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.

Re: 4W Maximum Power Servos? Huh?
 

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...)

Re: 4W Maximum Power Servos? Huh?
 

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

Re: 4W Maximum Power Servos? Huh?
 

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...

Re: 4W Maximum Power Servos? Huh?
 

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

Re: 4W Maximum Power Servos? Huh?
 

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.

Re: 4W Maximum Power Servos? Huh?
 

I know, it's terrible:mad: , but it is used.

http://www.onlineconversion.com/torque.htm

Re: 4W Maximum Power Servos? Huh?
 

Maybe it turns the other way when you flip it over.

Re: 4W Maximum Power Servos? Huh?
 

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

Would be helpful for our claw actuator.

Re: 4W Maximum Power Servos? Huh?
 

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.

Re: 4W Maximum Power Servos? Huh?
 

I guess the real question then is do these two count as the same thing? the three wire motor is essentially a continuous rotation "servo" called a motor in this instance by vex.. it all depends on definition now.

3 Wire Motor
http://www.vexrobotics.com/products/.../276-2163.html
3 Wire Servo
http://www.vexrobotics.com/products/.../276-2162.html

Here is one that is labeled "correctly" depending on your viewpoint...

Continuous rotation servo
http://www3.towerhobbies.com/cgi-bin...&I=LXSYK3&P=FR

I also posted a similar question on the usfirst forum, we will see what they have to say also..
http://forums.usfirst.org/showthread.php?t=16325

Our team would like to use a few "continuous rotation servos" to spin some wheels in our claw mechanism this year.. Trying to figure out if this is legal..

Re: 4W Maximum Power Servos? Huh?
 

Quick tip on your thread location: Try posting it in the Q&A instead of the general area. You'll need to have your team's main contact do it, though--they're the ones with the passwords to ask questions.

A servo and a motor are not the same thing. A servo goes to a position it's commanded to go to and stays there. A motor spins when powered. You can make a motor into a servo by adding sensory feedback onto whatever it's actuating. It's a whole lot harder to make a servo into just a motor. It's that commanded motion that makes a servo a servo.

(From the VEX servo page linked to earlier, emphasis mine.)

Servo != motor. Motor != servo.

Re: 4W Maximum Power Servos? Huh?
 

I agree a servo != a motor.

I guess what matters is what the rule makers want the rule to mean..

It seems to me that most of the rules limit what motors are allowed, in this case the same internal motor would be used just with a different output mechanism.

The motor controller is still built into it like a servo.

I have modified servos to turn continuously in the past, depending on the internal gearing this is fairly easy, or impossible if the teeth on the main output gear are not complete.

Re: 4W Maximum Power Servos? Huh?
 

you guys prob already saw this since this thread is old, but if you haven't team update 3 addresses this issue

http://usfirst.org/uploadedFiles/Rob..._Update_03.pdf

Re: 4W Maximum Power Servos? Huh?
 

For those like me looking to understand team update # 3 from last year, the update reads

calculate servo max power as stall torque x no load speed

and then they give an example for the HS-322 which, at 6v, has a stall torque of 3.7 kg/cm and a speed of 0.15 sec for 60 degrees, and they give the final formula as

torque = 3.7 kg/cm = 0.36 Nm
RPM = 0.15s @ 60 degrees = 66.7 RPM
and the result is

0.36 * 66.6 * 0.1046 = 2.5W, and thus under the 4.0W limit.

But where did some of those numbers come from I/you ask?

Well 1 Kg/cm = 0.0980665 Nm which you know from looking it up, say at http://www.onlineconversion.com/torque.htm, or from basic physics. A newton is the measure of force necessary to move a mass of one kilogram a distance of one meter in one second. The force of gravity on 1 kg is about 9.8 newtons, depending on your location, etc.

So that is how 3.7 Kg/cm * 9.8N/kg * 1m/100cm = 0.36 Nm

RPM is just algebra, no physics... 60 degrees is 1/6th of a circle so 0.15sec to travel 60 degrees becomes 0.15*6=0.9 sec for 1 revolution.

1rev/0.9sec * 60 sec/min = 66.7 rpm

So far we have N-m * Rev/min and we want an answer in watts. Ah, a watt is by definition one N-m/sec.

The last item is 0.1046 which, again, we know from a conversion table like at http://www.micromo.com/motor-calculations.aspx, but what it represents is the conversion of RPM to radians/second.

I hope this explains how stall torque x no load speed, or 3.7 kg/cm * 0.15s @ 60 degrees, became the equation 0.36 * 66.6 * 0.1046

Let me know if I messed any math or physics. Ignore me if I caused more confusion than I resolved.

Re: 4W Maximum Power Servos? Huh?
 

Just for the record:

The peak power for a DC motor is not equal to stall torque times no load speed.

Assuming the torque vs speed curve is linear, it is equal to 1/4 of that:

The peak power occurs at the middle of the torque vs speed curve, where torque is half the stall torque and speed is half the no load speed.

Re: 4W Maximum Power Servos? Huh?
 

Right, that's just what they chose to call it in their rules.

Re: 4W Maximum Power Servos? Huh?
 

According to Update 3:

The servo industry rates servos using
power rating = stall torque X no load speed

Does anyone know if there is any reason for using a different formula than the one for DC motors?

Re: 4W Maximum Power Servos? Huh?
 

Maybe it doesn't have a DC motor in it? Or it does but it's highly nonlinear?

Re: 4W Maximum Power Servos? Huh?
 

I might be jaded, but I believe it is purely for marketing reasons. This formula gives a number 4x as large as reality.

Please take my grumblings with a grain of salt though - I'm still a bit peeved that that industry tries specifying torque in gram centimeters. Ounce inches is bad enough, but please don't inflict the travesty that is ounces* upon the metric system!


* To my count, there are 11 definitions for ounce: 6 for mass (28.3495231g, 31.1034768g, 28.0668g, 28.75g, 100g, 50g), 1 for force, 3 for volume (28.4130625mL, 29.5735295625mL, and 30 mL ), 1 for fabric (1/64th inch), and 1 for copper thickness (which is non-linear!)

Re: 4W Maximum Power Servos? Huh?
 

The servo industry also tends to specify speed in seconds (as in, seconds to travel 60 degrees) and torque in oz. (as in, force assuming a 1" moment arm). It's wacky.