Not automotive but very intriguing - Team Camaro Tech
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post #1 of 12 (permalink) Old Sep 28th, 07, 01:03 PM Thread Starter
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Not automotive but very intriguing

Looking for an online calculator to figure voltage drops, pre- load.

Pre-load is the best way I can describe it.

Here's what I did
A PLC is outputting 24vdc 4 to 20 ma signal to 3 1400 HP, 1400 RPM engines of mine for speed control input. The engines have ECM's and you interface with a laptop and a program that is reminiscent of Excel(but it's not). You can tell the engine which speed control signal to use. 0-5VDC is customary, as there is a potentiometer on the engine panel that uses 0-5VDC. 4 to 20ma is enabled for PLC or other operations.

It's an either/or selection. Can't have both. This customer wanted both and I don't blame him.

We found a print of dual speed control(local and remote) that converts the 4 to 20 to 1 to 5 VDC, by sending the 4 to 20 positive to ground through a 250Ω resistor. You grab positive upstream of the resistor and send it to a selector switch to choose local or remote output to the ECM.

What is intriguing me is how to predict what the affect of a variation in the 24VDC as well as varying the 4 to 20 signal will have. It is performing, I am just intrigued by it.

Any insights?

If I have this clear as mud, ask away and I'll try to clarify.

Tim Smith
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post #2 of 12 (permalink) Old Sep 28th, 07, 01:10 PM Thread Starter
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Re: Not automotive but very intriguing

Not so worried about an online calc, as understanding what principle applies, and the math.

Thanks

Tim Smith
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post #3 of 12 (permalink) Old Sep 28th, 07, 02:14 PM
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Re: Not automotive but very intriguing

Well, after years of training on Naval aircraft up to and including updated F/A-18's...I would think that it would depend on how much electrical load is pulling on the 24VDC.
?
And if you vary the (hp/wattage) amps, it either works harder/better or less.

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post #4 of 12 (permalink) Old Sep 28th, 07, 02:16 PM
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Re: Not automotive but very intriguing

Electrical math can be googled. I cant show you what I have...

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post #5 of 12 (permalink) Old Sep 28th, 07, 05:41 PM
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Re: Not automotive but very intriguing

The nice thing about a 4-20ma current loop is that it's pretty well regulated. So even if your length of line is 1000' long, you can be assured of getting your 4 to 20ma current. The driver simply overcomes the circuit resistance until the desired current level is reached. What's more, even if your 24volts floats around, the current driver should maintain the correct current level for the setting.

The 250 ohm resistor gives you the right voltage at the 20ma setting (20mA * 250 = 5volts). But at the 4mA setting you get 1 volt. It's pretty close, but may be one source of error.

The real question is how much of an RPM change does 1 volt create (not related to the 4mA case, just a unit of measure). If you know the current regulation spec (say it's +/- 0.1&#37 then the voltage change will be the same +/- 0.1%. By knowing how much of an RPM variance 1 volt gives, you can estimate the variance in RPM.

I speculate that the speed control is a closed loop anyway, so whatever the speed control signal (voltage or current) might say, the actual RPM signal from the motor makes the controller adjust up or down until the right RPM is achieved. As a result, current regulation differences won't have a significant effect since the closed loop design will offset any errors. What's more, the voltage control signal will be effected equally by changes in current - if the current regulation changes by 1%, the voltage signal will change by 1% also.

Most likely, internal to the engine controller, when you put it in the 4-20ma mode, it's switching a 250 ohm resistor into the circuit and using the 0-5volt signal anyway.

Does that help?

Dave
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post #6 of 12 (permalink) Old Sep 28th, 07, 07:09 PM Thread Starter
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Re: Not automotive but very intriguing

Quote:
Originally Posted by dnult View Post
The nice thing about a 4-20ma current loop is that it's pretty well regulated. So even if your length of line is 1000' long, you can be assured of getting your 4 to 20ma current. The driver simply overcomes the circuit resistance until the desired current level is reached. What's more, even if your 24volts floats around, the current driver should maintain the correct current level for the setting.

The 250 ohm resistor gives you the right voltage at the 20ma setting (20mA * 250 = 5volts). But at the 4mA setting you get 1 volt. It's pretty close, but may be one source of error.

The real question is how much of an RPM change does 1 volt create (not related to the 4mA case, just a unit of measure). If you know the current regulation spec (say it's +/- 0.1%) then the voltage change will be the same +/- 0.1%. By knowing how much of an RPM variance 1 volt gives, you can estimate the variance in RPM.

I speculate that the speed control is a closed loop anyway, so whatever the speed control signal (voltage or current) might say, the actual RPM signal from the motor makes the controller adjust up or down until the right RPM is achieved. As a result, current regulation differences won't have a significant effect since the closed loop design will offset any errors. What's more, the voltage control signal will be effected equally by changes in current - if the current regulation changes by 1%, the voltage signal will change by 1% also.

Most likely, internal to the engine controller, when you put it in the 4-20ma mode, it's switching a 250 ohm resistor into the circuit and using the 0-5volt signal anyway.

Does that help?
You spotted the bug in the ointment. You know juice. Actually you saw all of them. A+

But they were bugs I knew I would have and we can live with.

The 1 volt at the bottom side does cause a little grief but we can live with it. For 4ma we get 1063 rpm instead of 1000. I program the floor and ceiling into the ECM, where 0 volts is 1000 and 5 volts is 1400. We can hit the ceiling but not the floor(from the PLC).

The other bug is that the PLC is not closed loop in regard to RPM, it is closed loop in regard to system inlet pressure. It is direct acting, if pressure is high then output is max.

It doesn't have to be perfect, all they really need these machines to do is slow down some if gas flow falls off.

Your last paragraph re internal switching and still using 0-5 volts... no sir. The 0-5 volts lands at different terminals than the 4 to 20. So it ignores any 0-5 input and looks at 4 to 20. We want to use either input and I added an external resistor to convert the 4 to 20 into 1 to 5 vdc, added a selector switch(0-5 on top, 1-5 on bottom, and selected output in the middle going to the ECM), and when the operator starts the unit he switches it to local and uses the 0-5 speed pot until everything is happy, then he switches it to the PLC(which I converted to 1 to 5). And I have enabled the 0 to 5 volt speed input, nothing is connected to the 4 to 20 terminals now.

I was just intrigued by what would happen if the 24vdc carrying the 4 to 20 fluctuates by a volt or 2. You have me wondering if I have a gap in my understanding of this 4-20 juice. It is always 24vdc(more or less) I thought, but only capable of supporting a given (ma) load. So when you put a load on it, that exceeds the current driver then the voltage falls off. And intrigued by what math is employed here. It's not OHM's Law as I normally use it. We may have discovered that I don't know what 4 to 20 is...Line me out please.

At 4ma I read 1.1 VDC on a Fluke 87, 1063 rpm
At 8 ma I get 2.1 vdc and 1160 rpm
at 12 ma I get 3.1 vdc and 1260 rpm
At 16 ma I get 4.1 vdc and 1360 rpm
And I hit 1400 rpm before it gets to 20 ma, but also before I should voltage-wise, so the Cat end of it is not perfect.

So it's behaving quite well, just has an errant floor.

Are you telling me that the 24 vdc carrier will be nice and constant? This is in the sticks, and power fluctuations are the norm. I would have thought a line current fluctuation may carry into it.

If it is 24 volts, and altering the ma alters the voltage by having a load to ground(the resistor), then that tells me the voltage at the driver end is maintained constant and the current is increased until it handles the load. Or am I heading the wrong way?

Thanks for your help.

Tim Smith
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post #7 of 12 (permalink) Old Sep 28th, 07, 10:22 PM
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Re: Not automotive but very intriguing

I believe you will never see 0 current because an idle circuit will always have quiescent (static) current, just enough to operate the circuits.

If you have 0 current, then the input to the circuit will go high because of no current flow.

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post #8 of 12 (permalink) Old Sep 28th, 07, 10:35 PM
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Originally Posted by wiskeesour View Post
.... I cant show you what I have...
Im hoping you gents found humor in this in sayin I never have any idea what Im talkin about.

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post #9 of 12 (permalink) Old Sep 28th, 07, 10:38 PM
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Re: Not automotive but very intriguing

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The other bug is that the PLC is not closed loop in regard to RPM, it is closed loop in regard to system inlet pressure. It is direct acting, if pressure is high then output is max.
Ok, so it's a close-loop pressure control system. Closed loop systems will overcome any error in the loop - provided they have more than proportional control. But thats a whole nuther subject. Suffice it to say that closed loop systems account for offsets in the control loop.

Quote:
Your last paragraph re internal switching and still using 0-5 volts... no sir. The 0-5 volts lands at different terminals than the 4 to 20. So it ignores any 0-5 input and looks at 4 to 20.
Not so fast...you're talking about terminals on the outside of the controller. I'm talking about the inside. It is doubtful that the controller implements two different methods of control. It may provide two different interfaces - voltage and current control. But in the guts both get distilled down to one control point however it may be implemented. You option of 0-5v or 4-20ma is simply an interface change, but the internal implementation remains the same. Most likely it's voltage control fed into a digital to analog converter.

Quote:
I was just intrigued by what would happen if the 24vdc carrying the 4 to 20 fluctuates by a volt or 2. You have me wondering if I have a gap in my understanding of this 4-20 juice. It is always 24vdc(more or less) ...Are you telling me that the 24 vdc carrier will be nice and constant? This is in the sticks, and power fluctuations are the norm. I would have thought a line current fluctuation may carry into it.
A perfect 24volt source can supply infinate current. Obviously that isn't possible in real life. The supply itself has resistance, and the components that carry the current have a limit as well. The 4-20ma current loop is a regulated current source. To get 20mA out of a 24volt source you need 1200 ohms of resistance internal to the controller. In reality, that 1200 ohms of resistance is a transistor or a FET that varies it's resistance to maintain the desired loop current. As a result, your voltage supply could vary widely and not affect the loop current. You're looking for 5volts at the load end accross your load resistor. Disregarding limitations in the controller itself, your supply voltage could dip perhaps as low as 6 or 7 volts and still be able to maintain 20mA in the loop. The point is that voltage has very little to do with it - it's a current regulator not a voltage regulator.

Dave
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post #10 of 12 (permalink) Old Sep 28th, 07, 11:42 PM Thread Starter
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Re: Not automotive but very intriguing

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Originally Posted by wiskeesour View Post
Im hoping you gents found humor in this in sayin I never have any idea what Im talkin about.
I understood Harley, it's coo

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post #11 of 12 (permalink) Old Oct 21st, 07, 06:14 PM Thread Starter
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Re: Not automotive but very intriguing

Wanted to give this thread a little closure in case somebody comes along wants to know....

I was applying Ohm's Law wrong. The 24VDC has SQUAT to do with it, as Dnult kept trying to tell me. Hey, I keep telling folks I'm like a Lab, sometimes it takes a 2 by 4 to get my attention and sometimes a whisper does the trick

You Do NOT put 24 volts in the equation. You input the ma and the voltage you DESIRE, Ohms spits out the resistor you put to ground and poof you have your desired voltage on the upstream leg of the resistor. I think it's pretty cool stuff anyway. I doubt if too many people care about it though. To me, it's just another sign of math and science at work around us everyday. Even when you don't get it

Tim Smith
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post #12 of 12 (permalink) Old Oct 21st, 07, 10:59 PM
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Originally Posted by wiskeesour View Post
.......I would think that it would depend on how much electrical load is pulling on the 24VDC.
?
And if you vary the (hp/wattage) amps, it either works harder/better or less.

Hmmmm....maybe I got lucky?

AE2(AW) Harley Moody, USNavy
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November 26th, 2008 was my last day to serve my country as a member of the World's Greatest Navy.
Thanks Chief.
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