Chapter 10: speedJ and speedL

About This Chapter

The moveJ and moveL commands discussed so far instruct the robot to move to a target position.

By contrast, speedJ and speedL are commands for moving at a specified velocity.

They are used for joystick operation, visual servoing, tracking control, and similar applications.

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10.1 Differences from Move Commands

moveJ:

Target position

↓

Controller generates trajectory

↓

Stop

speedJ:

Target velocity

↓

Maintain velocity

↓

Continue moving until a stop command

This is the most important difference.

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10.2 What Is speedJ?

speedJ specifies joint velocities.

It specifies the velocity of each of the six axes.

Example:

qd = [
    0.1,
    0.0,
    0.0,
    0.0,
    0.0,
    0.0,
]

rtde_c.speedJ(qd, 1.0, 2.0)

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10.3 Arguments

speedJ(
    qd,
    acceleration,
    time
)

Argument	Description
qd	Joint velocities
acceleration	Acceleration
time	Execution time

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10.4 qd

Joint velocities:

[
dq1,
dq2,
dq3,
dq4,
dq5,
dq6
]

Unit:

rad/s

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10.5 time

For example, a value of:

2.0

maintains the specified velocity for two seconds.

Some implementations also use a value of 0. However, the handling of execution duration depends on the ur_rtde version, so check the API for the version in use.

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10.6 speedL

speedL specifies the TCP velocity:

[
vx,
vy,
vz,
wx,
wy,
wz
]

Units:

m/s

rad/s

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10.7 speedL Example

speed = [
    0.1,
    0.0,
    0.0,
    0.0,
    0.0,
    0.0,
]

rtde_c.speedL(speed,0.5,2.0)

The TCP moves in the X direction at 0.1 m/s.

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10.8 speedStop

Unlike Move commands, velocity control does not stop automatically.

At the end of the operation, call:

rtde_c.speedStop()

Alternatively, use:

stopJ()

stopL()

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10.9 Characteristics of Velocity Control

Move commands:

Position control

Speed commands:

Velocity control

The final position is not guaranteed.

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10.10 Applications

Joystick:

Joystick

↓

Velocity

↓

speedJ

Visual tracking:

Camera

↓

Velocity calculation

↓

speedL

Force control:

Force

↓

PID

↓

speedL

These are typical examples.

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10.11 Comparison with Move Commands

Item	moveJ	speedJ
Input	Position	Velocity
Termination	Automatic	Stop command
Application	Teaching	Real-time control

Item	moveL	speedL
Input	Pose	TCP velocity
Termination	Automatic	Stop command
Application	Processing	Tracking control

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10.12 Common Mistakes

If you execute only:

speedJ(...)

as though it were a Move command, the robot may continue moving until a stop command is sent.

For real-time control, always design the complete stopping sequence.

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10.13 Design in ur_ctrl.py

For general applications, it is convenient to expose high-level APIs such as:

speed_j()

speed_l()

stop_motion()

It is recommended to use velocity commands together with corresponding stop commands.

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Summary

speedJ and speedL are APIs for controlling velocity rather than position.

They are more important than Move commands for joystick operation and real-time tracking control.

On the other hand, moveJ and moveL are used more frequently for ordinary teaching and automatic operation.

The next chapter explains servoJ and servoL.