IEEE Robotics & Automation Magazine - December 2015 - 102

Remote Computer
User Program
Input Device

KRC
JOpenShowVar

Input Driver

TCP/IP

CrossComClient
xt

writeVariable

KRL
KRC
Actuator
Kinematics
Program

KUKAVARPROXY

(a)
Remote Computer

Input Device

KRC

User Program
xt
Control Algorithm
(Kinematics)
it

JOpenShowVar

TCP/IP

KRL Actuator
Program

CrossComClient
writeVariable

ia
readVariable

KUKAVARPROXY

(b)
Figure 4. (a) The user program utilizes JOpenShowVar to set the desired end-effector position and then the robot joints are calculated
by the KRC using the standard kinematic model. (b) A custom control algorithm can be implemented by the user to calculate the
joint values for the robot and then send these angles to the KRC to be actuated. (Photo courtesy of KUKA Robotics Corporation.)

o = f v-1 (i a, xo d ),

if operating in position control mode, or by:
it

= i a + io d Dt,

(4)

if operating in velocity control mode.
When the operator maneuvers the manipulator, a vector signal with no semantic, s, is sent from the input device to the user
program. Here, according to the operational scenario, the vector
signal is interpreted as the target position x t . If the intent is to
use the standard kinematics provided with the KRC, the user
program simply works as a driver for the input device and uses
the writeVariable method of JOpenShowVar to forward x t to a
KRL program where the standard KRC kinematics is used to
calculate the joint angles i d . Alternatively, a custom control algorithm can be implemented within the user program to calculate the joint values for the robot according to x t . Essentially, the
custom control method has to implement classic inverse kinematic functions that can be generalized as follows:
id

= f -p 1 (x d ),

(5)

concerning position control, and

Algorithm 2. A use case for the new simpleRead
and simpleWrite methods.
try (CrossComClient client = new CrossComClient
("robotIPaddress", 7000)) {
System.out.println(client.simpleRead("$OV_
JOG"));
System.out.println(client.
simpleWrite("$OV_JOG", "90"));}

102

IEEE ROBOTICS & AUTOMATION MAGAZINE

DECEMBER 2015

(6)

for velocity control, where i a is the actual joint angles vector
that can be retrieved using the readVariable method of
JOpenShowVar. These values are then forwarded to a KRL
program where the standard KRC functions are used to actuate the robot.
Note that the possibility of implementing certain control
features does not influence the design for the presented interface. Instead, JOpenShowVar extends the functionalities of
the KRL language.
Additional Functions
To simplify the low-level communication protocol and improve reliability, some additional methods are provided with
the CrossComClient class as follows.
● The simpleRead and the simpleWrite methods are simpler
versions of the sendRequest function. In particular, these
methods do not execute any data parsing as opposed to the
sendRequest method. They allow for an easier and faster
access, as shown in Algorithm 2. The two new methods return a raw string without parsing the information. The aim
of these two new methods is to provide an easy way to
monitor the status of the robot making it possible to print
the raw information returned from the KRC.
● The readJointAngles method uses the readVariable method retrieve the actual joint angles vector, i a, of the controlled robot, all at once.
● The readJointTorques method allows for monitoring the
load of each joint actuator by retrieving the current
torque of each axis of the arm, all at once. In particular,
readJointTorques retrieve the global KRL array variable

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - December 2015