There&#x2019;s a lot going on when you bounce a ping-pong ball on a paddle. Tiny movements in the muscles of your hand and arm adjust to keep the ball centered, as you watch, feel and listen in order to react quickly and with enough pressure to not goof it up.

Arduino-tinkerer &#x201c;tkuhn&#x201d; [created a machine that juggles a ping-pong ball](https://electrondust.com/2018/07/22/stepper-juggler/) on a platform by listening to the variations in sounds the ball makes when it strikes the wood. Like a human hand, it makes small adjustments to keep the ball center. Unlike us, it does it only using sound.

Every tap the ball makes on the platform is a little different, and picked up by four microphones surrounding the platform. Depending on where it lands, the ball&#x2019;s sound determines where it&#x2019;s located relative to the microphones, which tell the motors how to adjust the platform and keep the ball bouncing.

It&#x2019;s easier to see how fine the adjustments are in a side view of one of tkuhn&#x2019;s earliest iterations of the robot:

A wooden plate juggles (and balances, while juggling) a ping pong ball (PID controlled).

Read more here: https://electrondust.com/2017/10/28/the-quest-to-get-a-machine-to-juggle/

things used on the plate:
-4 mics (for the determination of the ballposition)
-4 Savöx servos (for the movement)
-1 Mbed LPC1768 microcontroller (for the PID controlling)
-some analog signal processing circuits (for the mics)

PID Controlled Ping Pong Ball juggling Robot

They made [another ping pong bouncing bot](https://electrondust.com/2017/10/28/the-quest-to-get-a-machine-to-juggle/) that&#x2019;s pretty slick, too. This one uses LED lights to determine how to keep the ball balanced:

This is a Raspberry Pi Ping Pong Juggling Robot. This time it doesn't use a plate but a real ping pong racket. I use Nema17 stepper motors for the movements. I bought some belt drive components and ball bearings. I used those to convert the axial motion of the steppers into linear motion. It's the same setup as in an inkjet printer. 

The main controller of this project is a Raspberry Pi 2. It has a Raspberry Pi camera module NoIR connected to it. Because it is NoIR, there's no IR filter on the thing. I originally planned to use infrared light to brighten up the ball in front of the camera. But it didn't work well. The IR elements I used were way to weak. The infrared noise in the background (sunlight etc.) would overweight the infrared light getting reflected from the ball. Another problem was, that the IR signal gets picked up on the red-channel of the camera. This means that even if there is no infrared noise from sunlight, the infrared light's intensity still has to surpass the normal red light's intensity to be recognizable. So that idea got dropped. I ended up using the green-channel on the camera with green light from a RGB Power LED pointed at the ping pong ball. Sure enough, normal white light would've done the job as well, but I just happened to have a RGB LED laying around so I ended up with green.

Another important thing is this: You might be asking yourself "Why did you use a light source anyway? Couldn't you just use OpenCV and get the thing to recognize the white ball with normal lightning conditions?". The answer to this is: Probably not. At least not with a Raspberry Pi 2. The reason being that we need 90 fps real-time image processing to track that ball. The image processing has to be done fast. I didn't really use OpenCV for anything at this Point in time but my guess is, that it wouldn't work well, because I don't think I could get the processing rate up to 90 fps with OpenCV. Maybe I am wrong about this, but it was my gut feeling, so I went on to program a really simple algorithm that could take care of the image data really fast. The algorithm looks directly at the data stream, so no conversion to a different datatype (e.g. NumPy) is needed. This is one reason this algorithm is quite fast. The other reason is, of course, that it is a dumb algorithm. All it does is look for a clump of bright green pixels on a fairly black background and then compute the center of it.

After the algorithm extracted 3D coordinates out of the images it examines that data and, in the case the ball is going downwards and gets quite near to the racket, it uses that specific data set in a PID controller to get some useful correction values on the racket as the ball is moving towards it (the machine tries to get the ball into the center of the racket). The movement values are then sent to the Arduino Uno over the serial port. The Arduino Uno runs grbl (a program for running stepper motors very efficiently) and all it does most of the time is waiting for some new instructions from it's master; The Raspberry Pi.

Some lines about the tilting mechanism: The tilt in the racket is produced by different heights of the 3 belts on which the racket is mounted. The joint "belt to racket" is a rather elastic one. This is why a tilted racket can be accomplished without ruining the joints or other parts of the machine. 

It's worth mentioning that the thing isn't working perfectly at all. A 2 hour juggling session is definitely not one of the things this machine is capable of doing. It is able to juggle the ball for about 2 mins. Then it makes some misjudgment and the ball drops. So it isn't great. But then again: It only uses one single camera to compute 3D coordinates of a flying object. using 2 cameras would improve the performance. But it would also make the thing more complicated.

The Stepper motors are Nema17 Steppers! It Also uses an Arduino Uno with Arduino CNC-Shield.

The Arduino Stepper Motor Controller uses the great grbl software! here a link:
https://github.com/grbl/grbl

And here's the complete Raspberry Pi source code (Python3):
https://github.com/T-Kuhn/PingPongPi

And here's a blog post about the thing:
https://electrondust.com/2017/10/28/the-quest-to-get-a-machine-to-juggle/

Music:
"Covered In Oil" by Broke For Free
http://www.brokeforfree.com

PID Controlled Ping Pong Ball Juggling Robot With 90FPS Realtime Image Processing

There are plenty of good [Arduino steppers](https://www.arduino.cc/en/Reference/Stepper) available as open-source libraries, but their reaction times aren&#x2019;t quite fast enough to juggle a ball. &#x201c;The only problem was that the ones I know all accelerate linearly and take their sweet time whenever a change in direction occurs,&#x201d; tkuhn writes on their blog. &#x201c;Time is precious. The thing has to go up, stop, and almost instantly start going down again.&#x201d;

As Hackaday notes, it&#x2019;s reminiscent of a [maze platform](https://hackaday.com/2013/08/11/wireless-marble-labyrinth-uses-ti-dev-hardware/) or [Stewart platform](https://hackaday.com/2014/08/22/stewart-platform-ball-bearing-balancer/), which balance balls but don&#x2019;t have to bounce them. The added function of listening makes this one seem like it has a mind of its own.

An Arduino Project (Using an Arduino Uno & Arduino Nano) that tries to keep a Ping Pong Ball bouncing with 4 mics.

The thing uses: 
- 2 Arduinos
- 4 Stepping motors
- 4 mics
- stuff

Some words about how it works:
The location is calculated from the time difference between mic pairs (one pair for each axis on the 2-dimensional plate). The noise from the ball hitting the plate is a very loud one; It was surprisingly easy to distinguish it from other sounds. You can talk casually nearby the machine and it won’t be affected.

There is a small analogue circuit to process the signals from the mics. Each mic sets a flip-flop (a thing to store a either a “1” or a “0”, digitally speaking) if it’s amplitude surpasses an adjustable voltage level. An Arduino Nano is watching all the flip-flops and resets them after they all got set (the blue LEDs in the video show the current flip-flop states). The Arduino checks the states as fast as possible, this allows for a reasonable accuracy. Another Arduino then uses this data to tilt the plate in such a way that the ball will – hopefully – not fall down. It uses a PID controlling approach for this.

Some code:
Here’s the Github repo with all the code for this project:
https://github.com/T-Kuhn/Stepper-Juggler

I made the data visualization on the left with Unity. It shows the data points gathered by the 4 mics. The plate is driven by 4 steppers.

Here's a blog entry about the thing:
https://electrondust.com/2018/07/22/stepper-juggler/

FYI.

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Watch a Mesmerizing Robot Bounce a Ping-Pong Ball Using Sound

FYI.

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Watch a Mesmerizing Robot Bounce a Ping-Pong Ball Using Sound

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