Are these servos legal?

[dotbran]

My team needs to get a more powerful servo for our game piece, and the rules are somewhat vague on how they calculate maximum power.

[R48] states:
L. an unlimited number of COTS servos with a maximum power rating of 4W each at 6V.

The specs of the servos listed show the maximum current drain at 6V with no load, but is the 4W considering no load or the load that will be used?

Here are the servos in question. I would appreciate your advice.

3W max with no load
2.7W max with no load

[Bob Steele]

don't forget that you can use 2 of the VEX motors... although I am not sure how you power them at this point... they will only get 5 volts from the sidecar so you would have to downgrade their rating (which is based on 7.2 volts)

they are continuous rotation and could be geared to gain advantage..
They are really good little motors.. and much more robust than any servo that fits the FIRST guidelines..and they are very similar to servos in size and shape

These are not the old VEX motors ... these are new and more powerful...
worth a try

[dotbran]

Originally Posted by Bob Steele

they are continuous rotation and could be geared to gain advantage..
They are really good little motors.. and much more robust than any servo that fits the FIRST guidelines..and they are very similar to servos in size and shape

I need a servo because I will be rotating something 90-135 degrees and it has to be precise every time. If my team gears a continuous rotation motor to be practical and accurate within that range of motion it will take too long to actuate, and would require an encoder with PID programming to set the position in the program (a large task for a team with one seasoned programmer and one beginning programmer).

Do you think the servos I listed would be legal?

[EricH]

First: It doesn't matter what I think. It matters what the inspector at your event thinks.

That said I think it would be very useful for you to look at the blue box accompanying the rule, which specs out how to calculate the power for FRC applications:

Servo Max Power Rating = (Stall Torque) X (No Load Speed)

(emphasis mine)

[dotbran]

Originally Posted by EricH

That said I think it would be very useful for you to look at the blue box accompanying the rule, which specs out how to calculate the power for FRC applications

Thank you sir, you are a gentleman and a scholar. Please excuse my carelessness.

[artdutra04]

Originally Posted by EricH

That said I think it would be very useful for you to look at the blue box accompanying the rule, which specs out how to calculate the power for FRC applications:

That's incorrect. It needs to be:

(Stall Torque) X (No Load Speed) / 4

since max power output of a DC motor is at half stall torque and half free speed.

[DonRotolo]

Art, what do the rules say?

[EricH]

Originally Posted by artdutra04

That's incorrect. It needs to be:

(Stall Torque) X (No Load Speed) / 4

since max power output of a DC motor is at half stall torque and half free speed.

Yes, that is true, as I recall the curves. But, the GDC has seen fit to go straight for no-load speed and stall torque, so for this application, we don't divide by 4. If I remember correctly, there was some to-do about this last year or the year before when there wasn't a power definition at all in the rules.

[Joe Ross]

Originally Posted by Bob Steele

don't forget that you can use 2 of the VEX motors... although I am not sure how you power them at this point... they will only get 5 volts from the sidecar so you would have to downgrade their rating (which is based on 7.2 volts)

You will get 6 volts from the digital sidecar PWM plug (with the servo jumper installed)

Originally Posted by dotbran

My team needs to get a more powerful servo for our game piece, and the rules are somewhat vague on how they calculate maximum power.

[R48] states:
L. an unlimited number of COTS servos with a maximum power rating of 4W each at 6V.

The specs of the servos listed show the maximum current drain at 6V with no load, but is the 4W considering no load or the load that will be used?

Here are the servos in question. I would appreciate your advice.

3W max with no load
2.7W max with no load

It appears that you are using the electrical power for your calculations. As others posted, there is a specific definition of power that [R48] requires. When I apply that calculation, both servos would be above 4 watts.

[Dale]

I'm confused about the units here. If torque is measured in oz/in and speed in rpm how does the result in watts? I'm looking a servo that has 172oz/in of torque and 72rpm no load. Is that legal?

[Joe Ross]

Originally Posted by Dale

I'm confused about the units here. If torque is measured in oz/in and speed in rpm how does the result in watts? I'm looking a servo that has 172oz/in of torque and 72rpm no load. Is that legal?

Watts have units of N-M/s (among many). Convert your torque to N-M, and your rotation rate to radians/sec, (remember that radians are dimensionless) and it all works out. By my calculation, those servos are around 9W, using FIRST's formula.

[Dale]

Thanks! It makes sense now.