I'm looking for a relatively inexpensive solution to control/drive four stepper motors via USB or RS232 serial -- something I can connect to a PC, which will be doing the actual control. Everything I've found through google and part-searching online is in the $200+ per axis range, and I really can't afford to pay that kind of cash right now. Does anyone know of any cheaper solutions? I don't know the details of the motors I'll be driving yet, unfortunately, but in theory things should be relatively well balanced so they should be fairly low torque/amperage.

Also, how do closed-loop servos compare to stepper motors on power and accuracy? I understand that controller hardware can be found cheaper, but I don't know where to get a solid comparison of the differences between the two technologies.

 

The drivers depend a great deal on your stepper motors (obviously), but normal printer/floppy drive motors can usually be driven from four small transistors. You could use a PIC to receive the RS232 data and feed the drivers with the required stepper pattern.

It depends entirely what you want to do with them, they are very different devices. A stepper rotates completely, a servo usually only a limited angle. A servo is far more powerful than a stepper, comparing similar pricess, and a servo provides absolute positioning, a stepper only provides relative positioning.

They both have advantages in certain situations, it's not really very often you have a choice of both - due to their totally different methods of action.

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Well, here's the deal: I'm working on engineering a motion-control system for a camera jib I'm planning to build for my film and DV cameras -- I'm a film student. I'd like 4-axis control -- pitch and yaw at the base (to rotate the base and raise the arm) and pitch and yaw at the head (to position the camera itself). I say this is a relatively low-torque application because with proper counterweighting, very little force should be required to move the base (pitch), although I haven't yet done the analysis to prove it. I need about 180 degree range of motion for everything but the base pitch (which is limited to about 45 degrees by the physical design.

I was thinking stepper motors first, because they offer finer control, but servos obviously have the price per power advantage.

 

This is an obvious job for a servo, stepper motors don't provide absolute positioning - so you've no reliable idea where the camera is pointing. With servos you can tell it to go to a specific point (such as go to 25 degrees left of centre) - with a stepper you could only give it relative commands (such as go to 25 degrees left of where you presently are).

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Good point, but with two caveats:

1st Caveat: I know stepper motors can have their speed smoothly varied with relative ease. Can servomotors' speed be varied similarly? My only experience with servos comes from RC hobby use, and in that context, servos move to the final position quite rapidly, and tend to "twitch" slightly if moved slower (say, by very slowly moving the control stick on your RC transmitter). This minor twitching isn't a big deal for an RC plane's ailerons, but with a camera that needs extremely smooth motion, it could pose a problem. Is there a reasonable way around this issue?

2nd caveat: most servomotors (at least RC servos) have a 180 degree range of motion. The base pivot for my design is probably going to use a large bearing assembly. I was originally hoping to machine my own base, with gear teeth around the outer edge that would gear to a smaller gear on the motor to turn the whole assembly. A hobby servo wouldn't cut it for this. Can anyone suggest a different base design, or a different type of servomotor, that would do the deed?

 

Servo's, by their very nature, have a 'dead band', a small area where they don't move, this is to stop the servo continually moving once it gets to it's intended destination.

Stepper motors don't actually 'turn', they move in single steps - so a slow movement is just a series of small steps.

You can obviously use a mechanical mechanism off a servo to increase the amount of travel, simple gearing, or even a rack and pinion system (with the servo sliding the rack). But any such system is gearing the motor up, reducing it's torque.

You could incorporate the entire base in the servo loop. with a potentiometer connected to the moving part of the base, and a motor (either DC or stepper) driving the gear assembly. There's nothing to stop you using a stepper motor in a servo system (apart from cost).

You could do it quite easily using a PIC, this could drive a stepper or DC motor (via suitable drivers), read the position of the pot via one of it's analogue inputs, and compare it to the reference position it's been asked to move to. The input could come via RS232, or any other method you cared to use.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

Thank you much for the helpful feedback, Nigel. Cheers!

-Ryan Spicer