turntable: need some advice on motor etc

Hi all, contemplating my first arduino project!! getting excited :)

I would like to make a simple turntable setup, to do some photo/video small scale projects.

What I envision is this:

an 18" (about 45 cm) diameter turntable, perhaps based on a lazy susan, that can carry a 1 to 2 lb. (about 1 kg) load at it's perimeter (not centered). It's also important to me that I can produce ramped motion (I guess I'd call it "easing" of the motion curve, in animation lingo--smooth accel/deccel at start and stop, rather than linear acceleration, or jerky movements).

I'm also hoping that I can reach a max angular speed of one revolution in 6-8 seconds?

I'm guessing an Uno and a stepper motor with driver are the main things I need to get started? What about power? Any suggestions on particular models would be fantastic! Or any other advice really, on gearing, etc :)

If this is a project it is customary to state your project objective. You are obviously not building the turntable because you woke up one day and decided that's what you should do. This is going somewhere and now is the time to explain where so we don't find out later that you withheld information that would have changed what we may recommend now. Does that make sense ? So what are doing and why ?

thanks for your reply, and sorry for not providing more info

I want to be able to do 2 things:

1) take photo/video of an object that rotates at the center of the turntable (camera mounted on a tripod)

2) take photo/video of an object with a light weight camera rotating around it (camera mounted on perimeter of the turntable, focused on the subject at the center of the turntable--for this I would have to figure out a way to decouple the subject from the rotation--perhaps there's a hole in the center and this allows for the subject to remain motionless)

that help? thanks again

LML

Does that make sense ? So what are doing and why ?

No it doesn't in this case. The op stated what is desired and it doesn't really matter if pizza or a dashboard Jesus is on the turn table.

That being said, a servo modified for continuous rotation might be of interest in rotating the turntable. There have been a number of previous post in the forum concerning turntable projects that might have useful info.

https://www.google.com/search?as_q=turntable&as_epq=&as_oq=&as_eq=&as_nlo=&as_nhi=&lr=&cr=&as_qdr=all&as_sitesearch=http%3A%2F%2Fforum.arduino.cc%2Findex&as_occt=any&safe=images&tbs=&as_filetype=&as_rights=

ok, then why not Hawian Hula girl?

thanks guys ;)

so here's something important I didn't mention that I would also like to be able to do:

program, and be able to repeat, the precise rotational movements--as in a motion control device.

So that's why I was heading down the stepper route... from my limited knowledge, these give precise, repeatable control of rotation, correct?

LML

Your stepper doen't know where it is . You would have to count the steps in one direction and save that in a directioncount variable like Foward_Counts/Reverse_Counts etc. and increment/decrement accordingly. Alternately, you could use a shaft encoder somewhere or add a rubber wheel driven by the turntable and attache the shaft encoder to the rubber wheel. This would require a custom mount for the wheel and encoder. If you add an index opto sensor with a HOME flag like most stepper driven hardware, then you would have a HOME funtion executed on power up that reverses the motor slowly until the flag backs into the opto-sensor slot and then the motor reverses direction and drives "n" counts forward from the flag. This is the Home offset count. If you then keep track of forward and reverse counts the system (not the stepper) should be able to calculate steps in any direction to any point. This is how most stepper driven systems work unless they have incremental or absolute shaft encoders.

You could get a stepper motor and driver combo like below and do some up front testing and code development to see if stepper motors (possibly larger ones) would work for your situation.

http://www.ebay.com/itm/DC-5V-Stepper-Motor-ULN2003-Driver-Test-Module-Board-for-Arduino-28BYJ-48-/171125775759?pt=LH_DefaultDomain_0&hash=item27d7e41d8f

http://www.ebay.com/itm/arduino-L298N-stepper-motor-drive-28BYJ-48-stepper-motor-drive-circuit-C-/370880546505?pt=LH_DefaultDomain_2&hash=item565a3406c9

In general , when someone starts using words like “precise” , you expect some clarification as to the precision required.

@ zoomkat thanks for these links, gives me somewhere to start!

@ raschemmel by precise, I mean the ability to repeat a camera move twice, and have those moves be identical, ala motion control. and maybe that's just not possible for a noob-tinker-setup like this will be--but I can dream :) So the motion can be duplicated programmatically, just getting the starting point to align each time will be difficult?

thx both for the help

@ raschemmel by precise, I mean the ability to repeat a camera move twice, and have those moves be identical, ala motion control.

That depends on your ability to keep track of the direction and pulses, which is more complicated than reading a shaft encoder for absolute position. Accuracy depends on how much trouble you want to go to. I would opt for the absolute position shaft encoder but they're not cheap so maybe an incremental encoder. Either way accuracy is a function of the resolution of the encoder (255/1024/2048/4096/8192/16384 etc.etc.etc..)

You might have a problem with stepper motors being a bit jerky particularly if you are trying to make video.

There is an inherent conflict in designing turntables, between having enough mass for smooth operation and constant speed, and having low mass so you can change the speed easily.

@michinyon thx, that makes sense, hmmm, perhaps using an over-size stepper motor, and microstepping is the solution for this? just a wild guess

@ raschemmel now that I'm doing more reading on steppers, I'm confused. You're describing a closed-loop system using an encoder. And I'm reading that one advantage of the stepper motor is its accuracy in an open loop system...? Explain that for me?

I mean the ability to repeat a camera move twice, and have those moves be identical, ala motion control. and maybe that's just not possible for a noob-tinker-setup like this will be--but I can dream smiley So the motion can be duplicated programmatically, just getting the starting point to align each time will be difficult?

This is what you said. You didn't specify how much time might elapse between the first shot and the repeat shot. If your stepper doesn't skip any steps and if you repeat the shot immediately after the first shot, then yes you can probably do it in open loop. You asked what the difference is and the difference is that with closed loop and especially with an absolute encoder , you can repeat the shot six months later and it will be identical. With a stepper, you can't guarantee the initial starting point unless you have some system to know the starting position with a reasonable amount of accuracy. As I stated previously. Most stepper driven systems used in the industrial systems , be it a wafer lift in a semiconductor processing tool or an x/y table, have home flags to initialize the system to an exact starting location repeatably , 24/7/365 , year round. Some have more than one opto for reading the flag and can sense the extreme limits of the motion , called end-limits or end-flags. There is no limit (aside from space) to how many flag reading sensors you can have, but if you want to home it , you can only have one unless you electronically disable all but the home flag (which is not hard to do). If the power for all the other flags has a common source or ground you can switch them off with a mosfet leaving only the home flag working. If you have a home flag and you know the first shot was x counts cw from HOME (which is NOT the flag location but some number of steps , like 10 or 15 from the home flag) then when you want to repeat the shot you repeat the distance from Home. Another difference with closed loop or encoder approach is it is immune to skipped steps where a stepper overshoots due to inadequate drive current. If you want to calibrate your system. Buy a laser pointer and stall a clip holder on the turntable with drilled and tapped holes. Mount the clips, snap in the laser pointer, do your calibration then remove the screws and the clip and no one will even know they are there..The accuracy of the calibration is a function of the distance to the walls. If you have a large enough room the accuracy of the position of the dot on the wall is extremely precise, meaning one stepper step changes the position of the dot by several inches to up to a foot on the wall. If you want save money , use a home flag, the laser clips and mark the dot on the wall when you take your first shot and make sure the dot is in the same place when you repeat the shot. It's not rocket science.

OK, I appreciate the clarification–and all the extra info

thanks again!

LML

LML. I actually just finished building half of what you are talking about. My son-in-law wanted a set-up where he could set something on the turntable and rotate "X" degrees take a pic, rotate "X" degrees take a pic, etc. for one revolution. I used an Uno, Easydriver Stepper Motor Driver and the $14.95 stepper motor on Sparkfun. I bought a used ION USB turntable off craigslist and dismantled and removed what I didn't need so I didn't have to reinvent the wheel. I replaced the DC motor in it with the stepper. It also had a 9V & 16 Volt internal power supply and I was able to find places to mount the Uno & Easydriver inside and connect to power supply. It also has a USB port so I soldered a short length of USB cable inside to plug into UNO allowing capability to reprogramming if desired, but not have the USB cable always hanging out. I wired up a Nikon Coolpix so I could take the pictures automagically. Actually came out really clean I think.

Current operation sequence is basically: Focus TakePic Rotate 18 degrees (no significance, just what he wanted) Focus Take Pic Rotate 18 Degrees yada, yada, for a total of 20 steps in this case.

I can provide you more detailed info if you like.

thanks for that info Stevomv, much appreciated.

I am now scratching my head a bit about how likely it is to get the smooth motion (including speed ramps) w/ a stepper that I want for the video project. So I'm kind of on hold with it, but I'll definitely hit you up for more info if I run across something related to your project. Glad it turned out well!

cheers

ML