system identification +friction modelling

[sadiah]

Dear Sir,

I am going to determine the transfer function of an Automated Storage and Retrieval System (ASRS). It is a three axis system.

How about the friction modelling? Does this also included in the TF already or I have to conduct another test using stimulus and response method.

Thanks.

SADIAH

[Jochen]

Sadiah,

as long as your motor doesn't move in a stick-slip manner while determining the TF, dynamic friction should be part of the transfer function of your system. On the other hand dynamic friction is typically not linear and there is a lot of literature available about friction modelling (just search for "friction modelling" in Google or at Amazon...).
Still in most cases you don't have to think about dynamic friction too much as it just damps the system a bit which can be addressed by adjusting the control parameters (e. g. PID).

Static friction is another issue. NI Motion Control boards provide several control parameters to overcome static friction. For the case that you are developing your own motion control algorithm you still may have a look at this link to get an idea how to address this issue.

I hope that helps,

Jochen Klier
National Instruments Germany

[sadiah]

Thanks. I'm creating a custom motion control using NI Softmotion, with Fieldpoint facilities. to compare PID and Fuzzy algorithm.  Any good advice.

Thanks.

[Jochen]

Sadiah,

I just want to make sure that you have the right expectations regarding the control loop speed that you can achieve with a FieldPoint device. Compared to other PAC-systems like cRIO or PXI FieldPoint controllers provide the lowest loop rate as they are designed for relativly slow processes. In a three axis motion control system you shouldn't expect control loop rates faster than about 100 to 200 Hz.
This might be ok for slowly turning motors with significant inertia.

Typically motion control systems work with loop rates of 1 kHz and above which can be easily achieved by NI Motion boards (up to 16 kHz), PXI (up to 50 kHz and up to 200 kHz with FPGA board) and cRIO (up to 200 kHz).

Jochen