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  • Writer's pictureMarcos T Rubino

To servo or not to servo - that is the question

Much like Hamlet was, I am faced with a great existential question .

As we all know, in the monologue Hamlet wondered whether to terminate his anguish and suffering of an unfair world, and replace it with dreams in the eternal sleep.

I now find myself wondering whether to terminate my anguish and suffering of a lengthy design process, and replace my bi-directional overrunning clutch concept with a servo motor as anybody with a grain of sense would.

Let me explain:

The slings and arrows of outrageous fortune to which Hamlet referred have put my wife’s workstation in a position relative to the sun whereby, in the afternoons of Spring and Summer, the sun shines directly on her eyes as she is grading her students' papers.

Ever the resourceful one, she devised a simple solution of taping a sheet of paper to the window.

All was well, but then I retired ... and COVID keeps me inside.

Now the sheet of paper is taped to the window by voice activated remote control: “Honey, please go and tape a sheet of paper to the window, the sun is blinding me”.

After a couple of days I decided a shade was in order. And on the same day a shade was on order.

I looked at motorized shades, and found a few quite clever ones in the market. Alas, they are far too clever for my wallet (there’s Hamlet’s anguish and suffering of an unfair world again).

Well, retired or not I’m still an engineer, surely I can come up with a simple 12V gearmotor/coupling arrangement to lower and raise a window shade, right?

Darned right I can!

But I won’t! At least not a simple one!

If I wasn’t retired and was on the clock that’s exactly what I would do: make a simple and functional design with a disconnect coupling, so we could build and ship fast and make money for the stockholders.

But I am retired, and I’m not on the clock, so let’s have some fun in the process.

A simple, inexpensive design would consist of 4 components:

- AC/DC adapter

- Gearmotor

- Coupling

- Switch

The user would flip the switch up to raise the shade, flip it to the middle to stop it and flip it down to lower it. In the advent of a power failure he/she would move a small lever to disconnect the coupling and operate the shade manually without backdriving the gearmotor.

My design consists of:

- AC/DC adapter

- Gearmotor

- Coupling

- Bi-directional overrunning clutch

- Inertia ring to actuate the clutch

- Rare earth magnet arrangement

- Copper plate

- Adjustable bracket for the copper plate

- Mechanism with limit switches

- Motor controller

- Blue LED

- Passive buzzer

- Infrared receiver

- Resistors

- Arduino Nano board

- Printed circuit board

- Remote control

The user (my wife) will press the down button on the remote to lower the shade, release when it reaches the desired height, and use the up button to raise it.

When the shade reaches its highest or lowest point the motor will reverse direction for one revolution and stop, and the LED will flash three times with the buzzer beeping each time.

Well, by now those who still have this blog up and aren’t snoring or dead from boredom are probably wondering “what about the servo motor question?”.

I want to disconnect the motor from the shade, so we can move the shade manually in case of a power failure without backdriving the motor.

There are more than one way to skin this particular cat, and I considered a few:

A disconnect coupling manually actuated by a lever :

  • Would require that we remember to toggle before moving the shade manually, and anything that requires remembering to do something isn’t an option for me.

A disconnect coupling actuated by a solenoid:

  • The solenoids I could find in the market of suitable size and price can’t be on for more than a couple of seconds before starting a fire.

A disconnect coupling actuated by a small servo motor:

  • Easy, inexpensive, practical, motors are readily available, would require only 3 or 4 lines of simple Arduino code. Boring.

A centrifugal clutch:

  • I couldn’t find a suitable one in the market.

  • It would have to be larger that the space I have available.

  • I would have to manufacture the flyweights out of metal and I no longer have the metalworking workshop I used to have.

A bi-directional overrunning clutch that would drive the shade when the motor runs and freewheel when the shade is moved manually.

  • I couldn’t find a suitable one in the market.

  • Designing one would be complicated and time consuming.

  • The clutch would have to be spring loaded into overrunning mode, meaning I would have to design and probably manufacture the springs – a pain in the neck.

  • The clutch would have to be actuated either by friction or inertia. Friction means lubrication, heat and wear, so I’d be left with inertia.

  • In order to actuate by inertia I would have to figure out a way to determine experimentally what the gearmotor angular acceleration is, given the moments of inertia of the clutch components, which is a new project in itself.

  • Once the motor reached a constant speed the torque against the springs would drop to zero and the springs would disengage the clutch. There would need to be some sort of mechanism to keep the clutch engaged against the springs. Centrifugal force isn't practical as it is a small clutch and the motor runs slowly. Eddy current comes to mind, the principles of which I mastered in college 40+ years ago and forgot 39+ years ago.

  • All in all, it would be a Rube Goldberg to lower a shade. We have a winner!!

Good bye simple servo motor solution, hello Rube, old buddy!

Sorry Shakespeare, the slings and arrows of outrageous fortune do sound pretty exciting to this old retired engineer looking for ways to keep his brain going!

I'll post additional blogs with updates on the project status periodically.

This and previous blogs are in my web site Please feel free to hook a brother up, log in and add any comments, opinions, suggestions, ... you like. I will not share any personal information entered, nor will I send any e-mails to my blog contributors.

Thanks for your interest, and see you soon!

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Jan 03, 2021

Marcos, you are an amazing designer! So detailed! Looking forward to the progression of this project!


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