Motorized Zoom: Motor & Battery Suggestions

Hello,
I plan to do some “remote-controlled closeup wild photography” of red deers next september. I will operate a DSLR over a 25 m active USB cable, but I would like to have a possibility of zoom in/out my 17-50 mm lens. So I am thinking about deigning a device, which could be mounted to the lens and managed to rotate with zoom ring.

Currently, I am really not decided which motor should I use. According to my “thread and weights” measurement, I need 1100 g of mass to safely zoom the lens from every position (yeah, it goes quite stiff in certain position, but it is probably over-estimated, as more weight on ring results in stiffer operation). I plan to use pre-loaded timing belt with pulley to transmit a torque. Taking into consideration zoom ring and pulley diameters (ratio ~ 7.15:1), and “safety” factor of 2, I need around 0.1 Nm of torque. Demands on revolutions are not high. From 17 to 50 mm, it takes around 1/6 of revolution. It would be nice to zoom it form min. to max. in up to 3 seconds. Considering our ratio again, it results in roughly 25 RPM on a motor shaft.

I am absolute newbie in electronics, but recently I read a lot about controlling stepper motor, which is my option number one. A4988 Driver looks really comfy to interface with arduino, there are plenty of examples over the internet. The main problem with this solution is the power source - it needs to be a battery. According to driver data sheet, there should be at least 8V at motor input. That means 8x recharable AA (9.6 V) for me, which is quite spacious, compared to lens size for example. I also have 3S Li-Po baterry, but that is not much smaller, and definitely there comes more difficulties with it. :slight_smile: I am not concerned about insufficient capacity, because zoom would be used only occasionally and I do not need to hold motor (and driver) powered except zooming.

Now dive into picking the right stepper motor: as the device should be more-or-less compact, my favorite is the smallest Nema 08 I found, specifications in the attached image. It can only provide ~ 0.014 Nm of holding torque, which is around 7 times lower than I claim to need. There could help a set of gears - I would prefer to use 1:10 worm gear set. With that, I should have 0.14 Nm of holding torque. Question is, how much of “operational” torque :slight_smile: With that, of course, needed revolutions rises to 250 RPM now.

As I read this, I wrote quite a lot. But this is only one option. Others could be modified servo, brushed DC motor with reeeaaaly high reduction gear set, or BLDC motor as well reduced, …

You probably have a lot of concerns regarding reading zoom position, endstops, wireless operation, etc. I would like to leave this for later, as this is not a prior problem now. I would like to know your opinions and suggestions regarding choosing right type of motor and battery for this application. Feel free to write me that such a device powered by battery is a nonsense, if needed :slight_smile: Thank you!