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# Current Control for a DC Motor??

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#### Jacky_78

##### Newbie level 2
I am a senior student of Electrical Engineering Dept., and I am working on my senior design project which is Current Control for a DC Motor". However, I still have no idea how to design it. First of all, I have questions about how to measure or calculate motor components, such as motor resistance, inductance, torque, and angular speed etc.. The following is the precise description of my design:
Apply a DC voltage (about 5V) to a DC motor, which contains the internal resistance R, and inductance L. The current goes through the motor is i=(v-kw)/(Rm+Rs), where Rm is motor resistance, Rs is a small resistance (about 1 or 2 ohm) connects motor to the ground if we ignore L. The voltage appeared on the motor will be e=kw. The torque of the motor is proportional to the current i, so torque=ki. The output of the motor is the angular speed w.
This is a close-loop current controller, so we need to feedback the current i, the error current (input current – feedback) through an amplifier to control the current i to stay the same.

Now the questions are:
How am I going to measure the motor resistance, inductance, torque, and angular speed?
What steps should I take to appraoch this design?
How should I design this amplifier so that it can amplify the error current to control the DC current that goes to the motor?

I hope someone here can give me some hints or help me to accomplish my design.
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Thanks for the website you provided, but it doesn't have infomation I need. Could anyone give me some other info??

You can measure by avometer the Motor resistance but this varying by temperature.

Enductance changes by current becouse iron core not a linear.

DC motor has second order characteristic. Rotor resistans and enductance
too small. Electrical time constand smaller then mechanical time constand and You can neglet it and system simplify.

Now motor has first order characteristic and we have only friction and ibertia.

You can determine this parameters by experiment.

connect the tacho genarator to motor.
Build an ADC unit and connect the PC.

Give a fix voltage to motor and Take sample at every 100 microsec from the tacho voltage and store at the PC.

until the motor acsess the nominal speed take sample.

Now you have speed variation data. Draw the speed time graphics.

You will see this same as capacitor charge graphics. (dc voltage capacitor, resistor circuits.)

Now you can find the which parameters give same graphics using numeric methodes.

Here is a nice explanation to found motor parameters :

Others informations about current control could be found in datasheets of some discrete H-bridge.
I don't have any reference in my mind ?... sorry

Hi Jacky_78

I would also sujest that you consider pulse driving the motor as this will give you more conrol and prevent the posibility of your dc motor servo loop hunting.
When you drive the motor you can feedback a pulse to an intigrator that produces a correction voltage that adjusts the servo to the correct speed.

You will need
1 motor drive dc pulse amplifier.
2 pick up pulse Amp+ pickup
3 Intigrator to correct the motor drive amp

Barrybear

I have seen that there are specially made chips around for the control of DC motors, mainly intended for the exact rpm-control of small DC motors for cassette players and such. A hint would be to find some datasheets for such chips and examine the application notes. Hopefully you will find by what methods they determine the motor's speed and how they control the motor. When you measure the motor current you may be able to see small fluctuations in it. I vaguely recall that one of the chips that I have in mind makes use of these fluctuations to sence the motor's rpm.

/Pim

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