4W Maximum Power Servos? Huh?

[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

[Kevin Sevcik]

Originally Posted by EricVanWyk

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...

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.

[PAR_WIG1350]

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.

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

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

[kamocat]

Originally Posted by EricVanWyk

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...

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

[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.

[hilo90mhz]

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..

[EricH]

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.

Servo motors are a type of motor that can be directed to turn to face a specific direction, rather than just spin forward or backward.

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

Servo != motor. Motor != servo.

[hilo90mhz]

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.

[mcbean]

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

[LinuxArchitect]

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.

[Ether]

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

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.