kokody2@me.utoronto.ca (Gerry Kokodyniak) (12/28/89)
I originally posted this to comp.unix.i386 and received no replies, just
alot of requests to post what I find out. I will try this particular
newsgroup to see if I have any better luck.
Does anybody in netland have any experience with using A/D & D/A systems
for data acquisition & control? If so, I would like to hear about it.
Please e-mail me, and if there is enough interest I will summarize to
the net. I am interested in existing hardware/software vendors and the
flavour of unix that these products operate under. Also of interest is
how robust the data acquisition/control is under varying machine loads.
GREATLY
Any help will be appreciated.
^^^
Gerry Kokodyniak
--
Gerry Kokodyniak Department of Mechanical Engineering
Ph.D Student University of Toronto
kokody2@me.utoronto.ca Structural Integrity Fatigue and
kokody2@ME.UTORONTO.BITNET Fracture Research Laboratory
{linus,allegra,decvax,floyd}!utcsri!me!kokody2 (416) 978-6853bill@bert.Rosemount.COM (William M. Hawkins) (01/05/90)
In article <1990Jan4.074652.1026@stag.UUCP> trb@stag.UUCP ( Todd Burkey ) writes: >In article <1989Dec28.104004.5697@me.toronto.edu> kokody2@me.utoronto.ca writes: >>Also of interest is >>how robust the data acquisition/control is under varying machine loads. > >Very robust, since our controllers use an extremely tunable analog PID >configuration. Todd, what do you mean by "extremely tunable"? It has been my experience that PID's are not tuned to constants (P, I, and D) with more than a few significant digits, because the real world conspires to change the gains and time constants of electro- mechanical devices. The Nyquist stability criterion is usually met with some safety factor, called phase margin. The original question dealt with stability under varying machine loads. This becomes a problem when the dead time between taking a measurement and manipulating the output is more than 10 percent of the dominant lag time. If the external process has transport lag, add that to the processing time. The sampling and controlling interval is not dead time. It can be a problem when it is more than 25% of the dominant lag. That is, it is not dead time unless you wait until the start of the next interval to send the outputs from the last interval. bill@bert.rosemount.com P.S. We make process control systems that usually operate pneumatic valves from 1/2 to 48 inches in diameter. We use Motorola 68xxx processors, with a proprietary operating system written in C under UNIX. It is optimized to minimize delays in handling clock tick interrupts.
trb@stag.UUCP ( Todd Burkey ) (01/06/90)
In article <8461@rosevax.Rosemount.COM> bill@bert.Rosemount.COM (William M. Hawkins) writes: >In article <1990Jan4.074652.1026@stag.UUCP> trb@stag.UUCP ( Todd Burkey ) writes: >>In article <1989Dec28.104004.5697@me.toronto.edu> kokody2@me.utoronto.ca writes: >>>Also of interest is >>>how robust the data acquisition/control is under varying machine loads. >> >>Very robust, since our controllers use an extremely tunable analog PID >>configuration. > >Todd, what do you mean by "extremely tunable"? It has been my >experience that PID's are not tuned to constants (P, I, and D) >with more than a few significant digits, because the real world >conspires to change the gains and time constants of electro- >mechanical devices. We use 8 bit D/A converters to control the PID of analog controllers. The computer can adjust the PID, but the actual PID servo error feedback loop mechanism is real-time (i.e. no delay between sampling). >The original question dealt with stability under varying machine >loads. This becomes a problem when the dead time between taking >a measurement and manipulating the output is more than 10 percent >of the dominant lag time. If the external process has transport >lag, add that to the processing time. The analog PID adjustment inherently solves this problem. Note that we build our controllers for materials testing using hydraulic actuators. The only time we see the affects of transport lag is if we are performing complex real-time curve-fitting of data in order to dynamically adjust the control signals (i.e. as is required in crack growth rate tests). -Todd Burkey Software Development Manager Interlaken Technology Corporation