Finding the Right Stepper Motors

[airman00]

OK so I need to use stepper motors for a project of mine, since I need to have control over the position

Now I have a few questions for you guys:

1. All types of stepper motors are reversible, correct?

2. According to Admin's RMF calculator I need two motors each with an RMF value of 12.9 lb*ft*rps. The torque of a stepper motor can be found easily but how can I find the RPS of the stepper motor?I understand the RPS depends on the controller circuit, but what number for RPS do I put in the calculator?

3. What are the pros / cons of using steppers vs. encoders on DC motors ( yes i've searched the forum , but could get no definite answer)

Thanks,
Eric

[ed1380]

stepper motors you tell them where to go.; encoders tell you where they are.

so if you want to move forward 2 feet.
 for stepper you tell it to move X cycles.
for motor w/ encoders. you start moving forward until that distace it reached

[airman00]

stepper motors you tell them where to go.; encoders tell you where they are.

so if you want to move forward 2 feet.
 for stepper you tell it to move X cycles.
for motor w/ encoders. you start moving forward until that distace it reached

I understand that. What would you recommend to use , and what would be cheaper?

[izua]

Here's a consequence of using steppers.
Example: You have a 7.5 degree per step so you get 48 steps. With a uc, it's pretty easy to go halfstep, so that gives you 96, let's say 100 steps.
At a 100 mm wheel diameter, you have 314 mm circumference, so at each step, your robot will leap 3 mm. If you run at a high speed, the leaping won't be so obvious, but at low speed, it will appear to jump instead of rolling.
So one thing you'll need for your steppers is a high number of steps.

If that number of steps still won't be enough, you might need a microstep driver (it can be done on a uc, but it's a real pain).

Finally, if you have steppers, on a flat surface you pretty much have odometry. However, it's hard to know if the motors actually made a step. You can check the induced current in the opposing coils, or you can add encoders. By only a combination of stepper + encoders, you can map out the environment (and do collision "reaction") in a simple way: send a few steps, if the encoders jump back and forward by a similar amount, you're not moving, we need to change direction.

All steppers are reversible, all you need to do is reverse the sequence.

Using steppers you'll get a good holding torque, that is, your robot might be able to sit on an angled plane without requiring power to the motors.

A disadvantage of steppers is that they require a complex drive mechanism for efficiency (commanding one in bipolar, you'll need two h bridges, that's four h bridges for two wheels). The are ICs for this job, though.

As for buying them, I'd definitely say ebay. You can get really nice deals there.
nanotec.de has the real stuff, if you have the money for it.

edit: also, steppers aren't very speedy (i noticed the rpm just now). From my experience, to reach their maximum speed, they require some acceleration. it's an odd type of acceleration, meaning they can get to full speed in a few tenths of ms, but they can't get there instantly. also remember that the higher the speed, the lower the torque. plus, if you go too fast, when they stop, the steppers will have to counter the robot's momentum, so the wheels might drift a little, so you'll have to account for acceleration and deceleration too in your final firmware

[airman00]

thanks that clear it up a lot! 

so should I go with steppers or with DC motors and encoders?

[izua]

what are you trying to achieve?

[airman00]

what are you trying to achieve?

odometry on a robot

[ed1380]

have you thought of using a mouse's optical sensor

[izua]

You'll get some degree of precision with steppers, but the control just ain't worth it if microstep is needed.
I think you're better off going with DC/hacked servos with encoders.
Why? Well, steppers give you some accuracy, butI can never know if a step is realised or failed. So I'll need some encoders. And since I'm already using encoders, why should I bother myself with an additional level of complexity adding ICs for the stepper, when I can just use a DC motor?

Of course, you'll need to write software to process the information from the encoder, and you'll also have to look for a decent encoder

[Admin]

The RPS will be in the datasheet.

There are some disadvantages to steppers mechanically, too. At low speeds, a robot using stepper motors will vibrate. Why? Because a stepper will jump at full speed to each step, then stop until it is commanded to go to the next step. At low speeds this could cause your robot to make clatter sounds, or cause nuts to come loose . . .

I'd recommend just taking the option that is cheapest and easiest for you to do, but still does the job. This means you need to design both systems before you can compare . . .

[wdbarnwell]

You probably have your robot built by now, but you can get stepper motors with encoders. Soda machine made by Royal use them on all of their models, including the newer glass door vendors. Dixie Narco, and the Vendo company uses DC gear motors with encoders on their models. How do I know this? I work for Royal Reman and we remanufacture soda machines.