Servos on Battery project...How much current do they pull?

Steel

I am working on a project that is battery powered and requires 2 servos.· I have looked at many servo dealers spec sheets, and I am not able to find how much current they draw under user conditions.

I am 100% certain that people here have dealt with this and I was wondering if I could get some advice.

-If I have 2 of Parallax's full-rotation servo motors and a 2lb load on them...and they are in constant rotation for 3-4 hours...What size of battery should I expect to use?·· 500mAh? 1Ah?· 5Ah?· 10Ah?

-I looks like servos are rated to 6V.· Should I attach them straight to a 6V battery, or can I power them off of my existing 5V 1A voltage regulator(s)?

Any help would be greatly appreciated.

Shaun




Post Edited (Steel) : 3/17/2009 5:32:34 PM GMT

kwinn

Measure the current when the servo is in operation and use that for calculating the battery size. I would connect them directly to the battery. Less power dissipated by the regulator and the servos get 6V minus the drop across the drivers.

To calculate the battery capacity add the two current readings together, multiply that by the number of hours they will run, double that to allow for aging of the batteries and you have a ballpark amp/hour rating for the battery.

Mike Green

Figures I have heard and seen: 1A peak current (under heavy mechanical load), 0.25A average current under normal load

I'd use a 5Ah battery, maybe a 10Ah battery. 2 pounds is a moderate load, so figure 0.25A to 0.5A. Two servos makes 0.5A to 1A. For 4-5h, that's a 5Ah battery. As kwinn suggested, measure the current under load if you want accurate estimates of battery life.

Carl Hayes

Your best bet is to measure the current actually used.

It's impossible to determine the load when it is stated as two pounds, because servos (like other rotating machines) are loaded in terms of moment (torque), not force or weight.

You could, for example, determine the required torque, then calculate the work done per unit time (in horsepower or in ergs/second or in watts or in any other power unit), then assume, say, 50% efficiency in the servo, finally converting if necessary to come up with the required power in watts.· Double that because you don't want to run batteries down more than 50%.· Divide by 6 for the 6-volt supply, and -- voila! -- that's the current.· Multiply by the time you need from the batteries, and that's the AH rating you need.

But you can't start with just "pounds".

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
· -- Carl, nn5i@arrl.net

Steel

Mike-Thanks for the help. It gives me a great ballpark figure for a first prototype. My big fear was buying a $50 battery when I only need a $12 battery and I am able to narrow down what I will need based off of the current draw estimates.

I sure wish servo providers would provide a torque/current table to show the performance of the servos...but I guess that is just not something that I am in control of.