Convert Steps to Counts (7344 Motion)

[RWA]

Does anyone on here know if there is a set conversion from counts to steps.  I know that a servo has output in counts and a stepper has output in steps, but is there some relationship between the two?

I have a relationship between steps to mils, but wish to know if I can convert that over to counts-mils without having to recollect data.

[Bob_Y.]

With the servos that I am using, one count = ~.000035mm for about 28K counts per milimeter of linear travel.  Your servo's gearing and encoder will give you your counts/unit.  It is something that varies with the servo or stepper. 

Perhaps if you told us your servo make and model and your stepper make and model, we could look this information up for you.  If you are looking for a formula, that could probably be developed if you gave us more information as well.

Hope that this helps (a little),

Bob

[RWA]

I have done some testing to find the relationship of steps to mils.  I have just been reading a lot about stepper motor units being in steps and the units of a servo being in counts.  As far as I can tell, they seem to be the same thing, or closely related to the same thing.  So, I was wondering if there was some sort of conversion between the two.

For instance, I set up my axis in an absolute position mode.  I load a target destination into the computer, and start motion.  The output comes to be something on the order of 0.033 mils/step.  When the axis is configured in velocity mode and the position returned, the axis has moved something close to 500 000 counts/steps/encoder position/some unknown unit (i know its not steps because the axis did not displace 16 inches).  So I assumed that since it was moving in a continuous manner, it must be acting as a servo and the returned values must be in counts.   I have come across Flexmotion VI's that allow a user to set steps/counts or act as a servo or stepper motor, but nothing that relates counts to steps.

[Jochen]

It looks like there is a misunderstanding. You can't make a motor behave like a servo or like a stepper. These are two completely different types of motors based on completely differen physical working principlest. Thus there is no way to convert the steps of a stepper motor to the (encoder) counts of a servo (=DC) motor. Please have a look at this tutorial.

Best regards,

Jochen Klier
National Instruments Germany

[Buechsenschuetz]

It depends on the motor driver unit. There are servo motor driver units which can be fed with step/dir signals and thus imitate a stepper motor. Some servo motor drivers also convert the motor's generic resolver signals to quadrature encoder outputs, so the whole system looks like a stepper motor setup.

However, I think the original poster is somewhat confused with the definition of steps and counts, and we do not know exactly which set up he is using. Detailed information about the motor and driver unit he uses would be helpful.

[Bob_Y.]

The confusion is one of terms, I think.  The terms stepper and servo are for the mechanical workings of the motor.  It is how they are driven to make them move that differs.  What the original poster was asking about was position feedback.  This is usually related to the type of drive but does not have to be.  A stepper can have an encoder.  A sevro does not have to have an encoder.

That all being said, usually the postion feedback for a stepper is the number of steps.  The position feedback for a servo is often an encoder.  The linear movement for the number of steps or encoder counts varies with the manufacturer and model of the motor and its feedback device. 

I hope that this helps,

Bob

[RManion]

Just to further expand on what has already been said, a stepper motor's "steps" are based upon the way the motor is physically built.  When you tell it to spin, you are physically sending the motor a pulse for each step.  On the other hand, a servo motor's "counts" are based upon the encoder you use.  Encoders can be compared to a wheel with spokes.  While the motor spins, each spoke that passes would be considered a count.  Servo motors spin when you apply a DC voltage, and the encoder tells us how far or how quickly it has turned.

Because of this, a direct conversion from steps to counts is not necessarily possible although if you know the number of steps per revolution on your motor, as well as the number of counts per revolution on your encoder, you can so a math translation.

The drives that were mentioned allow you to control a servo motor as though it were a stepper motor.  These drives are often called "smart drives."

Hope this helps,

Robert

[RWA]

Thanks for the responses! 

I thought the differences were all in how the motor was configured, not built.  The gentlemen who was overseeing this project a few weeks ago kept referring to the motors as stepper motors, so I assume he is correct. 

One thing still concerns me.  Why is there such a difference in the output of the "Read Position" when the axis is configured in Velocity mode as opposed to Absolute Position?

[RManion]

What kind of difference are you seeing?  The Read Position VI should give you information on the current position as read by your feedback source (encoder typically).

Robert