How to measure current through servo and how to calculate it's power

[pyroknife]

Hi guys.

I am using a servo motor in one of my projects. I would like to calculate the power the motor is inputting into my system. I will use the equation P=IV to calculate my power.
Voltage across my motor will be constant. It's about 5V since that's the voltage required to run the motor.
That leaves me with measuring current.

One of my questions is, is the resistance of the servo constant? If so, couldn't I use the equation P=V^2/R to get the power? But that doesn't make sense to me. I think it requires more power to run the motor at higher speeds than at lower speeds. So the previous equation wouldn't make sense.

My question now is how would I be able to measure my current through my servo?

My circuit is pretty simple. The servo has 3 pins. 2 of the pins goes to the + and - on my power supply. The 3rd pin goes to my microcontroller.

Or maybe I'm thinking about this all wrong and the power is actually the same and independent of how fast the servo is rotating?

 

[Baluncore]

The voltage across the servo motor may be constant but the internally generated back EMF proportional to RPM will be subtracted from that internally, so there will be no virtual fixed resistance.

You must measure the current flowing, which by the way will be proportional to torque, power happens to be torque times RPM but that is not particularly useful here.

There are a few ways to measure current. The simplest would be to use a series resistor and measure the voltage drop across that resistor. Multiply that by the voltage across the motor and you have power. Most of that power will hopefully be delivered through the motor shaft, else it will heat the motor.

 

[Bobbywhy]

Hi guys.

I am using a servo motor in one of my projects. I would like to calculate the power the motor is inputting into my system. I will use the equation P=IV to calculate my power.
Voltage across my motor will be constant. It's about 5V since that's the voltage required to run the motor.
That leaves me with measuring current.

pyroknife, you seem to be mixing two different forms of power; one electrical and one mechanical. Using that equation to measure the “power” as you described will give the electrical power input to the system. The motor, however, converts that electrical power into mechanical power which is then “inputted” to the system. And since motors do not convert electrical power into mechanical power at 100% efficiency, the mechanical power input to your system will be some fraction of the measured electrical power.

 

[meBigGuy]

You can measure the power dissipated by your system by measuring the voltage drop and current. I don't know how to convert that to mechanical power without more information. You need a dynamometor to calibrate the system's power consumption at different loads.

 

[Baluncore]

There is a way to separate the shaft and resistive loads. By running the motor on a lower voltage and measured current while stalled, you can measure torque with a simple spring balance at the end of a bar attached to the shaft. From that you can calibrate the static current to torque relationship of your motor.

Then later, when the motor is running you measure the voltage, current and RPM. From the current and voltage you compute the total input power. You then use your earlier stall test data to calculate torque from the measured current. Shaft power is then the product of torque and RPM. Subtract the shaft power from the total power and you will have your motor's resistive power loss. The much less significant bearing and brush losses, plus rotor wind drag, will be included in the computed shaft power result.

 

[meBigGuy]

That assumes that the motor efficiency is constant with rpm, including 0. See http://mplab.ucsd.edu/tutorials/DC.pdf Am I missing something?

 

[Baluncore]

Am I missing something?

The efficiency is not assumed constant with RPM.
The current to torque relationship is linear and is not dependent on RPM. That is the key.
By measuring current, voltage and RPM you can then calculate efficiency in that particular situation.

Which bit of your linked 17 page DC-motors article are you referring to?

 

[meBigGuy]

I got it. Learned something about motors.