Working with Continous Servo

Hi All,
I am very new to electronics and Robots. I have been trying to run a continous servo motor 4v to 6v. I have a battery power source of 9v. I have the following question.
Since my power source is higher than my required voltage I wanted to use a resistor to lower the voltage, so my motor is not going to get damaged. I used the following formula to find out the resistor required to get this voltage.
R = (Vin - Vout)/current required which is (9-6)/.01 = 300 ohm
Since I didn't have a 300 ohm resistor I used a 330 ohm resistor.
When I connected the circuit without the motor an checked the voltage across the circuit with the resistor on, it gave me a reading of 8.90v. I was not very sure what was going on, so I went on to connect my servo motor to see if it works by passing the signal and voltage. To my surprise the motor didn't work. So checked the voltage across the circuit it was reading 2v.
I was a bit confused at this point, so I removed the resistor from the circuit and connected the servo motor straight to the 9v power supply and the motor started working.
Please can any of you tell me, if I am doing something wrong here. Why the motor didn't work with connecting to resistor Or do let me know if I have done something wrong with the calculations.
Thanks

Curly schrieb:
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A resistor is not a practical solution. The "resistance" of a motor depends on its burden (to drive), and other dynamic factors. In the simplest case you better control the *current* through the motor, by a (programmable) current source.
When you want to stop the motor quickly, you'll find out that the turning motor acts as a current source, that wants to maintain its previous current flow. This can result in a high *reverse* voltage at the device that breaks the current flow.
Real world servo motors are usually driven by impulses, of the full (maximum required) voltage, and of a variable width. The current then is limited by the dynamic "resistance" of the motor, not by the peak voltage of the impulses. Then the ratio between power on and off times determines the force and speed of the motor. Almost every microcontroler supports such Pulse Width Modulation (PWM), all you need are additional transistors, capable of switching the high motor current, and (optionally) an power limiting (and shortcut protection) circuit.
When you want to turn the motor also in the opposite direction, you have to (virtually) reverse the polarity of the motor power source. In practice the motor then is connected to two switching devices, capable of sourcing and sinking current. In such an "H" bridge circuit the drivers form the vertical lines of the H, the motor the horizontal line. The power supply (battery) is connected to the top and bottom ends of the H. Both drivers receive inverse pulses, so that the motor receives continuous impulses of opposite polarity. As mentioned above, a 50% ratio keeps the motor stalled, with almost no current flow due to the current generator property of the motor. Unlike your resistor, which has to sustain the DC current times voltage difference (P=U*I), the switching devices (transistors) don't have to sustain a high power, because they are always either turned fully on (U=0) or off (I=0), with a minimal power product U*I.
Have a look at readily available electronic devices, usable for PWM driven motors.
> I used the following formula to find out the resistor required to get this
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Such observations may depend on the angle of the motor shaft, because the polarity of the coils inside the motor must be reverted all the time, when the motor is running. And between the transitions these switches can be both off, so that you'll read an infinite (DC) motor resistance. Or the motor already contains some servo circuitry, that does part of the actions described above.
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A motor isn't a simple (linear) resistor (R), it's more a coil (L) with dynamic (burden dependent) inductive resistance. Without complicate (complex) calculations you better try to control the (average) current through the motor, within its specified current ratings.
Even if I know a bit more about the motor theory, I've no special experience in motor practice yet, just starting my own experiments. I've already used fischertechnik motors, with the PWM circuitry contained in the controller unit. Currently I'm playing with the Arduino Starterkit, my personal christmas gift, that also includes an motor, but I've not yet reached that chapter in the manual. Such kits are great for roboting newbies like us :-)
BTW I plan to construct an puzzle assistant, that inspects and returns the pieces I need for completing a puzzle.
DoDi

replying to Hans-Peter Diettrich , Curly wrote: Hi All,
Thanks for your response.
I am using Arduino to control the motor my only problem is controlling the voltage supplied which I think I can control it using voltage regulator. I have to look into the reverse voltage see how it might affect.
I am trying out small projects with Arduino.
Thanks