Advantages of using 0-10 vDC signal I/O?

Hello,
Knowing the advantages of 4-20 mA, why would anyone use 0-10 vDC as a control signal?
In my field of building automation, I notice that a lot of end device
manufacturers now use 0-10 vDC or 2-10 vDC control signals.
Are they cheaper or simpler to build, or electronically more reliable?
Plus, many controllers now have either fixed 0-10 vDC outputs, or outputs that can be changed to 4-20 mA by changing a jumper. I am curious as to what circuitry is bypassed or switched internally when the jumper is added. Does anyone know of a website where I could get a peek at the electronic schematics of a switchable transmitter?
(I have found schematics of 4-20 mA transmitters that allow a jumper to change the range to 0-20 mA, or the range of the internal inputs, but my searches were fruitless for the above scenario).
Thank you,
Ross McGregor snipped-for-privacy@yahoo.ca
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 8 Feb 2004 11:06:29 -0500, snipped-for-privacy@yahoo.ca wrote:

0-10v signals are potentially more precise. To convert a 4-20 ma signal to a voltage prior to A/D conversion, a precision resistor must be used. This resistor may be temperature dependent, which would introduce inaccuracies.

They might be using the precision resistor method to convert the 4-20 ma signal to a voltage before sending it out, or they might be generating the voltage using totally independent circuitry from the 4-20 ma loop circuitry.
-Robert Scott Ypsilanti, Michigan (Reply through this forum, not by direct e-mail to me, as automatic reply address is fake.)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
0-10 Vdc, +/- 10 V dc, 0-5 V dc, 0-1 V dc and others are standards that developed out of different places and industries, along with the later 0-20 (I think) and 4-20 mA standards. A book could be written in this subject, and some have been. In some cases vendors reinvented the wheel to beat patents and as improvements.
I myself like the 4-20 mA as it more immune to noise and often intrinsically safe. It can also be used for voltage I/O by adding a resistor.
Dennis McHenney
wrote:

address is fake.)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

This is not a valid reason. Firstly, I assume you mean "accurate" not "precise". Good resistors are available. Furthermore, every circuit is full of other resistors and components that affect accuracy. It is the presence of the resistor in a current loop that makes the loop immune to a number of error sources that affect voltage loops.
Walter.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

control signal?

Because it is easier to understand by people that don't know anything about instrumentation. You've got to admit, 4-20 mA is not the first thing that pops into your mind. Also, if someone is inventing a new type of transmitter, making it operate as a two wire, 4-20 mA device adds a level of sophistication that may seem like a totally unnecessary complication.
In systems with very short wire runs and in a low electrical noise environment, the advantages are not evident.
Or was your question rhetorical?
Walter.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
We prefer to use/design voltage outputs because they require less power. We can drive the voltage outputs easily with a DC to DC converter. 10 volts driving a 10K input requires .01 watts. 20 ma across 250 ohms is .1 watts. That is a big difference in power. If I had my way analog signals would be sent on a RS-485 type line as signed 16 bit numbers similar to the way SSI transducers report positions. Then the lines could be easily optically isolated too.
Peter Nachtwey
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
says...

Congratulations, you've just re-invented the fieldbus...
--Gene
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

volts
watts.
signed
Then
I am pefectly aware of field buses. A field bus allows for multiple devices on the bus. However, updates can be slow with a lot of jitter and the microsecond level. If I have a motion controller that is controlling a valve then I may want to update the command to the valve in a much more deterministic means than is available with a field bus. Some of the big hydraulic manufacturers have tried using CAN to control valves. I haven't seen any success here in the US except maybe in relatively static applications.
If the valves could support SSI type control signals then the controller can SIMULATANEOUSLY update many servo valves. The would be only one device on each cable just as with 4-20ma now.
Peter Nachtwey
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
says...

Some fieldbuses are deterministic in their timing, CAN not being one of them. RS-485, being asynchronous, will not be repeatable to 1 microsecond below 1 Mbit/sec, a speed rarely obtained over any distance. Field buses with deterministic behavior are non-trivial, which is why "fast enough" tends to dominate the market. There's also "feature creep". Once you have a digital link for data, it becomes very tempting to use that link for fault detection, firmware updates, two-way comms, etc. All of which require management packets as well as data to travel over the wire.
--Gene
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

power.
10
.1
as
and
controlling a

big
haven't
can
on
Which field buses? Keep in mind that phase delay is a killer in a motion control application.
Peter Nachtwey
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
says...

WorldFIP has demonstrated 1 usec accuracy, although it requires a master timebase (they claim that slave clock error is a larger contributor to timing error than the bus itself).
Delta Tau's MACRO was explicitly designed for motion control.
TTTech's Time Triggered Protocol (not exactly a fieldbus, but designed for determinism).
--Gene
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

power.
10
.1
as
and
controlling a

big
haven't
can
on
The CAN bus can be coerced into deterministic timing -- the automotive guys have an extension to it that competes with the time-triggered protocol. But just saying "CAN" doesn't give the whole story, and the entire system has to be designed with deterministic timing in mind.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Peter,
Once again we are discussing across industries that use the same words for different things. I'm talking process control and when I hear "fieldbus" I understand "Foundation Fieldbus". In our field we would not attempt to control the position of a valve using an FF signal. The FF signal would tell the valve where to go and the valve's internal controller (a part of the FF system) would position the valve where is was requested. It would also report this position and the required air pressure back to the central control system.
Walter.

and
a
can
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Actually, no, I wouldn't buy them. The fact that the signal uses less power is not relevant. The instrument as a whole still needs power and I suspect that it needs a lot more that .01 watts. What type of instruments do you design? How much power do they require to operate? How do they get it? It requires a rather high degree of sophistication to design a measurement instrument that operates on the 4 mA * 12 volts = 48 m Watts that is typical of 4-20 mA instruments.
The fact that the entire instrument power is carried by the signal itself provides high degree in noise immunity. What is your typical wire run length?
If you used an RS-485 signal, how would the device receive its operating power? That would mean adding another pair of wires. The user's costs start to run up.
Walter.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Peter,
This can be a confusing newsgroup since it deals with such different industries which work on such different scales. In the process industries 48 mWatt (4-20 mA) is sufficient to operate the great majority of instruments. External power is only used for very complex devices such as chromatographs. Loop power comes directly from the DCS. 4-20 mA also comes from the DCS and is sufficient to power even the largest valves. I mean 24" diameter, 1000 psi throttling valves. But that is because the real motive power is in the form of compressed air at 100 psi. We almost never use direct motor operated throttling valves.
Our wire runs are typically in the hundreds to thousands of feet. We avoid field located electronics as much as absolutely possible. This is because:
a) We deal with potentially explosive atmospheres and maintenance absolutely hates opening X-proof boxes. They also hate closing them which is another problem.
b) The boxes are outdoors in temperatures form + to - 40 deg C. It's very difficult getting -40C rated electronics. It's even harder to get maintenance out there on days like that. They'd rather wait until spring.
c) Supplying DC power is another nuisance we don't need. We would get serious voltage drops. Local DC supplies have serious reliability problems. Our central power supply is fully battery backed and redundant with auto transfer.
d) Lightening does weird things.
e) Ground loops do even weirder things.
f) Etc.
So each of our preferences have their reasons.
Walter.
PS. I still don't like 0-10 V. I prefer the live zero for even more reasons. ;-)

power.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@yahoo.ca wrote in message
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

signals.
Are you refering to transmitter manufacturers or control system (receiving end) manufacturers? Having worked for a DCS manufacturer, the input range depends upon what part of the hardware you are looking at. The actual circuit board with the A/D converter may take a 0-10v (or other zero based voltage input) because that is what the A/D circuit takes. However, the terminal block is looking for a 4-20 ma signal. Somewhere between the terminal block and the circuit board is a high precision resistor to convert current to voltage. The A/D circuit converts voltage to counts (an integer number, perhaps from 0 to 4095), the software then looks at the counts that would come from 4 ma as 0% and 20ma as 100%. (eg. 750 to 3750). So the system is expecting a 4-20 ma signal, but there is 0-10 volt hardware present. This method will also handle signals slightly below 0% (4ma) or above 100% (20ma) due to slight calibration errors in the transmitter.
To avoid error due to voltage drop, the resistor is located only a few feet from the A/D, perhaps in the same or a nearby cabinet.
The many very goog reasons for using 4-20 ma rather than zero based volts as the signal from the field have been fully discussed in earlier posts.
John Shaw Process Control Solutions http://www.jashaw.com/pid
snipped-for-privacy@yahoo.ca wrote in message
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.