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Originally Posted by Trevor
This thread, or comic cut with irrelevant imagery, has been published in the hope of swaying the gullible. There is history involved hopefully confirmed within the archives.
Some years ago the author must have stumbled upon an article describing the operation of linear, or possibly proportional solenoid valves. Unfortunately the text was not properly understood and a notion established that the principals detailed, involved solenoid valves in general, whereas the information was exactly specific.
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It was in 1975 when I was doing some design work for K-Mac Suspension in Sydney. They wanted to wind their own auto springs. I designed and built an automatic spring winding machine, that was run by a digital controller, designed and built by myself. (Micros and PICS weren’t around then). The red-hot wire was fed along the mandrill by a Hydraulic ram that was controlled by an Italian, Pulse Width Modulated solenoid valve, I forget the brand.
The Solenoid needed a variable Pulse Width, at 50Hz. The frequency had to revibrate the armature to prevent striction. For those in the know, I used two 555s, one to set the frequency of the signal, and the second, to turn this into the Pulse Width signal, controlled by the voltage threshold pin.
While I was testing this unit, to get the ram to move smoothly, I noticed that when I varied the frequency of the PW, the ram would start to vibrate if it was too slow, as I increased the frequency of the PW, the ram was smoother, if the frequency was too high, the ram would not move at all.
It was here that I realised that the whole operation of this hydraulic solenoid was in the frequency, that the PW was delivered at.
You can get any solenoid and by turning it on/off, the armature will do full travel. As you turn it on/off faster, you get to a point where you notice that the armature will not do the full travel, it is reducing as the frequency is increasing. There is a point where the armature will stop moving and just sit there exerting the same pressure on the return spring, and if increased too high, the armature will return to the rest position.
This is due to the PW signal varying the current flow through the windings, to vary the magnetic pull that it can exert on the armature. As full travel needs full magnetic strength, full current has to flow. Inductive reactance from the windings, prevents the full current flowing instantly at switch on, it takes time for the current to reach maximum.
As the PW modulation can alter the time that the current flows for, we can reduce it, to below that required, to give full magnetic strength, and travel. If we work in this ‘rising current time’, we can vary the magnetic fields pull on the armature. If the frequency is too high, the ‘reluctance’ of the electromagnets iron core, to change state, will be too high to change its magnetic field that fast, so there will be no field that the armature can react to.
As far as I know, this machine is still winding springs, 30 years later.
Harvey.
