Second Time Derivative Of Current

[Bret Cahill]

Is there any name for this other than "Second Time Derivative Of
Current?"


Bret Cahill

 

[John Larkin]

On Mon, 17 Aug 2009 22:55:04 -0700 (PDT), Bret Cahill
<BretCahill@peoplepc.com> wrote:

Is there any name for this other than "Second Time Derivative Of
Current?"

I don't think there's even a name for the first derivative of current.
We call that one "dee eye dee tee."

John

 

[John Larkin]

On Thu, 20 Aug 2009 08:29:19 -0700 (PDT), gearhead
<nospam@billburg.com> wrote:

On Aug 18, 11:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 17 Aug 2009 22:55:04 -0700 (PDT), Bret Cahill

BretCah...@peoplepc.com> wrote:
Is there any name for this other than "Second Time Derivative Of
Current?"

I don't think there's even a name for the first derivative of current.
We call that one "dee eye dee tee."

John


then it must be
dee squared aye
(over) dee tee squared

I suppose, but it has so little physical use in electronics that it's
usually not referred to at all.

di/dt is commonly of concern because it induces voltage drop in
intended or parasitic inductances.

I don't recall using the second derivative of voltage much, either.
dv/dt is real common, as in opamp slew rate.

John

 

[gearhead]

On Aug 18, 11:29 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Mon, 17 Aug 2009 22:55:04 -0700 (PDT), Bret Cahill

BretCah...@peoplepc.com> wrote:
Is there any name for this other than "Second Time Derivative Of
Current?"

I don't think there's even a name for the first derivative of current.
We call that one "dee eye dee tee."

John

then it must be
dee squared aye
(over) dee tee squared

 

[BobW]

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:5csq85pcgrcd8sa1nejpahtjjnan823fl0@4ax.com...

On Thu, 20 Aug 2009 08:29:19 -0700 (PDT), gearhead
nospam@billburg.com> wrote:

On Aug 18, 11:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 17 Aug 2009 22:55:04 -0700 (PDT), Bret Cahill

BretCah...@peoplepc.com> wrote:
Is there any name for this other than "Second Time Derivative Of
Current?"

I don't think there's even a name for the first derivative of current.
We call that one "dee eye dee tee."

John


then it must be
dee squared aye
(over) dee tee squared

I suppose, but it has so little physical use in electronics that it's
usually not referred to at all.

di/dt is commonly of concern because it induces voltage drop in
intended or parasitic inductances.

I don't recall using the second derivative of voltage much, either.
dv/dt is real common, as in opamp slew rate.

John

I use the term "accelericity" for d2i/dt2.

Bob
--
== All google group posts are automatically deleted due to spam ==

 

[Jon Kirwan]

On Thu, 20 Aug 2009 11:59:15 -0700 (PDT), gearhead
<nospam@billburg.com> wrote:

On Aug 20, 11:58 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Thu, 20 Aug 2009 08:29:19 -0700 (PDT), gearhead



nos...@billburg.com> wrote:
On Aug 18, 11:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 17 Aug 2009 22:55:04 -0700 (PDT), Bret Cahill

BretCah...@peoplepc.com> wrote:
Is there any name for this other than "Second Time Derivative Of
Current?"

I don't think there's even a name for the first derivative of current.
We call that one "dee eye dee tee."

John

then it must be
dee squared aye
(over) dee tee squared

I suppose, but it has so little physical use in electronics that it's
usually not referred to at all.

di/dt is commonly of concern because it induces voltage drop in
intended or parasitic inductances.

I don't recall using the second derivative of voltage much, either.
dv/dt is real common, as in opamp slew rate.

John

Yeah, little use for second derivatives of i or v.
I remember my old math prof said that there was a physical use for, I
think it was the fourth derivative of position (some derivative beyond
acceleration). Something called "bounce," like in an elevator.

The term I was taught was 'jerk' for anything at the 3rd derivate of
time and beyond. And yes, for elevators.. but also cars.

Jon

 

[Jon Kirwan]

On Thu, 20 Aug 2009 08:58:31 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 20 Aug 2009 08:29:19 -0700 (PDT), gearhead
nospam@billburg.com> wrote:

On Aug 18, 11:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 17 Aug 2009 22:55:04 -0700 (PDT), Bret Cahill

BretCah...@peoplepc.com> wrote:
Is there any name for this other than "Second Time Derivative Of
Current?"

I don't think there's even a name for the first derivative of current.
We call that one "dee eye dee tee."

John


then it must be
dee squared aye
(over) dee tee squared

I suppose, but it has so little physical use in electronics that it's
usually not referred to at all.

di/dt is commonly of concern because it induces voltage drop in
intended or parasitic inductances.

I don't recall using the second derivative of voltage much, either.
dv/dt is real common, as in opamp slew rate.

I would assume that you haven't worked on control methods for switched
power conversion, then.

Jon

 

[John Larkin]

On Thu, 20 Aug 2009 19:06:13 GMT, Jon Kirwan
<jonk@infinitefactors.org> wrote:

On Thu, 20 Aug 2009 08:58:31 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 20 Aug 2009 08:29:19 -0700 (PDT), gearhead
nospam@billburg.com> wrote:

On Aug 18, 11:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 17 Aug 2009 22:55:04 -0700 (PDT), Bret Cahill

BretCah...@peoplepc.com> wrote:
Is there any name for this other than "Second Time Derivative Of
Current?"

I don't think there's even a name for the first derivative of current.
We call that one "dee eye dee tee."

John


then it must be
dee squared aye
(over) dee tee squared

I suppose, but it has so little physical use in electronics that it's
usually not referred to at all.

di/dt is commonly of concern because it induces voltage drop in
intended or parasitic inductances.

I don't recall using the second derivative of voltage much, either.
dv/dt is real common, as in opamp slew rate.

I would assume that you haven't worked on control methods for switched
power conversion, then.

Jon

I have done a lot of switching regs, servo motor controllers,
microsteppers, and a few ballpark 32,000 horsepower/80,000 ton control
loops. Lots of temperature controllers, too, where the process
dynamics can be really dreadful: transport plus diffusion.

Even derivative control is tricky... gets unstable in a lot of
real-life situations. 2nd derivative sounds really hairy.

John

 

[gearhead]

On Aug 20, 11:58 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 20 Aug 2009 08:29:19 -0700 (PDT), gearhead



nos...@billburg.com> wrote:
On Aug 18, 11:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 17 Aug 2009 22:55:04 -0700 (PDT), Bret Cahill

BretCah...@peoplepc.com> wrote:
Is there any name for this other than "Second Time Derivative Of
Current?"

I don't think there's even a name for the first derivative of current.
We call that one "dee eye dee tee."

John

then it must be
dee squared aye
(over) dee tee squared

I suppose, but it has so little physical use in electronics that it's
usually not referred to at all.

di/dt is commonly of concern because it induces voltage drop in
intended or parasitic inductances.

I don't recall using the second derivative of voltage much, either.
dv/dt is real common, as in opamp slew rate.

John

Yeah, little use for second derivatives of i or v.
I remember my old math prof said that there was a physical use for, I
think it was the fourth derivative of position (some derivative beyond
acceleration). Something called "bounce," like in an elevator.

 

[Jon Kirwan]

On Thu, 20 Aug 2009 13:05:26 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 20 Aug 2009 19:06:13 GMT, Jon Kirwan
jonk@infinitefactors.org> wrote:

On Thu, 20 Aug 2009 08:58:31 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 20 Aug 2009 08:29:19 -0700 (PDT), gearhead
nospam@billburg.com> wrote:

On Aug 18, 11:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 17 Aug 2009 22:55:04 -0700 (PDT), Bret Cahill

BretCah...@peoplepc.com> wrote:
Is there any name for this other than "Second Time Derivative Of
Current?"

I don't think there's even a name for the first derivative of current.
We call that one "dee eye dee tee."

John


then it must be
dee squared aye
(over) dee tee squared

I suppose, but it has so little physical use in electronics that it's
usually not referred to at all.

di/dt is commonly of concern because it induces voltage drop in
intended or parasitic inductances.

I don't recall using the second derivative of voltage much, either.
dv/dt is real common, as in opamp slew rate.

I would assume that you haven't worked on control methods for switched
power conversion, then.

Jon

I have done a lot of switching regs, servo motor controllers,
microsteppers, and a few ballpark 32,000 horsepower/80,000 ton control
loops. Lots of temperature controllers, too, where the process
dynamics can be really dreadful: transport plus diffusion.

Even derivative control is tricky... gets unstable in a lot of
real-life situations. 2nd derivative sounds really hairy.

I gathered from reading (not doing) that it is NECESSARY in some
cases. For some things like uninterruptable supplies, active power
factor compensation, power filters and gyrators, the control input
itself, as a function of time, doesn't appear in the first time
derivative of the output voltage, which is something like:

d/dt V_out = (I_in - I_out) / C_out

But the control term it does appear in the second derivative. (I just
tried to work it out on paper, again, and it's 'longish' but the term
including the input control function of time, del(t), is:

d/dt [(I_in - I_out) / C_out] = V/(L_out*C_out)*del(t) - ....

Apparently, it's necessary to recover that term to apply switching law
and devise a control strategy. I remember reading something about
choosing a "sliding surface" here.

Sounded interesting on skimming before, but I've never required that
level of understand for anything so never went any further than to
just remember a few interesting tidbits. The one thing I definitely
remember well is that the second derivative was absolutely required in
order to even begin to develop a proper control strategy, though.

Jon

 

[John Larkin]

On Thu, 20 Aug 2009 21:41:41 GMT, Jon Kirwan
<jonk@infinitefactors.org> wrote:

On Thu, 20 Aug 2009 13:05:26 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 20 Aug 2009 19:06:13 GMT, Jon Kirwan
jonk@infinitefactors.org> wrote:

On Thu, 20 Aug 2009 08:58:31 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 20 Aug 2009 08:29:19 -0700 (PDT), gearhead
nospam@billburg.com> wrote:

On Aug 18, 11:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 17 Aug 2009 22:55:04 -0700 (PDT), Bret Cahill

BretCah...@peoplepc.com> wrote:
Is there any name for this other than "Second Time Derivative Of
Current?"

I don't think there's even a name for the first derivative of current.
We call that one "dee eye dee tee."

John


then it must be
dee squared aye
(over) dee tee squared

I suppose, but it has so little physical use in electronics that it's
usually not referred to at all.

di/dt is commonly of concern because it induces voltage drop in
intended or parasitic inductances.

I don't recall using the second derivative of voltage much, either.
dv/dt is real common, as in opamp slew rate.

I would assume that you haven't worked on control methods for switched
power conversion, then.

Jon

I have done a lot of switching regs, servo motor controllers,
microsteppers, and a few ballpark 32,000 horsepower/80,000 ton control
loops. Lots of temperature controllers, too, where the process
dynamics can be really dreadful: transport plus diffusion.

Even derivative control is tricky... gets unstable in a lot of
real-life situations. 2nd derivative sounds really hairy.

I gathered from reading (not doing) that it is NECESSARY in some
cases. For some things like uninterruptable supplies, active power
factor compensation, power filters and gyrators, the control input
itself, as a function of time, doesn't appear in the first time
derivative of the output voltage, which is something like:

d/dt V_out = (I_in - I_out) / C_out

But the control term it does appear in the second derivative. (I just
tried to work it out on paper, again, and it's 'longish' but the term
including the input control function of time, del(t), is:

d/dt [(I_in - I_out) / C_out] = V/(L_out*C_out)*del(t) - ....

Apparently, it's necessary to recover that term to apply switching law
and devise a control strategy. I remember reading something about
choosing a "sliding surface" here.

Sounded interesting on skimming before, but I've never required that
level of understand for anything so never went any further than to
just remember a few interesting tidbits. The one thing I definitely
remember well is that the second derivative was absolutely required in
order to even begin to develop a proper control strategy, though.

Jon

Depends on the system. Most control loops (power supplies, industrial
process) are happy with just proportional and integral terms.
Derivative amplifies noise and can do nasty things in cases like
deadbands and static friction.

A prudent amount of lead/lag compensation can improve phase margin;
every situation is different.

Derivative is often necessary in maximum-dynamics electro-mechanical
servos, like high-performance aircraft controls. Everything else -
actuators, sensors, machanics - must be precise enough to allow
serious derivative action.

One alternate to a tight control loop is to apply feed-forward
corrections from known error sources, like the unregulated input to a
voltage regulator, or ambient temperature into the control loop of a
crystal oven. That can buy you a 5 or 10x improvement without adding
the potential instabilities of a very tight loop.

John

 

[Jon Kirwan]

On Thu, 20 Aug 2009 16:19:37 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 20 Aug 2009 21:41:41 GMT, Jon Kirwan
jonk@infinitefactors.org> wrote:

On Thu, 20 Aug 2009 13:05:26 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 20 Aug 2009 19:06:13 GMT, Jon Kirwan
jonk@infinitefactors.org> wrote:

On Thu, 20 Aug 2009 08:58:31 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 20 Aug 2009 08:29:19 -0700 (PDT), gearhead
nospam@billburg.com> wrote:

On Aug 18, 11:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 17 Aug 2009 22:55:04 -0700 (PDT), Bret Cahill

BretCah...@peoplepc.com> wrote:
Is there any name for this other than "Second Time Derivative Of
Current?"

I don't think there's even a name for the first derivative of current.
We call that one "dee eye dee tee."

John


then it must be
dee squared aye
(over) dee tee squared

I suppose, but it has so little physical use in electronics that it's
usually not referred to at all.

di/dt is commonly of concern because it induces voltage drop in
intended or parasitic inductances.

I don't recall using the second derivative of voltage much, either.
dv/dt is real common, as in opamp slew rate.

I would assume that you haven't worked on control methods for switched
power conversion, then.

Jon

I have done a lot of switching regs, servo motor controllers,
microsteppers, and a few ballpark 32,000 horsepower/80,000 ton control
loops. Lots of temperature controllers, too, where the process
dynamics can be really dreadful: transport plus diffusion.

Even derivative control is tricky... gets unstable in a lot of
real-life situations. 2nd derivative sounds really hairy.

I gathered from reading (not doing) that it is NECESSARY in some
cases. For some things like uninterruptable supplies, active power
factor compensation, power filters and gyrators, the control input
itself, as a function of time, doesn't appear in the first time
derivative of the output voltage, which is something like:

d/dt V_out = (I_in - I_out) / C_out

But the control term it does appear in the second derivative. (I just
tried to work it out on paper, again, and it's 'longish' but the term
including the input control function of time, del(t), is:

d/dt [(I_in - I_out) / C_out] = V/(L_out*C_out)*del(t) - ....

Apparently, it's necessary to recover that term to apply switching law
and devise a control strategy. I remember reading something about
choosing a "sliding surface" here.

Sounded interesting on skimming before, but I've never required that
level of understand for anything so never went any further than to
just remember a few interesting tidbits. The one thing I definitely
remember well is that the second derivative was absolutely required in
order to even begin to develop a proper control strategy, though.

Jon

Depends on the system.

Yes, that's why I took a moment in the earlier post to include some of
the systems that I was reading about, at the time. Apparently, for
those it is important to consider. I agree that "not always" is also
true.

Most control loops (power supplies, industrial
process) are happy with just proportional and integral terms.

I've done a LOT of PID control. Not as much as Tim W., I'm sure (I
like reading his posts on control theory and feel as though I learn
things from him here and there), but enough. There are some commonly
found problems where it works horribly, if at all. It's dirt easy,
for example, to make it to "go to hell in a hand-basket" merely by
making the output function a nice, time-delayed copy of the input.
Like maybe a second's delay.

One of my favorite tricks, in fact, when presented with a "difficult"
case with PID control is to take a quick look at the delays and their
variability, as well, from measurement source to closed loop control
signal output. If there is any way to shorten it, I do that first.
Not infrequently, it starts working well enough with nothing more done
to it.

Derivative amplifies noise

Yes. However, in the cases I cited above, I remember the author
making an excellent case that this was the ONLY way to set up the
state-space equation and then properly design the control system for
it. If he's right about it, you bite the noise to get there.

and can do nasty things in cases like
deadbands and static friction.

I don't think I said it was appropriate in all cases (nor would I
know, to be honest) and instead chose to list those where the author
discussed needing the 2nd derivative to develop proper control theory
for them.

A prudent amount of lead/lag compensation can improve phase margin;
every situation is different.

I understand and agree with this much.

Derivative is often necessary in maximum-dynamics electro-mechanical
servos, like high-performance aircraft controls. Everything else -
actuators, sensors, machanics - must be precise enough to allow
serious derivative action.

One alternate to a tight control loop is to apply feed-forward
corrections from known error sources, like the unregulated input to a
voltage regulator, or ambient temperature into the control loop of a
crystal oven. That can buy you a 5 or 10x improvement without adding
the potential instabilities of a very tight loop.

To my unpracticed eye, feed-forward is a very important tool to be
aware of and able to apply. I didn't mean to take anything away from
that and I'm glad you brought it in. But that's a separate subject.

Because I saw the equation development by the author, mentioned
earlier, I definitely accept the author's assertion in those cases
that the 2nd derivative of output voltage was necessary for proper
control loop design. I don't know better from experience, but the
equation development looked solid to me if and only if the assumptions
the author brought into it were valid, too. I simply accepted those,
at the time.

Jon

 

[Bret Cahill]

Is there any name for this other than "Second Time Derivative Of
Current?"

I don't think there's even a name for the first derivative of current.
We call that one "dee eye dee tee."

Generally they have better, more agreed upon names and symbols in EE
than any other field except math.

On the other hand the names that appear in software are so bad you
wonder when they'll form a committee to purge them from the language.

The French have some expression for using the exact right word. It's
really a waste of time wondering over some post on heat exchange
equipment and then realize usage is different in the U.K.


Bret Cahill


"The human mind invents things more easily than words; that is why
many improper terms and inadequate expressions gain currency."

-- Tocqueville

 

[Michael Robinson]

"Bret Cahill" <BretCahill@peoplepc.com> wrote in message
news:0d55fa3b-76f5-4d27-825f-51cfdbd00e28@f20g2000prn.googlegroups.com...

The French have some expression for using the exact right word.

"Le mot juste" was Flaubert's big thing, as I recall.