Motion controlled loudspeaker

[David Grant]

I was reading back on rec.audio.pro about the feasibility of a
motion-controlled loudspeaker and found this cited as the main drawback:

"The biggest problem with this approach is that, above self resonance
(where most of the energy is pumped into a speaker) it is an force or
velocity device, not a position device. That is, the force applied to
the mass of the speaker (and the considerable mass of the air in front
of it) is proportional to the current delivered to the speaker. So
there is a considerable phase shift between the speaker current and
the position of the cone, as well as an integrator type frequency
response (reduced excursion at higher frequencies for similar
current)."
(John Popelish)

But wouldn't a PID controller address this? I seem to recall in robotics
lab a similar dilemna: We had a 16" flexible ruler mounted
cantilever-style to a servo shaft. Our input control signal was a square
wave which was supposed to cause the tip of the ruler to move suddenly
from position 1 to position 2 (some number of degrees away). Without the
PID controller (open loop control) the response was out of phase and had
horrible ringing, overshoot, etc... with it, it was vastly improved.

 

[Eeyore]

I was reading back on rec.audio.pro about the feasibility of a
motion-controlled loudspeaker and found this cited as the main drawback:

"The biggest problem with this approach is that, above self resonance
(where most of the energy is pumped into a speaker) it is an force or
velocity device, not a position device. That is, the force applied to
the mass of the speaker (and the considerable mass of the air in front
of it) is proportional to the current delivered to the speaker. So
there is a considerable phase shift between the speaker current and
the position of the cone, as well as an integrator type frequency
response (reduced excursion at higher frequencies for similar
current)."
(John Popelish)

But wouldn't a PID controller address this? I seem to recall in robotics
lab a similar dilemna: We had a 16" flexible ruler mounted
cantilever-style to a servo shaft. Our input control signal was a square
wave which was supposed to cause the tip of the ruler to move suddenly
from position 1 to position 2 (some number of degrees away). Without the
PID controller (open loop control) the response was out of phase and had
horrible ringing, overshoot, etc... with it, it was vastly improved.

What would you actually hope to gain in reality ?

Graham

 

[Genome]

......

But wouldn't a PID controller address this?

Probably not... PID control was invented to help people who sell PID
controllers to people who use PID controllers and it's become sort of self
replicating.

It's a development from PLC controllers through the realms of contactors
through to RLRL logic which has reached state of the art with new graphical
interfaces which operate through the parallel port.

These days we use PICS but you can only get access to the appropriate
registers through proper demonstration of trustworthyness. This is
information that you will not buy.

Winky Winky.

DNA

 

[Scott Dorsey]

But wouldn't a PID controller address this? I seem to recall in robotics
lab a similar dilemna: We had a 16" flexible ruler mounted
cantilever-style to a servo shaft. Our input control signal was a square
wave which was supposed to cause the tip of the ruler to move suddenly
from position 1 to position 2 (some number of degrees away). Without the
PID controller (open loop control) the response was out of phase and had
horrible ringing, overshoot, etc... with it, it was vastly improved.

You can improve it a lot, but you can't eliminate all the problems. You
can effectively increase the control of the voice coil (basically identical
to decreasing the output impedance and increasing the coupling of the coil)
considerably at low frequencies, though.

Remember your robotics lab servo is running well below 20 Hz. The higher
the frequency you get, the closer to a cycle a given amount of delay in
milliseconds is.

You can make feedback systems work pretty well at 20 Hz... Velodyne has done
it on subwoofers since the 1980s, and the modern accelerometers sold for
automotive air bags make it easier and cheaper than ever before. You can't
make them work at 1 KHz because the delay is too severe.
--scott

 

[David Grant]

You can improve it a lot, but you can't eliminate all the problems. You
can effectively increase the control of the voice coil (basically identical
to decreasing the output impedance and increasing the coupling of the coil)
considerably at low frequencies, though.

Remember your robotics lab servo is running well below 20 Hz. The higher
the frequency you get, the closer to a cycle a given amount of delay in
milliseconds is.

But can't the delay be shortened using a controller? That is, if a steep
slope is detected that the controller knows the driver will struggle to
accurately reproduce due to it's sluggish response time, it can output a
much steeper slope to ensure the coil accelerates faster?

 

[David Grant]

What would you actually hope to gain in reality ?

Graham

The same as I'd hope to gain in theory but with relaxed expectations due
to the non-ideal nature of reality...i.e to significantly reduce the
error in the position of or pressure created by the driver - thereby
reducing distortion.

 

[[email protected]]

Probably not... PID control was invented to help people who sell PID
controllers to people who use PID controllers and it's become sort of self
replicating.

those of us who on a daily basis command 30,000 positions per second
out of moving mirror galvanometer scanners would like to know what we
could replace our feedback sensors and PID with?

Steve Roberts

 

[Ron Capik]

< ....snip.. >

Remember your robotics lab servo is running well below 20 Hz. The higher
the frequency you get, the closer to a cycle a given amount of delay in
milliseconds is.

You can make feedback systems work pretty well at 20 Hz... Velodyne has done
it on subwoofers since the 1980s, and the modern accelerometers sold for
automotive air bags make it easier and cheaper than ever before. You can't
make them work at 1 KHz because the delay is too severe.
--scott

I do believe some of the new FPGA devices can do
closed loop control in to the kHz range. Feed forward
techniques may also be used to mitigate some of the
delay issues.

Can't say if it's practical for this application yet.

Later...

Ron Capik
--

 

[Ron Capik]

.....


Probably not... PID control was invented to help people who sell PID
controllers to people who use PID controllers and it's become sort of self
replicating.

It's a development from PLC controllers through the realms of contactors
through to RLRL logic which has reached state of the art with new graphical
interfaces which operate through the parallel port.

These days we use PICS but you can only get access to the appropriate
registers through proper demonstration of trustworthyness. This is
information that you will not buy.

Winky Winky.

DNA

So you're saying this will only work with trusted signals from
artists and engineers that are registered as being certifiable?


Later...

Ron Capik
--

 

[Clifford Heath]

I was reading back on rec.audio.pro about the feasibility of a
motion-controlled loudspeaker and found this cited as the main drawback:

"The biggest problem with this approach is that, above self resonance
(where most of the energy is pumped into a speaker) it is an force or
velocity device, not a position device.

No comment on position feedback, but John Popelish was wrong when
he said that a speaker is a position device below resonance. He
neglects the effect of back-EMF, which means that at a (close-to)
constant voltage, the coil is a constant *velocity* device. Try it
sometime with a broken woofer - remove the cone and apply a volt
across the coil. You'll see the coil rises at a fixed rate. I've
done this with 4" voice coils from an ancient "washing machine"
disk drive head mechanism, which makes the effect very visible.

 

[Genome]

those of us who on a daily basis command 30,000 positions per second
out of moving mirror galvanometer scanners would like to know what we
could replace our feedback sensors and PID with?

Steve Roberts

Those of us who do not believe in the flakey definition of what part of P, I
or D you need to change to get the answer you think you have would like you
to define your interpretation of 30,000 positions per second.

DNA

 

[Scott Dorsey]

But can't the delay be shortened using a controller? That is, if a steep
slope is detected that the controller knows the driver will struggle to
accurately reproduce due to it's sluggish response time, it can output a
much steeper slope to ensure the coil accelerates faster?

But what if it changes? The controller can't see ahead in time... it has
no way to anticipate the signal.
--scott

 

[Eeyore]

You can improve it a lot, but you can't eliminate all the problems. You
can effectively increase the control of the voice coil (basically identical
to decreasing the output impedance and increasing the coupling of the coil)
considerably at low frequencies, though.

Remember your robotics lab servo is running well below 20 Hz. The higher
the frequency you get, the closer to a cycle a given amount of delay in
milliseconds is.

You can make feedback systems work pretty well at 20 Hz... Velodyne has done
it on subwoofers since the 1980s, and the modern accelerometers sold for
automotive air bags make it easier and cheaper than ever before. You can't
make them work at 1 KHz because the delay is too severe.

Philips ( of all people ! ) tried motional feedback on loudspeakers decades ago.

Clearly the results weren't good enough for it to become widely used and Philips
dropped it too .

Graham

 

[Terry Given]

But what if it changes? The controller can't see ahead in time... it has
no way to anticipate the signal.
--scott

it can if each speaker driver includes a fixed delay, allowing the
controller to effectively look ahead in time.

I looked at doing this a few years ago for the smps for an audio
amplifier: add a 10-20ms delay, do a convex hull on the audio input
(hull bounded by the smps frequency response) and dynamically adjust the
smps output voltage to maximise efficiency.

Cheers
Terry

 

[Scott Dorsey]

Philips ( of all people ! ) tried motional feedback on loudspeakers decades ago.
Clearly the results weren't good enough for it to become widely used and Philips
dropped it too .

Part of the thing is that amplifiers with low output impedances have really
changed things since those days. Amps now have a much better ability to
control the driver at low frequencies.

But motional feedback IS pretty widely used today. Not only is Velodyne using
it, but the Carver Sunfire Subwoofer uses it and a few other subwoofers do
as well. It's a reasonable way to extend the low frequency corner on a speaker
down a little bit. It reduces some kinds of distortion, but not those caused
by cone breakup. Still, it allows you to build a driver with an outrageously
long Xmax without that driver having to be linear across the whole displacement
range.
--scott

 

[Terry Given]

Scott Dorsey wrote:




Philips ( of all people ! ) tried motional feedback on loudspeakers decades ago.

Clearly the results weren't good enough for it to become widely used and Philips
dropped it too .

Graham

your conclusion doesnt necessarily follow. Such an argument "proves" VHS
has much better image quality than Beta.

decades ago, computationally intensive tasks were not practical. as an
example, vector control of induction motors is based on work done in the
20's and 30's, but for the aforementioned reasons was unable to be
implemented until the 70's, and didnt really "make it" until the late 80's.

Cheers
Terry

 

[Eeyore]

it can if each speaker driver includes a fixed delay, allowing the
controller to effectively look ahead in time.

I looked at doing this a few years ago for the smps for an audio
amplifier: add a 10-20ms delay, do a convex hull on the audio input
(hull bounded by the smps frequency response) and dynamically adjust the
smps output voltage to maximise efficiency.

Nice idea.

What do you do about cone breakup though ?

Graham

 

[Genome]

So you're saying this will only work with trusted signals from
artists and engineers that are registered as being certifiable?


Later...

Ron Capik

Sure, join the IEEE and you might get it half price.

Winky Winky

DNA

 

[Genome]

Sure, join the IEEE and you might get it half price.

Winky Winky

DNA

Oooooooops. Mind you, be careful. There are rumours going on about advance
fee frauds and some of these 'institutions' are about to catch a serious
cold. Just make sure you tag on the extra 20% and it goes to the right
party.

Sweet


DNA

 

[Scott Dorsey]

Oooooooops. Mind you, be careful. There are rumours going on about advance
fee frauds and some of these 'institutions' are about to catch a serious
cold. Just make sure you tag on the extra 20% and it goes to the right
party.

The IEEE is a lot like a fraternity. You go through this initiation rite,
and they give you a lapel pin and teach you a secret handshake called
"Right Hand Rule."
--scott