Stepper Motor - Newbie Alert

I am about to start working on a timelapse dolly project.
The slider itself will be based on this:
http://www.igus.sg/wpck/2004/DryLin_W_Fuehrungsschlitten_montiert

I want to make it using three wheels to lock a timing belt in order to secure it tightly.
Something like that: http://www.diyphotography.net/files/images/7/150p-time-lapse-dolly-2a1.jpg

I am clueless where it comes to choosing the stepper motor (torque etc.).
It is supposed to lift a load of 3kg vertically.

Moreover, other than an Arduino board, what other shields and extras do I need? Any quick guide on how to plan the circuit (resistors, MOS/BJT, capacitors etc.)?

EDIT: I know how to reduce voltages and whatever is required, but since I'm not really familiar with stepper motors and their specs I'm not sure how to tell what voltage they require, how do I know their input\output resistances and such.

Thanks a lot,
Help to newbies is always appreciated :smiley:

For lifting a mass of 3kg vertically you vill need a force of F=mg -> F=39,81=30N. This is absolute minimum since there is some friction in the system as well that must be coped with and you want to have some margins as well. So between 40N and 50N would be reasonable.

The torque you need to create this force depends on the lever, in this case the pitch radius of the pulley presuming that you plan to a toothed belt.
For example: We have a XL type timing belt (pitch 5,08mm) and a 12 tooth pulley. This pulley has a pitch diameter of 19,4mm giving a pitch radius of 9,7mm. The required torque is M=Fd->M=509,7e-3=0,485Nm. For comparison, tightening a screw with a screwdriver gives you between 1 and 2 Nm.

Assuming that you can spin the motor at 60rpm or 1 revolution per second the speed will be V=#teethpitch=PitchDiampi ->V=12*5,08=61mm/s.

The speed of a stepper motor depends largely on the driver. I recommend using a current controlling(chopping) micro stepping driver like the Big Easy Driver with a as high as possible supply voltage as possible for best performance

Thank you very much.
May I ask, what is the difference between Unipolar and Bipolar motors?

A bipolar motor has to be driven by a H-bridge in order to drive current in both directions through the winding. In the old days a H-bridge was considered an very advanced circuit so turnarounds were designed such as unipolar motors. These have a center tap on the windings and can be driven with open collector drivers. But this only utilizes half of the winding so what you save in driver design you will have to pay by getting a bigger motor. Modern microstepping integrated chopping drivers have excellent performance and moderate cost so in my opinion the unipolar motor is going to be obsolete. But as long as the center taps are not internally connected (5 wire motor) you can use a unipolar motor as a bipolar, just leave the center taps unconnected.

Ok, thanks a lot for the detailed answers.
Will any of these motors be sufficient for my project?
http://www.sgbotic.com/index.php?dispatch=products.view&product_id=1465
http://www.sgbotic.com/index.php?dispatch=products.view&product_id=1464

If you look at the torque rating both will suffice but the NEMA 23 will have larger margins. Both can be driven with the Big Easy driver. I recommend a unregulated power supply of 30-35V which will be easy to build by using a 24VAC transformer, 1A will do fine for driving 1 or 2 motors, a bridge rectifier and a large reservoir cap. I also recommend using the accelstepper library because it minimizes the risk of loosing steps during acceleration and retardation

The links for those motors say that they need 1.7amps and 2.0amps.

I would have thought a BigEasydriver would be struggling at those current levels.

I also note that @nilton61 (who probably knows more than I do) suggests a 1amp power supply would be sufficient. I don't understand how?

...R

Robin2:
The links for those motors say that they need 1.7amps and 2.0amps.

I would have thought a BigEasydriver would be struggling at those current levels.

I also note that @nilton61 (who probably knows more than I do) suggests a 1amp power supply would be sufficient. I don't understand how?

...R

A Big Easy Driver handles 2A without problem. The component that get hottest is the voltage regulator and that is not current depending.

1A is sufficient because the inductance of the winding acts as an energy converter. A NEMA 23 motor has a voltage drop of maybe 3V (dependent of speed) at 2A. That means that the power dissipation is 6W. If you have a supply of 30V you will theoretically need 0,2A but that is to challenge things too much. As long as the reservoir caps can deliver the current transients the circuit will work fine. That is also the reason why a unregulated supply is preferable in this situation

@nilton61 - why would you need an AC source and a rectifier + capacitor? Why won't a DC source do?
How did you calculate the 30-35V value?
When looking at this guys project, he only chose a 18Ncm motor, significantly lower than 50Ncm, how come?
http://www.pocketslider.de/DIY_slider_en.html

Note that I want to make it a Move-Shoot-Move mode, so I want to be able to move the motor in smaller steps (this is in regards to your first message).

Do you have a better, rather affordable, motor to suggest? I really am clueless about this and learning a lot from you. So thank you very much!

nilton61:
A Big Easy Driver handles 2A without problem. The component that get hottest is the voltage regulator and that is not current depending.

This is interesting (and useful) because I have seen other comments suggesting a Pololu A4988 (the BED uses the same chip) can't manage 2A. I wonder if your comment relates to the BED specifically?

1A is sufficient because the inductance of the winding acts as an energy converter. A NEMA 23 motor has a voltage drop of maybe 3V (dependent of speed) at 2A. That means that the power dissipation is 6W. If you have a supply of 30V you will theoretically need 0,2A but that is to challenge things too much. As long as the reservoir caps can deliver the current transients the circuit will work fine. That is also the reason why a unregulated supply is preferable in this situation

This makes sense but I hadn't thought of it that way. Thanks

...R

elti:
@nilton61 - why would you need an AC source and a rectifier + capacitor? Why won't a DC source do?
How did you calculate the 30-35V value?

Pure DC sources are batteries and DC generators. When it comes to generating DC from the mains voltage (which is AC) you will have to choose between a traditional transformer followed by a rectifier a resevroir cap and an optional regulator on one hand and a switching adapter on the other. Most DC adapters that you buy today are regulated, this is because for most uses they have a good cost/performance ratio because regulating the voltage diminishes the need for costly reservoir caps.

But for this application large caps are exactly what we are after and there is no need to regulate since the driver already regulates the current. The BED has a maximum voltage of 35V you should be as close to this as possible. If you rectify the voltage from a (easily available) 24V transformer you will get sqrt(2)*24V= 34V subtract two diode voltage drop of 0,6V and you get about 32,8 wich is exactly right.

When looking at this guys project, he only chose a 18Ncm motor, significantly lower than 50Ncm, how come?
http://www.pocketslider.de/DIY_slider_en.html

As far as i can see he uses a DC gearmotor with a stall torque of 6,48 Nm. That's a lot. In addition to that it look like he is using a 10 tooth Module 1 pinion. This will have a pitch radius of 5mm. The force will be 136N. Using A DC motors means having a higher torque but no means of controlling the distance traveled, unless using a encoder and some quite advanced code to control motion. Also the desing seems to me to be used for horizontal travel, not vertical as you specified.

Note that I want to make it a Move-Shoot-Move mode, so I want to be able to move the motor in smaller steps (this is in regards to your first message).

When you use the BED or similar you will get 3200 steps/revolution. With a 12 tooth XL pulley that will give you 0,019mm/stepwich is good enough for light CNC and 3d print work, i bet it will suffice for photography.

Do you have a better, rather affordable, motor to suggest? I really am clueless about this and learning a lot from you. So thank you very much!

The motor parameters for your project depend on a number of parameters:

  • Power, that is the product of torque and speed. You have only specified the force for linear motion, not the speed. I suppose speed need are not that high.
  • Precision of control
  • System complexity (that includes your ability to handle that complexity). Stepper motors are a good tradeof between the two previous points
  • Sourcing, the components have to be available in your part of the world with reasonable deliverance times and costs