Choice of Motors for Cable Cam

I’m making a cable-cam, similar to the one in this video:

http://www.youtube.com/watch?v=nyEOttqo0eA

It has a slightly different design, and uses two cables instead of one so it can carry more weight (a full DSLR). The only thing that needs to be figured out now is what type of motor to use.

I need it to do two things:

  1. Move accurately and slowly over a long period of time (something like 0.01m/s)

  2. Move very fast (but not accurately) for shorter periods of time (something like 5+m/s) as seen in the video

My question is: should I try to find a very fast stepper motor which can do both, for instance:

http://www.ebay.co.uk/itm/DC-12-40V-2-5 … 43bbbfb283

Or should I instead build a motor assembly that can be swapped out, and instead use a standard stepper motor as well as a much faster (but similarly power-rated) DC Motor:

e.g. http://sparkfun.com/products/9238 and http://www.ebay.co.uk/itm/DC-12V-3000RP … 1240307290

These, when both attached to a 30mm diameter pulley designed for the 8mm rope should produce something like the needed speeds.

Or am I doing something completely wrong? The whole setup (Arduino, motor and radio receiver) needs to be powered from one 12V battery source.

Have you got a starting calculation for the motor RPM needed to get 5m/s ? Ditto for the torque needed to overcome the resistance at that speed ? Double ditto on the torque needed to accelerate the whole moving mass from 0 to 5m/s in a time and distance that you deem reasonable ?

Not yet, no. I’m not very experienced with mechanical engineering and tend to stay in the software world.

I’m yet to find a drive wheel suitable for the 8mm climbing rope I’ll be using, although here’s the process I’d use to find the necessary RPM for 5m/s-ish:

e.g. 30mm diameter drive gear (for example) * pi * RPM / 60 / 1000 = 5 so I’ll need something like 3000rpm.

On the other hand, I have no idea how you calculate torque produced by the motor or how to calculate torque required to move the entire mass. Would you mind helping out? The mass of the entire system will likely be 2-3kg depending on the materials used.

Start by reading about Torque in Wikipedia then do some web searches.

Here is a link to some beginner Robot info that would be useful to you:

http://www.societyofrobots.com/

Thanks, that’s helpful.

I’ve found another motor which seems like a better choice:

http://www.ebay.co.uk/itm/12v-DC-Brushe … 0825426652

As far as I can figure out the “RMF” needed for my design is:

(3kg * 1m/s/s * 5m/s)/2*Pi = 2.4 kg m rps

And the motor’s torque = 77 Nm = 7.1kg m

So the motor’s “RMF” = 7.1kg m * 75 rps = 532kg m rps with no load, less with.

So from my understanding, the motor above should be more than powerful enough to accelerate the 3kg of cable cam at 1m/s/s to ~5m/s?

OK you’ve specified an acceleration of 1m/sec/sec as an acceptable acceleration. I’ll also interpret that as a 5 sec time, 0 to max speed. I do this because the torque vs RPM curve of a stepper isn’t a flat line, a constant number. It starts out “high” and decreases w/RPM. Like a car, your platform will accelerate more quickly at low speeds vs higher speeds.

Your relation btw platform linear speed and RPM was almost right. I assume there’s a gear on the stepper to engage a gear on the drive pulley. Most likely (or not, see below) this will reduce the RPM from shaft to pulley, but it will increase the torque (by the inverse ratio).

So I would approach the question of motor requirements thusly;

  • It’s got to have enough torque at max RPM to overcome the resistance to motion your platform will have at max speed. The pulley size (where it contacts the line) and the above gear ratio play into this.

  • The integrated torque curve must be great enough to get your 0 - 5m/s in 5 secs. The pulley size, gear ratio, platform mass and resistance all play into this along w/the torque curve. This may be the hardest one to get a handle on.

  • The max RPM, accounting for gear ratio and pulley size, must get you your 5m/sec max speed.

Let me assume your 30 mm pulley has a 30 mm diameter. To go 5m/sec means it has to revolve 5000/(30 x pi) = 53 times in 1 sec. That’s (x 60) 3183 RPM. So with a 3600 RPM motor you could run > that speed with 1:1 gear ratio, assuming the motor had enough torque to overcome whatever resistance there was. So what might that resistance be ?

Frankly I don’t know. I do know that you could measure it fairly easily if you have your platform built and a tramline setup. For the moment let me ASSume it’s 0.1 kgf (only because it makes the math easy). Let me also assume a safety factor of 2, meaning the motor, through gearing, must produce at least 2x that retarding force. So 0.2 kgf at the radius of your pulley (30/2 = 15 mm = 1.5 cm) is .2/1.5 kg cm = 0.14 kg cm @ ~3200 RPM. Alas you’ll have to find a very complete datasheet to get that sort of number for a motor. FWIW 1 kgf = 9.8N, as in Newtons of force, the proper SI unit.

Your 1’st link to an Ebay stepper would get you a motor roughly equivalent to this one;

https://www.sparkfun.com/products/10846

From the manufacturers site I guesstimate it makes 20 N cm at 1000 RPM and a peak of 43 N cm at 175 RPM. That 43 N cm is close enough (= 61 oz in) to the 68 oz in claimed that I’ll call it the same. So that’s a min torque (in non-SI units) of 20/9.8 = ~2.0 kgf cm. But it’s a 1000 RPM motor and you need about 3x that. So the 3:1 gear ratio means that 2 is only 0.66 kgf cm at the pulley. More than enough over the 0.14 kgf cm, assuming the SWAG for resistance/retarding force is correct.

So what kind of acceleration might you expect ? Let’s model the torque curve (vs RPM) as a right triangle on top of a box, the height (torque) of the box being the min torque of 20 N cm. The height of the triangle is then 43 - 20 = 23 N cm over a base of 1000 RPM. A box of equivalent area with an equal base would have a height (torque) of 1/2 the height of 23, or about 11.5 N cm, which when combined with the min torque of 20 N cm means, on average, across the entire 1000 RPM, the motor will produce 33.5 N cm of torque. That means a force of X at the pulley groove, X = 33.5 N cm / 9.8 N/kgf / 3(gear ratio) / 1.5 cm (pulley radius) = 0.76 kgf. Subtract the ASSUMED resistance of 0.1 kgf and you’ve got a net force of ~ 0.66 kgf to accelerate your 3 kg of platform. The definition of 1 N of force is that it will accelerate 1 kg of mass at 1 m/sec/sec. Since 0.66 kgf = 6.53 N, you should have more than enough torque, on average, to meet your accel requirement, ASSUMING the guess for retarding force is anywhere near correct. Also I’ve neglected the moments of all the rotating bits. That may or may not be a good assumption but expect real life acceleration to be reduced.

So far it appears on casual examination that the SF, or Ebay (??), stepper might work.

Your last question asked if the same motor could do 0.01 m/sec. I don’t know. Obviously you could step, stop ‘n’ wait, step, stop ‘n’ wait, … to achieve any slow average speed, but I don’t think that’s what you want (unless it’s time lapsed w/motion photography, speeded up to make a ‘movie’ … in which case it’ll do). A step is typically 1.8 deg or ~1.5 mm of linear travel w/your pulley and 3:1 gearing. Given microstepping can maybe give you 10x finer resolution, I don’t know at what speed such stepping becomes too variable in size and timing and also becomes stop ‘n’ wait jerky. Perhaps someone else can answer.

I want to correct part of the post above, repeated here.

For the moment let me ASSume it’s 0.1 kgf (only because it makes the math easy). Let me also assume a safety factor of 2, meaning the motor, through gearing, must produce at least 2x that retarding force. So 0.2 kgf at the radius of your pulley (30/2 = 15 mm = 1.5 cm) is .2/1.5 kg cm = 0.14 kg cm @ ~3200 RPM.

So that’s a min torque (in non-SI units) of 20/9.8 = ~2.0 kgf cm. But it’s a 1000 RPM motor and you need about 3x that. So the 3:1 gear ratio means that 2 is only 0.66 kgf cm at the pulley. More than enough over the 0.14 kgf cm, assuming the SWAG for resistance/retarding force is correct.

I went the wrong way, it should have been ...

For the moment let me ASSume it’s 0.1 kgf (only because it makes the math easy). Let me also assume a safety factor of 2, meaning the motor, through gearing, must produce at least 2x that retarding force. So 0.2 kgf at the radius of your pulley (30/2 = 15 mm = 1.5 cm) means your motor must have a rating of 0.2*1.5 = 0.3 kg cm @ ~3200 RPM (not 0.14 kg cm as stated above).

So that’s a min torque (in non-SI units) of 20/9.8 = ~2.0 kgf cm. But it’s a 1000 RPM motor and you need about 3x that RPM at the pulley. So a 3:1 gear ratio means that min of 2 kgf cm @ 1000 RPM is only 0.66 kgf cm at the pulley and 3000 RPM. Still more than enough over the 0.3 kgf cm “needed”, assuming the SWAG for resistance/retarding force is correct.

I should also mention the above assumes a level tramline. If you're going uphill, you'll need to fight gravity as well.

It also occurs to me you could have swapable gear sets, one for high speed, low torque work and another for low speed, precision and/or hill climbing work.

It also occurs to me you could have swapable gear sets, one for high speed, low torque work and another for low speed, precision and/or hill climbing work.

I think Mee_n_Mac hit it on the head. Motors are designed to operate efficiently over a small range of speeds. Too fast or slow and they either do not work at all or work very poorly. Think about the gas motor in a car. It needs a transmission to allow for car speeds ranging from 5 to 75 MPH. Over that range (one order of magnitude) the car has 3 or 4 gears. Now you are asking for a 2.5 order of magnitude range of speed without gears.

Might I suggest that you run with two motors? Based on what pulley you loop the tram cable over you can go slow or fast. Or you if you have the mechanical ability you could mess with gearing.

I briefly looked for a hobby level CVT. Apparently there was one made for gas RC cars a while ago but it never caught on and isn’t available now. Sounds like an intriguing project for some mechanical type.