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need help: DC pulse train has to be converted to a DC voltage

C

Cosmo

Jan 1, 1970
0
I will have a constant amplitude pulse, that will increase and decrease
depending on my system. I need to take this pulse, take the average
value (DC voltage) and put it into my PID controller. Any suggestions!
Thank you in advance.
 
T

Tim Wescott

Jan 1, 1970
0
Cosmo said:
I will have a constant amplitude pulse, that will increase and decrease
depending on my system. I need to take this pulse, take the average
value (DC voltage) and put it into my PID controller. Any suggestions!
Thank you in advance.
Do you mean that the amplitude will increase and decrease, or the duty
cycle?

If it's the amplitude then you can rectify the pulse and filter it, as
is done with AM detectors.

If it's the duty cycle then you can filter it and read the analog
voltage. If you're implementing a PID controller on a microcontroller
then you may be able to measure the duty cycle with a counter on the chip.

If it's the frequency of the pulses then you can use a frequency to
voltage converter, or you can use your on-chip counter.

More detail would help us help you.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
J

John B

Jan 1, 1970
0
I will have a constant amplitude pulse, that will increase and
decrease depending on my system.

If it increases and decreases then it's not constant

I need to take this pulse, take the
average value (DC voltage) and put it into my PID controller. Any
suggestions! Thank you in advance.

My suggestion is that describe what you want in a clearer way.
 
C

Cosmo

Jan 1, 1970
0
Sorry I wasn't very clear. What I have is a optical sensor sending DC
square waves at some positive constant amplitude, unfortunately the
pulse will be different widths depending on the system. I will combat
this problem with a LM555 timer that will give me a positive constant
amplitude and width every time it is triggered. Now I have these square
wave pulses of all positive DC values, constant width whose frequency
is proportional to the speed of the system and need to get an analog DC
value. I'm very limited with in terms of components and experience; I
would like to do this with diodes, resistors, capacitors, op-amps or
other basic devices.

Basically yes! I need a frequency to voltage converter. I need to make
one with very limited supplies, and I thought since it is only a
positive square wave pulse there may be a way to get the dc value out
of it. I can't buy a FVC.

Thanks again!
 
J

Jim Thompson

Jan 1, 1970
0
Sorry I wasn't very clear. What I have is a optical sensor sending DC
square waves at some positive constant amplitude, unfortunately the
pulse will be different widths depending on the system. I will combat
this problem with a LM555 timer that will give me a positive constant
amplitude and width every time it is triggered. Now I have these square
wave pulses of all positive DC values, constant width whose frequency
is proportional to the speed of the system and need to get an analog DC
value. I'm very limited with in terms of components and experience; I
would like to do this with diodes, resistors, capacitors, op-amps or
other basic devices.

Basically yes! I need a frequency to voltage converter. I need to make
one with very limited supplies, and I thought since it is only a
positive square wave pulse there may be a way to get the dc value out
of it. I can't buy a FVC.

Thanks again!

You are producing (out of the 555) a series of fixed height, fixed
width pulses??

If you simply low-pass the pulses from the output of the 555, you will
have what you want.

...Jim Thompson
 
J

Joerg

Jan 1, 1970
0
Cosmo said:
Thanks Jim I will do that!

All you may need is one resistor and a capacitor to ground. Probably
something like 10k from the 555 output to the capacitor. Then make the
capacitor large enough so you do not see much ripple from the pulses but
small enough so the DC value can change fast enough when the speed changes.

Keep in mind that the pulse width out of a 555 is not very precise and
has drift. This will directly affect you measured DC output.
 
T

Tim Wescott

Jan 1, 1970
0
Cosmo said:
Sorry I wasn't very clear. What I have is a optical sensor sending DC
square waves at some positive constant amplitude, unfortunately the
pulse will be different widths depending on the system. I will combat
this problem with a LM555 timer that will give me a positive constant
amplitude and width every time it is triggered. Now I have these square
wave pulses of all positive DC values, constant width whose frequency
is proportional to the speed of the system and need to get an analog DC
value. I'm very limited with in terms of components and experience; I
would like to do this with diodes, resistors, capacitors, op-amps or
other basic devices.

Basically yes! I need a frequency to voltage converter. I need to make
one with very limited supplies, and I thought since it is only a
positive square wave pulse there may be a way to get the dc value out
of it. I can't buy a FVC.

Thanks again!
If that's your only feedback your application is just crying out to be
done with a microcontroller. Most decent microcontrollers have input
capture units that you can use to time the arrival of your pulse, then
you can just subtract the two arrival times to get a frequency, and feed
that number straight into a PID controller.

Or you can use a few op-amps, each the same size as the microprocessor
you don't want to use, along with caps, resistors, and all those nasty
analog problems*.

* Of course you'll be avoiding all those nasty digital problems going
the other way...

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
Cosmo said:
... I have these square
wave pulses of all positive DC values, constant width whose frequency
is proportional to the speed of the system and need to get an analog DC
value.

Use a charge pump; series limiting capacitor from the
square-wave-generator
into (1) diode clamp to ground and (2) pass diode to output. Connect
an
op amp to the output in current-to-voltage converter configuration.
Add capacitors
to the feedback resistor if you want some low-pass filtering in this
stage.

The limiting capacitor discharges to GND (through clamp diode) on the
negative
slope of the square wave, puts Q = (V-2*Vf)*C charge through the second
(pass)
diode into the current-voltage converter on the positive slope of the
square wave.
The output voltage is then proportional to F*Q. Your Vf (forward
voltage drop of
a conducting diode) can be made small by use of various tricks, or
compensated for.

And frequency-voltage converters of other sorts (like CD4046
phase-locked loop chip) are
cheap and easy enough to use; specialized 'voltage-frequency converter'
modules are
pricey instrumentation gizmos (for sub-1-percent precision use); ignore
'em.

The third possibility is to 'clean up' the square wave by using a '555
as a monostable (fixed
pulse width); then the average value of the output voltage is
proportional to the frequency
at which it's triggered.
 
J

jasen

Jan 1, 1970
0
Basically yes! I need a frequency to voltage converter. I need to make
one with very limited supplies, and I thought since it is only a
positive square wave pulse there may be a way to get the dc value out
of it. I can't buy a FVC.

you already have half of one (given that your pulses are fixed width)
add a low pass filter and the output voltage will be prop[ortional to
the speed.
 
G

GPG

Jan 1, 1970
0
jasen said:
Basically yes! I need a frequency to voltage converter. I need to make
one with very limited supplies, and I thought since it is only a
positive square wave pulse there may be a way to get the dc value out
of it. I can't buy a FVC.

you already have half of one (given that your pulses are fixed width)
add a low pass filter and the output voltage will be prop[ortional to
the speed.
LM2907/17
 
S

Scubasteve

Jan 1, 1970
0
Correct me if I am wrong, but cant your problem be solved by using an
opamp-based integrator. You will have to tweak the values depending on
your application, but it should work.

Is your PID controller opamp based?

Steve
 
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