Will this Circuit Work?

ndzied1

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Aug 2002
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Chicago, Illinois
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I have been asked by the boss to make a test box for the service and shop guys to test proportional valves. I have to have an output for 24V to power valves with on board electronics and another output that should be switchable between a 0-10V signal and a +/-10V signal.

Will the attached circuit work? I know it will really give me 0-12V and +/- 12V but I'll have a meter on the system and didn't want ot have to deal with a separate DC supply for the +/- 10. Is my thinking right that:
  • Since the valve command signals are feed into a differential amplifier circuit that it doesn't matter that the ref. for this signal is not the 0V from the 24V supply?
  • With the dividor resistors an order of magnitude less than the signal pot that they won't interfere too much with each other... (I'm not looking for anyting perfectly linear here, just something where I can increase and decrease the signal and observe what happens)? Maybe I should use a 10k pot?...
  • Looks like I'll need 1/2 Watt resistors for the 320 ohm dividers. I took the current through the dividers as 24V / (640 total ohms)=37.5mA. Then I calculated the resistor power as 320 ohm X 0.0375^2=0.45 Watt. Is this right?
As many know, my main background is mechanical so any help you could give me would be appreciated. Thanks a lot!!
Circuit1.JPG
 
The numbers are right.

Your design assumes the inputs are floating relative to the power supply. Sadly this is not always the case. If the 0V reference of the power supply and 0V reference of the input are tied together you will have problems. Back in the 80's I used something like that. I wouldn't let anybody use this device unless they knew they had to check the impedance between the two 0V references before using.

Can't you just buy a valve driver of the shelf now? It seems like many of the hydraulic shops sell valve drivers. I would search the web. I just use the motion controller output now.

Ron, hydraulic guys often use a 'twiddle' box to move the hydraulic actuator back and forth to make sure is works the way they intended before the control guys attach their analog control signals.
 
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Peter,

Rexroth sells a box. Of course, it is very German and is designed to control everything under the sun ever designed by Rexroth or Bosch. It has more knobs and doodads than an Apollo command module.

They wanted something simple ... like you said, a twiddle box. I'm also hoping to keep the size down so it can be easily carried on service. That rules out most multiple output bench power supplies....

Back to the drawing board.
 
That will get you the power supplies you need. For the proportional valve's control voltage, you'll want to make that adjustable I assume. A potentiometer with a resistor in series on each end will work. Example: From the +12V, a 100 ohm then a 1k pot then another 100 ohm to the -12V will setup the voltage divider you need for the +/-10Vs. (PS: I don't know the current requirements on the PV's control voltage so those values may need to be tweaked but the ratios are good).
 
The loading of the resistor networks may give you problems, using 24V in to a 0-12V and 12-0-12V dc-dc converters will allow the signal 0V reference to be anchored to the supply 0V. use a 2 pole switch to change voltages. I don't have time to do a sketch.
 
Norm it all depends on how accurate and "portable" you want to make this thing. I am enclosing a picture of a type of DC-DC converter (forward converter) that can use a battery and provide a higher DC output..ie a 9vdc battery could offer +10vdc and -10vdc outputs. This is a simple circuit that could use variable regulators to create what you want.
dc-dc.gif


National Semiconductors offers an online design program called Winbench that may help http://www.national.com/appinfo/webench/scripts/my_webench.cgi/
 
If the box has to be moved around, why not use batteries for the power source. This way it is portable, the batteries are floating, so you can easily earth the mid point current. Also, depending on the batteries you use, they are low power so there is little likelyhood of anything being destroyed if someone makes a wrong connection.

Maybe Norm's original circuit with 3*9V batteries as the power source will do the trick.

Doug

(Edit, Missed the bit about needing 24V power to actuate the valves, maybe 9V batteries will not do)
 
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Doug_Adam said:
(Edit, Missed the bit about needing 24V power to actuate the valves, maybe 9V batteries will not do)
There's no reason why you can't use batteries for JUST the analog portion. I like the separate battery idea. Use an AC powered 24VDC PS for valve power. You can't ask for better isolation than that!... :nodi:

🍻

-Eric
 
ndzied1 said:
Will the attached circuit work? ... Is my thinking right that:

  • Since the valve command signals are feed into a differential amplifier circuit that it doesn't matter that the ref. for this signal is not the 0V from the 24V supply?
  • With the dividor resistors an order of magnitude less than the signal pot that they won't interfere too much with each other... (I'm not looking for anyting perfectly linear here, just something where I can increase and decrease the signal and observe what happens)? Maybe I should use a 10k pot?...
  • Looks like I'll need 1/2 Watt resistors for the 320 ohm dividers. I took the current through the dividers as 24V / (640 total ohms)=37.5mA. Then I calculated the resistor power as 320 ohm X 0.0375^2=0.45 Watt. Is this right?
You need to include the complete circuit for the prop valve to see if the different references matter.


At 1/2 watt those 320 ohm resistors will get very hot! I hope you weren't thinking of putting it all in a plastic box.
 
For cheap I like the battery ideas that have been suggested.

As long as the batteries are required to supply the spool shifting power I think you will be OK.

Otherwise you need a circuit like the ones we use on our board. We use a DC to DC power supply that takes +24 volts and converts it to +/- 12 volts. We use an op-amp to drive the valve. This way you can use higher values for the resistors you use to bias the voltage. The op amp only needs to be a simple unity gain circuit but it will keep the valve from drawing current from the bias circuit.

Where is Willpower100? He is hydraulic/PLC knowledgeable and I thought he was designing a valve driver test unit.

What does Rexroth charge for their unit?
 
At 1/2 watt those 320 ohm resistors will get very hot! I hope you weren't thinking of putting it all in a plastic box.

I have not felt good the last couple of days so did not pay attention to the circuit very close. The divider using the 2 320 Ohm resistors would develop 37.5ma of current, 24 times .0375 is .9 watts ...the 1/2 watt resistors would get too hot.

I do not know enough about the valves (at this time) to offer a precise unit. It all depends on several factors...ie will a 24vdc power supply always be avaible, if so then my last circuit could easily be modified to create your variable +/- 10vdc using that supply or a 9v battery.

If a 24volt dc supply is not going to be available but 120/240 will be then it could actually get easier to make....depends. In this case you could go a couple of ways, my preference would probably be use 2 transformers. One xfmr would supply the fixed 24vdc and the other would be CT to develop your dual ouput +/-10vdc. All this could be mounted in a small box since the current requirements are small.

This is a ckt that could use an AC supply with a CT transformer or could be modified...note the ganged potentiometer used for varying the voltage.
pwrsup.jpg

I wish I had the components laying around like I usually do, I would just build something for you.
 
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Hello, Hello, yes I am back. Yes I built a test unit for servo's and proportionals. My unit was designed for On Board Electronic valves and testing valves without amplifiers using PWM. As well as +/- 200ma for servo testing.

I had to have multiple power supplies, the original concept was to test valves in the field and on the bench. With the emphasis on the bench testing. Worked pretty well once I got the servo amp figured out. It was about 1/3 the price of the Bosch-Rexroth units.

Havent really pursued selling any of them lately, as I have been just to busy with service and installations.

Willpower
 

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