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.
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.”
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.
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.
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.
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.
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.
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.
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.
I too wish that torque ratings were standardized. I’ve been on websites that have a different unit for every product they sell. However, newton meters are HUGE units. Servos, in general, are really weak. Milli Newton Meters are used more more often in this case (or kg-cm, or oz-in, or g-cm, or…)
Wouldn’t the electrical power draw (max) be stall current times the operating voltage? And by stall, I mean not just current drawn when holding position but just barely holding position against the maximum torque load.
I blame the imperial system and its conflation of mass and force units. Someone’s clearly trying to foist that horrible convention on the metric system under the guise of it being “easier”. It might be in retaliation for the slug… but that clearly WAS a good idea that would’ve made things easier.
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.