INTRO TO STEPPER AND MOTION CONCEPTS

ADC steppers are all open-loop, which means they do not have any feedback to verify that the motor actually moved the correct distance. But, they can be very reliable

and repeatable when correctly applied and sized, and can often make very cost-effective choices for motion control, when the accuracy and speed requirements are modest.

Our new advanced stepper drives (4850 and 80100) can do point-to-point positioning, by using serial ASCII commands (SCL mode), which our 06, 260 and Click plc's do very

well. This was not available with our previous stepper drive, which could only follow a pulse train input.

What do we mean by "point-to-point positioning"? Well, the user is usually concerned with feeding something a certain length ( dispensing tubing, feeding plastic sheet, etc), or

moving from point A to point B. With our new stepper drive, we can do either ( and more advanced variations of each).

Imagine a punch press, or a cutting machine that performs a certain operation on a section of "material", and then needs a new section fed into the machine for the next cycle.

This would be "Feed to Length", since we are concerned only about the length of the current move. This is a variation that is also called "Relative" or "Incremental"

positioning.

Now, imagine a milling machine table, or the ways of a lathe. We can only go a certain distance in either direction, and our position is usually very important in relation to our

previous or our next position. If we are drilling a series of holes, we usually have a very important dimension from the center of any adjacent holes, but also, from the end of the

part as well. This is known as "Absolute" positioning, since we will probably have important distances to other positions, and we want to maintain the overall relationship

between all the positions ( "Absolute" relationships).

Visualizing a ruler is a useful concept. The 5" line specifically means 5" from one end. If we were to setup our drive for Absolute positioning, and we have a Home sensor at the

end of the ruler ( 0") and after Homing is complete, the drive considers itself to be at 0". By sending a position of "5", the drive would go to the 5" mark. Now, the crucial

difference between Incremental and Absolute positioning: In Incremental mode, by sending a position of "2", the drive would feed in the same direction and come to a stop at

position "7" ( 5+2=7). In Absolute mode, by sending a position of "2", the drive would actually reverse itself and move backwards to the 2" line. So, remember, use a stationary

ruler as your visual reference for Absolute mode, and you shouldn't have any problems (other than the actual application, of course!).

Now , a quick explanation of how the stepper actually makes a move ( the "profile"). The drive can accept values for Accel, Decel, Velocity, and Distance for a basic move. The

drive accelerates at the rate set by Accel (AC), until it reaches the Velocity (VE) setting. It will then continue until it reaches the point in the move at which it must start

decelerating (DE) to stop at the move overall distance (DI). These 3 commands are based on revs per second. If we set AC (Accel) to 5, that means that the unit will gain

5 revs per second, each second it is still below the Velocity setting. After 1 second, it will be at 5 rps (revs per sec) and after 2 secs, 10 rps, and so on. Decel operates in the

reverse: If we were traveling at 10 revs per sec, and our DE (Decel) setting was at 5, it would take the drive 2 secs to come to a rest.

Normally, in point-to-point positioning with a stepper, the precise values for AC, DE, and VE aren't critical, and the user sets the values to whatever his load or application will

allow, or what works best. Since our stepper systems don't have feedback, a more conservative move profile will provide better consistency.

Note the two profiles at the left. The far left one has symmetrical ramps,

meaning that Accel and Decel rates are the same.

The next profile has a longer Accel ramp, and goes to a higher velocity,

and has a steeper Decel ramp.

But, both profiles cover the same distance, and your particular load/app

may need a longer time accelerating, but can actually decelerate faster.

You have control of the motion you want your load to follow.

The most commonly used commands of the drive will be the following:

(Note that all characters are in capital letters. This is the required format)

Accel

Distance

Feed with Speed Change

Wait

Commence Jogging

Decel

Distance Change

Feed to Length

Wait for Input

Change Speed

Velocity

Velocity Change

Feed to Position

Stop Jogging

Electronic Gearing

Feed to Sensor

Motor Disable

Motor Enable