Trajectory planner

Trajectory planner in GD drives is an internal function to produce smooth acceleration and velocity trajectories based on user input, that may be just a rapid setpoint value change. The main purpose of trajectory planner is to allow creating simple point-to-point motion with minimal complexity from the controller.

Operation

Trajectory planner has two main parameters which define the motion profile limits:

• Velocity limitCVL
• Acceleration limitCAL

Following example illustrates a case where user setpoint changes instantly, and causes trajectory planner to generate acceleration & velocity limited motion.

Signal change inside Trajectory planner during typical point-to-point operation

Trajectory planner has no limit of how often setpoint may be changed. It is possible to give new setpoint even when motion has not reached it's target position, and output motion still follows the given speed limits.

Application

When to use

Trajectory planner is mostly used in following cases:

• Point-to-point positon control (controller gives just target position and let drive to perform the motion)
• Simple velocity control tasks (such as spindle)
• Velocity limiting for safety reasons on all position & velocity control mode applications
  • Additional velocity safety limits may be also used, such as Over speed faultFEV and Velocity tracking error thresholdFVT

When to avoid

As trajectory planner causes limited rate of change, it is not desirable in applications that demand high bandwidth position or velocity control such as:

• Dual-loop feedback position control, or any system where setpoint of drive is output of another closed loop system
• Systems where external motion controller takes care of acceleration limit calculation and trajectory generation

To prevent trajectory planner from distorting or causing lag in such systems, it can be effectively disabled by setting Acceleration limitCAL to maximum value. Velocity limit may be still used for safety reasons as it will not alter the signal unless setpoint velocity exceeds the Velocity limitCVL limit.

Extending CAL and CVL parameter range

Especially when using high resolution feedback devices, one may hit the maximum settable range of CAL and CVL parameters.

Due to the structure of signal path of drive, effective scale of these parameters depend on parameter Setpoint dividerDIV. In other words, adjusting DIV will give different sensitivity to velocity and acceleration limits.

To extend range, use smaller DIV value. It's worth noting that if one adjusts only DIV, then setpoint scaling changes unless also Setpoint multiplierMUL is adusted proportionally.

See also

• Signal path of motor drive

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