Is a dimmer switch on the mini bot legal?

[Mr V]

Originally Posted by davidfv

Anyone have any issues this weekend with Mini-bot switches during inspection?

We are looking for a switch so when the mini-bot hits the top, we can then back drive the motors. Anyone know of one that would work?

The basic "3 way" household and the common "limit switch" with 3 terminals can be used to put the motors in "brake" mode to slow its' fall.

[Doc Wu]

There are a number of kinds of "Light Switches" that are sold that would work and have a push in snap action. Think of the light switches found on bathroom fixtures, or in the base of a table lamp. You may even be able to find them in a 3-way or SPDT configuration.

We were told for inspections this weekend to be liberal in the interpretation of the light switch rule. As long as it is called a "light" switch where sold it was allowed. I saw one that was like an old fashioned car headlight switch.

But the vast majority used either the conventional wall-switch toggle style, a "limit switch" such as the microswitch in the KOP, or the Tetrix switch.

Whether a dimmer switch would be allowed is a matter of interpretation and probably needs clarification. Since the dimmer part of it would do you no good on a DC motor, using it as only a switch seems like overkill, but I think would be within the spirit of the rule. Modifying the guts to provide some kind of brake or speed regulator would probably be over the line.

This is just my opinion, of course...

[xisybyl]

Re response by Mr. V: How would one put a motor in "brake' mode without using a resistor? Wouldn't that damage the motor?

[jvriezen]

Originally Posted by xisybyl

Re response by Mr. V: How would one put a motor in "brake' mode without using a resistor? Wouldn't that damage the motor?

I'm a different Mr. V, but our minibot used only one switch total-- a simple household 3-way wall light switch. Battery leads were connected to the non-common terminals, motors connected to Common and to one of the other terminals.

Motor acts as brake when motor leads are shorted together -- no damage observed.

John Vriezen
FIRST team 2530 "Inconceivable"
Mentor, Drive Coach, Inspector

[Mr V]

You really asked 2 questions so I'll break it up.

The reason you don't need the load (resistor) to make the motor into a brake. Spinning the motor "backwards" turns it into a generator. The polarity of the output is such that it makes the motor want to turn in the "forward" direction or provide resistance to being back driven. This is the exact same method method used by the motor controllers in the brake mode, no resistance is involved other than that of the motor windings and the nominal connection resistance.

The reason it won't burn up the motor is that the output voltage and thus current is proportional to the speed of the "generator". The resistive or braking force is proportional to the input voltage. So the faster it turns the more braking force it generates, meaning that it will reach an equilibrium and fall at a constant velocity. The caveat is that if you were to spin the motor faster than it's free speed at its rated voltage, for any length of time you could harm the motor. However to do so would require a load greater than the motor's capacity.

On our minibot in brake mode it takes about 3X as long to come down the pole as to go up.

We initially used the three way "common household" sw but in the interest of weight savings we went to the limit sw. It required a little more polycarb to make a lever system that allowed the wheels/motors closing to turn it on but still latch in the off/brake position. The extra sq" or 2 of pc was more than offset by the weight savings of the micro sw.

The wiring is very similar to the three way. The com term goes to the motor terminal you want (+) for up. If you want the sw depressed to be the "on" state connect the NC terminal to the motor up (-) and B-, the NO terminal to B+.