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hboy007

Mon Jun 27 2011, 11:38AM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

to improve this. Even if you've clicked the thread out of curiosity, check out the following problem:

Trying to build a really fast motorized nodal head (the usual ones look like this

or like this one here

, I've already confessed my sins (in terms of build quality):

Now let's try to get the heavy motor off the rotor ...

My first sketch uses an obscene count of gears and what-not to implement a differential. The vertical bar can be rotated down 180Â° for transport and is fastened when in operation. Please help me redesign this ... or at least I can nerd snipe you with this problem.

Thanks for having a look at this!

hboy

hboy007

Mon Jun 27 2011, 07:13PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
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Posts: 374

I think this looks better. You can see the two planetary gear sets joined at the planetary rings (actually I've only drawn one ring in green, I'm not sure where to allow some space for more planetary rings). Dots represent bearings, the tooth belt transmission is colored in orange.

Who thinks I'll ever have a chance to build the gearbox for a moderate amount of money? Any suggestions where to buy planetary gear sets? Thanks in advance!

Sulaiman

Mon Jun 27 2011, 08:10PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Based on your working unit, I assume that there will be a camera or similar?
If so then the weight of the motors will be almost negligible.
Your bearing do seem a little wobbly though.

Ash Small

Mon Jun 27 2011, 08:13PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I think you need to give a bit more info regarding spacifications, etc.

eg gear material, etc. (plastic, aluminium, bronze), etc.

What will this be used for?

hboy007

Mon Jun 27 2011, 10:54PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

I am trying to build a motorized tripod head for a digital SLR which weighs about 0.8kg up to 2kg depending on which camera body, lens and add-ons (battery grip) are used.

The first prototype was built just to get some hands-on experience. I've evaluated the concept of a foldable L profile and the suitability of directly coupled motors. The bearings themselves are satisfying (I've cut naves into pieces and reassembled them with the wooden plates sandwiched in between). However, I thought it might be good enough to just drill holes through the shafts of the gear motors, cut threads and mount them to the M9x1 axes of the naves. The shafts themselves are only press-fit into POM gears and there's the rub.

I've tested the Gigapano Epic Pro, that is a commercial solution

and most unsatisfactory when trying to shoot sunsets or rapidly changing weather situations because the pan speed is only a few degrees/sec.

Speed is my major motivation to go through all this. The motors I am planning to use are either stepper motors or servo motors with a gearbox (expensive). There are some cheap DC gear motor assemblies available from the automotive sector but I've had it with PWM motor control. It just doesn't live up to the amount of control provided by stepper motors (the DC motors have built-in rotary encoders but they are no good, delivering 8 counts per rev.).

So there are only a few design options left: put the motor on the rotor opposite to the vertical bar to compensate the imbalance of the vertical bar, which adds to the inertia of the rotor or
live with the subtle imbalance and make the rotor as light as possible.

My tripod weighs about 2.5kg but that's still too light in some cases to withstand the forces generated by the head.

Mounting the motor so it rotates along with the rotor limits speed and increases the drive current of the other motor.
Mounting both motors at the base of the head helps lower the center of mass, reduces the load on the azimuthal motor but makes the differential (as I've tried to illustrate above) a real pain.

Assuming the gears (made of steel or brass) weigh about 200g in total, they would save about 400g on the rotor.

As a last option, I could use helical bevel gears to transfer the force of the central motor to the polar axis and turn both motors in sync to produce azimuthal rotation but that just doesn't feel right... this would mean that I'd have to control the motors so well that the path can be time-independent... and that scares me a bit.

hboy007

Tue Jun 28 2011, 07:57PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

Just for the fun of it, have a look at these drawings I made.

1309291036 1667 FT118680 Differential Exploded Planetary

lightlinked

Tue Jun 28 2011, 08:57PM

Registered Member #2087 Joined: Tue Apr 21 2009, 08:32AM
Location:
Posts: 115

Nice, is that solid works? how accurate and fast does it have to be? Big steppers could do it direct drive, smaller ones if you use simple timing belts. Planetary gears seem quite excessive for this application, but if you want something off the shelf there are some electric bike gear boxes that might be nice

Dr. Slack

Tue Jun 28 2011, 09:21PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I suppose mounting the camera so it looks vertically upwards, then pan/tilting a mirror above it is out of the question? A mirror is much lighter than a camera. First-surface mirrors aren't cripplingly expensive if you are worried about the multiple reflections from a standard back-silvered type.

If you do insist on waving the camera about, then I'd strongly suggest you go for the two-sided GigaPan style, rather than the single-sided Revolution or PanoMachine style, it will be much stiffer for the same mass.

hboy007

Fri Jul 01 2011, 04:58PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

Dr. Slack wrote ...
, it will be much stiffer for the same mass.

Yes, that'll be the catch. I will go for the double-sided mount when I've figured out a way to install the quick release system that doesn't suck duing operation and that doesn't cost a fortune.

Look what arrived yesterday. This used to be an automotive component, a spindle drive for seat positioning system. Unfortunately, they come with these lousy POM gears and a forged/rolled worm gear driving it. I cut the worm gear shaft as close to the armature as possible, this leaves a few millimeters for a sleeve with a grub screw.

hboy007

Sat Jul 09 2011, 08:45PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

I've done my best to come up with this design now:

There is a pulley with a cylindrical recess milling to accommodate the lower axial bearing not shown in the model, which is driven by the second motor. The upper radial bearing is press-fit in the rotor. the shaft that sticks out at the top is for the worm gear (as shown in my last post).

Kind words and constructive criticism are always warmly welcome. Thank you!

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