M O T I O N C O N T R O L P R O G R A M M I N G
CONTROL VARIABLES &
I/O INTERFACE
Motion Control Programming
103
The host downloads this program to the controller
memory, where the program remains inactive. To execute
the program, the host sends the command:
XQ#A
which causes the controller to execute the program labeled
#A.
The DMC can receive commands from both the host
computer and the stored program and execute these instruc-
tions simultaneously.
Control Variables
Many motion applications include variable parameters. For
example, a cut-to-length application often requires that the
cut length be variable. The motion process is the same, but
the length is changing. To accommodate these applications,
advanced controllers provide symbolic variables. A program
can be written in which certain parameters, such as position
or speed, are defined as variables. The variables can later be
assigned by the operator or determined by program calcula-
tions. Variables allow the motion controller to perform cer-
tain mathematical functions and to make decisions accord-
ingly. This capability increases the intelligence of the motion
controller and allows it to perform some supervisory func-
tions in addition to the simple motion control.
A variable can be defined as a constant or can be equat-
ed to a controller parameter. For example, the instruction:
V = 3
sets the value of the variable V to 3. The instruction:
P = _TPX
reads the position of the X motor and equates the variable P
to that value. Variables can be defined in a variety of ways.
For example, the instruction:
YERROR = _TEY
equates the variable YERROR to the position error of the Y
motor.
Once the variable is defined, it can be used in mathemat-
ical operations. The DMC can perform mathematical func-
tions which typically include algebraic, trigonometric, and
logical operations.
When the operation is completed, the controller can use
the computation result to adjust the system parameters. For
example, the results can be used to change the speed, set the
distance, or change the filter gain. The use of variables is best
illustrated by the following example.
The following program shows an example where the X
motor follows the position of the Y motor. This is done
by driving the X motor at a speed proportional to the
position difference.
Instruction
Interpretation
#FOLLOW
Program name
DP 0,0
Define X,Y position as zero
JG 0
Set initial X speed to zero
BGX
Start X
#LOOP
Label
VE=_TPY-_TPX Find the position difference
VEL=VE*10 Compute the speed
JG VEL Modify the speed
JP #LOOP
Repeat the process
EN
End of program
I/O Interface
It is often desirable to synchronize motion with input/ out-
put (I/O) events, which enables the controller to control a
complete process. As such, the DMC can read input signals,
both digital and analog, and can generate digital output sig-
nals. Inputs may be control signals from digital push-buttons
or analog potentiometers. Outputs can be used to activate
solenoids or valves or to turn on indicator lights.
Input signals, both digital and analog, can be read by the
controller and their values can be stored in control variables.
These variables may be used later in motion programs.
The reading of digital inputs may be performed with the
instruction:
DIGITAL = @ IN[2]
