I need to control two servo motors simultaneously from a portable battery pack. I would like the combined torque between the two servo motors to be able to output around 200 ozin (or 0.1440 kgm) to support a weight.
I find the no load angular velocity from the data sheet to be w = 0.14 sec/60° (or 7.4799 rad/s).
Calculating the mechanical power I find
P = Tw = (0.1440 kgm)(7.4799 rad/s) = 1.077 watts
and then using that to calculate total current I find
I= P/V = (1.077watt)/(6V) = 0.1795 A = 179.5 mA
Am I calculating my total current consumption properly? I don't think I am because I know torque depends linearly on current and when I look at the data sheet I see,
Running current (at no load): 500mA
Stall torque (at locked): 17 kg-cm
Stall current (at locked) : 2500mA
Using this information and interpolating I find to get T = 7.20 kg-cm from one motor I would need I = 1347.15 mA. This is a lot more current!
Motors are NOT 100% efficient. The electrical power will always be higher than the mechanical power. When the servo is holding there it not doing any mechanical work so it has an efficiency of zero with all of the electrical energy converted to heat.
Ok that makes sense thank you! So now I know each servo will have to draw roughly 1.4 Amps. I have a battery that is 12V @ 3A max current. Can I put the two 6V servos in series and drive them both that way?
Unfortunately you cannot put 6V servo in series, power supply wise.
You would be asking the servos the share the same current, and that is not possible with such a dynamic load as servos.
Ok thank you! Ok so my servos need to be wired in parallel. That means I’m going to need to draw roughly 2.8A for my application. Is using a battery pack that caps out at 3A too close for this application? If not maybe it could still work by using a voltage regulator to step my voltage down to 6V and connecting my servos to it in parallel.
I'm not sure the P=Tw thing works at the stall anyway, since by definition w=0 and P=T0=0.
At stall there's no motion, so no mechanical work and therefore no mechanical power.
That's not to say, of course, there's no electrical power consumption, but I don't think it can be calculated from the mechanical power, since there isn't any.
The electrical power will always be higher than the mechanical power. When the servo is holding there it not doing any mechanical work so it has an efficiency of zero with all of the electrical energy converted to heat.
