Moving On To The Ping Pong Launcher
Hi everyone and welcome back to the blog! This week I will present the process behind making a ping pong ball propelling device which will be used in an obstacle course later. If you want to see me complete this obstacle please follow this channel in the coming weeks! So you may be wondering about my disorganisation and blatant lies this week when my topic of choice isn’t the robot assembly in real life just as I promised last week?? Well it turns out that my robot still hasn’t arrived in the post yet (I know I sound like a broken record saying this every week). In the meantime I decided to move on with this project by making a vital part of a future planned obstacle course, namely a ping pong launcher.
At first three methods of propulsion came to mind, these were springs, compressed air and a battery powered motor. Before building anything I decided to analyse each of these in order to find out which is most suited for my type of application. The battery powered motor seemed like a great idea as I thought I could even use the Arduino from the robot to program the motor if needed. After some thinking I learned this would prove highly impractical as the robot would be using the Arduino at the same time of operation as the launcher. Also with current restrictions due to COVID in place it would be fairly difficult to buy the most basic components of this device such as terminals and motors without waiting a lifetime for shipping. Therefore I decided to rule out the battery powered motor and focus on springs and compressed air. Both of which I could upcycle from my house.
After some consideration and gathering any useful materials from around my house this is my version 1 of a spring propelled launcher. I found my old exhaust from my car which I will use as the guidance tube for the ping pong ball. Since this tube is made out of steel I will weld it onto a weighted bottom piece at an angle. Inside the tube, mounted at right angle to it, there will be a floor on which the spring will sit on. Above the spring there will be a platform on which the ping pong ball will sit on. This platform will be mounted to the spring and will be free to move up and down the barrel of the tube. The spring will be compressed to gain potential energy, next the spring will be released turning potential energy into kinetic energy, thus propelling the ping pong ball.
In the compressed air prototype I will use the same frame of the weighted bottom, guidance tube and tube floor as in the previous prototype. However in this model I plan on incorporating an air valve into the side of the tube. An air storage device such as an air compressor, balloon or a syringe will be connected to this valve. A ping pong ball platform will be placed in the tube, this platform will once again be free to move upwards and downwards along the tube. The ping pong ball will rest on this lightweight platform. The platform is a vital component of this model as it will be a perfect fit to the diameter of the tube therefore ensuring the compressed air doesn’t travel around it. If the ping pong ball was to be left by itself the air would find the path of least resistance around the ball therefore launching it with a smaller force. The working process of this model is as follows, firstly the valve would be closed while connected to an air storage device. Next the valve would be suddenly opened and the high pressure air rushing into the low pressure tube would accelerate the platform, hence launching the ping pong ball.
After these two prototype models were built in CAD I decided to pursue the spring version because of a few major factors in its favour. Firstly I had all the necessary components at home to build the spring prototype in real life. Whereas I didn’t have an air valve or any type of air storage device. Secondly the tension springs that I had would produce a much bigger force when stretched than a balloon or syringe even if I was to invest in the pneumatic model. We will see the calculations for these forces below, (I obviously wasn’t going to buy an air compressor now). Thirdly a spring would produce a consistent force once it is stretched a certain distance, this will make the accuracy of landing the ping pong ball in one spot a lot easier. Whereas with a balloon it would be much more difficult to control a constant air volume and pressure, therefore the landing spot of the ball would vary greatly. Lastly it is very difficult to get a ping pong ball platform to be a perfect fit inside a tube in order to maximise the force of the rushing air. With all these factors in mind I decided to start making improvements to my 1st version of the spring prototype.
I conducted an experiment to find the spring constant using Hooke’s Law. I placed weights on the bottom of the spring incrementally moving up by 100g. Each time I would measure the extension of the spring. Next I made a plot of the Force in Newtons applied by the weights and the displacement of the spring in metres. From this experiment I found the spring constant to be approximately 1.9. Using Hooke’s Law F=kx and an estimate that I can extend the spring 10cm, I calculated that the force applied will be 0.8N. The ping pong ball has an average weight of 0.0023kg, therefore it will experience an acceleration of 42m/s/s. Using the kinematic equations of motion I then calculated that the ping pong ball will have a velocity of 5.7m/s coming out of the tube which seems fit for purpose on the obstacle course. It is important to mention that I assumed no losses in this calculated so this velocity will decrease greatly in real life application.
In comparison with the compressed air model I assumed a pressure of 0.1MPa as the pressure in a balloon at nearly full capacity is approximately atmospheric pressure. Next using the equation F=PA I calculated that the Force acting on the ping pong ball would be approximately 0.09N. Even assuming that I can make a perfect seal in the tube for all this force to act on the ball, it is still half as much as the spring model. Therefore I decided to pursue and improve the spring prototype.
I used vernier calipers to measure the diameter of the ping pong ball which equaled 38mm, next I did the same to the exhaust pipe I planned to used. Unfortunately the exhaust pipe only measured 35mm and so the ball wouldn’t fit inside of it. Thankfully I found an old silicone glue tube which measured 40mm in diameter. Next I tackled the problem of the spring, namely one big spring inside the tube as shown in the version 1 prototype would prove very impractical to compressed from the outside. Therefore I decided to use 2 smaller tension springs mounted on the outside. These springs would be mounted to a cylindrical platform which would be free to move along the barrel of the tube.
In this model all components except for the tube, springs and ping pong ball will be made from wood. The working principle of this version is as follows: the ping pong ball would be placed inside the barrel of the tube. The cylindrical platform would be pulled downwards, thus putting tension in the springs. Finally the cylindrical platform would be let free, this will cause the springs to return to their original state, in turn pulling the cylinder upwards with force. This kinetic energy would then transfer to the ball, hence launching the ping pong ball out of the tube.
In my next blog post I will be assembling this model in real life using all upcycled parts I found around my house. If you want to see more of this project or similar engineering endeavors to this, please consider giving my page a follow. See you next week!
