Small motor used as sensor

[JoelP]

In order to measure the RPM of the wheel used as the shooter, would it be legal to connect a small dc motor, not in the KOP, to the shaft of the wheel so that when the wheel is turned by a KOP motor, the small motor generates a current which is connected to the RC analog inputs.

I'm pretty sure using a small motor as a sensor is legal by the rules regarding to custom circuitry, but it doesn't hurt to double check.

[Mike Betts]

Originally Posted by JoelP

In order to measure the RPM of the wheel used as the shooter, would it be legal to connect a small dc motor, not in the KOP, to the shaft of the wheel so that when the wheel is turned by a KOP motor, the small motor generates a current which is connected to the RC analog inputs.

I'm pretty sure using a small motor as a sensor is legal by the rules regarding to custom circuitry, but it doesn't hurt to double check.

Joel,

It is best to check with FIRST Q&A regarding legality. I am guessing that they would not allow this device as it is a motor and would generate a signal more powerful than other sensors available.

Why not use an encoder or other sensor designed for this type of measurement?

Mike

[KenWittlief]

its a great idea.

You would want to load the motor lightly to get a more linear response, and maybe filter it some with caps to squash the commutator spikes.

I dont know if FIRST will allow a motor not in the KOP. If its being used as a tachometer.... maybe?

[sciencenerd]

The reply to part B of this question in the Q&A would seem to imply that if it is not a kit motor, this is not legal. You could post a clarification question just to be sure, though.

[Al Skierkiewicz]

Motors that are not in the kit beyond the allowed two additional small chalupas are not allowed on the robot for any reason. The rotational sensors or even using banner sensors aimed at a reflective area of the shooter wheel give a far more accurate determination of speed than a spinning motor.

[Mark McLeod]

We are using the small KOP Mabuchi motor as a tach for velocity control on our shooter and it works very well. The response is linear and the Mabuchi spec is a near perfect fit for our design speeds. There is additional noise from the commutator, but it's regular and we smooth that somewhat in software and set our tolerances to avoid reacting to it.

We added a diode to limit current to one direction and an adjustable pot to bring the output into the 0-5v range. You can also add caps as Ken suggested, but it's not necessary.

[Joe Johnson]

If it is used as a sensor and it is done safely, I assume it would be legal.

The question referenced above specifically says you can buy and use a tachometer and you can use a KOP motor as a tachometer. Note that it does NOT say that non-KOP motors cannot be used as a tachometer.

A tachometer is a device that provides a voltage proportional to rotational speed.

Many tachometers are just fancy motors. A motor is really just a simple tachometer.

As long as you are using the motor as a tachometer, I can't imagine that it is not legal.

Now, if you are using the motor as a motor, then that is another matter. Just as if you bought a tachometer and used it as a motor you'd be afowl of the rules.

Callin' 'em as I see 'em.

Joe J.

[ntroup]

I think this Q&A question is more directly related:

Q&A - DC Tach Generators

Question:
Are DC Tach Generators allowed as sensors?

If so, can DC motors be used as tach generators solely to provide a signal source to an analog input?

Answer:
DC Tach Generators are allowed as sensors.

DC Motors are not allowed per <R44>.

I would take that to say that any DC motor (non-KOP) can not be used at all, even if for use as a tachometer.

-Nate

[Joe Johnson]

Originally Posted by ntroup

I think this Q&A question is more directly related:

Q&A - DC Tach Generators


I would take that to say that any DC motor (non-KOP) can not be used at all, even if for use as a tachometer.

-Nate

Thanks for the quote. The quote that I was referring to did not have the R44 rule.

I was wrong.

This is a strange ruling to me because, as a practical matter,
I really don't know how to distinquish between a motor used as a sensor to turn rotational velocity into a voltage and a tachometer.



The above "generator/tachometer" is legal, but a simple non-KOP Mabuchi is not. Seems strange...

But... the rule is the rule.

Joe J.

[Richard Wallace]

As Joe says, the rule is the rule.

I am concerned that this Q&A ruling could create difficulty for inspectors, since (like Joe) we will have trouble distinguishing a DC Tachometer from a DC Motor. I just hope any DC Tachometers that I see when inspecting robots have legible manufacturers labels that allow me to identify them as such, since this rule doesn't allow me assume they are tachometers based on how they are connected to the control system.

As mentioned in several earlier posts, uniform application of the rules at all events should be an important consideration.

[Matt Krass]

I'm missing something I think, does this mean KOP motors cannot be used as tachometers as well?