position feedback device
position feedback device
(OP)
I am controlling a 48in butterfly valve with a 6in dia neumatic cyclinder.I am using two solenoid valves and creative programming to throttel this valve to the desired water flow. I replaced the positioner because it just didnt work for this valve. There is one drawback, I don't have feed back on the physical position. I have to rely on the expected flow at the position it "should" be. I remember somewhere at a trade show a 4-20mA slide rail position feedback device that I can mount paralle to the motion of the valve. Has anybody seen a device like this and where can I buy one. Thanks.





RE: position feedback device
RE: position feedback device
One wouldn't want any accidents...
RE: position feedback device
Here is one of the latter. These are very commonly used in hydraulic valves.
http
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: position feedback device
RE: position feedback device
Before you wander away, you might look at some of the scales
uses for Digital Read Out. There are some inexpensive
chinese scales that might fit your needs. They have a
wierd digital interface, but have some inexpensive feedback
and display units. You might want to reference the 8"
version found at this site:
http://li
which might provide you the resoltion that you require.
Unfortunately, the DRO displays that are shown do not have
anything other than a visual display. They don't have
a computer interface.
Jraef, what are the prices of the LDT's that you mentioned?
I spent a good 15-20 minutes trying to get a rough
estimate of the cost of some of these units. Second,
what sort of indexing/calibration is required for this
sort of sensor?
With the scales that I have mentioned above, when power is
lost to the devices, there is no "zero index" to them.
In the "interface spec" from the internet from someone
who has "reverse engineered" the interface, the absolute
signed value is set when power is restored to the scale.
Is a similar sort of thing required for the LDTs? Does
the control electronics have to recalibrate itself by
sending down pulses to establish?
cheers,
Rich S.
RE: position feedback device
You could also look at magnetostrictive and LVDT sensors from (e.g.) Balluff, Penny & Giles and Temposonics for position transducers which fit internally to the actuator. I'm a big fan of Balluff's products in hydraulic applications where I need precision and a ruggedised installation. These are fairly expensive too, but if you need precision these are about the best of the mass-market transducer technologies.
Variohm make a variety of resistance-based transducers which sound similar to the type you described. Cost is low relative to the previous suggestions.
http://www.variohm.com/position/index.htm
----------------------------------
Sometimes I only open my mouth to swap feet...
RE: position feedback device
Go to www.jordancontrols.com to find your feedback device. There are a variety of choices.
rmw
RE: position feedback device
Look at the original post. He has a 48" valve and the pneumatic actuating cylinder is 6" diameter, not 6" long! The travel distance is probably 2 feet or more.
To give you an idea, here's a similar setup although not pneumatic and only 18" pipe. The pneumatic actuator would go in place of that big spring. He want to be able to detect how far he has moved the actuator to determine the amount of restriction he is putting on the flow with that valve. He would simply put the LDT parallel to the actuator.
To answer your other questions;
About $1000 for a 24" unit
Yes, it recalibrates itself on power up. It takes a few milliseconds. No manual adjustments are necessary, that's one of the beuties of them.
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376
RE: position feedback device
It would help to know the following:
Available budget
Intended lifespan
Purpose of feedback (indication only or closed loop control?)
Environmental conditions & susceptability to damage.
----------------------------------
Sometimes I only open my mouth to swap feet...
RE: position feedback device
Potentiometers are cheap, but they do tend to spend more of their time than you'd like telling you that your actuator is hard up against one of its end stops. You need to know whether this is going to upset you when it happens.
A.
RE: position feedback device
You mentioned:
"Look at the original post. He has a 48" valve and the pneumatic actuating cylinder is 6" diameter, not 6" long! The travel distance is probably 2 feet or more."
Opps. my bad. I did read 6" throw.
Yep, gottcha on the mounting of the LDT.
And thanks on the pricing. About what I figured, give or
take 50% to 150% or so......
The little scales mentioned above are dirt cheap, 8" $69.00,
16" $108.00, 1 mil accuracy. They also have 4" and 12"
units similarly "scaled" in price (sorry about the pun).
It opens up a lot of possiblities. Using etched glass and
other linear encoders is an older alternative for the
semiconductor industry. One of the clients I worked with
make laser interferometers, and those are not cheap by
any shape or form
Since the OP is "programming" the interface, a little more
"creative programming" could be used to interface the
scales. As mentioned, it's not a "standard" interface,
although not too bad. Again as a downside, when power is
lost to the scale, absolute position reference is lost,
requiring a seperate "home" position to recalibrate it.
Probabily shutting down the water supply to the system would
be a "show stopper", but I can certianly think of
alternatives that would circumvent this problem allbeit with
temporary increases or decreases to water flow.
I'm "playing" with a DRO interface design myself, so these
scales are somewhat in the forefront of my mind right now.
I'm using a digital caliper from Harbor Freight for $17.00
as the interface right now (well, as soon as I either get
a good connetor for it or get this little homebuilt
connector I'm using right now more reliable).
Since they are a "hand tool" they seem robust enough and I'm
sure with a suitable housing could be used in an
industrial setting.
I'm not affiliated with either the manufacturer of the
scales nor the littlemachineshop, although I am a customer
of the latter.
It's an interesting alternative. ScottyUK has some of the
real interesting questions that should be answered first.
Indeed, number of units would also be a question asked as
there might be multiple copies of each required, although
it is implied that this is a one off situation.
Cheers,
Rich S.
RE: position feedback device