problem with following sine wave with analog feedback

[wayne g]

I have a servo motion system where I am following a sine wave with analog feedback. Thus the driving system will keep adjusting the required velocity of the motor to achieve the set point until it is instructed to go the other direction.

My problem is that when the sample starts to fail, the system will never achive the analog feedback and simply reverse direction and head off the other direction.

Specifically you could think of this as testing a sample that starts life very hard and then gets softer and softer.

I think my solution is, at some stage I need to switch into velocity mode or more simply prolong the time my system has to achieve the desired analog feedback maximum value (lower the frequency of t
he input driving sine wave).

This test/flag is the only portion of the system that is confusing me.

[ColinAE]

Hello.

Sounds like you are discovering that you get some problems when your system characteristics change over time, namely: When the material begins to get 'soft,' the amount of time that it takes to achieve the position that you are wanting is increased. In order to compensate for this, you have the following choices:

1. Decrease your 'frequency' of oscillation (i.e. slow down your moves).
   
2. Adjust your PID settings accordingly as the stiffness of the system changes. Again, fundamentally what is happening is that your system is changing, and thus the control algorithm to make moves will change. This is likely to be a complicated process, but it is something I would take a look at.

Hopefully this clarifies what is happening and some possibilit
es for fixing it. This is a somewhat tricky problem to correctly handle, though, I will say!

Regards,
Colin C
Applications Engineering