In the second part of his blog about building 'Shirley', his Big Bang Fair robot, Rapid's Education Manager Chris Calver (right) concludes the build and hands it over to Stefan Gates for its starring role in the 'Quantum Mechanical Chocolate Factory'.
Shirley was starting to take shape in the last blog. The sugar spinning and thermal imaging parts were done. Now I needed to come up with a way to fire a Malteser - as you do.
Now, as it happened, I was refereeing at a VEX Robotics Competition at the time of the build and the 2015/16 challenge, Nothing but Net, required teams to fire foam balls into a goal. Whilst these balls are much bigger and heavier than a light and airy chocolate snack, the concept is the same. Teams were using all sorts of mechanisms to fire the balls and I am not at all afraid to say that I shamelessly stole the idea for my launcher from team 3117, the Sterling Sharks of Stanborough School.
The mechanism is very simple. It consists of a linear slide with a rack gear mounted on the top. The slide can be moved along by rotating the gear.
By cutting a few of the teeth off the gear, there is a period in each rotation where the slider can run freely under the gear. The slider itself is spring-loaded with elastic bands and the kinetic energy in the bands is released quickly in the few moments where there are no teeth on the drive gear holding the slide in place.
It took a few different versions to get this working reliably but after a few mangled Maltesers, it was up and chucking! I started off driving the 36-tooth gear directly but the motors did not quite have enough grunt so I added a 3:1 gearbox which gives it enough power. Later on, I upgraded it to four motors. It’s probably overkill, but I wanted it to fire reliably in the show.
Now all the functional parts of the robot were complete, I started adding some finishing touches. The right arm will have a shoulder and elbow joint as well as a claw that can open and close to grab objects.
The shoulder works in exactly the same way and utilises the same gear ratios as the left arm. It’s powered by two motors which are placed down in the base of the chassis next to the left shoulder motors.
The elbow joint uses a single motor which drives a first stage gearbox using a 3:1 reduction. The motor is placed as close to the shoulder joint as possible, to reduce the weight at the end of the upper arm which means the shoulder will have less strain on it. Power is then transferred to the elbow joint via a chain and sprockets with a 4:1 reduction in gear ratio. This gives a total elbow gear ratio of 12:1.
With most of the skeleton complete, it was time to add the VEX IQ Brain that would control the motors and then get it wired up.
The arms, head and launcher use a total of 12 motors which just so happens to be the number of ports on a VEX IQ Brain so I can run all the upper body stuff from the same Brain. The drive base to make the robot move around the floor will need four motors as well; these will be driven from a separate Brain.
I was using a pair of 4" VEX EDR wheels on each side of the front of the robot with omni-directional 4" EDR wheels on the back which essentially act as a castor. Each pair of front wheels is driven by two motors. The two motors are linked together using two 36-tooth gears so both motors drive the same shaft.
With the robot now mobile, I went back to the launcher and added two additional motors giving it a total of four. In this image, you can see the teeth that have been cut off the green gear to give it the “slip gear” functionality.
The robot then needed programming so that the controls were as simple as possible for Stefan and his team to operate on stage. This meant writing a program that would calibrate the arms when the robot was first switched on so that I had some datum points to work from. I could then make single button presses that moved various parts of the robot to the correct positions for the show. You can check out the ROBOTC code here.
Stefan's show was called the 'Quantum Mechanical Chocolate Factory' and ran three times a day for the four days of the Big Bang Fair. Shirley was certainly one of the stars:
Shirley's fame has been more than fleeting - since the Big Bang Fair my robot has appeared with Stefan on Channel 5's The Wright Stuff, played shows in Butlins and entertained the Director General of the BBC at the BBC Food Awards!.@stefangates has a new girlfriend - and she fires maltesers! #ShirleyTheRobot pic.twitter.com/ZFfQ6lUdY0— The Big Bang Fair (@BigBangFair) March 19, 2016
That @stefangates introduces BBC DG Tony Hall to the joys of sugar work #bbcfoodawards pic.twitter.com/GjLSf4thDo— Charlie Hicks (@totalcharlie) April 28, 2016
1 comment :
Wow, excellent post. I'd like to draft like this too - taking the time and real hard work to make a great article. This post has encouraged me to write some posts that I am going to write soon.
linear slide system
Post a Comment