Graham Grindlay wrote...
This is actually for a force-feedback drum project that I've been
thinking about recently. The idea is to seat the magnet just behind
the head of a drum, pulse the field direction rhythmically such that
it attracts and repels a permanent magnet embedded in a drum stick tip.
The original motivation was purely creative: see if this arrangement
could be used to produce a sort of 'player drum' effect (e.g. you strike
the drum once, but at the same time the magnet is used to rapidly attact
and then repel the permanent magnet in the drum stick tip, say in the
pattern of a fast triplet). Lately, I've been thinking that, if I can
get this thing to work, it would be interesting to look at pedagogical
applications. If you could 'quantize' a beginning player's sloppy
timing you might affect their learning rate by giving them a feel for
the correctly played rhythm. There are also some musical concepts such
as 'playing behind the beat' that just about impossible for a teacher to
convey verbally, so 'learning by feel' might be a real advantage in some
situations. In any case, this is obviously all very early stage stuff.
Very interesting, and highly relevant to your design. I was very
concerned over the issue of coil heating, because 192 watts (8A*24V)
will overheat and destroy a small coil like yours in the end. You
may not notice trouble during short tests as the heat is taken by
the copper's thermal mass, but the equilibrium temperature will get
you. I envisioned that you had a steady maximum field and suddenly
wanted to collapse the field and then reverse it. But if what you
need is short, fast musically-timed field pulses, the average power
dissipation could be acceptable. That's good.
Second, your need for as fast as a 100ms repetition rate may be for
only a few cycles at a time, good, and you may be able to live with
slightly reduced force pulses after the first pulse, which we saw in
Tony's simulations of fast reversals.
I'm wondering if you do want a reversal. The shape of the field
pulse must be a strong factor in the drummer's sensation, surely
in normal use you don't want to interfere too strongly with the
drum stick's natural bounce off the surface. That must be why
you want to collapse the field quickly, to stop the attractive
forces by the time the stick hits the surface. But do you also
want a repulsive force? Perhaps a weakened one, to gain stick
distance for the fast triplet beat you're thinking about, etc?
I also wonder if you want a standing full-strength field before
the drum-beat event? What's the time-frame you envision for the
onset of your attractive field? The natural t = L/R = 53ms time
constant from simply switching your 24V supply seems pretty slow.
You can push single powerful half-sine uni-polar pulses of current
into an electromagnet with a charged bipolar capacitor and an SCR.
.. half-sine positive current pulse
.. +HV supply ---/\/\------,
.. + C1 C2 + | --- coil --- t = pi sqrt L*C
.. ,---|(---+---)|---+ 160mH 3-ohms
.. ----+---|<|--+--|>|---+--#####--/\/\--, Ip = V sqrt C/L
.. gnd |______________|/|________________|
.. trigger__/|\| scr
The bipolar capacitor came up earlier in this thread. Since the
value you'll need is high, it's made up from two electrolytics.
As the voltage on initially-charged capacitor C1 approaches zero
halfway through the pulse, the field current reaches its peak.
The coil maintains a declining current flow in the second half
of the pulse, charging C2. When the current reaches zero the
SCR shuts off. C1 and C2 are both equal to C in the equations.
After the pulse you still have most of the energy, but it's in
the second cap, and the full bipolar capacitor has a reversed
voltage (diodes protect the electrolytics during the voltage
reversal). This is another place a high-voltage H-bridge may
be useful, to recover and reuse the energy.
To make a single-cycle bipolar current pulse, replace the SCR
with a TRIAC, and provide gate drive through the first half of
the cycle, plus a bit, then remove it so the TRIAC will turn off
after the second half finishes. A significant portion of the
charge will be back in C1, ready for the next pulse.
If you make C2 larger than C1, the attack will be fast but the
middle portion of the cycle will be longer than the attack. Or
if you make C2 smaller than C1, the current cutoff and reversal
will be faster than the attack and decay, which might be better
for your drumsticks. C2 will need a higher voltage rating.
BTW, you can get TRIACs with current ratings to 6000A.
This must be a very loosely-coupled magnetic system, so the
electromagnet's inductive Q will not be mechanically reduced.
