[MOC] TrayRacer 2.0 | Lego robot that brings around snacks at a party

[896gerard]

My new MOC is probably the weirdest Lego Technic car (I) ever uploaded- it is made to be invisible and to work autonomously. Its use case is based on the fact that at a party, you never really get the snack you want without interrupting talks of others. My solution is a small robot that follows the edge of the snack table while carrying the snack tray(s). That is where the name comes from, it 'races trays' around the table and I liked the software-origined word 'Raytracer' a lot. For the same reason why a car exists with the name 'Interceptor'. 

I have done some exploring in this area, a few years ago TrayRacer 1.0 was published. But it could only follow rounded tables. And that got me thinking: can you build a Lego Thing that is able to understand when there's a corner coming up? 

The first answer was 'no'. It was never done before and I couldn't think of any way. The second answer was 'Well, for the intended size, you will need a mechanical sensor and a mechanical 'brain' that is able to translate the 'mechanical sensor data' into 'Let's start a steering action NOW'. That's a lot of quotes, how to convert this into hard plastic bricks? After a lot of thinking, tinkering and tuning TrayRacer 2.0 was born. It is really hard when you throw the remote out of the door, to make the robot think for itself without using the convenient Mindstorms sensors. TrayRacer 2.0 uses only a single PF M motor and is full mechanic Lego Technic. There's an important reason why I only used an M-motor: power consumption. A nice robot system with a lot of sensors and actuators will be always empty as the testing is done and the party is starting. A simple rechargeable battery box with a PF M motor is small and reliable. I added PF navigation lights because it looks good in the dark, which looks good on the party. The new TrayRacer 2.0 is 4 studs high, exposing the 4-stud batterybox as a load on the back. This had to be accepted as I needed the speed control: you HAVE to be able to set the 'feeding speed' on parties otherwise people eat way too much. In other cases, maybe an old battery  would have been a solution. 

[As a sidenote: with the new Lego Technic electronic systems coming up, I don't see battery boxes and motors becoming any smaller. This is a bad development that TLG should be aware of. ]

High quality photos (Brickshelf files not yet public, so direct links:) 1 2 3 4 5 6 7 8

The video explains it all: 

The deep technical details- a dynamic system

Usually, 'mechanical sensor data' is fed into a clutch/driving ring or a differential, which means the device using the data also influences it. The 'mechanical imput impedance' is too low (This robot is built using mostly electronic engineering concepts..). In the RayTracer 2.0, this effect is visible in the sensor wheel. It takes some effort to shift the driving ring, which means the rubber band needs to be stronger. As said in the video, the sensor wheel needs to be pressed to the RIGHT (so robot pressed LEFT) for straight driving, which is harder when the rubber band is stronger. Because the robot is very light (lithium battery, no big electric components) a stronger rubber band means it needs to push harder to the LEFT, which is only possible when its rotational inertia is higher, so then it needs to be heavier. But weight kills power consumption. A lower weight causes oscillations, as the robot slams to the left using its inertia after a succesful steering action - enabling straight driving- , then the rubber band springs back because the robot is not able to press hard enough to the left continuously- enabling the left wheel brake again-. When the rubber band is too strong, it oscillates forever, when it is just a little too strong, the robot is designed to reach a stable orientation in a few oscillations. In the video this effect is sometimes visible, you can see different dynamics with its bodywork removed! Also, as you can see, the sensor wheel is on a long lever. The lever enables tuning of the force needed to press it to the RIGHT, thus for tuning it together with the rubber band system. To summarize: because the 'mechanical information' is influenced by its 'processor', there is a fine optimum in rubber band strenght vs sensor wheel lever length vs overall robot weight. For this reason, I could give you instructions, but there's no guarantee that any reproduction would work. Any axle that is a tiny little bent, any driving ring with some small damage, even the friction of individual pins, these are all factors that influence the final tuning and need to be optimized. 

Instructionless building with Lego Technic is not only a unique combination of parts, it's also the way it's built and tuned that makes it a final product.

Some quick photos: 

 

 

Edited November 21, 2019 by 896gerard
edit: typo check

[MangaNOID]

That is very cool. That mechanical action to turn...that's pure genius!

First I thought it was for tables where people sat so I was going to suggest to call it the 'keep your elbows off the table' but I see where its useful now 

anyway well done I think its function is great 

[Conchas]

Very cool and well thought engineering!

I just see the risk of the protruding sensor to get stuck on the hip/haunch of someone near a corner of the table, and the it will continue to move straight and will fall off. 

[MAB]

I'm not too sure it is a problem but anyway the solution is excellent.

[SamuelYsc]

Cool! Super cool! Now this is real engineering! 

[deraven]

Well, I also don't know that this is much of a problem in my life, but I definitely agree that the engineered solution is clever and well-executed!  I love mechanical solutions like this.  Great build!  

[Krol Zmija Sebastian]

You should get your rank situated to "Citizen." @896gerard

[Permo]

Very cool !

Especially the name of the project, almost made me choke on my coffee 

[rekreK]

I like it, but one practical problem, it will knock over all your drinks if they are on the edge of the table.

Maybe it needs a front bumper that detects obstacles and stops all drive, then resumes once the obstacle is removed... TrayRacer 3.0

[KirTech LAB]

Awesome system!

[aeh5040]

That's brilliant!  I love mechanical sensing, and I understand how much thought is needed to work in a confined space like this!

I wonder about the long-term robustness of using axle connectors (in the sensing arm and in the little arm that holds the wheel in place at the bottom of the picture).  They have a tendency to work loose...

[896gerard]

On 11/22/2019 at 12:38 AM, rekreK said:

I like it, but one practical problem, it will knock over all your drinks if they are on the edge of the table.

Maybe it needs a front bumper that detects obstacles and stops all drive, then resumes once the obstacle is removed... TrayRacer 3.0

You're right. But there's always a catch. I agree with you that some additional sensor would be nice. Maybe will turn this in a project at some time. 

[jrx]

Really cool! Great mechanical engineering!