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.