Chapter 4: RTDEControlInterface

About This Chapter

This chapter explains the motion-control APIs provided by RTDEControlInterface in SDU Robotics’ ur_rtde library.

This is the class most frequently used when controlling a UR robot from Python.

By the end of this chapter, you will understand the differences among:

moveJ
moveL
moveP
speedJ
speedL
servoJ
servoL
Stop commands

4.1 What Is RTDEControlInterface?

RTDEControlInterface is a PC-side motion-control interface provided by SDU Robotics’ ur_rtde. It is not a motion API belonging to Universal Robots’ native RTDE protocol.

For example, when you call:

rtde_c.moveJ(...)

The PC-side RTDEControlInterface uses registers in Universal Robots’ official RTDE interface to pass commands and parameters to the controller. On the robot side, SDU’s control URScript reads these values and executes the corresponding URScript motion command, such as movej. URControl executes that command and connects it to actual robot control.

Thus, RTDEControlInterface does not directly invoke the Servo Controller through the official RTDE protocol.

RTDEReceiveInterface handles state acquisition.

4.2 Categories of Motion Commands

Representative RTDEControl commands can be classified as follows.

Category	API	Purpose
Joint interpolation	moveJ	Most common motion
Linear interpolation	moveL	Processing and measurement
Circular interpolation	moveC	Circular trajectories
Combined interpolation	moveP	Trajectories using blending
Velocity control	speedJ	Joint velocity
Velocity control	speedL	TCP velocity
Servo control	servoJ	Real-time control
Servo control	servoL	Real-time control

Understanding moveJ and moveL first is particularly important.

4.3 moveJ

moveJ means Joint Move.

The robot moves toward specified joint angles.

Initial configuration

↓

All joints rotate simultaneously

↓

Target configuration

The intermediate TCP trajectory is not guaranteed.

Arguments

moveJ(
    q,
    speed,
    acceleration,
    asynchronous
)

Example:

q = [
    0.0,
    -1.57,
    1.57,
    0.0,
    1.57,
    0.0,
]

rtde_c.moveJ(q)

Advantages

Fast
Does not require inverse kinematics
Most stable
Suitable for moving between taught positions

Disadvantages

The TCP follows a curved trajectory.

It is not suitable for machining operations.

4.4 moveL

moveL means Linear Move.

The TCP moves along a straight line.

A

────────────

B

The TCP path is guaranteed.

Applications:

Machining
Dispensing
Laser processing
Measurement
Pick and place

Arguments

pose =

[x,y,z,rx,ry,rz]

rtde_c.moveL(pose)

Orientation is represented using a Rodrigues vector.

4.5 Difference Between moveJ and moveL

moveJ

TCP

○

╲

╱

○

moveL

TCP

○────────○

moveJ performs joint interpolation.

moveL performs TCP interpolation.

4.6 moveP

moveP means Process Move.

It smoothly connects multiple points using blending.

○────○────○

↓

Smooth trajectory

Use it when you want to reduce the time spent stopping at intermediate points.

4.7 Acceleration

Both moveJ and moveL allow you to specify:

acceleration

Units are:

rad/s²

or

m/s²

The units depend on the API.

4.8 Speed

speed

specifies the target speed.

Even if you specify a large value, the controller automatically applies limits based on factors such as:

Safety restrictions
Payload
Joint limits

4.9 Asynchronous Execution

When you specify:

moveJ(..., asynchronous=True)

the function returns immediately without waiting for the motion to finish.

moveJ()

↓

Returns immediately

↓

Robot continues moving

With synchronous execution:

moveJ()

↓

Motion completes

↓

Returns

4.10 Stop Commands

Representative stop commands include:

stopJ()

Stops joint motion.

stopL()

Stops TCP motion.

These commands perform a decelerated stop, not an abrupt stop.

They are different from an Emergency Stop.

4.11 Return Values

Commands such as moveJ and moveL normally return:

True

or:

False

False can indicate:

Communication failure
Control failure

However, conditions such as a Protective Stop may also raise an exception.

Therefore, you must handle exceptions as well as check return values.

4.12 Recommended Pattern

try:

    ok = rtde_c.moveJ(q)

    if not ok:
        print("Move failed")

except RuntimeError as e:

    print(e)

This pattern is recommended.

4.13 Usage in ur_ctrl.py

In this library, calls are automatically dispatched as follows:

move()

↓

moveJ()

or

moveL()

Users do not need to work with RTDEControlInterface directly.

Summary

RTDEControlInterface is the class that sends motion commands to the robot.

The most important commands are:

moveJ
moveL

Understanding these two commands is enough to build most applications.

The next chapter explains RTDEReceiveInterface.