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Topic: Using a solenoid for variable force (Read 123 times) previous topic - next topic

Johan_Ha

I have an idea of creating a train horn like device with a variable pitch. I need a variable force that acts on the membrane of the horn. I have the idea of the construction ready. In short, a ring pushes the membrane with a variable force.

My question is, can a solenoid be used here? I'd drive the solenoid with a power transistor and PWM. The function of the solenoid would not be to move the whole throw. It would move only a fraction, while the membrane would act as a counter force.

Paul_KD7HB

I have an idea of creating a train horn like device with a variable pitch. I need a variable force that acts on the membrane of the horn. I have the idea of the construction ready. In short, a ring pushes the membrane with a variable force.

My question is, can a solenoid be used here? I'd drive the solenoid with a power transistor and PWM. The function of the solenoid would not be to move the whole throw. It would move only a fraction, while the membrane would act as a counter force.
In the US, electrically driven train horns consist of two horns, each a different frequency. You get three different tones by using them individually or both together.

Paul


jremington

Quote
I'd drive the solenoid with a power transistor and PWM.
That would probably work, but would require experimentation to get the right force (force is proportional to average coil current). Don't forget the flyback diode across the solenoid coil.

Johan_Ha

#3
Jun 20, 2017, 09:31 pm Last Edit: Jun 20, 2017, 09:54 pm by Johan_Ha
Yes. I'm prepared to do a lot of experimentation. Here's a guy that makes a train horn of pvc pipes and joints:

https://www.youtube.com/watch?v=MNaZSaT5J5c

I'd have to place the solenoid and the ring inside the cup, pushing the membrane from behind (seen at video 2:10).

So it's probably ok to run variable current through the solenoid just to create a force rather than a movement. The full movement or throw of a solenoid plunger might be some 15 mm but the membrane might let it move only 2-3 mm at full force. I guess that's ok for the solenoid. I mean, it's just a coil and the same current runs through it no matter if the plunger moves all 15 mm or if it is restricted by a counter force. It's not like an electric motor, which might burn its coils, if it can't rotate freely while loaded with a voltage.

Experimentation would include determining the force needed to raise the pitch of the vibrating membrane say two octaves (which is pretty much for a trumpetist). Then choosing the right solenoid that could create that force with 5 V (or 12 V or whatever I have to use). And after building the cup with the solenoid and everything (except the horn-bell), I'd have to build a circuit with one potentiometer controlling the solenoid force and another potentiometer controlling the supplied air pressure. The solenoid force determines the pitch or the frequency of the vibrating membrane. The air pressure controls the quality of the produced sound (too low pressure - no sound at all, too high pressure - too much air in the sound). When this is accomplished, I'll have a table containing the solenoid values and the air pressure values for each desired tone.

Since the length of the horn kind of determines the pitch, I should be able to vary the length of the horn, too, not only the tension of the membrane and the pressure of the supplied air. For this I'd attach a real trumpet (or any other brass instrument) to the cup. Then I'd need three more solenoids to push the valves of the trumpet.

Paul_KD7HB

Actually a solenoid IS an electric motor and will burn out it's coils if activated too long. Also may need to be heat sinked if used too long and too often. You will have to experiment with this.

Paul

INTP

Solenoids do have published maximum on states. They are quite short. But it could be fine for your use if you aren't holding tones for long. The solenoid would be off after each activation.

jremington

Some solenoids are rated for continuous operation. Study the data sheet carefully and obey the recommendations.

INTP

I'd imagine those continuous solenoids to be conservatively powered with big heatsinks.
A solenoid is much like a speaker, and a constant state is like a speaker killing square peak.

For the purposes of just holding a point against a surface, my first thought wouldn't be a solenoid.
I'd've considered a stepper motor with an eccentric cam stuck on it. Holding power is great, doesn't need to drain power to hold the states, and steps are highly reliably repeatable.

Johan_Ha

For some reason, most continuous duty solenoids are also heavy duty solenoids. They take tens of amps, whereas the pulse duty solenoids might only take some 2-3 A.

The step motor might work. Though it has its steps. The faster action I want, the more direct it has to be and the more steps I'd hear in the produced pitches. The smoother I'd want the produced pitch changes to be, the more gears I'd need between the motor axis and the contact to the membrane and the slower the pitch change would be.

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