Continuous Rotation Servos

[nathannfm]

Hi everyone,
We are looking for "continuous rotation servos", something similar to VEX motors but that fall into the category of the unlimited number of

COTS servos with a maximum power rating of 4W each at 6VDC

Any suggestions of 2013 FRC legal servos that will spin forever (not standard 90/180 degree servos) would be greatly appreciated, preferably as powerfully as possible.

Thanks

-3929 Atomic Dragons

[Kevin Sevcik]

My preferred solution seems to be out of stock at ServoCity: Hitec HSR-1425CR. It seems to be in stock at RobotShop, however. ServoCity will mod most of their analog servos into a continuous rotation version for you for a nominal fee, but I'm uncertain as to the legality of that. On the one hand, it's not you modifying it the servo. On the other hand, the resulting servo definitely isn't factory spec after that.

EDIT:There's also this monstrosity. That's a legal HS-5485 servo modded for continuous rotation and slapped into a gear reduction setup. You can get more torque out of it that way, though it's even slower. I think I might question your sanity if you picked one of those things over a window motor or something similarly sized but rather more powerful.

[Retired Starman]

As a robot inspector, you are going to have to work real hard to convince me that this is a servo and not a motor.

I'd love to see what Big Al has to say about this!

Dr. Bob
Chairman's Award is not about building the robot. Every team builds a robot.

[Kevin Sevcik]

Originally Posted by Retired Starman

As a robot inspector, you are going to have to work real hard to convince me that this is a servo and not a motor.

I'd love to see what Big Al has to say about this!

Dr. Bob
Chairman's Award is not about building the robot. Every team builds a robot.

Well the HSR-1425CR is branded a continuous rotation servo by Hitec. I would assume they'd know.

To be extra pedantic, I play with 400V continuous rotation servos at work all the time. The primary difference between a servo and a motor is the feedback and closed loop controller.

[nathannfm]

Thanks! We will look into the HSR-1425CR, the other one is a bit too ridiculous and expensive for our needs.

[team222badbrad]

If a company is modifying an off the shelf servo for continuous rotation then as an inspector I'd say it's illegal. It doesn't matter who does the modifications as per R33. Modification is modification.

If a manufacture (not a reseller) of the servo is marketing a servo as a continuous rotation servo then you might be in luck, but I'd certainly ask FIRST in the Q/A to be sure and bring the answer and servo specs to competition with you.

[Michael Hill]

By "Continuous Rotation Servo", do you mean "Stepper Motor"

(actually, is there a difference? I feel like it's just semantics or something.)

[Kevin Sevcik]

Originally Posted by Michael Hill

By "Continuous Rotation Servo", do you mean "Stepper Motor"

(actually, is there a difference? I feel like it's just semantics or something.)

Huuuuuge difference. Stepper motors have a very specific construction involving two or more independent stationary magnetic coils with "teeth" and an unmagnetized rotor with matching teeth. Energizing the coil in a specific order makes the teeth in the rotor line up with the teeth of the coils in a specific order. So you energize the coils in a pattern and the rotor makes tiny little steps of rotation.

A continuous rotation servo is a more standard brushed or brushless motor with integrated feedback device and control logic.

[Mark McLeod]

Originally Posted by Kevin Sevcik

...
The primary difference between a servo and a motor is the feedback and closed loop controller.

The HSR-1425CR continuous servo does not have feedback (no potentiometer, no closed loop control), so it would seem to be a motor by your definition.

[Tristan Lall]

I've always wondered about these, and think separate Q&A submissions are warranted.

Originally Posted by Kevin Sevcik

My preferred solution seems to be out of stock at ServoCity: Hitec HSR-1425CR. It seems to be in stock at RobotShop, however.

On one hand, they're clearly sold as being COTS servos, but on the other hand, they don't really act like a servomotor. I don't think FIRST specifies what a servo is in a way that lends itself to drawing a hard line. If inspecting, I would be inclined to accept the team's contention that the labelling of the part is enough to classify it as a servo.

The core question here is whether a servo is defined in terms of its characteristics or its nomenclature—or in some other way.

Originally Posted by Kevin Sevcik

ServoCity will mod most of their analog servos into a continuous rotation version for you for a nominal fee, but I'm uncertain as to the legality of that. On the one hand, it's not you modifying it the servo. On the other hand, the resulting servo definitely isn't factory spec after that.

While the servo is modified from its factory configuration, its the vendor (ServoCity) that did the modification. Also, I'm not sure what we should consider integral—the mechanical stops are removed, but they also mess with the control electronics, which are inside the servo case, but separate from the tiny can motor contained within. If the modification is treated as a team request (like a contract with a fabricator), it's unclear whether it would be legal. I'd still be inclined to pass it, given the uncertain rule.

Here, one core question is whether a vendor-modified item can be COTS, even if the modification would have been illegal if performed by a team. Also, is that proposed modification even illegal for a team to perform?

[Kevin Sevcik]

For the record, this came up in the Q&A in 2010. The Q was is if a continuous rotation servo was legal. And if the team could modify a servo to be continuous rotation. The answer to the first part was that it was legal if it followed R52-B, which was "An unlimited number of COTS servos with power less than 4W." The second part was deemed illegal, so a team can't modify their own servo for continuous rotation.

[sjspry]

I would mention the existence of continuous rotation Vex servos and those made by Parallax with a built in zero-point calibration pot.

They are both made that way.

As for the definition of COTS, as long as everyone can buy it with equal opportunity, how is it not COTS? I won't go on a rant about the rules right now, but... If teams can't lawyer the rules, why are inspectors doing it?

[Mark McLeod]

I don't think Vex labels anything as a continuous rotation servo.
Those are labeled as motors, and one of them is called out as a legal motor this year. We're allowed two.

[Joe Ross]

Originally Posted by Mark McLeod

I don't think Vex labels anything as a continuous rotation servo.

Vex used to sell a continuous rotation servo, but discontinued it and now only sells motors.