Thread: 3 Axis Skull RCservo vs Stepper motor

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I have looked around here, and I have not seen this idea "duked out".

What is the feasability of using stepper motors instead of RCservos in a 3axis skull?

My guess it would be quieter, stronger, and cheaper, but consume more electricity.

Has anybody here tried?
How well/cheap can I control a stepper motor with VSA?


Anybody?

Bueller?

Not sure if you have thought this out.

First what size stepper motors? Do you have some examples? Keep in mind that servos are basically gear motors with position control. Check the oz inch torque of a standard servo.
What stepper motor will offer say 60 oz inches of torque?

Then you have the driver that is required for a stepper motor. Could be placed out side the skull of course, but you are talking a minimum of 4 wires pr motor I believe. I imagine some could be common.

How about the cost? You can get Hitec servos that will work in a skull for $12. Then get a ssc32 board that can control 32 servos $50. VSA will cost another $100. or so. So for less than $ 500 you can have the majority of hardware required to control 8 3 axis skulls with jaw movement.

Where can you get 32 stepper motors with drivers for under $400?

Here is some data on stepper motors.


MODEL NO. MOTOR
CONNECTION
1 = SERIES
2 = PARALLEL
3 = UNIPOLAR MOTOR
LENGTH
mm (in)
HOLDING
TORQUE2
(oz-in) LEADS STEP
ANGLE
(DEG) VOLTS AMPS OHMS MH ROTOR
INERTIA
(oz-in2/
g-cm2) MOTOR
WEIGHT
g (lb)
OMHT11-013 2 48 (1.87) 15 4 1.8 2.0 1.0 2.0 2.6 0.098/18 177 (0.39)
OM5014-842 2 40 (1.57) 26.0 4 1.8 4.8 1.0 4.3 5.5 0.109/20 213 (0.47)
OMHT17-075 1 47 (1.85) 62.8 8 1.8 5.7 0.85 6.6 12.0 0.37/68 331
(0.73)
2 2.8 1.70 1.7 3.0
3 44.4 4.0 1.20 3.3 3.0
OMHT17-275 1 48 (1.90) 62.3 8 1.8 5.7 0.85 6.6 10.0 0.44/82 357
(0.79)
2 2.8 1.70 1.7 2.5


Check out the unit that offers 62.8 Oz Inch of torque. It weighs .73 lbs. With 4 servo's that would add 3 lbs to the skull. A standard servo like the HS425BB weighs 1.6 OZ. 57 Oz Inch of torgue at 6 volts.

What about something like this;
http://www.alltronics.com/cgi-bin/it...-stepper-motor

with an IC based controller;
http://www.alltronics.com/cgi-bin/it...ectronics-L297

Yes there is more work involved than just these parts, but even so you are still in the ballpark of an RC servo.

I guess the real questions about torque needed is this, if the rod end bearing is shouldering the weight of the skull, how much torque is needed to move it in the specific axis?

Not trying to reinvent the wheel here, or am i?

What is the torque of the motor please.

That one doesn't give torque rating, it takes big money (from alltronics) to get a torque rating for the motor;
http://www.alltronics.com/cgi-bin/it...-stepper-motor

THis one give 10 oz-in. Although this looks to be a larger chassis.

Again, what is the amout of torque that is usually required?

A couple things not being addressed here...

1) Steppers aren't absolute positioning devices. Servos 'know' where they are at any time, because of the built-in feedback pot. Steppers, on the other hand, have no idea. You can only tell them to move relative to where they started up. So, you normally need a 'homing' routine (keep moving one direction, until you hit a switch - then you know where you are).

2) Steppers use just as much current, or more, and they use it all the time. With servos, the lighter the load, the less current they draw.

Servos have been used in retail R/C for forever. They're wayyyy down the line, on the technology/cost curve. As far as bang for the buck, you probably can't do better. On the other hand, if you don't mind spending some more, digital hobby servos might be a step up. Beyond that, industrial servomotors would probably be the ($$$) next step after that.

- Hook

With the nod axis and a bucky skull, anything less than 50 oz inch is not enough. At least not if you want decent movement. In my design I use a HS425BB with a 2 to 1 ratio. I get 90 Degrees of nod. I also have a counter spring that puts the skull at level with no load on the servo. The standard three axis you see does not provide that much movement. But often even with the 60 oz inch servo a counter spring is required. So bottom line. We need 50 oz inches of torque. Now with a stepper motor you could always gear it to get the torque. But you also have limited room inside the skull. Stepper motors are almost 2 inches in diameter. And once again let me remind you that a servo weighs 1.6 oz. You have 4 stepper motors inside a skull, you just added minimum 2 pounds. That just increased the torque requirement a ton.

Hooked,

I think the lack of feedback is equally important. I just felt the weight and torque issue was the first and foremost obstacle to deal with.

Thanks to both of you. I am just looking for ways to cheapen my wish list. I guess that steppers isnt it.