paper: Servo Power Worksheet

Thread created automatically to discuss a document in CD-Media.

Servo Power Worksheet
by: Ed Sparks

An Excel spreadsheet that calculates the power in watts of specified hobby servos. Several Hitec models have been pre-loaded.

A tool to help with that pesky question of which servos are legal in FRC robots given the requirements outlined in the game rules. Several Hitec models have been added but you can add any servo you like.

Servo Power Worksheet.xlsx (16.7 KB)

I added this “Servo Power Worksheet” to CD today in an effort to help those still struggling with the 4W limit on servo power. Just add the common specs for the servo in question to the first 2 columns and look at the calculated power in the last column.

Could you possibly add a note that stall torque * free speed is NOT power, even though that’s the formula the FRC rules require?

Actual power is torque * speed @ a given operating point. The point of peak power for a brushed DC motor is around 1/2 speed (1/2 torque as well, since the curves are linear), so the actual mechanical power will be 1/4 of the “servo rated power” using the FRC specified formula.

I don’t know why FRC specifies the formula they do, it’s wrong.

I do like the worksheet, though.

I suspect it’s because the servo manufacturers are lying cretins. There’s no excuse for a manufacturer making that misrepresentation: it’s either deliberate or negligent.

FIRST should definitely make the rule based on the actual power rating, and then include a brief explanatory note in the rules. This is conservative, from a competition point of view, because the team that blindly accepts the manufacturer’s rating won’t exceed the specified (real) power limit. Teams that read and understand the rule and explanatory note will benefit from the use of servos right up to the real power limit.

(Incidentally, this is a dead-simple change. I wonder if FIRST has frozen the rules yet for 2013, or if straightforward improvements are still possible? Perhaps we should inquire?)

I should have stated that this assumes that FIRST will still allow servos in 2013 and they define the max power the same as prior years.

I realize that I am 3 years late to reply to this, but this is where the rule stands today:

PWM COTS servos with a maximum power rating of 4W each at 6VDC
Per the Servo Industry, Servo Max Power Rating = (Stall Torque) X (No Load Speed)

I don’t know what “No Load Speed” means… I have not looked at previous years game manuals yet, but wonder if this issue was ever addressed.

No Load speed for a servo is the same as “free speed” for a motor - the speed that it moves when there is no load (other than the internal friction and inertia of the motor and gearing). Another way of viewing this is as zero output torque (all of the torque of the motor has been used up in friction). As was noted earlier, if you assume that the torque and speed scale in an inverse linear fashion (usually a decent approximation), the peak mechanical power output is at one-half of the no load speed, and one-half of the stall torque, which is one-quarter of the max power rating.

Ah… no load as in… “NO” load… rather than no as in No. (an abbreviation for number). Got it… thanks.