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Robot Concepts 10
10.1 Understanding Robot Motion Parameters
When programming a robot, there are several factors that play an important part in
performance. In order to achieve optimum performance from your robot, it is helpful to
have an understanding of how these factors work.
Speed, Acceleration, and Deceleration
Robot speed is usually specified as a percentage of normal speed, not as an absolute
velocity. The speed for a single robot motion is set in N240:4 or <strong>Adept</strong> Speed (see page 81)
for each individual location. Note that the result obtained by the speed value depends on
the operating mode of the robot - joint-interpolated versus straight-line (see
“Joint-Interpolated Motion vs. Straight-Line Motion” on page 140 for details). Whether
in joint-interpolated mode or straight-line mode, the maximum speed is gated by the
slowest-moving joint during the motion, since all the joints are required to start and stop
at the same time. For example, if a given motion requires that the tool tip is rotated during
the motion, which requires joint 4 to rotate, that joint could gate the maximum speed
achieved by the other joints, since joint 4 is the slowest-moving joint in the mechanism.
Using the same example, if joint 4 was not rotated, the motion could be faster without any
change to the speed value.
NOTE: The motion speed specified in N240:4 or <strong>Adept</strong>_Speed must
always be greater than zero for a regular robot motion, or in the range
-127 to 127 when jogging the robot. Otherwise, an error will be returned.
You can use the acceleration parameter to control the rate at which the robot reaches its
designated speed and stops. Like speed, the acceleration/deceleration rate is specified as
a percentage of the normal acceleration/deceleration rate. To make the robot start or stop
less abruptly, set N240:5 or <strong>Adept</strong>_Acceleration to a low value. To make the robot
accelerate and decelerate more quickly, set N240:5 or <strong>Adept</strong>_Acceleration to a higher
value.
NOTE: The value of N240:5 or <strong>Adept</strong>_Acceleration must always be
greater than 0. Otherwise, an error will be returned.
The speed and acceleration parameters are <strong>com</strong>monly modified for cycle-time
optimization and process constraints. For instance, abrupt stops with a vacuum gripper
may cause the part being held to shift on the gripper. This problem could be solved by
lowering the robot speed. However, the overall cycle time would then be increased. An
alternative is to slow down the acceleration/deceleration rate so the part does not shift on
the gripper during motion start or stop. The robot can still move at the maximum
designated speed for other movements. Another case would be a relatively high payload
and inertia coupled with tight positioning tolerances. A high deceleration rate may cause
overshoot and increase settling time. Higher acceleration/deceleration rates and higher
speeds don't necessarily result in faster cycle times.
<strong>Adept</strong> <strong>Cobra</strong> <strong>PLC600</strong>/PLC800 Robot User’s <strong>Guide</strong>, Rev B 137