Ball - sense that is falling

Jaguarjoe

Senior Member
Yes, the speed of the ball changes,
Acceleration is defined as change in speed over time (dV/dT), so the acceleration on the ball is changing.

BCJKiwi

Senior Member
Sorry Boriz but your 'springy wire' is effectively reproducing an accelerometer's functional element. It also will suffer from orientation issues in at least one direction.

Perhaps if you could organise your wires from something like piano wire you may be able to produce a coil spring at one side and a Zed shaped spring at the other oriented so both ends move covering all 3D. However this would then be an impact detector.

For my money, distance sensing is the only way, as suggested by springer. However I don't think you would need 3D. The workings could be in a bag at your feet or on your belt or wherever and the rest is as springer suggests but only for linear distance from the bag. When the maximum distance is reached you have the apogee. you could also do anything you wanted at any distance, not just at the top.

Also I am not sure if calculating averages from the catch is realistic. What about the juggling where the ball moves from hand to hand before being thrown up, and the different throws where the cycle time changes between throws (from behind, high then low etc etc).

Perhaps unigamer could give us some input on what sort of impact actually occurs at the catch to see if you could reliably detect this and further insight into what is actually required.

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mikek

Member
JaguarJoe
The speed change (dV) per time interval (dT) is the same for all time intervals for which no other force (hand) is acting on the ball. The acceleration due to gravity is constant.
Mike

boriz

Senior Member
Perhaps if you could organise your wires from something like piano wire you may be able to produce a coil spring at one side and a Zed shaped spring at the other oriented so both ends move covering all 3D. However this would then be an impact detector.
Exactly what I was suggesting.

I use guitar string because I happen to have some lying around. I picture two interlocking loops, both passing through each other like links in a chain, but not touching. The rest of the wire will need to have at least one 90 degree bend before entering the circuit board. I have had success using something similar before.

BCJKiwi

Senior Member
Sorry boriz, was not trying to be critical - I was not sure that guitar string would hold shape as well as piano wire but since I'm not a guitarist I don't know guitar strings that well. I guess if the wires were at 90 deg to each other (forming an 'L' when viewed from above), then you would probably cover all three axes of movement.

flyingnunrt

Senior Member
Here is a way
Transmitter & Reciever in each ball
Ball A transmits to ball B, Ball B to Ball C and Ball C back to Ball A
The appogee will always be pretty close to half the flight time unless you are doing tricks or circular juggling but would be fine for cascade.
Count pusles for fight time in ball A and send half this time to ball B etc,etc.
This way you would in fact be 1 throw ahead and now 1 throw behind as previous.
Reverse the launch sequence for some interesting effects.

moxhamj

New Member
Presuming that everyone is happy with the physics showing there are no net forces within the ball that can detect the apogee of the ball's flight, I am wondering if there might be a way to detect this point with direct tracking. In tennis matches they seem able to replicate the balls flight in a computer model and bring up graphics of the ball landing on a line with great accuracy. Ditto in cricket. I don't know how these systems work but at least it does show that it is possible. It might be using multiple cameras looking for a specific yellow or red of the ball.

There also might be an ultrasound solution. I have seen a high voltage transmission line technician walk under a wire, point a scanner at the wire 40 metres above and read out the distance accurately (wires sag if they get hot). That wire is not much thicker than a thumb.

Ralpht

New Member
Boriz,
Gravity is not the only force acting on the ball. If it were, the ball would never leave the jugglers hand. When he throws the ball up, he is applying energy to the ball sufficient to overcome gravity (temporarily). The ball will accelerate away from his hand because of the energy he applied to the ball and that acceleration is measurable. Because the energy is not continuous, gravity will eventually take over and the ball will come to a rest for a split second before falling.

The falling is initially not steady, the ball will accelerate at 32mtrs/sec squared till it reaches it's terminal velocity at which point in time it will then fall at a constant rate. That acceleration to terminal velocity is also measurable. The only time when your theory applies is when terminal velocity is reached and the ball is no longer accelerating downwards. This assumes the the ball reaches terminal velocity before the juggler catches it again.

Gravity stays the same, it's the energy applied to an object that changes over time.

Do not confuse gravity with acceleration.

This acceleration can be measured by an appropriate instrument in all axis. How do you supose aircraft "G" meters work, at all angles and attitudes of an aircraft.

According to your theory I should never feel a thing when making my aircraft do a loop or pulling out of a dive. Gravity acts the same so according to you it is always 1G.
Explain to a combat pilot doing a steep turn, pulling 9 G's with his face almost coming out of his butt, that he is only feeling gravity at 1G.

Evertime you change direction you are exerting force on an object, even if it is your own body. At low speeds you will not feel much, but put on a 100Kg backpack and try to change dirction fast when you are running. Good old inertia and newtons law will give you the acceleration/deceleration effects quite nicely and all of it is measurable.

The only time acceleration is not measurable is when an object has reached a steady state - ie not moving or moving at a constant speed and constant direction.
In the case of the juggler, the balls are always changing speed or direction.

Michael V

Senior Member
Smart Balls or smart juggler?

My thoughts

Once the ball is in the air, without a source of propulsion i don't think its movement can be controlled. (is that right, or is that what you want to do?)

Therefore i would be investing energy into the device that throws and catches the balls. If the mass and size of the ball is known and the propulsion force and direction is known, through controlling it, then from a purely mathematical perspective you should know where and when it is going to land, therefore be able to "catch" it, then just repeat the cycle.

So rather than smart balls, you want to build an electromechanical juggler. Would be a lot easier.

Some clever lighting in the balls themselves, perhaps something timed bt the "throwing" g force would create the illusion of the balls appear smart. - Magic!

Michael V

hax

New Member
The ball will accelerate away from his hand because of the energy he applied to the ball and that acceleration is measurable.
Hi Ralph,
The ball only accelerates as it is in his hand. The moment it is released, the ball is in zero effective gravity till it is caught again.

Imagine you were launched from a cannon inside a round ball.... Imagine that you can't see through the ball, and have no way of knowing exactly where you are relative to the earth.

You will experience high G forces as you are being shot out of the cannon, but the moment you leave the cannon, you will experience 0 gravity for the entire length of the journey.

You will not know the point at which you stop rising and start falling....

Now taking air resistance into account, yes when you reach terminal velocity you will start to feel gravity again, but in the case of the juggling balls, terminal velocity due to air resistance can be disregarded.

Michael 2727

Senior Member
Dr Ac, are you sure the HV Line tech was looking for distance ?
They can also detect invisible arching over with Ultrasonic sensors.
I'll Shutup now

boriz

Senior Member
RalphTeichel.
Dear me. Where to begin.

â€œGravity is not the only force acting on the ball. If it were, the ball would never leave the jugglers hand.â€ ???

Just to repeat. When the ball is in freefall, between the moment it leaves the hand, and the moment it is caught, the only force acting on the ball is gravity. When it is in the hand TWO forces are acting. Gravity and the push of your hand. The reason the ball does not move when it is in your hand, is because these two forces are equal and act in opposite directions.

â€œapplying energy to the ball sufficient to overcome gravityâ€.
This is the situation when the ball is at rest in the jugglers hand. If he were applying less than â€˜sufficient energy to overcome gravityâ€™ the ball would move towards the ground.

â€œThe ball will accelerate away from his handâ€
Only if it has a built in propulsion system.

â€œThe only time when your theory applies is when terminal velocity is reached and the ball is no longer accelerating downwards.â€
No. Terminal velocity is reached when the force of aerodynamic drag is equal and opposite to the force of gravity. In that special case, the accelerometer would read 1G and the ball, although moving would not be accelerating. It would read exactly as if it was in your hand or on the ground. The two forces are equal and opposite so the ball does not accelerate. But the forces are still very real.

â€œGravity stays the sameâ€
I already said that several times!

â€œDo not confuse gravity with acceleration.â€
Iâ€™m far from confused my friend. I know they are both exactly the same thing! As I described in a former post, the occupant of a space ship (with no window) accelerating at 1G cannot bring any sense to bear, not use any instrument that can tell him he is not on the surface of earth. He would have to get outside the craft and observe the movements of stellar objects, such as the rapidly receding earth. His frame of reference is self consistent and complete. Only by reference to another frame can he determine if he is moving at all.

â€œHow do you supose aircraft "G" meters work, at all angles and attitudes of an aircraft.â€
Err. Magic? (Sorry, but it is a silly question)

â€œAccording to your theory I should never feel a thing when making my aircraft do a loop or pulling out of a dive.â€
Not so. Try reading it again.

â€œExplain to a combat pilot doing a steep turn, pulling 9 G's with his face almost coming out of his butt,â€
Why would I want to do that?. He probably knows a great deal more about centripetal forces, inertia and gravity than you or I, and will understand easily what my post is attempting to explain.

â€œEvertime you change direction you are exerting force on an object, even if it is your own body.â€
Like for example the moon? Sometimes it is moving towards the sun, sometimes away from it. Surely it takes a lot of energy to reverse the direction of such a massive object. Where does this energy come from?

Bah!. I give up. Itâ€™s getting too hard. Iâ€™m having to repeat myself and I feel like Iâ€™m wasting my time. Everything you need to know is on the web. Just spend a couple of hours on Wikipedia.

The original question was about using an accelerometer to measure when the ball has reached the apex of itâ€™s flight. The answer is no. The whole time the ball is in flight, an internal accelerometer will read a constant ZERO. If you donâ€™t believe me, try it. An accelerometer CAN however read forces during the catch and throw, and therefore the moment of apex can be estimated with a timer as half the interval between catches. Though, as I have said, I favour the simple mechanical spring impact detector.

Dippy

Moderator
Poor Boriz, you sound exhausted.

I think the problem is that non-physicists (or non-experienced) have an analogy in their minds that contradicts real life.
They imagine the feeling they get in an elevator but forget that it is powered.

Gravity, as far as I (and Arthur C Clarke) know is the only force that acts on every element of a body simultaneously. I think it was old Art who suggested that if you could power a space ship using a gravitational force then you could accelerate hugely without leaving your face behind.

But the effect of wind resistance on the external aspect of the body compared to the forces within the body may have some mileage in some things, but, I suspect, not with spinning balls.

It sounds like a lot of people here should take a physics course.
But it's been an interesting/amusing discussion and, as I predicted, will shortly be reaching Nesbitt proportions.

I just wish I could juggle...

BrendanP

Senior Member
Hell, I'm too frightenned to buy back into this thread lest I get the sharp edge of Boriz tongue! (or keyboard)

demonicpicaxeguy

Senior Member
haven't looked at this thread for a while..... have we got a set picaxe'd balls that does somthing somewhat close to what it's supposed to? .........,,,,,..... or are we now debating the existance of every little force acting on a ball getting thrown up and down and wether an accelerometer would pick them up or not

i think a few people on here would benefit greatly by simply taking a crash course in "vector forces" and while a google window is open maybe then "couple forces" that topis is rather riviting ,and if the subject is still awake "centrifugal forces" is also an interesting read

one particular point i want to make is that with an accellerometer in a ball getting tossed in the air, several things must be taken into account

1,the accellerometer will have to be in the middle of the ball otherwise it will pick up centrifugal forces as well and will have to be well balanced so that if it were to spin it would do around the dead centre of the ball.

2,if the ball is not balanced one of the "couple forces"(google it if you don't know) on the ball will be larger than the other, thus the ball will spin about it's point of balance (which will be to one side of the ball) thus making it pointless for the accelerometer to be in the dead centre of the ball.

some engineering points here

1, the ball while stationary has two forces acting on it gravity which wants to push it towards the earth then there is another force pushing it up eg somones hand, the ground.....

2, when the the ball is tossed up into the air a force is exerted on it greater than the downward force(gravity) sending it up into the air.

3, as the ball is being tossed there are varying forces on the ball due to the nature of the human hand not being perfect and rather more of an uneven surface

4, while the ball is travelling it will encounter forces from things like wind resistance, things it will hit and of course forces causing it to spin due to the tossers hand not being able to toss the ball without making it spin

5, when the ball gets to the height of the travel it has reached the point where the force extered on the ball initially is equal to the force exerted by gravity this doesn't last very long

6, when the ball gets back to the hand or the ground it is simply a case of the ground or hand providing an equal and or higher upward force on the ball this may resulting on what is known in "laymans" terms as a bounce

Dippy

Moderator
Personally, I think (really?) that trying to measure up-down with accelerometers is a waste of time , and as said before you will only get a nice blip when the ball is being caught-moved-thrown by the hand. If that's good enough then fine and time to build, but with balls spinning all your in-flight stuff will be full of spuriosity (thanks George Dubbya).
But I bet a fiver that a working set of balls doesn't get made.

Michael 2727

Senior Member
Here is my version of a Capacitive Zero G Sensor.
This is placed at the core of the Juggling Ball.
BeanieBots has done some work with capacitive
type sensors, try SEARCH, or ask him nicely.

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boriz

Senior Member
Interesting solution, I like the way you think. But consider this. As the inner ball moves closer to the outer ball, there is a local increase in capacitance where the gap is closing, but at the same time on the opposite side, there is a decrease of capacitance because the gap is widening. Is it possible that this would balance out to give no overall change?

Jeremy Leach

Senior Member
Personally I think Boriz's two interlocking loops idea is good, and if there was a 'dead spot' because of orientation then it's a simple enough solution to perhaps have two of them and ORing the output. However I'm with Dippy in that I bet this won't get made anyway !

Another point here that I don't think has been discussed is the intention to use LED(s) on the ball. Well that means needing a big enough battery etc ....so not sure the ball would fit in your hand []

BeanieBots

Moderator
I found a ball in the park a few years ago. It was made of a hard water clear material like the old "power balls" of the 70's.
Inside (and not removeable) was about 1/2 dozen red LEDs and two lithium coin cells.
As soon as the ball hit the ground, the LEDs would flash in a rapid sequence for about 10 - 15 seconds or until it was bounced again.
The 'impact' detector was a coil of wire with a centre pin.
It was bigger than a golf ball but smaller than a tennis ball.
Catching it hardly ever triggered but bouncing on concrete always did.

If whoever lost it is reading this, sorry mate, I don't seem to be able to find it now

flyingnunrt

Senior Member
"I just wish I could juggle..."
Well Dip
Just build yourself some balls with leds that light up when they reach the top of the throw, put a couple of these in one hand and one in the other. throw one from the hand with two balls in it then kept throwing from opposite hands when the leds light up ... Easy!

boriz

Senior Member
"I just wish I could juggle..."
Well Dip
Just build yourself some balls with leds that light up when they reach the top of the throw, put a couple of these in one hand and one in the other. throw one from the hand with two balls in it then kept throwing from opposite hands when the leds light up ... Easy!
OF COURSE!

It sounds so simple. I'll get started right away.

Dippy

Moderator
Great!
I'll bit some little bells in the balls. Every time a ball hits my hand it'll go 'ting' and a mic will pick it up and make my balls flash red/green. I'll learn in no time.

catalina

New Member
Coming in with a late post on this topic, and getting away from force and acceleration considerations, what about a pressure sensor integrated into the ball to register changes in altitude. I've not had experience with such small pressure variations but recall reading somewhere about pcb mounted sensors able to provide this order of pressure. Sorry, but unable to point to a source but a search of the relevant literature should may be worthwhile.

Dippy

Moderator
Depend how high you can throw I guess.

init 6

New Member
"Dear me. Where to begin."

Ditto.

" When the ball is in freefall, between the moment it leaves the hand, and the moment it is caught"

Its not in freefall when it leaves the hand.

"the only force acting on the ball is gravity."

Not true, you've just applied a force with your hand. A force that is greater than gravity, otherwise it would stay in your hand.

"If he were applying less than â€˜sufficient energy to overcome gravityâ€™ the ball would move towards the ground."

But again, you've applied a bigger than gravity force - hence upward movement.

"Only if it has a built in propulsion system."
Propulsion systems don't need to be built in. If the ball starts at 0mph (in your hand) and moves up to say 10mph (I'm guessing at this speed) then it has accelerated.

"The original question was about using an accelerometer to measure when the ball has reached the apex of itâ€™s flight. The answer is no. "

The answer is yes. We use it in rocketry all the time. BTW the apex is called the apogee.

"The whole time the ball is in flight, an internal accelerometer will read a constant ZERO. If you donâ€™t believe me, try it. "

I'll dig out my accelerometer based altimeter and try to throw it (or more importantly catch it )

The biggest problem you'll have is the small forces you'll be applying - although 2 G is quite easy to obtain.

toxicmouse

Senior Member
well, i reckon the accelerometer won't pick up the drag on the ball, so the only force on it once leaving the hand is gravity. taking the ball as the reference frame (shouldnt really take accelerating bodies for reference frames), the accelerometer will read 0G. it is a bit like those astronaut training aircraft that "fall" out the sky at high altitude and the passengers feel 0G.

the rockets tend to tip over at their apogee, so the accelerometer would read slightly negative (drag pushing UP) after their apogee.

i think the altimeter might work. the SCP1000 from sparkfun has a resolution of around 20cm that i have definately tested. more importantly you could use it to find airspeed, maybe a strange pitot tube of some sort. problems may be the update rate and cost, look for some samples and try them.

nice problem.

boriz

Senior Member
*Rolls eyes*

I&#8217;m sorry Init 6, but every point you have made is wrong. No point in trying to explain. Google+Wikipedia have everything you need to know about orbital mechanics and why (aerodynamic factors aside), a thrown ball, an artillery shell, the international space station, the moon, and yourself when you jump from a building are ALL in earth orbit.

Odd that you would use the word apogee. It implies that you understand the orbital nature of the balls trajectory. But you don&#8217;t.

init 6

New Member
ToxicMouse, when the ball is pushed by the hand there is a force acting in the opposite direction to gravity ie the push. Otherwise it would stay in the hand. Try getting the ball higher than your hand without imparting a force on it.

Boriz:

So how DO accelerometer based altimeter's detect apogee? Is the answer in wikipedia?

I'll dig out a graph from one of my flights and show you that during motor burn (or hand pushing ball up) there is a positive acceleration measured by the accelerometer. When the motor stops burning, the rocket starts to deccelerate and shows as a negative acceleration on the accelerometer. This drifts towards 0 which is when the rocket has reached apogee.

Glad real rocket science isn't based on what people read on the internet.

init 6

New Member
There we go. The first 2 seconds is the motor burning and the rocket accelerating. At 2 seconds the motor stops burning and the rocket starts to deccelerate (negative acceleration) this continues until the rocket gets to apogee and is neither accelerating or decellerating. The vertical line at 15.5 seconds is when the rocket reached apogee.

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boriz

Senior Member
aerodynamic factors aside
Any negative acceleration detected by an onboard sensor during the climb after the impulse, is due entirely to aerodynamic drag. Similarly with any non-zero readings on descent. Itâ€™s the exact same effect exploited by re-entry vehicles, experiencing deceleration forces of up to 10G due entirely to aerodynamic drag.

If you repeat the flight on a planet with the exact same mass, but no air, then your onboard accelerometer will read exactly zero from the moment of engine shutdown, until the moment of impact with the ground. During the time Mr Newton is in the driving seat. The object can be said to be in freefall, or if you prefer, ORBIT. (there is no significant difference)

A rocket travels much faster than a juggling ball. Aerodynamic drag (a function of the square of the velocity) is so small with the ball that it need not be taken into account.

init 6

New Member
Back to the original post, if you search Google for "Accelerometer juggling" you'll find a few links about doing what you asked about. There's even a link about The Flying Karamazov Brothers using accelerometers in their clubs to control amongst other things music. There's a few MIT papers as well.

On the Freescale site you should be able to find an app note on detecting freefall using a 3-axis accelerometer. Not sure if the picaxe could keep up with the calculations required though.

boriz

Senior Member
An excerpt from one of the accelerometer application notes on the Freefall site:

S-FACTOR
At all times, the accelerometer will sense the acceleration
of gravity. Depending on the orientation, each axis of the
accelerometer will see a range of accelerations from 1g (when
the axis is parallel to gravity) to 0g (when the axis is
perpendicular to gravity). The S-factor is a way to consider the
total acceleration acting on the device at once, acting for all
axes combined.
When the accelerometer is held in any orientation, at least
one of the sensing axes will be parallel with the acceleration
of gravity. Therefore, at least one of the axes will be more than
0g. For example, when the accelerometer is tilted at an angle,
a portion of the acceleration of gravity will be sensed.
Therefore, the S-factor will always be equal to 1 when the
accelerometer is static.
During freefall, all 3 sensing axes converge to 0g. Since the
S-factor is the total acceleration on all axes and all three axes
are zero, the S-factor is 0. The S-factor will only be zero when
freefall is occurring.
REF

----------

EDIT.

A. Before engine fires. Rocket is at rest. Sensor reads 1G *

B. Engine is firing. Sensor peaks at 14G. However, thrust peaks at only 13G. (* subtract 1G)

C. Engine exhausted. Deceleration due to drag. Sensor output approaches zero as the drag decreases.

D. Sensor reads 0G. Candidate location of apex. Could be anywhere in this zone. Assuming apex is reached BEFORE chute deployment.

E. (15.3 seconds) Explosive deployment of chute. Severe shock confuses sensor.

F. (15.6 seconds) Shock of chute opening. Note, the force is large and in one direction.

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Dippy

Moderator
He works for NASA I reckon.

init 6

New Member
No E is apogee ie where the accelerometer measure 0G. Its an artificial line put in by the graphing program - would have been nice if they used a different colour.

But you're missing the point entirely. The accelerometer can measure the acceleration and deceleration of the rocket/ball and therefore can detect it moving with the hand, leaving the hand, reaching apogee, falling and landing in the other hand.

Your interpretation of the graph shows this. Perhaps we should use m/s/s rather than G when talking about acceleration.

So
A: Stationary
B: Accelerating
C: Decelerating
D: Still decelerating
E: Apogeee
F: Accelerating.

Just what background do you have in terms of rocketry?

BCJKiwi

Senior Member
Perhaps I can refer you back to my earlier post (Page 4, #36).
This covers it all at the types of speeds etc applicable to juggling.

I have a gizmo which normally lives in my car mounted on the windscreen. It incorporates accelerometers and can be used in two modes - either for Drag racing (which I don't do) or road racing (which I don't really do either but do do some laps on the local track). When driving, it shows acceleration/deceleration and Lateral G's from cornering. It can also be set to record these g forces as well as RPM and time of day. It can therefore be used to analyse and compare different laps, cornering forces etc to improve ones driving and lap times.

If you have it in your hand and rotate it gently on any of its accelerometer axes, the indicators shift about as the g forces change with respect to the calibrated reference orientation - i.e. if it is tilted in relation to its axis then it will register a change.

If it is suspended on a cord so it forms a pendulum and is swung back and forward in the direction of one of its axes, it records no changes even though it is changing direction and accelerating and decelerating through each cycle of the pendulum.
However if you reproduce the equivalent action while holding it in your hand you need to change direction quite rapidly so you feel the mass against your hand resisting the change. Only then is the change of direction registered.
When in the car there are external forces acting (acceleration from the engine, engine/hydraulic braking etc). These register but pendulum type activity doesn't. I would equate pendulum type activity to be close to juggling.

I did also just toss this thing up and down in the air and where I could manage to do so without it spinning about, it also registered nothing.

So if I were to publish the chart of the entire sequence of three cycles of a 'juggle?', it would be a straight line!

I should perhaps add that this thing is sensitive - it happily registers 0.01g.

Dippy

Moderator
Well, we can summarize that it may measure something. A ball decelerating vs a fast rocket decelerating is a tad different of course. Furry balls will give a better result.

So, get building , try it, no-one knows the solution - otherwise this thread is going to become Nesbittesque with no conclusion.