Six-Axis Robot Configuration Singularities Use of the V+ MV.SL_MOVE Routine

Six-Axis Robot Configuration Singularities Use of the V+ MV.SL_MOVE Routine
Six-Axis Robot Configuration Singularities
Use of the V+ MV.SL_MOVE Routine
and the SPEED.LIMIT Parameter
March 30, 2007
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Table of Contents
1 .....Introduction................................................................................................. 4
2 .....Understanding Robot Configuration Singularities ................................. 4
Types of Singularities ....................................................................................................... 4
Wrist Singularity ......................................................................................................... 4
Alignment Singularity................................................................................................. 5
Elbow Singularity ....................................................................................................... 5
3 .....Six-Axis Robot Configurations .................................................................. 5
RIGHTY versus LEFTY................................................................................................... 6
ABOVE versus BELOW .................................................................................................. 7
FLIP versus NOFLIP ........................................................................................................ 8
CONFIG Function ............................................................................................................ 8
4 .....Dealing with Robot Configuration Singularities ..................................... 9
Define the areas where the robot is moving close to singularities.................................... 9
Change the motion from straight-line to joint-interpolated .............................................. 9
Use the V+ Routine MV.SL_MOVE.......................................................................... 9
Use the SPEED.LIMIT V+ Parameter...................................................................... 11
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1 Introduction
This document describes configuration singularities experienced by six-axis robots during straight-line
motion.
The V+ MV.SL_MOVE routine and the V+ SPEED.LIMIT parameter are discussed, as well as other
measures for avoiding configuration singularities.
2 Understanding Robot Configuration Singularities
A configuration singularity can be defined as a location in the robot workspace where two or more joints no
longer independently control the position and orientation of the tool. As a robot executes a straight-line
motion that moves close to a configuration singularity, the robot joint speeds necessary to achieve that
motion become excessive.
Types of Configuration Singularities
The types of configuration singularities that can be experienced by a robot depend on the physical
relationships between the robot joints. The following sections describe the most common types of
configuration singularities that can occur.
Wrist Singularity
Wrist Singularity occurs when the axes of Joints 4 and 6 are aligned.
Figure 1. Wrist Singularity
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Alignment Singularity
Alignment Singularity occurs when Joint 6 (wrist) and Joint 1 axes are aligned. (This has not yet occurred
in the following figure, but is about to.)
Direction of tool-tip
motion
Joint 6 Axis
Joint 1 Axis
Figure 2. Alignment Singularity
Elbow Singularity
Elbow Singularity (not illustrated here) occurs when the arm is fully extended. In this case, as the elbow
joint becomes further extended, higher joint speeds are required to maintain constant Cartesian speed. The
robot cannot extend beyond its reach.
This document does not deal with this type of singularity, because it is not considered internal to the robot
workspace.
3 Six-Axis Robot Configurations
A six-axis robot can move to a location defined by a Cartesian (World) transformation with different joint
positions and, consequently, different robot configurations. If you want the robot to move to a location in
space with a specific configuration you must use a precision point to define that point and orientation.

Precision points specify the joint values for all the joints of the robot, so the configuration of the
robot is specified explicitly. (Note the exception that follows.)

Cartesian coordinates and orientation angles specify the tool position and orientation, but the
configuration is not specified.
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A robot tool-tip can move in a straight line to a location defined by a Cartesian transformation or precision
point. This is achieved, in V+, by using APPROS, DEPARTS, and MOVES instructions. It is important to
note that the robot configuration can not change during straight-line motions.
NOTE: These three commands calculate the position specified by a precision point, and convert that
into a transformation. The configuration will remain as it was before the command, even if the joint
positions specify otherwise.
Below are the different robot configurations for the Adept Viper s650 robot. Note that, for all the of
configurations shown, the position and orientation of the robot tool-tip is represented by the same Cartesian
transformation. That is, the robot tool can be moved to this location and orientation with all the different
configurations shown.
RIGHTY versus LEFTY
The following figures illustrate RIGHTY versus LEFTY configurations of a Viper s650.
RIGHTY
LEFTY
Figure 3. RIGHTY/LEFTY Configurations
The following V+ code snippet demonstrates the use of the RIGHTY and LEFTY program instructions:
RIGHTY
MOVE loc_a
; Request change in robot configuration during next motion
; Move to loc_a transformation in RIGHTY configuration
LEFTY
MOVE loc_a
; Request change in robot configuration during next motion
; Move to loc_a transformation in LEFTY configuration
NOTE: The RIGHTY and LEFTY program instructions can include the keyword ALWAYS, which
causes the instruction to continue until it is explicitly disabled. For example:
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RIGHTY ALWAYS ; Request change in robot config. during subsequent motions
MOVE loc_a
; Move to loc_a transformation in RIGHTY configuration
ABOVE versus BELOW
The following figures illustrate ABOVE versus BELOW configurations of a Viper s650.
ABOVE
BELOW
Figure 4. ABOVE/BELOW Configurations
The following V+ code snippet demonstrates the use of the ABOVE and BELOW program instructions:
ABOVE
MOVE loc_a
;Request change in robot configuration during next motion
;Move to loc_a transformation with ABOVE configuration
BELOW
MOVE loc_a
;Request change in robot configuration during next motion
;Move to loc_a transformation with BELOW configuration
As with the LEFTY and RIGHTY program instructions, the ALWAYS keyword can be used with ABOVE
and BELOW.
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FLIP versus NOFLIP
The following figures illustrate FLIP versus NOFLIP configurations of a Viper s650.
FLIP
NOFLIP
Figure 5. FLIP/NOFLIP Configurations
The following V+ code snippet demonstrates the use of the FLIP and NOFLIP program instructions:
FLIP
MOVE loc_a
;Request change in robot configuration during next motion
;Move to loc_a transformation with FLIP configuration
NOFLIP
MOVE loc_a
;Request change in robot configuration during next motion
;Move to loc_a transformation with NOFLIP configuration
The ALWAYS keyword can be used with FLIP and NOFLIP.
CONFIG Function
To learn the robot’s configuration, you can use the V+ CONFIG real-value function:

CONFIG(0) – The returned value represents the current (instantaneous) configuration of the robot.

CONFIG(1) – The returned value represents the configuration the robot will achieve at the end of
the current motion.

CONFIG(2) – The returned value represents configuration the robot will achieve at the end of the
next motion.
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Examples:
CONFIG(0) = 0 indicates that the robot has the following configuration:
LEFTY, ABOVE, NOFLIP
CONFIG(0) = 4 (only bit #3 set) indicates that the robot has the configuration:
LEFTY, ABOVE, FLIP
Bit # Bit Mask
Indication if bit SET
1
1
Robot has RIGHTY configuration
2
2
Robot has BELOW configuration
3
4
Robot has FLIP configuration
4 Dealing with Robot Configuration Singularities
This section describes several methods for avoiding robot configuration singularities.
Define the areas where the robot is moving close to configuration singularities
The first step in dealing with configuration singularities is to characterize where in the robot work-envelope
the singularities exist. These configuration singularities are described in the section “Understanding Robot
Configuration Singularities”. Try to design your application to avoid these areas of singularity.
Change the motion from straight-line to joint-interpolated
Straight-line motion is achieved with the V+ language through the APPROS, DEPARTS, and MOVES
instructions. During motion driven by these instructions no changes in configuration are allowed, and the
robot joints may require excessive speeds to achieve the requested Cartesian motion.
To avoid this situation:

Use the MOVE instruction instead of the MOVES instruction, (since the robot can change
configuration during a MOVE instruction to avoid a singularity). See the section “Use the V+ Routine
MV.SL_MOVE” for information on using this routine.

If you must use straight-line motion near robot configuration singularities, set the SPEED.LIMIT
parameter so that the robot will stop when moving close to a configuration singularity, instead of
attempting to move certain joints at excessive speeds in order to maintain the straight-line motion. See
the section “Use the SPEED.LIMIT V+ Parameter” for information on setting this parameter.
Use the V+ Routine MV.SL_MOVE
The V+ routine MV.SL_MOVE, which is available in the SLMOTION utility, is provided as an alternative
to the MOVES instruction, for use when the robot might move close to a configuration singularity.
The MV.SL_MOVE routine divides a transformation vector into smaller vectors. It then takes each small
vector (transformation) and, using the V+ program instruction SOLVE.ANGLES, computes the robot joint
positions for this transformation. After performing the relevant checks, the robot is moved, in jointinterpolated mode, as defined by the calculated joint positions. In other words, the program uses many
MOVE instructions to move the robot by small steps along the desired straight-line path.
It is recommended that the MV.SL_MOVE routine be used when the robot is moving close to a Wrist
Singularity (see Wrist Singularity), because during this motion, Joint 4 speed is controlled and there is little
risk of other robot joints moving unexpectedly to maintain straight-line tool-tip motion.
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If the MV.SL_MOVE routine is used when the robot is moving close to an Alignment Singularity (see the
section “Alignment Singularity”), Joint 1 could rotate by 180° in order to maintain straight-line motion.
This could damage cell tooling or peripheral equipment in the cell.
For more details, see the description of the SLMOTION utility in the Instructions for Adept Utility
Programs.
A Note on Conveyor Tracking
Conveyor tracking requires straight-line motions, but the MV.SL_MOVE routine is generally too slow to
keep up with conveyor parts. Therefore, the program does not support use with conveyor tracking. The celldesigner must be aware of the possibility of configuration singularities as the robot tracks a moving belt,
because the robot motion is effectively governed by the belt motion.
The following cell-design guidelines can prevent the robot from moving close to configuration singularities
while conveyor tracking:

Design the end-effector so that, while tracking parts on the conveyor, Joint 5 stays well away from
zero, for example between 45˚ and 90˚. This will prevent Joint 4 – 6 wrist singularities. You can
use the V+ ALIGN program instruction to align Joint 6 with the belt surface.

Design the cell so that the Joint 1 and Joint 6 axes will never align, thus preventing alignment
singularities.

Define wait locations with precision-points, and use the V+ MOVE instruction to go to the wait
locations. This will ensure consistent robot configurations.

Choose the conveyor belt window so that the robot cannot fully extend itself.
Figure 6 shows a wrist singularity (described in section 2), which occurrs as the robot tracks a part on the
conveyor belt. The robot has an end-effector that is offset from the robot flange by a distance of Tz in the
tool Z direction, and Tx, in the tool X direction.
Figure 6. Wrist Singularity During Conveyor Tracking
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This type of singularity can be avoided by adjusting the belt height (h) . To find the optimum belt height:
1.
Move the arm so that Joints 4 and 6 are aligned (as shown in Figure 6).
2.
Note the distance (d) where the robot will access the part on the near edge of the conveyor.
3.
Calculate the minimum belt height (h) using the following equations:
L12  (d – Tx) 2  (L2  Tz – h) 2
 h  (L2  Tz) – L12 – (d – Tx) 2
Use the SPEED.LIMIT V+ Parameter
A system parameter is provided in the V+ system (version 16.3D3 and later) to offer more functionality in
controlling robot behavior as the robot moves close to a configuration singularity.
For a one-robot system, the parameter can be set with an instruction or Monitor command in this form:
PARAMETER SPEED.LIMIT = value
For a system with multiple robots, the desired robot must be specified as follows:
PARAMETER SPEED.LIMIT[robot number] = value
NOTE: If the robot number is omitted or zero in a PARAMETER command or instruction, the settings
for all robots will be altered.
In either case, the value specifies the maximum allowable joint speed as a percentage of the maximum joint
speed of the robot.
Use of this parameter requires care, as it may introduce undesired limits on other existing robot motions.
When the SPEED.LIMIT parameter has not been set, a constant speed is enforced on the tool-tip motion for
every trajectory setpoint along straight-line motions, and additionally a speed limit is enforced on the joint
motions on an average basis for the straight-line motion. This permits joints to experience transitory
periods of exceeding the joint speed limits.
However, after the SPEED.LIMIT parameter is set, the joint speeds are limited to the specified percentage
of their maximum values on every setpoint of every subsequent straight-line trajectory, instead of on an
average over the straight-line motion. As a result, this might restrict some straight-line motions that were
previously achievable.
This functionality is a secondary check on each setpoint of the V+ trajectory before it is sent to the servos.
A trajectory will be started and tracked until the SPEED.LIMIT value is exceeded – when the setpoint is
frozen (causing an abrupt stop in motion) and a V+ error is displayed as: “*Maximum setpoint speed or
acceleration exceeded*”. Note that there is no way to “pre-trap” this error, since it is declared as the
setpoints are sent to the servos. It is possible, however, to use an error reaction routine to catch the error
and determine how the system responds to the error.
The default boot-up value of this parameter is zero (checking disabled) to avoid limiting robot motions
unnecessarily. Therefore the SPEED.LIMIT parameter must be explicitly set in user programs to be active.
The current setting of the SPEED.LIMIT parameter can be determined with these Monitor commands:
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PARAMETER SPEED.LIMIT
PARAMETER SPEED.LIMIT[robot_number]
(When the PARAMETER command is entered with no arguments, the SPEED.LIMIT parameter is not
listed in the output. This particular parameter must be explicitly specified in order to see its setting.)
The current setting can be determined within a program with an instruction in one of these forms:
variable = PARAMETER(SPEED.LIMIT)
variable = PARAMETER(SPEED.LIMIT[robot_number])
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