4W Maximum Power Servos? Huh?

[Bryscus]

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

[sanddrag]

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

[Foster]

Originally Posted by sanddrag

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

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.

[Bryscus]

Originally Posted by Foster

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.

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

[Foster]

Originally Posted by Bryscus

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.

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.

[Bryscus]

Originally Posted by Foster

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.

- Bryce

[EricVanWyk]

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.

[EricVanWyk]

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.

[Kevin Sevcik]

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.

[hilo90mhz]

Does this mean we can use VEX servos/drive motors in the main robot?

Would be helpful for our claw actuator.

[EricH]

Originally Posted by hilo90mhz

Does this mean we can use VEX servos/drive motors in the main robot?

Would be helpful for our claw actuator.

Servos, yes, provided that they are under 4W. VEX motors, not so much, due to <R46> disallowing anything not allowed under <R45> and <R45> not specifically allowing VEX motors.

[PAR_WIG1350]

Originally Posted by EricVanWyk

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 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...)

[artdutra04]

Originally Posted by PAR_WIG1350

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...)

I sure hope they aren't using kg-cm or g-cm for torque measurements. (Kilo)grams are a unit of mass, not force.

[EricVanWyk]

Originally Posted by artdutra04

I sure hope they aren't using kg-cm or g-cm for torque measurements. (Kilo)grams are a unit of mass, not force.

Check out the kg-cm on this sheet:
http://www.servocity.com/html/hs-325hb_bb_deluxe.html

I wonder if their performance depends on local gravity...

[SteveJanesch]

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.

- Steve