Charging Mechanism

Jeff,

I'm surprised nobody's asked this question: Are you a student who's part of this team or a teacher running the team?

Some of the things you are talking about are potentially dangerous. The voltages and currents you propose to operate at can make components explode (literally) and also hurt people. From the level of questions you are asking I can tell that you are not, at this time, qualified to play with these kinds of circuits at these power levels. Not a criticism, just an observation. You can learn, of course. But you also indicated that you are just a few weeks away from having to have all of this work for a competition.

The parallel charge/series-discharge scenario you paint requires some pretty sophisticated MOSFET switching and power control circuitry. You need to maintain full enhancement at the gates of several MOSFET's as the sources are taken up and down in voltage while the caps charged, switched and discharged. There are relatively simple ways (in terms of component count and control) of attempting this, but I don't feel compelled to tell you about them because of the danger involved.

If you've never seen an electrolytic capacitor explode, well, believe me when I say that the type of caps you will be using can do serious damage. Don't mess with this unless you have a very solid understanding of the subject. If you want to learn, fine, do so, but don't attempt this level of design work a few weeks away from a competition. It's simply not necessary. And, if you are a teacher, don't expose your students to potentially dangerous voltages and currents. Anything over 40 to 50 volts and I start to have serious concerns. Keep it low, 12 volts is more than enough. You just have to get clever about how to solve the problem.

Regarding your kicking mechanism and required power. If I were you I'd visit your local hobby shop and get a powerful R/C car motor with a suitable motor controller. These things can develop upwards of 200 watts and gobs of torque. Gear it down 6:1, 10:1 or more (experimentation required) and I'm sure you can produce the kind of kicking device you are seeking. More importantly, it would all be done at low voltage (12V) and with power control components that were professionally designed and field proven. Different mechanical approaches might be required to deliver the amount of energy you want per kick. You can approach this knowing that the off-the-shelf power system is not somethign you have to worry about.

I don't know how big your machine is, but here's another idea (and it would not require a very powerful motor). Use a geared-down motor as a winch. The winch has an electro-magnet at the end of the line. Use this arrangement to pick up an adeqately sized steel ball. Lift it to a height based on the amount of energy you'll need (pretty simple potential energy formula). To kick you would release the ball. The ball would fall onto a simple sprung lever mechanism that would convert the potential energy into a kick. The ball would travel in a tube, so the winch would simply be lowered to retrieve it and fire again.

Of course, you could also have a preloaded chamber with a number of steel balls in it. A little servo mechanism would serve to release one at a time.

If you want to use air power, again, visit your local hobby shop and ask about neumatic retracts for R/C airplanes. You might have to find them over the internet as many hobby shops would not carry these products. The neat thing about neumatic retracts is that they include all the valving you need to use a standard R/C servo to both extend and retract the landing gear. You could use this hardware as the basis for a pretty powerful compressed-air or CO2 kicker.

Have fun!

Thankyou for your concern martin. I am not a teacher, but a student on the team. I have had a little experience with electronics before, but this is realy over extending my reaches into what I know, so I think that we will take your advice and change to a lower voltage solution kicker.

TIA

Jeff

Jeff,

Above all be safe. Part of the fun should be in trying to solve the problem creatively and within attainable and reasonable constraints. That's what engineering is about. You are ahead by a mile because you don't hesitate to ask for help. You will learn a lot taking that approach.

So, go with 12 volts and figure out how to store energy for release during the power stroke. Since you said that you can live with one kick every 30 seconds ideas such as the electro-magnet equipped winch might be applicable and very simple from a Physics point of view.

If you decide to go with a motor-driven approach I would highly recommend you contact ModelAir-Tech and look into using one of Tom Hunt's belt-driven gearboxes. These are designed for electric-powered airplanes, however, the higher ratio units might be just fine for what you are trying to do (lots of torque). Here's the link:

I have no affiliation with this company. I used to be in the business and know Tom Hunt from back then. He's a very capable engineer working for Grumman Aerospace.

Amen.

A long long time ago I was playing at drawing arcs from a very large charged capacitor. Neat fun.

I woke up on the floor, not quite sure what had happened. A week later I found a screwdriver I had been holding buried about 3" in the concrete wall behind me. I had no idea I could throw so hard or be so lucky.

Experience is what you need most right before you get it.

Jeff,

Had another idea today while driving. How about using a cars electric antenna mechanism to raise the aforementioned steel ball? I think they produce enough force to lift a reasonably sized object. You can probably get one at a junk-yard out of an old Ford or Chevy for a few bucks. Might be worth a shot.

I don't think it is very responsible to give this sort of advice to a student. As a father, I particularly resent you characterizing my advice to Jeff as "crap", when you obviously don't know the subject well enough to make such assertions. You are also not giving any weight to the fact that this is a school kid exploring robotics. Do you honestly think that it is OK for a kid to play around with switched-mode power supplies generating upwards of 80 Volts? We must live in radically different worlds.

Twelve volts is more than enough for what he wants to do. In fact, a

12V-powered system can rip your arm off (every play with a winch on a truck?).

A 12V NiCd pack with a high-performance motor and an off-the-shelf motor controller can easily (and safely) draw upwards of 35 to 50A. Taking voltage droop into account, let's say that you get somewhere between 350 and

500 Watts out of the setup. Of course, motor efficiency isn't constant. But, in general terms, you can get a lot of power out of the setup. Safely. It shouldn't be all that difficult to convert over 1/4 of a horsepower into a decent kick.

The difference in the approach here is that he would be using off-the-shelf components and not getting his hands into >40V circuitry while trying to get it all to work. Furthermore, a machine with a grounded frame and >40V potentials inside can be absolutely lethal without anyone even suspecting it. Imagine accidentally grabbing the frame with one hand and touching the

The problem is then reduced to a mechanical excercise. And, while 1/4 of a horsepower can hurt you, people tend to keep their appendages away from fast moving mechanical assemblies. So, the end result might be safer still.

A common solenoid is the wrong way to do it, of course. Notice I never said to use a solenoid.

12 Volts is safer than, say, 80 Volts because, of course, CURRENT is what kills, not voltage. However, current is a function of resistance, isn't it? So, with higher voltages the chances of getting hurt increase.

Here's an excerpt from the Electrical Safety guide at the Lawrence Livermore Laboratory:

"Conditions for a serious (potentially lethal) shock across a critical path, such as the heart, are:

1- More than 30-V root mean square (rms), 42.4-V peak, or 60 V dc at a total impedance of less than 5000 ohms. 2- 10 to 75 mA. 3- More than 10 J."

Wet hands holding a wire are equivalent to about 3,000 to 5,000 ohms. Wet doesn't mean in a swimming pool, it can be sweaty hands. Or, say, holding non-insulated pliers or tools: 1,000 to 3,000 ohms. Deadly at voltages over

30V RMS.

Not so. You need to do some reading. Here's a link to the Lawrence Livermore safety document I mentioned above:

Look towards the bottom of the document, Appendix B "Effects of Electrical Energy on Humans". There are many like it on the WWW from reputable organizations, you don't have to take my word for it.

DC, of course, is a lot less lethal than AC. And it takes a lot more effort to get hurt with DC. But, the minute you start playing with switched circuits you've potentially turned things into AC. I would not put a bunch of students in a position to have to be aware of whether or not they are creating a potentially deadly environment. Currents as low as 10mA are possibly fatal. By operating at lower voltages (say 12V) you have a built-in safety margin of nearly 8x with respect to the proposed 80V operating voltage.

This is true. I'm not sure how to reconcile that. It seems to me that people are a lot more careful around things with hoses and valves, having more of a sense of danger because they understand that there's a high pressure gas inside. A lot of people have seen what a water hose can do. They understand it. Most folks don't know that they can get killed (or seriously hurt) by voltages as low as 30 to 40V. They lack the empirical frame of reference.

I urge you to consider the audience when voicing such opinon. Jeff is probably a very smart 13 to 18 year-old with interest in a wonderful field. I'm sure many like him read this group. Don't tell them to go play with voltages that can kill them, or that advice that says to be cautios is "crap". That's not right.

KIDS: Be safe, learn, explore and always seek the supervision of knowledgable adults so that you may experience Robotics in the safest and most fullfilling way possible. Don't act on any advise, ideas or opinions given in these newsgroups without first checking with someone you know you can trust.