Building an Idea
Don’t you hate it when rain gets on your BBQ? Why not just make a track for it, and hook it up to a rack and pinion system from an old car gate?
When a friend at work saw my LED project, he decided to bring in his old car gate system for me to tinker with. The system contained a motor, 5 racks and a pinion, a motor controller board with various I/O, and an IR reader (without the remote control).
My roommate just purchased a BBQ, and we did not have a convenient place to store it. These two events resulted in the idea to make an electric BBQ track.
Clean and Wire
First thing was taking the motor controller outside. I blasted it with air, and vacuumed out all of the spiders. I will spare you the pictures of the carnage.
After a bit of searching online, I found the datasheet for the motor controller and started to test all of the I/O to see how well it worked.
Gate Control Board
The plan for the controller involved a start button, two limit switches, and a motor. Press the button, and the platform would move forward until it hit the first limit switch, and then it would stop. Press the button again, and the platform would move back until the second limit switch was hit.
There was an issue I had when I first plugged it in. When pressing the button to start, the motor would run for one second, and then turn off. I tried changing the settings and potentiometers, but nothing seemed to change that 1 second period.
Reverse Engineering the Chip
The next troubleshooting plan was to test the I/O of each of the pins for the micro-controller on the board.
Unfortunately it was not a dedicated IC, so the datasheet only gave me the pins for VCC, GND, and OSC, leaving me to assess the rest.
With no other idea coming to mind, and the project being abandonded if I couldn’t find a solution, I went proceeded to try obscure methods.
I tried pressing the start button, and then as the motor turned on, I quickly applied 5V to the OSC pin, thus freezing the state of the micro-controller. It worked, and I was surprised.
Because the motor kept running, I concluded the issue is what of the input puts on the micro-controller that was affecting the settings. I was unable to find it, but if I replaced the micro-controller with one that I programmed myself, I could run the system effectively.
Replacing the Chip
I needed 6 I/O for my proposed system, so I used an ATtiny84 that I had from a previous project.
It was not the same size as the previous controller, so I wired it up to easily remove it.
This was just done for fun at home, and I do not advocate wiring like this.
The code for this is pretty basic, and probably not worth posting, but I will anyway.
Fixing the Motor
Now was the time to test out the motor, and come across the second part that didn’t work.
The motor would stop working when a minimal amount of resistance was applied to it. I did not understand. I tried measuring power consumption and I also listened to the motor as I was stalling it. The motor was still running when the gear was stopped. This made me decide to open up the gearbox and take a look.
It seemed that a groove had worn in the middle gear (beige) as a result of the ball bearing (purple). This was probably due to someone only half torquing the clutch pin (green). The result was the the ball bearing would not transfer torque from the gear to the shaft. In order to cause the ball bearing to re-engage with the gear, I shimmed the pinion & shaft (pink). This caused the ball bearing be moved axially within the gear, causing it to catch on the locating grove in the gear.
The laziest solution I could come up with was to just shim the clutch gear. This positioned the motor such that it would stay engaged, and solved my problem.
Building the BBQ Base
The BBQ would need to be on a base frame which could also contain the motor and track wheels as an entire unit.
After doing some quick measurements, I found out that the BBQ barely fit into the storage alcove by itself, let alone on a track. This inspired me to disassemble the bottom of my friend’s BBQ (to his dismay) to reduce its overall height. This also provided me with threaded holes to mount the bottom of the BBQ securely to the slider base.
The base needed wheels that were low profile, as height was an issue. I found wheels for $1.50 each at Home Hardware.
The system also needed a constraint on lateral motion, as the motor would only apply a force on one side of the track. The constraint had to be smooth and of low friction, otherwise the whole track would bind and possibly deform. I chose to solve this problem by placing strips of UHMW along the sides of the base to fit inside the track.
Building the Track
I had access to some scrap wood, so all designs were based off of what was available.
The area which the track was to be installed was not necessarily level, causing me to shim until I met my tolerance of +/- half a bubble on my level. Precision was not required.
I also needed to ensure the track was secure, but the entire project was temporary, so nothing too extreme was done to secure it.
The uneven force due to the motor meant that the track had to be rigid enough to keep its shape and be repeatable when in use. Cross beams were installed in the optimal locations to reduce deformation.
The rack bars were installed to fit the height of the motor gear at each location.
Adding a Hinge
The purpose of this track was to use the BBQ on the grass, while storing it under the balcony when not in use. Leaving the track on the grass would have been just as much of a problem. The solution to this was a hinge in the middle of the track, allowing it to be stored when the BBQ was not in use. I did not decide to automate this component.
I needed to ensure that the hinge split was still smooth and level for the wheels to run along during use. I did this by recessing the wood where the hinge sat with a Dremel. I also had to make sure that the inside of the tack was consistent along this gap, for the UHMW track to slide along without binding.
The rack teeth needed to be continuous for the application to work. Because the rack was located above the axis of rotation for the hinge, they would collide with each other when retracted. My solution to this was to notch each of the pivots on opposite sides, as the rack was still strong enough when some material as removed.
The limit stops had to stay out of the way of the track, but still hit the limit switch on the BBQ track. I did not see a need for anything too strong, so I used whatever took the least amount of time to build.
And Voila! An automated BBQ track.