Advantage of closing the position loop in LinuxCNC
27 Oct 2022 10:50 #255175
by Ausschuss-Manufaktur
Replied by Ausschuss-Manufaktur on topic Advantage of closing the position loop in LinuxCNC
Thanks for all the opinions.
I think I’m going to use a 7i76e for now and go with Step/Dir controlled Delta Servos.
Moving the Schaublins axis by Hand is probably not necessary I think. But I never really thought about it before.
If I’m going to encounter problems with the Index signal or want to be able to move the axis like tommylight likes, I can always add a 7i85 and use the encoder signal like arvidb does
I think I’m going to use a 7i76e for now and go with Step/Dir controlled Delta Servos.
Moving the Schaublins axis by Hand is probably not necessary I think. But I never really thought about it before.
If I’m going to encounter problems with the Index signal or want to be able to move the axis like tommylight likes, I can always add a 7i85 and use the encoder signal like arvidb does
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27 Oct 2022 16:38 #255203
by arvidb
No, the loops are closed only in the drives. That LinuxCNC receives feedback does not mean it's closing the loop since it does not use that feedback to produce a control signal. (It only outputs an open-loop position reference.)
I guess you could argue that it's kind of a time-shared control where the drive closes the loop while the servos are active and LinuxCNC closes the loop when the servos are inactive. But besides being a bit silly it also misses the point that you can get the performance benefits of closing the loops in the drives and still be able to move the joints by hand without any problems.
Replied by arvidb on topic Advantage of closing the position loop in LinuxCNC
It does, here is the code:
I do not think so, changing feedback while LinuxCNC is running would cause a lot of issues in a lot of situations.And if I'm not mistaken LinuxCNC will internally set actual (i.e. commanded) position to fed back position while drives are disabled.
case EMCMOT_MOTION_DISABLED:
/* set position commands to match feedbacks, this avoids
disturbances and/or following errors when enabling */
emcmotStatus->carte_pos_cmd = emcmotStatus->carte_pos_fb;
for (joint_num = 0; joint_num < ALL_JOINTS; joint_num++) {
/* point to joint struct */
joint = &joints[joint_num];
/* save old command */
joint->pos_cmd = joint->pos_fb;
/* set joint velocity and acceleration to zero */
joint->vel_cmd = 0.0;
joint->acc_cmd = 0.0;
}
In that case, it is a closed loop system at the LinuxCNC side, also most probably loop is closed in the drives so LinuxCNC gets just the actual position.I do know that I can run my EtherCAT drives like that: the loops are closed in the drive (no pids loaded in HAL at all), I get actual position feedback from the drive (that works also when the drives are disabled), and there is no problem with turning the drives back on again after they've been moved; LinuxCNC keeps track of where they are also while disabled.
No, the loops are closed only in the drives. That LinuxCNC receives feedback does not mean it's closing the loop since it does not use that feedback to produce a control signal. (It only outputs an open-loop position reference.)
I guess you could argue that it's kind of a time-shared control where the drive closes the loop while the servos are active and LinuxCNC closes the loop when the servos are inactive.
But besides being a bit silly it also misses the point that you can get the performance benefits of closing the loops in the drives and still be able to move the joints by hand without any problems. Please Log in or Create an account to join the conversation.
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