AMCI SD4840EK, User Manual

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M
OVE
P
ROFILES
SD4840EK User Manual
ADVANCED MICRO CONTROLS INC.
20
Definitions (continued)
Starting Speed
The Starting Speed is the speed that most moves will begin and end at. This value is set while configuring the
unit and it has a valid range of 1 to 1,999,999 steps/second. This value is typically used to start the move
above the motor’s low frequency resonances and, in micro-stepping applications, to limit the amount of time
needed for acceleration and deceleration. AMCI does not specify a default value in this manual because it is
very dependent on motor size and attached load. With that said, a starting speed between 0.25 and 0.5 RPS is
usually a good starting point.
Target Position
The Target Position is the position that you want the move to end at. There are two ways to define the Target
Position, with relative coordinates or absolute coordinates.
Relative Coordinates
Relative coordinates define the Target Position as an offset from the present position of the motor. Most
SD4840EK moves use relative coordinates.

The range of values for the Target Position when it is treated as an offset is ±8,388,607 counts. Assum-
ing that the motor is wired to the SD4840EK as shown in this manual, positive offsets will result in
clockwise moves, while negative offsets result in counter-clockwise moves.

The Motor Position value reported back to the host can exceed ±8,388,607 counts. The only way to
move beyond ±8,388,607 counts is with multiple relative moves or jog commands.
Absolute Coordinates
Absolute coordinates treat the Target Position as an actual position on the machine. Note that you must set the
Home Position on the machine before you can run an Absolute Move. (See
Home Position
on the previous
page.)

The range of values for the Target Position when it is treated as an actual position on the machine is
±8,388,607 counts. The move will be positive if the Target Position is greater than the Current Position
and negative if the Target Position is less than the Current Position.

The Motor Position value reported back to the host can exceed ±8,388,607 counts. However, you can-
not move beyond ±8,388,607 counts with an Absolute Move. The only way to move beyond
±8,388,607 counts is with multiple relative moves or jog commands.
Definition of Acceleration Types
Most of the move commands allow you to define the acceleration type used during the move. The SD4840EK
supports three types of accelerations and decelerations. The type of acceleration used is controlled by the
Acceleration Jerk
parameter.
What is jerk?
Just as speed is a measurement of change in position per unit time and acceleration is a measurement of
change in speed per unit time, jerk is a measurement of change in acceleration per unit time. Likewise, just as
a change in position equals speed * time,

p = s(t), and a change in speed equals acceleration * time,

s =
a(t), a change in acceleration equals jerk * time,

a=j(t). Jerk has units of steps/sec
3
.
The SD4840EK uses the jerk property to smoothly change the acceleration applied during the move. In this
case, the speed of the move does not increase linearly, but exponentially, resulting in an “S” shaped curve.
This limits mechanical shocks to the system as the load accelerates.