DIY Servo Follow Focus

[chris szlachetka]

I'm working on a new project and had a few questions about servos & how to make a control box to move it.

Servo Motor

Found this hack, which will allow 360 degrees of movement on a servo.

http://www.youtube.com/watch?v=CtLN8BoMIlQ

-  Noise seemed a little high.  I'm not sure how to find out the noise level these servos make, but this would be one of my main concerns.

-  How to control the speed of the servo (response time)   Maybe with a adjusting power feeding switch of some sort to control the response time.  More power, faster movement   Less power, slower movement   However the torque would need to stay the same.

Controller Box

- Knob to control Focus

 Not really sure where to look, but I might have to make one of my own.

- Control Box

http://eolsurplus.com/images/ProjectBox.jpg

http://www.pcstats.com/articleimages/200306/bbaudio_pspc.jpg

Electronics

- On/off switch
- On/off LED light
- Potentiometer (Direction control)
- Battery compartment & Batteries

And can't think of what else is needed.

Any ideas ?

[David L. Holmes]

BIG PROBLEM...  If you remove the feedback pot from the servo like the guy did in the YouTube video, you wont have anything that can reference the location of the servo.  Follow focus works by making reference points in which your distance changes from the object your filming and the camera and adjusting the focus ring to follow the reference path.  Unless you had an external pot that would feedback the location of the focus ring, you will never have complete control of your focusing.

Just something I noticed...

Also, I would like to follow anything you come up with because this is something I will be playing with in the future, so good luck!!!

[chris szlachetka]

I finally found a 90° beveled gearbox sold in the us.    I'm going to start of manual first, then look into what servo to buy and so on.

Any suggestions ?

Any servos for dummies books or anything like that ? 

[Seth Kersey]

I have much more experience controlling stepper motors, so I can't really help much besides providing a couple references.

I have several books on robotics, there are many out there, just look for one with a good servo section. A book entitled "Robot Builders Bonanza" (second edition) has a very nice section regarding controlling and modifying servos.

You may also want to browse the Seattle Robotics Society for some similar (and free) info... just do a search for "servo" and you will get many pages of useful info. http://www.seattlerobotics.org/

Instead of modifying the servo, you may want to consider a 360° servo so you retain angular control (like David pointed out). Here is an example I found with a quick search http://www.acroname.com/robotics/parts/R298-1T-SERVO.html

Also, gearing can also be used to obtain 360° rotation, but that could get bulky.

As for a control knob, I usually use Mouser electronics and they do have a knob that may be appropriate, it is 1.75" in diameter and has a "finger spinner"... but is nearly $30 USD. http://www.mouser.com/Search/ProductDetail.aspx?qs=KaZmdw9s7UflBjcuyVq03Q%3d%3d

They also have a knob with the small perpendicular positioning knob, but it is plastic... http://www.mouser.com/Search/ProductDetail.aspx?qs=EIgIgHbwPd1pNwpyJpowFg%3d%3d

FixtureWorks also carries similar knobs with a variety of larger sizes, also plastic though. http://www.fixtureworks.net/Home/TechSheet/tabid/57/Default.aspx?catID=06268-11CM&thumbPath=7|3|1|0

You will also need a transmitter and receiver, I would assume that you will be using R/C transmitter and receiver and just modifying to fit them inside the project box. If you wanted to get really complex, you could add a microcontroller and a motorized potentiometer, then you would be able to have buttons for preset focus positions... but if you are not experienced with electronics and programing that may be a bit much... maybe version 2.0

I am curious, where did you find the 90° beveled gearbox? I haven't looked for a while, but I found it difficult locating a decent cheap gearbox that didn't have too much play in the gears. Also, I have been unable to locate the standard specs for the follow focus whip socket (the square hole in the center of the ff knob), would you happen to know the standard dimensions?

[chris szlachetka]

I just picked up electronics for dummies and it has a great servo part in it. 

"You will also need a transmitter and receiver, I would assume that you will be using R/C transmitter and receiver and just modifying to fit them inside the project box. If you wanted to get really complex, you could add a microcontroller and a motorized potentiometer, then you would be able to have buttons for preset focus positions... but if you are not experienced with electronics and programing that may be a bit much... maybe version 2.0 "

I was told from a guy that sells nothing but servos for hobby stuff, that they didn't make a 360° servo.  From what I'm seeing in the photo, it looks like some sort of pulley on the shaft, is this how it's able to rotate 360° ?

To start off with some kind of movement, I've seen this test box they sell that allows you to move the servo in a forward or reverse.  I was thinking of a wired unit to start off with and then eventually move on the the whole wireless thing (transmitter & receiver).   Something like a switch that will attach to the post or a rail system.   I've never build a transmitter or receiver, so this is something I'll need to learn.

"Also, gearing can also be used to obtain 360° rotation, but that could get bulky."

I was looking at a few companies to make me gears, but can be very expensive in low quantities.   I'm setup for just about everything except gears.  I'm eventually going to that 4 axis....

"As for a control knob, I usually use Mouser electronics and they do have a knob that may be appropriate, it is 1.75" in diameter and has a "finger spinner"... but is nearly $30 USD. http://www.mouser.com/Search/ProductDetail.aspx?qs=KaZmdw9s7UflBjcuyVq03Q%3d%3d

They also have a knob with the small perpendicular positioning knob, but it is plastic... http://www.mouser.com/Search/ProductDetail.aspx?qs=EIgIgHbwPd1pNwpyJpowFg%3d%3d

FixtureWorks also carries similar knobs with a variety of larger sizes, also plastic though. http://www.fixtureworks.net/Home/TechSheet/tabid/57/Default.aspx?catID=06268-11CM&thumbPath=7|3|1|0"

 I have a few of these laying around from another project, but couldn't find the lager ones.   Thanks !!

"I am curious, where did you find the 90° beveled gearbox? I haven't looked for a while, but I found it difficult locating a decent cheap gearbox that didn't have too much play in the gears. Also, I have been unable to locate the standard specs for the follow focus whip socket (the square hole in the center of the ff knob), would you happen to know the standard dimensions?"

Rino.   Check your PM, I'll send you the links.

[Seth Kersey]

Regarding the 360° servo, I believe that the pulley you are referring to is simply attacked to the servo instead of the standard "servo horn". I have read of servos that do 360° rotation and are used primarily for R/C aircraft landing gear. It is possible that it is a normal servo with a modified gearbox (internal gears), or it could have electronics that allow it to turn a full 360°, or a combination of both. I am not really an R/C person, so I am not sure.

I was looking at a few companies to make me gears, but can be very expensive in low quantities.   I'm setup for just about everything except gears.  I'm eventually going to that 4 axis....

4 axis... Nice! The last time I made gears I hobbed aluminum worm gears on my drill press with an acme tap I made from a section of acme threaded rod. I just buy off-the-shelf gears otherwise

[David L. Holmes]

I have a question for those who own or have used a Follow Focus system.  Do the servos need to go 360 degrees or can you get an acceptable amount of focus with less then that amount of travel?

[chris szlachetka]

"Do the servos need to go 360 degrees or can you get an acceptable amount of focus with less then that amount of travel?"

Depends on what you're using the servo for.  If you're using the servo for a 35mm adapter, then no.  You can make up for the loss by adding bigger gears, but more weight and less room.  Now if it's for the camera, then yes you need something that is going to rotate 360°.