Small motor used as sensor

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. :slight_smile:


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?


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?

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.

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.

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.

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.

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

Q&A - DC Tach Generators

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?

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.


Thanks for the quote. The

quote 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.[/quote]

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.

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

No, on the contrary, I think one message explicitly allows the use of KOP motors as tachometers.


Are purchased DC tachometers allowed as sensors?

Is it legal to use a KOP motor as a DC tachometer rather than as a motor?

a) Yes

b) Yes, if it is a KOP motor

While some reading the other message may (wrongly, I believe) infer that all motors may not be used a a tachometer. I think that it is in reference to using a (implied non-KOP) motor as a tachometer.


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?

DC Tach Generators are allowed as sensors.

DC Motors are not allowed per <R44>.

That is how I read the rules/Q&A anyway…

Joe J.

The way I read it, a KOP motor can be used as a tach and DC tach can be be purchased and used as a tach. And that’s all.

Thanks for that, had me worried for a bit there :slight_smile:

Here is my take.

If there is a NON-KOP motor on the robot, the wires must be connected to an input (IFI or custom).

I would have agreed with you but not anymore.

I believe that the answer to the Tachometer question implied that R44 forbids all motors other that KOP motors and those specifically allowed by R44 (which reference the R43 exceptions).

Joe J.

I have to disagree. With what I have read on the Q&As, and with the discussion here, I think that Joe is right on when saying that NO non-KOP motor may be used at all.

If it is a motor, not allowed. If it is listed and branded as a DC tachometer, then it is allowed.

It may sound like an odd statement, since some, if not all, DC tachs will have a motor/generator internally, but I think they want to not allow any non-KOP ‘motor’ from being on the robot in its purest form.


So you get a page of label paper for your inkjet printer

make a nice little label that says:

Kens Kustom Analog Tachometer
Model 2006F
Serial # 123456789
(not a motor)

and put it on a DC motor and Wallah! you have a tachometer.

This rule is really splitting hairs and getting lost in semantics. If a device with coils and magnets is not wired to a power source, then it is not functioning as a motor.

This reminds me of a black-powder kit I got for christmas one year. When assembled it was a black-powder pistol. But you must have a pistol permit to own a pistol. According to the manual that came with the kit, if you assemble it, but never fire it, then its not a pistol and you dont need a permit

but the first time you put black powder and a bullet, and fire the thing, then it instantly transforms into a pistol, and a permit is now required!

Ken, I guess your idea would be OK if someone who qualified as a supplier could supply the motor (re-badged as a tachometer) to all teams as a COTS item.

My problem as an inpector would be how to tell a ‘real’ (!?) tachometer from a functionally identical device that was sold as a motor. Like Joe, I suspect this would be difficult.

Of course I agree with Ken and Joe that the rule is not technically sound; I also think it may be difficult to police.

Let’s not get too worked up about this “Motors can’t be Tachs” rule. I am not ready to go storm the gates and get these people to see reason.

In actual fact, motors make a pretty lousy tachometers compared to how easy an alternative is on a FIRST robot. Basically all you need to do is get a Banner Sensor or an encoder or maybe even a reed switch mounted to the thing that was going to drive the motor/tachometer and then use the RC calculate speed by either counting pulses per given time or by timing each pulse (which is a piece of cake, given Kevin Watson’s many fine examples).

I would argue that there is almost always a better solution to the tachometer problem than using a motor as a generator.

It is an unexpected rule interpretation, but it is one I think reasonable people can live with.

Joe J.