Electromechanical gearbox

[Zerobricks]

So I got this crazy idea, why not connect 2 motors in serial?

If one motor gets stalled, the other one would kick in, so this would be vry good for a gearbox. 30 minutes later and here was my first setup:

Using old 9V wires I manged to connect both motors without any modifications. The Xl motor has a clutch gear in order to give the M motor priority, so that the Xl motor doesnt turn when not needed.

I installed the setup in the simple chassis, and here is the result:

Notice, there are some other PF ideas in video.

so how usefull si this gearbox? I think its a good idea, but practicaly stalled motor still use some power, leaving less for the working motor. Also the current is shared between motors and the bigger Xl motor can draw more current than the small M can take, so its not very recommended. But I think it was a good idea and experiment!

[allanp]

You always come with some great ways to think outside the box, love it.

[freakwave]

hi Zblj, two main comments, let me first adress the latter part of the video, this is a simple but great idea! It looks hilarious having the two or even one "system" control itself. Next step would be building a discreet NXT

Now to the first part concerning your gearbox.

I'm very split on seeing it as a good or bad idea. The good part is coming from my construction heart and I like the idea behind it to achieve a turbo/power boost function. The bad part is coming from my electrical engineering heart which is a bid sad for seeing a high stalling current going through the both, but mainly the PF-M motor.

Now why I thinkg so: As your diagram shows and you wrote, the motors are in serial connection. Usually they would each need 9V, however as they are in serial, they are effectively sharing the voltage which makes it 4,5 V per motor. Now you have stalled the PF-XL. It would "just" draw current, the voltage would drop a bit. Leaving a bit more for the PF-M.

Next step is stalling the PF-M. Here in my opinion the problem starts. The current is increasing in a stalled motor, voltage drops for the PF-M, leaving more for the PF-XL which can overide the mechanical blocking by the cluth gear. For some time now the PF- has may be a higher current through it that it was designed for. So far for the theory, I did not do any measurement to proof my arguments, but thats what theory says. (Philo did a lot of measurements so may be he could help with figures)

May be I go down to the basement and measure myself later today

fW

[Cheng Fei]

I've never thought of the self-triggering mechanism!!!

[Zerobricks]

hi Zblj, two main comments, let me first adress the latter part of the video, this is a simple but great idea! It looks hilarious having the two or even one "system" control itself. Next step would be building a discreet NXT

Now to the first part concerning your gearbox.

I'm very split on seeing it as a good or bad idea. The good part is coming from my construction heart and I like the idea behind it to achieve a turbo/power boost function. The bad part is coming from my electrical engineering heart which is a bid sad for seeing a high stalling current going through the both, but mainly the PF-M motor.

Now why I thinkg so: As your diagram shows and you wrote, the motors are in serial connection. Usually they would each need 9V, however as they are in serial, they are effectively sharing the voltage which makes it 4,5 V per motor. Now you have stalled the PF-XL. It would "just" draw current, the voltage would drop a bit. Leaving a bit more for the PF-M.

Next step is stalling the PF-M. Here in my opinion the problem starts. The current is increasing in a stalled motor, voltage drops for the PF-M, leaving more for the PF-XL which can overide the mechanical blocking by the cluth gear. For some time now the PF- has may be a higher current through it that it was designed for. So far for the theory, I did not do any measurement to proof my arguments, but thats what theory says. (Philo did a lot of measurements so may be he could help with figures)

May be I go down to the basement and measure myself later today

fW

Did you read my last sentence? I know the current problem, so its not good for real life.

[fmmjqtft]

The current would not be a problem. Assuming a stalled current cannot be higher at a lower voltage, the total current is limited to 1/(1/M + 1/XL), where M and XL are the stalled currents of the M and XL motors at voltage V. If V=9v and M=0.85A and XL=1.8A, the maximum current is 0.577A. This assumes a voltage drop of 9v over each motor, which is impossible, so the current will be lower.

Anyway, heres another idea. Use two motors in series as a differential. Two identical motors connected in series have very similar properties to one motor driving a differential, but with double the ohmic (that is the minimum/stalled) resistance, so with half the maximum torque.

Edited April 2, 2011 by fmmjqtft