperature due to the positive TCR of the motor coils. This means that if the tuning is set to equalise motor impedance when the motor is cold it will sag when the motor gets warm, or if it set to equalise when the motor is warm it will oscillate when the motor is cold.

The set speed will drift with time and temperature and need to be reset frequently.

These are not problems with this particular circuit alone, they are endemic to the class of circuits.
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