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From: Rune Allnor on 18 Dec 2009 04:30
On 18 Des, 10:26, pnachtwey <pnacht...(a)gmail.com> wrote:
> On Dec 17, 6:11 pm, Tim Wescott <t...(a)seemywebsite.com> wrote:> The other part of it is because the field is somewhat
> > specialized; for most people if you don't know what questions to ask
> > you're not going to understand the answers.
>
> This is my fear.  I know that this will not be easy and I have a
> pretty good idea of how to do this but I am not sure that my customer
> understands what he is getting into.  The lastest Control Systems
> Magaziine has a chapter on Kalman filters applied to a maritime
> application.  If have also forwarded pdf articles on similar
> applications so that the customer will not be surprised by the
> calculations involved.
> I am not buying the sensors.  I/we are just, hopefully, selling a
> motion controller but I fear that I will need to get involved in far
> more.

If so, you might want to get a copy of Fossen's book
on control of marine vessels,

http://www.amazon.com/Guidance-Control-Ocean-Vehicles-Fossen/dp/0471941131/ref=sr_1_2?ie=UTF8&s=books&qid=1261128590&sr=8-2

It costs an arm and a leg, but it contains a lot of
very useful stuff.

Rune
From: pnachtwey on 18 Dec 2009 10:01
On Dec 18, 1:30 am, Rune Allnor <all...(a)tele.ntnu.no> wrote:
> It costs an arm and a leg, but it contains a lot of
> very useful stuff.
Yes, but mistakes cost much more.
Thanks for the link. I read the book review. Looks good.

I still would like more feedback resolution and a SSI interface.

Peter Nachtwey
From: Fred Marshall on 22 Dec 2009 11:20
pnachtwey wrote:
> On Dec 18, 1:30 am, Rune Allnor <all...(a)tele.ntnu.no> wrote:
>> It costs an arm and a leg, but it contains a lot of
>> very useful stuff.
> Yes, but mistakes cost much more.
> Thanks for the link. I read the book review. Looks good.
>
> I still would like more feedback resolution and a SSI interface.
>
> Peter Nachtwey

It might really help if you told us "why?".... to both.

Wanting an SSI interface implies a rather strongly constrained situation
- thus the question there.

Wanting higher resolution implies a rather demanding application - what
is it or why is it so demanding?

Fred
From: pnachtwey on 22 Dec 2009 17:13
On Dec 22, 8:20 am, Fred Marshall <fmarshallx(a)remove_the_xacm.org>
wrote:
> pnachtwey wrote:
> > On Dec 18, 1:30 am, Rune Allnor <all...(a)tele.ntnu.no> wrote:
> >> It costs an arm and a leg, but it contains a lot of
> >> very useful stuff.
> > Yes, but mistakes cost much more.
> > Thanks for the link.  I read the book review.  Looks good.
>
> > I still would like more feedback resolution and a SSI interface.
>
> > Peter Nachtwey
>
> It might really help if you told us "why?".... to both.
>
> Wanting an SSI interface implies a rather strongly constrained situation
> - thus the question there.
>
>
> Fred
The pdf file says it has a serial interface but normal RS232 or RS485
interfaces aren't fast nor efficient. Transferring 8 bits at a time
isn't efficient. Our controller doesn't have 4 serial interfaces to
read 4 inputs at the same time. We can read multiple channels of SSI
though. We can use the analog inputs without problems but noise on
top of 12 bit resolution is not good. This data will need to be
filtered to provide positions, velocities and accelerations to gear
to. Also, our controller is a general purpose motion controller. The
closed loop updates are MUCH faster than 10 milliseconds. I can slow
the controller down to 2 ms but it will not like seeing the same
reading for 5 times in a row. I have worked out how to filter the 10
millisecond data and estimate positions, velocities and accelerations
for the times the motion controller updates. It is just another thing
to hassle with.

The quality of motion control is often dependent on the quality of the
feedback. Sample jitter and quantizing of feedback or reference
signals prohibit high gains that may otherwise work.

In this case we are trying to keep positions stables in possible
rolling waves.
After that there are still question I have for the customer because I
think we can keep our positions stable but how does one compensate for
the motion of there nearby objects? Obviously there is more I need to
know about the application.

My simulations indicate that the estimated position, velocity and
acceleration should be good enough with the current feedback device.

> Wanting higher resolution implies a rather demanding application - what
> is it or why is it so demanding?
Hydraulic systems like to use the second derivative. For instance
active damping is done by measuring the the pressure on either side of
the piston and computing a net force. Likewise it is good to be able
to compare the resulting acceleration with the acceleration of the
reference. Accelerometers work well for this. In the past we have
used accelerometers that update as fast as our controller and our
controller has a 16 analog inputs. Estimating the reference
acceleration using a slow update and low resolution feedback is just
another hassle but I have it worked out.

A better MRU would reduce the need for software.

Peter Nachtwey




From: Rune Allnor on 26 Dec 2009 13:30
On 22 Des, 23:13, pnachtwey <pnacht...(a)gmail.com> wrote:
> but  noise on
> top of 12 bit resolution is not good.

Again, check what data the MRU actually produces. 12 bits is
about 4 significant digits, which ought to be sufficient if the
device produces attitude angles, as it would do if it reads
gyro alignments.

Rune
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