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Topic: Estimating Power for Servos (Read 3630 times) previous topic - next topic

-Railgun-

Hi guys. I'm trying to calculate how long I'll be able to run my build.
I'm using these two servos:

http://www.servocity.com/html/hs-5685mh_servo.html
http://www.servocity.com/html/hs-7954sh_servo.html

At 7.4 volts, both holding about 4 lbs or less or so.

They're also sharing power with an UNO but I'm not so worried about this as far as power is concerned.

After doing the calculations factoring in the servo arms I will be using, their torque ratings should be about:
HS-5685mh - 11.19 lbs (no change)
HS-7954sh -10.75 lbs

For the majority of the time, they will both be carrying their loads at a specified position (basically, the servo arms will have moved away from their neutral position and be holding their load at a specific position with gravity working against them).
In addition to this, the servos will have to do a rotation and move their loads about 180 degrees periodically (maybe once a every 20+ minutes).

I'm planning on using a lithium polymer rated 7.4V 8000mah. Battery space is at a premium right now. I understand that with both servos at lock/stall, this setup wont even last me 2 hours. What I don't know is how to figure out the power required when the servos are just holding their loads at a specific position (does this count as idle current draw?) and the power required to move the loads from time to time.

I understand the parameters are vague. Not looking for anything exact, just an estimate. Thank you!!!!!!!!!!!!!!!

zoomkat

Since you will be paying $$$ for those servos, why not just ask servo city what the stall current is for the servos?
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MarkT

When they are holding position the current is a lot less than at stall unless a lot of load torque - the
gear train has enough friction to do a lot of the holding work...  Without a current v. torque graph
from the manufacturer you may have to do some measurements to find out.
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-Railgun-

Thanks for the replies

Unfortunately, I haven't had a great experience working with Servo City's technical support.

The stall currents are about 3A and 5A for the 2 servos (latter being for the stronger one).

MarkT - the load will be about half the stall torque for each servo. They stall at about 11 lbs each and I'll be carrying 4 lbs or less. Ballpark estimate - do you think I'll get at least 4 hours out of the battery? If it's possible, I'd like to downgrade to a smaller, 4000mah battery too.

Erdin

It is impossible to calculate.

The stall current is a lot, but the idle/hold current is low.
They have a gear inside, so they can hold a position without effort.

If they are used for half of the maximum torque, they still could require the maximum current at the moment they start moving.

Assuming both 200mA idle current, and one second per 20 minutes maximum current.
3 * 8000mA / 3600 = 7mAh
7mAh + 200mAh = 207mAh.
Say 40...50mA extra for Arduino and a led, makes it 250mAh.
Your battery would last 16 hours.

If my calculation is right, and you have a unreliable battery from Ebay, it could be 4 to 20 hours.

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