I have an wooden turntable whose platform diameter is 33 cm, thickness - 2.5 cm, mass - (no idea, but made of plywood).
Now, I have to automate the rotation of the platform for 3-D scanning of objects weighting up to 10 kg with maximum diameter of 25 cm.
So, what are the minimal rough specifications (specifically torque) for the stepper motor that is required?
You will have to measure the torque required for the motion.
So, what's the easiest way to measure (roughly) that? I don't have fancy tools to measure! 
If the platform is mounted on low friction bearings most of the torque will be required to accelerate the loaded platform and to decelerate. Very little torque should be required to keep it moving.
You can calculate the force required to accelerate the load by working out the moment of inertia (try Google) of the platform plus the load. I would be a reasonable guess to assume the plywood has a density of 500kg/m3
If you do not have low friction bearings you will have to measure the torque required with different loads on the platform.
If your stepper motor does not have enough torque (and some to spare) it will miss steps when accelerating and decelerating.
I wonder if you really need a stepper motor? If the scanning can happen while the platform is moving a geared DC motor with a rotary encoder to detect the position of the platform might be better.
...R
You don't need fancy tools, just some understanding of force and torque.
To measure torque required to rotate the turntable, you can use a standard hand-held force sensor (for example, a luggage scale) applied at an outside edge of the turntable.
The torque is the force times the distance from the center of the turntable to the point at which the force is applied. See this explanation of force and torque.
Reply to: I wonder if you really need a stepper motor?
No. No. I do not need the motor to rotate continuously. I actually have to rotate the model by around say 5 degrees and stop the motor and take images and repeat the same.
Stepper motor is a reasonable approach to this. Reduction gearing/belt is useful
for more precision (if low backlash).
You need to measure the frictional torque of your turntable under load - this is easy if
you have a small spring balance and a piece of string. Almost all of the torque
is friction in whatever bearing system you use, this is not something readily calculated.
You then have to derate the stepper's pull-out torque by perhaps a factor of 3 for good measure.
And if using gearing/belt measure the torque at the motor side of the system.
Make sure to select quite a low max acceleration for the stepper or it will mis-step - turntables
have a lot of inertia.
Thanks every one! I have ordered a spring balance. Once it is delivered, I will measure the frictional torque (starting) under load.
I actually have a NEMA17 stepper motor which I had bought long ago with a rating of 4.2 kg-cm holding torque. It seems I need a more powerful stepper motor of holding torque around 19 kg-cm.
Measure first, decide once you have the numbers... Have you an idea of acceleration rate
needed? Presumably quite low is OK (if not then acceleration may dominate the torque
required - you'll need to know the worst-case moment of inertia for your load, which can
be very roughly estimated as the total mass times the turntable radius squared (in kg and
m, giving a value in kg-metres-squared).
Total_torque = angular_acceleration * moment_of_inertia + frictional_torque
I forgot to include a link to stepper motor basics earlier - it has some simple ideas for measuring torque.
However I suspect that the main force to be overcome is the inertia / momentum associated with acceleration and deceleration and that is not easy to measure.
...R
Ok guys! So, I have measured the max. torque under maximum load condition using a spring balance as follows:
Under max load condition, I used a thread attached to and rolled around the platform (diameter = 33 cm) at its periphery at one end and the spring balance at the other. Then I pulled the spring balance to start rotating the turn-table.
While the spring balance meter reading was not stable enough, I was able to read the maximum force during the starting which was around 0.11 kg force.
Hence the max. load torque required (frictional+starting) = 1.82 kg-cm (which is far less than 4.2 kg-cm).
Does this mean that I can use my older bipolar stepper motor (NEMA17 4.2Kgcm Stepper Motor (With D Shaft) - Premium NEMA17 4.2Kgcm Stepper Motor (With D Shaft) - Premium [RMCS-1008] - ₹585.00 : Robokits India, Easy to use, Versatile Robotics & DIY kits) with holding torque of 4.2 kg-cm?
Also, I am driving it using a pololu a4988 stepper motor driver with a power supply of 12V 1A for the motor (using the included current limiter).
Is this power supply sufficient enough as I am not able to obtain the coil current per phase any near to the rated current which is 1A (according to http://robokitsworld.com/documentation/RMCS-1008.pdf)?
(Max. phase current that I can obtain is around 0.75 A).
It sounds to me like that motor would be fine.
I don't know what you mean by "(using the included current limiter)" - is it part of the power supply, or is it the current limiter on the A4988 ?
The relationship between the power supply current and the coil current is not simple. The motor needs 1.4 amps at 2.8v or about 4 watts. 12v x 1 amp is 12 watts. The stepper driver chops the power to ensure that the limit is not exceeded and in doing so the average seen by the power supply will be the current that provides the required number of watts. You need an oscilloscope to measure that stuff properly.
Try it and see what happens.
...R
Please see my new post regarding this New Post.
panini:
Please see my new post regarding this New Post.
Please don't split your project across multiple Threads. It is much better to have all the info in one place.
Please click Report to Moderator and ask to have the new Thread merged with this one.
...R
Panini,
The other thread you started was regarding stepper motor wiring, etc. But, I am also building a turntable for a scanner and also am trying to calculate load / motor capacity.
I am interested in how your calculations went? Using the spring scale gave you a fairly low inertia load requirement. So, how did those calculations work out? Was you motor able to drive the table under load as you calculated? Were you driving it directly? I need to scan some heavy items, upward of 100 lbs, and want to design for max of 200. I am using my own design for turn table with separate centering and load bearings. The table should turn quite easily.
But, knowing if your calcs worked would help me out. (My preliminary - off the cuff - idea, with no real idea of what i was thinking, I was going to use a 84oz/in motor and gear it 49 x (2 7:1 belt pulley's), thinking that ~5 lb/in torque would translate to a 200 lb++ load rating at 49:1 BUT, that is shaft holding power and now i see that turntable inertia weight is totally different. Maybe i can get by with just 7:1 ratio? The ramp up and down times can be reasonable - I mean i don't want to jerk it on each move.
Thanks.
PS [added Edit] I am sitting on my office chair, and noticed, a real simple concept of leveraged force, but, I tried turning my chair with my 150 lb body sitting in it, and realized that when i push off (or pull toward) my desk, if I do so closer to the center of the chair, it takes a heck of lot more torque then if i put my arm out farther from the center and push off with little effort.
So, my question is about using the string around the Circumference of the turntable: If you measure the force needed at the outer edge of circumference it is going to be far less then if the motor were direct driving it from it's shaft at the center of the table at the center, wouldn't that take so much more torque? I am not that knowledgeable in this, but someone who knows more, am i right or missing something?
So, measuring with a string at circumference isn't the same as what a direct drive motor would need? am i way off on this, or not?
I would prefer a DRV8825 rather than an A4988 (compare datasheets);
follow the steps to limit the current for the motor (see the instructions on Pololu web site.
If I understood you right you are using the turntable for 360 degrees scanning (5 degrees per movement).
If your application allows to have "delays" between the stops you could power off the driver for the delays; re-enable the driver after a short delay, move to the next stop etc.
Thus your driver would stay pretty cool in comparison with having the driver powering the motor all the time.
I have an application which makes use of this; before the drivers got pretty hot, even that I limited the motor current to less than 300mA. Now I can power it up to >1000mA and the driver stays cool (my application is so that there are 5-10sec between moving the stepper for about 1-2 sec).
If you can go that path, you should check if the driver should provide the full torque (current) a little bit longer than the second your motor stops to allow the moved mass to get to a full standstill.
unltdsoul:
Torque is not measured in lb/in or oz/in. It is the product of force and distance, not a ratio.
My strong advice is always use Nm for calculations, since there are no arbitrary constants when working in SI.
Convert to SI, work in SI, convert result back if you need to.
Rotational inertia is simple in SI too, torque = MoI x angular acceleration. (Nm, kg m^2, rad/s/s)
So, my question is about using the string around the Circumference of the turntable: If you measure the force needed at the outer edge of circumference it is going to be far less then if the motor were direct driving it from it's shaft at the center of the table at the center, wouldn't that take so much more torque? I am not that knowledgeable in this, but someone who knows more, am i right or missing something?
You need to learn what torque is - its the energy per radian of turn. So you can use J/rad, but everyone
uses Nm which is the same thing without making the rotation explicit. The size of the shaft is irrelevant
to the energy per radian, but the tangential force is inversely proportional to radius for a given torque,
due to conservation of energy.
Measure the force, multiply by the radius. That's the torque.