a strange question

[irving2008]

Stepper motor controlled gas valves... hmm nice idea, and not hard to rig up... but i wouldnt use a stepper.. adds complexity... i would use a RC servo and generate a pulse width based on the average signal level at each frequency I want to sample. The only issue might be the reactiveness of the valves..would they be able to move fast enough?

What sort of fire display?

[Dan Brown]

the valves shouldnt be a problem in regards to how stiff they are as i can have a second manual valve to shut of the system completly therefore the stepper/servo driven valve can be loosend as it dosen't need to be totally gas-tight as there will also be a constant pilot light that besides makeing sure everything ignights properly will also burn of any lost gas.

the only problem with useing servos instead of steppers is that since i realised it was possable for someone like me to build a cnc machine the learning curve has been fairly steep and this follows for ALL my current ways of making a living and i'm starting to get the feeling that maybe i was issued with a pretty small internal H.D

it seems for everything i learn i have to forget something else

so servos may be economical in one way but in brain space they are proberly more than i can afford

[Dan Brown]

answering your question irving the display i have in mind is maybe six indapendant 10 foot (above the stage platform)poles (well away from the crowd obviously) on oposite sides of the stage area giving off varying size puffs of flame (the max being roughly no more than 1 foot in diamiter) in line with the treble, mid and base,
this would preferably be run from a standard analogue light control unit but if another option is more viable then so be it.

as for the type of valve i am thinking of useing something like a simple quarter turn water stopcock, hob taps are o.k but not enough flow unless i try and modefy in some way

[irving2008]

the problem with an analogue light control unit is that its output is triac controlled AC, they are merely on/off switches. I suppose the gas control valve as used on a boiler could be utilised as that is solenoid driven, but i doubt they'd last very long.

I stil think a radio-control model servo would be a good bet for driving the stopcock and simpler to implement than a stepper motor control...

i think your first experiment is to obtain a suitable valve and find out just how much torque is required to operate it... then we could work out necessary accelerations etc and see what is the best driving technology. The rest, getting analogue signal, frequency splitting it, and converting to a control signal is a seperate issue and maybe an existing analogue light contol could be adapted.

(btw I designed and built an analogue light control chaser for a mobile disco I used to run in the 70's.. it was quite sophisticated for the time, with an 8-channel frequency analyser, each channel controlling either intensity or strobe rate, and a seperate beat follower channel.)

[wires]

rc servos are great, they come in many varieties. brushed motor or brushless, plastic, metal or titanium gear. different reaction speeds and torque ratings. might sound a little complicated but its not really, its all just a price thing - more expensive servos tend to be stronger and quicker and to make them stronger they put in metal or titanium gears.

i could only see one potential problem with using rc servos. i can tell you from experience that a servo pushing against an imovable force will burn out its internal driver board. for example, if you had one attached to a valve and your control circuit was calling for the servo to open the valve. if your program was not calibrated right and was getting the servo to try and turn the valve passed open(assuming the valve has a stop) the servo wont last long, you can count on that.

if you go down this route you will need some sort of spring loaded coupler. if the valves are nice and loose then this is no biggie.

this is a typical rc 'servo saver'
http://www.rc-area.co.uk/blog/wp-con...ervo-Saver.jpg
you will see in the pic two cams pressed together by a spring. the spring tension is adjustable. in your application i guess the servo would attach to the top,black, part(you can see a lever on the back of it) and then some linkage from the orange part to the handle on your valve.

there is a simpler design http://news.killerhurtz.co.uk/servo_saver.jpg but its hard to explain how it works. you have to see it in your hand.

if you got something like this in your design you would have no problems, except for programming the PIC. i might be able to help you out by getting some specs for controlling the servos. i can tell you now that they are three wire, most are max 6volts but work away quite happily at 5volts.
i cant remember if it is PCM or PWM they use. anyway, if you need info i might be able to help.

a decent rc servo will cost £80 -£100. really good ones even more. £40 - £80 will get something reasonable but stay away from anything cheaper unless you read good reviews about them.

btw, do you think there will be much lag between the valve opening and the flame shooting up in the air. if there will be some lag, like half a second or more then it might not look right. imagine a nice loud kick drum beat hitting you and half a second or more later the flame shoots up out of the stage. im just saying it might be something to consider.

[wires]

just as an example here is the servo i use in most of my rc cars...
http://www.hitecrcd.com/servos/show?name=HS-7955TG

you will see speed and torque ratings have two values depending on input voltage. this servo is usually £60 - £70
there are other brands but hitec is just what i use, never had any problems with these servos.