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Tattoo machine electromagnetics

by Phaedrus
Tags: electromagnetics, machine, tattoo
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Phaedrus
#1
Oct24-03, 03:54 PM
P: 26
Hello, New to this forum, thanks for having me. I have been researching electromagnetics to better understand the workings of the tools of my trade (tattoo artist) and have come upon a number of questions and cannot seem to find the answers. I would appreciate any help to point me in the right direction. This will be my first question.
A tattoo machine is basically a dc circuit containing two magnetic coils in series with a capacitor in parallel which operate an armature on a spring contact. Think doorbell. The capacitor is basically there to suppress the spark generated by the rapid collapse of the magnetic field as it switches itself on and off. My question at this point (I will have more concerning magnetics later) is...why does changing the value of the capacitor cause the machine to run faster for smaller and slower for larger values of capatance. As I understand it the energy created by the collapse of the magnetic field (since dc will not go through a cap.) is kind of sloshed back and forth through the coils until it is reduced to zero by the resistance. Why would the speed of the armature be related to the speed at which the cap. discharges this energy back and forth. Thanks
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HallsofIvy
#2
Oct24-03, 04:17 PM
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I will confess to not being very familiar with tattoos or the "implements of torture" but I'll take a stab at it!

If I understand correctly, a tattoo needle goes a short distance into the skin and then back out again. That's controlled basically by an electromagnet. The magnetic field is one direction for the "stab", the opposite direction for the retraction. That means that the field has to reverse at regular intervals.

I suspect the capacitor is there not just to "suppress the spark" but to hold the electrical charge until it is time for the reversal in direction. The larger the capacitance, the more charge it can store and so the more time before it triggers the reversal (i.e. slow). Smaller capacitance, less charge stored, faster reversal.
Phaedrus
#3
Oct24-03, 04:28 PM
P: 26
Actually the reverse motion is a function of the spring tension. The spring returns the armature and contact to a closed point at which the circuit is closed and the motion starts all over again. the magnet is off when the circuit is open and that is when the field collapses producing the opposing voltage. Remember how a capacitor functions in a dc circuit. Thanks

Integral
#4
Oct24-03, 05:08 PM
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Tattoo machine electromagnetics

The circiut you describe is called a Resonate Tank. It essentially alternatly stores energy in the field of the coils and between the plates of the capacator. Thus it is creating AC from the DC current. The frequency at which the tank circiut runs is determined by the combination of the coils and the capacator, change the value of either of them will change the operation frequency of your needle.
EDIT:
I went looking for the relationship between Inductance, Capacitance and frequency.

ω=1/([sqr]LC)
Phaedrus
#5
Oct25-03, 12:36 PM
P: 26
That really helps...I spent all night researching tank circuits trying to understand. So if I use that equation and come up with a frequency value of 120 (for example) does that mean that my machine will run at optimum performance at 120 hz.
Also my equation for inductance is rather cumbersome(L=Number of turns * Permeability of soft iron core(600?)* Area of coil in square meters divided by Length of coil in meters) Does this seem accurate enough or is there something else)
thanks
Integral
#6
Oct25-03, 12:55 PM
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Inductance is tough, I would not advise unwinding your coils to attempt to learn this number. To make it worse inductors are the worst for having a value printed on them. If you are able to measure the operating frequency (or know it for some value of capacitance) you can compute the inductance. This would be the best way to go. I am not sure how much you want to spend to study this, the best tool would be an oscilloscope, these are very expensive and require substantial training/knowledge to use. Radio Shack does sell Multimeters which can measure frequency. You seem pretty knowledgeable about circuits (ie you were able to identify caps and coils) so should be able to master it with no difficulty.
Phaedrus
#7
Oct25-03, 01:26 PM
P: 26
3 weeks ago I knew nothing about electromagnetism or circuits....now it seems I know even less.
I can get a pretty accurate number for turns as I know there are about 44 turns per inch with 22 guage wire... The coils are sold as 6 wrap, 8 wrap or 10 wrap usually, so I can figure the number of turns without unwraping them. Most machines come with a 47uF 35v capacitors. I bought a good multimeter at radio shack last week (having problems getting accurate Hz readings, most of the time it jumps around a lot, not sure why, maybe the spring is bouncing at the contact on return) So the coils I have in front of me have about 336 turns per coil (there are 2 coils and one cap.) There are several things I can do to change the operating freq. I can adjust spring tension/stiffness, armature weight. The problem comes down to...If the machine is not tuned correctly it can get very hot and just not work as well. Also there seems to be some misunderstandings in the tattoo world concerning the function of the capacitor. So I am trying to figure out why diff. cap. sizes cause diff. performance and to arrive at an algorythm or value for a well tuned machine.
thanks
Integral
#8
Oct25-03, 02:09 PM
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You also need to consider the interaction with the spring. The mechanical system also has a resonate frequency. If you get a mismatch between the electrical oscillations and the mechanical you will indeed lose efficiency. Figuring the resonate frequency of the mechanical system will be more difficult then the electrical. I'll do a bit of digging see if I can find a formula for it. It is interesting that that the fundamental equations for electrical and mechanical oscillation problems are identical. This fact is frequently used when modeling systems.

Ok. with that said, I found this.

In mechanical systems m plays the part of inductance and Capacitance corresponds to Mechanical compliance= 1/k were k is the Hook's Law spring constant so your mechanical resonate frequency is

ωm= [squ]k/m
k can be measured if you can remove the spring and carefully measure its extension with addition of mass.

k= Mg/x here M is that added mass, g is the acceleration due to gravity.
Phaedrus
#9
Oct25-03, 07:33 PM
P: 26
ok...I looked into Hooke's Law and it seems that it applies to coil type springs. What we have here is a flat spring.

This diagram may shed some light on the problem. But I would think that you are still right about the mechanical resonance. Because when things are out of sync on this system it creats a nightmare.
Integral
#10
Oct25-03, 08:37 PM
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Hook's law is always shown with a coil spring but it holds for your spring also. It has a spring constant and the resonate frequency is determined by the equation I gave. It may be very difficult to determine your spring constant.
zoobyshoe
#11
Oct27-03, 04:28 AM
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I'm curious: the capacitor in the drawing looks like it might be eletrolytic. Is the one on the actual tool electrolytic?
Integral
#12
Oct27-03, 04:45 AM
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I would bet it is. Why would that be of any particular significance?
zoobyshoe
#13
Oct27-03, 07:10 AM
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Here's my thinking: the basic vibrator circuit behind this machine is as old as the hills.
Peole have, since they were invented, been trying all kinds of ways to prevent the spark that occurs when the points come apart. Everyone has resorted to the capacitor, which while it doesn't eliminate the spark, seems to cut it down.

I've been trying to think if arranging it as a tank circuit, as Integral suggests is what is going on, would stop the spark somehow, but I can't see that it would. I must arrive at the conclusion that the capacitor is not put there to make it into a tank circuit.

For whatever its worth, the person who put this site together believes the capacitor is for controlling the spark as well:

HOW DOES A MODERN TATTOO MACHINE WORK?
Address:http://www.faqs.org/faqs/bodyart/tat...section-5.html

I don't know how the capacitor is affecting the performance. I do know you could remove it altogether and it would work fine, except that the spark would be at its maximum.

Thomas Edison invented this basic machine in 1876 for engraving or some such thing, and about 20 years later someone modified it to make tattoos. It is also the same circuit used in the Model T spark coil to make and break the juice to the primary.
Phaedrus
#14
Oct27-03, 12:19 PM
P: 26
Now your hitting on what I am trying to figure out. You would think that taking the cap. off would not change the way it runs...but it does...significantly. Aside from the blue light show...the machine requires more voltage just to operate then is rather erratic. I have a machine that I have modified as to be able to clip in various capacitors or none at all. With a 47uf 35v cap. at 5 volts it runs with a duty cycle of about 60%. when you remove the cap. it will draw more in amps run very weak and the duty cycle goes to 80%.
Phaedrus
#15
Oct27-03, 01:17 PM
P: 26
Some thoughts I have had on the problem. When the contact is broken the magnetic field collapses and generates a significant spike of voltage. The way the cap. suppresses this spike is that instead of arcing across the contact it is builds up on one side of the cap...at some point enough has built up on one side that it wants to go to the other side. Since dc current will not travel through a cap. it takes the next best route which is back through the coils. When enough has built up on the other side the process begins again sending the energy back and forth each time creating and collapsing a magnetic field. The coils can have about 2.5 ohms of resistance from one side to the other, so within x number of passes the energy is completely dispersed. So then some time later the contact returns to its original position...the circuit is closed and the whole thing starts again. So this is where the resonance may come in...the rate at which this spike is sent back and forth at the moment the contact is broken seems to drastically effect how the machine runs.
Also keep this in mind...it is not enough that the armature simply go up and down...I use two machines for nearly every tattoo...sometimes 3-4. Some artists use as many as 5-6. Each machine has it's own characterists involving speed, power etc. depending on how it is set up.
Integral
#16
Oct27-03, 02:29 PM
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Clearly the coils and capacitor from a tank circuit, for optimal performance the natural frequency of the electrical must match the natural frequency of the spring mechanism. I would guess that the best of all worlds would occur if you time the points to open near the zero current point of the tank circuit. This is called zero crossing and is in common use with Solid State Relays. If you can achieve this condition then there will be little or no spark when the contacts open or close. To get this with a mechanical system would be difficult, but not impossible.

Here is the schematic of the circiut as shown in the link above. Schematic Diagram This is a driven harmonic oscillator, the electrical system dirves the mechanial spring system. Do a search on that you should find lots of infomation. Maximum efficeny will come when the electrial and mechanical systems are tuned to the same frequency operation. The tank circiut should be able to drive the mechanical system for a short period with the points OPEN. If this circiut is tuned correctly the current draw from the source will be small, it is only needed to replace losses. If you are not tuned current form the source will increase and operation will be compromised.

Think of the electrical circiut as a fellow pushing his child in a swing, the mechanical system is the swing. If you push at the wrong time it requires more work and you simply stop the swing, if you push at just the right moment, it requires little effort and the swing goes further. That is the goal of the tank circiut, it needs to apply max current to the coils just as the points are opening. When the mechanism is at its maximum displacement the coil current must be near 0, thus releasing the mechanism to begin the upward travel.

When the spring mechanism (which will include everything that moves with the needle) is allowed to operate at its natural frequency (as given in a previous post) it will require little energy to sustain motion. So the goal of the electrical system is to provide gentle pushs at the right moment.
zoobyshoe
#17
Oct27-03, 05:54 PM
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Integral makes excellent points here, and I don't see any way to get around the fact that adding the cap to suppress the spark creates a tank circuit as well, which will have to be tuned for optimum performance, in conjunction with the tuning of the mechanical oscillations: all stuff that should have been worked out by whoever engineered this particular machine.

One thing to bear in mind, if you aren't already, is that the current created when the magnetic field collapses always goes in the same direction the original current was going. This is why I asked if the cap was electrolytic. Another is that sparks, whenever they occur, are not straightforward, unidirectional propositions: they actually oscillate at high frequencies but with a general overall direction. This makes Integral's goal of a no spark situation definitely the one to shoot for.
Phaedrus
#18
Oct27-03, 06:37 PM
P: 26
Engineered...are you kidding...the worst running machines on the market are the ones stamped out in factories made by engineers. The best and most sought after machines are hand made by people who understand the feel of a fine tuned machine. Once you have the geometry correct then all you have is the electronics and mechanics. But I'm willing to bet that some of the best machine makers in the world have never heard of a resonant tank or a harmonic oscillator. Like a violin maker from 100 years ago. That is what has brought me here...If you research tattoo machines and tattoo forums you will find tons of misinformation, whether it be from misunderstanding, superstition and/or just plain old ignorance. At this point it is a very misundertood area amongst even the people who use it everyday.

Also...Are you sure about the spike going in the same direction as the current that created it...I was under the impression that the whole idea was that it was in opposition to the current that created it in an effort to equalize the experienced loss. I've got it in my notes somewhere and will try to find the reference.
Thanks for the interest.
integral...with that new info on springs and driven harmonic oscillators I am finding tons of pertinant information. Thanks


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