Daire Byrne
New Member
My name is Max. My world is fire and blood.
Posts: 24 
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Post by Daire Byrne on Mar 30, 2015 19:17:39 GMT
Conor/Adam
Breno informed me that the Servo will not work as a catapult due to low speeds. This question is two-pronged: when doing some preliminary work on the equations that you gave me, I found that the TowerPro SG-5010 with 6 Volts of power will provide enough speed to throw the ball 55 cm with a 45 degree angle, while the Torque is comfortably over the maximum required for the ball and spoon. There are other such models but the TowerPro is the cheapest by far, and retails Stateside for less than $10. (I am unsure of availability in Ireland, and am probably misinterpreting some of the specifications).
Secondly, should that not work out, I was wondering if you could suggest some alternative methods so that we can return to the drawing board as soon as possible.
Sorry for missing the classes last week. Thanks very much for all the help you have given us and I apologise for bombarding you with questions.
Regards,
Daire.
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Daire Byrne
New Member
My name is Max. My world is fire and blood.
Posts: 24
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Post by Daire Byrne on Apr 2, 2015 23:09:46 GMT
Hi Conor/Adam,
I am just sharing my workings in defence of the Servo-only system:
The following calculations assume a TowerPro SG-5010 Servo with 6 Volts of power:
Maxmium torque of model: 11.0kg cm
Speed: 0.14/60 degrees
Ball Weight:50g
Radius: 2cm
Spoon Length: 0.14m
Weight: 15g
Assume a launch angle of 45 degrees for this sample:
45 degrees = (Pi/4) radians
Speed is equal to 1 rotation in .84 seconds = 45 degrees in 0.105 seconds
Rotational Velocity is equal to (Pi/4)/.105 = 7.479982509 radians per second
Conversion to linear velocity: `v=wr` therefore ((Pi/4)/.105) X 0.14 = 1.05 m/s
Distance = v cos( angle of launch) = 1.05 X cos(45) = 0.74m
As the angle is 45 degrees in this sample, the maximum height, which uses sine, is the same. A higher angle gives an increased height but a lower distance and vice versa. This fact of physics will allow us to generate different power levels. Personally, I feel that 74 cm will be more than sufficent for the hoop, and even allows us to use a slower speed in the code.
Torque: (Mass(arm) X Gravity X Length)/2 + (Mass(ball) XGravity X Length)
((0.015kg X 9.81m/s(s) X 15cm)/2) + (0.05kg X 9.81m/s(s) X 15cm) = 1.103625 + 7.3575 = 8.461125kg cm
This figure, the maximum torque required for any point of our project, is under the 11.0 kgcm of the model I am using as an example. This is not the only model with a fast enough speed, a reasonable size and the necessary torque, but I am merely using it as a sample.
Conclusion: By using a powerful enough servo, the necessary speed and torque required can be reached.
Regards,
Daire Byrne
P.S. Once again, the group and I are extremely open to any alternate suggestions. Engineering is not an exact science and we will only learn by trial and error, and we are constantly wishing to improve. Thanks
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Post by Admin on Apr 17, 2015 23:32:08 GMT
Hey,
Sorry its taken a few days to get back to you - been busy writing libraries for the sensors that we bought it.
Anyway I mentioned to Breno that I suggest using an elastic mechanism as the first approach. By using elastic its easy to customise to ensure that it meets your criteria (i.e. add elastics or stretch until you get the right performance). With servos, you have much more limited control over performance. Also servos wont provide the acceleration that taught elastic will. With the timeline for the project being quite short, I encourage teams to build it simple first. Once you have something simple working, we can always make it more complex later.
Lets go through some basic designs tomorrow. Try make it in for 1pm so we can find time to do this.
Cheers,
C
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Daire Byrne
New Member
My name is Max. My world is fire and blood.
Posts: 24
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Post by Daire Byrne on Apr 30, 2015 7:08:37 GMT
Any luck on a mechanism for releasing and catching the spoon?
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Post by Admin on May 4, 2015 16:00:32 GMT
Hey,
Sorry for the delay in getting back to you. yes I've pretty much figured it out. What I suggest is that we use pneumatics (compressed air) to draw the catapult back and a latch (servo operated) to release the mechanism. The pneumatic system can be operated manually with a bike pump and the latch controlled with an Arduino. I will try to have something ready for this weekend to show you guys.
Hows progress on the catapult?
C
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Daire Byrne
New Member
My name is Max. My world is fire and blood.
Posts: 24
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Post by Daire Byrne on May 5, 2015 20:50:29 GMT
We are building the catapult and hoop out of wood. All the materials are ready and we have settled on a design so it is just a matter of putting it together, which will be done over tomorrow, Thursday and Friday.
Thanks for all the help. Will we need to incorporate any specific connection to the throwing arm for the pneumatics?
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Post by Admin on May 5, 2015 23:10:03 GMT
Great - in fact I think it might make sense to adopt a servo mechanism instead.
No once I have the catapult I can build something that will work with it. I'm afraid to do much until the catapult is ready in case the sizes don't match etc. At the very least, I will have the design on Saturday to show and discuss with you guys.
Just make sure that you dont make the catapult too big/strong. For the live demo in a few weeks its important that the game can be played indoors. If the catapult is a monster and has the potential to cause damage this may be a problem.
Talk soon
C
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