Shirley the sensational sugar spinning robot: Part 1

For this year's Big Bang Fair, Rapid's Education Manager Chris Calver (right) collaborated with TV presenter Stefan Gates on his live show. The brief: to build a sugar-spinning, Malteser firing, thermal imaging robot. This is the first part of how he did it. 

I think it is fair to say that this is one of the most bizarre robot builds I have done but at the same time, also one of the most interesting.

Stefan Gates, perhaps better known as The Gastronaut, is a television presenter, author and live performer who describes himself as a Food Adventurer. To find out more about Stefan, check out his website.

Stefan does a live stage show at the Big Bang Fair each year. His shows are well known for providing an energetic, exhilarating yet hugely educational experience and 2016 was no different. This year’s show was entitled Gastronaut and the Quantum Mechanical Chocolate Factory and Stefan decided it would be cool to incorporate some robots into the performance.

Stefan Gates & 'Shirley'

I got chatting with Stefan at the Bett Show in January and we worked out a couple of ways that we could incorporate a robot into the stage show. What we needed was a robot that could fire Maltesers, spin sugar and give the audience a thermal imaged view of the world ...

It was harder than it sounded. This is how I built the robot that came to be known as 'Shirley' ...

The design

Stefan had pretty much left the design up to me. There were some basic criteria with size and he also sent me a knife steel which needed to be incorporated for the sugar spinner. Other than that, I was on my own. I wanted to make the robot look as much like a large VEX IQ robot as possible and I was also keen to use the VEX IQ motors to make the wiring simpler. However, the size of it and the fact that it needed to travel well and be relatively maintenance-free meant that I would build the bulk of the frame out of VEX aluminium and screw it all together with nyloc nuts.

The first task was to screw together the basic metal chassis so that I had a basis to design all the rest of the parts to. It’s made from 7 pieces of standard 1x5x35 hole VEX aluminium C-Channel and bolted together using nyloc nuts. I also bolted a couple of pieces of IQ plastic to the chassis just to check that it was going to be easy enough to interface the VEX IQ motors with the VEX EDR metal.

The chassis

Next job was to take the knife steel and work out a way to rotate it at approximately 45rpm as requested by Stefan - this forms the sugar spinner of the robot. It’s fairly weighty so I needed to come up with a way to support the steel at either end of the handle.

Conveniently, the knife steel that Stefan provided has a nice square profile on the handle which will make it easier to clamp to and rotate it. The clamp part is mounted to a VEX IQ turntable. I then built a frame to house the whole of the handle of the steel and at the other end of this frame is another IQ turntable. I drilled a hole in the centre so that the blade of the steel will protrude through and this will provide a perfect rotatable support.

What to do with the camera?

Next, I started to look at how to house the thermal imaging camera on the robot. The camera I am using is the FLIR i3, partly because this is what I had to hand and partly because it can be used as a USB webcam. This will allow us to connect it to a laptop and ultimately, to the large screen at the back of the stage. A quick test at my desk was looking good so I set about working on a way to mount it on the robot.

Now the FLIR i3 is a rather unwieldy beast and not really the ideal camera for this application. However, I like a challenge so I stuck with it!

It’s too big to use as one of the robot’s eyes, so I decided to give the robot a wide mouth and have the camera there instead. This also means I could hide the long handle of the camera in the neck area.

The first job was to build a frame around the camera that held it securely and could then be used to mount it to the robot. Once I had a secure frame, I then needed to make it a little bit more like a face so added some eyes using VEX IQ wheels and temporary eyebrows. I’ll change them for coloured plastic later and also add a mouth once it is fixed to the robot and everything is working.

Shirley's thermal imaging mouth

So back to the sugar spinner - I now needed to mount this to the robot. The whole thing is pretty weighty so I decided I'd use 2 VEX IQ motors using a fairly substantial reduction gear ratio to raise and lower the arm.

The shoulder joint pivots on an axle with two 60-tooth gears mounted to the frame of the arm. These 60-tooth gears will be rotated by a 12-tooth pinion giving a gear ratio of 5:1 at the shoulder.

To make sure there was plenty of space for the head mechanism at the top of the robot, the two motors that would actually control the raising and lowering of the arm were mounted in the bottom of the robot’s body. The two motors are connected together with a pair of 36-tooth gears and then chain and sprockets are used to transfer the rotational movement up to the shoulder. This uses an 8-tooth sprocket at the motor end and a 32-tooth sprocket at the shoulder end giving a first-stage gear ratio of 4:1. This means the total gear ratio of the whole mechanism is 20:1 which will give it plenty of grunt.

When in use, the arm needs to stick out at 90 degrees in front of the robot and when rested it sits at approximately 20 degrees so with the 20:1 ratio, it will take approximately 2.3 seconds to raise and lower.

The sugar spinner

Look out, here comes the maths part!

A VEX IQ motor has a maximum output speed of 100rpm. Our gear ratio is 20:1 so the speed at the shoulder is 100/20 = 5rpm.

5rpm is equal to 1800 degrees per minute (360 x 5) or 30 degrees per second (1800/60). Our arm needs to move through approximately 70 degrees in total and 70 / 30 gives us 2.3 seconds. In reality, it will be a bit slower than this as the motor won’t hit 100rpm under load but it will be fast enough for me!

Now to make the head move.

It doesn’t need to rotate left and right, just raise up and down so that Stefan and his team can aim the camera at an object in front of the robot. I needed to keep the motor out of the way of the handle of the thermal imaging camera so it didn’t block its path as it swings through. To do this, I created a chain of three 36-tooth gears. The first gear in the chain is driven by a 12-tooth pinion which is connected to the motor. The third gear shares an axle with another 12-tooth pinion forming a compound gear which drives a final 36-tooth gear giving a total gear ratio of 9:1. We don’t need much torque here so I’ll control the speed by slowing the motor down in my program.

About halfway through the build

In the next blog I will tell you how the rest of the build went, and what happened when Shirley took to the stage!