Controls Block Diagrams

Hey Wescott, you out there?

Are there any generally accepted practices for showing gains in feedback
and polarities at summing junctions for a simple control loop?

I like to see a positive gain in the feedback branch feeding a negative
(subtraction) input to the comparator. Mathematically, it doesn't matter
if the signs arethe other way around. But it makes visualizing open loop
gains and generally understanding what's going on a bit simpler.

What do the pros do?

On Tue, 24 Jan 2012 12:33:36 -0800, Paul Hovnanian P.E. wrote:

Hey Wescott, you out there?

Who??

Are there any generally accepted practices for showing gains in feedback
and polarities at summing junctions for a simple control loop?

Quite a few of them. Which do you want to use?

I like to see a positive gain in the feedback branch feeding a negative
(subtraction) input to the comparator. Mathematically, it doesn't matter
if the signs arethe other way around. But it makes visualizing open loop
gains and generally understanding what's going on a bit simpler.

What do the pros do?

In general you show a summing junction with a circle. This circle is
often decorated with one thing or another (older texts cross the circle
diagonally, dividing it into four parts -- which is confusing, because in
a control block diagram that means addition, while in a RF block diagram
it means mixing).

At any rate, you decorate the signals coming into the circle with their
sign in the summation, by putting a little '+' or '-' by the signal (or,
if you quarter the circle, by putting a '+' or '-' in the appropriate
circle quarter).

Here's an article on the subject:
http://www.wescottdesign.com/articles/BlockDiagrams/BlockDiagrams.html

Note that the block diagram method that I present is somewhat different
from what is commonly seen -- but it makes things explicit that have been
used in ad-hoc ways elsewhere, and is (in my opinion, of course),
superior to what's normally used.

On Tue, 24 Jan 2012 12:33:36 -0800, Paul Hovnanian P.E. wrote:

Hey Wescott, you out there?

Who??

Are there any generally accepted practices for showing gains in feedback
and polarities at summing junctions for a simple control loop?

Quite a few of them. Which do you want to use?

I like to see a positive gain in the feedback branch feeding a negative
(subtraction) input to the comparator. Mathematically, it doesn't matter
if the signs arethe other way around. But it makes visualizing open loop
gains and generally understanding what's going on a bit simpler.

What do the pros do?

In general you show a summing junction with a circle. This circle is
often decorated with one thing or another (older texts cross the circle
diagonally, dividing it into four parts -- which is confusing, because in
a control block diagram that means addition, while in a RF block diagram
it means mixing).

At any rate, you decorate the signals coming into the circle with their
sign in the summation, by putting a little '+' or '-' by the signal (or,
if you quarter the circle, by putting a '+' or '-' in the appropriate
circle quarter).

Here's an article on the subject:
http://www.wescottdesign.com/articles/BlockDiagrams/BlockDiagrams.html

Note that the block diagram method that I present is somewhat different
from what is commonly seen -- but it makes things explicit that have been
used in ad-hoc ways elsewhere, and is (in my opinion, of course),
superior to what's normally used.

Hey Wescott, you out there?

Are there any generally accepted practices for showing gains in feedback
and
polarities at summing junctions for a simple control loop?

I like to see a positive gain in the feedback branch feeding a negative
(subtraction) input to the comparator. Mathematically, it doesn't matter
if
the signs arethe other way around. But it makes visualizing open loop
gains
and generally understanding what's going on a bit simpler.

What do the pros do?

--
Paul Hovnanian mailto:Paul_at_Hovnanian.com
------------------------------------------------------------------
Error: Keyboard not attached. Press F1 to continue.

On Tue, 24 Jan 2012 12:33:36 -0800, Paul Hovnanian P.E. wrote:

Hey Wescott, you out there?

Who??

Are there any generally accepted practices for showing gains in feedback
and polarities at summing junctions for a simple control loop?

Quite a few of them. Which do you want to use?

I like to see a positive gain in the feedback branch feeding a negative
(subtraction) input to the comparator. Mathematically, it doesn't matter
if the signs arethe other way around. But it makes visualizing open loop
gains and generally understanding what's going on a bit simpler.

What do the pros do?

In general you show a summing junction with a circle. This circle is
often decorated with one thing or another (older texts cross the circle
diagonally, dividing it into four parts -- which is confusing, because in
a control block diagram that means addition, while in a RF block diagram
it means mixing).

At any rate, you decorate the signals coming into the circle with their
sign in the summation, by putting a little '+' or '-' by the signal (or,
if you quarter the circle, by putting a '+' or '-' in the appropriate
circle quarter).

Here's an article on the subject:
http://www.wescottdesign.com/articles/BlockDiagrams/BlockDiagrams.html

Note that the block diagram method that I present is somewhat different
from what is commonly seen -- but it makes things explicit that have been
used in ad-hoc ways elsewhere, and is (in my opinion, of course),
superior to what's normally used.

On 1/24/2012 12:58 PM, Tim Wescott wrote:
On Tue, 24 Jan 2012 12:33:36 -0800, Paul Hovnanian P.E. wrote:

Hey Wescott, you out there?

Who??

Are there any generally accepted practices for showing gains in
feedback and polarities at summing junctions for a simple control
loop?

Quite a few of them. Which do you want to use?

I like to see a positive gain in the feedback branch feeding a
negative (subtraction) input to the comparator. Mathematically, it
doesn't matter if the signs arethe other way around. But it makes
visualizing open loop gains and generally understanding what's going
on a bit simpler.

What do the pros do?

In general you show a summing junction with a circle. This circle is
often decorated with one thing or another (older texts cross the circle
diagonally, dividing it into four parts -- which is confusing, because
in a control block diagram that means addition, while in a RF block
diagram it means mixing).

At any rate, you decorate the signals coming into the circle with their
sign in the summation, by putting a little '+' or '-' by the signal
(or, if you quarter the circle, by putting a '+' or '-' in the
appropriate circle quarter).

Here's an article on the subject:
http://www.wescottdesign.com/articles/BlockDiagrams/BlockDiagrams.html

Note that the block diagram method that I present is somewhat different
from what is commonly seen -- but it makes things explicit that have
been used in ad-hoc ways elsewhere, and is (in my opinion, of course),
superior to what's normally used.


That is a serious amount of work you put into that webpage. About all
that is missing is instructions on Mason's rule.

It isn't as pretty, but signal flow graphs work fine instead of block
diagrams. I find they are easier to do with Inkscape. You draw a vector
and plop down a label. Gain is a number, integrator is 1/s, etc. And the
form is more of a standard for design, but it is not acceptable to the
style police for publication. [Publication is where accurate technical
diagrams become pretty but full of errors.]

http://en.wikipedia.org/wiki/Signal-flow_graph

Tim Wescott wrote:

On Tue, 24 Jan 2012 12:33:36 -0800, Paul Hovnanian P.E. wrote:

Hey Wescott, you out there?

Who??

Are there any generally accepted practices for showing gains in
feedback and polarities at summing junctions for a simple control
loop?

Quite a few of them. Which do you want to use?

I like to see a positive gain in the feedback branch feeding a
negative (subtraction) input to the comparator. Mathematically, it
doesn't matter if the signs arethe other way around. But it makes
visualizing open loop gains and generally understanding what's going
on a bit simpler.

What do the pros do?

In general you show a summing junction with a circle. This circle is
often decorated with one thing or another (older texts cross the circle
diagonally, dividing it into four parts -- which is confusing, because
in a control block diagram that means addition, while in a RF block
diagram it means mixing).

At any rate, you decorate the signals coming into the circle with their
sign in the summation, by putting a little '+' or '-' by the signal
(or, if you quarter the circle, by putting a '+' or '-' in the
appropriate circle quarter).

Here's an article on the subject:
http://www.wescottdesign.com/articles/BlockDiagrams/BlockDiagrams.html

Note that the block diagram method that I present is somewhat different
from what is commonly seen -- but it makes things explicit that have
been used in ad-hoc ways elsewhere, and is (in my opinion, of course),
superior to what's normally used.

I like your use of the Sigma for summation, Pi for product, etc. You
didn't answer my question directly, but your examples have given me a
hint.

Most (all?) of your feedback summing junctions use a minus (-) input for
the feedback signal. See your Figure 12 on the above page. That would
make your H(z) transfer function 'positive'. That's the way I've seen
most controls people do things.

I just got into an argument with someone who wanted to change an
existing specification, defined as described above, to show a negative
feedback function gain (H(z)) to a positive summing junction input. I
said 1) that's the way its usually done and 2) why revise an existing
document?

Its a reactive load feedback signal for generator voltage regulation, if
anyone needs to know.

On Wed, 25 Jan 2012 01:19:33 -0800, miso wrote:

On 1/24/2012 12:58 PM, Tim Wescott wrote:
On Tue, 24 Jan 2012 12:33:36 -0800, Paul Hovnanian P.E. wrote:

Hey Wescott, you out there?

Who??

Are there any generally accepted practices for showing gains in
feedback and polarities at summing junctions for a simple control
loop?

Quite a few of them. Which do you want to use?

I like to see a positive gain in the feedback branch feeding a
negative (subtraction) input to the comparator. Mathematically, it
doesn't matter if the signs arethe other way around. But it makes
visualizing open loop gains and generally understanding what's going
on a bit simpler.

What do the pros do?

In general you show a summing junction with a circle. This circle is
often decorated with one thing or another (older texts cross the circle
diagonally, dividing it into four parts -- which is confusing, because
in a control block diagram that means addition, while in a RF block
diagram it means mixing).

At any rate, you decorate the signals coming into the circle with their
sign in the summation, by putting a little '+' or '-' by the signal
(or, if you quarter the circle, by putting a '+' or '-' in the
appropriate circle quarter).

Here's an article on the subject:
http://www.wescottdesign.com/articles/BlockDiagrams/BlockDiagrams.html

Note that the block diagram method that I present is somewhat different
from what is commonly seen -- but it makes things explicit that have
been used in ad-hoc ways elsewhere, and is (in my opinion, of course),
superior to what's normally used.


That is a serious amount of work you put into that webpage. About all
that is missing is instructions on Mason's rule.

It isn't as pretty, but signal flow graphs work fine instead of block
diagrams. I find they are easier to do with Inkscape. You draw a vector
and plop down a label. Gain is a number, integrator is 1/s, etc. And the
form is more of a standard for design, but it is not acceptable to the
style police for publication. [Publication is where accurate technical
diagrams become pretty but full of errors.]

http://en.wikipedia.org/wiki/Signal-flow_graph

Signal flow graphs are superior for linear, time-invariant systems, but
real-world systems often boil down to things that are easier to express
in a block.

And I left out Mason's rule on purpose -- the page is for beginners, and
I didn't want to scare them off. Besides -- by the time I get to the
level of complexity where I'm thinking of applying Mason's rule, I've
long since put the linearized system description into a state-space
representation, and solved that. I've been out of school for over 20
years now, and I think I've used Mason's rule for pay twice in all that
time.

On Wed, 25 Jan 2012 10:53:48 -0600, Tim Wescott <tim_at_seemywebsite.com
wrote:

On Wed, 25 Jan 2012 01:19:33 -0800, miso wrote:

On 1/24/2012 12:58 PM, Tim Wescott wrote:
On Tue, 24 Jan 2012 12:33:36 -0800, Paul Hovnanian P.E. wrote:

Hey Wescott, you out there?

Who??

Are there any generally accepted practices for showing gains in
feedback and polarities at summing junctions for a simple control
loop?

Quite a few of them. Which do you want to use?

I like to see a positive gain in the feedback branch feeding a
negative (subtraction) input to the comparator. Mathematically, it
doesn't matter if the signs arethe other way around. But it makes
visualizing open loop gains and generally understanding what's going
on a bit simpler.

What do the pros do?

In general you show a summing junction with a circle. This circle is
often decorated with one thing or another (older texts cross the
circle diagonally, dividing it into four parts -- which is confusing,
because in a control block diagram that means addition, while in a RF
block diagram it means mixing).

At any rate, you decorate the signals coming into the circle with
their sign in the summation, by putting a little '+' or '-' by the
signal (or, if you quarter the circle, by putting a '+' or '-' in the
appropriate circle quarter).

Here's an article on the subject:
http://www.wescottdesign.com/articles/BlockDiagrams/
BlockDiagrams.html

Note that the block diagram method that I present is somewhat
different from what is commonly seen -- but it makes things explicit
that have been used in ad-hoc ways elsewhere, and is (in my opinion,
of course), superior to what's normally used.


That is a serious amount of work you put into that webpage. About all
that is missing is instructions on Mason's rule.

It isn't as pretty, but signal flow graphs work fine instead of block
diagrams. I find they are easier to do with Inkscape. You draw a
vector and plop down a label. Gain is a number, integrator is 1/s,
etc. And the form is more of a standard for design, but it is not
acceptable to the style police for publication. [Publication is where
accurate technical diagrams become pretty but full of errors.]

http://en.wikipedia.org/wiki/Signal-flow_graph

Signal flow graphs are superior for linear, time-invariant systems, but
real-world systems often boil down to things that are easier to express
in a block.

And I left out Mason's rule on purpose -- the page is for beginners, and
I didn't want to scare them off. Besides -- by the time I get to the
level of complexity where I'm thinking of applying Mason's rule, I've
long since put the linearized system description into a state-space
representation, and solved that. I've been out of school for over 20
years now, and I think I've used Mason's rule for pay twice in all that
time.

I had a class from Sam Mason, taught from Mason & Zimmerman, "Electronic
Circuits, Signals, and Systems".

And a class taught by Jim Melcher, with textbook, Zimmerman & Mason,
"Electronic Circuit Theory".

Both very nice guys, and both died young, Mason from a cerebral
hemorrhage and Melcher from colon cancer :-(

But I never knew of anything called Mason's rule... just flow-graph
circuit analysis... which I never much cared for as an analysis tool.

On Wed, 25 Jan 2012 10:18:07 -0700, Jim Thompson wrote:

On Wed, 25 Jan 2012 10:53:48 -0600, Tim Wescott <tim_at_seemywebsite.com
wrote:

On Wed, 25 Jan 2012 01:19:33 -0800, miso wrote:

On 1/24/2012 12:58 PM, Tim Wescott wrote:
On Tue, 24 Jan 2012 12:33:36 -0800, Paul Hovnanian P.E. wrote:

Hey Wescott, you out there?

Who??

Are there any generally accepted practices for showing gains in
feedback and polarities at summing junctions for a simple control
loop?

Quite a few of them. Which do you want to use?

I like to see a positive gain in the feedback branch feeding a
negative (subtraction) input to the comparator. Mathematically, it
doesn't matter if the signs arethe other way around. But it makes
visualizing open loop gains and generally understanding what's going
on a bit simpler.

What do the pros do?

In general you show a summing junction with a circle. This circle is
often decorated with one thing or another (older texts cross the
circle diagonally, dividing it into four parts -- which is confusing,
because in a control block diagram that means addition, while in a RF
block diagram it means mixing).

At any rate, you decorate the signals coming into the circle with
their sign in the summation, by putting a little '+' or '-' by the
signal (or, if you quarter the circle, by putting a '+' or '-' in the
appropriate circle quarter).

Here's an article on the subject:
http://www.wescottdesign.com/articles/BlockDiagrams/
BlockDiagrams.html

Note that the block diagram method that I present is somewhat
different from what is commonly seen -- but it makes things explicit
that have been used in ad-hoc ways elsewhere, and is (in my opinion,
of course), superior to what's normally used.


That is a serious amount of work you put into that webpage. About all
that is missing is instructions on Mason's rule.

It isn't as pretty, but signal flow graphs work fine instead of block
diagrams. I find they are easier to do with Inkscape. You draw a
vector and plop down a label. Gain is a number, integrator is 1/s,
etc. And the form is more of a standard for design, but it is not
acceptable to the style police for publication. [Publication is where
accurate technical diagrams become pretty but full of errors.]

http://en.wikipedia.org/wiki/Signal-flow_graph

Signal flow graphs are superior for linear, time-invariant systems, but
real-world systems often boil down to things that are easier to express
in a block.

And I left out Mason's rule on purpose -- the page is for beginners, and
I didn't want to scare them off. Besides -- by the time I get to the
level of complexity where I'm thinking of applying Mason's rule, I've
long since put the linearized system description into a state-space
representation, and solved that. I've been out of school for over 20
years now, and I think I've used Mason's rule for pay twice in all that
time.

I had a class from Sam Mason, taught from Mason & Zimmerman, "Electronic
Circuits, Signals, and Systems".

And a class taught by Jim Melcher, with textbook, Zimmerman & Mason,
"Electronic Circuit Theory".

Both very nice guys, and both died young, Mason from a cerebral
hemorrhage and Melcher from colon cancer :-(

But I never knew of anything called Mason's rule... just flow-graph
circuit analysis... which I never much cared for as an analysis tool.

Mason's rule gives a fixed algorithm for computing a transfer function
from a flow graph or block diagram, given that you know the transfer
functions of each of the legs or blocks.

It's one of those things that's superbly handy in an academic setting,
but doesn't seem to have much application in real life.

(At least not for me -- by the time I get a control system block diagram
that's complicated enough for Mason's rule it's covering enough of the
system that it has significant nonlinearities, and linear analysis has
been tossed out the window.)

Tim Wescott wrote:

On Tue, 24 Jan 2012 12:33:36 -0800, Paul Hovnanian P.E. wrote:

Hey Wescott, you out there?

Who??

Are there any generally accepted practices for showing gains in feedback
and polarities at summing junctions for a simple control loop?

Quite a few of them. Which do you want to use?

I like to see a positive gain in the feedback branch feeding a negative
(subtraction) input to the comparator. Mathematically, it doesn't matter
if the signs arethe other way around. But it makes visualizing open loop
gains and generally understanding what's going on a bit simpler.

What do the pros do?

In general you show a summing junction with a circle. This circle is
often decorated with one thing or another (older texts cross the circle
diagonally, dividing it into four parts -- which is confusing, because in
a control block diagram that means addition, while in a RF block diagram
it means mixing).

At any rate, you decorate the signals coming into the circle with their
sign in the summation, by putting a little '+' or '-' by the signal (or,
if you quarter the circle, by putting a '+' or '-' in the appropriate
circle quarter).

Here's an article on the subject:
http://www.wescottdesign.com/articles/BlockDiagrams/BlockDiagrams.html

Note that the block diagram method that I present is somewhat different
from what is commonly seen -- but it makes things explicit that have been
used in ad-hoc ways elsewhere, and is (in my opinion, of course),
superior to what's normally used.

I like your use of the Sigma for summation, Pi for product, etc. You
didn't
answer my question directly, but your examples have given me a hint.

Most (all?) of your feedback summing junctions use a minus (-) input for
the
feedback signal. See your Figure 12 on the above page. That would make
your
H(z) transfer function 'positive'. That's the way I've seen most controls
people do things.

I just got into an argument with someone who wanted to change an existing
specification, defined as described above, to show a negative feedback
function gain (H(z)) to a positive summing junction input. I said 1)
that's
the way its usually done and 2) why revise an existing document?

Its a reactive load feedback signal for generator voltage regulation, if
anyone needs to know.

(At least not for me -- by the time I get a control system block diagram
that's complicated enough for Mason's rule it's covering enough of the
system that it has significant nonlinearities, and linear analysis has
been tossed out the window.)