Actuator control signal

[Eric Chambers]

I am trying to control 2 valve actuators with one control signal (0 to 10 Vdc). Actuator #1 must be driven first to full open before Actuator #2 can start to open. On closing,it is Actuator #2 then #1. I thought I could do it by taking the control signal and making 2 signals. Input of 0 to 2.5 Vdc = 0 to 10 Vdc (proportional) to Actuator #1. Input of 2.5 to 10 Vdc = 0 to 10 Vdc(proportional) to Actuator #2. I think I need some sort of OP AMP circuit. 24 Vac is available to power project. Parts must survive 120DegF environment. Is there a better way?<p>[ January 08, 2002: Message edited by: Eric Chambers ]<p>[ January 11, 2002: Message edited by: Eric Chambers ]</p>

[russlk]

It will take several milliseconds for the valve to close. The input signal will have to have a very slow risetime, 10 to 20 mS, for that proceedure to work. A better way would be to use a 555 timer to delay the input signal by enough time to ensure that valve #1 has closed before energizing valve #2.

[Eric Chambers]

I Left out some details.
They are steam valves, Valve #1 is 1/3 and valve #2 is 2/3 of capacity. In order to get 100 of capacity these valves must open and close in sequence. 25% call for steam = valve #1 open about 80%, valve#2 is closed. If the load changes to 50% call for steam then valve #1 opens 100% and THEN valve #2 will open to about 30%. If the load then changes to 30% steam valve#2 will close tight and valve#1 will close to about 95%. The input is controlled by a microcontroller, the microcontroller knows how long both valves(Actuators) (1+2) take to open full stroke.
Sounds like fun huh?

[russlk]

How does the controller know how far open the valve is? There must be feedback, right? Is this a motor valve? With stepper motor drive?

[Eric Chambers]

First, I made a mistake with the control signal, it is 0 to 10 Vdc not 2 to 10 Vdc. I edited the original post.
I am making hot water for heating purposes. The valve actuator has an internal controller than strokes the valve to the position indicated by the control signal. The microcontroller is looking at water temperature leaving the heat exchanger. If the water temp is 2DegF too low it opens up for more steam say, 2.1 to 2.2 Vdc. If water temp is 50DegF. too low, it would ask for 100% steam (starting say,2.1, jump to 10 Vdc). This is the only feedback unfortunately. If the valve gets stuck or moves without the signal it might be in the wrong position. Eg. Signal = 10 Vdc, valve = 95%. The microcontroller thinks the valve is wide open, but its not. So, every 3 hours (adjustable)the microcontroller will close the valve (0Vdc) and then go back to the last setting (?Vdc). I was informed the actuator is not a stepper motor. I it just a motor with a gear drive however I will double check this.
At first, I figured I could make 2 signals out of the input with a pair of transisters. But, for this application I might need more percision. Then I though OP AMP. I looked through nuts & volts back issues and books but, could only find a 4-20 mA to 0-10Vdc interface.

[russlk]

Aha, it is becoming more clear: the valves have an internal position servo. If the microcontroller had two outputs, it would be a simple programming problem, but since it appears that there is one output of 0 to 10 volts which corresponds to 0 steam and 100% steam, then it is only necessary to drive the two valves in parallel. With 5volts into each valve, #1 will be 5/10 of 33 1/3% open and #2 will be 5/10 of 66 2/3% open = 50% open.<p>Originally, it was stated that #1 had to fully open before #2 opened. Was there a reason for that? It can be done with two op amps, let me know if you really want to do it.<p>[ January 11, 2002: Message edited by: Russ Kincaid ]</p>

[Eric Chambers]

Sorry about the lack of info, I'm a first time poster!
When the control job was done the contractor was given the specs for the wrong job & simply wired the two in parallel. I can remember him saying "What's the point of having a 1/3 & 2/3 valve and wiring them in parallel. You could do the same job with one valve?".
The correct spec is, 1/3 valve will open untill it's 100% and then the 2/3 valve will open up if it needs more steam. the 1/3 valve will not close untill the 2/3 valve is fully closed.
Yes!, I must do it this way.
(I you think this job was a mess, you should see the rest of the buildings)
Thank you for your help.