Flow Switch with Pulse

[TechGladiator]

I am designing an application for an embedded computer (PC/104) and I
need to attach a FLOW switch w/ Pulse. It basical generates pulse
output proportional to the rate of flow. One example is
http://www.clarksol.com/html/crfo_crfa.htm ..

I have a Digital I/O board connected to my computer via RS232 and I can
read other types of inputs such as Start/stop switches, etc..

My question is, since the flow switch generates pulses so quick, is
there any type of circuit that I can use to read this pulses other than
my I/O board? The reason I am asking is because I am afraid that
becase these pulses can happen so quick that I will miss some reading
them with this I/O board.

Thanks in advance,

Miguel

 

[martin griffith]

I am designing an application for an embedded computer (PC/104) and I
need to attach a FLOW switch w/ Pulse. It basical generates pulse
output proportional to the rate of flow. One example is
http://www.clarksol.com/html/crfo_crfa.htm ..

I have a Digital I/O board connected to my computer via RS232 and I can
read other types of inputs such as Start/stop switches, etc..

My question is, since the flow switch generates pulses so quick, is
there any type of circuit that I can use to read this pulses other than
my I/O board? The reason I am asking is because I am afraid that
becase these pulses can happen so quick that I will miss some reading
them with this I/O board.

Thanks in advance,

Miguel

Try posting to comp.arch.embedded, include make and model etc,pretty
good bunch of people, but look for "external interrupt" in your
documents. Dont really know much about PC104


martin

 

[Sjouke Burry]

I am designing an application for an embedded computer (PC/104) and I
need to attach a FLOW switch w/ Pulse. It basical generates pulse
output proportional to the rate of flow. One example is
http://www.clarksol.com/html/crfo_crfa.htm ..

I have a Digital I/O board connected to my computer via RS232 and I can
read other types of inputs such as Start/stop switches, etc..

My question is, since the flow switch generates pulses so quick, is
there any type of circuit that I can use to read this pulses other than
my I/O board? The reason I am asking is because I am afraid that
becase these pulses can happen so quick that I will miss some reading
them with this I/O board.

Thanks in advance,

Miguel

Try to find a quadrature counter board,it gives you
a running total for the flow.
Most of those come with 4 counters,and can read
most incremental up/down (digital) potmeters.

 

[Frank Bemelman]

I am designing an application for an embedded computer (PC/104) and I
need to attach a FLOW switch w/ Pulse. It basical generates pulse
output proportional to the rate of flow. One example is
http://www.clarksol.com/html/crfo_crfa.htm ..

225 Hz max, according to that page. That is not high.

I have a Digital I/O board connected to my computer via RS232 and I can
read other types of inputs such as Start/stop switches, etc..

Is that I/O board connected directly to your PC/104 computer, and do you
have a PC connected to that? The program running on the PC/104 should
be able to poll the inputs fast enough for an accurate count of
225 Hz.

OTH, if your (dumb) I/O board is connected via RS232 and the program
runs on the other side of the RS232 link, 225 Hz is indeed a bit too
much.

In that case you can use an 8 or 16 bit counter. The counter is
clocked by the flow meter. Then you can read the counter, at
a slower pace. You will suffer a delay between the measured and
actual result; if that is a problem you have to get rid of that
RS232 connection.

 

[TechGladiator]

225 Hz max, according to that page. That is not high.


Is that I/O board connected directly to your PC/104 computer, and do you
have a PC connected to that? The program running on the PC/104 should
be able to poll the inputs fast enough for an accurate count of
225 Hz.

The I/O board is connected to RS232 port in my PC/104 which is really a
full blown computer running my application). Excuse me if I sound dumb
but I can read the serial port ever 20-25ms, how do I relate that to
Hz?

OTH, if your (dumb) I/O board is connected via RS232 and the program
runs on the other side of the RS232 link, 225 Hz is indeed a bit too
much.

In that case you can use an 8 or 16 bit counter. The counter is
clocked by the flow meter. Then you can read the counter, at
a slower pace. You will suffer a delay between the measured and
actual result; if that is a problem you have to get rid of that
RS232 connection.

I think this is possibly the best way because just in case there is a
delay for some reason within the OS (since it's not a RTOS) then I
still will be able to read the counter. Now my question is, do you
know where I could find these counters?

Thanks
Miguel

 

[petrus bitbyter]

I am designing an application for an embedded computer (PC/104) and I
need to attach a FLOW switch w/ Pulse. It basical generates pulse
output proportional to the rate of flow. One example is
http://www.clarksol.com/html/crfo_crfa.htm ..

I have a Digital I/O board connected to my computer via RS232 and I can
read other types of inputs such as Start/stop switches, etc..

My question is, since the flow switch generates pulses so quick, is
there any type of circuit that I can use to read this pulses other than
my I/O board? The reason I am asking is because I am afraid that
becase these pulses can happen so quick that I will miss some reading
them with this I/O board.

Thanks in advance,

Miguel

Miguel,

Don't know the properties of your I/O board, but if it is meant to read the
status of a binary input you can't do accurate time measurements with it.
That is, not using a normal I/O pin. A possibillity to circumvent the
restrictions for the normal I/O is using an input pin of the serial
interface directly. So CTS, DCD or DSR. You'll need a MAX232 or similar to
convert the voltage levels, but the rest is done in software. You need to
set up the UART to generate an interrupt on an edge of the input pin you use
and measure the time between to interrupts. Alternatively you can count the
pulses in a certain amount of time.

Of course you can use hardware as wel. In the old days you'd build a
counter/timer using TTL logic circuits and a UART to transfer the measured
value to the PC. These days, a microcontroller is the way to go. If you want
a serial interface, you'll still need a MAX232, but if you have a free
parallel printerport you can do easier in EPP-mode.

petrus bitbyter

 

[TechGladiator]

Miguel,

Don't know the properties of your I/O board, but if it is meant to read the
status of a binary input you can't do accurate time measurements with it.
That is, not using a normal I/O pin. A possibillity to circumvent the
restrictions for the normal I/O is using an input pin of the serial
interface directly. So CTS, DCD or DSR. You'll need a MAX232 or similar to
convert the voltage levels, but the rest is done in software. You need to
set up the UART to generate an interrupt on an edge of the input pin you use
and measure the time between to interrupts. Alternatively you can count the
pulses in a certain amount of time.

Of course you can use hardware as wel. In the old days you'd build a
counter/timer using TTL logic circuits and a UART to transfer the measured
value to the PC. These days, a microcontroller is the way to go. If you want
a serial interface, you'll still need a MAX232, but if you have a free
parallel printerport you can do easier in EPP-mode.

petrus bitbyter

Petrus,

Thank you for your advice. I think that I am going to have my designer
guy(EE or whatever he is) build the counter into the Input board as
this seems to be the best approach. Currently we are in the process of
having this board designed for us but since I am a software guy and not
hardware I want to make sure that I got different opinions and try to
figure out which one will work out the best.

Thank again
Miguel

 

[Chris]

I am designing an application for an embedded computer (PC/104) and I
need to attach a FLOW switch w/ Pulse. It basical generates pulse
output proportional to the rate of flow. One example is
http://www.clarksol.com/html/crfo_crfa.htm ..

I have a Digital I/O board connected to my computer via RS232 and I can
read other types of inputs such as Start/stop switches, etc..

My question is, since the flow switch generates pulses so quick, is
there any type of circuit that I can use to read this pulses other than
my I/O board? The reason I am asking is because I am afraid that
becase these pulses can happen so quick that I will miss some reading
them with this I/O board.

Thanks in advance,

Miguel

Hi, Miguel. If you use a toggle flip-flop to get a divide-by-2
counter, each pulse will result in a change of logic state, which will
be much easier to capture than a brief pulse.

If your latency is such that you're sure you won't miss noting the
change during the time between pulses, this should work. Any faster,
and you'll need a counter as described.

Remember that you're interested in rate, rather than exact positioning.
If you were to use a divide-by-16 counter and just monitor the MSB,
you'll still get a good idea of how fast the flowmeter is spinning.

Make sure you also use some signal conditioning to prevent multiple
toggles or counts in the presence of electrical noise, for either this
toggle FF or a counter.

Good luck
Chris

 

[Frank Bemelman]

The I/O board is connected to RS232 port in my PC/104 which is really a
full blown computer running my application). Excuse me if I sound dumb
but I can read the serial port ever 20-25ms, how do I relate that to
Hz?

With 25mS interval, you can capture upto 20Hz. Fmax = (1/(0.025+0.025)).

I think this is possibly the best way because just in case there is a
delay for some reason within the OS (since it's not a RTOS) then I
still will be able to read the counter. Now my question is, do you
know where I could find these counters?

Missing a pulse from a flow meter is not desireable, but it is
often not a big deal either, if it doesn't happen too often of
course. These kind of flow meters are pretty inaccurate, 10% error
is quite usual.

The binary outputs of the counter are connected to the binary
inputs. Depending on how your board reads the inputs, there
are some things you should be aware of.

Suppose you use an 8 bit counter, connected to 8 digital inputs.
You need to read all the inputs at once, at a moment the outputs
are all stable. It requires a counter and a latch and... You also
need a filter to remove possible glitches from the flow meter signal.

My advice is to buy a dedicated counter board, one that sits directly
on top of your PC104, for example:

http://pc104.winsystems.com/products/pc104/pcmctc.html

In fact, if you use a board that connects to the PC104 directly,
you can use a normal I/O board to count your flow meter
pulses, because in that case you can poll it much faster than
20ms.

I could shout some partnumbers to build your own counter, but since
you need to ask for partnumbers there is very little chance you would
be able make a reliable circuit.

 

[Fred Bloggs]

With 25mS interval, you can capture upto 20Hz. Fmax = (1/(0.025+0.025)).




Missing a pulse from a flow meter is not desireable, but it is
often not a big deal either, if it doesn't happen too often of
course. These kind of flow meters are pretty inaccurate, 10% error
is quite usual.

The binary outputs of the counter are connected to the binary
inputs. Depending on how your board reads the inputs, there
are some things you should be aware of.

Suppose you use an 8 bit counter, connected to 8 digital inputs.
You need to read all the inputs at once, at a moment the outputs
are all stable. It requires a counter and a latch and... You also
need a filter to remove possible glitches from the flow meter signal.

My advice is to buy a dedicated counter board, one that sits directly
on top of your PC104, for example:

http://pc104.winsystems.com/products/pc104/pcmctc.html

In fact, if you use a board that connects to the PC104 directly,
you can use a normal I/O board to count your flow meter
pulses, because in that case you can poll it much faster than
20ms.

I could shout some partnumbers to build your own counter, but since
you need to ask for partnumbers there is very little chance you would
be able make a reliable circuit.

They have the analog output version- then he could just use a stock A/D
on the I/O board; or if he's stuck with the pulse output, there are such
things as F->V converters ;_)- input F range is only 1:15, he can signal
condition with that and then go into stock A/D. This avoids all that
synchronization and latching nonsense. What I want to know about is that
20oF water.

 

[Frank Bemelman]

They have the analog output version- then he could just use a stock A/D
on the I/O board; or if he's stuck with the pulse output, there are such
things as F->V converters ;_)- input F range is only 1:15, he can signal
condition with that and then go into stock A/D. This avoids all that

I don't know if he wants to measure flow and/or needs a totalizer.
Totalizing by counting pulses is easy, but deriving a total from
a converted F/V will cost him another 5-10% accuracy if not more.
It seems he may have problems ready an A/D converter at steady and
predictable intervals, with his RS232, his OS not being realtime,
and whatever other problems he may have to deal with.

synchronization and latching nonsense. What I want to know about is that
20oF water.

Typically comes in ice cubes But you could pump 20oF oil through
it also, slowly.

 

[Fred Bloggs]

I don't know if he wants to measure flow and/or needs a totalizer.
Totalizing by counting pulses is easy, but deriving a total from
a converted F/V will cost him another 5-10% accuracy if not more.

Not what Joerg says- claims 0.1% accuracy with cheap parts

It seems he may have problems ready an A/D converter at steady and
predictable intervals, with his RS232, his OS not being realtime,
and whatever other problems he may have to deal with.

"That's what I'm saying." The F->V provides precise and continuous
control of the integration interval, whereas the totalizer does not.

 

[TechGladiator]

Frank Bemelman wrote:

I don't know if he wants to measure flow and/or needs a totalizer.
Totalizing by counting pulses is easy, but deriving a total from
a converted F/V will cost him another 5-10% accuracy if not more.

Actually what I am trying to do is measure the speed of a conveyor
powered by a hydrailic motor. Since conveyors have variable speed
which is controlled by the flow of oil throught the lines I need the
pulses from the switch in my program as a reference so I can perform
certain functions with the output board. For me it's more important
that i read every pulse even tho it may be like 20-100ms delayed than I
miss a pulse. On average probably only about 4-8 pulses per second
will be generated by this flow switch so for the most part I will have
no problem reading from the program with a counter circuit, but I
always run the possibility of missing pulses without a counter.

--Miguel

 

[Michael A. Terrell]

Try posting to comp.arch.embedded, include make and model etc,pretty
good bunch of people, but look for "external interrupt" in your
documents. Dont really know much about PC104.

martin

PC104 is an industrial version of the old EISA buss but the boards
stack instead of using slots on a motherboard. http://www.aaeon.com/
made the boards that I worked with.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Stanislaw Flatto]

Frank Bemelman wrote:
<...Text cut out.>

Actually what I am trying to do is measure the speed of a conveyor
powered by a hydrailic motor.

Now you are talking!
You have the shaft of the motor which IS the reading directly related to
speed of conveyor. Any toothed wheel and magnetic sensor will give you
the needed result. Flowmeters have a nasty habit of beying influenced by
the piping configuration(s) and can give errors from here to eternity
depending on the hydraulic expertise (read learning from mistakes) of
the constructor.

Since conveyors have variable speed
which is controlled by the flow of oil throught the lines I need the
pulses from the switch in my program as a reference so I can perform
certain functions with the output board. For me it's more important
that i read every pulse even tho it may be like 20-100ms delayed than I
miss a pulse. On average probably only about 4-8 pulses per second
will be generated by this flow switch so for the most part I will have
no problem reading from the program with a counter circuit, but I
always run the possibility of missing pulses without a counter.

--Miguel

HTH

Stanislaw
Slack user from Ulladulla.