4W Maximum Power Servos? Huh?
 

Guys,

I've been an RC enthusiast for probably 10 years now and I've never (I MAY have seen a power curve, I can't remember) come across a power rating for a servo. I've been on the phone with the Hitec and JR tech support and they have no idea how FIRST has come up with such a number and I have no idea on how I would prove that a different servo meets the 4W Maximum requirement.


In the Rules:

<R45> Motors specifically permitted on 2011 FRC ROBOTS include:

B. an unlimited number of COTS servos with a maximum power rating of 4W ,
- The burden of proof that the servo meets the criteria is on the team. The team must show the appropriate data sheet to the inspector.


Any thoughts or insights? We would like to find some more powerful servos for gear shifting, but I've come do a dead end. We would prefer to try to find an electrical solution instead of a pneumatic one. Thanks.

- Bryce

Re: 4W Maximum Power Servos? Huh?
 

James Prescott Joule would argue that a reasonable estimate could be calculated based on other commonly available specifications. ;)

Re: 4W Maximum Power Servos? Huh?
 

How about this (not exactly simple but just a thought): The servo is basically a DC motor, so you can create the motor power curve from 2 points.

1) run it no load to get the no-load speed
2) measure the stall torque - you'll need to create a test rig

Peak power is 0.25 * stall torque * no-load speed (get the units right to convert it to Watts)

Be prepared to show your data.

Re: 4W Maximum Power Servos? Huh?
 

Guys,

I have no problems with taking measurements. But I want to know how FIRST came up with the number. Maybe they measured the servo under controlled conditions?

Also, the manual states that:

"The burden of proof that the servo meets the criteria is on the team. The team must show the appropriate data sheet to the inspector."

Do self-written datasheets count? :)

- Bryce

Re: 4W Maximum Power Servos? Huh?
 

As his friend James Watt would have helped by saying it's 0.00536 horsepower.

All of the servo people give you rotation speed and force output, so you convert and go.

Take a screenprint of the servo specs.

For what (not watt) it's worth, those are pretty small servos.

Re: 4W Maximum Power Servos? Huh?
 

NOt sure, but can you get the servo companies to give you no load speed and torque? Surely they have that information.

Re: 4W Maximum Power Servos? Huh?
 

:) Doesn't one need an efficiency rating as well? I don't think they mean power in terms of mechanical power, but rather in terms of electrical load on the digital side car.

- Bryce

Re: 4W Maximum Power Servos? Huh?
 

From previous experience, inspectors will probably not check this unless they have reason to believe that the servos you're using are more powerful than specified. It would slow down the inspection process even more.

Now, this is not to say go ahead and use whatever you want. DO have the proper paperwork for your servos. I believe this is more of a GP rule.

A real inspector should be able to tell you whether or not what I have said is true.

Please don't take this as concrete evidence. My thought is that inspectors have much more to worry about rather than seeing the performance data on all the servos on your robot. They are more worried about safety. Are all the pinch points covered? Are there any sharp edges that need filing down? Are all electrical conductors covered?

Again, my word is not FIRST's. Don't stretch the rule to any extent. I did not say that they will not check it.

I do not believe self-written sheets apply. I can say that my servo (which is could be the size of a CIM motor) only outputs 3W of power.

Re: 4W Maximum Power Servos? Huh?
 

Where can I get one of those?! :) That's exactly what I need!

- Bryce

Re: 4W Maximum Power Servos? Huh?
 

Motor ratings are done as power output, not power consumed. So you don't need the efficiency. Had they cared about the power input they should have said "draws max of 0.33 amps at 12 volts".

But since you brought it up, drag the question over to the Q&A site and ask, power in or power out and see what watt they want.

Re: 4W Maximum Power Servos? Huh?
 

I agree with you on the power output bit. I don't know why they would care about limiting the output power, but I'll post the question and see what I get.

- Bryce

Re: 4W Maximum Power Servos? Huh?
 

Servocity.com gives you ounce-inches of torque, and seconds to turn 60 degrees. A little conversion gives us:

Power = 0.25 * oz-in * (0.00706 N-m/oz-in) * (1/(6 * t)) RPS * (2pi rads/R)

Or,

Power = 0.001849 * oz-in / (sec per 60 deg)

Judging by that, there aren't many servos on ServoCity that AREN'T legal. So I'm doubting that's right. I think the electrical load is the more likely definition, though that's more difficult to determine, since there's less specs on that.

All in all, probably a good Q&A to see if the GDC was thinking electrical or mechanical power there.

Re: 4W Maximum Power Servos? Huh?
 

Man, this is harder than I thought - these servo vendors sure don't like to make things easy for us. What the heck is an ounce inch? seconds per 60 degrees? Seriously? I miss newton meters and radians per second :(



I poked a bit at some of the servos that have been legal in the past to see if my calculation method was ballpark valid. First I converted to Nm and rad/s and then multiplied to get Nm/s (W). I used whatever the companies advertised for 6V.

HS-475HB:
76.0 oz-in Stall
0.18 sec/60° No load
.536 Nm, 5.81 rad/sec, 3.12 W


HS-332HD/HS-325HB
51.0 oz-in Stall
0.15 sec/60° No load
.360 Nm, 6.98 rad/sec, 2.51 W


From this, it looks like no load * max torque might be the formula they use. Wouldn't hurt to officially ask.

Also, can someone please please please check my math? Its quite possible that I dropped something during unit conversion.

Re: 4W Maximum Power Servos? Huh?
 

Eh, each sidecar can push 18 Watts total to the servos, and only takes out the servos if it is over loaded. I think it is mechanical power.

Re: 4W Maximum Power Servos? Huh?
 

Your math is correct for stall torque * no-load speed. Which, of course, isn't the same as max power, which is (stall torque * no-load speed)/4. My post above uses the latter convention. I take it you were looking for whatever formula the GDC might have been using when they developed the 4W limit. I think it's definitely worth the Q&A to see what they were thinking of when they wrote this one.