precision servos (digital) vs. precision stepper (<3.6degree step angle)

[bennyben]

Hello,

I'm new to the world of servos and stepper motors. However, I will be taking on a project that will require a very precise servo/stepper. Part of the project will be a laser mounted to the motor and we need the laser beam to move in as short of increments as possible at moderate distances.

Reasonable performance would be an increment of about 3-5cm when shooting 2.5m away. If my calculations are correct, a stepper motor with 3.6 degrees step angle would move the laser beam 15.7cm on the object. That's not going to be good enough.

I have found that digital servos are supposedly more precise than conventional servos, but I haven't found any data that tells me how precise they are.

Thanks, Ben

[Stevie]

you need to micro step; there are plenty of boards out there to do it

[John S.]

Most stepper motors are 1.8 degrees per step.
So given from your figures that 3.6 degrees will move 15.7 cm then 1 degree will move 4.36cm
Taking 1.8 degrees and microstep this by 10 will give 0.18 degrees which will give 0.79cm on the target.
Is this good enough?
See http://www.geckodrives.com for more info on micro stepping motor drives, You need the G201.

John S.

[Boros]

Hi bennyben

Digital servos are no more percise than conventional servos.
When I say this I must bring up the fact that digital servos are newer technology and therefore are used in the newer and more accurate systems.

If the new systems used conventional servos they would be able to acheive pretty much the same accuracy.

Servo motors and stepper motors are completely different in the way they go about position control.

Servos use velocity a command system.
Steppers use a position control system.

What determines the accuracy is the encoder pulses per rev.
That is not to say that encoder pulses translate to the accuracy of the servo.

The servo drive also has a great amount of influance. If the servo drives bandwith and gain are not capable of the resultion required then its useless to use a high count encoder.

I've used a standard DC servo motor with a 81000 PPR encoder which acheived much greater accuracy than a digital servo with a 2048 PPR encoder.

Just a note:
Digital servo drives usually use analog encoders.
Conventional(analog) servos use digital encoders.

Thanx

Boros

[Al_The_Man]

That is one heck of a lead! Can you not introduce some reduction of some kind to get higher resolution, if not you should be able to get higher resolution with servo's, depending on what type of controller you intend using, encoders with 10,000 counts and higher are available a 10,000 count used in quad mode gives 40,000 counts/rev so divide your lead distance (distance/rev) by 40,000 gives you the smallest move distance possible.
Al

[bennyben]

I want this to be from scratch as much as possible, so I'd rather not get into designing any microstepping circuits if possible. You said that I would get highest resolution from a servo if i wasn't using any reduction. I'm sure it varies from servo to servo, but I need at least an approximation of what they can do (degree-wise).

????

[Al_The_Man]

No, I said you would get more resolution with reduction. The finest resolution would depend on the type of controller you would use and the resolution of the encoder. For example, if I have a servo with a 1000 line encoder that hase a linear move of .25"/rev then the smallest measurement would be .00025" or 1 rev/1000 that is why with reduction you could achieve even smaller resolution.