- What is the meaning and usage of the "Endless Movement" bit 15 of EPOS4's / IDX's Controlword (0x6040)?
It is possible by the "Endless Movement" bit to start a motion in "PPM - Profile Position Mode" without knowing the final "Target position" in advance.
If the "Endless movement" bit is set, there is just the requested direction of motion checked based on the configured "Target position" (0x607A) and the motion is started based on the configured "Profile acceleration" and "Profile velocity" parameters but it does not stop on the "Target position".
Please find a brief description of the "Endless movement" bit by chapter "3.3.2 How to use «PPM»" of the "EPOS4 Firmware Specification" too:
PPM versus PVM:
- Advantages by "PPM - Profile Position Mode":
- PPM offers a more stiff and dynamic control due to PID position control than possible by "PVM - Profile Velocity Mode" which is just based on velocity feedback.
- In case of PPM the drive always tries to follow the internally calculated "Demand position" which is updated within each position control cycle (i.e. every 0.4ms in case of an EPOS4 or IDX), i.e. any speed variation (e.g. due to load changes) will be compensated and the drive catches up the motion profile's position again.
- In case of PPM the "Following error actual value" (0x60F4) is internally monitored and a "Following error" (0x8611) is triggered if the configured "Following error window" (0x6065) is exceeded. It can be easily recognized by an increasing "Following error actual value" or a triggered "Following error" when the drive is not able anymore to follow the commanded acceleration, deceleration, or speed (e.g. in case of too high torque demands or too high commanded speed values).
- Disadvantages by "PVM - Profile Velocity Mode":
- PVM just focuses to keep the speed stable but does not catch up with "lost" position in case of any short speed drop (e.g. due to sudden load changes).
- There is no automatic detection and no error state present when the actual speed does not match with the commanded speed value. This can just be checked by a master system by polling and comparing the objects "Velocity demand value" (0x606B) and "Velocity actual value" (0x606C) or monitoring "Velocity actual value averaged" (0x30D3/01) at expected constant speed phase.
Application examples for PPM plus "Endless movement" bit:
- Applications demanding for some quite stiff speed control based on position control, e.g. precise volume pumps by laboratory or medical applications.
- Pumps which use PPM - Profile Position Mode" and "Endless Movement" (instead of PVM) follow precisely the internally calculated "Demand position" and catch up again to compensate short sporadic speed resp. pumped volume drops which might happen in case of load changes.
- Due to position control the "Following error actual value" is observed and a "Following error" is triggered if the pump is not able to follow the profile (= demanded pumped volume) anymore.
- Applications demanding for some operation with fixed speed and finally stopping at a defined position after some period of time or based on user's or process interaction, e.g. like by a centrifuge.
- The final "Target Position" is configured when it is known and the "Endless movement" bit is reset and new "Target Position" is triggered by the Controlword (and toggling bit "New setpoint").