Stepper motor or DC motor?

Discussion in 'The Projects Forum' started by pavelnedved, Apr 23, 2012.

  1. pavelnedved

    Thread Starter New Member

    Apr 17, 2012
    Hi all!

    For a project I need a motor that makes small turns of about 0.2° and stops (+stay put), another turn of 0.2°, stop etc...

    Size is important. It has to be rather small as the motor must fit into a solenoid coil. Let's say it could have a length of 6cm max.

    The load mounted on top of the axle is quite lightweight, let's say about 500g.

    Speed is not important at all. It will never reach any high speeds.
    Positional accuracy is more important.

    I have informed myself about stepper motors (using microstepping or a gear to achieve the appropriate step angle). However, these small steppers seem to have very low torque. Closed loop is not possible because of the dimensional limitations, but OK I can do it open-loop.

    I don't know much about DC motors though. Are they generally smaller/bigger than steppers. Common sense tells me they must be bigger because of the optical encoder included on DC motors, but I'm not sure.

    If somebody has some good knowledge about DC motors (or both) and could give me some advice about the choice of type, that would be greatly appreciated!

  2. panic mode

    Senior Member

    Oct 10, 2011
    what is 6cm? restriction in any direction? or just length?
    what is the travel? is the price one of constraints? you can get nice quicksilver servo for ~ $1500...
    did you consider hobby servos?
  3. jimkeith

    Active Member

    Oct 26, 2011
  4. BillB3857

    Senior Member

    Feb 28, 2009
    How about the head positioning motor from and old floppy drive?
  5. pavelnedved

    Thread Starter New Member

    Apr 17, 2012
    Hi, thanks for the replies.

    Well 6cm is like the distance (length) from downside of the motor to the top of the axis.
    Money is some kind of constraint as the university is not that rich (or willing to spend too much).

    Many recommend a servo motor. I guess I should consider that path too.

    About the stepper motors, I'm having difficulties finding one which meets the following criteria:

    small dimensions: length = 6cm max
    step angle of 0.2° (possible with gear or microstepping)
    torque high enough for a load of 500g (max)

    for example : torque = 230 oz-in = 1.6Nm = only 160grams
  6. panic mode

    Senior Member

    Oct 10, 2011
    the data you post is not very helpful:
    1. 500g is mass - not torque; what do you need torque or mass (payload) to be moved?
    2. what is the travel? (eg " linear travel of 20cm" or "rotary travel of 270deg")
    3. what is dimensional constraint? world is 3D and so far we only know one dimension. if i give you motor that from back to the tip of the shaft is 5.5cm but it is 20x20cm in other dimensions, would that work?
    4. if the motion is rotary, what is length of the arm?
    5. is there any other restriction (voltage, mass etc.)?

    since speed is not important at all, you can have small device moving slowly and practically moving a mountain. your assessment of torque is incorrect, don't forget to keep track of units, because you can't convert torque to mass.

    if you are adult male with an iron grip and you use that strong hand to grab motor shaft that is about 12mm diameter, and only 2-3cm long (so you cannot wrap entire hand around it), the most torque you can produce is only about 4..4.5Nm. that is barely 3x more than mentioned 1.6Nm.

    1.6Nm is torque produced by 1.6N weight where this force has reaction arm that is 1m long. that 1.6N does correspond to a mass of 160g pulled by Earth's gravity, but using leverage of 1m.

    if the distance is reduced to 20cm (reasonable length for common tool wrench for example), then this would correspond to a mass of 800g. if the radius is down to 5cm, we get 3.2kg. small motors have shafts that are much thinner, maybe 8mm (0.008m), then to get the same 1.6Nm, we'd need mass of 20kg. so if you wrap fishing line around 8mm shaft driven by 1.6Nm motor, we can lift 20kg.

    and - that is without using any gear yet (wormgear, planetary gear or whatever).

    but if we add 50:1 gear, that same motor can lift a ton (20kg*50=1000kg, of course, shaft material would have to stand such load). but, see where this is going?

    check video of Jamie from Myth-busters getting stuck under the ceiling. He used small battery operated device with DC motor to lift himself. Hilarious moment was as he was all the way up and wanted to come down. He forgot to install reversing switch so he had to be rescued.
  7. pavelnedved

    Thread Starter New Member

    Apr 17, 2012
    Thank you very much, this is really helpful!

    Let me try to more specific:

    On top of the motor shaft we will mount a circular platform carrying the load (also circular). Diameter of the platform will be about 6cm. So, maximum 3cm away from the axle.

    Platform + load weighs less than 500g.
    However, the centre point of gravity will be in line with the axle because of the symmetry, so I guess we only need mass to be moved.

    2/ Rotary travel is about 100° : 50° in CW direction and 50° in CCW direction

    3/ All other dimensions have to be 8cm or smaller, because the motor as a unit has to fit into a solenoid.
    It is possible to use standalone controller though to make the dimensions smaller. Some wires can leave the coil.
    The length (up to top of the shaft) preferable no longer than 6cm.

    4/ the motion is rotary, but I assume the netto load torque is zero because of the symmetry.
    You can correct me if I’m mistaken.

    5/ Another restriction is electromagnetic influence produced by the motor as small as possible.

    Somehow I'm a little worried about the inertia torque in this project.
    I'm really lost how to calculate that though..
  8. John P

    AAC Fanatic!

    Oct 14, 2008
    Would it be possible to drive this thing via a timing belt, from a motor that would be located some distance away? Then the issue of placing the motor in the center wouldn't exist. If that's possible, I'd suggest putting an encoder inside the solenoid, and running the motor as needed to place the output where you want it. That way any non-linearity that the transmission introduces wouldn't affect the accuracy.
  9. panic mode

    Senior Member

    Oct 10, 2011
    inertia does not play role is slow system and even in highly dynamic one, adding reducing gearbox makes it negligible , specially as the ratio goes up.

    if the target is circular and symmetrical, net holding torque is zero and we only need torque while turning. also this torque would be much smaller than in case of asymmetrical load.

    hobby servos have DC motor, high ratio gears, and controller all built in. their position is controlled by PWM (in case of analog ones, there are digital ones too):
  10. panic mode

    Senior Member

    Oct 10, 2011
    compact industrial grade servos are very nice but not cheap. usually they have external drive that is another complication when it comes to size but there are also some compact ones where drive, controller, motor and encoder are integrated into one compact unit, for example:
  11. pavelnedved

    Thread Starter New Member

    Apr 17, 2012
    But even without net holding torque, isn't there like some sort of resistance of shaft/gear/load to a change from standstill to motion? I just fear this would be an issue, that the weight of the load would be too high for the rotor to start moving.

    For example, on the website I check a certain motor. The torque at a speed of 0Hz (and also at really low step frequency) is about 0,15Nm.
    Is there some way to calculate whether this is high enough for my application?

    Theoretically speaking the load is all symmetrical. It consists of a magnetic compass on a circular platform. However, in practice there might be a very slight deviation in some directions.
  12. panic mode

    Senior Member

    Oct 10, 2011
    you can measure torque required to turn your system. this is torque required to overcome static friction. as long as motor torque is greater, it will be able to turn it.